Late Neogene lacustrine record and palaeogeography in the ...diposit.ub.edu/dspace/bitstream/2445/34428/1/145794.pdf · Tertiary continental sedimentary successions charac-terise

Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) 5–37

Late Neogene lacustrine record and palaeogeography in theQuillagua–Llamara basin, Central Andean fore-arc (northern Chile)

A. Saez a,Ł, L. Cabrera a, A. Jensen b, G. Chong b

a Grup de Geodinamica i Analisi de Conques, Departament d’Estratigrafia i Paleontologia, Facultat de Geologia,Universitat de Barcelona, Campus de Pedralbes, E-08028 Barcelona, Spain

b Departamento de Ciencias Geologicas, Universidad Catolica del Norte, Avda. Angamos 0610 – casilla 1280, Antofagasta, Chile

Received 15 September 1996; revised version received 15 December 1997; accepted 30 November 1998

Abstract

The Cenozoic Quillagua–Llamara basin (northern Chile, Central Andes) is an asymmetrical, intramassif fore-arc basinwith a relatively wide northern sector separated from a narrower southward extension by a basement threshold. The north-ern sector was characterised by a noticeable Oligocene?–late Neogene alluvial-fan and lacustrine dominated depositionwhich resulted in sequences up to 900 m thick, whereas the southern sector was often a bypass zone with thinner fluvialand lacustrine sediment accumulation. The basin infill includes two third-order alluvial–lacustrine unconformity-boundedunits which include other higher-frequency (4th to 5th order) sequences. The evolution of the Late Miocene–Pliocenelacustrine episodes in the Quillagua–Llamara basin was not only controlled by the regional variations from arid tohyperarid palaeoclimate conditions, due to the combined influence of the Pacific high pressure cell, the rain shadow effectexerted by the rising Andes and the northward flowing cold oceanic currents, but also by: (a) the extensional tectonics andevolution and uplift of the fore-arc region which defined the location and size of the depocentres; (b) the resulting basementpalaeorelief which affected sediment thickness and facies distribution during the late basin-infill episodes; and (c) thetectonic modifications of watersheds, water divides and drainage networks in the Precordillera which caused considerablechanges of water income in the lacustrine systems. Understanding of this regional tectonosedimentary evolution is anecessary first step before analysing of the low- to high-order lacustrine sequence changes in the region. Lacustrine watersupply was very sensitive to tectonics; even gentle tectonic tilting and uplifting in critical water-divide zones could resultin changes in water balance in the lacustrine basins and trigger variations in the depositional record. The very conspicuous,lacustrine regime changes recorded in the Quillagua–Llamara basin infill cannot be considered in themselves conclusiveproof of an exclusive climatic forcing, since they took place close to either major regional drainage changes or to gentlebut noticeable tectonic reactivation in the fore-arc region. 1999 Elsevier Science B.V. All rights reserved.

Keywords: lacustrine sequences; Central Andes; northern Chile; late Neogene; intra-massif fore-arc basins

1. Introduction

The fore-arc region of the Central Andes in north-ern Chile is located in one of the most arid to hy-

Ł Corresponding author. E-mail: [email protected]

perarid regions in the world. General agreement ex-ists that this current extreme aridity is because theregion is under a high atmospheric pressure cell,the rain-shadow effect of the high Andes cuts offAtlantic moisture contributions, and the northward-flowing cold oceanic waters affected by upwelling

0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 1 3 - 9

6 A. Saez et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) 5–37

prevent moisture from the Pacific Ocean penetratingonshore. But how and when these factors affected cli-matic evolution in the Miocene to Recent is still de-batable (Alpers and Brimhall, 1988; Ortlieb, 1995).

Non-marine intramassif fore-arc basins have beenevolving from Tertiary time in northern Chile in afringe which extends from the present volcanic arc(Western Cordillera) as far as the Coastal Range(Chong, 1988). The Quillagua–Llamara basin is oneof these basins where regional tectonic and palaeocli-matic evolution affected lacustrine systems. Recentand ancient lacustrine records have often been citedas substantial sources of pristine data for analysingregional and global environmental changes (Keltsand Talbot, 1990; Anadon et al., 1991; Gierlowskiand Kelts, 1994). Nevertheless, the differentiation ofthe relative influence of diverse driving forces onlacustrine evolution is not always obvious, due eitherto the absence of high-resolution records or to poorunderstanding of the tectonic and morphological set-ting of the lacustrine systems.

This paper deals with the stratigraphy, main sed-imentological features and palaeogeographic evolu-tion of the ancient lacustrine systems which devel-oped in the Quillagua–Llamara basin. It also pro-poses a tectonic and morphological framework forthem. The late Neogene lacustrine sequences of theQuillagua–Llamara basin were deposited in a sce-nario of ongoing regional to global climatic events,but these were also coeval to critical structural andmorphological changes in the southern Central An-dean region. A good grasp of regional tectonosed-imentary evolution is needed before interpretationof the low- to high-order sequential changes. Thisis also a pre-requisite of analysis of the potentiallykey high-resolution sedimentological, geochemicaland palaeobiological records in lacustrine sequences,in order to detect higher-order, climatically forcedperiodical changes.

2. Geological setting — the Quillagua–Llamarabasin

The southern Central Andes is a typical orogenicbelt segment which rises between the oceanic Nazcaplate and the western margin of South Americasince the Cretaceous (Fig. 1). The convergent mar-

gin of the Central Andes is characterised by a high-to moderate-altitude western fore-arc region, an ex-tensive high plateau area with Cenozoic volcanicactivity increasing to the east (volcanic arc), andan easternmost retro-arc fold and thrust belt. ThickTertiary continental sedimentary successions charac-terise the tectonosedimentary evolution in this partof the Andes (Schmitz, 1994; Lamb et al., 1997).

In northern Chile, between 18º and 23ºS, theonshore fore-arc zone of the Central Andes hasthree main N–S-oriented morphostructural units: theCoastal Range, the Longitudinal Valley (Central De-pression) and the Precordillera Range. Several Ceno-zoic alluvial–lacustrine and volcanoclastic basins de-veloped in each of these domains (Reutter et al.,1988, 1991; Cabrera et al., 1995; Chong et al., 1999;May et al., 1999 and Gaupp et al., 1999).

The Longitudinal Valley includes several of N–S-oriented, elongated, asymmetrical fault-boundedbasins and heights. Normal faulting has been putforward as the main process in the formation ofthis basin zone (Mortimer, 1973). These exten-sional faults have been related either to generalregional extensional regimes developed in the intra-massif fore-arc zones (Arabasz, 1971; Reutter et al.,1988, 1991; Hartley et al., 1988; Flint et al., 1991;Scheuber et al., 1995; Hartley and Jolley, 1995;Santanach et al., 1996), or to the local to regionalextensional resolution of lateral motions along majorregional strike-slip faults (Flint et al., 1991; Cabreraet al., 1995; Jensen et al., 1995).

The Quillagua–Llamara basin extends from 21ºto 23º000S and is a non-marine basin of 150 km inlength. Its eastern margin is bounded by the west-ern Precordillera Range which mainly consists ofPrecambrian basement rocks, Mesozoic terrigenousand carbonate sequences, and Cenozoic intrusive andextrusive rocks. The Precordillera Range slopes from4000 m to 1000 m high in a region mantled byMiocene rhyolitic ignimbrites. The western edge ofthe basin extends along the foot of the Coastal Rangeand consists of a Palaeozoic metamorphic basementand Mesozoic igneous rocks, which rise to 1800m. The Atacama Fault System to the west and thePrecordillera Fault System to the east are two ma-jor N–S-oriented ancient fault systems (Mesozoic toPalaeogene) which are close to the boundaries of thebasin (Fig. 1). The Tertiary Quillagua–Llamara basin

A. Saez et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 151 (1999) 5–37 7

Fig. 1. Regional geological setting of the Quillagua–Llamara basin in the Central Andes and geological sketch of the Quillagua–Llamarabasin (Longitudinal Valley–Central Depression) and surrounding Coastal Range, Precordillera Range, Central Andean Depression(including Calama basin) and Western Cordillera zones. AFZ D Atacama fault zone. WFZ D Western fault zone. Note location of studiedsections, significant basin zones and of the chronostratigraphic frameworks a–a0 and b–b0 in Fig. 2. ME. D Maria Elena, Q. D QuillaguaVillage. Oil wells: 1 D Lomas de la Sal, 2 D Hilaricos.


Fig. 2. General longitudinal and transverse chronostratigraphic frameworks of the Cenozoic basin infill of the Quillagua–Llamara basin.Topographic and structural features are not represented. See Fig. 1 for location.


was formed by the combined activity of N–S- andNW–SE-oriented faults, which in most cases wereconcealed by basin infill and became supratenuousfaults. The subsurface data on the basement depth(Rieu, 1975; Dorr et al., 1995) and the basementoutcrop distribution (Fig. 1) suggest that the NW–SE faults split the basin into several sub-basins andstructural highs, and define two sub-basins (north-ern and southern) characterised by different struc-tural, stratigraphic and palaeogeographic featuresand bounded by a basement threshold (Encanadahigh, Figs. 1 and 2). The northern sub-basin is 75km wide and 75 km long and has a rhomb-shapedpattern which has been interpreted as the result ofa pull-apart, strike-slip-related evolution (Jensen etal., 1995). Data from two oil exploration wells inthe Lomas de la Sal area (20 km east of the LoaRiver, see Fig. 1) indicate that the pre-Cenozoic sub-stratum depth ranges from 603 to 930 m deep, withthe maximum located in the southern part of thenorthern sub-basin (Rieu, 1975). These subsurfacedata show that the basin bottom is asymmetrical witha gentle western margin and a steeper eastern one.The southern sub-basin was a narrower zone (30km wide and 75 km long). The frequent occurrenceof substratum highs in this zone points to the factthat the maximum basin substratum depth is not asdeep as in the northern sector. In consequence, thestratigraphic successions of the northern and south-ern basin sectors are different, with the northernsector characterised by thicker, continuous deposi-tion, whereas the southern sector was often merelya bypass zone with less accumulation (Fig. 2). Inaddition, the lower basin infill units, which are wellrepresented in the northern areas, are lacking orthinner in the southern ones (Jensen, 1992).

Therefore, the overall geometric features of theQuillagua–Llamara basin infill is clearly a con-sequence of the fault activity which affected thefore-arc region, but the upper basin infill occurredwhen fault activity was much less. Therefore, theUpper Neogene basin fill successions onlap a sub-stratum palaeorelief which was carved during earliererosive phases. The main sedimentary infill rangesprobably from Oligocene (Jensen et al., 1995) toPliocene–Pleistocene. Terraced alluvial-fan, fluvialand lacustrine deposits ranging from Pleistocene toRecent overlie unconformably the Late Miocene–

Pliocene sequences. Neogene exposures crop outmainly along the N–S-oriented Loa river valleytrenches (Fig. 4), which extend across the westernbasin zones, and also along ephemeral creeks (the so-called ‘quebradas’). The ‘quebradas’ flow westwardfrom the Precordillera Range and contribute to theLoa River or end in closed drainage zones (Fig. 1).The entrenched Precordillera ‘quebradas’ reach thepre-Cenozoic substratum in several areas and mostlyexpose the successions closer to the western basinmargin which range from 100 to 300 m in thick-ness. Extensive exposures of Cenozoic successionsare lacking in the northern and central basin zoneswhere the quebradas have not cut so deeply.

3. Stratigraphy

3.1. Methodology and techniques

The study of the basin infill has been focused onits stratigraphy and major sedimentological features.This has been the subject of previous work (Rieu,1975; Naranjo and Paskoff, 1982; Marinovic and Lah-sen, 1984; Jensen, 1992). Field mapping was carriedout, supplemented by satellite images and aerial pho-tography analysis of the laterally extensive exposuresalong the Loa valley and near the basin margins.Some oil exploration well data were also available(Rieu, 1975) and were used to establish the max-imum recorded thickness and the lateral extent ofthe subsurface successions. Stratigraphical–sedimen-tological logging provided the framework for selec-tive sampling of petrological, sedimentological andpalaeobiological (diatomites, palynomorphs) analy-ses. More than 150 samples from diverse sedimentaryand volcanic facies were taken for further mineralog-ical (XRD) and petrological analysis both by opticmicroscopy and SEM (Jensen, 1992). Biotite crystalsfrom various pyroclastic deposits interbedded in thealluvial–lacustrine sequences were K=Ar-dated. Thesedimentological and palaeoenvironmental interpre-tations of the sequences studied were integrated intothe regional tectonic setting and were the basis ofpalaeogeographic basin reconstructions.

