Geoarchaeology and Geochronology of Pluvial Lake Chapala, Baja

Geoarchaeology and Geochronology of Pluvial LakeChapala, Baja California, Mexico

Loren G. DavisDepartment of Anthropology, Oregon State University, Waldo 238, Corvallis, Oregon 97331

Stratigraphic exposures in natural profiles, archaeological excavation units, backhoe trenches, and an uncasedwater well from the Laguna Seca Chapala basin in the Central Desert of Baja California (29/N, 115/W) recordlake level and climate changes and provide a context for prehistoric occupation predating 9070 yr B.P. andextending through the Holocene. Lith-ofacies analysis points to the presence of a large (ca. 66 km ) lake prior to2

9070 yr B.P., which desiccated by 7.45 ka yr B.P., promoting rapid dune growth. New dating and redefinition ofStratigraphic units in the basin refutes earlier models of lacustrine history and prehistoric occupation including aproposed series of Pleistocene lake levels with associated cultural occupations. The geologic record from theLaguna Seca Chapala basin compares well with other paleoenvironmental records in southwestern NorthAmerica, supporting interpretations of wet and cool conditions in Baja California during the late Pleistocene andearly Holocene. © 2003 Wiley Periodicals, Inc.

INTRODUCTION

The Laguna Seca Chapala basin, comprising two hydrologically closed subbasins, is located inthe Central Desert region of Baja California, ca. 320 km north of the border of Baja CaliforniaSur (Figures 1 and, 2). Following intense rainfall, runoff from flanking formations of Mesozoicgranitics and Plio-Pleistocene age andesites and basalts (Moran-Zenteno, 1994) collects on theLaguna Chapala Playa (4.6 km ), the Western Playa (5.5 km ), and on a smaller unnamed playa2 2

(2 km ) immediately south of the Laguna Chapala Playa. Lakes formed after brief, intense2

rainfall are typically shallow (ca. 20 cm) and may persist for several months if created during thecooler winter season. Evidence suggests that in the past, the Laguna Chapala basin held largerbodies of water for long periods of time. Because rates of precipitation and runoff were greaterthan evaporitic losses, deepening waters formed ancient Lake Chapala. Stratigraphic evidencereported here will show that Lake Chapala probably had a maximum surface area of ca. 66 km2

and a depth of nearly nine meters above the present playa floor (ca. 666.9 m above sea level[masl]). A lower stand of Lake Chapala (661.0 masl), as seen in basin stratigraphy, records thedecline of pluvial conditions in the area. Today, artifacts form discrete in situ clusters along theedges of the playa, reflecting past human occupation in the basin.

Geochronology of Pluvial Lake Chapala, Mexico

Figure 1. Map of study area with Baja California inset map. Dashed pattern shows projected maximum limits of Pluvial Lake Chapala.


Figure 2. Overview photo of Laguna Chapala Basin looking east from eastern embayment of Laguna Chapala Playa (seeFigure 3 for photographer position and orientation). Parallel dikes of siliceous rock used as toolstone source in basinoutcrop at surface of flanking mountains at (a) and trend downslope to right.

This paper discusses the late Quaternary geologic history and geoarchaeological significance ofthis pluvial lake.

HISTORY OF GEOARCHAEOLOGICAL RESEARCH IN THE CHAPALA BASINRogers (1939, 1966) and Massey (1947) conducted the first formal investigations in the

Laguna Chapala basin through limited surveys of surficial sites. In his study of Laguna Chapalageomorphology and archaeology, Arnold (1957) provided the first detailed descriptions of LakeChapala and its associated archaeological record. From a study of undated stratigraphic sections,geomorphic features, and archaeological materials found in both surficial and buried contexts,Arnold argued that Pleistocene-age sites existed along the shores of a large "pre-lateWisconsinan" Lake Chapala. Ritter (1976) conducted surficial surveys of Laguna Chapala basinarchaeological sites and dated calcium carbonate samples from suspected lacustrine sediments.

Arnold (1957) identified four earlier stands of Lake Chapala at 12-15 m (40-50 ft), 6-7.6m (20-25 ft), 3-4.5 m (10-15 ft), and 1.8-2.4 m (6-8 ft) above the present playa surface, based onthe position and elevation of slope breaks in talus deposits, flattened summits of "sand hills," andgravel deposits; the lowest lake stand is identified from an erosional truncation of lacustrinedeposits. Arnold correlated the growth of Lake Chapala with late Pleistocene climaticconditions, suggesting a pluvial origin for its lacustrine deposits. On this basis, he correlatedthetiming of the different lake stands "with stadials and interstadials of Wisconsin glaciation"(Arnold, 1957:269). Although he did not give specific ages, this span of time literallycorresponds with marine oxygen isotope stages 4-2, which date between 75,000-13,000 yr B.P.(Shackleton and Opdyke, 1973).

