Quaternary International 219 (2010) 79–94
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Palaeoenvironmental significance of diatom and vertebrate fossils from Late Cenozoic tectonic basins in west-central Me´xico: A review ˜ o-Monroy a, J. Barron c, M.A. Rodriguez-Pascua d I. Israde-Alca´ntara a, *, W.E. Miller b, V.H. Gardun a
´ s de Hidalgo, ´ rgicas, Edificio U. Ciudad Universitaria, Universidad Michoacana de San Nicola Departamento de Geologı´a y Mineralogı´a, Instituto de Investigaciones Metalu 58060, Morelia, Michoacan, Mexico b Department of Geological Sciences, Brigham Young University, Provo, UT 84602, USA c USGS, MS 910, Menlo Park, CA 94025, USA d ´ rea de Peligrosidad y Riesgos Geolo ´gico y Minero de Espan ´n y Prospectiva, A ´ gicos, C/Rı´os Rosas 23, 28003 Madrid, Spain ˜a (IGME), Dpto. de Investigacio Instituto Geolo
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 28 January 2010
Pronounced lacustrine sedimentation developed in west-central Me´xico during the late Miocene, between approximately 11 and 7 Ma. This was in response to tectonic extension associated with the initial emplacement of the late Miocene substrata of the Trans-Mexican Volcanic Belt. Climatic conditions in west-central Me´xico during this interval were relatively warm and humid based on the widespread distribution of interpreted lacustrine beds. Following a latest Miocene (8.0–5.4 Ma) stage of arid conditions and greatly reduced deposition of fine-grained lacustrine sediments, extensive, east–west oriented, relatively deep, perennial lakes ensued. They mark the early Pliocene (5.3–4.0 Ma). Lower Pliocene diatomites contain the same diatom species (e.g., Stephanodiscus carconensis and Tertiarius aff. baikalensis) found in rocks of this age in the western United States. The relatively warm and humid conditions that characterized this interval in central Me´xico coincide with a period of high-latitude warming, higher global sea level, and a reduction in size of the Antarctic Ice sheets. Because the Central American Seaway persisted until at least the latest Miocene, it might have acted to increase precipitation in central Mexico. This could have continued into the earliest Pliocene. Mexican Pliocene mammalian faunas also support a savanna setting with moist and warm conditions prevailing at the time. Shallow lakes and fluvial conditions dominate after 4.0 Ma, until the end of Pleistocene. A combination of reduced precipitation, due to general global cooling and drying, as well as volcanic and tectonic processes, are presumed to have been the cause for this mid-Pliocene reduction in lake size and extent in central Me´xico. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Since Paleocene to middle Eocene times, central Mexican physiography has evolved as a result of the complex geodynamic evolution of the Pacific margin and fragmentation of North American plate (Aranda-Gomez et al., 2005). The main physiographic provinces in central Mexico are constituted by Basin and Range tectonic depressions oriented N–S and NNW–SSE (Aranda-Gomez et al., 2005) and the Sierra Madre Oriental. This last province, together with the Oligo-Miocene province of Sierra Madre Occidental and Altiplano Central, constrains the limits of the central * Corresponding author. Tel.: þ52 443 223500; fax: 52 443 3167414. E-mail addresses:
[email protected],
[email protected] (I. Israde-Alca´ntara),
[email protected] (W.E. Miller),
[email protected] (J. Barron), ma.rodriguez@ igme.es (M.A. Rodriguez-Pascua). 1040-6182/$ – see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.01.012
Trans-Mexican Volcanic belt (TMVB). This magmatic arc evolved in central Me´xico between Miocene and modern times and has shown significant changes in its composition and geometry through time. However, in general there has been a tendency for the development of NE–SW structures (Ferrari, 2003; Gomez-Tuena et al., 2005). The main activity of the TMVB evolved after 12 Ma in the central western region, building large volcanic edifices at first, subsequently, North American fragmentation generated grabens and semigrabens as products of a NW–SE transtensive regime during Miocene–Pliocene. Depressions present an overlapping of the Basin and Range NNW–SSE andN–S system observed in Guanajuato basins with the NE–SW systems of the TMVB of northern Michoacan. During the Pliocene–Holocene, magmatism became monogenetic in character, building the modern Trans-Mexican Volcanic Belt. The volcanism, together with the fault reactivation and pyroclastic emissions of the
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calderas Amealco, Azufres, Huichapan and La Primavera, contributed to the infilling of the NW–SE and NE–SW lacustrine basins with fluvial and alluvial deposits. The Lerma River is another determinant feature in the physiographic configuration of central Mexico. It follows the transtensional structures and together with the volcanic edifices and lava emitted, controlled the limits of the main lakes and has been important in maintaining the infilling of lakes during Neogene to Quaternary times. A review of the history of basin extension and volcanism in central Me´xico during this time reveals that at least six separate eruptions of rhyolitic ash occurred (Adams et al., 2006). The lacustrine basins developed during the late Miocene and early Pliocene under favorable hydrological conditions within rift basins of the Chapala–Citala Graben (19 500 to 20 300 latitude; Fig. 1). Humid conditions beginning at this time enabled the basins to begin filling with water. These basins now range in elevation from 1535 m (Chapala) to 2500 m (Chincua and Ixtlahuaca). Discontinuous periods of faulting and volcanism have produced differences in elevation between these lacustrine basins that arose at different times during the rifting trans-tensional process. Bounding extensional faults of the ancient deeper lakes, Chapala and Cuitzeo, are still active, and their axial grabens have moved southward and northward respectively, resulting in a progressive north or south translocation of their axial depocenters (Rosas et al., ˜ o-Monroy, 1999) (Fig. 2a, b). 1997; Israde-Alca´ntara and Gardun Along the fault scarps of the east–west depressions, outcrops of lacustrine sediments containing diatoms, fossil vertebrates and volcanic products occur, allowing reconstruction of the evolution of the tectonic lakes of central western Me´xico. The ancient diatom floras in the bedded strata of the Chapala– Cuitzeo depression and Chincua basin include distinctive species and genera that have been calibrated with radiometric ages (Fig. 1). As for diatoms and terrestrial vertebrates from other ancient lacustrine regions around the globe, the taxa in these Mexican
