Late Neogene stratigraphy and tectonic control on facies evolution in the Laguna Salada Basin, northern Baja California, Mexico

Sedimentary Geology 144 (2001) 5±35

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Late Neogene stratigraphy and tectonic control on facies evolution in the Laguna Salada Basin, northern Baja California, Mexico A. MartõÂn-Barajas a,*, S. VaÂzquez-HernaÂndez a, A.L. CarrenÄo b, J. Helenes a, F. SuaÂrez-Vidal a, J. Alvarez-Rosales c a

Departamento de GeologõÂa, Centro de InvestigacioÂn Cientõ®ca y EducacioÂn Superior de Ensenada, Baja California CP 22830, Mexico b Instituto de GeologõÂa, UNAM, Ciudad Universitaria, CoyoacaÂn 04510, Mexico, DF, Mexico c Residencia de Estudios del Campo GeoteÂrmico de Cerro Prieto, ComisioÂn Federal de Electricidad, Mexicali BC, Mexico Received 1 May 2000; revised 17 December 2000; accepted 19 December 2000

Abstract The Laguna Salada Basin (LSB) in northeastern Baja California records late-Neogene marine incursions in the Salton Trough and progradation of the Colorado River delta. Early subsidence and subsequent tectonic erosion are related to evolution of the Sierra El Mayor detachment fault during late Miocene time (,12 Ma). The stratigraphy of uplifted blocks on the east-central margin of the Laguna Salada Basin and from three exploratory wells allows reconstruction of the main sedimentary and tectonic events. Marine mudstone and sandstone, and subordinate conglomerate of the Imperial Formation tectonically overlie metamorphic and granitic basement. Microfossils, lithology, and sedimentary structures in the Imperial Formation de®ne Upper Miocene (,6 Ma) outer-shelf facies that grade up-section into inner-shelf and tide-dominated delta plain deposits of the ancient Colorado River. Lower Pliocene (,4±2 Ma) reddish, sub-arkosic ¯uvial sandstone and siltstone of the Palm Spring Formation de®nes progradation of non-marine ¯uvio-deltaic deposits over the marine Imperial Formation. Continuous outcrops of the Palm Spring are less than 170-m thick, but correlative deposits are more than 570 m thick in the lower part of a 2400-m deep geothermal exploratory well on the eastern margin of LSB. Inter®ngering ¯uvial-sandstone deposits and prograding alluvial fanglomerates with coarse debris-¯ow and rock-avalanche deposits crudely mark the onset of vertical slip along the Laguna Salada fault and rapid uplift of Sierra Cucapa and Sierra El Mayor. Up to 2 km of Quaternary alluvial-fan and lacustrine deposits accumulated along the eastern margin of LSB, whereas lower subsidence rates produced a thinner sedimentary wedge over a ramp-like crystalline basement along the western margin. In early Pleistocene time (,2±1 Ma), the Laguna Salada became progressively isolated from the Colorado River delta complex, and the Salton Trough by activity on the Elsinore and Laguna Salada fault zones. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Gulf of California; Stratigraphy; Neogene; Laguna Salada; Salton Trough

1. Introduction * Corresponding author. Address: Departamento de GeologõÂa, Centro de InvestigacioÂn Cientõ®ca y EducacioÂn Superior de Ensenada, P.O. Box 434843, CA 92143 San Diego, Mexico. Tel.: 1526-174-4501; fax: 152-6-175-0557. E-mail address: [email protected] (A. MartõÂn-Barajas).

The Laguna Salada Basin and the Imperial Basin farther north constitute the southwestern limit of the Salton Trough. Both provide insights into the evolution of marine and non-marine sedimentary basins that developed at the head of the Gulf of California

0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0037- 073 8(01)00133- 6

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Fig. 1. Tectonic framework of the southwestern Salton Trough and the northern Gulf of California (modi®ed from Winker and Kidwell, 1996). SAF, San Andreas Fault; CCF Coyote Creek Fault; SMF, Superstition Mountain Fault; SHF, Superstition Hill Fault; IMF, Imperial Fault; CPF, Cerro Prieto Fault; LSF, Laguna Salada Fault. Textured areas are Neogene localities with marine and non-marine deposits. Dark grey areas are active pull-a-part basins.

rift system during Late Neogene time (Fig. 1). Tectonic and sedimentary events within the Laguna Salada and Imperial Basins register similar episodes of late Miocene to Pliocene detachment-related extension, which evolved to dominantly wrench tectonics along the Imperial and Cerro Prieto faults and intervening pull-a-part basins (Axen and Fletcher, 1998; Fuis and Kohler, 1984). Both basins received their ®rst marine incursion in late Miocene (Winker and Kidwell, 1996; VaÂzquez-HernaÂndez et al., 1996; Bell-Countryman, 1984; Quinn and Cronin, 1984), and contain evidence of the progradation of the Colorado River delta. In Pleistocene time, the Laguna Salada Basin became isolated from the Imperial Basin

and from the delta complex by tectonic activity on the Elsinore and Laguna Salada faults and the Sierra El Mayor detachment-fault (cf. Johnson et al., 1983; Axen et al., 2000). This isolation produced lacustrine conditions alternating with episodic marine and ¯uvial ¯ooding from the south. Continued subsidence and lower sedimentation rates maintained the basin ¯oor below sea level. Compared to the well-known Imperial Basin stratigraphy and tectonics, the evolution of the LSB is still poorly de®ned. Recent studies have provided information on the structure and the tectonics of the basin (Mueller and Rockwell, 1991; Siem and Gastil, 1994; Axen and Fletcher, 1998; GarcõÂa-Abdeslem et al., 2001;

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Axen et al., 1998, 1999, 2000). Uplifted blocks on the eastern margin of the LSB contain good exposures of Plio-Pleistocene sedimentary deposits that provide unique insight into the basin. One of these is the Cerro Colorado basin, which constitutes an uplifted block on the footwall of CanÄon Rojo fault. This fault is a releasing bend on the Laguna Salada Fault system (Fig. 2). The Cerro Colorado outcrops have been the main topic of several stratigraphic studies (Siem, 1992; VaÂzquez-HernaÂndez, 1996; VaÂzquez-HernaÂndez et al., 1996; Dorsey and MartõÂn-Barajas, 1999). Additionally, lithologic logs of three geothermal exploratory wells from ComisioÂn Federal de Electricidad (Alvarez-Rosalez and GonzaÂlez-LoÂpez, 1995 (Fig. 2) provide new stratigraphic data on key locations within the basin and offer new constraints to our model on the geometry and structure of the basin. Based on existing and new information, we can now de®ne the stratigraphic sequence in Laguna Salada as formed by terrigenous deposits, essentially devoid of carbonate and volcanic rocks. Most stratigraphic sections include alternating ®ne-grained to very coarse-grained deposits that represent the interplay of various sedimentary environments, ranging from outer shelf, prodelta to intertidal delta-plain to ¯uviodeltaic, alluvial and lacustrine deposits, which alternate with avalanche and rock-slide deposits. The aim of this paper is to provide a comprehensive depositional history of the LSB based on our own stratigraphic and paleontological data from the Cerro Colorado basin and from three exploratory wells together with published and unpublished structural and geophysical studies. We integrate the stratigraphic and sedimentary data available to propose a conceptual model for the evolution of sedimentary facies in Laguna Salada. We use this conceptual model to evaluate the structural controls on the development of the western boundary of the Gulf Extensional Province at this latitude. Finally, we provide new insights on the role that supradetachment basins play in transtensional domains where marine, ¯uvio-deltaic, and alluvial depositional systems interact to form complex tectono-stratigraphic sequences. 2. Methods Detailed mapping and stratigraphic analysis of the

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main sedimentary units in Cerro Colorado basin constitute the basis for this work. Lithologic descriptions and micropaleontological determinations on stratigraphic sections provided important data to constrain the evolution of the eastern margin of LSB and its sedimentary facies. Fifty samples were processed for benthic and planktonic foraminifera and ostracoda. A number of slides were made for the study of calcareous nannoplankton and playnomorphs, following the usual techniques (Bolli et al., 1985; Wood et al., 1996). Lithological descriptions and palynological analyses of 16 samples selected from well ELS-1, provided us with information on the relative proportions of marine and terrestrial species in the main lithological units in this well. A marine palynological index (MPI) was de®ned by the ratio of the number of marine species and the sum of the terrestial, marine, and reworked taxa. However, detailed correlation between well logs and exposed sections on the eastern LSB margin is not yet possible because of poor resolution in the exploratory wells, due to mud drill circulation and lack of detailed petrological studies. Nevertheless, the lithological and paleontological data in ELS-1 allow tentative correlation of the main stratigraphic units and units studied in more detail on outcrops along the eastern margin of LSB. The exploratory wells provide important constraints to our models on the geometry and structure of the basin. 3. Tectonic and structural setting Both ancient and active rift-basins within the Gulf of California Extensional Province (GEP) are commonly bounded by faults having both strike-slip and dip-slip displacements in response to transtensional strain associated with oblique rifting of Baja California (Angelier et al., 1981; Stock and Hodges, 1989; Umhoefer and Dorsey, 1997). Extensional faulting dominated during proto-Gulf time (12±5 Ma), and this early extension probably resulted from strain partitioning between faults across the area now occupied by the Gulf of California and the shortlived Tosco±Abreojos transform fault along the Paci®c side of Baja California (Spencer and Normark, 1979; Lonsdale, 1989; Stock and Hodges, 1989; Lee