In his study of Laguna Chapala archaeology, Arnold (1957) defined three technologicaltraditions, represented by the oldest Elongate Biface Assemblage (EBA), the Scraper-Plane


Assemblage (SPA), and the youngest Flake-Core-Chopper Assemblage (FCCA). A relativechronology was constructed based on the degree of weathering and patination on artifacts andtheir association with projected stands of Lake Chapala. The namesake of the elongate bifaceassemblage is "made by removal of large thick flakes from either side of a core. . . .Supplementary flaking of edges is minor, and physical evidence of use is not clearly established"(Arnold, 1957:250). According to Arnold, the EBA is found on the flanks of the basin, to thenortheast and southeast of the Laguna Chapala playa (Arnold, 1957: Map 6); and is associatedwith the first rise of Lake Chapala at 12-15 m. Scraper-planes associated with the SPA are"roughly hemispherical, thick tabular, or somewhat elongate forms . . . with a flattish base,generally (created by removal of) a single flake" opposite to "a high back which is wholly ormostly a network of flake scars" with a working edge created by "striking off flakes steeply fromthe margin of the base toward the back" (Arnold, 1957:253-254). The SPA is found entirelyaround the southern edge of the Laguna Chapala playa (Arnold, 1957: Map 6); and immediatelypreceded a rise in lake levels to 6-7.5 m (20-25 ft), when it was buried by sands either depositedor reworked by lake currents (Arnold, 1957:261). Soon after this rise, Lake Chapala receded toan elevation 3-4.5 m (10-15 ft) above the modern playa, eroding older deposits and exposingSPA artifacts. Flake-Core-Chopper Assemblages are found as surficial scatters along thesouthwestern edge of the Laguna Chapala subbasin, and along the western and southwesternedge of the Western Playa (Arnold, 1957: Map 6). Because the FCCA is largely found in theWestern Playa subbasin, near the hypothesized position of the final bodies of lake water, andincludes small serrated projectile points and marine shells, Arnold interprets it as being mostrecent and likely late Holocene in age. Arnold's association of Laguna Chapala archaeologicaloccupations with late Pleistocene lake levels (Arnold, 1957, 1978, 1984) attracted the interest ofother archaeologists seeking evidence of early human occupation in the New World (Rogers,1966; Davis, 1968; Ritter, 1976; Ritter et al., 1978, 1984; cf. Willey's map in Joukowsky, 1980:Figure 4-5).

To test Arnold's geoarchaeological model, Ritter (1976) conducted a limited strat-igraphic study of lake sediments exposed along the southwestern margin of the Laguna ChapalaPlaya (at SH-1 in Figure 3). Ritter collected calcium carbonate, suspected as pedogenic in origin,from a hard, coarse angular blocky silt bed 220-230 cm (ca. 664.8-664.7 masl) down from thetop of the exposed profile. This sample returned a radiocarbon date of 14,610 ± 270 yr B.P.(GaK-4362). Ritter viewed the results with caution, citing the many problems associated withinterpreting carbonate dates. Bryan and Gruhn (1999) and Gruhn and Bryan (2001)


Figure 3. Closeup map of the Laguna Chapala basin with positions of archaeological sites, stratigraphic exposures, backhoetrenches, and abandoned well. Explanation: BT 23 = backhoe trench #23; SH-1 = Sand Hill section 1; HH-A, HH-B =Haplopappus Hill site test pits A and B; DS1 = Dune Site 1; GB = Granite Bay site; AP = Abrigo Paredon site; RRx = RedRocks site; TDV-2 = Tinaja de Villegas section 2; P = position and orientation of Figure 2 photograph.

report on excavations conducted at the Abrigo Paredon site, located in the eastern embayment ofthe Laguna Chapala subbasin (Figure 3). Two early Holocene radiocarbon dates of 8650 ± 60 yrB.P. (Beta-115421) and 9070 ± 60 yr B.P. (Beta-115420) (Table II) were returned from charcoalassociated with a lithic production station.

My field investigations of Lake Chapala were conducted during field sessions in 1996,1997, and 1999. Studies were made of stratigraphic exposures provided in quarries, roadcuts,and in the uncased walls of a 50 m deep well. Additional exposures were included fromarchaeological excavations at four sites in the Laguna Chapala basin. Twenty-six backhoetrenches were excavated in the central, southern, and eastern portions of the Laguna Chapalaplaya and basin (Figure 3). Detailed stratigraphic records were collected from nearly allexposures, and datable samples collected from a portion of these sections. Elevations ofstratigraphic exposures were established with a transit and stadia rod.

STRATIGRAPHY AND GEOCHRONOLOGYNineteen lithofacies established from stratigraphic units observed in the Laguna Chapala

basin are subdivided into aeolian, alluvial, lacustrine, and colluvial units (Table I). Chronometriccontrol on the lithofacies is provided by 13 radiocarbon dates (Table II) and fourthermolummescence (TL) dates (Table III). The oldest unit (Qal) is a clast-supported cobblegravel seen in the wall of an uncased well at an abandoned ranch ca. 1.0 km north of the Laguna


Chapala playa (Figure 3). The Qal gravel lies 37 m below ground, at an elevation of 629.0 masl;and, although undated, likely represents Pleistocene alluvial channel gravels in a hydrologicallyopen Laguna Chapala Basin.