The depositional record in the basin includes Ter-tiary and Quaternary deposits. The stratigraphic sub-division adopted here for the Tertiary basin fill builds


on previous proposals (Jensen, 1992; Cabrera etal., 1995) and consists of two low-order unconfor-mity-bounded units (Lower and Upper), whereas theexpansive–retractive relationships between the allu-vial and lacustrine sequences enable three geneticsubunits (A to C) in the Lower Unit to be distin-guished.

3.2. Lower Unit

The Lower Unit includes the alluvial, lacus-trine and volcanogenic sequences deposited betweenthe basement-Tertiary unconformity and the uncon-formably overlying evaporite-dominated sequencesof the Upper Unit. It ranges from 500 to 800 m thickin the northern basin sector and is lacking or is verythin in the southern zones (Figs. 1 and 2).

Subunit A includes the alluvial successions de-posited between the basement-Tertiary unconformityand the lowermost lacustrine deposits which definethe base of Subunit B (Fig. 2). The area distribu-tion and lithological features of the sequences of thissubunit are poorly known since it only occurs inthe subsurface. Nevertheless it has to be restrictedto the northern basin sector where well-log data(Hilaricos and Soledad wells) show that Subunit Aincludes red-bed-dominated, coarse-grained alluvialassemblages, which were deposited on proximal tomiddle alluvial fan fringes. The alluvial fan systemsspread mainly from the Precordillera, although mi-nor contributions could also reach the basin fromthe Coastal Cordillera, and the northern and southernends of the northern sub-basin.

Subunit B includes alluvial–lacustrine sequencesdeposited between the lowermost lacustrine depositsoverlaying Subunit A and those which underlie Sub-unit C. These sequences are only found in the north-ern basin sector. Outcrop and well-log data show thatin this zone Subunit B consists of alluvial red bedscontaining conglomerates, sandstones and mudstoneswhich were deposited on proximal to distal alluvialfan fringes. Moreover shallow lacustrine, playa andplaya-lake sequences up to 450 m thick and con-sisting of diatomites, carbonates and sulphate evap-orites also occur. Most of these deposits have beenreported in the subsurface and only the uppermost la-custrine evaporite episode crops out in some zones ofthe northern sub-basin (Hilaricos anhydrite, Figs. 2,

4 and 6). In the linking zone between the Quillagua–Llamara and Calama basins the sub-unit B depositsare not so well developed but a slight increase in thesubstratum depth allowed the deposition of a terrige-nous and evaporite alluvial and playa lake sequence,up to 100 m thick, which is broadly equivalent tothe middle-upper part of the Lower Unit (Batea For-mation; Jensen, 1992). Preliminary radiometric K=Aranalysis performed on biotites with 7.621% of K con-tent, 1.782 nl=g of radiogenic Ar and 63% of atmo-spherical Ar indicate an age of 6:0 š 0:4 Ma for vol-canic layers interbedded in the lower part of Hilaricosanhydrite, which dates the upper part of the LowerUnit as Late Miocene (Fig. 2).

Subunit C includes the sequences deposited be-tween the settling and early spreading of the low-ermost lacustrine deposits which overlie subunit Band the development of the bounding unconformitybetween the Lower and Upper Unit. A variety of lat-erally related alluvial fan, fluvial, fluvial–lacustrine,lacustrine and volcanoclastic facies occur in this unit(Figs. 2–5 and Figs. 6–9). Most of these terrigenous,diatomitic, carbonate and epiclastic facies assem-blages are included in the previously defined Quil-lagua Formation (Rieu, 1975; Jensen, 1992) and in thePuente Posada limestones (Figs. 2 and 3). Some of thealluvial deposits included in this subunit have beendescribed as depositional units (Arcas alluvial fan,Dorr, 1996). The lacustrine diatomite, carbonate andterrigenous deposits extend through the northern andsouthern basin sectors (Figs. 1–3). The sequences ofthis subunit also thin towards the western and south-ern basin margin sectors where they onlap the regionalbasement (Figs. 2–4). The Puente Posada carbonateunit is assumed to be laterally equivalent to the car-bonate-dominated sequences which crop out exten-sively in the linking zone between the Quillagua–Llamara and Calama basin (El Loa limestone, Mayet al., 1999). Preliminary radiometric K=Ar analy-sis carried out on biotites with 7.215% of K content,1.617 nl=g of radiogenic Ar and 44% of atmospheri-cal Ar indicates an age of 5:8 š 0:4 Ma for volcaniclayers interbedded in the lower part of the QuillaguaFormation in the Cerro Mogote section, which datesthe lower part of the Upper Unit as Late Miocene.Other preliminary magnetostratigraphic data suggestthat the upper part of this unit could be Pliocene (Gar-ces et al., 1994).


Fig. 3. Stratigraphic correlation between the lithological–sedimentological logs of the late Neogene (Late Miocene–Pliocene) successionsin the southern sub-basin of the Quillagua–Llamara basin. See Figs. 1 and 2 for location.

3.3. Upper Unit

This unit is up to 100 m thick and overlies un-conformably the Late Miocene–Early Pliocene se-

quences of the Lower Unit, which are affected bygentle tectonic deformation (Figs. 1 and 2). Themostly evaporitic Soledad Formation (Bobenrieth,1979) is included in this unit and in the north-


Fig. 4. Oblique aerial view of the Loa River crossing Longitudinal Valley in the northern sub-basin from 21º350 (bottom) to 21º150S(top). The north is located at the top of the photograph. S. D pre-Tertiary substratum; L. D Loa Canyon breccias (mainly scree andalluvial fan coarse deposits); A. D Hilaricos Unit (ephemeral playa-lake anhydritic deposits); L. D distal Precordillera alluvial fan red-bedfacies. Note as this unit wedges out to the north. Q. D Quillagua Formation (mainly lacustrine marls, diatomites and travertines), R. DQuaternary red alluvial fan deposits.

ern Quillagua–Llamara sub-basin it overlies un-conformably different Late Miocene–Pliocene units,which are gently tilted or folded. Thus, in CerroHilaricos, the Soledad Unit overlies the distal Pre-cordillera alluvial fan successions, whereas in Lomasde la Sal it overlies the Hilaricos anhydrite. The de-posits of this unit also onlap the basement on thenorthern and western basin margins (i.e. in CerroSoledad and Cerro Termino, Figs. 1 and 2). Theupper boundary of this unit is the erosive surface re-lated to the earlier lowering of the regional base leveland the subsequent entrenchment of the drainagenetwork. Its relative stratigraphic position and theavailable radiometric–magnetostratigraphic dating ofthe underlying sequences suggest the age of theSoledad Formation deposits as Pliocene–Pleistocene(?). Although this unit is clearly dominated by haliteand anhydrite deposited in ephemeral playas and in

playa-lake environments, minor alluvial fan terrige-nous facies and pyroclastic and epiclastic beds occuras well (Fig. 10).

The Soledad Formation has been extensively dis-solved and eroded in widespread basin zones, butthe sequences of this unit can be observed in severalhill ranges (Lomas de la Sal, Cerro Soledad, CerroHilaricos, Cerro Termino) where this unit is up to100 m thick. The westward decrease of the altitudein the summits of these ranges (Lomas de la Sal1097 m a.s.l., Cerro Soledad 1095 m, Cerros Hilari-cos 1060 m, and Cerro Termino 1048 m) indicates avery gentle westward dip in this part of the basin.

3.4. Quaternary deposits

The Pliocene–Pleistocene? lacustrine evaporitesof the Soledad Formation were affected by ero-


Fig. 5. General depositional styles of the channels in the fluvial and fluvial–lacustrine facies of the Quillagua Formation (LowerUnit, Subunit B). (a) Multistorey ribbon-like channel infills in the Puente Posada area (see stratigraphic location in Fig. 3). (b) Minormulti-storey, ribbon-like channel infills affecting underlying, abandoned laterally accreted point bar bodies which show pervasive rootcolonization in Encanada (see location in Fig. 1).

sion during the transition from internal to externaldrainage conditions in extensive basin zones. Thischange meant the end of the extensive evaporite sed-imentation (nowadays limited to some minor karsticcollapse ponds), the deep entrenchment of the LoaRiver and some of its tributaries, and the onsetof a process of degradation of the formerly de-veloped evaporites by dissolution and karstic col-lapse (Rieu, 1975; Cabrera et al., 1995). MinorPleistocene-to-Holocene terraced deposits, which aremostly restricted to the neighbouring Loa valley,overlie unconformably the Neogene successions of

the Quillagua–Llamara zone. Here several fluvialand fluvial–lacustrine terrace levels, with a wideclast compositional spectrum occur (Rieu, 1975).The thickness of the terraces and their clast lithologyvary depending on: (a) the pre-Tertiary and Tertiarysubstratum composition in close drainage areas oflocal tributaries; (b) changes in river profile. In theolder continuous terrace (San Salvador Unit, Rieu,1975), local deposition of macrophyte travertines,domal stromatolites, oncoids, diatomite marls andpure diatomites took place; these record the devel-opment of fluvial–lacustrine environments in some


Fig. 6. Sedimentological log of Hilaricos Unit in Lomas de la Sal. Northern sector of Quillagua–Llamara basin. See Fig. 1 for location.

ancient river transects. The younger lower terracelevels (Las Vegas Unit, Rieu, 1975) are mainly ter-rigenous and more continuous laterally: they arepresent throughout the entire Loa valley and theLoa Canyon, including the outlet area where marineinfluence is recorded.

4. Depositional framework

The most significant lacustrine successions in theLower and Upper Units of the Quillagua–Llamarabasin were fed by alluvial and fluvial systems anddeveloped ahead of their terminal zones (Fig. 11).

Moreover, occasional volcanic events, which gener-ated widespread pyroclastic and epiclastic deposits,were also significant and noticeably affected the finalcomposition of the detrital and=or biogenic alluvialand lacustrine facies. A regional overview of thealluvial and volcanic frameworks is now given asa prerequisite to understanding the evolution of thelacustrine record.

4.1. Alluvial fan and fluvial systems

Changing sediment and water contributions fromthe alluvial systems were some of the most impor-tant features which either triggered or hampered the


evolution of lacustrine systems in the fore-arc basins.Alluvial fan contributions spread into the Quillagua–Llamara basin directly from the neighbouring Pre-cordillera and Coastal ranges. The Precordillera allu-vial-fan watersheds were more extended than thosein the Coastal Range (i.e. Arcas fan watershed up to750 km2). Other water and sediment incomes werecontributed by a northward-draining fluvial systemfed by wider, higher Precordillera catchment ar-eas located to the east of the southern end of theQuillagua–Llamara basin.

The major alluvial fan systems were restrictedmostly to the Northern Basin sector where thick,significant red-bed successions were sedimented,whereas thinner colluvial scree deposits, alluvialfan and fluvial successions were deposited in thesouthern basin sector (Figs. 2 and 3). Alluvial fandeposition in the northern basin sector was the resultof the erosion of the Precordillera and Coastal Rangerelief caused by extensional faulting which gave riseto a set of basins and ranges (Santanach et al., 1996).Some gentle, minor internal progressive unconformi-ties occur in the upper alluvial fan sequences whichcrop out along the Precordillera and Coastal Rangebasin margins. This suggests that minor reactivationof the structures previously generated took placeduring the sedimentation of the upper alluvial fansequences which finally concealed the faults.

The Coastal Range alluvial fan systems gave riseto red and grey coarse-grained facies assemblages upto 300 m thick (Loa Canyon breccias), with exten-sive exposures visible in the Loa Canyon. Thesesequences consist of interbedded coarse-grained,poorly to well sorted and matrix- to clast-supportedbreccias and conglomeratic breccias. These depositsshow a rather changing arkose- to slate-dominatedclast composition, depending on whether the localsource areas are igneous intrusive or metamorphicin composition. These facies assemblages were de-posited by mass and stream flows in low-efficiencysmall alluvial fans radiating out a few km. The largethick exposures observed in the Loa Canyon showthat the alluvial fan sequences are arranged intothree fining upwards sequences which range from 20to 80 m in thickness and which record successivestages of fault scarp generation and degradation.