Tertiary lake deposits can be arranged geochronologically as useful biostratigraphic markers. In the case of diatoms, the central Mexican diatom biochronology agrees with the evolution and appearance of Neogene freshwater diatoms elsewhere in North America, and generally conforms to the phylogenetic inferences and chronological evolutionary framework established by molecular biology, particularly 285 ribosome RNA (Philip et al., 1994; Surhannus et al., 1995 (and nuclear and chloroplast DNA markers using parsimony and Bayesian methods (Alverson et al., 2007). The environment and physiographic configurations have also played important roles in the evolution of morphology in diatoms. The opportunity for crossing ecological barriers and dispersal abilities amongst diatoms allows gene flow between different isolated or interconnected water bodies and environments that lead to separate evolution and speciation patterns of endemic taxa. Cladistic and phylogenetic analysis can offer a better understanding of ancestor–descendant relationships in diatom genera (Theriot, 1992; Theriot et al., 2006; Alverson et al., 2007). In the Mexican region considered here, it is important to study continuous stratigraphic records. A problem is a constantly changing environment that often does not allow fossilization or faster directional selection. The existence of continuous diatomite sections will permit a better understanding of the transgressive changes in diatom evolution and the processes of extinctions/dispersal, and the role of environmental change in the evolution of diatoms. This will allow morphologic and stratigraphic continuity to be determined between lineages from the fossil record (Theriot et al., 2006). Further study based on the knowledge of their distribution must be done in Me´xico in the future. Diatoms together with several species of horses represent the most abundant taxa from late Miocene through Pliocene time in central Me´xico. Equids make good index fossils, and like diatoms reflect environmental conditions. The Mexican diatom biochronology complements knowledge about the continental distribution of some extinct Mio-Pliocene centric diatom taxa (Bradbury,
Fig. 1. Geodynamic sketch of the Neogene-Quaternary lacustrine basins of west-central Me´xico and location of the studied sites 1.Chapala; 2. Cuitzeo; 3. Chincua; 4. Acambay; 5. Ixtlahuaca.
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Fig. 2. Tectonic environment associated with the active Chapala graben and Cuitzeo half graben. Lava flows and down-dropped sides of faults form barriers and basins for accumulation of Neogene and Quaternary alluvial sediments and lakes.
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1984), including Mesodictyon (Theriot and Bradbury, 1987), Tertiariopsis (Khursevich et al., 2000, 2002), and Tertiarius (Hakånsson and Khursevich, 1997). Several thousands of terrestrial vertebrate fossils of Hemphilian and Blancan age (Bell et al., 2004), have been collected in central ˜ eda and Miller, Me´xico over the past few decades (Carranza-Castan ˜ eda, 1984, 1994, 1980, 1992, 1998, 2004; Miller and Carranza-Castan 1998a, 1998b, 2001a, 2001b). These ages constitute two of the North American Land Mammal Ages (or NALMA), as first defined in Wood et al. (1941). The Hemphillian (latest Miocene–earliest Pliocene) and Blancan ages (Pliocene, excluding the Hemphilian portion) are the two most prolific ones known for central Mexican vertebrates. Sediments from which they were obtained range from coarse sandstones through mudstones/claystones, with intercalated volcanic ashes containing fossils at many sites (Fig. 3). These volcanic ashes have provided a number of radiometric ages (Kowallis et al., 1998; Flynn et al., 2005). The fossil vertebrates have been collected mostly from basins within the grabens associated with the Trans-Mexican Volcanic Belt in the states of Hidalgo, Puebla, Me´xico, Quere´taro, San Luis Potosı´, Guanajuato, Michoaca´n ˜ eda and Miller, 1998; Miller and and Jalisco (Carranza-Castan ˜ eda, 2001b). Southernmost Baja California Sur, Carranza-Castan which has yielded many late Tertiary and Pleistocene vertebrate fossils of generally similar taxa, lies outside the TMVB.
Geochronologic as well as paleomagnetic studies of the volcaniclastic, fluviolacustrine and lacustrine sediments along the TMVB have permitted a more precise determination of fossil mammal ages to be made (Flynn et al., 2005; Adams et al., 2006). This also holds true for the diatoms recovered from Neogene basins. Both studies provide the basis for a preliminary fossil biochronology of west-central Me´xico (Serieyssol et al., 1998; Michaud et al., 2000). The collected fossil vertebrates and diatoms of biochronologic interest in this paper come mainly from the lacustrine basins of the states of Guanajuato, Jalisco, Michoaca´n and Hidalgo. The stratigraphy, chronologic ranges of distinctive fossil diatoms and vertebrates, as well as the tectonic activity of these lacustrine basins are well documented (Fig. 4a and b). These factors are all related to the paleontological and climate data for the time involved. The presently reported research has been done with consideration of Neogene climates, lacustrine, and marine sea levels, and the related paleoenvironments. 2. Geological setting Landscapes of the late Neogene volcanic belt in central western Me´xico are characterized by a series of distinct tectonic depressions. This topography is directly associated with the formation of the TMVB that was controlled by two independent oceanic plates,
Fig. 3. Stratigraphy of the main lacustrine basins of west-central Me´xico.