The abandoned ranch well profile shows Qal gravels overlain by an undated series offine- to coarse-textured clastic sediments (Figure 4a). Hard reddish carbonate-rich clay seen inthe upper reaches of the well profile correlates closely with Qls3, Qls6, and Qls9 marls at thesections SH-1, BT 17, BT 18, and BT 23 (Figures 4-6). Because they lack associated dates andwere not described to the same level of detail as other profiles, the abandoned well sediments aredifficult to correlate to other stratigraphic units.

Section SH-1 shows an alternating series of marls and coarse lacustrine sands (Qls2-Qlsl2) reflecting depositional environments during six periods of rising and falling lake levels(Figure 6). The marl units exposed in the SH-1 profile (Qls3, Qls6, Qls9, and Qlsll) are hard withmoderately developed blocky prismatic structure and common small carbonate concretions. Adispersed charcoal sample from a flaser-bedded sand, silt, and clay deposit (unit Hal3) at theHH-B section (Figure 6) returned a date of 7600 ± 70 yr B.P. (Beta-115417). In the eastern partof the Laguna Chapala basin, an extremely hard brown to very pale brown Qlsl marl is overlainby a charcoal-rich hearth feature in BT 25 (Figure 7), which returned a radiocarbon date of 7100± 190 yr B.P. (TO-8198). Closer to the edge of the basin, the Qlsl unit occurs as a very hardpoorly sorted carbonate-rich loamy sand. Although the veracity of Ritter's 14,610 ± 270 yr B.P.date (Ritter, 1976) on Qls9 carbonates at SH-1 cannot be established here, an upper limit age isprovided by the 7600 ± 70 yr B.P. date from Hal3 at HH-B, providing indirect support (Figure6).

Along the southern edge of the playa, the Qlsl unit is unconformably overlain byalternating well-sorted 0.5-cm-thick planar beds of coarse dune sand and 0.2-cm-thick planarbeds of fine dune sand (Hae2-Hael), which accumulated to an elevation of ca. 682.0 masl.Bedding in these dune sands dips between 12/ and 16/, trending to the southwest. Deposition ofHae2 dune sands was interrupted by two periods of surficial stability (Figure 6), marked by areddish brown sandy clay loam soil with single grain structure and weak carbonate development.The color is probably derived from the aeolian redeposition of playa silts following desiccationof an ephemeral lake. This author observed such a phenomenon during the fall of 1997 afterintense rains created a shallow (ca. 20 cm deep) lake on the Laguna Chapala playa. Severalweeks later, Santa Ana winds estimated at 96-160 kmph (60-100 mph) pushed the shallow laketo the western edge of the playa and blew clouds of playa sediment to the southwest in theimmediate direction of the large dunes. Four TL samples were taken from the walls of HH-A andHH-B, producing a range of dates between 2.33 ± 0.22 ka yr B.P. to 7.45 ± 0.75 ka yr B.P.(Table III). Because the age ranges of TL samples from the bottom of HH-A overlap with thosein HH-B, the TL chronology is considered to show the relative rapidity of dune growth (ca. 14 mhigh, measured at SH-1) after the deposition of Hal3 at 7600 yr B.P.

Stratigraphic profiles in the eastern Laguna Chapala basin (e.g., BT 19-BT 22) showburial of the Qlsl unit beneath poorly sorted light and dark yellowish brown alluvial pebbly siltsand sands (Hal2), which contain the 7100 yr B.P. hearth feature (Figure 7a). The creation ofthese pebbly silts and sands is analogous to modern runoff events following intense rainfall,


Table I. Description, age, and distribution of Laguna Chapala basin lithofacies.

Lithofacies Description Age Section Examples

Hael Bioturbated medium and coarse dunesands

modern-1200 yr B.P. RRx-B, DS1, GB-A HH-A,HH-B

Hae2 Dipping alternating beds of coarse andmedium dune sands overlying Qlsl

<2.5 ka- <7600 B.P. BT 6, BT 7, BT 19-BT 22

Hall Poorly sorted pebbly alluvial silts andsands with brownish yellow, lightyellowish brown, to grayish browncolors

<5340-<7100 yr B.P. BT 19-BT 22, BT 24, BT25

Hal2 Poorly sorted pebbly alluvial silts andsands with dark and light yellowishbrown color; reddish brown silt loamand sandy alluvial gravel facies seen inTinaja de Villegas quarry exposures

< 5340-7100 yr B.P. SH-1, HH-B

Hal3 Flaser beds composed of ripple crosslaminated sand, silt, and clay;truncated silts and clays drape troughsof sandy ripple beds; base of unitretains angular unconformity with Qlsunits at SH -1

ca. 7600 yr B.P. Abandoned ranch well

Qal Clast supported medium to coarse alluvial cobble gravels extending below629.0 masl