The Precordillera alluvial fan sequences includethick (up to several hundreds metres), coarse- to

fine-grained red bed sequences the upper part ofwhich (up to 300 m) has been observed along theeastern marginal basin zones and the Loa river valley.They have been also reported in the two available oilwell logs, where they are up to 400 m thick (Rieu,1975). A striking feature of the depositional recordof this alluvial fan assemblage is that one of itssingle genetic units seems to preserve its nearly finalfan-shaped depositional surface long after becominginactive (i.e. Arcas alluvial fan, Jensen et al., 1995;Dorr et al., 1995; Dorr, 1996). The Precordilleraalluvial fan systems were more efficient than thoseof the Coastal Range and their contributions werewidely spread over nearly the entire northern sectorof the Quillagua–Llamara basin. The largest Arcasalluvial fan reached the foot of the Coastal Rangeduring its maximum spread and radiating out to 40km from apical to fan-toe zones (Figs. 2 and 11).

The Precordillera proximal alluvial fan faciesassemblages consist of coarse-grained, boulder- togravel-dominated conglomerate sequences with mi-nor interbedded sandstones and red mudstones. In-terbedded lenticular dune-like aeolian sands occurfrequently in the upper part of the sequences. Threeto four discontinuous but noticeably spread volcaniclayers are also interbedded in the upper successionsof these proximal alluvial sequences, where alsosome internal unconformities occur (Fig. 2). Thesemarginal unconformities are spatially restricted andcannot be traced basinward. The proximal alluvialfan sequences grade laterally into medial to distal,poorly sorted fine-grained conglomerate channels,sheet sandstones and mudstones. This facies assem-blage crops out extensively in the Quillagua zonewhere it reaches a thickness of 40 m and rapidlythins southward and westward, onlapping the basinsubstratum in the Coastal Range and the La En-canada threshold and interfingering with the lacus-trine successions (Figs. 2 and 4).

The overall sedimentological features of theCoastal Range alluvial fan sequences suggest thedominance of flash flood deposits probably laiddown during storms. Some of the depositional fea-tures in the Precordillera alluvial fan systems alsosuggest ephemeral water contributions, which werenot so flashy as in the western alluvial fan sys-tems. Moreover, the larger radial spread of thePrecordillera alluvial fans suggests that water and

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Fig. 7. Stratigraphic–sedimentological logs of the Cerro Mogote and of Quebrada Temblor sections showing low- to high-frequency sequence trends of the QuillaguaFormation. Curve a represents 4th-order cycles of increasing–decreasing of persistence of the lacustrine conditions. Curve b represents the recorded transgressive–regressive5th-order lacustrine cycles. See Figs. 1 and 2 for location and Fig. 3 for legend.


Fig. 8. Upper part of the Quillagua Formation in the Quebrada Temblor section. White area corresponds to marl, diatomites and travertinelacustrine facies. Dark dark corresponds to detritic deltaic facies. Triangles indicate 5th-order sequences. Locations in Figs. 1 and 2.Notice the rapid lateral changes of the high-frequency sequences and the stratigraphic position of sequences C, D and E in Fig. 7. R. DQuaternary red alluvial fan deposits.

sediment contributions from the eastern source areaswere significatively larger. The position of the Pre-cordillera Range, its greater height and larger catch-ment areas would have resulted in greater surfaceand groundwater water supply into the lacustrinezones.

The late Neogene fluvial facies are mostly relatedto the Quillagua Formation and crop out all alongthe western margin of a narrow zone which stretchesfrom the southern end of the Quillagua–Llamarabasin (i.e. Maria Elena–Puente Posada area) to thenorthernmost outcrops located nearby the La En-canada threshold (Figs. 1–3). The most proximalsandy and coarse-conglomerate fluvial facies relatedto these axial, northward-directed fluvial contribu-tions are up to several dozen metres thick and oc-cur along the southern basin zones (Puente Posadasection, Figs. 3 and 5). The remaining middle todistal fluvial facies of this system are distributedmore widely and crop out extensively. The distal–terminal fluvial assemblages grade northwards intothe marginal lacustrine zones where diatomitic and

carbonate marginal assemblages developed. Lateraltransition into the inner lacustrine sequences of theQuillagua Formation occurs near the boundary be-tween the southern and the northern sub-basins (Que-brada Temblor section, Fig. 3).

Both facies distribution and palaeocurrent trendsof the fluvial sequences in the Quillagua Formationsuggest that most of their deposits came from north-ward-flowing semi-perennial to ephemeral channelsystems. The axial northward-spreading trend of flu-vial facies along the southern basin sector suggeststhat water and sediment contributions were fed fromextensive, higher Precordillera catchment areas (In-termediate Basin), which would have fed a largerwater supply into the lacustrine systems developedin the northern sub-basin.

4.2. The volcanic influence

There are several laterally continuous (up to somekm) tabular, stratiform volcanic and volcano-sed-imentary deposits interbedded in the Quillagua–


Fig. 9. (a) Smectite (s) and micritic calcite (m) crystals in diatomite facies of the Quillagua Formation. (b) Vitric volcanic shards in thediatomitic lacustrine facies of the Quillagua Formation.


Llamara successions, which are good local to re-gional key beds. These deposits are the product ofpyroclastic flows (ignimbrites) and falls and theirequivalents reworked in the lacustrine environment.The pyroclastic falls are coarse to fine ash tuffsthat can be classified as vitric tuffs and ashes and,since they are fragmentary, as vitric-crystal tuffs andashes. The degree of weathering is low to null, andtherefore most of these lithofacies are friable and canbe considered in a broad sense as tephra. In spiteof this, the pristine characteristics of the depositswere frequently modified, leading to bentonitizationor carbonation of the glass fragments; often, too,interstitial cement hardened the rock.

Clear pyroclastic and epiclastic textures can beobserved under the microscope (selective concen-tration of crystal fragments, cross-lamination, etc.).The glass fragments are mainly minute shards, some-times tightly packed and with delicate gradations ingrain size. These textures look like pristine fall ones,and are interpreted as such. The pumice fragmentsare less abundant, and generally below lapilli size(coarse ash). These fragments are mostly vesicu-lated, showing sometimes some degree of flatteningof the vesicles (and of the fragments themselves).The arrangement of pumice clasts enables hot, in-situ deformation to be excluded. Quartz, plagioclase,biotite and, to a lesser extent, amphibole and al-kali feldspar occur between the crystal fragmentsin most lithofacies. Therefore, the mineral composi-tion suggests that some of those deposits are daciticto quartz-andesitic. Nevertheless, it must be kept inmind that most of the rock consist of glassy frag-ments and can be noticeably more silicic than thecrystal content (e.g. rhyolitic in composition). Theorigin and meaning of these ignimbrite layers shouldbe seen in the context of the Late Miocene evo-lution of the Central Volcanic Zone of the Andes,which extends from 14º to 28ºS and has been activesince the Late Miocene (10.4 Ma) up to the Recent(De Silva, 1989; De Silva and Francis, 1989). Thespread of ignimbrites from their original volcanicfocuses was conditioned by the pre-existing topog-raphy (Guest, 1969). In Miocene–Pliocene times thereliefs of the Precordillera normally acted as barri-ers to the pyroclastic flows coming from the east.Nevertheless, some of these flowed over the Pre-cordillera Range and reached the eastern parts of

the Quillagua–Llamara basin. Volcanic processes inthe volcanic arc region are often recorded in thefore-arc depositional record with the development ofwidespread pyroclastic and epiclastic deposits whichin some cases attain large areal spreading even fromvery distant volcanic focuses. Some punctual pyro-clastic episodes may be significant as potential keysto establish a correlation between arc and fore-arcevolution but they do not seem to have resulted indramatic modifications of the fore-arc depositionalframework.

4.3. The lacustrine episodes

Three major late Neogene (Miocene–Pliocene)lacustrine episodes can be distinguished in the Quil-lagua–Llamara region.

(1) The Lower Unit–Subunit B lacustrine episodewhich includes among others the ephemeral, salineplaya-lake sequences of the Hilaricos anhydrite, andevaporite- and diatomite-bearing subsurface succes-sions. The whole lacustrine episode was provablyMiocene and developed between the early settlingand spreading of the lacustrine zones in the north-ern basin sector and the widespread progradationof the Precordillera alluvial fans, which caused theobliteration of the evaporite zones (Figs. 2 and 11b).

(2) The Lower Unit–Subunit C lacustrine recordincluding the fluvial and perennial lacustrine se-quences of the Quillagua Formation and the PuentePosada Unit (Late Miocene–Early Pliocene?). Thislacustrine episode took place after significant pyro-clastic layers were deposited in the basin. This isthe best known lacustrine episode in the region withthe largest facies variety and a lot of outcroppingsequences (Figs. 2 and 11c,d).

(3) The Upper Unit playa and playa-lake record ofthe Soledad Formation (Pliocene–Pleistocene?). Thelacustrine sequences related to this episode over-lie a tectonic unconformity which records changingdepositional gradients and regional drainage. Theevolution of this lacustrine stage was character-ized by a spreading of the depositional zones andthe clear connection between the Quillagua–Llamarabasin and other neighbouring depocentres (e.g. SalarGrande, Figs. 2 and 11e).

The amount of information available on thesewell-distinguished lacustrine stages is diverse, due to


the changing quality and extent of the exposures andthe variety of facies in the lacustrine zones. How-ever, it is possible to define their most significantgeneral features and establish some comparison be-tween the ephemeral, saline–evaporitic systems andthe perennial oligohaline systems.

5. The evaporite lacustrine record

5.1. Lower Unit lacustrine evaporites (Subunit B)

The outcrops of the evaporite lacustrine sequencesdeposited during the early evolutionary stages of thebasin (Subunit B) are only found in the upper part ofthe succession and near the Hilaricos anhydrite. Nev-ertheless the available oil well data (Hilaricos, Fig. 1)show that up to 450 m thick evaporite-bearing se-quences occur in this unit (Rieu, 1975) and suggestthat the main depocentre of lacustrine evaporites dur-ing the earlier basin evolutionary stages was locatedaround the Lomas de la Sal area (Figs. 2 and 11b).Thus, during the lacustrine episodes linked to Sub-unit B, evaporite-dominated facies were depositedmainly in the southern part of the southern sub-basin, in the marginal, peripheral evaporitic fringesof a perennial, likely saline lake located more to thenorth and where carbonate and diatomite sedimen-tation took place (Fig. 11c,d). Sulphate evaporitesbecame dominant and more widespread in the upperpart of this Subunit B, leading to the deposition ofthe Hilaricos anhydrite.

The Hilaricos anhydrite crops out in the north-ern basin sector in the Loa valley exposures and toa lesser extent, in Cerro Hilaricos and Lomas dela Sal, with a thickness ranging from 11 up to 20m. From these northern basin areas, the anhydritesuccessions thin westward and southward, onlap theregional basement along the western basin margin inthe Coastal Range (Figs. 2 and 4) and may reach theLa Encanada threshold (Figs. 2 and 3). This unit isoften detected in the subsurface thanks to the col-lapsed dolines which affect the overlying QuillaguaFormation sequences.

This unit is composed mainly of laterally exten-sive anhydrite beds interbedded with red mudstonesand pyroclastic and epiclastic beds which are morefrequent in the eastern basin sectors closer to the Pre-

cordillera (Fig. 6). The interbedded red mudstonesand lithic, epiclastic wackes and sandstones indicaterapid and repeated progradation of the terminal allu-vial fan zones fed from the Precordillera and CoastalRange and which surrounded the playa and playa-lake zones. That interbedded volcanoclastic bedswere more frequent in the anhydrite successionsthan in the coeval alluvial-dominated assemblagessuggests that volcanoclastic deposits were preservedbetter in lacustrine–palustrine zones sheltered fromterrigenous contributions. In the Quillagua villagearea near the Loa valley (Figs. 1 and 2) the anhy-drite beds are predominantly white and display di-agenetic massive and nodular (chicken-wire) facies.However, some decimetres-thick, shallowing-upwardplaya-lake sequences occur in the Hilaricos zone(Fig. 6). The most simple of these sequences con-sists of anhydrite beds topped by desiccation cracksand vertical, root-like traces filled by anhydrite in-traclasts in a red mudstone matrix. More complexplaya-lake sequences display four main lithologicalterms: (a) laminated anhydrite, (b) selenitic in-situgrowing crystals, (c) nodular anhydrite supported inred mudstone matrix, and (d) red mudstones. Someof these anhydrite sequences show their tops par-tially brecciated or affected by dissolution causedprobably by later influence of diluted flows (Fig. 6).