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Fig. 4. (a) Age range of stratigraphic sections compared with tectonic, climatic, and oceanic conditions in west central Mexico and the eastern North Pacific. (b) Distribution of diatom and vertebrate fossils and associated paleoenvironments.
the Pacific plate and the North American plate (Gomez-Tuena et al., 2005). These plates produced three tectonic depressions that created a triple point with a northwest branch (known as the Tepic Zacoalco graben), a southern branch (known as the Colima graben), and an east-west trending branch (known as the Chapala graben; Demant, 1981). The Chapala graben includes the Chapala Tula fault (Sutter et al., 2001, Fig. 1) and the Chapala–Ixtlahuaca system, which roughly follows the Lerma River (Szynkaruk et al., 2004). Present-day Lake Chapala, within the Chapala basin, occupies the westernmost limit of the east–west oriented Chapala–Tula fault. An east–west trending branch of the Chapala graben runs from the state of Jalisco to the state of Me´xico. It contains lacustrine deposits of diatomite (Fig. 1). All the grabens mentioned above have been filled with fluvial and fluviolacustrine sediments and inter˜ o-Monroy and Gutierrezmediate basic volcanic ejecta (Gardun Negrin, 1992). The related Neogene grabens trend east–west and are limited by sub-vertical fault scarps up to 300 m high. The region has been affected by extensional deformation along faults mostly trending approximately east–northeast to west–southwest and east–west. The east–west deformation is controlled by an old morphology of regional north, northwest, and south, southwest faults that steadily decline towards the west of the TMVB depressions (as well as causing reactivation into right lateral faults). These two major deformations have influenced the regional morphostructures (Szynkaruk et al., 2004) and the Lerma River configuration. Tectonic activity took place from approximately the middle Miocene until the late Pliocene. However, older northwest–southeast trending Miocene basins apparently have developed along more ancient north–south oriented Basin and Range structures. These include ones in the Ocotla´n, Cuitzeo Queretaro and probably the Hidalgo and Guanajuato zones. These earlier depressions represent the structural limits for the younger Miocene and Pliocene lacustrine basins (Fig. 1). Both east–west and older northwest–southeast structures conform to a regional strike-slip fault
system (trans-tensional deformation), which contain a left-lateral component (Pasquare´ et al., 1991). In the east–west branch of the Chapala graben, volcanic structures and lava flows altered the fault bounding Acambay basin as well as the Cuitzeo–Charo and Chapala basins. This changed the characteristic landscapes of the region. The Acambay basin, part of a 45 km east–west trending fault system, is located inside the Morelia–Acambay fault (Sutter et al., 2001). Seismites and liquefaction processes affected the normal sedimentation reworking, faulting and folding the sediments at different times within this system (Rodriguez-Pascua et al., 2004a, 2004b). In the Cuitzeo region’s eastern sector, the graben is almost symmetric (Fig. 2a). However, in the western sector the depression constitutes a half graben with a tilting of 30 to the south, being more pronounced towards the basin’s depocenter (Fig. 2a). Further from the depocenter in the Morelia Atapaneo region, the tilting measures 10 south. The Cuitzeo graben and half graben are separated by the northwest–southeast Tzitzio–Valle de Santiago fault. This fault in the Cuitzeo lake basin produces an extension of the lake towards the northwest (Moroleo´n region) in Pliocene– Holocene times under more humid conditions. The most evident structures in the Chapalqa–Acambay region are potentially seismic in origin. These east–west oriented normal and strike-slip faults produced vertical displacements up to 250 m that date from the early Pleistocene. Modern seismic activity along these faults, such as the Acambay–Tixmadeje Fault, is generating co-seismic displacements of centimeters. One example is the Acambay earthquake of 1912 (6.8–7.0 magnitude) which produced a seismic induced vertical scarp of 105 cm (Landgridge et al., 2000) (Fig. 3). The northwest–southeast trending basins of Miocene age belong to the Guanajuato–Allende province (Kowallis et al., 1998). Deposits of the easternmost Miocene–Pliocene lacustrine basins
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Fig. 4. (Continued).