Pleistocene SH-1, HH-B

Qlsl Very pale to pale brown carbonaceouslacustrine silt or clay;hard,carbonaceous loamy sand occursas lateral equivalent at basin margins

ca. 7600 yr B.P. SH-1, HH-B

Qls2 Coarse reddish lacustrine sand, oftencemented with upper eroded contact

> 7600 yr B.P SH-1, Rrx-B, BT7

Qls3 Very pale brown to pale brownlacustrine silty clay and silt loamtypical, coarse sand rare

>7100 yr B.P BT9, BT 10, BT 19-BT 25

Qls4 Reddish lacustrine clay often seen witheroded upper contact

> 7600 yr B.P. SH-1, BT 17, BT 18, BT 23

Qls5-Qls 12 Alternating beds of lacustrine sand,silt, and clay, recording cycles of LakeChapala transgression and regression

> 700 y r B.P. SH-1

Qcv Poorly sorted colluvial accumulationof reddish brown clay and sub-roundedto angular pebbles and cobbles; matrixor clast supported, deposit drapesflanks of basin sloping steeply towardplaya; over lies Qls4 clays in BT 17,BT 18, and BT 23

Holocene BT 16-BT 18, BT 23


Table II. Radiocarbon dates from the Laguna Chapala basin.a

Lithofacies

Laboratory Material C/ Q12 13

ratioC age 14

(yrB.P.)Sample Locality

Hae l Beta-115413 Charcoal -25.0% 70 ±60 Granite Bay/A3Hae l Beta-115412 Charcoal -25.0% 220 ± 50 Dune Site 1/AW5Hae l Beta-115411 Charcoal -25.0%o 260 ± 100 Dune Site 1/AE12Hall Beta-115418 Charcoal - 15.6% 370 ± 40 Abrigo Paredon/1Hall Beta-115419 Charcoal - 16.3% 340 ± 50 Abrigo Paredon/2Hael Beta-115416 Charcoal - 13.8% 400 ± 40 Granite Bay/A13Hael Beta-115422 Charcoal -25.0% 410 ± 70 Red Rocks/A4Hael Beta-115415 Charcoal -25.0% 620 ± 80 Granite Bay/A12Hael Beta-115423 Charcoal - 15.8% 890 ± 40 Red Rocks/A11Hael Beta-115414 Charcoal - 13.4% 1170 ± 100 Granite Bay/A8Hal2 Beta-115424 Charcoal -25.0% 5340 ± 160 TDV-2Qls2 TO-8112 Charcoal -25.0% 6800 ± 580 Abrigo ParedonHal2 TO-8198 Charcoal -25.0%o 7100 ± 190 BT 25/98-100 cm

below surfaceQlsl Beta-115417 Charcoal -27.5% 7600 ± 70 Haplopappus Hill/B7Qls2 Beta-115421 Charcoal - 27.9% 8650 ± 60 Abrigo Paredon/20-25

cm below surfaceQls2 Beta-115420 Charcoal -25.0%o 9070 ± 60 Abrigo Paredon/36 cm

below surface Explanation: Beta = Beta Analytic laboratory; TO = IsoTrace laboratory at the University of Toronto. Lithofaciesa

follow Table I.

Table III. Thermoluminescence dates from Haplopappus Hill test excavation profiles.a

Sample Elevation (masl) TL age (ka yr B.P.)

HH-A 670.92 2.33 ± 0.22

HH-A 669.85 7.69 ± 0.82

HH-B 665.89 6.17 ± 0.67

HH-B 664.95 7.45 ± 0.75

"Ages shown in thousands of years before present (ka yr B.P.); error limits shown at 68% confidence level.


Figure 4. Stratigraphic profile of abandoned ranch well (a) and Tinaja de Villegas Arroyo Cut 2 profile (b). Sections oriented to

elevation in meters above mean sea level (masl). Sediment textures compared to Wentworth (1922) system; and include clay (cl),

silt (si), sand (s), granule (g), pebble (pb), and cobble (cb) clastic sizes. Width of deposit corresponds with dominant texture

category. Lithofacies designations (e.g., Hall) follow Table I. Radiocarbon dates shown in uncalibrated C years before present.14


Figure 5. Overview photo of SH-1 Stratigraphic section, at southwestern edge of Laguna Chapala Playa (Figure 2), showing

position of lithostratigraphic units and erosional unconformity beneath Hal3. Figure at far right is ca. 1.60 m tall.

Figure 6. Stratigraphic profiles and correlation between SH-1, HH-A, and HH-B sections. Sections oriented to elevation in meters

above mean sea level (masl). Sediment textures compared to Wentworth (1922) system; and include clay (cl), silt (si), sand (s),

granule (g), pebble (pb), and cobble (cb) clastic sizes. Width of deposit corresponds with dominant texture category. Lithofacies

designations (e.g., Hall) follow Table I. Radiocarbon dates shown in uncalibrated C years before present. Thermoluminescence14

dates are uncalibrated, and shown in thousands of years (ka) before present.


