A new fossil mammal assemblage from the southern Chilean Andes:

implications for geology, geochronology, and tectonics

John J. Flynna,b,*, Michael J. Novacekc, Holly E. Dodsond, Daniel Frassinettie, MalcolmC. McKennac, Mark A. Norellc, Karen E. Searsb,a, Carl C. Swisher IIIf,1, Andre R. Wyssd

aDepartment of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, IL 606052496, USAbCommittee on Evolutionary Biology, The University of Chicago, 1025 E. 57th Street, Chicago, IL 60637, USA

cDivision of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USAdDepartment of Geological Sciences, University of California at Santa Barbara, Santa Barbara, CA 93106, USA

eDepartamento de Geologa, Museo Nacional de Historia Natural, Interior Quinta Normal, Casilla 787, Santiago, ChilefBerkeley Geochronology Center, Berkeley, CA 94709, USA

Received 1 October 2001; accepted 2 January 2002

Abstract

A diverse (36 taxa), new fossil terrestrial mammal assemblage has been recovered from the Santacrucian South American Land Mammal

Age (SALMA; latest Early Miocene) in the southern Andes of Chile. This is the westernmost high latitude mammal fauna known in South

America and the first in a string of new mammal assemblages discovered in Chile after a lapse of nearly a century. The terrestrial mammal-

bearing sequence conformably overlies a marine section of Late Oligocene to Early Miocene age. The combined marineterrestrial

sequence, as well as a locality with fossil whales and bracketing basalts, bear significantly on theories regarding the extent of the late Tertiary

Patagonian epicontinental seaway and the onset of later Cenozoic phases of uplift in the southern Andes. Uplift in this region likely began by

Santacrucian SALMA (,1617.5 Ma) time, but it remains uncertain whether this occurred in two phases (Pehuenchic and Quechuic) or one.These discoveries substantiate propositions of sharp geologic contrasts north and south of the Lago General Carrera/Lago Buenos Aires area

(Magellanes basin to the south and Ro Mayo embayment to the north). Minimum estimates of uplift rate are approximately 0.05

0.07 mm/yr (but as high as 0.22 mm/yr), comparable to or slightly lower than those from other parts of the Andes (e.g. Bolivia). The timing

and location of uplift may be correlated with major plate tectonic events associated with the Chile Margin Triple Junction. q 2002 Elsevier

Science Ltd. All rights reserved.

Keywords: Tectonic events; Fossil mammals; Geochronology; Miocene; Paleontology

1. Introduction

In contrast with neighboring regions in Argentina, the

Andean region of southern Chile has previously contributed

little to knowledge of South American Cenozoic ver-

tebrates. Cenozoic terrestrial sequences, broadly exposed in

Patagonian Argentina, are scattered and isolated in the

southern Chilean Andes. Early exploration for vertebrate

fossils in the region (Hatcher, 1903) was largely unsuccess-

ful. Accordingly, the history of Cenozoic vertebrate

paleontology in Chile was little known. Notwithstanding

this general paucity of evidence, highlights include

description by Simpson (1941) of the Miocene sloth

Nematherium birdi from exposures east of Laguna Blanca

near Punta Arenas, a specimen of Astrapotherium magnum

from Los Cruceros on the southern margin of Laguna del

Toro (Hemmer, 1935), and studies of mammal faunas that

typify the Friasian South American Land Mammal Ages

(SALMA; ?middle Miocene) from northwest Patagonia,

near the ArgentineChilean frontier (Roth, 1908, 1925;

Kraglievich, 1930; Marshall et al., 1983; Vucetich, 1984;

Flynn, et al., 1989a,b; Marshall, 1990; Cifelli, 1991;

Madden, et al., 1997).

We report here the discovery of Miocene mammals from

high altitude sequences in the southern Andes of Chile. At

the time of its discovery, this represented the first new

0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 89 5 -9 81 1 (0 2) 00 0 43 -3

Journal of South American Earth Sciences 15 (2002) 285302

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1 Present address: Department of Geological Sciences, Rutgers

University, New Brunswick, NJ 08901, USA.

* Corresponding author. Address: Department of Geology, The Field

Museum, 1400S. Lake Shore Drive, Chicago, IL 606052496 USA. Tel.:

1-312-665-7620; fax: 1-312-665-7641.E-mail address: jflynn@fieldmuseum.org (J.J. Flynn).

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Tertiary fossil mammal assemblage recovered from the

Chilean Andes since enigmatic reports by Roth (1908) of

the fauna from the Ro Fras/Cisnes area of Patagonia nearly

a century ago. The presence of mammal fossils in this region

was first brought to our attention in the fall of 1984 by

inhabitants of the region, Dr Paul Raty and Carlos de Smet

de Olbeck de Halleux, who found fossil whale vertebrae in

mountainous terrain southwest of the town of Chile Chico

(Fig. 1). Our exploration of this area in January 1986 yielded

only limited additional whale material, but reconnaissance

of isolated exposures southeast of the town of Puerto Guadal

(Fig. 1) yielded an abundance of terrestrial fossil mammals.

The main terrestrial localities lie near a summer grazing

pasture known locally as Pampa Castillo (not to be

confused with the Eocene (Casamayoran) Pampa de

Castillo Este in Patagonian Argentina (Simpson, 1934,

1948; Savage and Russell, 1983)). Further prospecting and

collection of this area continued in January 1987 and 1988.

This article is a preliminary report on the composition of

the mammal fauna, correlation of the marine macroinverte-

brate assemblages (Frassinetti and Covacevich, 1999),

stratigraphic relationships, geochronology, and implications

for paleogeography and Andean tectonics. The SALMA

informal biochronologic units, characterized by unique

mammalian assemblages, have proved useful for intracon-

tinental correlations and subdividing Cenozoic time in

South American sequences (as have the original NALMA,

or North American Land Mammal Ages, first proposed by

Wood et al., 1941). Friasian SALMA refers to a restrictive

Friasian sensu stricto, which excludes faunas and time

represented by Colloncuran or Mayoan (Flynn and Swisher,

1995; Madden et al., 1997). Friasian refers to Friasian

sensu lato, which is a broader conception that includes

Colloncuran, Friasian, and Mayoan ages or subages.

Catalog designations (SGO PV#) refer to specimens from

the vertebrate paleontological collections of the Museo

Nacional de Historia Natural, Santiago, Chile.

2. Regional geology

The study region is roughly 400 km2 of mountainous

terrain south of Lago General Carrera (Lago Buenos Aires

on Argentine maps) and north of Cochrane (Fig. 1). The area

lies at approximately 478S and between 738 and 728W inAisen Province, Chile. The Andean Cordillera in this region

is composed mainly of basement metamorphics; Jurassic,

Cretaceous, and Tertiary volcanics; and intermittent Ter-

tiary marine and terrestrial sediments (Niemeyer et al.,

1984). To the west, the main Cordillera is made up of a Late

MesozoicEarly Tertiary batholith that includes some of

the highest peaks (e.g. Mt San Valentn, 4058 m) of the

Patagonian Andes. The study area is roughly 250 km SE of

the present position of the Chile Margin Triple Junction

(CMTJ), the point of convergence of the Nazca, Antarctic,

and South American plates (Fig. 1).

3. Stratigraphy

The rich assemblage of terrestrial fossils is derived from

a thick stratigraphic sequence, locally exposed as a

prominent butte (1349 m elevation), just east of Pampa

Castillo in forested mountains about 25 km southeast of

Puerto Guadal (Fig. 1). The butte exposes an approximately

300 m thick sequence of northward-dipping fluvial silt-

stones and sandstones with minor claystones and conglom-

erates (Fig. 2). This sequence represents part of a large

Fig. 1. Map of the study area. Black box indicates the main study area, including Pampa Castillo, within the Meseta Guadal.