crop out in the Chincua and Ixtlahuaca zones. Part of the westerly TMVB area studied belongs to the Hidalgo sector, which includes the Chapala–Tula rift. This rift extends into the state of Guanajuato. The Hidalgo and Guanajuato strata consist mainly of lacustrine, fluviolacustrine, volcaniclastic, and alluvial deposits interbedded with volcanic ash. In summary, it appears that the Miocene–Pliocene regional lakes are part of the nearly flat morphologies produced along the tectonic east–west to northeast–southwest trending transtensional depressions. Faults and numerous volcanic structures determined the limits of the depressions as is observed in the Chapala–Ixtlahuaca system (Szynkaruk et al., 2004). This system roughly follows the Lerma River. The trans-tensional depressions are strongly controlled by older north–northwest and south– southwest faults that caused a lessening of all depressions on the western portion of the TMVB
3. Stratigraphy Stratigraphic sections of the west–central lacustrine basins in Me´xico have been studied in reasonable detail over the past 25 years. A number of these sections, which include volcaniclastic deposits, have provided radiometric ages from various localities in ˜ o-Monroy et al., 1993; Ferrari et al., 1994; the Chapala basin (Gardun Rosas et al., 1997; Michaud et al., 2000; Miller and Carranza˜ eda, 2001a; Flynn et al., 2005). The Ixtlahuaca, Chincua Castan (Israde-Alca´ntara, 1999) and Cuitzeo–Charo basins of the TMVB ˜ o-Monroy, (Israde-Alca´ntara, 1995; Israde-Alca´ntara and Gardun 1999), which include those that comprise the east–west rift branch were studied in detail. They help to illustrate the evolution of the late Miocene–early Pliocene west-central basins (Figs. 1, 4a and 4b).
Several fossil vertebrate sites are exposed north and northwest of Atotonilco el Grande (Amajac site). They occur in a volcaniclastic depositional sequence that becomes more fluvial to alluvial grading upwards. This sequence is considered Pliocene–early Pleistocene in age (Arellano Gil et al., 2005). Fossil diatoms from dated outcrops and cores, with associated or interbedded volcanic material found in the present study, strongly support the stratigraphic ranges of lacustrine diatoms obtained from the literature (Fig. 5a, b and c). This includes western North America (Krebs et al., 1987; Krebs, 1990, 1994), western South America (Fourtanier and Gasse, 1988), Africa (Servant et al., 1986; Fourtanier and Gasse, 1988), Eurasia in the regions of Byelorussia (Khursevich et al., 2003), Bulgaria (Temniskova-Topalova et al., 1988, 1997), and France (Serieyssol and Gasse, 1991). In a general, the overall biochronologic studies of Neogene lacustrine diatoms suggest that nearly contemporaneous first appearances and extinctions of certain species of centric diatoms occur around the globe. The prospects of establishing a useful diatom biochronology for continental deposits remain excellent based on the stratigraphic ranges found the world over. In Me´xico, only a few Neogene fossil diatoms older than the Quaternary have been sufficiently documented (Serieyssol et al., 1998). Four taxa belonging to the Thalassiossiraceae (Fig. 5) are reported in this work. They have potential stratigraphic and biochronologic significance and appear in the eastern, central and western basins (Israde-Alca´ntara et al., 2004). 3.1. Eastern Miocene–Pliocene basins Extensive searching for and collection of vertebrate fossils has been done in the central Mexican states of Hidalgo and Guanajuato between 1976 and the present. Almost exclusively these fossils in the state of Hidalgo (Zietla) are of Hemphilian and Blancan age. At
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Fig. 5. a) Diatom marker for the Miocene (Mesodyction, Theriot and Bradbury, 1987), detail of the valve shown in SEM view; (b) Tertiariopsis Khursevich et al., 2002, bas. T. cuitzeonensis (Serieyssol et al., 1998) index fossil for the Upper Miocene in several world regions. T. cuitzeonensis was found in the upper Miocene deposits of Cuitzeo paleolake. View of the upper mantle and inner valve; (c) Tertiarius aff. baikalensis (Khursevich et al., 2000). Similar taxa appear in the Pliocene deposits of Chapala lake. View of the upper mantle and inner valve.
Hidalgo (Zietla) the strata containing fossils are observed in a clayey silt sequence that is intercalated with coal lenses (Carranza˜ eda and Miller, 1993, 2000). All these strata are capped by Castan basaltic lava (Castillo-Cero´n et al.,1996). The fossils found in the strata occur within the Hemphilian and Blancan NALMA. Numerous localities in the state of Guanajuato (Jime´nez-Hidalgo and Carranza˜ eda, 2005) have yielded abundant fossils that have been Castan found in lacustrine, fluviolacustrine, and volcaniclastic sediments, ranging from late Miocene(e.g., Coecillo and Rinconada sections), to early Pleistocene (e.g., Rancho El Ocote, Rancho Viejo, Arroyo Seco, and La Pantera sections). Volcanic ashes are present in some localities that yield vertebrate fossils and are the basis for several radiometric dates (Kowallis et al., 1998). Where no volcanic ash is present, it is
possible to infer an age based on the fossils present. Horses are especially useful for this. Calippus castilli, Nannippus minor, and Dinohippus cf. interpolatus, are among the earliest types known, and might date back to about 6.0 Ma. Fortunately, horses are the most abundant taxon in these deposits. 3.1.1. Ixtlahuaca basin The Ixtlahuaca basin, 2530 m above sea level, close to the city of Toluca, is incised by the Lerma River. This basin is surrounded by volcanic domes. On the west and to the east, fluviolacustrine outcrops reveal almost 70 m of coarse-grained sediments and sand that include interbedded ash and pumice (Fig. 3d). These deposits change transitionally to more lacustrine facies with diatomite