Figure 7. Stratigraphic profiles and correlation seen between BT 23 to BT 19 (a), and from BT 16 to BT 22 (b), in eastern portion of theLaguna Chapala basin. Sections oriented to elevation in meters above mean sea level (masl). Sediment textures compared to Wentworth(1922) system; and include clay (cl), silt (si), sand (s), granule (g), pebble (pb), and cobble (cb) clastic sizes. Width of depositcorresponds with dominant texture category. Lithofacies designations (e.g., Hall) follow Table I. Radiocarbon dates shown inuncalibrated C years before present.14


when fine clastic sediment moves down-slope towards the playa. In the Tinaja de Villegas area (at sectionTDV-2), Hal2 sediments appear as a reddish silt loam and contain dispersed, likely redeposited, charcoalfragments dated to 5340 ± 160 yr B.P. (Beta-115424) (Figure 4b). A later period of equally widespreadalluvial deposition (Hall) is seen at TDV-2 in a slight change to lighter colored pebbly silts and sands (seeBT 19-BT 22), which often retains 1-5 cm-thick bedding units.

Smaller, low-lying dunes composed of poorly-sorted light brown medium loamy sand and coarsesands (Hael) are found along the southeastern and eastern edges of the Laguna Chapala playa, revealed inprofiles at Dune Site 1 (DS-1), the Granite Bay site (GB), and the Red Rocks site in test pit A (RRx-A)(Figure 8a). Archaeological excavations recovered evidence of human occupation on these dunes. Charcoalsamples from a hearth and fragments dispersed in the Hael matrix returned a vertically discontinuous seriesof radiocarbon dates ranging between 70 ± 60 yr B.P. (Beta-115413) and 1170 ± 100 yr B.P. (Beta-115414)(Table II), pointing to extensive bioturbation. The Hael sands unconformably overlie a hard Qlsl marl,which retains a highly irregular surface, suggesting erosion occurred prior to its burial by late Holocene dunesand. Figure 9 shows a generalized reconstruction of basin stratigraphy.

LACUSTRINE HISTORY OF LAKE CHAPALAArnold (1957) suggested that lava extruded from a volcanic vent in the southwestern portion of the

Western Playa blocked the original outlet of an earlier alluvial basin, thus creating a hydrologically closedLaguna Chapala basin up to an elevation of ca. 670 masl. The difference in elevation from the outlet to aprojected lake elevation above 667.75, established earlier on the basis of Qls3 marls in BT 23, may relate towestward neotectonic tilting of the Laguna Chapala basin (cf. Arnold, 1957).

A series of lacustrine sediments at SH-1 (Qls3-Qlsl2) and in the abandoned ranch well point toseveral earlier undated stages of Lake Chapala. Discussion will be limited to those deposits that are closelyassociated with or can be correlated to dated samples. Polygenetic deposition of Hal2 and Qcv sediments atAbrigo Paredon contains archaeological materials associated with three radiocarbon dates on charcoal of6800 ± 580 yr B.P. (TO-8112), 8650 ± 60 yr B.P. (Beta-115421), and 9070 ± 60 yr B.P. (Beta-115420)(Figure 6; Table II) (Bryan and Gruhn, 1999). The presence of alluvial sheetwash (Hal2) and colluvium(Qcv) at Abrigo Paredon originated from adjacent slopes and represents a terrestrial depositionalenvironment. Coupled with extensive evidence of human occupation (e.g., bifaces, lithic debitage andmanuports), site stratigraphy suggests that Lake Chapala stood below 661.0 masl between 9000 and ca.7000 yr B.P.

Using the dates from Abrigo Paredon, reconstruction of the Lake Chapala strat-igraphic sequencemay be extended somewhat. Some time before 9000 yr B.P., during deposition of Qls3 marls preserved atBT 23 and SH-1, Lake Chapala prob ably rose to the maximum limit of the basin around 670 masl when itwould have had an area of ca. 66 km (Figure 1). As lake levels fell after deposition of Qls3, a contraction of2

the lake margin from 666.9 masl to ca. 661.3 masl is recorded in the deposition of Qls2 sands throughout thebasin. Prior to 9000 yr B.P., Lake Chapala rose again, depositing Qlsl marls up to ca. 666.3 masl at DuneSite 1-A. Lake Chapala likely stood just below 661.0 masl by 9000 yr B.P., as reflected in the presence ofQlsl marls in BT 19-BT 22.