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et al., 1996). Recent studies now indicate that PlioPleistocene extension was largely accommodated on low-angle detachment-fault systems in northeastern Baja and Southern California, synchronous with wrench tectonics along the southernmost San Andreas fault-zone (see Axen and Fletcher (1998); for a summary and discussion). This indicates that detachment faults likely played a major role on basin evolution along the rift margins. The LSB currently is an active asymmetric graben with major subsidence along the eastern margin (Fig. 2) (Savage et al., 1994; Dorsey and MartõÂn-Barajas, 1999; GarcõÂa-Abdeslem et al., 2001). The western range front along northern Sierra JuaÂrez lacks a major bounding fault (Romero-Espejel, 1997) and this range was interpreted by Axen (1995) as a faulted rollover structure antithetic to underlying west-directed detachments. Detailed geologic mapping across the escarpment, in the southern Sierra JuaÂrez indicates two distinctive extension directions (Mendoza-Borunda et al., 1995). An earlier ENE±WSW direction may have controlled the development of the Gulf escarpment but the larger faults have kinematic indicators resulting from a younger extension event having as ESE±WNW direction. Both fault populations cut 10.5 Ma old lava ¯ows (Mendoza-Borunda et al., 1995) and this data constitutes one of the earliest age constraints on basin subsidence. Basinal subsidence likely initiated in late Miocene time (,15±11 Ma) along a detachment fault system named the Sierra El Mayor Detachment Fault (Axen and Fletcher, 1998; Axen et al., 2000), which was ®rst recognized at CanÄada David by Siem (1992) and Siem and Gastil (1994). Since latest Pliocene time, subsidence in the LSB was maintained by the dextral ±oblique Laguna Salada Fault (Axen et al., 1998; Axen and Fletcher, 1998; Dorsey and MartõÂn-Barajas, 1999), but fault scarps on Holocene alluvium along the western foothills of Sierra El Mayor suggest that the detachment may still be active beneath Laguna Salada (Axen

et al., 1999), and still controls the depocenter in the southern half of the basin (Fig. 2). An early estimate of the depth to basement, based on gravity data (Kelm, 1971), predicted 5±6 km of sediments in the central part of the basin, which presents a strong asymmetric gradient across the eastern margin. In a more recent gravimetric and magnetometric study, GarcõÂa-Abdeslem et al. (2001) proposed a 2D model of the crustal structure, which is consistent with a half-graben model. The geophysical data suggest a basement depth of 2.5±3 km in the eastern LSB, whereas the sedimentary ®ll along the western margin de®nes a thinner wedge and the basement progressively merges into the main Gulf escarpment (GarcõÂa-Abdeslem et al., 2001; MartõÂnAtienza, 2001). This geometry is consistent with a smooth gravimetric and magnetic gradient along the western margin and indicates lack of a major bounding fault. The geological and geophysical studies indicate that the top of the western Sierra El Mayor detachment, and the dextral-oblique Laguna Salada Fault are the master faults in this basin since late Miocene and Quaternary time, respectively. 4. Stratigraphy The sedimentary units described here follow the nomenclature proposed for Plio-Pleistocene stratigraphic units in the southeastern Imperial Basin (Bell-Countryman, 1984; Dibblee, 1984). Winker and Kidwell (1996) proposed to raise the long-standing Split Mountain, Imperial and Palm Spring units from Formation Rank to Group Rank, and classify various distinctive facies throughout the Salton Trough into many Formation-Rank units. However, outcrops in the Laguna Salada region are much more restricted and constitute dismembered structural blocks with limited aereal distribution that make the interpretation of lateral and vertical transition among depositional environments dif®cult. We thus consider convenient to maintain the formal names of Imperial

Fig. 2. Simpli®ed geologic map of Laguna Salada and outcrops of Plio-Pleistocene marine and non-marine sections discussed in this paper. The geologic background is from Gastil et al. (1975) and Axen and Fletcher (1998). CD, CanÄada David detachment fault; SMD, Sierra El Mayor detachment fault.

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and Palm Spring Formations to designate marine and ¯uvial deposits in Laguna Salada, respectively. Sedimentary rocks of the LSB crop-out at various locations within what is termed the Laguna Salada Basin. The use of different basin names is a casual reference to de®ne local sequences that once shared depocenters and a large common sediment source, but that evolved differently as the rift margin became structurally disrupted during Plio-Pleistocene time. These local sequences crop-out on uplifted and faulted blocks along the foothills between Sierra Cucapa and Sierra El Mayor (Fig. 2). This area was named the Cerro Colorado basin by Axen et al. (1998) and Dorsey and MartõÂn-Barajas (1999). The Cerro Colorado basin was likely part of the Laguna Salada Basin in Late Miocene±Late Pliocene as well as the Yuha basin to the north, along the west side of the Laguna Salada Fault (Isaac, 1987). We thus infer that these units form a continuous belt at depth along the eastern LSB. In this paper we refer to the Cerro Colorado basin, which include the CanÄoÂn Rojo section (Fig. 2), to describe the stratigraphy of late Neogene and Quaternary deposits on the eastcentral margin of the Laguna Salada Basin (Dorsey and MartõÂn-Barajas, 1999; Siem, 1992; VaÂzquez-HernaÂndez, 1996). Small outcrops also occur on the western foothills of Sierra El Mayor to the south (Fig. 2), and tilted PlioPleistocene fanglomerate and alluvial deposits crop-out in the LoÂpez-Mateos Basin on the eastern Sierra El Mayor (Axen et al., 1998). These deposits are probably related to the Cerro Colorado basin through the detachment fault system (Axen et al., 1998). However, our study focuses on the Cerro Colorado and CanÄoÂn Rojo sections and on three exploratory wells of ComisioÂn Federal de Electricidad (CFE) in Laguna Salada Basin (see Fig. 2). These sections provide critical information

on the sedimentary facies and thickness of the Mio-Pliocene to Recent stratigraphic units, and allow reconstruction of the main sedimentary and tectonic events in the Laguna Salada Basin and its link to the SW Salton Trough. 5. Stratigraphy of the Cerro Colorado basin The footwall block of the CanÄoÂn Rojo Fault (Fig. 3a) contains a stratigraphic sequence representing deposition in the Cerro Colorado and CanÄoÂn Rojo areas (Siem, 1992; VaÂzquez-HernaÂndez, 1996; Dorsey and MartõÂn-Barajas, 1999). The sequence includes four main stratigraphic units from base to top including (1) the Imperial Formation, (2) the Palm Spring Formation, (3) the CanÄon Rojo redbeds, and (4) the Grey Gravel unit. The three ®rst units are strongly disrupted by an array of north±south striking, low- and high-angle normal faults (Fig. 3a and b), which produced variations in thickness and local unconformities (VaÂzquez-HernaÂndez, 1996; Dorsey and MartõÂn-Barajas, 1999). Both syn-depositional and post-depositional deformation is likely related to activity in the CanÄada David segment of the detachment fault and the Laguna Salada Fault systems (Dorsey and MartõÂn-Barajas, 1999). 6. Imperial Formation 6.1. Stratigraphy and lithology In the Cerro Colorado basin, undisrupted blocks of the Imperial Formation indicate a minimum thickness of ,200 m (Fig. 4). We distinguish three informal members based on grain size, sedimentary structures, and fossil content. A coarsegrained member (Tim1) occurs only in restricted outcrops, and is in fault contact with metamorphic

Fig. 3. (a) Simpli®ed geologic map of the Cerro Colorado basin (adapted from VaÂzquez-HernaÂndez, 1996). This structural block constitutes the upper plate of the CanÄada David detachment segment (inactive) now being exhumed on the footwall block of the CanÄon Rojo Fault. This fault is a release step of the Laguna Salada Fault system and records rapid uplift during Holocene time. No. 1±4 are representative stratigraphic sections shown in Fig. 4. (b) Structural cross-section of the Cerro Colorado basin: Tim2 and Tim3 are members of the Imperial Formation; both contain several low amplitude folds with axes trending N±S to NNW that also affect Tps (not shown on Fig. 3a). Contact between Imperial and Palm spring deposits is generally faulted. An angular unconformity is locally observed between Palm spring (Tps) and the CanÄon Rojo redbeds (PQc), elsewhere this contact is transitional (no vertical exaggeration).

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Fig. 3. (continued)

rocks of the Monte Blanco Dome (Fig. 3a). Member Tim1 locally is derived from high-grade metamorphic rocks and granitic intrusions in the northern Sierra El Mayor (Siem and Gastil, 1994; VaÂzquez-HernaÂndez, 1996). The thickest section of Tim1 (approximately 70 m) crops out in the northeastern ¯ank of Monte Blanco dome (Figs. 3a and 4), where a coarsening-upward sequence grades from medium to thick bedded, matrix-supported conglomerate having normal and reverse grading, to clast-supported conglomeratic breccia. In this site, bedding strikes NNE and dips 25±458E, and the conglomerate is reported to contain reworked oyster, coral and marble clasts with borings (Siem, 1992). We did not ®nd microfossils in the sandy matrix. Although these deposits characterize the lower member it passes laterally into ®ne-grained equivalents. For example, at CanÄada David (Fig. 3a), Member Tim1 consists of approximately 10 m of matrix-supported breccia that grades laterally and up-section into a poorly bedded gravely sandstone (Fig. 5a) and into thinly bedded sandstone and mudstone couplets (Fig. 4, log 2 and 2 0 ). Lateral variations to mudstone and claystone facies are unclear because of faulting. Other isolated outcrops of Member Tim1 occur on the northwestern side of Sierra El Mayor, and include matrix- to clast-supported conglomerate and coarse-grained sandstone. These deposits have the same stratigraphic position and have similar

sedimentological features as outcrops on the northern and eastern ¯anks of Monte Blanco dome; both have internal massive bedding and/or normal grading, and sharp bases on individual beds, (Fig. 3a). The mudstone member of the Imperial (Tim2) is composed chie¯y of a yellow-greenish claystone± mudstone sequence having penetrative anastomosing cleavage and abundant ®brous gypsum in fractures (Figs. 4 and 5b). The claystone is massive and shows contorted cleavage that makes dif®cult to measure bedding attitudes, however, thin silty laminae locally de®ne dips between 15 and 508, consistently to the north. The upper part of Tim2 includes silty to sandy layers and beds and thin lenses of coquina dominated by oysters and pectinids. Although contact with the overlying sandstone±mudstone facies (Tim3) is faulted in most places, a transition from clay and silt to more uniform sandy and changes in the fossil content beds were clearly documented (see Section 6.2). The Imperial upper member (Tim3) crops out in the western and northwestern side of the Cerro Colorado basin (Fig. 3a). The thickest undisrupted sequence is up to 140 m thick (Fig. 4) and it consists of yellow-beige colored, ®ne-to mediumsized sub-arkosic sandstone and mudstone with prominent conquina beds and lenses of pectinids and oysters (Fig. 5c). There, the upper member is thinly bedded and contains planar±parallel

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Fig. 4. Stratigraphic logs with continuous sections and sample locations from representative areas on the Cerro Colorado basin (see Fig. 3a for location of measured sections).

bedding, cross-bedding, and ripple-cross strati®cation. A 10-m thick, massive coherent block of alabaster gypsum crops out in western Sierra El Mayor. This deposit overlies, in fault contact, the basement rocks and its stratigraphic position relative to other members described here is uncertain. However, it probably represents marine sedimentation prior to introduction of terrigenous sediments from the Colorado River. If so, these deposits would be the lowermost marine sedimentary unit in Laguna Salada.