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302286

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region of exposure of the synclinal Meseta Guadal (Niemeyer,

1975; Niemeyer et al., 1984). In the Meseta Buenos Aires

(usage of Charrier et al., 1978, 1979 and Niemeyer et al., 1984;

Ramos (1982b), Fig. 4, applies the term Meseta del Lago

Buenos Aires more narrowly to an Argentine meseta lying

southeast of Chile Chico and Lago General Carrera/Buenos

Aires), which begins less than 30 km to the east of Meseta

Guadal and continues into Argentina, marine strata and

bracketing basalt intervals (100200 m thick) lie just to the

west of and interfinger with terrestrial strata (Ro Zeballos

Formation). There are no extrusive volcanics associated with

either the upper marine strata or the terrestrial units at Pampa

Castillo (Niemeyer et al., 1984).

Formal stratigraphic assignment of the terrestrial

sequence at Pampa Castillo is problematic because of the

complex formational nomenclature and stratigraphic

interpretation of units in the area around Lago General

Carrera. The terrestrial sequence at Pampa Castillo was

assigned to the Galera Formation (Niemeyer et al., 1984),

but its equivalents in other areas south of Lago General

Carrera previously had been referred to the Ro Zeballos

Formation (Niemeyer, 1975; Charrier et al., 1978, 1979;

name modified from El Grupo de Ro Zeballos of Ugarte

(1956)). Niemeyer et al. (1984) and Charrier et al. (1978)

considered the Galera (Espinosa and Fuenzalida, 1971;

Skarmeta, 1976) and Ro Zeballos formations to be

equivalent. The latter name, based on a type section east

of Ro Jeinimeni in Argentina, has priority. However, most

recent studies and geologic maps of the region continue to

use the Galera Formation designation, possibly because its

type section is in Chile (north of Lago General Carrera, on

the frontier between Coihaique and Balmaceda).

A lithostratigraphic unit with which the Galera/Ro

Zeballos Formation is sometimes correlated requires

comment. Ramos (1981) provided the first comprehensive

description and designation of a type section for the

fossiliferous unit bearing the name Formacion Ro Fras.

This formation was based on the fossil-bearing, biostrati-

graphically conceived Ro Fras-Stufe of Roth (1908),

Formacion Friaseana of Kraglievich (1930), and Friasian

Formation of Simpson (1940), later formalized as a

lithostratigraphic unit (Formacion Fras of Ramos (1976);

Formacion Ro Fras of Ploszkiewicz and Ramos (1977);

Ramos, 1981). Work by the first author and a team of other

colleagues documented that the type area of the Ro Fras

Formation is the region that produced the mammalian

material reported by Roth (1908, 1925) and initiated

biochronologic and geochronologic studies to constrain

the faunal occurrences and age of this sequence better

(Flynn et al., 1989a,b; Madden, 1990; Marshall, 1990;

Marshall and Salinas, 1990; Cifelli, 1991; Madden et al.,

1991; Ortiz Jaureguizar et al., 1993; Flynn and Swisher,

1995; Kay and Madden, 1997; Madden et al., 1997). Fossils

from this region form the basis of the poorly known and

problematic Friasian (s.l.) or Friasian (s.s.) SALMA

(Kraglievich, 1930; Simpson, 1940, 1971; Feruglio,

19491950; Pascual et al., 1965; Pascual and Odreman

Rivas, 1971, 1973; Bondesio et al., 1980; Vucetich, 1984;

Marshall, 1990; Marshall and Salinas, 1990; Vucetich et al.,

1993; Flynn and Swisher, 1995; Madden et al., 1997).

Ramos (1976) correlated the Galera Formation with the Ro

Fras Formation, the latter of which is exposed 130 km north

of Lago General Carrera/Buenos Aires, along the Ro Cisnes

(earlier called the Ro Fras) drainage. Our observations of

the Rio Fras and Galera Formations in their type areas and

of the putative Galera Formation (sometimes also called

Santa Cruz Formation), south of Lago General Carrera/

Buenos Aires in Chile, confirm that these units are distinct

lithologically (Ramos, 1976; Niemeyer et al., 1984), which

suggests that the terrestrial unit in the area of the Pampa

Castillo may be better considered a new formation or

assigned to the Ro Zeballos Formation (or another

Fig. 2. Photograph of the entire thickness of terrestrial strata exposed at Pampa Castillo; view to the NE from steeply tilted marine strata on the flanks of the

Meseta Guadal (Frassinetti and Covacevich, 1999). E-O quarry levels lie low in the terrestrial sequence and are exposed in the mid-ground near the base of the

butte. Equivalent strata elsewhere on Meseta Guadal are in the distance above the treeline.

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302 287

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formation in the Ro Zeballos Group, according to the

conceptualization of Ugarte (1956)).

Ramos (1982b, 1989) later considered the units assigned

to the Galera and Ro Zeballos formations from the

Magallanes basin south of Lago General Carrera/Buenos

Aires (in both Argentina and Chile) to all be assignable to

the Santa Cruz Formation. Similarly, units that he

considered temporally equivalent to the Santa Cruz

Formation (e.g. Galera, Ro Fras, Ro Mayo, Pedregoso)

but from the Ro Mayo embayment north of Lago General

Carrera/Buenos Aires were all assigned to the Ro Fras

Formation. We note that these conclusions were based in

part on lithologic similarity and inferred common lithoge-

netic origin, but also in part on suggestions that many of the

previously separated units are of the same age (Santacrucian

SALMA). These stratigraphic synonymies may be an

oversimplification of a more complex geologic setting,

according to observations of (1) large variation in lithologic

characteristics, facies, and thickness (and therefore inferred

lithogenetic origin); (2) a lack of direct physical continuity

between the exposures of the presumed single formation

(contrary to the requirements of the International Guide of

Stratigraphic Nomenclature); (3) difficulties in establishing

formational equivalence of units formed in the possibly

locally isolated Andean foreland basins, with the Santa Cruz

Formation in its type region on the Atlantic coastal plain of

Argentina; and (4) the requirement that lithostratigraphic

units are recognized solely on the basis of physical

characteristics, not inferences about age.

The assignment of all units in the Magellanes basin

immediately overlying strata of the final Patagonian

transgression (Centinela Formation and equivalents) to

the Santa Cruz Formation has been further complicated by

the formalization of a new stratigraphic unit, the Pinturas

Formation, whose type section is geographically interposed

between the Meseta Guadal/Pampa Castillo and coastal

Santa Cruz Formation sequences southeast of Lago General

Carrera/Buenos Aires (Bown et al., 1988; see also

Pinturense of Castellanos (1937), which is based on the

Astrapothericuleen or Astrapothericulus beds of Ame-

ghino (1906)). Barrio et al. (1984) discuss a fossil mammal

assemblage from the Santa Cruz Formation in the same area,

assigning it to the Piso Astrapothericulense of Ameghino

(1906), or early Santacrucian SALMA. The Pinturas

Formation occupies a stratigraphic position equivalent to

the Santa Cruz Formation, but the presence of dune facies

deposits within the former differentiates it from typical

coastal Santa Cruz Formation deposits even more than do

the other presumed correlative units of the Andean

precordillera. If all the units in either basin (Magellanes

basin or Ro Mayo embayment) immediately overlying the

final Patagonian transgression indeed constitute a single

formation, it will necessarily be very broadly conceived and

will vary widely in lithologic characteristics, underlying

tectono-sedimentary genetic mechanisms, and possibly age

throughout its geographic extent.

We concur with Ramos (1982b, 1989) that the sharp

lithologic and genetic differences between the superposed

series of terrestrial strata in the Magellanes basin and Ro

Mayo embayment likely reflect distinct tectonic histories in

the two regions, which influenced their filling by various

temporally equivalent Middle Miocene stratigraphic units.

The Deseado Massif may have represented a tectonic and

topographic barrier that isolated stratigraphic sequences

forming within these two basins; major plate tectonic

movements may have played an important role in the

location, origin, timing, and magnitude of uplift of the

barrier.

Given these tectonic and stratigraphic complexities, we

regard the terrestrial strata at Pampa Castillo as distinct from

the Santa Cruz Formation, as typified in coastal deposits of

Argentina, and from the Ro Fras Formation, as typified

north of Lago General Carrera/Buenos Aires. We conserva-

tively refer to the Pampa Castillo terrestrial deposits as an

unnamed formation, possibly equivalent to the Ro Zeballos

Formation (Niemeyer, 1975; Charrier et al., 1978, 1979;

including the southern exposures referred to the Galera

Formation by Niemeyer et al. (1984) and Frassinetti and

Covacevich (1999)).