Anhydrite sequences of the Hilaricos unit recordthe sedimentation in playa-lake environments rang-ing from subaqueous to vadose and subaerial zones.These lacustrine assemblages were deposited be-tween the terminal fringes of the alluvial fan systemsrooted in the Precordillera and in the Coastal Rangeand record the development of sulphate-dominatedplayas and playa-lakes. The recorded lateral faciesrelationships between the alluvial fan successionsand the lacustrine evaporites suggest that the wa-ter and solute feeding of the ephemeral–saline lakeswere largely dependent on these surrounding Pre-cordillera and Coastal Cordillera fan systems. Notraces of significant water and sediment contribu-tions from other catchment areas have been detected.(Fig. 11b).

The smaller-scale sequence arrangement of thelacustrine record in the Hilaricos anhydrite (Fig. 6)shows the spreading and retreat of the subaqueousephemeral lacustrine zones, and the influence exertedby the terminal alluvial fan facies. The sequences ob-


served could be the result either of shallow lake levelchanges or of autogenic evolution (i.e. sediment in-fill, progradation–retrogradation and lateral shifts ofthe terminal fan zones which spread into the la-custrine zones). Some especially noticeable erosivesurfaces related to entrenched channels or dissolu-tion could be due to incisions forced by lacustrinewater-level drops and changes in the water soluteconcentration.

5.2. Upper Unit lacustrine evaporites (SoledadFormation)

Saline deposits of the Soledad Formation (Boben-rieth, 1979) are the uppermost, non-terraced Neo-gene lacustrine sediments recorded in the Quillagua–Llamara Basin (Fig. 2). The halite- and sulphate-dominated sequences of this unit only occur in thenorthern basin sector, which also includes the up-permost saline deposits in the Salar Grande area(Chong et al., 1999). Pyroclastic and epiclastic, vol-canic lithofacies also occur in the sequences studiedin the Quillagua–Llamara basin (Figs. 2 and 10).The thickest sections of the Soledad Formation wereformed in Cerro Soledad and in Lomas de la Sal,where they reach at least 100 m. A low halite mem-ber (about 80 m thick) and an upper, much moreextensive sulphate–halite member can be distin-guished. The lower halite member crops out widelyin Lomas de la Sal where it has been intermittentlymined. The upper sulphate–halite member crops outalong the western and northern basin margins, reach-ing the Salar Grande basin margins (Figs. 1 and 11e).All the evaporite successions of the Soledad Forma-tion in the Quillagua–Llamara Basin display earlydiagenetic textures which show that these depositswere intensively diagenised under vadose conditions.This caused massive displacive playa halite facies,whereas subaqueously generated facies did not form(Fig. 10). Only in the neighbouring Salar Grandebasin do chevron-like crystals and other features ofoutcropping upper halite deposits prove subaqueoussedimentation in very shallow saline lakes (Cabreraet al., 1995; Chong et al., 1999).

According to Bruggen (1950), the saline lacus-trine sequences of the Soledad Formation (i.e. theevaporite sequences in the Quillagua–Llamara zoneand in the Salar Grande halite-dominated sequence)

Fig. 10. Sedimentological log of Soledad Formation in the CerroSoledad area (northern sector of Quillagua–Llamara basin). De-tailed facies observation is prevented by recent dissolution pro-cesses. See Figs. 1 and 2 for locations.

mark the existence during the early Quaternary ofa deep, perennial saline palaeolake. On the basisof some stepped lineaments recorded in the CerroSoledad zone and which involve the Soledad For-mation deposits, Bruggen (1950) also suggested theexistence of abandoned lacustrine shorelines whichwould have developed during successive water leveldrops of the perennial saline lake, which would haveattained a water depth of up to 80 m. Other authors(Hollingworth, 1964; Stoertz and Ericksen, 1974)accepted this idea and even increased the estimateddepth to 200 m. Later contributions (Rieu, 1975) nu-anced that the stepped arrangement could have beencaused by the activity of normal faults. Moreover,the sedimentological and early diagenetic features ofthe facies which make up the evaporite sequences inthe Soledad Formation do not support the hypothesis


of a deep, perennial saline lake. Any interpreta-tion of the observed morphological features has totake into account the persistently shallow, ephemeralcharacter of the lacustrine zones. Thus, tectonic step-ping (Rieu, 1975) and=or erosive terracing of thepreviously deposited Soledad evaporites during thelate erosive stages of the basin are the most likelyexplanation for the morphologies observed. In anycase, the highest topographical position of the evap-orite deposits of the Soledad Formation records thehighest infill level attained in the basin during theclosed-drainage evolutionary stages of the basin.

The large area reached by the Soledad Formationsequences (Fig. 11e) in the Quillagua–Llamara andSalar Grande basins culminates the spreading trendof sedimentation in this fore-arc region, since lacus-trine depositional zones were more extensive than inprevious episodes (Fig. 11b–d). The halite–anhydritedominated sequences record sedimentation in shal-low playa-lake environments ranging from subaque-ous to vadose and subaerial zones. In the Quillagua–Llamara basin these lacustrine assemblages were de-posited in a belt between the terminal fringes ofalluvial fans draining the Precordillera and Coastalranges. The water and solute feeding of these lacus-trine systems seems to have been again dependentmostly on these surrounding fan systems; there is noevidence of water and sediment contributions fromthe southern basin. In the Salar Grande basin theplaya and playa-lake successions received surfacewater and solutes from the Quillagua–Llamara zone(Chong Diaz et al., 1999).

5.3. Meaning of the evaporite lacustrineassemblages

The distribution of the evaporite lacustrine faciesassemblages in the Quillagua–Llamara zone duringthe sedimentation of the Lower and Upper Unitsshows that, although the area distribution of theephemeral playas and playa-lakes clearly increased,the lacustrine zones were always restricted to thenorthern subbasin (Fig. 11b,e). The Late MioceneHilaricos anhydrite and the Pliocene–Pleistocene (?)Soledad Formation were deposited in extensive (upto 1300 and 2000 km2, respectively) shallow playaand playa-lake systems (Fig. 11b,e) which only oc-casionally included steady, shallow open water. This

considerable area correlates well with both the west-ward increase of the depositional zones and thecoeval Precordillera alluvial fan shrinkage.

Among other late Neogene lacustrine units theevaporite assemblages of the Hilaricos anhydriteand the Soledad Formation are clear examples ofthe widespread ancient ephemeral, saline–evaporiticlakes in the region. They clearly show negative waterbalances over an extensive region whose hydrologywas probably determined by a low to very low lo-cal surface runoff which was not compensated bysurface or groundwater inflow from neighbouringor more distant high zones. If it is so, it must beemphasised that the expansion of the evaporite la-custrine zones was coeval to the retrogradation of thePrecordillera alluvial fans.

The widespread dominance of shallow subaque-ous to vadose facies in the evaporite deposits of theephemeral–saline lakes in this region has importantconsequences, since it means that the water inputand accumulation in the Quillagua–Llamara basinduring long time spans of the late Neogene was justenough to keep place with subsidence. Thus for thePliocene–Pleistocene (?), when the Soledad Forma-tion was deposited, this water income was signifi-cantly lower than previously hypothesized (Bruggen,1950; Hollingworth, 1964).

6. Perennial lacustrine record

Well developed perennial lacustrine facies assem-blages occur in the Lower Unit (Subunits B andC, Figs. 2 and 11c,d), as is shown by subsurfacedata and the successions recorded in the QuillaguaFormation and Puente Posada Unit (Figs. 2 and 3).The N–S-oriented Loa river valley provides overmore than 100 km near-continuous exposures ofthe Quillagua Formation sequences (Figs. 1 and 4).Moreover, in the Cerro Mogote section (northernsector) the lacustrine successions reach up to 65 min preserved thickness. In the southern basin zones,the Quillagua Formation becomes fluvial-dominatedand passes laterally to the Puente Posada LimestoneUnit (Figs. 2 and 3). The area of the carbonate-dom-inated sequences of the Puente Posada Unit in thestudy zone is not precisely known, but their expo-sures are restricted to the southernmost end of the


basin (Figs. 1–3). Six major facies assemblages havebeen distinguished in the deposits interpreted as la-custrine and fluvial–lacustrine on the basis of theirlithological features, thickness, geometry, sequentialarrangement and floral and faunal contents.

6.1. Shallow lacustrine–palustrine carbonateassemblage

This carbonate-dominated assemblage of thePuente Posada Unit overlies the lowermost red screeand alluvial fan deposits which onlap the basementin the southern basin sector and pass laterally north-ward into the diatomite-bearing facies of the lowerQuillagua Formation (Figs. 2 and 3). This lime-stone-dominated assemblage is laterally spreadingwestward and interfingering with the Quillagua For-mation sequences of the carbonate-dominated El LoaFormation which was deposited in the neighbouringCalama Basin (Naranjo and Paskoff, 1981; May etal., 1999), but no physical continuity has been estab-lished to date between the two units. The successionstudied is up to 12 m thick and made up mainly of cmto dm thick limestone and minor interbedded sand-stones and green-grey mudstones (Fig. 3). Limestoneis mostly sandy biomicrites (mudstones–packstones)with minor bioclastic gastropod shell accumulation.They show typical shallow lacustrine and palustrinesedimentary and early diagenetic features. The top ofthis unit is partially incised by fluvial palaeochannelsandstones of the Quillagua Formation. The upperpart of the limestone packet is irregularly brec-ciated and karstified, and shows dissolution porosityranging from small vugs to larger caverns filled bygreen mudstone. This facies assemblage records de-position in shallow, carbonate-dominated lacustrine–palustrine zones, in floodplains sheltered from activechannel zones. The sequential relationships observedbetween this carbonate-dominated unit and the over-lying channel-dominated facies record the shift orspreading of these fluvial active channel zones intothe lacustrine–palustrine floodplains (Fig. 3).

6.2. Intraclastic lacustrine breccia assemblage

A widespread breccia-dominated facies assem-blage occurs in the northern basin sector along theeastern Loa riverside in the lower part of the Quillagua

Formation (Fig. 3). The breccias are whitish, verypoorly cemented and make up lenticular bodies sev-eral kilometres wide and up to 15 m thick. They rangefrom massive to well-stratified and show large inter-nal erosional surfaces. Single beds in the stratifiedbreccia bodies are sheet-like to lenticular with theirthickness ranging from a few centimetres to 0.5 m.The breccias are poorly sorted, polymodal, rangefrom clast- to matrix-supported and include boulder-to gravel-sized, angular to sub-angular clasts com-posed of volcanics, lacustrine marls and diatomiteswhich vary from centimetres to decimetres in size andare often silicified. The matrix is marly with variableamounts of volcanic epiclastic particles and includesa mixed diatomite assemblage, which comes from thereworking of the lacustrine materials. No significantterrigenous, extrabasinal contributions are recordedin these deposits, but some clastic contributions fromthe neighbouring ranges cannot be ruled out.

This breccia assemblage is the consequence oferosion and cannibalistic reworking by mass flowsor highly concentrated flows of partly lithified py-roclastic, epiclastic and lacustrine deposits occurringin marginal lacustrine zones. This cannibalistic, in-trabasinal reworking was subsequent to the prioraccumulation and consolidation of pyroclastic andepiclastic deposits which entered the lacustrine sys-tem through major volcanic activity. The gradualupwards transition between these breccias and theoverlying lacustrine sequences suggests that theirdeposition took place during the early stages of de-velopment of the Quillagua lacustrine depositionalframework. They may reflect the reworking and re-deposition of marginal lacustrine deposits in innerlake zones due to tectonic destabilization of the lakebottom floor or to storm-generated flood events.

6.3. Inner-marginal lacustrine facies assemblage

The inner-marginal lacustrine facies consist ofsequences up to 7 m thick of massive to poorly lami-nated, white to very pale brown marls, diatomaceousmarls and diatomite beds. This assemblage is wellrecorded in the middle part of the Cerro Mogotesection (Fig. 7).