Arnold (1957) suggested that activation along parallel normal faults in the area of the old outletdropped a large graben block (Figure 1), which removed the lava obstruction that originally closed theLaguna Chapala basin. Although not dated,


Figure 8. Stratigraphic profiles and correlation between DS-1 and GB-A, and from RRx-B to BT 6 (a); expanded view of AbrigoParedon profile shown with position of radiocarbon dates (b). Sections oriented to elevation in meters above mean sea level (masl).Sediment textures compared to Wentworth (1922) system; and include clay (cl), silt (si), sand (s), granule (g), pebble (pb), and cobble(cb) clastic sizes. Width of deposit corresponds with dominant texture category. Ldthofacies designations (e.g., Hall) follow Table I.Radiocarbon dates shown in uncalibrated C years before present.14


Figure 9. Generalized Stratigraphic cross section of Laguna Chapala Basin, as projected along two transects (A-A’ and B-B’ [see Figure 3 for position of transects]). Cross sections are not toscale. See Table 1 for description of lithofacies.


this scenario appears probable, because vertical displacement of a reddish lava flow is clear today in thesouthwestern corner of the Western Playa. Evidence for tectonic alteration of Laguna Chapala basinhydrology may be held in Hal3 sediments at SH-1 and HH-B. The Hal3 unit is identified as flaser bedscomposed of ripple cross-laminated sand, silt, and clay, which suggest alluvial deposition in fluctuatinghydraulic conditions (Reineck and Singh, 1980; Boggs, 1995). One scenario can explain the presence ofHal3 flaser bedding: After the graben block fell, the hydrological limits of the Laguna Chapala basin werereduced to an elevation below 660 masl. Continued precipitation and runoff into the hydrologically openLaguna Chapala basin eroded and reworked lacustrine sediments toward lower elevations as Hal3 units(Figure 5). At HH-B, deposition of Hal3 sediments ceased immediately before ca. 7.45 ka, followed byburial beneath aeolian sand, marking a shift to drier conditions. Once the hydrologic limits of the basin werechanged, exposed lacustrine sediments provided abundant clastic material for dune growth.

Today, evidence of past lake stands remain only where buried beneath younger sediments. Itappears that large-scale erosion, deflation, and alluvial reworking of lacustrine sediments has destroyeddeposits from past lake stands throughout much of the Laguna Chapala basin during periods of desiccation(cf. Holliday, 1997). For example, the 667.75 masl elevation of Hal3 sediments seen at SH-1 cannot betraced beyond their protected context beneath sand dunes; however, reconstructions of this associated lakelevel would project such sediments to have accumulated several kilometers to the north and south (Figure1).

GEOARCHAEOLOGICAL SIGNIFICANCE OF PLUVIAL LAKE CHAPALAEvaluating Arnold's Model of Pleistocene Prehistory

Stratigraphic and geochronologic evidence fails to support many of Arnold's (1957) findings,particularly as they relate to the timing of lake stands. Arnold interpreted an initial lake stand 12-15 m abovethe modern playa (i.e., 670.86-673.86 masl) from sediments at the SH-1 section, assuming that cross-beddedsediments (Hal3) exposed at the top of the existing SH-1 profile extended to the top of the landform. On thebasis of exposures at SH-1, HH-A, and HH-B, I have argued that the cross-bedded deposits are probablyflaser sediments (HalS) related to the erosion and redeposition of lacustrine sediments and that they quicklygive way to Holocene-age dune sands. Nevertheless, preservation of Qls3 marls at BT 23 (666.75 masl)suggests that a lake stand closely approximating Arnold's 12-15 m estimate actually existed; however, thetiming of this highest lake level is challenged by considering other paleoenvironmental records fromnorthern Baja California and the southwestern United States. Macrobotanical remains in a 10,219 ± 160 yrB.P. packrat midden from the Sierra de San Francisco, located ca. 300 km south of the Laguna Chapalabasin, point to cool and wet climate conditions (Rhode, 2002). Analyses of a 9.5 m core from the LagunaSeca San Felipe playa, located ca. 320 km north of Laguna Chapala, revealed the presence of a pluvial lakefrom 34,000 to 7000 yr B.P. (Ortega Guerrero et al. 1999). Details of this stratigraphic record show amarked increase in precipitation across the late Pleistocene-early Holo-cene transition, punctuated by a briefreduction in lake level at 11,000 yr B.P. At ca. 7000 yr B.P., Laguna San Felipe appears to have desiccatedentirely, followed by aeolian sedimentation. Marine cores from the Santa Barbara basin of southernCalifornia show similar climatic conditions, with maximum precipitation inferred between 13,000 and10,000 yr B.P. (Rack et al., 1995). Urrutia-Fucugauchi et al. (1997) and Metcalfe et al. (2002) report apluvial lake existed in the Alta Babicora Basin of Chihuahua prior to ca. 10,000 yr B.P., which desiccatedduring the early Holocene. Investigations of Lake Cochise, in southeastern Arizona, revealed two majorpluvial periods at ca. 13,750-13,400 yr B.P., and at or soon before ca. 8900 yr B.P. (Waters, 1989).