It is noteworthy that the mammal fauna from Pampa

Castillo (see Table 1 and subsequent discussion) differs

from the much smaller assemblage reported from Ro Fras/

Cisnes (Kraglievich, 1930; Marshall et al., 1983; Vucetich,

1984; Marshall, 1990; Marshall and Salinas, 1990; Cifelli,

1991; Madden et al., 1997), which forms the basis for the

presumed younger Friasian SALMA. A much broader range

of taxa are now known to exist in the Ro Cisnes fauna, and

additional geochronologic and biostratigraphic work on the

strata was undertaken by Flynn, Kay, and colleagues (see

subsequent discussion). These studies will clarify the degree

of similarity of these faunas.

The terrestrial sequence at Pampa Castillo overlies a

650 m section of marine strata assignable to the Guadal

Formation (Heim, 1940; Niemeyer et al., 1984). The Guadal

Formation may be temporally and stratigraphically equiv-

alent to the Centinela Formation (Furque and Camacho,

1972) and to units termed Piso Patagoniano (Feruglio,

19491950) and Patagoniano (Ugarte, 1956) in Argen-

tina, as well as to the Segundo nivel marino con Ostrea in

the Meseta Buenos Aires south of Chile Chico and in

Argentina (Niemeyer et al., 1984). The top of the marine

section is exposed at the base of Pampa Castillo (Fig. 2),

where a conformable transition to the terrestrial sequence

occurs as a thin (510 m) interval of oyster-bearing sands

overlain by plant-bearing sands, silts, and clays. This

transition marks the final withdrawal of the middle

Cenozoic Patagonian seaway after it reached its maximum

northward and westward extent (Fig. 3). The lower parts of

the fossiliferous marine section are exposed as a large

southward-facing escarpment over the Ro Chacabuco

drainage and a series of more easterly buttes of steeply

dipping strata. The base of the marine section is in

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Table 1

Mammalian fauna from Pampa Castillo (occurrences of taxa within the Colhuehuapian, Santacrucian, or Friasian SALMAs noted by X; questionable

occurrences noted by ?; new species, likely Santacrucian, marked by p ; Marshall et al., 1983; McKenna and Bell, 1997; Tauber, 1997a,b; additional data

from Vucetich, 1984, rodents; Bown and Fleagle, 1993, palaeothentids; Madden, 1990, toxodontids)

Colhuehuapian Santacrucian Friasian

Marsupialia

Palaeothentidae

Palaeothentes pascuali X

Palaeothentes minutus X X

Palaeothentes intermedius X X

Palaeothentes smeti (new species) p

Borhyaenidae

Cladosictis sp. X X

Edentata

Dasypodidae

Prozaedius sp. X X X

Proeutatus sp. X X X

Glyptodontidae

Propalaeohoplophorus sp. X X X

Megalonychidae

Hapalops sp. X ?

Notoungulata

Homalodotheriidae

Homalodotherium ? X X

Toxodontidae

Adinotherium sp. X

Nesodon sp. X

Interatheriidae

Protypotherium sp. X X X

Hegetotheriidae

Hegetotherium sp. X X X

Astrapotheria

Astrapotheriidae

Astrapotherium sp. X X X

Litopterna

Proterotheriidae

Proterotherium sp. X

Macraucheniidae

Theosodon cf. T. gracilis X X X

Rodentia

Chinchillidae

Prolagostomus pusilus X X

Prolagostomus profluens X

Prolagostomus divisus X

Pliolagostomus notatus X

Echimyidae

Stichomys sp.? X X

Spaniomys riparius X

Acarechimys cf. A. minutus X

Acarechimys new sp. p

Dasyproctidae

Neoreomys australis X ?

Scleromys sp. (Scleromys in Laventan) X

Octodontidae

Sciamys principalis X

Eocardiidae

Eocardia perforata X

Schistomys erro X

Erethizontidae

Steiromys duplicatus X

Steiromys new sp.

Neoepiblemidae

Perimys impactus X

Perimys scalaris X

Perimys procerus X

Perimys onustus X

Perimys perpinguis X

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302 289

discordant contact with the underlying metamorphic com-

plex (Chile Chico Ibanez Formation, see Niemeyer et al.(1984)). Late Cenozoic uplift and volcanism have folded

and faulted this sedimentary sequence. A faunal list, based

on the collection and study of the Guadal Formation

macroinvertebrate assemblage by Frassinetti and Covace-

vich (1999), is shown in Table 2 and discussed sub-

sequently. Samples from the marine sequence were

analyzed for microfossils, but only one diatom was

recovered, likely from a reworked fragment (Burckle,

personal communication).

In contrast to fossil localities nearer Pico Sur and

Chile Chico, the marinenonmarine section at Pampa

Castillo does not contain reliably datable volcanics. An

extensive sample of rocks from both the marine and

terrestrial sections were collected for paleomagnetic

analysis (in progress).

4. Mammalian faunas

Isolated fossil whale vertebrae and rib fragments

represent the only mammals found in an unnamed

marine sequence exposed near Pico Sur, south of Chile

Chico (Fig. 1, see below). Although whales were not

found in direct association with the diverse marine

macroinvertebrates recovered in the Pampa Castillo

section (Frassinetti and Covacevich, 1999), the over-

lying nonmarine strata at this locality yield abundant

terrestrial fossil mammals. This mammalian fauna is

notable for its diversity (36 taxa; Table 1), quality of

preservation, and striking resemblance to classic Santa-

crucian SALMA faunas in Argentina. As noted by

previous authors (Simpson, 1940; Pascual et al., 1985;

Pascual and Ortiz Jaureguizar, 1990), the Santacrucian

and Pan-Santacrucian cycle represent the southernmost

record of South American Cenozoic mammals (and

extreme southward extension of more subtropical

assemblages). The Pampa Castillo fauna extends this

high-latitude record farther west than previously known.

Niemeyer et al. (1984) remark that the Galera

Formation is characterized locally by abundant mammals

but do not mention the Meseta Guadal sequence in this

context. Although fossil mammals likely occur abundantly in

other exposures of the Galera Formation in this region, the

richest concentration identified to date is from the unnamed

strata at Pampa Castillo. Reconnaissance of adjacent outcrops

yielded only scattered and less well-preserved fossils. Further

exploration of the difficult terrain (exposed ridges are often

separated by kilometers of densely forested canyons generally

impassable on horseback) is warranted.

Nine major mammal-bearing horizons have been

identified from the terrestrial sequence at Pampa

Castillo, though the sequence is fossiliferous throughout.

Initial description of the fauna is facilitated by its

resemblance to well-studied Santacrucian SALMA

mammal assemblages elsewhere (Marshall et al., 1983;

Savage and Russell, 1983; Tauber, 1997a,b), but a more

comprehensive taxonomic review is beyond the scope of

this preliminary faunal report. The following groups

thus far have been recorded from the Pampa Castillo

terrestrial strata (Table 1).

Fig. 3. Paleogeographic map of middle Cenozoic Patagonian epicontinental seaway, in the study area (modified from Ramos (1982a,b)).

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Table 2

Composite macroinvertebrate fauna from marine strata at Pampa Castillo

(after Frassinetti and Covacevich (1999))

Mollusca: BivalviaNuculidae

Nucula reticularisNucula (Leionucula) sp.

MalletiidaeNeilo ornate

ArcidaeArca (Arca) patagonicaArca sp.

ParallelodontidaeCucullaria darwini

CucullaeidaeCucullaea alta

LimopsidaeLimopsis insolita

MytilidaeBrachidontes andinusModiolus cf. ameghinoiModiolus sp.

PinnidaePinna magellanica

IsognomonidaeIsognomon quadrisulcatus

PectinidaeChlamys centralisZigochlamys geminatus

OstreidaeCrassostrea aff. Hatchery

CarditidaeCardiocardita inaequalisPleuromeris elegantoides

CrassatellidaeCrassatella cf. longiorCrassatella patagonica

CardiidaeTrachycardium puelchumPatagonicardium philippiiPatagonicardium iheringiPatagonicardium? guadalensis

LahilliidaeLahillia sp.