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deposits 20 m thick in the central part of the basin. The horizontally bedded strata are overlain by pumice and paleosols. Three main units comprise the main stratigraphy of the region. The basal member consists of 8 m of fluviolacustrine deposits which are composed of sands, dune sands, microconglomerates (of volcanic affinities that present primary structures like ripple marks), and massive intercalated pumice deposits. The pumice clasts tend to be rounded and range up to 3 cm in diameter. They occur in the main outcrops and are exposed along the course of the Lerma River. The succession changes transitionally to member 2, which consists of a 25 m sequence of diatomite which is more laminated at the base and top. The characteristic diatom taxa are planktonic and include Stephanodiscus medius and Stephanodiscus aff. minutulus. These species indicate a deep lacustrine level at the base. The lacustrine facies are interrupted in the center of the stratigraphic column by a 6 m fluviolacustrine deposit that presents a reworked diatomite within slump structures. Above the slump structures, diatomites are characterized by Punctastriata pinnata and other perifitic diatoms indicative of shallow lacustrine levels. The lacustrine facies are overlain in transitional contact by a fluvioalluvial sequence (member 3) containing common sands and pumice deposits and a few diatomite lenses (Fig. 6a). The entire
sequence is capped by 6 m of paleosols and pumice deposits. Radiometric ages are currently being determined for the volcanic rocks of the Ixtlahuaca lacustrine graben. The relationship with the Acambay graben and the absence of marker Miocene diatoms, however, suggests a Plio-Quaternary age. 3.1.2. Acambay basin In this region, a long-lived lake developed which is now represented by nearly 60 m of interbedded diatomaceous, lacustrine, fluviatile, and volcaniclastic deposits (Fig. 3c). Equus simplicidens fossils occur in these sediments, implying a late Blancan age for at least the upper member (Mercer et al., 2003). The layered sequence is deeply deformed both at the base and at the top (Fig. 6c). This basin appears to be one that would be very useful for paleoseismologic studies (Rodriguez-Pascua et al., 2004a, 2004b). The greater than 70 m-thick sequence is characterized by three main units. The base is composed of a nearly 50 m succession of diatomite and volcaniclastic sediments along with clayey deposits which have been folded and intruded by sand dikes. This lower unit (1) is characterized by planktonic diatom taxa. Unconformably overlying unit 2 is a 22 m thick sequence of diatomite and silts containing characteristic Stephanodiscus and Aulacoseira. These beds
Fig. 6. Features associated with the west-central Me´xico lacustrine basins. (a) Landscape in Ixtlahuaca region showing the uplifted fluviolacustrine sediments; (b) Deeply deformed. diatomite strata in the Acambay graben; c) Escarpment near the eastern end of Cuitzeo lake; (d) Charo–Atapaneo fluviolacustrine deposits inside the Cuitzeo graben; (e) Uplifted block along the north limit of Chapala lake showing a laccolith intruding Neogene diatomites.
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grade progressively upward into a laminar diatomite that is characterized by the taxa Aulacoseira granulata and Stephanodiscus excentricus. This thick diatomite in turn transitions into a 1 þm thick clay stratum. At the top of this unit is a 10 cm-thick volcanic ash fission-track dated at 1.2 0.13 Ma (Mercer et al., 2003; Mercer, 2004). Several molars and premolars of E. simplicidens, partial teeth and tusk fragments of Rhynchotherium sp. and parts of camelid bones were found in upper units beneath this ash. These taxa are indicative of a Blancan age. Above this 10 cm-thick volcanic ash lies a w2 m thick, very deformed diatomite stratum, the base of which shows de´collement structures (slumps). The overlying lacustrine sequence (unit 3) is capped by intercalated silts and sands which show liquefaction structures (Fig. 6b). This appears to be a product of seismic activity (Rodriguez-Pascua et al., 2004a, in press). The upper 2 m of sediments shows evidence of abundant burrowing. These could represent ancient ground squirrel activity. Some unreported Pleistocene vertebrates were earlier collected by private individuals in the Acambay graben located here. One of the authors (WEM) looked at some of these fossils, which included horse, camel, bison, and mammoth. The lacustrine unit unconformably underlying Blancan deposits has not yet been dated. However, an ash located low in the section has recently been located and is being dated (results not yet available). Diatomites in this underlying unit include the very abundant central taxon S. excentricus (Caballero et al., 2000). Stephanodiscus excentricus (Hustedt), was described in upper Pliocene sediments of Oregon (Hakånsson and Stoermer, 1987). This taxon is also found in the upper Pliocene deposits in central Mexico basins, very often associated with vertebrate remains in the ancient lacustrine regions at Zacapu, and Patzcuaro, Michoacan (RoblesCamacho et al., 2009) and Acambay (Caballero et al., 2000; IsradeAlca´ntara et al., 2005). S. excentricus does not seem to be currently extant anywhere in the world (Hakånsson and Stoermer, 1987). 