Both marls and diatomaceous marls make uptabular to gently lenticular beds (tens to hundredsof metres in length) up to 0.5 m thick. These


are massive or thinly laminated and are quite of-ten connected to pure diatomite beds. Whitish toyellowish diatomitic marls often include thin, mil-limetre-thick lenses or pure diatomite. They oftengradually grade upwards into purer white-diatomitebeds which are up to a few decimetres thick. Theoptical microscope analysis shows pelloidal micriticaggregates (0.05 to 0.1 mm), rounded pumice grains(0.1 mm in diameter) often altered to smectite andscattered fragmented diatom frustules, charophytes,disarticulated ostracod valves and gastropod remains(Fig. 9a). Minor quartz, feldspar and volcanic vitricshards are also frequent components (Fig. 9b). Thescanning microscope shows that most of the claygrains are associated with vesicular volcanic parti-cles (20 mm), whereas the ostracods are sometimesmicritized. Widespread microsparitic and micritic ce-ments occur, forming euhedric–subeuhedric crystalsof calcite reaching up to 6 mm at maximum.

Pure diatomites form lenticular and decimetre-thick tabular beds which attain at least a hectometrein lateral extension. They are white, soft and veryporous although some of them are silicified and areassociated with decimetre- to centimetre-scale chertnodules. Diatom frustules can be well-preserved butthey are often rather fragmented. Microsparitic andmicritic calcitic cements, together with minor quartzcement precipitation, are also found. Preliminarydata from diatom assemblages found in these fa-cies record widespread oligohaline–mesohaline con-ditions in lake waters. Nevertheless they also suggestrapid changes of bathimetry and salinity both inthe single diatomite beds and in the whole verti-cal section (Servant Vildary in Jensen, 1992; Baoet al., work in progress). These diatomite-dominatedfacies record subaqueous, perennial, inner-marginallacustrine conditions with terrigenous contributionsranging from negligible to noticeable. The overalltaxonomical composition of the diatom flora and theabsence of well-preserved lamination in these in-ner facies suggest that these lacustrine zones wererather shallow (no more than a few metres deep) andnon-stratified, with holomictic conditions dominant.

6.4. Deltaic lacustrine facies

A 40-m-thick sequence of deltaic marginal lacus-trine facies occurs in the Quebrada Temblor section

in the southern area of the Northern Basin sec-tor (Figs. 3, 7 and 8). This assemblage is charac-terised by an interbedding of subaqueous marginalinterdeltaic lacustrine facies (marls, diatomaceousmarls, diatomites and travertines) and prodelta anddelta front channelled facies, which alternate witha clear and marked sequential arrangement form-ing decimetre- to metre-thick sequences. The deltaicfacies assemblage passes laterally northward into in-ner lacustrine facies dominated by finer-grained anddiatomitic deposits.

Prodelta facies are characterized by well-lami-nated intervals of sub-millimetre- to millimetre-thickrhythmites consisting of a diatomite silty lamina withbiotite grains which is overlain by a clay diatom-bearing episode. These deposits are turbidite-likeand probably record sedimentation from underflowcurrents active during fluvial flood episodes. Diatomfrustules may be well-preserved, but are often ratherfragmented.

Lenticular coarse-grained conglomerate and sand-stone channel infills are well recorded in the lacus-trine delta front deposits. Channel infill bodies areup to several metres thick and several tens of me-tres wide in a transverse section, with a high widthto height ratio. They are mainly shallow ribbon-likebodies with vertical monostorey to multistorey infilland erosional bottom surfaces which are from gentlyto deeply incised. The coarse-grained bodies mainlyconsist of gravelly sands and poorly cemented con-glomerates. Each is bounded by erosional surfacesand usually has a fining-upwards trend. Laterally ex-tensive trough-cross-bedding sets up to 10 cm highmake up the upper infill of channel cores and lateralwings which display sigmoid cross-bedding. The co-eval occurrence of several channel bodies at the samelevel, their vertical infilling and significant entrench-ment suggest that the channel patterns were proba-bly anastomosing. Cross-stratification data point tonorthward palaeocurrent trends and the scour direc-tions observed at the channel bottoms are coherent.These palaeocurrent trends point to the fact that thedeltaic facies were the result of the northward spreadinto the lacustrine zones of the fluvial system whichparallels the basin axis.

Thin sheet-like sands and fine-gravel facies occurfrequently in the Quebrada Temblor section and con-sist of dark grey to brown fine gravels and cross-lam-


inated sands with a clast composition spectrum dom-inated by volcanic clasts and grains. Minor whitishsilicified marl clasts also occur. These deposits are upto a few decimetres thick and are mainly sheet-like togently lenticular in shape with a lateral range of tensto hundreds of metres. Root traces infilled by finermarly material occur at the tops of this coarse, sheet-like sandstone term. This facies records the deposi-tion of subaerial to shallow subaqueous small-scaleterrigenous lobes at the mouth of the fluvial channel.The shallow subaqueous conditions favoured theirquite widespread colonization by marginal palustrinevegetation.

6.5. Littoral interdeltaic lacustrine facies

This facies assemblage is characterized by car-bonate macrophyte travertines, sands, intraclasticmicrobreccias and epiclastic volcanic beds with mi-nor thin interbedding of diatomites and diatomiticmarls. These facies make up more or less sym-metrical, deepening–shallowing-upwards lacustrinesequences. Upwards transitions from whitish toyellowish diatomitic and sandy marls to purerwhite diatomites are often observed. These purediatomitic facies intervals may display sedimentaryand palaeontological features similar to those of theinner lacustrine facies. Nevertheless, clastic coarseterrigenous and epiclastic volcanic contributions aremore frequent in these subaqueous fine-grained andbiogenic beds, which causes layers to be thinner(up to several decimetres thick) and narrower (up tosome hundred metres) than in the inner marginal la-custrine assemblage. Moreover, frequent root tracesat the top of the marl and marly diatomitic bedspoint to deposition in very shallow lacustrine zones(Fig. 7C–E, Fig. 8).

Travertine layers may be sometimes embeddedin sandy, intraclastic marls (Fig. 7E, Fig. 8). Theyare grey, lenticular, up to a few decimetres thickand consist of small carbonate tubules up to 0.8mm in diameter and a few centimetres long, whichtend to occur in a vertical position and occasionallydisplay bothrioidal textures and vug porosity. Onlycalcite is present in this facies which shows a veryporous clotty microfacies. Some intraclasts can alsobe coated with calcite–micrite laminae. This traver-tine facies records the development of a marginal

zone of vegetation which encroached on marginallacustrine zones and probably suggests shallowing ofthe water column.

Intraclastic microbreccias also occur and consistof massive to nodular grey sandy and marly intra-clastic beds, which display an upward increase inthe fine-matrix percentage. Microbreccias are almostunconsolidated, forming gently lenticular layers, cmto dm thick and tens of metres wide. Marl intr-aclasts make up most of the microbreccia frame-work together with volcanic pumice and ash clasts.Gastropod bioclastic lenses and accumulations offragmented travertine carbonate tubules are associ-ated with this facies. Vertically arranged root traces,which often are infilled by marly sediment, alsooccur. This facies records the action of tractive cur-rents which eroded and reworked marginal lacus-trine zones, producing mixed intraclastic, bioclasticand clastic primary deposits later affected by plantrooting in shoreline or shallow marginal lacustrine–palustrine zones.

6.6. Terminal lacustrine fan-delta assemblage

Thin sheet-like to gently lenticular, up to a fewdecimetres thick, dark grey to brown fine gravelsand ripple cross-laminated sands and silts occur insome of the sequences studied (e.g. A and B inthe Cerro Mogote section, Fig. 7). The lateral ex-tent of these layers ranges from tens to hundredsof metres. They were deposited in terminal fan–marginal lacustrine zones. These coarse-grained de-posits are dominated by igneous terrigenous con-tributions and epiclastic clasts. Root traces infilledby overlying very fine marly material are oftenrecorded at the top of these coarse-grained beds.This facies assemblage was deposited in subaerialto subaqueous small-scale terrigenous lobes devel-oped in the terminal, frontal parts of alluvial fans.The top of some of these coarse-grained lobeswas colonized by marginal palustrine vegetation,which suggests rather shallow environmental condi-tions for these deposits. Predominantly fine-grainedred sandstones and mudstones usually overlie thesethin subaqueous fan-delta assemblages and recordthe progradation of the terminal subaerial fan-deltazones over the shallow subaqueous lacustrine zones(Fig. 7B).


6.7. Meaning of the perennial lacustrine assemblages

The distribution during the sedimentation of Sub-unit C (Lower Unit) of the perennial lacustrine faciesassemblages which can be seem in the Quillagua–Llamara zone, enables two major depositional zonesto be distinguished (Fig. 11c,d).

(1) Zone of fluvial bypass and water and sedimenttransference all along the southern sub-basin. Fluvialand fluvial–lacustrine diatomite-bearing successionswere deposited where the influence of fluvial contri-butions was dominant, whereas lacustrine–palustrinecarbonate zones developed in zones sheltered fromterrigenous contributions.

(2) Main, extensive lacustrine zones developed inthe northern sub-basin. Deltas and fan-deltas devel-oped at the head of the terminal fluvial and allu-vial-fan systems, whereas interdeltaic and inner openlacustrine diatomite-dominated sequences developedin other marginal and central zones of the sub-basin.

The sequence arrangement of the lacustrinerecord in the Quillagua Formation enables the set-tling and pulsating expansion and retreat of theLate Miocene–Pliocene lacustrine system to be vi-sualized. Two low-order (3rd order, see review byEinsele et al., 1991) expansive–retractive lacustrinealluvial sequences can be observed in the CerroMogote succession and illustrate the overall evo-lution of the palaeolake system (Fig. 7). Theselow-order sequences were, in this case, the con-sequence of interaction between the lacustrine de-pocentres and the areally restricted alluvial fans,which spread from the Coastal Range basin margin.They split into minor, higher-order (4th to 5th order)transgressive=regressive sequences which record mi-nor progradations and retrogradations of the terminalfan delta and=or oscillations of the water level.

An overall trend of early lacustrine spreading andlater retreat is also recorded in the deltaic-domi-nated Quebrada Temblor section (Figs. 3, 7 and 8).This overall trend can be again split into lower-order (4th to 5th order) transgressive–regressive se-quences probably driven by processes similar tothose described for the Cerro Mogote. Therefore,the observed sequences could result either from lakelevel changes or from the autogenic evolution (i.e.progradation–retrogradation and lateral shifting) ofthe deltaic system which impinged on the lacustrine

zones. Some especially noticeable erosional surfacesrelated to deeply entrenched channels might be re-lated to forced regressions linked to falls in lakewater level, but no conclusive evidence has beenfound. Depending on sediment influx variations,some of the observed regressive progradational se-quences could in fact develop under both highstandand lake-level fall conditions.

The lacustrine assemblages of the Quillagua For-mation were deposited under perennial conditionswhich made them rather different from the morewidespread ancient and recent ephemeral evaporiticsystems in the region (Chong, 1988). Therefore thisperennial lacustrine record is an interesting elementof contrast with other ancient and recent, ephemeral,evaporitic lacustrine systems in the region. The LateMiocene–Early Pliocene (?) Quillagua system was anextensive (up to 2000 km2), shallow (mostly a fewmetres deep), low- to intermediate-altitude holomic-tic lake (Fig. 11c,d). Unlike most former and laterlacustrine systems in the region, the Quillagua lacus-trine system included a steady open waterbody whosehydrology was determined by a low to very low localsurface runoff which could be compensated by sur-face and groundwater inflow from neighbouring ormore distant high-altitude zones. The clear predom-inance of oligohalobe to mesohalobe diatom speciessuggests that the lake waters were mainly fresh tomoderately saline, and partially confirms evaporativewater concentration though this did not attain highenough concentrations to lead to either the widespreaddevelopment of diatom assemblages characteristic ofhigh saline concentrations or the deposition of evapor-ites. It is not possible to establish for sure whether theperennial lacustrine zones were hydrologically closedsystems. The non-occurrence of evaporite facies sug-gests either generally open conditions or a balancebetween water income and evaporation.

7. Sedimentary and palaeogeographic evolutionin the Quillagua–Llamara basin

With the above-described tectonosedimentary fea-tures in the basin and the observed depositionalchanges taken into account, the following five majorpalaeogeographic evolutionary stages can be estab-lished in this Central Andes zone (Fig. 11).