SolenidaeSolena sp.

TellinidaeTellina sp. 1Tellina sp. 2Macoma? sp.

VeneridaeDosinia aff. meridionalisEurhomalea? navidadiformis sp. nov.Chione argentinaChione darwiniChione cf. meridionalisChione patagonica

HiatellidaePanopea bagualesiaPanopea nucleusPanopea sp.

PholadidaePholadidea patagonica

PholadomyidaePholadomya (Bucardiomya) sp.

PeriplomatidaePeriploma sp.

MOLLUSCA: GASTROPODAFissurellidae

Fissurella? sp.

Table 2 (continued)

TrochidaeGibbula dalliTrochus laevis

TurretillidaeTurritella ambulacrum

VermetidaeSerpulorbis sp.

EpitoniidaeEpitonium sp.

CalyptraeidaeCalyptraea sp.Trochita sp.

StruthiolariidaeStruthiolarella ameghinoi

Naticidae, indet.Cassidae

Semicassis ovulumFicidae

Ficus carolinaMuricidae

Trophon cf. patagonicusTrophon sp.

BuccinidaeSiphonalia sp.

TaiomidaeTaioma cf. tricarinata

VolutidaeProscaphella cossmanniProscaphella quemadensisProscaphella sp. 1Proscaphella sp. 2Proscaphella sp. 3Proscaphella sp. 4Adelomelon? burmeisteri

CancellariidaeCancellaria sp.

ScaphandridaeScaphander sp.

Incertae sedisPeonza cf. torquata

COELENTERATA: ANTHOZOAFlabellidae

Flabellum aequalisBRYOZOABryozoa spp. indet.BRACHIOPODA: INARTICULATADiscinidae

Discinisca sp.BRACHIOPODA: ARTICULATATerebratellidae

Terebratella aff. patagonicaTerebratella sp. 1Terebratella sp. 2Iheringithyris sp.

ANNELIDA: POLYCHAETIASerpulidae

Serpula sp.ARTHROPODA: MAXILLOPODAScalpellidae

Scalpellum sp.Balanidae

Balanus sp.ARTHROPODA: MALACOSTRACAGeryonidae

Archaeogeryon cf. patagonicusECHINODERMATA: ECHINOIDEASchizasteridae

Schizaster cf. ameghinoiMonophorasteridae

Iheringiella patagonensis

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4.1. Marsupialia

The borhyaenid Cladosictis sp. is represented by a

fragmentary maxilla bearing a single tooth. The specimen

was found as float high in the section (level 7) exposed on

Pampa Castillo.

Four species of the caenolestid Palaeothentes are known

from Pampa Castillo. These are represented primarily by

jaws and teeth from the rich quarry sites at the base of the

terrestrial section (level E-O). Some specimens (e.g. SGO

PV 2214, 2302, 2304, 2305, 2307) have unusually well-

preserved dentitions (Fig. 4). The four species of

Palaeothentes appear to be similar in abundance in the

assemblage from Pampa Castillo. Bown and Fleagle (1993)

report that Santacrucian SALMA specimens of palaeothen-

tids are very rare in the pre-Andean Pinturas Formation but

quite common in the coastal Santa Cruz Formation. As

might be predicted, the small-bodied Palaeothentes are

relatively common in quarried sites (level E-O) at Pampa

Castillo but rare elsewhere in the section.

As many as nine species of Palaeothentes are currently

recognized, ranging from Deseadan to Santacrucian

SALMA in age (Marshall et al., 1983; Bown and Fleagle,

1993). Bown and Fleagle (1993) named three new species

and considered six of the nine species recognized earlier by

Marshall (1980) to be valid; the other three were placed in

new or resurrected genera. Mones (1986) recognized seven

species. All previously named species at Pampa Castillo

also occur in Santacrucian SALMA deposits in Argentina

(Pinturas and Santa Cruz Formations; Bown and Fleagle,

1993). Paleothentes minutus and P. intermedius also have

been reported from Chile (Alto Ro Cisnes/Fras; Marshall,

1990).

With their extensive samples and new stratigraphic field

data, Bown and Fleagle (1993) were able to document five

Palaeothentes (and two Propalaeothentes ) species from

Santacrucian SALMA deposits. The Pampa Castillo site in

Chile has produced the three smallest of these Palaeothentes

species: P. pascuali, P. minutus, and P. intermedius, as well

as a diminutive new species (see below). One of the two

specimens (both uppers), referred to here as P. intermedius,

differs slightly in dental proportions from Argentine speci-

mens assigned to that taxon, falling outside the 95%

confidence interval for a regression of M1 length versus

width (for both P. intermedius alone and all palaeothentines

generally). However, it is very similar in size and

morphology, so we conservatively assign it to P. inter-

medius rather than to a new taxon.

Bown and Fleagle (1993) found P. minutus to be present

at nearly all significant localities of Santacrucian age (and

the most abundant palaeothentid at many), whereas the

other previously named taxa occurring at Pampa Castillo (P.

pascuali (previously reported only from the Santa Cruz

Formation) and P. intermedius ) were noted as less common.

Prior to discovery of the Pampa Castillo fauna, only six

specimens (all lower dentitions) were positively referred to

P. pascuali. Pampa Castillo has yielded at least three P.

pascuali specimens, including at least one partial upper

tooth row, the first known for the taxon (assigned on the

basis of size; P. pascuali lower teeth are approximately 20%

smaller than those of P. minutus, as are these uppers). This

finding extends the range of this species (in addition to the

two other Palaeothentes previously reported from Alto Ro

Cisnes/Fras; Marshall, 1990) into Chile.

One small marsupial recovered at Pampa Castillo is

clearly referable to Palaeothentes, but it differs dramatically

from all other previously recognized species. Accordingly,

we briefly describe this new species (Fig. 4):

Marsupialia

Caenolestoidea

Palaeothentidae

Palaeothentes smeti new species.

Type specimen: SGO PV 2304 (Pampa Castillo 4 in Fig.

5), maxillary fragment bearing P2-M3.

Age: Santacrucian SALMA, currently known only from

Pampa Castillo, Chile.

Etymology: The species is named in honor of our host

and friend, Carlos de Smet de Olbeck de Halleux, an avid

naturalist and former mayor of Chile Chico, who discovered

and led us to the fossil whale site and assisted us in

exploring the region of the Meseta Guadal. The de Smet

family including Carlos, his wife Beatriz, and relatives

running the Estancia at Ro Baker near Pampa Castillo

provided great hospitality to the members of our

expeditions.

Diagnosis: A member of the palaeothentid caenolestoid

marsupial clade Palaeothentes, possessing upper dentition

features noted by Bown and Fleagle (1993) as diagnostic of

the Palaeothentidae (distinguished from Abderitidae in

having M13 tribosphenic, unsquared and divided into

distinct elevated hypocone platform and lower trigon basin,

and M13 lacking lophs connecting primary labial and

lingual cusps; distinguished from Caenolestidae in having

M1 much larger than M2 and molars decreasing in size

posteriorly, P12 very reduced (only P2 preserved in P.

smeti ), P3 enormous, and M1 with elevated hypocone

platform on unworn teeth, separated from trigon by deep

fissure at lingual midline of tooth) and Palaeothentinae

(differ from Acdestinae in having less widely separated

M12 protocone and hypocone, less triangular M24, and

less reduced M4 also possessing distinct cusps). It is

distinguishable from all other previously named palaeothen-

tines in its much smaller size (20% smaller than the smallest

previously known species, Palaeothentes pascuali, and at

least 40% smaller than all other Palaeothentes species).

Proportions of M1 differ from the general shape for other

palaeothentine and palaeothentid species (Fig. 5, which

illustrates a generally constant log linear shape allometry

with size for all other species averages) in being relatively

narrow.