3.1.3. Chincua basin The Chincua basin sections (Fig. 3) consist of conglomerates of andesitic composition at the base, which unconformably underlie roughly 60 m of silty, sandy clays that are interbedded with diatomite. These were deposited mainly under lacustrine conditions. An approximately 4.0 Ma ignimbrite (Robin and Pradal, 1994) uncomformably overlies these lacustrine beds. Near the middle of the section, an equid molar was found. It was identified by Carranza as D. interpolatus, indicating an age of late Miocene (Castillo-Cero´n ˜ eda, 2001b). The Chincua et al., 1996; Miller and Carranza-Castan diatomite deposits contain Mesodictyon, a centric diatom (Fig. 5a). This diatom was described from upper Miocene deposits in the Snake River Plain and in the Chalk Hill Formation of Idaho (Theriot and Bradbury, 1987; Khursevich and Van Landingham, 1995). This genus has also been reported from upper Miocene strata in an offshore core from Angola, Africa (Fourtanier and Gasse, 1988). Mesodictyon has not yet been discovered in Asia (Khursevich, 1994). A similar species of Mesodictyon has been recovered from the Cajabamba basin, Peru (Fourtanier et al., 1993). However, in Me´xico this genus has only been found in the Miocene Chincua basin of Michoaca´n. This diatom’s age is consistent with that of the horse, D. interpolatus, whose fossils have also been found in Chincua deposits. Additionally, other vertebrate fossils of the same age have been recovered in the Michoaca´n–Jalisco area. Some of the identified mammalian taxa include Notolagus (rabbit), Osteoborus (hyaenid dog), Machairodus (saber-tooth cat), Calippus (horse), Neohipparion (horse), Nannipus (horse), Prosthennops (peccary), Teleoceras ˜ eda and Miller, (rhinoceros) and Alforjas (camel). (Carranza-Castan ˜ eda, 2001b). 2002; Miller and Carranza-Castan
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3.2. Central Miocene–Pliocene basins 3.2.1. Cuitzeo–Charo basin This basin is located in the state of Michoaca´n towards the west of the Chincua locality (Fig. 1). Inside the Charo–Cuitzeo basin the Charo paleolake deposits constitute the uplifted flanks that were produced in the upper Miocene. Before the uplifting of this area the paleolake deposits were located close in altitude to the active Cuitzeo Lake basin depocenter. Andesite lava flows underlie these paleolake deposits and have been dated by 40K–40Ar at 8.0 0.2 Ma (Instituto Mexicano del Petroleo (I.M.P), 1986; Israde-Alca´ntara and ˜ o, 1999). Ancient landscapes are occupied by a series of Gardun diatomite hills altered by later tectonic movements that formed incised valleys at different elevations. Several sections of silty–clay and lacustrine deposits, which are intercalated with beds of diatomite, become more fluviolacustrine towards the top of the stratigraphic columns (Fig. 3). River terraces cut by normal faults are common in this basin (Fig. 6c). Successive tilting and reactivation of faulting enhances the burying of lake sediments with fluvial and ˜ o-Monroy, alluvial deposits (Fig. 3e) (Israde-Alca´ntara and Gardun 1999). The ages of sediments and contained fossils changes progressively with rifting activity. This activity appears more ancient in the southern Charo sector of modern Cuitzeo Lake (1920 m altitude) and becomes younger towards the modern depocenter (1820 m altitude). The diatom flora in the lower stratigraphic levels of these Charo–Cuitzeo basin deposits, close to the La Goleta locality, contains Tertiariopsis (Khursevich et al., 2000), basionym Thalassiosira cuitzeonensis (Serieyssol et al., 1998) and is assigned to the upper Miocene (Fig. 5b). This taxon has been reported from only one locality 2 km south of Lago de Cuitzeo, Michoaca´n, Me´xico (Israde-Alca´ntara, 1995). It is very similar to Tertiariopsis reported from the upper Miocene deposits of Eastern Europe (Khursevich et al., 2002). T. cuitzeonensis has the rimoportula directly on the margin of the valvar face, while Tertiariopsis (Khursevich et al., 2002) has a rimoportula directly over the mantle. Stephanodiscus (Ehrenberg, 1845) species have been found and associated with Thalassiosira species in the Cuitzeo basin of Mexico in the sediments assigned to the upper Miocene (6.0 Ma); however, there are ˜ ofew other taxa known here (Israde-Alca´ntara and Gardun Monroy, 1999). An archaic and presumably extinct species of Stephanodiscus has been recorded from lacustrine environments in Egypt, in lower Miocene sediments of the Moghra Formation (Zalat, 1996) and in Asia from middle Miocene outcrops from Tunka basin in Russia (Likhoshway et al., 1997). Stephanodiscus occurs at the same stratigraphic level as Mesodictyon in Angola within marine offshore sediments (Fourtanier and Gasse, 1988). This genus is common only in the Pliocene of Europe (Serieyssol and Gasse, 1991) and in the western United States (Bradbury and Krebs, 1982). A complete skull and mandible of a proboscidean, Rhyncotherium falconeri, was discovered from the upper part of the La Goleta section (Fig. 3). It was recovered from fluviolacustrine deposits of ˜ eda, 1976). In the fluvial terraces of Blancan age (Carranza-Castan Atapaneo, west of the La Goleta locality, Israde and Lopez discovered a skull and molar respectively that possibly represents a species of Gomphotherium (Agenbroad L, personal communication, Fig. 6d). Other published reports correspond to Repening (1962) on the giant ground squirrel, Paenemarmota, which was recovered along with an undescribed Goleta vertebrate fauna from the eastern part of the Morelia lake basin. This small local fauna was said to come from near the village of La Goleta. It was stated that the recovered fossils, especially Paenemarmota, indicated a Blancan age. Enclosing sediments were reported to be lacustrine and fluvial, comprised of clay, diatomaceous and tuffaceous components. The age given by Repening (1962) is in agreement with the 3.0 Ma
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40