Despite the likelihood of a late Pleistocene-age high lake stand, establishing the EBA as associatedwith an early shoreline is another matter. Because simple bifaces and bifacial cores are time-transgressive


technological traditions in New World prehistory, it is difficult to use them as markers for Pleistocene-ageoccupation in the Laguna Chapala basin. In many areas of the basin, surficial distribution of lithic quarriesand large bifaces corresponds with the location of siliceous dikes that commonly cross-cut granitic outcrops(Figure 2a). Because these dikes were probably a source of toolstone throughout the late Quaternary, and thetemporal context of the EBA has not been established independently, correlating the spatial distribution oflarge bifaces to past lake levels is tenuous, at best (cf. Ritter, 1992).

Archaeological excavation of the Dune Site 1, Granite Bay, and Red Rocks sites served to testArnold's assertion that SPA occupations predate a late Pleistocene-age 6-7.5 m lake level (664.86-666.36masl) (Arnold, 1957: Map 6). This hypothesis was disproven when excavations at all sites revealed culturalmaterials (L.G. Davis, unpublished data) in late Holocene-age Hael dune sands. Charcoal samples from thethree excavated sites returned dates between 220 ± 50 yr B.P. (Beta-115412) and 1170 ± 100 yr B.P. (Beta-115414) (Table II).

Evaluating the Potential for Early Occupation at Abrigo ParedonIf the lacustrine reconstruction offered here is correct, the following scenario at the Abrigo Paredon

site would apply: prior to 9070 yr B.P., and perhaps during the late Pleistocene, the site was submergedbeneath the surface of Lake Chapala; between 9070 and 6800 yr B.P., after the lake retreated and asHal2/Qcv was deposited, hunter-gatherers lived at the site on a sandy beach near a lower stand of LakeChapala; after 6800 yr B.P., and until the latest Holocene, the site may have been subjected to burial byaeolian deposition, erosion, and deflation; during the last 400 yr B.P., finally, slopewash and colluvial actionemplaced a poorly sorted mix of clastic sediments (Hall and Qcv), capping the site.

Finding Pleistocene-Age SitesFuture efforts to locate early sites in the Laguna Chapala basin will be assisted by considering the

history of Pluvial Lake Chapala. Based on comparisons with regional paleoenvironmental records presentedabove, and considering the strati-graphic record presented here, Lake Chapala probably reached itsmaximum levels sometime during the late Pleistocene. By 9000 yr B.P., the lake level stood below 661masl, allowing occupation of the Abrigo Paredon site. Sites predating 9000 yr B.P. will probably be foundas surficial sites on remnant shoreline features or as cultural deposits buried beneath Holocene-age clasticsediments (e.g., Hal2, Qcv), near or above the projected maximum limits of Lake Chapala (Figure 1).Because they lack extensive alluvial reworking, such as that seen in the Tinaja de Villegas area, and offerbroader shorelines than the Western Playa subbasin, eastern and southern portions of the Laguna Chapalasubbasin should provide the best chances for preserving Pleistocene-age sites particularly where ancientshorelines approximate outcrops of siliceous dike rock.

CONCLUSIONSThe geologic history of Pluvial Lake Chapala provides evidence of a long sequence of lake

environments, probably dating back to the late Pleistocene. Evidence of a high lake stand predates 9000 yrB.P. Normal faulting in the southwestern corner of the Laguna Chapala basin reduced hydrologic limits tomodern levels, and promoted erosion of lacustrine deposits under a pluvial precipitation regime. Desiccationand rapid growth of sand dunes occurred after 7600 yr B.P. The timing of Pluvial Lake Chapala's rise andfall compares closely with late Pleistocene-early Holocene lacustrine and precipitation records from thesurrounding region. Reev-aluating stratigraphic sections and acquiring new chronometric dates provides ameans to assess earlier models of lacustrine history, and claims for late Pleistocene-age cultural occupationin the Laguna Chapala basin. Results of the geoarchaeo-logical study presented here largely disprove


Arnold's (1957) three-part archaeological model. In particular, the SPA, hypothesized to representoccupation along an early pluvial lake stand, has been dated to the Holocene. Although Arnold'sinterpretation of a high lake stand is supported, his argument that EBA artifacts are associated with a latePleistocene-age Lake Chapala has not been substantiated. Because large bifaces and bifacial coresassociated with the EBA are typically found near silica-rich dike exposures on the flanks of the LagunaChapala basin, Arnold's association of EBA artifacts with high lake levels remains circumstantial andrequires independent evaluation (cf. Ritter, 1976).

This work was made possible by generous financial support provided by the Huber Trust. Researchwas conducted under archaeological permits issued by the Institute Nacional de Antropologia e Historia(INAH); and was greatly facilitated by the help of Arqlga. Julia Bendimez Patterson, Evangelina DavilaRivera, and the staff at the Ensenada INAH office. Special thanks to Alan Bryan, Ruth Gruhn, Paul Wilson,Celeste Henrickson, George Wisner, the field crews of the 1997 and 1999 field seasons, Eugenic GrossoPeralta, and the members of the Ejido Revolucion. Thermoluminescence dates were produced by NickDebenham, Quaternary TL Surveys. Alan Bryan and Ruth Gruhn provided useful comments and editorialsuggestions on earlier drafts of this paper. Lastly, thanks to referees Mike Waters and Eric Ritter whoprovided helpful and constructive comments.