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302292

Discussion: P. smeti (Pampa Castillo 4 in Fig. 5 plot of

M1 ln width versus ln length) is a tiny palaeothentid that

falls well below the size range of all known species for this

clade. The M1 area of the P. smeti holotype specimen is

2.17 mm3, 40% smaller than the P. minutus average M1 area

of 4.39 mm3. P. pascuali, the smallest previously known

palaeothentid, is only about 20% smaller than P. minutus

and thus 20% larger than P. smeti (based on the percentage

size difference of P. pascuali relative to P. minutus and the

size of the first known upper molars of P. pascuali ).

Discovery of P. smeti broadens the size range for the

palaeothentid clade. It also indicates that Palaeothentes

species diversity at Pampa Castillo (four species are known,

all from the quarries of the E-O level) equals or exceeds that

of any other single Miocene site. Seven palaeothentids

(including four Palaeothentes species) are reported cumu-

latively for the entire Pinturas Formation (Bown and

Fleagle, 1993), and the Pampa Castillo assemblage is

twice as diverse as that from the type Friasian SALMA

sequence.

4.2. Edentata

Cingulate remains were found throughout the section,

either as fragmentary osteoderms or nearly entire carapaces;

one reasonably complete skull was collected. The glypto-

dont Propalaeohoplophorus is by far the most common

edendate in the fauna (Ameghino, 1887; Castellanos, 1937).

Dasypodids are less abundant, but a well-preserved partial

skeleton of Proeutatus (SGO PV 2041) was recovered.

Megalonychid (e.g. Hapalops ) remains are rare.

4.3. Notoungulata

Isolated elements and partial skeletons of notoungulates

are numerous. Surface float material is dominated by tooth

fragments of the toxodontid Nesodon sp. Only one size

category for Nesodon has been identified, but the possibility

of more than one species cannot be discounted. Homalo-

dotherium, represented by one nicely preserved maxilla, is

rarer in occurrence, as is the toxodontid Adinotherium.

Species-level assignment of these forms is impeded by a

complicated taxonomy that needs broad revision (Mones,

1986; Madden, 1990).

Hegetotheriids are represented by several complete

skulls of Hegetotherium, best preserved in the quarry sites

at the lowest fossiliferous horizon of the terrestrial section

(e.g. SGO PV 2312). Excellent mandibles and upper

dentitions of the interatheriid Protypotherium were also

recovered from several horizons.

4.4. Astrapotheria

Remains of a large Astrapotherium are rare and primarily

limited to dental fragments found as float on the surfaces in

eastern exposures of the butte.

4.5. Litopterna

Occurrence of the litopterns Proterotherium and Theo-

sodon is also limited, though the former is represented by

several well-preserved dentitions (e.g. SGO PV 2209).

4.6. Rodentia

As in typical Santacrucian SALMA faunas, rodents are

extremely abundant and diverse at Pampa Castillo. Detailed

analyses of these taxa were undertaken in a masters thesis

by Dodson (1994) and in Dodson et al. (in preparation),

from which we present a brief summary of key information.

At least 20 species are recognizable from partial

skeletons, jaws, and isolated teeth recovered throughout

the section. Skulls are preserved mainly in the basal quarry

sites. The species diversity from Pampa Castillo matches

that reported from the most diverse Santacrucian SALMA

assemblages and is two to three times greater than that

known from the Friasian SALMA of Argentina and Chile

(Vucetich, 1984). Rodent diversity at Pampa Castillo also is

at least as high as that from the tropical La Venta Fauna

(Walton, 1997; approximately 1820 species), one of the

most diverse South American fossil rodent assemblages

known, though they differ markedly in composition.

Neoreomys australis (Ameghino, 1887) is particularly

abundant and well represented (e.g. SGO PV 2212; Fig. 6) at

Pampa Castillo. It is the most abundant mammal and occurs

throughout the stratigraphic section. There appear to be

some within-species morphologic changes through time in

the sequence (e.g. P4 changes from square to more

Fig. 4. SEM photograph of the maxillary dentition of the holotype of the new caenolestid marsupial Paleothentes smeti from Pampa Castillo.

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302 293

rectangular in outline with length greater than width;

Dodson, 1994). This species is also present in most

Argentine Santacrucian SALMA assemblages and has

been noted from the type assemblage of the Friasian

SALMA in Chile. Also present are several well-preserved

teeth of Spaniomys riparius (Ameghino, 1887); Spaniomys

is restricted to Santacrucian SALMA faunas (Savage and

Russell, 1983; Marshall et al., 1983; Vucetich, 1984). Other

common rodents include Eocardia perforata, Acarechimys

minutus, and Perimys. A large percentage of the rodent taxa

known from Friasian SALMA assemblages of Argentina

and Chile (Vucetich, 1984) is not present in the Pampa

Fig. 5. Proportions of Miocene palaeothentid M1s.

Fig. 6. Skulls of the rodents (A) Neoreomys australis and (B) Perimys procerus from Pampa Castillo.

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302294

Castillo fauna, including Alloiomys, Cardiomys, Simplimus,

and Disteiromys.

5. General mammalian faunal compositiontaxonomic

bias

Table 1 documents that a wide diversity of South

American Miocene mammal groups occur in the Pampa

Castillo fauna. Although the 36 taxa thus far identified at

Pampa Castillo are fewer than a composite list of taxa for

the numerous, diverse Santacrucian SALMA faunas (i.e. a

composite of 81 genera in Marshall et al. (1983, Table 9)),

the Pampa Castillo diversity exceeds that of most, if not all,

individual Santacrucian localities. Most of the major clades

found in Santacrucian SALMA faunas are represented at

Pampa Castillo. Clades not presently known from Pampa

Castillo are the Microbiotheriidae and the enigmatic and

extremely rare Necrolestidae (Marsupialia); the edentates

Megatheriidae, Mylodontidae, Entelopidae, and Myrmeco-

phagidae (Edentata); the Adianthidae (Litopterna); and the

Notohippidae (Notoungulata). A notable absence thus far is

the lack of any evidence for primates, though this group is

relatively rare (or absent) in occurrence in Argentine

Santacrucian SALMA faunas, and the most abundant

occurrences occur in facies that do not appear well

represented in the Pampa Castillo sequence (Bown et al.,

1988; Bown and Fleagle, 1993).

6. Biochronology

The preceding survey of the mammalian fauna is

preliminary, and species-level taxonomic studies have not

yet been undertaken for groups other than palaeothentids

and rodents. Nonetheless, there is sufficient information to

support a close correlation between the Pampa Castillo

fauna and Santacrucian SALMA faunas of Argentina. It is

apparent (Table 1) that 19 of the taxa, representing more

than half of the Pampa Castillo fauna, are known only from

the Santacrucian SALMA elsewhere. One occurs in only the

Colhuehuapian and Santacrucian SALMAs, six occur in

only the Santacrucian and Friasian SALMAs (two

questionably in the Friasian), eight taxa have broad

Early to Middle Miocene age distributions, and two are

new named taxa currently known only from Pampa Castillo.

The Santacrucian SALMA is late Early Miocene in age.

Although Marshall et al. (1986), Marshall (1990) and

Marshall and Salinas (1990) considered the Santacrucian

SALMA to span approximately 1518 Ma, additional

geochronologic data led Flynn and Swisher (1995) to

constrain its age to 16.317.5 Ma. These age constraints are

among the best available for any SALMA and are based on

magnetostratigraphy and radioisotopic dating of Santacru-

cian SALMA faunas (Flynn and Swisher, 1995) from the

Santa Cruz Formation of Argentina (including Monte Leon,

the nominal type locality of the Santa Cruz Formation and

the Santacrucian Land Mammal Age; Marshall et al., 1983,

1986; Fleagle, et al., 1995), the older, conformably

underlying Monte Leon Formation (at Monte Leon), and

the younger, overlying Collon Cura Formation (Colloncuran

SALMA; Marshall et al., 1983, 1986; Ortiz Jaureguizar

et al., 1993; Flynn and Swisher, 1995). Although Santacru-

cian SALMA faunas are the most diverse and best sampled

of the South American Cenozoic record, they have been

limited almost exclusively to Argentina until now. Dis-

covery of a Santacrucian SALMA mammal fauna in Chile,

near the northwesternmost margins of the final Miocene

marine embayment and an important geologic disjunction

near Lago General Carrera/Buenos Aires, extends under-

standing of taxonomic diversity patterns, paleobiogeogra-

phy, and paleoenvironmental distribution of these faunas.