K–40Ar date obtained from the airfall tuffs interbedded with the lacustrine beds in the upper part of the sequence (Carranza˜ eda, 1991). Castan After 3 Ma, reactivation of normal faulting and tilting towards the south, the low-lying areas began filling with alluvial deposits. Erosional processes caused a reworking of late Pleistocene–early Holocene sediments along the northern border of Cuitzeo Lake (now dry). Erosion at the la Cinta site for example uncovered abundant fossils in which late Pleistocene-early Holocene horse remains are characteristic. This apparently was due to tectonic uplifting of the area. South of of the ancient lake edge, fluviolacustrine deposits have been uplifted more than 20 m. 3.3. Western Miocene–Pliocene basins
3.3.1. Chapala basin Outcrops are well exposed in the north, northwest and northeastern portion of the Lago de Chapala basin (Figs. 1 and 2b). In the ˜ o and Israde (Fig. 6d) northeastern section of this basin, Gardun sampled a laccolite that intruded the thick lacustrine sediments. It has been dated at 7.0 0.3 Ma by 40K–40Ar (Fig. 3b). A 35 m-thick sediment sequence with intercalated lacustrine, fluviolacustrine, diatomaceous lacustrine, and epiclastic sediments outcrops 1 km north of Lago de Chapala. The lower unit of the Lake Chapala section is well exposed in the Las Brisas locality. It is 8 m in thickness and contains an interbedded ash layer dated at 5.2 0.2 Ma. Stephanodiscus sp. cf. Stephanodiscus carconensis and Tertiarius (Tertiarius aff. T. baikalensis, Khursevich et al., 2002) occur at the base of this lacustrine succession (Fig. 5c). T. baikalensis was defined in the Lake Baikal region from middle Pliocene sediments (Khursevich et al., 2003). As there are no reports of Tertiarius in the modern lakes of Me´xico, it is considered extinct in this region. Similar taxa characterize the late Pliocene and early Pleistocene in the United States, but these taxa need to be positively identified using the original descriptions comparing them with modern standards. These taxa include Cyclotella pygmea Pantocsek (Krebs et al., 1987) or Tertiarius sp. aff. baikaensis Khursevich and Fedenyal (Khursevich et al., 2003) and occur in sediments from Lost River Klamath, Oregon. Other taxa also have been found, such as Cyclotella hannaites (Van Landingham, 1967) in the Chapala Formation (Michaud et al., 2000) as well as in late Pliocene sediments from a long core of Lake Baikal (Khursevich et al., 2000). Additional ancient Stephanodiscus species (referred to as Stephanodiscus asteroides and Stephanodiscus carconensis) are common in the Pliocene and lower Pleistocene in the United States (Krebs et al., 1987) and Africa (Gasse, 1980; Fourtanier and Gasse, 1988). These taxa also occur in the Chapala Formation (Delgado Granados, 1992; Michaud et al., 2000). Characteristic late Pleistocene and modern diatom taxa are found in Lago de Chapala sediments. These include Stephanodiscus sp. aff. Stephanodiscus rotula, Stephanodiscus sp. aff. Stephanodiscus carconensis and Stephanodiscus spaff. nemanensis (Hermes et al., 2007). A great variety of late Pleistocene vertebrate fossils, including fish, amphibians, birds, reptiles and abundant mammals, are known from Lago de Chapala sediments (Rufalo, 1998). These fossils have been found at several sites surrounding the lake. Unfortunately, most of this material was gathered on the surface and without sufficient stratigraphic data. However, viewed as an assemblage, the fossils are indicative of more moist conditions in the region during the late Pleistocene than today. This appears evident from what must have been an abundant vegetation in this area (probably comprising a savanna), which would have supported a diverse and abundant fauna. Also, fish of late Pliocene to Pleistocene age have been reported from the Lake Chapala basin. Smith et al. (1975) described seven species of fish in six families from deposits near the western
end of Lake Chapala. Cavender and Miller (1982) identified a new species of salmonid fish from sediments at the northwestern end of this lake that was found associated with Pleistocene mammalian ˜ eda et al. (2005) reported fish from Lake fossils. Carranza-Castan Chapala sediments of Blancan age. One of the other fossils (that of a horse) earlier collected in deposits at Lago de Chapala is also representative of this age. Downs (1958) considered the presence of this horse (Nannippus) in the type Chapala Formation to be suggestive of at least some Pliocene [Blancan] deposits in the section. To the southwest of Lake Chapala in the Tecolotlan basin, a majority of the vertebrate fossils are found in upper Miocene (Hemphillian) and upper Pliocene (Blancan) to lowest Pleistocene (Irvingtonian) beds (Kowallis et al., 2003). These ages are consistent with those of the ancient diatoms mentioned above. This has been substantiated by numerous mammalian fossils that have been collected to date. A sample of fossil vertebrates from the Hemphillian deposits at Tecolotlan sites includes horses (Neohipparion eurystyle, N. minor, and Dinohippus mexicanus, a camel (Hemiauchenia vera), a rhinoceros (Teleoceras fossiger), a bear (Agriotherium schneideri), a dog (Canis ferox), a saber-tooth cat (Machairodus cf. M. coloradensis), and rabbits (Notolagus velox, and Hypololagus sp.). Those mammals indicative of Blancan/Irvingtonian beds in the Tecolotlan area have been identified as horses (Nannippus peninsulatus, and E. simplicidens), peccary (Platygonus sp.), xenarthrans (Glyptotherium sp., Plaina sp.), capybara (Neochoerus cordobai), and mastodon (Rhynchotherium sp.). In two recent articles, Lucas (2008a, 2008b) reported a variety of vertebrates from 23 localities in Jalisco basins which indicate Hemphillian through Rancholabrean ages. Many of these sites are in the Lake Chapala and Tecolotlan areas. 