6.2. Fossils and age Relative ages of late Cenozoic deposits in LSB based on fossils are still poorly de®ned, principally due to mixing of reworked and in situ species and the poor preservation of in situ species. However, we infer some age relationships based on lithology and the fossil association. The lower member Tim1 contains rare reworked fragments of coral, oyster shells, and clasts with evidences of boring, but no in situ macrofossils and microfossils are found and thus no age can be assigned to this unit.

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All microfossil samples from the Tim2 and Tim3 members of the Imperial Formation contain large amounts of reworked Cretaceous-lower Tertiary species, which almost certainly indicate a Colorado River source. In particular, planktonic foraminifera, calcareous nannoplankton and terrestrial and marine palynomorphs are very characteristic of a source on the Colorado plateau. Foraminifers are fragmented, and/or include small specimens similar to those reported by Cotton and Von der Haar (1979) from Quaternary deposits sampled on wells in the Cerro Prieto geothermal ®eld to the east (Fig. 2). More than 90% of the calcareous nannoplankton assemblages recorded contain reworked forms. We did not attempt to identify nannoplankton at the speci®c level, but some genera were easily distinguished, allowing their recognition as Cretaceous forms. Cotton and Von der Haar (1979) also reported many of the reworked calcareous nannoplankton recorded herein in Quaternary deposits in the area to the east (Fig. 2). Palynomorphs in the Imperial Formation of LSB are sparse and mostly reworked. We found mainly Cretaceous forms, such as Proteacidites, Tricolpites reticulates and many others of the normapollen type. In addition, the presence of dino¯agellate taxa with peridinioid af®nities indicates Cretaceous-lower Tertiary reworking. The same pollen and spores assemblages are also present in cuttings samples from well ELS-1, which we interpreted as representing the Palm Spring Formation in the eastern Laguna Salada Basin and having their source in the Colorado River. The in situ microfossil assemblage in the mudstone±claystone member (Tim2) includes the planktonic foraminiferas Globigerinita uvula and Globigerina bulloides plexus. The benthic foraminif-

eral assemblage is dominated by Bolivina spp. (Fig. 4, Table 1). The calcareous nannoplankton shows abundant signs of dissolution. The distal and proximal shields and the isolated central parts of nannoplankton specimens are missing making their taxonomic identi®cation dif®cult. Nevertheless, forms of Pontosphaera and Calcidiscus are identi®ed unequivocally, and a few Rhabdosphaera are questionably identi®ed. In the upper Tim3 member, the assemblage is characterized by abundant juvenile and adult stages of exclusively benthic foraminifera dominated by Cribroelphidium spp., as well as by the sparse occurrence of some isolated valves of ostracodes (Table 1). Index planktonic foraminifera are missing in the Imperial Formation from Cerro Colorado basin, probably due to diagenesis, so it is not possible to make a con®dent age assignment based on the observed planktonic foraminifera, calcareous nannoplankton or palynomorph assemblages. In spite of the time-transgressive nature of the benthic foraminifera, and according to the stratigraphic ranges reported by Finger (1990, 1992), many autochthonous foraminifers listed herein have their ®rst appearance datum in the Late Miocene. Although some planktonic foraminifers may be as old as Late Miocene, we consider they do not represent the true age of these deposits, since the ®ne-grained deposits of the Imperial Formation are primarily derived from the Colorado River. The age of the introduction of Colorado River sediments into the SW Salton Trough is believed to be close to the Mio-Pliocene boundary according to Winker and Kidwell (1996). This lithologic change in the sedimentsource has been documented at the base of the Wind Cave Member of the Latrania Formation.

Fig. 5. (a) Outcrop photo of breccia facies (member Tim1) of the Imperial Formation. This facies includes poorly organized, matrix-supported breccia, which grades laterally and upward into bedded sandstone. Arrow points to hammer for scale. (b) Claystone±mudstone facies of Imperial Formation (Tim2) near CanÄada David. Note the pervasive ¯aky anastomosing cleavage and fractures de®ned by gypsum veins. (c) View towards the WNW of the Cerro Colorado basin and the upper member of the Imperial Formation (Tim3). Extensive coquina beds and lenses form resistant beds within sandstone±mudstone strata. The sequence is tilted 15±208 north-northwest. (d) Transition zone between Palm spring formation (Tps) (thick sandstone beds at the base), and fanglomerate deposits from CanÄon Rojo redbeds (PQc). The cliff is ,30 m high. (e) Close-up of the mudstone±sandstone facies (Tps1) of the Palm spring formation. Layering is formed by alternate centimeter-thick beds of sandstone and mudstone laminations (darker layers). (f) Sandy conglomerate facies on the CanÄon Rojo redbeds; this facies represents sheet deposits of a prograding alluvial fan and grades northward into conglomerate and breccia as it approaches the range front of Sierra Cucapa. The outcrop is ,8 m high.

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Fig. 5. (continued)

A. MartõÂn-Barajas et al. / Sedimentary Geology 144 (2001) 5±35 Table 1 Checklist of the microfossils recorded at the middle (Tim2), and upper (Tim3) members of the Imperial Formation ( pReworked taxa, mainly Cretaceous). Repository: Micropaleontological Collection of the Instituto de GeologõÂa, Cd. Universitaria, Mexico, DF Foraminifera (Tim2) (Tim2) (Tim2) (Tim2) (Tim2) (Tim2) (Tim3) (Tim2) (Tim3) (Tim3) (Tim3) (Tim2) (Tim2) (Tim2) (Tim2)(Tim3) (Tim2) (Tim2) (Tim2) (Tim2)

Bolivina argentea Cushman B. brevior Cushman forma vaughani B. interjuncta bicostata (Cushman) B. spissa Cushman B. tumida Cushman Cancris baggi Cushman and Kleinpell p Chiloguembelina? (fragments) Cibicides ¯etcheri Galloway and Wissler Cribroelphidium discoidale (d'Orbigny) C. gunteri (Cole) Elphiidinae Globigerina bulloides (d'Orbigny) Globigerinita uvula (Ehrenberg) Globulina cf. gibba d'Orbigny p Hedbergella? (fragments) Islandiella californica (Cushman and Hughes) Loxostomum cf. bradyi (Asano) Psudononion basispinatus (Cushman and Moyer) Textularia schencki Cushman and Valentine

Calcareous nannoplankton p Ahmuellerella octaradiata (Gorka, 1957) (Tim2)(Tim3) p (Tim2)(Tim3) Ahmuellerella spp. (Tim2)(Tim3) Calcidiscus spp. (Tim2)(Tim3) Dyctioccocites spp. p (Tim2)(Tim3) Eiffellithus eximius (Stover, 1966) p (Tim2)(Tim3) E. turriseiffelli (De¯andre in De¯andre and Fert, 1954) (Tim2)(Tim3) Ellipsolithus spp. p (Tim2)(Tim3) Gartenago obliquum (Stradner, 1963) p (Tim2)(Tim3) Kamptnerius spp. p Lithastrinus sp. (Tim2)(Tim3) p (Tim2)(Tim3) Manivitella spp. (Tim2)(Tim3) Rhabdosphaera? sp. p (Tim2)(Tim3) Tranolithus manifestus? Stover, 1966 p Syracosphaera spp. (Tim2)(Tim3) Ostracoda (Tim3) (Tim3) (Tim3) Palinomorphs (Tim2)(Tim3) (Tim2)(Tim3) (Tim2)(Tim3) (Tim2)(Tim3)

Costa? sanfelipensis Swain Cyprideis spp. (fragments) Perissocytheridea meyerabichi (Hartmann) p

De¯andrea complex Normapollen type p Proteacidites spp. p Tricolpites reticulatus p

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The underlying Fish Creek Gypsum and most of the marine Latrania Formation are latest Miocene (Dean, 1988; Johnson et al., 1983; McDougall, 1998). We thus favour an Early Pliocene age for the Imperial Formation in Laguna Salada. However, this stratigraphic section is unrooted, and we do not rule out an upper Miocene age for the Imperial transgression, as has been documented at several localities in the northern Gulf margins and the northern Salton Trough (e.g. Quinn and Cronin, 1984; Ingle, 1974; Dean, 1988; Boehm, 1984; McDougall, 1998). 6.3. Depositional facies and history Our interpretation of late Cenozoic sedimentary facies and paleonvironments in the Laguna Salada Basin is based on lithological, sedimentological, and paleontological data. The Imperial Formation in the Cerro Colorado basin provided all three data sets. The lower member Tim1 includes sub-facies interpreted as proximal to distal gravity ¯ows in sub-aqueous settings. Matrix supported conglomerate and breccia are internally massive, with a sharp base, tabular bedding, and no clastimbrication. Sparse bioclast fragments, including coral, and sea-urchine spines, and bored clasts, indicate reworking of shallow marine deposits. However, microfossils in the sandy matrix are absent. West of Monte Blanco Dome (Fig. 3a) an outcrop of massive, thickly bedded, coarsegrained sandstone and conglomeratic sandstone tectonically overlies the metamorphic basement. This sandstone body is considered to represent sandy turbidite deposits due to its sharp base, the lack of internal traction-related structures, and normal grading. Both the conglomerate-breccia and sandstone deposits are locally derived from basement highs within the basin and are likely related to activity in the detachment. This member is in fault contact with the metamorphic basement. Thus, even though clasts in the Imperial lower member (Tim1) are derived from basement rocks it rest on, the deposits were apparently decoupled from this basement during detachment-related basinal subsidence.