7. The Santacrucian SALMA

Due to poor stratigraphic control associated with most

previous faunal studies, there have been few recent attempts

to subdivide the Santacrucian SALMA. Tauber (1997a,b)

provided the most recent detailed biostratigraphy for Santa

Cruz Formation sequences in Patagonia, erecting two

biozones within these classic sections but not proposing

formal subdivisions of the Santacrucian SALMA. Never-

theless, possible subdivisions of the Santacrucian were

erected as early as Ameghino (19001902), who included

Notohippidien and Santacruzeen stages in his con-

ception of the Formation Santacruzienne. Ameghino

(19001902) also recognized an Astrapothericuleen

stage for faunas from the area southeast of Lago Buenos

Aires (Barrio et al., 1984) and considered these strata above

the Formation Patagonienne and below the Notohippidien

and Santacruzeen stages. Ameghino (1906) later con-

sidered the Astrapothericuleen closer in age to the younger

Notohippidien than to the older Colpodoneen stage (the

lower part of the Formation Patagonienne; as mentioned,

Ameghino previously explicitly excluded the Astrapother-

iculeen from that formation). Kraglievich (1930) applied the

name karaikense to Ameghinos notohippidense and

included in it the astrapothericulense. The Colpodoneen,

or Colpodonense (called trelewense by Kraglievich

(1930)), forms the basis for the Colhuehuapian, which

currently is recognized as the mammal age immediately

preceding the Santacrucian. Therefore, if it is possible to

define and recognize the Astrapothericulense or Noto-

hippidense biochronologic units uniquely, they represent

(and would be best considered) a subdivision (or subdivi-

sions) of the Santacrucian SALMA, as has been proposed by

Pascual et al. (1965), Pascual and Odreman Rivas (1971)

and Barrio et al. (1984).

One recent attempt to subdivide the Santacrucian

SALMA (or identify a new, older SALMA) has been the

suggestion that a small fauna from Lago Puerreydon (Fig. 1)

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302 295

ALFONSOHighlight

hpResaltado

hpResaltado

in western Argentina (Hatcher, 1903; Marshall, 1982, p.

432; Marshall, 1985, p. 62) may represent a younger interval

of Santacrucian time than that represented by the most

extensive typical Santacrucian SALMA faunas farther east.

Barrio et al. (1984) explicitly considered a fauna from the

western side of the Deseado Massif (E of Lago Puerreydon

and SE of Lago General Carrera/Buenos Aires) to be

assignable to the Piso Astrapothericulense (stage). These

authors also stated that the Astrapathericulense stage was

Santacrucian in age and that uncertainties remained about

the temporal relationship among strata assigned to the

Astrapothericulense and Notohippidense. Marshall

(1990) also suggested that it may possible to distinguish

an early Santacrucian SALMA faunal subdivision (corre-

sponding to the Notohippidense horizon) on the basis of

marsupials, though he did not provide a formal definition or

clear basis for recognizing this interval. This proposal has

yet to be extensively tested by studies of other elements of

the type Friasian fauna; for example, a new adianthid

litoptern (Cifelli, 1991) was suggested to have close

relationships to a Santacrucian SALMA species and an

unstudied taxon from the Notohippus fauna (Ameghino,

19031904, 1906; Karaikense Karaiken, of Kraglie-vich (1930); representing an early phase of the Santacru-

cian; Simpson, 1940). In contrast, Madden (1990)

emphasized that several Ro Fras/Cisnes toxodontids are

found only in this type Friasian or younger faunas and that

several typical Santacrucian SALMA taxa are absent. Other

taxa, however (see below), also show clear distinctions in

taxon occurrences between the type Friasian fauna and those

from other SALMAs. Geochronologic studies (Flynn et al.,

1989a,b; Flynn and Swisher, 1995; Flynn, in progress) of the

type Friasian sequence also indicate that the date for the

Friasian fossil assemblages low in the section was incorrect

(roughly 16.5 Ma, near the younger boundary of the

Santacrucian SALMA, rather than the older estimate of

roughly 17.5 Ma cited by Marshall (1990)). Thus, though

the fossil-bearing part of the Friasian-type sequence could

overlap in time with a small part of the Santacrucian

SALMA, the entire sequence at Ro Cisnes/Fras may

represent a longer time span and reach much younger ages

than is represented by Santacrucian SALMA assemblages

elsewhere. Clarification of the relationship between the

Friasian and Colloncuran SALMAs requires additional

work on both.

Faunal distinction between the Santacrucian SALMA and

the presumed younger Friasian (sensu lato) faunas and the

older Colhuehuapian SALMA faunas remains poorly delim-

ited, in part because of lack of detailed study of those

bracketing faunas. Similar chronostratigraphic problems are

associated with many SALMAs (Marshall, 1985; Flynn and

Swisher, 1995; Madden et al., 1997). Determination of precise

temporal relationships among these faunas, and the resulting

establishment of land mammal ages (and primary stages on

which ages must be based) is further hampered by short

stratigraphic sections, lack of direct superposition between

faunas, and lack of magnetostratigraphic and radioisotopic

control in most areas. Kay and Madden (1997; Tables 30.8 and

30.9) provide faunal resemblance indices for various Miocene

assemblages and SALMA composites, indicating that the

Friasian type fauna (from Ro Cisnes/Fras) and Santacrucian

and Colloncuran SALMA assemblages are very similar to one

another (all 6981% similar), reflecting only slight differ-

ences in age or geographic provinciality for coeval assem-

blages. However, even with high levels of similarity in this

coefficient, it is not a range-based biochronologic method and

it excludes information about taxa restricted to the larger

assemblage (the coefficient is the number of shared taxa

divided by the size of the smaller assemblage). There are a

variety of taxa whose ranges (and thus, presence or absence in

specific SALMAs) clearly distinguish the three SALMAs

(Bondesio et al., 1980; Vucetich, 1984; Madden, 1990;

Vucetich et al., 1993; Ortiz Jaureguizar et al., 1993; Madden

et al., 1997; Tauber, 1997a,b). The thick and richly

fossiliferous sedimentary sequence at Pampa Castillo affords

an excellent opportunity to better constrain at least one portion

of the mammalian chronology and explicitly characterize its

faunal composition, thus complementing other recent studies

from Argentine sections at similar latitudes (Bown and

Fleagle, 1993; Tauber, 1997a,b). This section also has the

potential to provide the first rigorously documented analysis of

faunal change within the Santacrucian SALMA based on

directly superposed faunal horizons within a single strati-

graphic section.

At a broader scale, the presence of both diverse marine

macroinvertebrates and terrestrial mammal faunas clarifies

the geochronology of this sequence. Late OligoceneEarly

Miocene invertebrates are conformably overlain by late

Early Miocene (Santacrucian SALMA, 16.317.5 Ma)

mammals. Further chronologic refinements for this section

should be provided by paleomagnetic stratigraphy (in

progress).

The Guadal Formation represents the youngest marine

sedimentation in this area, and the faunas bracket a

transition from marine to terrestrial deposition, thereby

providing both a well-dated paleoelevational horizon (sea

level) and a maximum estimate of the timing of final

withdrawal of the Patagonian epicontinental seaway (late

Early Miocene or early Middle Miocene) in this area.

Withdrawal of the seaway must have been caused by

initiation of regional tectonic uplift and/or eustatic sea level

fall (significant drops in eustatic sea level occur at 15.5 and

16.5 Ma; Haq et al., 1987). In either case, tectonic uplift of

this part of the southern Andean cordillera must have been

coeval with or postdated the late Early Miocene marine to

terrestrial transition observed in the Meseta Guadal.