7.1. Stage 1

The main phase of extensional fault activity inthe region probably took place in the Oligocene–Early Miocene (?) and led to the activity of the N–Smajor normal faults (Precordillera fault system) andof other NW–SE-oriented faults. Normal movementalong these faults defined the basin margins alongthe Precordillera and the Coastal Range, resultedin broad basin asymmetry and gave rise to twomain sub-basins (northern and southern, Fig. 11a). Amain depocentre developed in the northern sub-basinsector and lasted around the Lomas de la Sal Areaduring successive evolutionary stages. The resultingtectonic palaeorelief in the area includes severalsubstratum inselbergs (e.g. Cerro Soledad, CerroMogote, Cerro Hilaricos, Encanada Range) that wereonlapped by the basin infill. One of these highs(Encanada Range) acted as a threshold separatingthe southern and northern sub-basins. The erosionof the tectonic relief and the degradation of faultscarps led to the deposition of coarse scree brecciasand of a first generation of alluvial systems in thenorthern sector of the basin (Lower Unit, Subunit A).No lacustrine deposits during this evolutionary stagehave been found.

7.2. Stage 2

This stage was characterized by the progressivevanishing of the activity of the faults bounding thebasin, which became supratenuated, and the result-ing onlap of the earlier basin margins by the LowerUnit (i.e. Subunit B) deposits. Oil well data indicatethat lacustrine sedimentation began with the devel-opment of both perennial saline lakes with diatomitedeposition (northern parts of the northern subbasin;Fig. 11b) and ephemeral playas and playa-lakes (inthe central zones of the northern basin sector andsouthern parts of the southern basin sector). Theselacustrine zones were closed and received water andsediment contributions mostly from the Precordilleraalluvial fans. After an early areally restricted devel-opment, these lacustrine zones expanded, as demon-strated in the eastern basin zones and in the westernbasin margins; Fig. 2). This spreading of the lacus-trine zones culminated in the Late Miocene .6:0š0:4Ma, Figs. 2 and 3) with the sedimentation of the up-

permost playa-lake episode recorded in Subunit B(Hilaricos anhydrite, Fig. 2) which was coeval with aretrogradation of the Precordillera alluvial fans. Themaximum area of the lacustrine system was about1000 km2. The lacustrine zones were bounded bythe Precordilleran distal alluvial fan environments,and to the west and south by the colluvial and al-luvial systems along the western basin margin andto the south of the Encanada Range. Although nomajor volcanoclastic influences have been recordedin this stage the volcanoclastic beds interbedded inthe anhydrite-dominated successions outline the im-portance of the active Western Cordillera volcanicfocuses.

At the end of stage 2, between 6:0 š 0:4 and5:8 š 0:4 Ma, a major progradational episode of thePrecordillera alluvial fans took place (Fig. 2). Theevaporitic areas shrank dramatically or disappearedcompletely.

7.3. Stage 3

During the Late Miocene new lacustrine environ-ments expanded southward to 23ºS from the north-ern basin depocentre. This major perennial lacus-trine episode was preceded by the emplacement ofan extensive ignimbrite layer and was characterizedby early intrabasinal cannibalistic erosive episodeswhich resulted in the local to sectorial reworkingof pyroclastic and lacustrine deposits. Interbeddedvolcanoclastic beds frequently occur in the overly-ing lacustrine sequences. All this suggests volcanicactivity in the volcanic arc region before and duringlacustrine sedimentation. Carbonate and siliceousbiogenic deposits and terrigenous facies were de-posited in this perennial, mostly fresh-to-oligohalinewater system. The evolution of the area of the la-custrine zones, the relation of carbonate to biogenicsiliceous facies, and the changing importance of thefluvial and deltaic lacustrine, led us to distinguishtwo successive palaeogeographic settings during thisstage. During the earlier episodes shallow lacustrinecarbonates (Puente Posada unit, Figs. 2, 3 and 11c)accumulated in the southernmost basin zones. Thesezones received water contributions and were crossedby a non-hierarchised fluvial system draining largeareas of the Precordillera Range. This fluvial sys-tem flowed northward more or less parallel to the


Fig. 11. Main late Neogene evolutionary stages of the Quillagua–Llamara basin area between 21º450 and 22º150S. Note the change fromephemeral playa-lake (recorded in Hilaricos anhydrite and Batea Formation evaporites, map b (stage 2)) to perennial, shallow lacustrineconditions (recorded by the Quillagua Formation diatomitic sequences, maps c and d (stage 3)) and from there again to ephemeralplaya-lake and playa environments (recorded by Soledad Formation evaporites, map e (stage 4)). The current situation stage is shown inmap f (stage 5). Note the important effects of the opening of the Loa River to the Pacific Ocean.

basin axis and the channelled parts graded laterallyinto flood-plain and lacustrine zones where biogenicsilica accumulation was significant. Eventually the

fluvial channels crossed the former Encanada thresh-old, bringing water and terrigenous input to thenorthern lacustrine basin and deposited fluvial-domi-


Fig. 11 (continued).


nated deltas. The passage from deltaic to inner lacus-trine zones is recorded in the facies assemblages atthe lower part of the Quillagua Formation. The lakewas surrounded by the minor alluvial fan systemsflowing from the Coastal Cordillera and by the largerPrecordillera alluvial fans. During the later episodesof the third evolutionary stage, inner lacustrine envi-ronments persisted in the northern basin areas closeto Cerro Soledad and Cerro Mogote. Deltaic sedi-mentation persisted in the Encanada and Quillaguazones, whereas fluvial channel, flood-plain and re-lated lacustrine environments occupied areas of thebasin more to the south (Fig. 11d).

Marly and diatomaceous deposition spread oversome 2000 km2 during the maximum extension ofthe lacustrine conditions. But this situation largelychanged and the lake underwent expansions andretractions, as is recorded by the high-frequencysequential arrangement of marginal and inner la-custrine facies in the Quillagua Formation. Freshto brackish, slightly saline conditions were domi-nant in this lacustrine system, where no very salineconditions have been found. If some palaeohydro-logical connection existed during this evolutionarystage between the Quillagua–Llamara basin and theSalar Grande zone (Figs. 1 and 11d), some of thesubsurface deposits of halite in the Salar Grandecould represent the development of chlorure-domi-nated playa-lake and playa facies, coeval with the di-atomites sedimented in the perennial lacustrine zonesof the Quillagua basin. Higher water-concentrationbrine evolving from Quillagua water contributionswould have been attained in the Salar Grande basin.

The third stage ended rather sharply due to gentletectonic movements, which probably changed wa-tersheds in the region and led to the disappearanceof the Quillagua lacustrine system disappearance. Inthe northern basin sectors these movements caused agentle unconformity between the Lower and UpperUnits (Fig. 2).

7.4. Stage 4

Ambiguous dating criteria prevent adequate dat-ing of the erosive surface linked to the gentle tectonicunconformity which developed at the end of the thirdstage and is recorded in the northern basin. Subse-quently, when lacustrine conditions recurred during

the Pliocene (?), a highly saline, chlorure- to sul-phate-dominated ephemeral system was established.This fourth evolutionary stage could range fromPliocene to Pleistocene in age. This depositionalstage was characterised by a noticeable spreading ofthe lacustrine zones, with the upper basin units on-lapping again the western basin margin. The new la-custrine zones occupied only the northern sub-basinand covered some 2500 km2, i.e. more than the for-mer Quillagua system. They were ephemeral playasand playa-lakes, which gave rise to the deposition ofhalite-dominated sequences with interbedded layersof anhydrite (Soledad Formation: no facies show-ing fresh or oligohaline conditions have been found;Fig. 11e). The ephemeral saline lacustrine zones ofthe northern basin sectors were connected with theSalar Grande basin and brine could be fed into thisarea from the Quillagua–Llamara zone.

The maximum height of the saline deposits inCerro Soledad and other isolated neighbouring hills(e.g. Cerro Salar) shows the final base level duringthe final evolutionary stages of the basin. It was in re-lation to this base level that the Precordillera alluvialfans developed, which likely experienced a notice-able shrinkage and are considered here the onlysignificant source of water, solutes and clastic sed-iments from the neighbouring Precordillera heights.No sedimentary record of this stage is preserved inthe southern basin zones and there is no record of anorthward-flowing fluvial system which could havesupplied some water and sediment to the northernsub-basin.

7.5. Stage 5

After deposition of the Soledad Formation evap-orites (Pliocene–Pleistocene?), the internal drainageof a few sectors of the Quillagua–Llamara basin re-mained closed, but very large zones were affected bythe opening towards the Pacific Ocean of the deeplyentrenched Loa Canyon (Fig. 11f). This canyon isup to 600 m deep and cuts at right angles throughthe N–S-oriented Coastal Range. As a consequencean extensive open, erosive drainage network (whichtakes as its lowest expansion point the eastern endof the canyon at 800 m a.s.l.) has been developingto date in the Quillagua basin. This change to opendrainage conditions resulted in widespread erosion


and dissolution of the ancient alluvial and lacustrinedeposits. Since then sedimentation has occurred inrather restricted zones, giving rise to terraced allu-vial-fan, fluvial and minor lacustrine deposits whichdeveloped at increasingly entrenched topographiclevels.

Most dissolution which affects the evaporite rocksof the Soledad Formation is produced by ephemeralsheet flooding of Precordillera alluvial fans and thegroundwaters which discharge in the basin from thePrecordillera Range recharge zone (Fig. 11f). Theentrenchment of the El Loa drainage is deeper inthe central zone of the northern basin sector, i.e. inthe areas where the evaporite Soledad Formation wasprobably thicker. In consequence, the entrenchmentof the Precordillera network of alluvial fans (whichdepends on the base level defined by the Loa Riverincision) is relatively shallow in those zones locatedfar away from this more deeply incised zone. Thisfact, together with the hyperaridity of the region,allowed the nearly final depositional surface of someof the Precordillera’s Late Miocene alluvial fans(e.g. the Arcas fan, Dorr, 1996) to be well preserved.The successive entrenchments giving rise to the LoaRiver terraces were probably caused by several fac-tors (e.g. vertical lithospheric rising, glacioeustaticsea level change, changing supply of water and ter-rigenous sediments), whose scale of importance hasnot been properly established (Ortlieb, 1995).

8. Discussion

The three major Miocene–Pliocene and Pliocene–Pleistocene (?) lacustrine episodes recorded in theQuillagua –Llamara basin were mainly affected byextensional tectonic evolution, volcanic activity andthe changing climate affecting the water balance inthe source and depositional areas of the fore-arcregion. Considered as a whole, the Late Neogenelacustrine record in this basin is significant sinceit records the repeated alternation during Miocene–Pliocene times of extensive evaporitic (ephemeraland saline) and non-evaporitic (perennial and freshto slightly saline) lakes in a climatically sensitive,arid zone which evolved to hyperarid conditions. Butdeeper understanding of the context within whichwhere these lacustrine systems evolved is crucial in

order to evaluate their palaeoclimatic and=or tectonicmeaning.

8.1. Tectonic and volcanic influence

Lacustrine evolution in the Tertiary Quillagua–Llamara was closely related to the development offault-bounded basins along fault lines active sincethe Oligocene that progressively became inactive.This resulted both in changes in the subsidence–sedimentation rate (i.e. of the accommodation space)and in basin sizes and morphology which modifiedthe extent and distribution of the alluvial and la-custrine environments. The location and north–southorientation of the basin depocentres and the alluvial–lacustrine depositional arrangement were clearly re-lated to these faults.

The onlap relationships observed between thebasin substratum and the upper basin infill alongthe basin margins suggest that most of the tectoni-cally generated topographical–depositional gradientsin the fore-arc were generated before the final stagesof the basin infill took place and that a vigorouspalaeorelief was filled up by the alluvial and lacus-trine sequences. Nevertheless, gentle tectonic activitystill affected the Quillagua–Llamara basin during thelater depositional stages causing minor intraforma-tional unconformities such as the one between theLower and Upper Units (Fig. 2).

Widespread development of biogenic, silica-richdiatomite facies is one of the most obvious influ-ences of volcanism in the lacustrine record of thisCentral Andean fore-arc region. The extensive lix-iviation processes which affected the huge volcanicformations after the Neogene igneous activity sup-plied large amounts of silica. The high solubility ofthe main vitric volcanoclastic deposits in the lacus-trine zones together with the hydrothermal activityof the volcanic centres was also able to provide sup-plementary incoming silica, which enhanced diatomblooms.