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In the Cerro Colorado basin, a shift to a shallower water environment from the claystone± mudstone facies (Tim2) and the mudstone±sandstone facies (Tim3) in the Imperial Formation is based on lithology and distinctive biofacies. The mudstone member (Tim2) de®nes an upper-bathyal to outer-shelf biofacies, as indicated by the association of the planktonic foraminifera Globigerinita uvula, and Globigerina bulloides plexus and by benthic species dominated by Bolivina spp. (Table 1). Some nannoplankton taxa identi®ed in the claystone member (Pontosphaera, Calcidiscus and Rhabdosphaera?) also suggest paleodepths of up to 100±150 m, according to the upper depth boundaries proposed by Finger (1990, 1992). This microfossil assemblage corresponds to the shelf/slope transition, which is consistent with a basin ¯oor receiving delta-borne ®ne-grained sediments. However, the lack of sandy layers is interpreted as particle settling from suspended load in a low energy environment. We interpret the upper member (Tim3) to represent delta facies, particularly prodelta±delta plain environments based on sedimentary structures, and coarser deposits. This is also consistent with a lack of planktonic foraminifera and the almost exclusive presence of benthonic foraminifera, mainly Cribroelphidium discoidale and C. aff. gunteri. Furthermore, the recorded euhaline ostracode species also corresponds to shallow, moderate-to high-energy environments, such as estuarine to inner-shelf settings, with rocky to sandy substratum. This shallow-water microfossil assemblage in Tim3 is accompanied lithologically by a shift to medium-size sand and interbedded silt with coquina lenses and beds of reworked pectinid, oyster and barnacle shells, all of which likely indicate tidal ¯ats and tidal channel facies. However, no delta-front facies is de®ned on these outcrops, and in spite of the stratigraphic relationship we could not document a full transition from prodelta facies to delta-plain facies. Contact between the Imperial and Palm Spring formations may be transitional in some areas, because in few sites the Tim3 includes thin strata of reddish brown sandy-mudstone with a sharp base and top. The contact between Imperial and Palm Spring Formations is faulted in all outcrops.

7. The Palm Spring Formation and the CanÄoÂn Rojo redbeds 7.1. Stratigraphy and lithology The Palm Spring Formation unconformably overlies the distinctively yellow mudstone±sandstone beds of the Imperial Formation. The largest continuous section of the Palm Spring Formation is approximately 170 m thick (section 3 on Fig. 4), but this unit is cut by numerous low-and high-angle normal faults and it may be considerably thicker. The Palm Spring Formation in the Cerro Colorado basin includes thin-to thick-bedded, poorly consolidated sandstone and siltstone beds. The selectively cemented sandstone is thin-to thick-bedded, with parallel and cross-bedding. This unit typically occurs in thick intervals of ®ning-up beds, with sharp basal contacts. Sandstone±siltstone intervals alternate with intervals, up to 1 m thick, of thinly bedded, reddish to purple mudstone and claystone (Fig. 5d and e). Upsection, the sandstone sequence includes thicker, well-sorted sandstone beds, which alternate with conglomeratic-sandstone and sandy conglomerate redbeds that have sharp bases in the upper part of the interval (Fig. 5f). Alternating sandstone± mudstone and coarser gravelly sandstone occurs in a vertical stratigraphic interval 20±25 m thick (Fig. 5d). Up-section the Palm Spring Formation fully grades into thick fanglomerate deposits locally derived from the north and east. The fanglomerate is up to 350 m thick in the southern half of the Cerro Colorado basin and overlies and inter®ngers with the Palm Spring Formation (Dorsey and MartõÂn-Barajas, 1999). The fanglomerate unit includes coarse-grained sandstone at the base, and rapidly increases in grain-size up-section (Fig. 5f). This conglomerate, including some inter®ngering ¯uvial sandstone deposits at the base (e.g. Fig. 5d) was named the CanÄon Rojo redbeds sequence by Dorsey and MartõÂn-Barajas (1999). To the north it includes more than 1000 m of medial to proximal facies of debris-¯ow, and sheet-¯ow dominated alluvial fan deposits. 7.2. Fossil and age No age determinations are provided for the Palm

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Spring or the redbed sequence in the Cerro Colorado basin. Sandstone and mudstone beds are barren of microfossils probably due to oxic sedimentary conditions. Additionally, this unit is devoid of primary volcanic deposits and no absolute age has been obtained up to now. Depositional facies and history of the Palm Spring Formation. Depositional facies and interpretation of the Palm Spring Formation is chie¯y based on sedimentological and lithological data. Outcrops in the Cerro Colorado basin include two main sedimentary facies; Facies Tps1 is composed of thick-to mediumbeds of well sorted quartzose sandstone with massive to planar and tabular cross-strati®cation, sharp basal contacts and sparse, gravely sandstone lenses. Silicacemented concretions are distinctively associated with this facies. These well-stored sandstone beds are interpreted as sand-bar deposits on ¯uvial dominated channels on the delta plain. Facies Tps1 predominates towards the top of the stratigraphic sections (Fig. 5d). Facies Tps2 is composed of rhythmically bedded sandstone and mudstone in centimetric couplets (Fig. 5e) and it is more representative of the lower part of the Palm Spring Formation. Sandy beds are generally thin, have planar±parallel strati®cation, and grade into muddy layers, which have sharp or disrupted tops, and are commonly bioturbated by ,1 cm thick calcite cemented branching tubes. These tubes are interpreted as plant roots, and we conclude that facies Tps2 may represent overbank deposits of intermittent ¯uvial ¯ooding. Analysed samples from outcrops are barren of calcareous microfossils and palynomorphs, probably due to diagenesis under oxic conditions as indicated by the distinctive hematitic matrix in both the Palm Spring Formation and in fanglomerate deposits from the CanÄoÂn Rojo redbeds (Dorsey and MartõÂn-Barajas, 1999). The progressive increase in the grain size in the fanglomerate sequence indicates progradation of the alluvial fan system (see Dorsey and MartõÂn-Barajas, 1999), which likely records a rapid uplift of Sierra Cucapa, and likely marks the onset of activity on the Laguna Salada Fault. This sequence accumulated in the hanging wall of a now-inactive strand of the CanÄada David segment of the Sierra El Mayor Detachment Fault.

19

8. The Grey Gravel unit A sedimentary unit that Dorsey and MartõÂn-Barajas (1999) informally refer to as the `Grey Gravel unit' overlies the CanÄoÂn Rojo fanglomerate with angular unconformity. This unit consists of clast-supported conglomerate having clasts consisting mainly of tonalite from the Sierra Cucapa. The Grey Gravel unit is a few meters thick in the southern end of the Cerro Colorado basin but it thickens to approximately 600 m toward the faulted range-front north of the Cerro Colorado (Fig. 3a). Further description of this unit is presented by Dorsey and MartõÂn-Barajas (1999). 8.1. Age and history of the Grey Gravel unit Like the redbed sequence, no age constraints on this unit are yet available, but we tentatively propose a late Pleistocene age because it overlies the whole sequence and constitute the younger sedimentary unit being eroded on the uplifted block on the Cerro Colorado basin. The Grey Gravel unit is interpreted as high-gradient alluvial-fan deposits shed from the footwall of Laguna Salada Fault in the Sierra Cucapa (Dorsey and MartõÂn-Barajas, 1999). The unconformity between the Grey Gravel unit and the underlying redbeds is likely related to the onset of the CanÄoÂn Rojo Fault, which records important activity in Holocene time (Mueller and Rockwell, 1991, 1995) and produced incipient footwall erosion in the grey unit (Dorsey and MartõÂn-Barajas, 1999). 9. Stratigraphy of geothermal exploratory wells Three exploratory wells ELS-1, ELS-2 and ELS-3 of ComisioÂn Federal de Electricidad indicate the presence of thicker sedimentary sections in the eastern margin of the LSB (Fig. 6) and well ELS-1 records up to 2400 m of post-Imperial deposits (Fig. 6). We divide the stratigraphic column in well ELS-1 into three main sedimentary units based on lithology and microfossil content. These units are labelled 2±4 in this section because we assign unit 1 to the Imperial Formation, probably at depth in this well, to preserve the stratigraphic order documented in the exposed Cerro Colorado section. The lower unit in well ELS-1 (unit 2) is composed

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of more than 600 m of reddish, ®ne-grained sandstone±mudstone beds. This sandstone sequence contains quartzose sand and a reddish, calcareous, muddy matrix. No microfossils were found in the lower 200 m, but the upper two thirds contain reworked marine and terrestrial palynomorphs ranging in age from upper Cretaceous to Pliocene (Table 2, Fig. 7). The ratio of non-marine versus marine palynological species increases upsection in both the lower and upper half of unit 2, however, this unit is lithologically similar throughout in spite of differences in the palynological assemblages. Unit 3 in well ELS-1 consists of very coarse conglomerates and megablock breccias interlayered with siltstone and mudstone beds (Fig. 6); the interval between the lowermost and uppermost breccias in this unit is approximately 850 m thick. The conglomerate and breccia beds are composed of tonalite and granodiorite blocks and boulders derived from Sierra Cucapa. Mudstone and claystone deposits between the breccia layers vary in colour from reddish to greenish; bivalve; fragments are common between 1100 and 1200-m depth in the well (Fig. 6). The upper unit in well ELS-1 (unit 4) consists of about 980 m of alternating gravel-conglomerate, arkosic sandstone, and mudstone (Fig. 6). Mudstone in the lower half contains marine (e.g. Selenopemphix nephroides) and/or brackish (e.g. Michrystridium fragile) microfossils (Fig. 7, Appendix A). The uppermost 400 m of unit 4 includes mudstone and sandstone deposits with subordinate pebble conglomerate (Fig. 6). These deposits de®ne two stratigraphic sequences G and F (Fig. 7) separated by a mudstone and claystone-rich layer at ,215 m. Well ELS-2 penetrated granitic basement at approximately 1590 m (Fig. 6). The lowest stratigraphic unit consists of approximately 100 m of breccia conglomerate and subordinate sandstone and mudstone having sparse unidenti®ed shell fragments. Mudstone samples in well cuttings are grey to greenish, and the conglomerate is formed chie¯y by rounded and angular, boulder to cobble granitic clasts. Up section, the well penetrates approximately 340 m of arkosic sandstone and mudstone, having variable amounts of gravel (as much as 10%). The mudstone is grey to greenish with sporadic horizons of reddish