8. Marine invertebrate assemblages from Pampa

Castillo

The marine macroinvertebrate assemblage collected

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302296

ALFONSOHighlight

ALFONSOHighlight

from the Guadal Formation in the vicinity of Pampa Castillo

(Table 2) is diverse (84 taxa) and indicates an undiffer-

entiated Late OligoceneEarly Miocene age (Frassinetti

and Covacevich, 1999). This assemblage is very similar to

faunas recovered from the uppermost Piso Patagoniano

(also incorrectly referred to as a Formacion Patagonica,

Patagoniana, or Patagoniense) levels of extra-Andean

Patagonia but is markedly distinct from Miocene faunas

elsewhere in Chile (Frassinetti and Covacevich, 1999).

Patagoniano assemblages are generally associated with

upper Oligocene to lower Miocene sequences in the

southern Chilean Andes and Argentina (Ortmann, 1902;

Feruglio, 19491950; Riggi, 1957, 1979; Ramos, 1982a,b;

Niemeyer et al., 1984; Frassinetti and Covacevich, 1999).

Frassinetti and Covacevich (1999) consider the Pampa

Castillo assemblage most similar to Argentine Patago-

niano assemblages from the Monte Leon Formation (Late

OligoceneEarly Miocene); they share 78% of the bivalve

species and 83% of the gastropod species in common

(Monte Leon Formation information from Camacho

(1995)). There were much lower similarities to the older

San Julian Formation (early Late Eocene; 59 and 42%) and

Estratos con Monophoraster y Venericor (EMV; Late

Eocene; 26 and 8%; Camacho, 1995). The Eocene

comparisons may vary due to temporal, total assemblage

diversity (e.g. the EMV have only eight bivalve plus

gastropod species, whereas there are 31 from the Monte

Leon Formation and 39 from Pampa Castillo, the most

diverse assemblage) and paleogeographic or paleoenviron-

mental effects.

The youngest Patagoniano level (e.g. Monte Leon

Formation) is considered Chattian (latest Oligocene to Early

Miocene) in age on the basis of planktonic foraminifera

(Bertels, 1975, 1979). In Argentina, the Monte Leon

Formation directly underlies the Santa Cruz Formation; in

the Pampa Castillo section, there is a similar superpositional

relationship between the invertebrate-bearing marine beds

and the Santacrucian SALMA mammalian fauna from the

overlying terrestrial sediments. On the basis of this, the

stratigraphic position of the Pampa Castillo marine strata at

the top of the Guadal Formation, and the presence of the

marine to terrestrial transition (representing the final stages

of the marine transgression and the beginning of withdrawal

of the seaway), Frassinetti and Covacevich (1999) further

argue that the age of the marine invertebrate fauna from

Pampa Castillo might be refined to Early Miocene.

Water depths within the Guadal Formation marine

section at Pampa Castillo vary from shallow marine

nonmarine (containing oysters and fragmentary plant

material) of less than 20 m depth (based on the occurrence

of the echinoderm Iheringiella ) to possibly nearshore and

greater than 20 m deep (based on diverse assemblages of

bivalve and gastropod molluscs, coelenterates, brachiopods,

crustaceans, etc.). The presence of the gastropod Ficus,

which is found today in subtropical waters (Covacevich and

Frassinetti, 1980), may indicate a paleoenvironment that

was relatively warm and subtropical.

The invertebrate assemblages from the Guadal For-

mation document the northernmost and westernmost extent

(Fig. 3) of the middle Cenozoic Patagonian epicontinental

seaway (Tarling, 1980, Fig. 16; Uliana and Biddle, 1988,

Fig. 7; Ramos, 1982b, Fig. 4), as represented by the Mid

Tertiary Transgressive Onlap Sequence of Williams and

Hubbard (1984). Assemblages from the Meseta Guadal (the

most comprehensively described assemblage, from Pampa

Castillo, Frassinetti and Covacevich, 1999; from Cerro Pato

Paro, Niemeyer, 1975; from western Meseta Guadal, Katz,

1963) are the westernmost occurrences, and the coeval

assemblages from the study area southwest of Chile Chico

mark the northernmost extent of this seaway (Fig. 3).

Although these localities are only 100 km east of the Pacific

margin of South America and the Miocene invertebrate

localities from the Golfo de Penas area, the macroinverte-

brate fauna shares very few species with Chilean Pacific

OligoceneMiocene faunas (e.g. Navidad, Arauco; la

participacion de especies procedentes de estas cuencas,

entre la fauna de Pampa Castillo, es minoritaria y poco

significativa; Frassinetti and Covacevich, 1999, p. 61). The

taxa potentially shared with Chilean Pacific assemblages are

either geographically wide ranging, found in both Atlantic

and Pacific Miocene assemblages, or have not been revised

taxomically for sometime and represent different species.

Therefore, it appears that the invasion of the Miocene

Atlantic in Patagonia extended as far northwest as the

Meseta Guadal (72.58W) but was separated from the Pacificby a continuous geographic barrier of the Andean Cordillera

formed by the late Mesozoicearly Tertiary batholith

(Uliana and Biddle, 1988, Fig. 7).

9. Marine mammals and radioisotopic dating of

bracketing basalts

The cetacean material recovered includes several large

vertebral and rib fragments referable to the Mysticeti (De

Muizon, personal communication). These were found in

association with marine macroinvertebrates, presumed to be

correlative with the better-known assemblages from Pampa

Castillo (Table 2) (Frassinetti and Covacevich, 1999). The

site lies at approximately 1900 m elevation near Pico Sur,

about 20 km southwest of Chile Chico. The fossiliferous

unit is part of an unnamed sequence of Miocene marine

sediments (including the Segundo nivel marino con

Ostrea), intercalated between two informal units of basalt

flows of early Cenozoic to MiocenePliocene age (Charrier

et al., 1978, 1979; Complejo Basaltico de Plateau of

Niemeyer et al. (1984)).

This marine fossil assemblage is derived from nearly

horizontal strata directly bracketed by four basalts dated by

whole-rock 40K40Ar methods (Table 3). The highest basalt

(sample 86-7) underlying the marine assemblage lies at

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1900 m altitude and is dated at 34.2 Ma, and the overlying

basalt (86-6) lies at 1950 m and is dated at 9.1 Ma, which

clearly indicates a significant depositional hiatus either

above and/or below the marine assemblage. Charrier et al.

(1978, 1979) and Ramos et al. (1982) provide additional40K40Ar analyses from the same basalt sequence with both

older and younger dates from the informal units Basaltos

Inferiores de Meseta Buenos Aires and Basaltos Super-

iores de Meseta Buenos Aires (Niemeyer et al., 1984;

Basaltos inferiores and Basaltos de la Meseta BuenosAires, respectively, of Charrier et al. (1978)) that are

consistent with the dates (and inferred hiatuses) reported

here. In addition, a continental conglomeratic unit (Ro

Zeballos Formation, Friasian Stage in Charrier et al., 1979,

Figs. 23) is considered to lie between the uppermost

marine strata and the upper basalt unit along the Ro

Jeinimeni on the southeast side of the meseta, though it does

not appear to be present at our sampled localities. The

precise superpositional relationships between those dated

basalts and the marine faunal horizons we collected are

uncertain; thus, the previously published dates are not

incorporated in our estimates of regional rates of uplift

(see below).

10. Andean tectonics

Uplift in the southern Andes has been related, in part, to

tectonic responses of subduction of the Chile Rise beneath

continental crust and northward migration of the CMTJ

since the middle Cenozoic (Forsythe et al., 1986). The ridge

subduction events are associated with increased heat flow

and thermally driven regional uplift. Three ridge segments,

offset by fracture zones, collided with the South American

margin at approximately 1410, 6, and 3 Ma (Ramos

1982b; Pilger, 1983; Cande et al., 1987; Ramos and Kay,

1992; Ramos and Aleman, 2000). Such activity is regarded

as closely tied to the creation of aesthenosphere windows

formed in compressional tectonic regimes, wherein young

oceanic crustal material (i.e. a ridge) is subducted beneath

an overriding continental margin. Because ridge segments

were subducted only south of 468S during the late Cenozoic,it may account for the strikingly different tectonic histories

observed in the regions north and south of Lago General

Carrera/Buenos Aires.