4. Central Me´xico vertebrate evolution In Neogene sediments from central Me´xico’s basins, abundant terrestrial vertebrates have been collected systematically for the past 35 years (Fig. 4b). They include fish (Osteichthyes) and four classes of tetrapods: Amphibia, Reptilia, Aves, and Mammalia. Mammalian fossils are by far the most abundant of these taxa. All are extinct at the species level, and many at the generic. Major groups of these include shrews, a bat, several types of rabbits, numerous kinds of rodents, a variety of carnivores (including saber-tooth and biting cats, bears, hyaenid-like dogs, an ancestral coyote and smaller forms), gomphotheres, mastodonts, a rhinoceros, at least eight species of horses, including ancestral Equus, tayassuids (or peccaries), camels, pronghorns, and a few types of xenarthrans (or edentates). A list of Hemphillian and Blancan age mammals from central Me´xico can be found in published articles (e.g., Miller and ˜ eda, 1984, 1998b, 2001a; Carranza-Castan ˜ eda and Carranza-Castan Miller, 2004). Many North American vertebrate taxa of late Miocene through Pliocene age have their southernmost known range extensions in the basins associated with the TMVB. The South American forms now recovered from these same basins began arriving in central Me´xico during the initial stages of Panamanian land bridge development, at least 4.7 Ma (Flynn et al., 2005). Surprisingly, none of these South American taxa have been reported from the United States prior to about 3.0 Ma (e.g., Webb, 1977; Marshall, 1985). Perhaps preferred habitats for the South American immigrants living in central Me´xico did not develop until significantly later in the United States due to different faunal provincialities in the two regions. Northward dispersals to the United States may have been greatly delayed until climate and habitat changes eliminated the provinciality. A significant amount of time, therefore, might have been involved in allowing for needed species’ adaptations before the northward dispersal of South American immigrants. The vertebrate fauna demonstrates marked changes from the late Miocene through the Pleistocene. It also assists in an
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interpretation of climatic conditions during latest Tertiary time. Webb and Perrigo (1984) pointed out a major extinction event for mammalian species in North America at the close of Hemphillian time (w4.8 Ma). This is at about this time that the Panama land bridge was forming which broke the connection between the Atlantic and Pacific Oceans. This must have caused pronounced climatic changes for much of North and South America. Subsequently, floral and faunal changes occurred, accounting for much extinction. Haug and Tiedemann (1998) report that there was a major reorganization of ocean circulation, and hence climatic patterns, due to the emergence of the Isthmus of Panama starting at 4.6 Ma (Fig. 4a). Their evidence for this was based on stable-isotope and carbonate sand-fraction records from Atlantic and Pacific palaeoceanographic data. Also at about this same time, the mammalian faunas across central Me´xico began showing changes. Horses for example, decline from six genera in the Hemphillian to two in the Blancan. The two Blancan genera appear to be descended from Hemphillian forms (N. peninsulatus, from N. minor and E. simplicidens from D. mexicanus). As reported earlier by Miller and ˜ eda (2001a), D. mexicanus seemingly evolved into Carranza-Castan E. simplicidens during earliest Blancan time (early Pliocene) somewhere in central Me´xico. Some of the other mammalian taxa disappearing from the central Mexican record at about 4.6–4.8 Ma include the bear Agriotherium, the hyaenid dog, Osteoborus, the saber-tooth cat, Machairodus, the rhinoceros, Teleoceras (a late Hemphillian skull and mandible of Teleoceras from near Juchitlan in Jalisco was recently described by Lucas (2008c), the peccary, Prosthennops, the camel, Alforjas, and the rabbit, Notolagus. Conversely, new mammalian taxa began appearing by the early Blancan (Pliocene). Some of these in addition to the horses listed above are the bear, Ursus, the hyaenid dog, Borophagus, the peccary, Platygonus, the camel, Camelops, the rabbit, Paranotolagus, the rodents Dipoides, and the capybara, Neochoerus, as well as xenarthrans, Glossotherium, Glyptotherium and possibly Plaina. 5. Paleoenvironments of diatoms and vertebrates A sequence of particularly large, freshwater lakes developed during the late Miocene and Pleistocene in the regions of Ixtlahuaca–Chincua to the west, south, and north of Lake Chapala along tectonic depressions (Fig. 3). The diatom assemblages of the late Miocene paleolakes reflect generally shallow, and sometimes turbid and alkaline conditions. These lakes received inflow by a combination of surficial drainage and groundwater input. Climates then were generally more humid than that of today. In the Ixtlahuaca basin, diatomites are characterized by large planktonic forms (e.g., Stephanodiscus niagarae and S. aff. minutulus) that towards the top of the section are replaced by a nearly monospecific community in which P. pinnata is associated with S. niagarae. This community, however, lies above an erosional discordance. These planktonic taxa display very fragmented valves in the slump deposit. This sudden change in diatom taxa indicates a drastic habitat change from high to low water to very shallow water levels for a brief period. Modern communities of P. pinnata can be observed in rocky habitats in water depths of less than 4 m (Bradbury, 2000). The period of high to very shallow waters represents a drier phase evidenced by the presence of Campylodiscus clypeus, Anomoeoneis sphaerophora and Surirella sp. C. clypeus is an epipelic form common in waters with a high mineral content, (pH of 7.0–9.5), high conductivity (3000 to