mudstone, and contains calcareous bioclasts and pyrite. A conglomeratic-sandstone interval approximately 50 m thick and located at approximately 1100-m depth is a distinctive interval in the well, because sandstone±mudstone deposits dominate the majority of the section. In well ELS-3 the stratigraphy consist of an 830-mthick sequence of monotonous, coarse-to ®ne-grained arkosic sandstone and subordinate mudstone, claystone, and gravel deposits overlying a crystalline basement of mica schist. No distinctive vertical lithological change in the sediments occurs in this well, and no microfossil determinations have been completed at this time. 9.1. Microfossils and age The palynological assemblages recovered from cuttings samples from well ELS-1 allow the recognition of paleoenviromental changes in this section studied. However, their low diversity (Table 2) and fair to poor preservation of the palynomorph assemblages preclude a reliable age assignment. In general, the palynomorph assemblages (Fig. 7) contain a combination of Late Cretaceous (Subtilisphaera pirnaensis, Isabelidinium glabrum, Dinogymnium euclaense and Senegalinium obscurum), Paleogene (Spiniferites cornutus, Selenopemphix nephroides and Selenopemphix armata) and Neogene to recent dino¯agellate species (Lingulodinium polyedrum, Operculodinium centrocarpum, Polysphaeridium zoharyi and Spiniferites mirabilis). The presence of reworked Cretaceous palynomorphs indicates a strong in¯ux of material from the Colorado River Basin. This reworking has been previously reported in the area, and has been associated to erosion of the Upper Cretaceous Mancos Shale (Merrian and Bandy, 1965) since this and equivalent units exposed on the Colorado plateau contain Cretaceous palynomorphs (Franczyk et al., 1990; Cushman and Nichols, 1992; Fleming, 1993, 1994). Additionally, Cretaceous to lower Tertiary planktonic microfossils have been reported in samples from Quaternary deposits in the Cerro Prieto geothermal area (Cotton and Von der Haar, 1979). The interval from 1130 to 1426 m (Fig. 7), is intriguing since the combination of some dino¯agellate (Palaeocystodinium golzowense, in sample

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21

Fig. 6. Lithostratigraphic logs from three exploratory wells drilled by ComisioÂn Federal de Electricidad (CFE) on Laguna Salada (cf. AlvarezRosalez and GonzaÂlez-LoÂpez, 1995). See Fig. 2 for location of well. Depths in meters.

at 1130, Cribroperidinium tenuitabulatum in sample at 1301) and pollen species (Pachydermites diederixi, uncertain identi®cation in sample at 1426 m) strongly suggests reworking from Miocene sediments.

9.2. Depositional facies and history of stratigraphic section on geothermal exploratory wells Our palynological determinations on selected samples from well ELS-1 indicate that unit 2 in this

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Table 2 Important palynomorph taxa included in Fig. 7. The reported stratigraphic ranges for each taxa are indicated in parentheses. Complete list of taxa are in Appendix A Dino¯agellates Cribroperidinium tenuitabulatum Lejeunecysta sp. Lingulodinium machaerophorum Operculodinium centrocarpum Palaeocystodinium golzowense Polysphaeridium zoharyi Selenopemphix armata Selenopemphix nephroides Spiniferites mirabilis Pollen Pachydermites diederixi

(Eocene±early Miocene) (late Eocene ±Recent) (early Eocene±Recent) (Oligocene ±Recent) (early Eocene±late Miocene) (middle Eocene±Recent) (middle±Eocene) (middle Eocene±Recent) (Eocene±Recent) (late Miocene±Recent)

well correlates with the Palm Spring Formation, which chie¯y include sandy ¯uvial deposits, but may also comprise brackish water settings towards the top. The ratio of marine to terrestrial palynomorphs (MPI) increases up section on unit 2 regardless of the lack of a distinctive lithological change on the well cuttings (Fig. 7). Unit 2 has a distinctive quartzose composition and hematite matrix, and fragments of reddish mudstone are distinctive of the Palm Spring Formation and the intervening CanÄon Rojo redbeds from the Cerro Colorado basin. Unit 2 in well ELS-1 lacks conglomerate deposits, and de®nes a sedimentary cycle of ¯uvial deposits with predominance of terrestrial palynomorphs, and a shift to brackish water conditions up-section. Additionally, the hematite-rich matrix, and sub-arkosic composition of sand allow tentative correlation with the Palm Spring Formation and the lower part of the CanÄon Rojo redbeds. The lower megabreccia deposit recorded in unit 3 indicates that a prominent topographic relief existed on Sierra Cucapa, and imply rockavalanche transport mechanisms for metric-boulder size granitic blocks in this deposit. Marine and (or) brackish deposits intercalated with the breccia-conglomerate deposits suggest high subsidence rates on the eastern margin. These intervals of mudstone±claystone show that rapid subsidence maintained the basin ¯oor at or below sea level, as indicated by the high MPI on several ®ne-

grained samples from unit 3. The ®ne-grained lithologies contain mixed marine and nonmarine palynomorphs with a broad age-range from late Cretaceous to Pleistocene (Fig. 7). Between 950 and 1840 m, the mudstone±claystone beds contain the higher proportion of marine microfossils, and indicate in¯uence of marine incursions from the south. This unit includes three stratigraphic sequences (B, C and D) that de®ne a distinctive sedimentary cycle wherein pulses of debris-¯ow deposits from alluvial fans or slide deposits have punctuated a sequence of lacustrine and marine deposits. A coarse-grained sandstone and gravel interval about 200 m thick overlies the last conglomerate-breccia deposit and a decrease in grain size and MPI follows up-section (Fig. 7). Although marine-palynomorph diversity is very low, a local increase in the MPI (sample at 444 m) suggests intermittent brackish paleoenvironments, which likely alternate with lacustrine, eolian and distal facies of alluvial-fan systems. A denser sampling on the upper lithologic unit would probably indicates more variation of the MPI, related to variations of sea level or seismic recurrence on the Laguna Salada Fault, which would yield rapid subsidence of the basin ¯oor. We interpret unit 4 to represent a separate cycle, which includes three stratigraphic sequences (E, F and G) based on MPI and lithology. Paleoenvironments may correspond to medial to distal alluvial-fan deposits shed from Sierra Cucapa with interbedded lagoon (brackish?) deposits related to river ¯ooding and or marine incursions from the south. Debris ¯ow deposits in unit 4 are not present probably because this transport mechanism has been less intense with time along the eastern margin of Laguna Salada. The upper 400 m of unit 4 include two subunits separated by a mudstone and claystone-rich layer; these two stratigraphic sequences, are interpreted as ®ne grained alluvial fan deposits probably separated by a lacustrine deposit. However, this interpretation is speculative because no microfossil determinations are available from this interval. Nevertheless, this interplay between lacustrine and alluvial-fan deposits is to be expected, as it occurs in present time, and represents an imbalance between tectonic subsidence

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Fig. 7. Stratigraphic sequences interpreted in well ELS-1 based on lithology and the marine palynological index (MPI). Paleoenv., paleoenvironments; T, terrestrial; P, paralic; M, marine. Taxa underlined and in bold suggest Miocene age. Reworked Cretaceous and Tertiary microfossils are commonly mixed with Plio-Pleistocene specimens.

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and sediment accumulation. Lacustrine deposits may have rapidly followed pulses of vertical displacements on the Laguna Salada Fault system and promoted growth of high-gradient alluvial-fan systems from the surface of a steep topographic relief. In well ELS-2, two distinctive conglomerate intervals may tentatively correlate with megabreccia deposits recorded in well ELS-1 (Fig. 6), whereas the thick sandstone and mudstone deposit above the upper conglomerate beds may represent the interplay of lacustrine and alluvial fan systems as in unit 4 from ELS-1. However, no microfossil determinations are yet available for well ELS-2. In Well Els-3, the stratigraphic section is more homogeneous and grain-size variations in sediments probably are related to small-scale strati®cation of beds corresponding to distal deposits of alluvial fans interbedded with lacustrine and eolian deposits. Modern alluvial-fan systems emerging from Sierra JuaÂrez extend more than 10 km into the basin and form large pediment slopes of 1±28. This well is located at the distal portion of a modern alluvial-fan system, and the entire section penetrated may represent the interplay of the same sedimentary environments as today. Additional stratigraphic, sedimentological, and structural reconnaissance at two locations on the eastern margin of LSB have been published; the LoÂpez Mateos Basin (Axen et al., 1998) and the Yuha Desert section (Isaac, 1987) (Fig. 2). These locations provide important clues for the understanding of the evolution of the LSB and a summary of the stratigraphy, sedimentological observations, and tectonic implications are included in the following sections. 10. LoÂpez Mateos basin A conglomeratic sequence crops out in the east-central part of Sierra El Mayor, and unconformably overlies metamorphic basement rocks in a board paleovalley (Axen et al., 1998). The lower part of the sequence is lithologically similar to the Palm Spring Formation. It comprises poorly cemented sub-arkosic sandstone with sparse reworked carbonate bioclasts and pebbles. The

sandstone grades rapidly up-section into pebble to boulder conglomerates, poly- and monomictic conglomerates, and breccias with clasts up to several meters in diameter (Axen et al., 1998). Clast imbrications indicate a transport direction to the west, from a source presently buried in the Mexicali Valley (Axen et al., 1998). In the LoÂpez Mateos basin, lower Palm Spring strata and overlying conglomerates are tilted 40±608 to the east, and faulted by mostly westdown normal faults. Up-section the bedding dips decrease to 208 east. The fanning dip up-section indicates that conglomerates of the LoÂpez Mateos basin were deposited syn-tectonically near the breakaway of the detachment, which is thought to be located to the east (Axen et al., 1998). Strands of the detachment-fault system bound the southern end of this Tertiary outcrop. The basal sandstone deposits are interpreted as representing ¯uvial-dominated delta plain deposits because of their lithological similarities with the Palm Spring sandstone from Laguna Salada. The conglomerates and breccias are interpreted as debris-¯ow deposits and bedded conglomerates with clast-imbrication are interpreted as traction deposits by Axen et al. (1998). No age determinations for these deposits are yet available, but based on its stratigraphic succession and lithology this section tentatively correlates with the Palm Spring Formation and the CanÄoÂn Rojo redbeds from the Cerro Colorado basin. 11. The Yuhu Desert basin East of Cerro Centinela, at the northern end of Laguna Salada (Fig. 2), the Imperial and Palm Spring Formations unconformably overlie Mesozoic crystalline basement (Isaac, 1987). There, the Imperial Formation includes three distinctive sedimentary facies based on lithology and grain size: (1) a conglomeratic unit, (2) a ®ne-grained unit, and (3) a megabreccia unit (Isaac, 1987). The nonmarine conglomerate unit underlies and inter®ngers with the ®ne-grained facies of the Imperial Formation, which generally overlies the basement rocks in low-angle fault contact. Barnard (1968) estimates a maximum thickness of 400 m, and