The occurrence of significant pyroclastic ign-imbrite deposits underlying one of the most im-portant lacustrine episodes (i.e. Quillagua Formationlacustrine successions) and the frequent occurrenceof pyroclastic and epiclastic volcanic deposits inthe lacustrine deposits suggest a certain relationshipbetween volcano-tectonic processes and the lacus-


trine record. However, although volcanism couldhave exerted a noticeable short-term influence onthe regional sedimentary frameworks, it is difficultto establish clear causal relationships between ma-jor single volcanic episodes and potentially coevaldepositional changes. Volcanic eruptions can mod-ify the morphology of drainage areas, change in anearly instantaneous way water contributions intodepositional zones and obliterate extensive lacustrinezones. These rapid changes can however be ratherquickly buffered or counter-balanced by regionaldrainage evolution and do not lead to long-lastingsituations, which are likely to be easily recorded orrecognized.

8.2. Climatically and tectonically driven waterbalance — drainage evolution

Tectonic plate reconstructions (Smith and Briden,1977; Smith et al., 1981; Parrish et al., 1982; Par-rish and Curtis, 1982; Scotese et al., 1988) andpalaeomagnetic data (Hartley et al., 1988) suggestthat the fore-arc region of northern Chile was lo-cated during Neogene times under the influence ofthe steady high-pressure southeastern Pacific cellwhich causes arid to semiarid climatic conditionsbetween 23º and 30ºS latitude. Parrish et al. (1982)and Parrish and Curtis (1982) suggested that aridpalaeoclimatic conditions started in the Palaeogene,so times favouring widespread and large evapor-ite deposition (Chong, 1992). Further hyperaridityconditions were triggered by both the Andean rain-shadow effect and the influence of cold oceaniccurrents. The effect of the rain shadow on precipi-tation from the Amazon Basin was increased by thegrowth in height of the Andean orogen during theOligocene and Early Miocene (Alpers and Brimhall,1988). Recent evidence from ocean drillings pointto huge increase of sediment flux into the westernAtlantic at 8 Ma (Filippelli, 1997). This would sug-gest that in the Late Miocene renewed uplift in theAndes triggered more orographic precipitation fromtradewinds from the Amazon basin and also rein-forced the rain-shadow effect on the Late Miocene toRecent fore-arc regions, which became more arid. Inits turn the thermal inversion related to cold oceaniccurrents (similar to the present Humboldt current)and coastal upwelling enhanced hyperaridity in the

fore-arc regions. The existence since the Paleoceneof a similar oceanic cold circulation pattern in theeastern Pacific has been suggested (Andel, 1979, inKennet, 1982; Haq, 1981). However, its effect wasenhanced especially from Middle Miocene to Recenttimes, as a result of glaciation in Antarctica (Kellerand Barron, 1983). Several authors suggested thatice sheets in Antarctica expanded a lot during thelatest Miocene, simultaneously with the first glacialspreading beyond the Andes (Mercer and Sutter,1982). This Southern Hemisphere cooling process inhigh latitudes would have led to increased AntarcticBottom Water (Hodell et al., 1986) and reinforcedcold oceanic currents and upwelling processes, all ofwhich is well-established thanks to Miocene phos-phorite and diatomite marine deposits in northernChile (Ferraris and Di Biase, 1978; Tsuchi et al.,1988; Martınez-Pardo, 1990).

Late Neogene climatic evolution in the SouthernHemisphere may have led to regional changes whichenhanced the dominant arid to hyperarid situation inthe region studied. Thus, some of the most conspic-uous lacustrine changes recorded in the Quillagua–Llamara basin could be tentatively related with thisglobal to regional palaeoclimatic evolution. One ofthe main problems in this analysis is that dating ofthe Late Neogene to Quaternary alluvial and lacus-trine sequences is still inaccurate and discontinuous,although several points can be emphasised. Transi-tion from ephemeral, evaporite-dominated lacustrinesystems into perennial, non-evaporitic systems tookplace several times during the depositional evolutionof the area, which implies that this transition wasneither single nor exceptional (Fig. 2). One of thesetransitions took place between the Hilaricos anhy-drite and the Quillagua Formation (Figs. 2 and 4).This transition was preceded by a widespread progra-dation of the Precordillera alluvial fans, which couldimply an increase in water and sediment contribu-tions. This transition was also characterized by theadditional water supply from the northward-flowingfluvial system. Increased water and sediment con-tributions from the Precordillera would mean eitherenlargement of the catchment areas or more pre-cipitation in the Precordillera source areas, i.e. awetter climate, which would have hampered progra-dation, and a larger-than-average radial spread ofthe Precordillera alluvial fans (Figs. 2, 4 and 11c,d).


Even when alluvial fans retrograded due to lacus-trine spreading and rising base level, they attaineda rather larger radial extent than during the earlierevaporite-dominated stages.

However, the increase in water input caused bythe system flowing northward means that not onlyclimatic changes but modifications of watershedsand drainage divides in the region could have playeda role in the evolution of water balance. Thus, wa-ter-sediment contributions to the diverse basin zoneswere also closely related to the tectono-sedimentaryand drainage network evolution in the southern basinsector and in its linking zone with the Calama basin(Chong, 1992; May et al., 1999). The Calama basinzone underwent changing tectonic and palaeogeo-graphic conditions, which resulted in evolutionarystages characterized in some cases by water barrageand storeying, whereas in others it acted as a by-pass area, allowing a noticeable input of water andsediment into the Quillagua–Llamara basin (Figs. 1and 12).

Other well-recognized lacustrine changes tookplace between the Lower and Upper Units, when thebiogenic silica sedimentation in oligohaline to fresh-water lakes which characterized the Quillagua la-custrine stage was replaced by evaporite-dominatedplaya and playa-lake deposits. The occurrence ofthis transition in the latest Miocene–Early Pliocenecould give support to a causal relationship betweenthe ongoing climatic and oceanic changes which af-fected by that time the Southern Hemisphere and therecorded depositional changes in the basin (i.e. Pre-cordillera alluvial fan retractions suggesting smallerwater contributions). However, it should be stressedthat this evaporite lacustrine episode was not onlycoeval with a retraction of the surrounding alluvialfans but also with a regional gentle reactivation oftectonic activity, which caused regional unconfor-mity. In consequence, tectonically induced changesin the regional drainage network and watersheds canbe claimed as at least an additional reason for thischange of lacustrine regime.

It can be concluded that when morphotectonicevolution in the fore-arc region triggered water bar-rage and spreading of lacustrine areas in water-sheds to the south of the Quillagua–Llamara basin(i.e. the Calama basin zone), water input into thebasin diminished drastically. This favoured the onset

of a negative water balance and the developmentof sulphate–chloride-dominated saline playas andplaya-lakes, especially when more arid–hyperaridregional climatic conditions were also occurring andless water was being fed from the neighbouringPrecordillera and Cordillera source areas (Fig. 11,stages b and e and Fig. 12a). When water storey-ing in the Calama Basin decreased and largerwater inputs flowed into the Quillagua–Llamararegion, widespread fresh-to-oligohaline lacustrinezones with diatomite and marly deposits were able todevelop, especially if this situation was coupled withrelatively wetter conditions (Fig. 11, stages c and dand Fig. 12b).

The two low-order (3rd order) Lower and Up-per unconformity-bounded units defined here in-clude hierarchically arranged, higher-order alluvial–lacustrine successions, which resulted mainly fromthe inter-relationship between alluvial and lacus-trine systems. These sequences recorded cyclicallychanging depositional conditions possibly triggeredby palaeoclimatic evolution, i.e. alternation of arid–semiarid pulses in the Central Andean region fromthe Late Miocene to Recent (Alpers and Brimhall,1988; Gaupp et al., 1999). The hierarchical se-quence arrangement in the upper basin infill inthe Quillagua–Llamara basin can be recognised inmost of the lacustrine successions, but the marginaldeltaic and fan-deltaic lacustrine sequences of theQuillagua Formation display especially clear, welldeveloped high-frequency (4th–5th order) sequencetrends, which suggest some climatic forcing, withpossible interference from tectonic and volcanic–tectonic processes (Figs. 6 and 8). Despite this, noconclusive evidence is available for an exclusive allo-genic climatic forcing on this sequence arrangement,since autogenic processes (progradation, retrogada-tion and shifting of the deltaic and fan-deltaic lobes)could have produced similar cyclical, though notnecessarily periodical, patterns.

9. Concluding remarks

Some general concluding remarks can be estab-lished on the base of the overall major features ofthe Late Neogene tectono-sedimentary and lacustrinerecord in the northern Chile fore-arc zones (Fig. 12).


Fig. 12. Conceptual block diagrams showing the overall relationships between the diverse fore-arc zones in the southern Central Andeanregion in northern Chile and the influence exerted by tectonic, volcanic and climatic driving features on the lacustrine record. Twoextreme situations are shown. (a) Negative water balance in the northern zone of Quillagua–Llamara basin, linked to drier climaticconditions (which resulted in playa-lake development and in retrogradation of surrounding alluvial fans and=or to water barrage in theCalama Basin because of the existence of a tectonic topographical threshold. (b) Clearly positive water balance in the same zone linkedto wetter climatic conditions (which would enhance perennial lakes spreading and progradation of the alluvial fans) and to a very distinctregional drainage which results in no water retention in the Calama basin and the development of a northward flowing fluvial system.


(1) Fault activity in the non-marine, intramassiffore-arc regions studied played a major role in theearly development of the basins and their patternof deposition since it controlled the location of theearly basin depocentres. Later sedimentary evolutionof the basins was not so strongly dependent on activefault tectonics, but rather on the interaction betweenthe evolving reliefs and the depositional systems.Relatively minor substratum palaeoreliefs of tectonicand erosive origin acted as topographic thresholds.These thresholds controlled accommodation spaceand water and sediment bypassing or accumulation,and gave rise to diverse sedimentary records in thediverse basin zones. Late basin aggradational stagespreceding regional drainage entrenchment were char-acterized by spreading of the depositional zonesand onlapping of the basin margins and topographichighs by sediments.

(2) The Precordillera catchment areas which fedthe alluvial fan and river systems were larger thanin the Coastal Range and in most cases contributedwith larger water and sediment amounts, which ledto clear depositional and hydrological asymmetry.The distance of some of the Precordillera watershedsfrom the lacustrine zones meant that local but sig-nificant tectonic modifications of drainage dividescould become driving forces of the sedimentary evo-lution. Even gentle tectonic tilting and uplifting incritical water divide or bypass zones could bringabout noticeable changes in the water income intothe lacustrine basins and trigger variations in thedepositional record.

(3) The Late Miocene to Pliocene global scenarioin the Southern Hemisphere was characterized by anoverall stepping trend of high-latitude cooling andvariation in the intensity of cold oceanic currentsaffecting the ocean close to this Central Andean re-gion. The alternating reinforcement and weakeningof these cold oceanic currents could have caused os-cillations in the arid–hyperarid conditions which inturn could have modified the regional water balance.Due to this, the coeval development of evaporite-dominated deposits and alluvial fan retrogradations,and the nearby deposition of perennial diatomite suc-cessions related to major alluvial fan progradationssupport the idea of low-frequency climatic forc-ing. However, these very conspicuous facies changesrecorded in the Quillagua–Llamara basin infill (i.e.

transition to Hilaricos anhydrite, to Quillagua For-mation and its later change to Soledad Formation)are not on their own conclusive evidence for exclu-sive climatic forcing, especially when they are soclose to either remarkable regional drainage changesand=or gentle but noticeable tectonic reactivation inthe fore-arc region

Acknowledgements

The research leading to the results shown inthis paper was funded by the Spanish DGCYTproject PB94-0901, ‘The Fore-arc Continental Neo-gene Basins of northern Chile: The Evolution of La-custrine Systems and drainage networks’ and the EUproject No. CI1*-CT94-0069. This research was un-dertaken within the framework of the earlier (1993–1995) ‘Proyecto de Cooperacion Iberoamericana delVo Centenario’ of the Spanish Ministry of Foreignaffairs. A. Jensen’s work was also funded by theUniversidad Catolica del Norte at Antofagasta, whosupported his Ph.D. research stay (1989–1992) inthe University of Barcelona. We are very grateful toJ.J. Pueyo and C. Dabrio for their deep and helpfulreading of the text and to P. Santanach, D. Gimenoand David Serrat for their useful comments. Thanksare also due to the Servicio Nacional de Geologıade Chile, where the K=Ar radiometric dating wascarried out. We would like to express our gratitude toall our colleagues in the Department of Geology ofthe UCN of Antofagasta for their help and support,and to Jorge Mena and his family in Quillagua fortheir warm kindness during our field work stays.