Previous estimates of time of initiation and rate of uplift

in this region of the Andes have been highly discordant and

poorly constrained (Charrier and Malumian, 1975; Skar-

meta, 1976; Charrier et al., 1979; Ramos 1982b, 1989;

Smith and Zinsmeister, 1982; Malumian and Ramos, 1984;

Ramos and Kay, 1992). Estimates for initiation of uplift in

this southernmost part of the Andes range between 10 and

26 million years. Skarmeta (1976) argues that orogenic

diastrophism resulted in uplift and the initiation of

sedimentation of terrestrial deposits in this region during

the Oligocene. Malumian and Ramos (1984) view maxi-

mum Andean uplift as occurring between 10 and 12 Ma but

note that continuous uplift began with deposition of the

Santa Cruz Formation. The age of this formation was

originally bracketed as between 16 and 22 Ma, on the basis

of 40K40Ar dates, but more recent analysis has revised this

range to as narrow as 16.317.5 Ma for Santacrucian

SALMA assemblages from this formation (see Marshall

(1985) and Flynn and Swisher (1995) for review). Ramos

(1982b, 1989) suggests a two-phase Cenozoic history for the

southern Patagonian Cordillera, with an initial uplift phase

(Pehuenchic) occurring in the Late Oligocene (postdating

the marine Centinela Formation deposits) and development

of the final structure of the Patagonian Cordillera in this

area during a main phase (Quechuic), sometime between 18

and 8 Ma (before deposition of the upper Miocene basalts,

about 89 Ma). Ramos and Kay (1992) incorporated more

recent data to constrain the timing of the main phase of

deformation somewhat more tightly, between 15 and 9 Ma.

Ramos and colleagues (Ramos, 1989; Ramos and Kay,

1992; Ramos and Aleman, 2000) also emphasized striking

differences between Miocene sediments and Andean

structure and tectonics, north and south of approximately

46.5478S (the region around Lago General Carrera/Buenos Aires). Work on marine macroinvertebrates

Table 340K40Ar radioisotopic data, whole-rock basalt samples from Pico Sur, Chile

Sample (Lab #) %K1 %K2 Wt (g)40Arp (mol/g) %40Ar Age (Ma ^ 1s )

86-6 (5641) 1.533 1.534 1.79693 2.39211 40.0 8.96 ^ 0.2

86-6 (5641R) 1.533 1.534 1.83900 2.43211 39.2 9.13 ^ 0.2

86-7 (5642) 0.638 0.633 1.84942 3.90211 56.8 34.99 ^ 0.6

86-7 (5642R) 0.638 0.633 1.90393 3.80211 36.7 34.15 ^ 0.7

86-7 (5642R-2) 0.638 0.633 1.87535 3.73211 45.7 33.52 ^ 0.6

86-13 (5643) 1.416 1.417 1.85164 9.79211 81.4 39.43 ^ 0.6

86-13 (5643R) 1.416 1.417 1.92899 9.77211 70.1 39.34 ^ 0.6

86-14 (5644) 0.628 0.622 1.92682 4.52211 55.2 41.22 ^ 0.8

86-14 (5644R) 0.628 0.622 1.83731 4.86211 51.3 44.30 ^ 0.9

86-14 (5644R-3) 0.628 0.622 1.90519 4.81211 41.4 43.85 ^ 0.8

Notes: Decay constants: l1 l10 0:581 10210 yr21; lb 4:962 10210 yr21; l 5:543 10210 yr21; and 40K=Ktotal 1:167 1024; 40Arp refersto radiogenic component.

J.J. Flynn et al. / Journal of South American Earth Sciences 15 (2002) 285302298

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hpResaltado

suggests the Late Miocene as the time of maximum uplift

(Smith and Zinsmeister, 1982). The presence at Pampa

Castillo of conformably superimposed marine macroinver-

tebrates, microplankton, and terrestrial vertebrate bearing

horizons, in conjunction with radioisotopic dates and

magnetostratigraphy, offer the potential for a highly

integrated correlation network to substantially refine dating

of tectonic events in this area. For example, our combined

data from Pico Sur and Pampa Castillo suggest that the

marine transgression may not have ended until late Early

Miocene (or even early Middle Miocene, because the

marineterrestrial transition is conformably and closely

overlain by Santacrucian SALMA fossils). These data also

suggest that the subsequent Pehuenchic phase of tectonism

might not have begun until the Middle Miocene rather than

the Late Oligocene. Radioisotopic (40K40Ar) dates for the

highest basalts (Basaltos Superiores de Meseta Buenos

Aires) in this region range as old as 16 Ma (Charrier et al.,

1979). If they are accurate, they greatly compress the

interval between the last Centinela-like (pre-Pehuenchic)

marine transgression and the post-Quechuic basalts. If the

upper basalts indeed postdate the end of the Quechuic phase,

it raises the intriguing possibility that there was but a single

phase of middle Cenozoic uplift in this region, driven by

dramatic changes associated with plate tectonic movements,

the Chile Ridge Triple Junction, and the structural

discontinuity near Lago General Carrera/Buenos Aires.

Geochronologic results, though preliminary, bear on

postulated minimum and maximum uplift rates for this

region of the Andes. At the study area southwest of Chile

Chico, marine strata containing a whale and the same Late

OligoceneEarly Miocene invertebrate fauna as is present

at Pampa Castillo have been uplifted from below sea level to

their current elevation of at least 19001950 m. Basalts

bracketing this horizon indicate that uplift began sometime

after 34 Ma (and possibly even after 9 Ma if the upper

basalts were deposited at low elevation/marine environ-

ments and/or did not postdate the Pehuenchic/Quechuic

phases of tectonism) and provide minimum estimates of an

uplift rate of 0.06 mm/yr (based on the age of the underlying

basalt) to 0.22 mm/yr (based on the age of the overlying

basalt). In the study area at Pampa Castillo (southern Meseta

Guadal), strata deposited at or below sea level during a

marine to terrestrial transition were uplifted sometime after

the late Early Miocene (about 1720 Ma, based on the age

of marine fossils below the transition and Santacrucian

SALMA fossils above the transition) to a present altitude of

1100 m, yielding an uplift rate of 0.050.07 mm/yr.

Even the minimum uplift rates of 0.050.07 mm/yr

calculated independently for two regions of the southern

Andes are comparable to rates calculated for various phases

of uplift in the Bolivian Andes (Nelson, 1982; Crough,

1983; Kohn et al., 1984; Benjamin et al., 1987). Because

they represent minimum estimates, actual rates may have

been much more rapid (0.180.63 mm/yr) if uplift of the

two study sequences began in response to subduction of the

Chile Ridge (Forsythe et al., 1986) beneath this region.

Rapid uplift estimates assume a much younger age for

initiation of the major pulse of uplift in response to ridge

subduction between 6 and 3 Ma (rather than to subduction

of the older ridge segment south of the Esmerelda fracture

zone at around 1410 Ma) for the two localities currently

lying at 1100 and 1900 m in elevation.

Acknowledgments

Fieldwork and research for the 19861988 seasons in

southern Chile were supported by a grant from the Eppley

Foundation for Scientific Research. We are indebted to Greg

Buckley, Roger Carpenter, Paul Raty, Paul Sereno, Carlos

de Smet, Cruz Vargas, and members and friends of the de

Smet family for their assistance in the field. Our mountain

guide, Rene Burgos, performed meritorious service under

extremely harsh conditions. Jeanne Kelly and Jane Shumsky

of the American Museum provided meticulous preparation

of the specimens, many from very hard matrix (especially

those of the E-O quarry levels), and Ed Heck provided Fig. 1

(and the basemap for Fig. 3). From the Field Museum, we

thank Ron Testa and John Weinstein for photography,

Marlene Donnelly for figure preparation, Elaine Zeiger for

typing and assistance in manuscript preparation, and Susana

Magellon and Betty Strack for SEM guidance. Guiomar

Vucetich, Rick Madden, Lloyd Burckle, and Christian de

Muizon provided valuable scientific discussion, and Guio-

mar Vucetich and Bruce MacFadden provided helpful

formal reviews of the manuscript.

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