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called this facies the older fanglomerate, which Isaac (1987) later re-assigned to the Imperial Formation because it inter®ngers with the marine ®ne-grained facies. Clast-composition is chie¯y tonalite, with subordinate gneiss and marble having boulder size up to 1.5 m. Most of this unit is very coarse conglomerate, with thin sandstone beds and lenses. The conglomerate is poorly bedded to massive and includes angular to well rounded pebbles and cobbles in a muddy to sandy matrix (Isaac, 1987). The megabreccia facies in the Yuha desert is similar to the lower member of the Imperial Formation in the Cerro Colorado basin. Both have limited aerial distribution and do occur in fault contact over the granitic or metamorphic basement. Both units are considered marine deposits because the breccia clasts contain clam-boring ichnofossils and barnacles in growth position and laterally pass into marine muddy sandstone. The unit is less than 10 m thick and includes angular boulder clasts that are several meters in diameter. Barnard (1968) estimates a thickness of 120±400 m for the ®ne-grained Imperial facies in a section west of Cerro Centinela (Fig. 2). This marine facies of the Imperial Formation is chie¯y composed of thinly bedded to massive green to yellow mudstone, siltstone, and ®ne-grained sandstone. Bedding attitudes are dif®cult to obtain except for the presence of thin (up to 30 cm thick) beds and lenses of sandstone and continuous coquina beds, which are described in the Coyote Mountains as the Yuha reefs by Bell-Countryman (1984). These bioclastic deposits in the Yuha desert include mainly shallow marine calcareous fossils such as oysters, gastropods and barnacles (Isaac, 1987). The mudstone±sandstone facies of the Imperial Formation in the Yuha area inter®ngers upsection with the Palm Spring Formation. The thickness of the Palm Spring Formation in the northern end of Laguna Salada is unknown, but it ranges from 0 to 2200 m in the southwestern Salton Trough (Dibblee, 1984). Approximately 4100 m of Palm Spring sediments where reported by Johnson et al. (1983) for the Vallecitos FishCreek area in a stratigraphic section ®rst sampled by Opdyke et al. (1977) for magnetostratigraphy. Thus, this unit records most of the basinal subsi-

25

dence of the southwestern Salton Trough during Plio-Pleistocene time. Sedimentation of the Palm Spring unit in the southwestern Imperial Basin and in the Yuha desert was interrupted by deformation and faulting on the Elsinore, Laguna Salada and San Jacinto faults. This tectonic event may have occurred less than 1 Ma before present (cf. Johnson et al., 1983), synchronous with progradation of the Colorado River delta and sediment accumulation on the delta-plain that produced separation of the Imperial Basin from the upper Gulf of California.

12. Discussion 12.1. Age constraints on basin subsidence Onset of extensional deformation that led to subsidence and syntectonic deposition in the Laguna Salada area is believed to have occurred between 16 and 10 Ma based on geochronology and structural data. Throughout the western margin of LSB normal faults cut arc-related basaltic lava ¯ows dated at approximately 16 Ma (two K/Ar whole-rock ages) in the northern range front of Sierra JuaÂrez (Lance Forsythe, unpublished data). These ages are consistent with ages on the Progreso Volcanics in northern Sierra Cucapa dated at 15:3 ^ 0:8 Ma (K/Ar) by Barnard (1968). However, lava ¯ows at both localities probably erupted from multiple dike feeders during a pulse of ma®c volcanism across the northern Laguna Salada area. Synrift volcanism (,12 Ma) appears to have been volumetrically small in this region because volcanic detritus in Pliocene sediments (e.g. Imperial, Palm Spring, and CanÄoÂn Rojo beds) accounts for ,5% (VaÂzquez-HernaÂndez, 1996). No primary volcanic deposits occur within the Cerro Colorado basin, or on well cuts studied here. In the southern end of Laguna Salada, trachyandesite ¯ows from Sierra Las Tinajas are approximately 10.5 Ma, and overlie three continuous welded to unwelded tuffs dated at approximately 12 Ma (Mendoza-Borunda et al., 1998). These lava ¯ows and tuffs form a continuous volcanic ®eld

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across the escarpment, which is disrupted by a series of east-and west-dipping normal and oblique faults (Mendoza-Borunda et al., 1995, 1998). The age of these volcanic rocks clearly provide an upper limit for onset of extension along the main Gulf Escarpment of Sierra JuaÂrez. A third line of evidence for extensional tectonics and basinal subsidence, come from exhumation ages on crystalline rocks. These new data constrain the early uplift associated with the detachment fault on Sierra El Mayor between approximately 15±10 Ma (Axen et al., 2000). Apatite ®ssion-track and (U±Th)/He ages of approximately 5 and 4 Ma, respectively, record ®nal cooling through approximately 110±708C. Until today up to 12±17 km of horizontal extension were necessary to unroof the Sierra El Mayor to present time (Axen et al., 2000). Ages constrained by microfossils vaguely suggest early Pliocene or Late Miocene deposition for the Imperial Formation in the Laguna Salada area. However, an indirect line of evidence for a Lower Pliocene age is indicated by the sub-arkosic composition of sediments (VaÂzquez-HernaÂndez, 1996) and the predominance of reworked Mesozoic microfossils in deposits introduced by the Colorado River. In the southwestern Salton Trough this lithological change in the Imperial Formation from locally derived sediments (local suite) to Colorado River sediment occurred near the Mio-Pliocene boundary (Winker and Kidwell, 1996). This lithological change can be extrapolated to the Laguna Salada area where members Tim2 and Tim3 are composed of sediments introduced by the Colorado River and thus may correspond to lower Pliocene deposits. Marine sedimentation in the Cerro Colorado basin likely occurred prior to introduction of sediments from the Colorado River as indicated by the alabaster gypsum on the west side of Sierra El Mayor. However, its relative stratigraphic position and age remain unknown. 12.2. Structural and sedimentological controls on facies evolution Two, very different structural sedimentologic models for the LSB can be developed. Some authors

interpret the Laguna Salada Basin as a pull-a-part basin with rombochasm geometry similar to the Cerro Prieto and Salton Sea spreading centers (Mueller and Rockwell, 1991, 1995). Fenby and Gastil (1991) speculated that marginal basins in the northern GEP initiated as extensional basins rooted by detachment faults having gravity-driven transport direction toward the Gulf. These authors proposed that as extension proceeded, rombochasm deformation like that observed in the modern Gulf, transform faults and pull-a-part basins initiated on precursor basins developed on the extended crust. Recent structural studies (Siem and Gastil, 1994; Axen and Fletcher, 1998; Axen et al., 1999, 2000) and re-interpretation of basin geometry (Axen, 1995; GarcõÂa-Abdeslem et al., 2000) strongly suggest that Laguna Salada constitutes an asymmetric graben initiated and driven by detachment faulting with a transport direction to the west (Fig. 8a and b). Thickness of the sedimentary sections and lithological variations among exploratory wells ELS-1, ELS-2, and ELS-3 indicates that the top of the basement lies deeper to the east and that major subsidence occurred along this margin. This interpretation agrees with models of basement con®guration based on gravimetric and magnetic studies (GarcõÂa Abdeslem, 2001; MartõÂn-Atienza, 2001). During Quaternary and Holocene times, this basin experienced rapid subsidence along the dextral± oblique Laguna Salada Fault. However, stratal geometry in the Cerro Colorado basin and structural, and tectonic constraints indicate that Laguna Salada initiated a supradetachment basin (Dorsey and MartõÂn-Barajas, 1999). In the Laguna Salada region, the stratigraphic record recording early basin subsidence has not been found; the breccia- conglomerate (Tim1) and the claystone facies (Tim2) and the alabaster gypsum unit are the stratigraphically oldest rift-related deposits yet identi®ed. These beds are truncated by underlying detachment faults and are not in place. The amount of tectonic transport along the detachment fault is still poorly constrained. However, Axen et al. (2000) estimated a minimum of 17 km of extension associated with the detachment, based on their

A. MartõÂn-Barajas et al. / Sedimentary Geology 144 (2001) 5±35

calculations of vertical tectonism and erosional unroo®ng of Sierra El Mayor. By late Miocene time the LSB was likely located ,250 km to the southeast of its present position relative to ®xed North America (cf. Winker and Kidwell, 1986; Oskin et al., 2001). Initial conditions for subsidence and sedimentation in this basin remain unknown, but we speculate that initial tectonic subsidence was probably related to detachment faults during protogulf extension (cf. Axen and Fletcher, 1998; Axen et al., 2000) (Fig. 8a and b). The microfossil assemblage in the claystone facies (Tim2) indicates outer-shelf water depths on a marine basin receiving terrigenous sediments primarily derived from the Colorado River. These sediments were likely reworked from the delta by tidal current and/or turbidite currents and deposited on the basin ¯oor principally during sea level low-stand. This implies that sediments derived from the Colorado River in Imperial times reached distances similar to the distance between the modern delta complex and the Wagner basin (see Fig. 1). The change to shallower waters up-section in the Imperial Formation from the Cerro Colorado basin also signals rapid progradation of the Colorado River delta to the south (Fig. 8b). Tectonic subsidence was rapidly compensated by fast sediment accumulation, which produced shallower water depths and a delta-plain environment represented by member Tim3, the upper member of the Imperial Formation. Deposition of ¯uvial-sandstone facies of the Palm Spring Formation probably took place at or close to sea level, as indicated by the association of marine and terrestrial palynomorphs in the lowermost unit (unit 2) of well ELS-1. However, no Palm Spring deposits occurs in wells ELS-2 and ELS-3, and we speculate that Laguna Salada contains the western limit of the ¯uvial delta plain during deposition of the Palm Spring Formation in this basin. A critical event in the geological history of the Imperial and Laguna Salada Basin occurred in early Pleistocene time. The increase of tectonic activity on the Elsinore and Laguna Salada faults (cf. Johnson et al., 1983), and the rapid subsidence of the Salton Sea, relative to the Imperial Basin, shifted major depocenters toward the east and south (e.g. Fuis and Kohler, 1984; Axen and Fletcher, 1998). Rifted-margin blocks in the southwestern Imperial Basin became structu-