References

Alpers, C.N., Brimhall, G.H., 1988. Middle Miocene climaticchange in the Atacama Desert, northern Chile: evidence forsupergene mineralization at La Escondida. Geol. Soc. Am.Bull. 100, 1640–1656.

Anadon, P., Cabrera, L., Kelts, K., 1991. Lacustrine FaciesAnalysis. IAS Spec. Publ. 13, 318 pp.

Arabasz, W.J., 1971. Geological and Geophysical Studiesof the Atacama Fault Zone, in Northern Chile. Ph.D.Thesis, California Institute of Technology, Pasadena, CA,264 pp.

Bobenrieth, L., 1979. Geologıa de los cuadrangulos Cerro De-samparado y Cerro Soledad. Proyecto Cordillera de la Costa


Tocopilla–Rıo Loa. Instituto Investigaciones Geologicas, es-cala 1 : 50,000, 97 pp.

Bruggen, J., 1950. Fundamentos de la Gelogıa de Chile. Inst.Geog. Milit (Chile), 374 pp.

Cabrera, L., Chong, G., Jensen, A., Pueyo, J.J., Saez, A., Wilke,H.G., 1995. Cenozoic and Quaternary lacustrine systems inNorthern Chile (Central Andes, Arc and Fore-Arc zones). In:Saez, A. (Ed.), Excursion Guidebook on Recent and AncientLacustrine Systems in Convergent Margins, GLOPALS–IAS,Meeting, 77 pp.

Chong, G., 1988. The Cenozoic saline deposits of the ChileanAndes between 18 degrees and 27 degrees south latitude.Lecture Notes Earth Sci. 17, 137–151.

Chong, G., 1992. Sistemas lacustres cenozoicos. Andes del Nortede Chile. VIII Congreso Latinoamericano de Geologıa. Sym-posios 1, 54–62.

Chong Diaz, G., Mendoza, M., Garcıa-Veigas, J., Pueyo, J.J.,Turner, P., 1999. Evolution and geochemical signatures in aNeogene forearc evaporitic basin: the Salar Grande (CentralAndes of Chile). Palaeogeogr., Palaeoclimatol., Palaeoecol.151, 39–54 (this issue).

De Silva, S.L., 1989. Geochronology and stratigraphy of theignimbrites from the 21º300S to 23º300 portion of the CentralAndes of Northern Chile. J. Volcanol. Geotherm. Res. 37, 93–131.

De Silva, S.L., Francis, P.W., 1989. Correlation of large ign-imbrites. Two case studies from the Central Andes of NorthernChile. J. Volcanol. Geotherm. Res. 37, 133–149.

Dorr, M.J., 1996. Der Arcas Facher und sein Erosinsgebiet inNordchile: Bilanz einer exogenen Massenverlagerung. BerlinerGeowiss. Abh. (A) 189, 121 pp.

Dorr, M., Gotze, H.-J., Ibbeken, H., Kiefer, E., 1995. The ArcasFan in Northern Chile: a 0.5 megayear event? Recent andAncient Lacustrine System in Convergent Margins, Abstracts,p. 7.

Einsele, G., Ricken, W., Seilacher, A., 1991. Cycles and eventsin stratigraphy — basic concepts and terms. In: Einsele, G.,Ricken, W., Seilacher, A. (Eds.), Cycles and Events in Stratig-raphy. Springer-Verlag, Berlin, pp. 1–19.

Ferraris, F., Di Biase, F., 1978. Hoja Antofagasta. Region deAntofagasta. Instituto Invest. Geol. Carta de Chile, escala1 : 250,000, No. 30, 48 pp.

Filippelli, G.M., 1997. Intensification of the Asian Monsoonand chemical weathering event in the late Miocene– EarlyPliocene: implications for late Neogene climate change. Geol-ogy 25, 27–30.

Flint, S., Turner, P., Jolley, E.J., 1991. Depositional architec-ture of Quaternary fan-delta deposits of the Andean fore-arc:relative sea-level changes and response to aseismic ridge sub-duction. Spec. Publ. Int. Assoc. Sedimentol. 12, 91–103.

Garces, M., Saez, A., Jensen, A., Cabrera, L., Pares, J.M.,Chong, G., 1994. Magnetoestratigrafıa de la Fm. Quillagua.Plioceno de la region de Antearco del N de Chile. Datos pre-liminares. II Congreso Espanol del Terciario, Comunicaciones,pp. 239–242.

Gaupp, R., Kott, A., Worner, G., 1999. Palaeoclimatic impli-cations of Mio–Pliocene sedimentation in the high-altitude

intra-arc Lauca Basin of northern Chile. Palaeogeogr., Palaeo-climatol., Palaeoecol. 151, 39–54 (this issue).

Gierlowski, E., Kelts, K., 1994. Global geological record oflake basins. Introduction. World and Regional Geology 4.Cambridge University Press, Cambridge, Vol. 1, pp. xvii–xxxiii.

Guest, J.E., 1969. Upper Tertiary Ignimbrites in the AndeanCordillera of Part of the Antofagasta Province, Northern Chile.Geol. Soc. Am. Bull. 80, 337–362.

Haq, B.U., 1981. Paleogene paleoceanography: Early CenozoicOcean revisited. Oceanol. Acta, Actes 26e Congr. Int. Geol.,Geologie des Oceans, Paris, pp. 71–82.

Hartley, A., Jolley, E.J., 1995. Tectonic implications of LateCenozoic sedimentation from the Coastal Cordillera of north-ern Chile (22–24ºS). J. Geol. Soc. London 152, 51–63.

Hartley, A.J., Turner, P., Williams, G.D., Flint, S., 1988. Palaeo-magnetism of the Cordillera de la Costa, northern Chile:evidence for local forearc rotation. Earth Planet. Sci. Lett. 89,375–388.

Hodell, D.A., Elmstrom, K.M., Kennett, J.P., 1986. LatestMiocene benthic δ18 changes, global ice volume, sea leveland the ‘Messinian salinity crisis’. Nature 320, 411–414.

Hollingworth, S.E., 1964. Dating the uplift of the Andes ofnorthern Chile. Nature 201, 17–20.

Jensen, A., 1992. Las cuencas aluvio-lacustres oligoceno–neogenas de la region de ante-arco de Chile Septentrional, en-tre los 19º y 23º sur. Ph.D. Thesis, Universidad de Barcelona,217 pp.

Jensen, A., Dorr, M., Gotze, H.-J., Kiefer, E., Ibbeken, H.,Wilke, H., 1995. Subsidence and sedimentation or forearc-hosted, continental pull-apart basin: the Quillagua trough be-tween 21º30 and 21º45S, Northern Chile. Recent and AncientLacustrine System in Convergent Margins, Abstracts, pp. 5–6.

Keller, G., Barron, J.A., 1983. Paleoceanographic implications ofMiocene deep-sea hiatuses. Geol. Soc. Am. Bull. 94, 590–613.

Kelts, K., Talbot, M.R., 1990. Lacustrine carbonates as geo-chemical archives of environmental change and biotic=abioticinteractions. In: Tilzer, M.M., Serruya, C. (Eds.), Large Lakes:Ecological Structure and Function. Springer-Verlag, Berlin,pp. 288–314.

Kennet, J., 1982. Marine Geology. Prentice Hall, EnglewoodCliffs, N.J., 813 pp.

Lamb, S., Hoke, L., Kennan, L., Dewey, J., 1997. Cenozoic evo-lution of the Central Andes in Bolivian and northern Chile. In:Burg, J., Ford, M. (Eds.), Orogeny Through Time. GeologicalSociety, Special Publication 121, pp. 237–264.

Marinovic, S., Lahsen, A., 1984. Carta Geologica de Chile.Escala 1 : 250,000. Hoja Calama (Region de Antofagasta).Servicio Nacional de Geologia y Mineria, 140 pp.

Martınez-Pardo, R., 1990. Major Neogene events of the South-eastern Pacific: the Chilean and Peruvian record. Palaeogeogr.,Palaeoclimatol., Palaeoecol. 77, 263–278.

May, G., Hartley, A.J., Stuart, F.M., Chong, G., 1999. Tectonicsignatures in arid continental basins; an example from theUpper Miocene–Pleistocene, Calama Basin, Andean forearc,northern Chile. Palaeogeogr., Palaeoclimatol., Palaeoecol. 151,55–77 (this issue).


Mercer, J.H., Sutter, J.F., 1982. Late Miocene– Earliest Plioceneglaciation in Southern Argentina: implications for a global ice-sheet history. Palaeogeogr., Palaeoclimatol., Palaeoecol. 38,185–206.

Mortimer, C., 1973. The Cenozoic history of the southern Ata-cama desert, Chile. J. Geol. Soc. London 129, 505–526.

Naranjo, J.A., Paskoff, R.P., 1981. Estratigrafıa de los depositoscenozoicos de la Region de Chiuchiu–Calama, Desierto deAtacama. Rev. Geol. Chile, 13–14, 79–85.

Naranjo, J.A., Paskoff, R.P., 1982. Estratigrafıa de las unidadessedimentarias cenozoicas de la Cuenca del Rıo Loa en laPampa del Tamarugal, Region de Antofagasta, Chile. Rev.Geol. Chile 15, 49–57.

Ortlieb, L., 1995. Paleoclimas cuaternarios en el Norte Grandede Chile. In: Argollo, J., Mourguiart, Ph. (Eds.), Cambioscuaternarios en America del Sur. ORSTOM, La Paz, pp. 225–246.

Parrish, J.T., Curtis, 1982. Atmospheric circulation, upwellingand organic-rich in the Mesozoic and Cenozoic eras. Palaeo-geogr., Paleoclimatol., Palaeoecol. 40, 31–66.

Parrish, J.T., Ziegler, A.M., Scotese, C.R., 1982. Rainfall patternsand the distribution of coals and evaporites in the Mesozoicand Cenozoic. Palaeogeogr., Paleoclimatol., Palaeoecol. 40,67–101.

Reutter, K.J., Giese, P., Gotze, H.J., Scheuber, E., Schwab, K.,Schwarz, G., Wigger, P., 1988. Structures and crustal de-velopments of the Central Andes between 21º and 25º. In:Bahlburg, H., Breitkreutz, Ch., Giese, P. (Eds.), The SouthernCentral Andes. Lecture Notes Earth Sci. 17, 231–261.

Reutter, K.J., Scheuber, E., Helmcke, D., 1991. Structural evi-

dence of orogen-parallel strike slip displacements in the Pre-cordillera of northern Chile. Geol. Rundsch. 80, 135–153.

Rieu, M., 1975. Les formations sedimentaires de la Pampa delTamarugal et le rıo Loa (Norte Grande du Chili). Fr. Off.Rech. Sci. Tech. Outre-Mer, Cah., Ser. Geol. 7 (2), 145–164.

Santanach, P., Serrat, D., Saez, A., Cabrera, L., Chong, G., 1996.Tertiary extensional faulting at the Lower Loa Valley. NorthernChile (21–22ºS). Abstr. 3rd Int. Symp. Andean Geodinamics(ISAG), St. Malo, pp. 489–490.

Scheuber, E., Hammerschmidt, K., Friedrichsen, H., 1995.40Ar=39Ar and Rb–Sr analyses from the Atacama fault zone,northern Chile: the age of deformation. Tectonophysics 250,61–87.

Schmitz, M., 1994. A balanced model of the southern CentralAndes. Tectonics 13 (2), 484–492.

Scotese, C.R., Gahagan, L.M., Larson, R.L., 1988. Plate tectonicreconstructions of the Cretaceous and Cenozoic ocean basins.Tectonophysics 155, 27–48.

Smith, A.G., Briden, H.C., 1977. Mesozoic and Cenozoic Paleo-continental Maps. Cambridge University Press, Cambridge, 63pp.

Smith, A.G., Hurtley, A.M., Briden, H.C., 1981. Phanerozoic pa-leocontinental world maps. Cambridge University Press, Cam-bridge, 102 pp.

Stoertz, G.E., Ericksen, G.E., 1974. Geology of salars in northernChile. U.S. Geol. Surv. Prof. Pap. 811, 65 pp.

Tsuchi, R., Shuto, T., Takayama, T., Fujiyoshi, A., Koizumi, I.,Ibaraki, M., Martınez-Pardo, R., 1988. Fundamental data onCenozoic biostratigraphy of Chile. Reports of Andean Studies,Shizuoka University, Vol. 2, pp. 71–96.

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