REFERENCESArnold, B. (1957). Late Pleistocene and recent changes in land forms, climate and archaeology in central

Baja California University of California Publications in Geography 10, 201-318.Arnold, B. (1978). Surface evidence of early man in arid lands. In A.L. Bryan (Ed.), Early man in America

from a circum-Paciflc perspective (pp. 181-192), Occasional Papers No. 1. Edmonton, Alberta:University of Alberta Department of Anthropology.

Arnold, B. (1984). Early man in Baja California—evidence and interpretation. Pacific Coast ArchaeologicalSociety Quarterly 20, 27-36.

Boggs, S., Jr. (1995). Principles of sedimentology and stratigraphy (2nd edition). Englewood Cliffs, NJ:Prentice-Hall.

Bryan, A.L., & Gruhn, R. (1999). Results of the 1997 excavations at the Abrigo Paredon, Laguna Chapala,Baja California, Mexico, Report on file. Edmonton, Alberta: Department of Anthropology,University of Alberta.

Davis, E.L. (1968). An archaeological reconnaissance in the Central Desert of Baja California, AnnualReport 1968, Volume 10. Los Angeles: University of California, Los Angeles, ArchaeologicalSurvey.

Gruhn, R., & Bryan, A.L. (2001). An interim report on two rockshelter sites with early Holocene occupationin the northern Baja California peninsula. Paper presented at the Second Bi-National Symposium,Ensenada, Baja California.

Holliday, V.T. (1997). Origin and evolution of lunettes on the High Plains of Texas and New Mexico. Quaternary Research 47, 54-69.

Joukowsky, M. (1980). A complete manual of field archaeology: Tools and techniques of field work forarchaeologists. Englewood Cliffs, NJ: Prentice-Hall.

Massey, W.C. (1947). Brief report on archaeological investigations in Baja California. Southwestern Journalof Anthropology, 3, 344-357.

Metcalfe, S., Say, A., Black, S., McCulloch, R., & O'Hara, S. (2002). Wet conditions during the last gla-ciation in the Chihuahuan Desert, Alta Babicora Basin, Mexico. Quaternary Research, 57, 91-101.

Moran-Zenteno, D. (1994). Geology of the Mexican Republic, AAPG Studies in Geology #39. Tulsa, OK:The American Association of Petroleum Geologists.

Ortega Guerrero, B., Caballero, M., Lozano Garcia, S., & De la O Villanueva, M. (1999). Palaeoenviron-mental record of the last 70,000 yr in San Felipe Basin, Sonora Desert, Mexico. Geofisica Interna-cional, 38, 153-163.

Rack, F.R., Heise, E.A., & Stein, R. (1995). Magnetic susceptibility and physical properties of sedimentcores from site 893, Santa Barbara Basin: Records of sediment diagenesis or of paleoclimatic andpaleoceanographic change? Proceedings of the Ocean Drilling Program: Scientific Results (Volume146, Part 2). College Station, TX: Ocean Drilling Program.

Reineck, H.E., & Singh, I.E. (1980). Depositions! sedimentary environments (2nd edition). Berlin:Springer-Verlag.

Rhode, D. (2002). Early Holocene juniper woodland and chaparral taxa in the central Baja Californiapeninsula, Mexico. Quaternary Research 57, 102-108.

Ritter, E.W. (1976). The antiquity of man in the Laguna Seca Chapala Basin of Baja California. PacificCoast Archaeological Society Quarterly 12, 39-46.

Ritter, E.W. (1992). Los primeros Bajacalifornianos: Enigmas cronologicos y socioculturales. In J. Ben-dimez Patterson (Ed.), Estudios fronterizos: Revista del Institute de Investicaciones Sociales,(pp. 9-30). Ensenada, Baja California: Universidad Autonoma de Baja California.


Ritter, W.E., Payen, L.A., & Rector, C.H. (1978). A brief note on an archaeological reconnaissance toLaguna La Gmja, Baja California. Pacific Coast Archaeological Society Quarterly, 14, 21-24.

Ritter, W.E., Payen, L.A., & Rector, C.H. (1984). An archaeological survey of Laguna La Guya, BajaCalifornia. Pacific Coast Archaeological Society Quarterly, 20, 17-26.

Rogers, M.J. (1939). Early lithic industries of the lower basin of the Colorado River and adjacent desertareas, Museum Papers No. 3. San Diego, CA: San Diego Museum.

Rogers, M.J. (1966). Ancient hunters of the Far West. San Diego, CA: Union-Tribune.Shackleton, N.J., & Opdyke, N.D. (1973). Oxygen isotope and paleomagnetic stratigraphy ofequatorial

Documents

Geoarchaeology and Geochronology of Pluvial Lake Chapala, Baja