27

rally disrupted and thick sections of Plio-Pleistocene sections became exhumed as sedimentation on the delta plain retreated east and southward (Winker and Kidwell, 1986; Johnson et al., 1983). During Quaternary time, the Laguna Salada Basin evolved differently from the western Imperial Basin in that LSB continued to subside. This subsidence was driven by the Laguna Salada fault and the Sierra El Mayor detachment faults, and accounts for a minimum of 1.8 km of vertical displacement on the Laguna Salada Fault based on the thickness of units 2 and 3 in well ELS-1 (Fig. 7). This estimate is an approximation that still needs to be corrected for compaction, which should produce larger total subsidence. Dorsey and MartõÂn-Barajas (1999) provided an independent estimate of approximately 2-km at 2±4 mm/year of tectonic subsidence along the Laguna Salada fault based on thickness of the CanÄon Rojo redbeds and the Grey Gravel unit. Since Pleistocene time, the Laguna Salada became progressively isolated from the delta complex due to uplift of Sierra Cucapa and Sierra El Mayor. Additionally, tectonic transport of this basin relative to the delta apex by right lateral strike-slip displacements on faults of the San Andreas system produced isolation from the delta complex. The sedimentological implication is that Sierra Cucapa and Sierra El Mayor formed a natural barrier in the delta front and rerouted ¯uvial sediment transport southward (Fig. 8c). Alluvial-fan systems merging from both margins and sediment input from episodic ¯uvial and marine incursions from the south constituted the main sediment supply after the LSB was isolated from the delta complex in late Pleistocene time. Alternating mudstone, conglomerate, and breccia in well ELS-1 indicate major episodes of debris-¯ow and possibly landslide deposits, which reached distances in excess of 5 km to the west from the range front of Sierra Cucapa. Granitic blocks several meters in diameter occur in thick breccia-conglomerate deposits in Unit 3 de®ned in that well (Fig. 7). Only rock slides and rock-avalanche mechanisms can explain the clast sizes found in well ELS-1 and ELS-2, several kilometers away from the range front and intercalated with brackish and marine mudstone±claystone deposits. A similar megablock breccia occurs in the CanÄon Rojo fanglomerate but within less than 500 m from

28

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the Laguna Salada fault, and grain size rapidly decreases away from the fault zone (Dorsey and MartõÂn-Barajas, 1999). Thus, the conglomerate and breccia in unit 3 of well ELS-1 may represent large debris ¯ows or avalanches that reached the central part of Laguna Salada as probably recorded in well ELS-2 as well. Rapid subsidence below sea level related to a vertical slip component on the Laguna Salada fault resulted in the migration of the depocenter close to the range front (Fig. 8d). Interbedded mudstone± sandstone deposits, conglomerate, and breccia indicate pulses of rapid subsidence and retrogradation of alluvial-fan facies during deposition of lacustrine sediments. Additionally, sea-level changes may have played an important role in marine ¯ooding and sedimentation, as probably recorded in unit 3 of ELS-1. However, detailed records of sea-level changes and their effects on sedimentation in Laguna Salada are unknown because biostratigraphic resolution is inadequate in samples from exploratory wells. 13. Conclusions The Laguna Salada Basin rocords the late Miocene(?)±early Pliocene marine incursion into the northern Gulf, and the progradation of the Colorado River delta. Stratigraphic units of Late Miocene and Pliocene age in both Laguna Salada and Imperial Basins are similar and indicate that these two basins are linked genetically by detachment faulting, early marine incursions in the northern Gulf, and deltaic sedimentation from the Colorado River. Based on benthic and planktonic foraminifera claystones and mudstones in member Tim2 of

the Imperial Formation in Laguna Salada are interpreted as marine outer-shelf deposits that accumulated in water depths up to 150 m. This formation is unrooted by the Sierra El Mayor detachment fault and no data from older syn-rift units are available. The upper contact with shallow-water Imperial facies (Tim3) is faulted or represented by a local angular unconformity, suggesting syn-sedimentary faulting on the detachment fault since early Pliocene time. Deltaic progradation and northwest-directed tectonic transport of the basin, relative to the delta apex, may have produced very rapid sediment accumulation and shallower facies up section. However, delta front facies and the transition from marine to non-marine settings are still poorly de®ned. The western limit of the deltaic ¯oodplain was likely located along the eastern side of Laguna Salada because of the presence of thick (.600 m) ¯uvial deposits from unit 2 in well ELS-1 and the lack of this unit in wells ELS-2 and ELS-3. Uplift of Sierra Cucapa and Sierra El Mayor, and southward progradation of the delta complex isolated the Laguna Salada Basin from the Imperial Basin and from the delta complex during late Pliocene±early Pleistocene time. Since then, the location of the main depocenter in Laguna Salada is controlled by the Laguna SalaÂda Fault as indicated by the thick (approximately 2 km) post-Palm Spring sections cored in the exploratory well ELS-1. Increase in the tectonic activity of the Laguna Salada and related faults to the north dramatically changed the depositional settings. Rapid vertical offset on faults along the eastern margin produced lacustrine conditions with sporadic marine/brackishwater sedimentation punctuated by rock avalanche or slide block deposits from Sierra Cucapa. Although

Fig. 8. Proposed model for evolution of the Laguna Salada area (view looking North): (a) Early subsidence in Late Miocene time probably produced by extension accommodated on low-angle faults. This subsidence permitted the ®rst marine incursion to the Salton Trough area in the form of Imperial Formation or its precursors (alabaster gypsum?). The hypothetical breakaway fault of this detachment would be located east of Sierra El Mayor. A transfer fault represents the northern limit of the detachment, and the Laguna Salada segment of the rift, however, the structure of this accommodation zone is more complex that represented herein (cf. Axen and Fletcher, 1998). (b) Progradation of the Colorado River delta and deposition of the Imperial Formation and the Palm Spring Formation. (c) Onset of the Laguna Salada Fault and rapid uplift of Sierra Cucapa. The CanÄon Rojo redbeds accumulate and large debris ¯ow and avalanche deposits reach the central part of the basin. Detachment-related subsidence occurs west of the Sierra El Mayor, which emerges during foot-wall uplift. (d) Onset of the CanÄon Rojo fault, and exhumation of the Cerro Colorado basin. The CanÄada David detachment became inactive. Schematic blocks not to scale. Position of exploratory wells ELS-1, ELS-2 and ELS-3 is approximated.

A. MartõÂn-Barajas et al. / Sedimentary Geology 144 (2001) 5±35

29

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large alluvial fan systems developed along the range front of Sierra Juarez on the western margin, sediment accumulation is probably three times larger along the eastern margin, where very rapid Quaternary and Holocene subsidence is produced by activity on the Laguna Salada and CanÄoÂn Rojo faults. Acknowledgements Jonathan Matti, Genaro MartõÂnez, and Jochen Halfar are gratefully acknowledged for their careful and constructive review of this manuscript. Special thanks to HeÂctor GutieÂrrez Puente and the Residencia de Estudios de Cerro Prieto, ComisioÂn Federal de Electricidad, for giving permission to use the well data from Laguna Salada. Dr Enrique MartõÂnez (Instituto de GeologõÂa, UNAM) kindly helped processing samples for palynology. We bene®t from thoughtful discussions with John Fletcher, Gary Axen, RamoÂn Mendoza, Juan GarcõÂa-Abdeslem, and Becky Dorsey that helped to integrate different information on the tectonics and sedimentation of the Laguna Salada Basin. Lance Forsythe provided information about unpublished K/Ar ages from volcanic rocks from northern Sierra JuaÂrez. This contribution was partially supported by CONACYT project 1224-T9203 to G. Axen and A. MartõÂn. Appendix A Registry of palynomorphs observed in each sample. Number in parenthesis indicates the amount of specimens of each taxon. (?) indicates questionable identi®cation because of poor preservation Sample: 199 m Barren Sample: 444 m Terrestrial Palynomorphs Betula sp. Chenopodiaceae Compositae Corylus sp. Deltoidospora sp. `Fungi' Laevigatosporites sp. Pinus sp. `Psilamultiporate' Psilatricolporites sp.

(1) (1) (1) (1) (1) (2) (1) (1) (2) (1)

Retimonocolpites sp. Retitricolpites sp. Taxodium sp. `Tricolpate granular' `Trilete granular' Marine Palynomorphs Leiosphaeridia sp Foraminiferal mold Cretaceous reworking Ephedripites sp. Gleicheniidites sp. Proteacidites sp. Rugutriletes sp. Isabelidinium glabrum Sample: 67 m Terrestrial Palynomorphs Chenopodiaceae Compositae Corylus sp. Deltoidospora sp. Laevigatosporites sp. Pinus sp. Psilatricolpites sp. Retimonocolpites sp. Retitricolpites sp. Taxodium sp. Marine Palynomorphs Leiosphaeridia sp. Cretaceous reworking Cyathidites mesozoicus Ephedripites sp. Gleicheniidites sp. Proteacidites sp. Dinogymnium euclaense Senegalinium obscurum Sample: 763 m Terrestrial Palynomorphs Chenopodiaceae Compositae Corylus sp. Deltoidospora sp. `Fungi' Pediastrum sp. Pinus sp. Psilatricolpites sp. Retitricolpites sp. Taxodium sp. Marine Palynomorphs Leiosphaeridia sp. Cretaceous reworking Cyathidites mesozoicus Ephedripites sp. Gleicheniidites senonicus Subtilisphaera pirnaensis

(1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1)

(1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1)

(1) (2) (1) (1) (2) (1) (1) (1) (1) (2) (1) (2) (1) (3) (?)

A. MartõÂn-Barajas et al. / Sedimentary Geology 144 (2001) 5±35 Sample: 862 m Terrestrial Palynomorphs Chenopodiaceae Compositae Corylus sp. Deltoidospora sp. `Fungi' Laevigatosporites sp. Monocolpites sp. Pinus sp. `Psilamultiporate' Psilatricolporites sp. Retitricolpites sp. Taxodium sp. `Triporate vestibulate' Marine Palynomorphs Leiosphaeridia sp. Foraminiferal mold Cretaceous reworking Cyathidites mesozoicus Dinogymnium cretaceum Ephedripites sp. Gleicheniidites senonicus Isabelidinium glabrum Subtilisphaera sp. Sample: 945 m Terrestrial Palynomorphs Chenopodiaceae Compositae Corylus sp. Deltoidospora sp. `Fungi' Laevigatosporites sp. Larix sp. Pediastrum sp. Pinus sp. Psilatricolpites sp. Retitricolpites sp. Taxodium sp. Tricolpites sp. Marine Palynomorphs Apteodinium sp. Leiosphaeridia sp. Operculodinium centrocarpum Cretaceous reworking Cyathidites mesozoicus Dinogymnlum cretaceum Gleicheniidites senonicus Isabelidinium glabrum Palaeohystrichophora infusorioides Proteacidites sp. Rugutriletes sp. Subtilisphaera sp. Sample: 1130 m Terrestrial Palynomorphs

(9) (4) (4) (3) (10) (4) (3) (18) (1) (1) (7) (9) (1) (6) (2) (2) (?) (2) (1) (1) (3)

(16) (3) (5) (3) (8) (4) (3) (3) (19) (8) (7) (20) (1) (?) (7) (1) (2) (?) (4) (5) (1) (2) (2) (3)

Chenopodiaceae Corylus sp. Cymatiosphaera sp. Deltoidospora sp. `Fungi' Juglans sp. Laevigatosporites sp. Larix sp. `Monocolpate echinate' `Monocolpate reticulate' Pinus sp. Psilatricolpites sp. Retitricolpites sp. Taxodium sp. `Triporate triangular' Verrucatosporites sp. Marine Palynomorphs Apteodinium sp. Lingulodinium machaerophorum Operculodinium centrocarpum Polysphaeridium zoharyi Cretaceous reworking Circulodinium distinctum Cyathidites mesozoicus Dinogymnium heterocostatum Ephedripites sp. Gleicheniidites senonicus Isabelidinium glabrum Oligosphaeridium complex Palaeocystodinium golzowense Rugutriletes sp. Subtilisphaera sp. Xenascus ceratioides Sample: 1169 m Terrestrial Palynomorphs Chenopodiaceae Cicatricosisporites sp. Corylus sp. `Fungi' Graminea Juglans sp. Laevigatosporites sp. Larix sp. `Monocolpate echinate' Pinus sp. Psilastephanoporites sp. Psilatricolpites sp. Psilatriletes sp. Psilatriporites sp. Retimonocolpites sp. Retitricolpites sp. Rugutricolpites sp. `Scabramonocolpate' Scabratriporites sp. Taxodium sp. `Tetrade'

31 (33) (14) (1) (14) (39) (4) (14) (11) (1) (1) (45) (25) (21) (45) (1) (1) (2) (2) (2) (5) (1) (4) (?) (6) (3) (8) (1) (?) (2) (4) (1)

(47) (2) (19) (49) (4) (1) (13) (19) (8) (55) (13) (38) (4) (1) (1) (23) (1) (?) (2) (42) (1)

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`Trilete granular' Marine Palynomorphs Lingulodinium machaerophorum Foraminiferal mold Operculodinium centrocarpum Spiniferites sp. Cretaceous reworking Cyathidites mesozicus Dinogymnium cretaceum Ephedripites sp. Gleicheniidites senonicus Hystrichosphaeropsis ovum Isabelidinium glabrum Kallosphaeridium helbyi Proteacidites sp. Senegalinium obscurum Spiniferites pseudofurcatus Subtilisphaera pirnaensis Sample: 1175 m Terrestrial Palynomorphs Chenopodiaceae Compositae Deltoidospora sp. `Echinate spore' `Fungi' Laevigatosporites sp. `Monocolpate reticulate' Monoporites sp. Pinus sp. Proxapertites sp. Psilatricolpites sp. Psilatriporites sp. Tricolpites sp. Tricolporites sp. Tricolporopollenites sp. `Triporate granular' Triporites sp. Verrucatosporites sp. Marine Palynomorphs Lingulodinium machaerophorum Foraminiferal mold Spiniferites sp. Cretaceous reworking Coronifera oceanica Cyathidites sp. Ephedripites sp. Gleicheniidites sp. Isabelidinium acuminatum Isabelidinium sp. Palaeohystrichophora infusorioides Palaeoperidinium cretaceum Tertiary reworking Selenopemphix armata(?, marine Eocene) Selenopemphix nephroides(4, marine, Eocene to Miocene) Sample: 1289 m Terrestrial Palynomorphs

(2) (16) (1) (?) (4) (11) (1) (4) (2) (?) (4) (1) (1) (?) (?) (?)

(8) (3) (11) (2) (26) (5) (1) (2) (24) (1) (1) (1) (4) (1) (3) (1) (2) (2) (4) (2) (2) (2) (17) (2) (3) (2) (1) (4) (2)

Chenopodiaceae Cicatricosisporites sp. Corylus sp. `Fungi' `Inaperturado echinate' Laevigatosporites sp. Larix sp. Pinus sp. Psilatricolpites sp. Retitricolpites sp. Taxodium sp. Verrutricolporites sp. Marine Palynomorphs Leiosphaeridia sp. Foraminiferal mold Operculodinium centrocarpum Spiniferites mirabilis Spiniferites sp. Cretaceous reworking Cyathidites mesozoicus Ephedripites sp. Gleicheniidites senonicus Isabelidinium glabrum Sample: 1301 m Terrestrial Palynomorphs Chenopodiaceae Cicatricosisporites sp. Compositae Corylus sp. Deltoidospora sp. `Verrucate spore' `Fungi' Laevigatosporites sp. `Monocolpate echinate' `Monocolpate reticulated' Monocolpites sp. Pinus sp. `Retitetracolpate' Retitricolpites sp. Taxodium sp. `Tetrad' `Tricolpate gemmate' `Tricolpate rugulate' `Triporate vestibulate' Marine Palynomorphs Leiosphaeridia sp. Lingulodinium machaerophorum Polyspaeridium sp. Polysphaeridium zoharyi Cretaceous reworking Appendicisporites distocarinatus Cyathidites sp. Exochosphaeridium sp. Gleicheniidites senonicus Isabelidinium glabrum Odontochitina operculata

(16) (1) (19) (22) (1) (3) (1) (42) (13) (5) (17) (1) (4) (2) (?) (?) (1) (8) (1) (2) (1)

(8) (4) (1) (12) (1) (2) (26) (10) (3) (1) (1) (34) (1) (15) (13) (1) (1) (1) (1) (2) (4) (?) (4) (?) (13) (?) (3) (?) (?)

A. MartõÂn-Barajas et al. / Sedimentary Geology 144 (2001) 5±35 Proteacidites sp. (1) Senegalinium bicavatum (?) Spiniferites cornutus (2) Teritary reworking Cribroperidinium tenuitabulatum (1, marine, Eocene±Miocene) Sample: 1426 m Terrestrial Palynomorphs Alnus sp. Chenopodiaceae Corylus sp. `Fungi' Laevigatosporites sp. Larix sp. `Monocolpate echinate' Pinus sp. Psilatricolporites sp. Retitricolpites sp. Taxodium sp. `Triporate' Marine Palynomorphs Michrystridium fragile Foraminiferal mold Operculodinium centrocarpum Operculodinium sp. Spiniferites mirabilis Spiniferites multibrevis Spiniferites ramosus Cretaceous reworking Appendicisporites sp. Isabelidinium glabrum Palaeoperidinium cretaceum Proteacidites sp. Tertiary reworking Pachydermites diederixi(?, terrestrial Miocene) Sample: 1840 m Terrestrial Palynomorphs Chenopodiaceae Cicatricosisporites sp.. D(1) Cicatricosisporites sp.. G(2) Corylus sp. Deltoidospora sp. `Reticulate spore' `Fungi' Laevigatosporites sp. Lycopodinium sp. Mauritiidites sp. Monocolpites sp. Pinus sp. Polysphaeridium zoharyi Proxapertites sp. Taxodium sp. Tricolpites sp. Tricolporites sp. Marine Palynomorphs Lejeunecysta sp. Foraminiferal mold

(?) (5) (4) (6) (2) (1) (2) (8) (2) (1) (2) (1) (?) (2) (1) (?) (2) (?) (?) (1) (1) (?) (1)

(3) (5) (5) (1) (10) (1) (?) (?) (2) (20) (1) (?) (3) (2) (3) (?) (1)

Operculodinium centrocarpum Operculodinium sp. Spiniferites sp. Cretaceous reworking Ephedra sp. Gleicheniidites sp. Isabelidinium acuminatum Phelodinium magni®cum Proteacidites sp. Tertiary reworking Selenopemphix nephroides(3, marine, Eocene to Miocene) Sample: 2033 m Terrestrial Palynomorphs Alnus sp. Chenopodiaceae Corylus sp. `Fungi' Laevigatosporites sp. Larix sp. Momipites sp. Pinus sp. Psilatricolporites sp. Salix sp. Stereisporites sp. Taxodium sp. Tricolpites sp. Marine Palynomorphs Michrystridium fragile Nematosphaeropsis sp. Spiniferites sp. Cretaceous reworking Gleicheniidites sp. Sample: 2205 m Barren

33 (3) (?) (3) (?) (2) (1) (1) (?)

(1) (1) (29) (4) (3) (4) (1) (9) (1) (5) (1) (6) (8) (?) (?) (1) (1)

Sample: 2370 m Barren

References Alvarez-Rosalez, J., GonzaÂlez-LoÂpez, M., 1995. Resultados de los pozos exploratorios en Laguna Salada, BC. III International Meeting on the Geology of the Baja California Peninsula, La Paz, BCS, MeÂxico, pp. 4±5. Angelier, J., Colletta, B., Chorowicz, J., Ortlieb, L., Rangin, C., 1981. Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez, Mexico. J. Struct. Geol. 3, 347±357. Axen, G., 1995. Extensional segmentation of the main Gulf Escarpment, Mexico and the United States. Geology 23 (6), 515±518. Axen, G.J., Fletcher, J.M., 1998. Late Miocene±Pleistocene extensional faulting, northern Gulf of California, Mexico and Salton Trough, California. Int. Geol. Rev. 40, 217±244. Axen, G.J., Fletcher, J.M., MartõÂn-Barajas, A., 1998. Late

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