Taphonomic analysis in lacustrine environments: Two different contexts for Triassic lake paleofloras from Western Gondwana (Argentina)

Sedimentary Geology 222 (2009) 149–159

Contents lists available at ScienceDirect

Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Taphonomic analysis in lacustrine environments: Two different contexts for Triassic lake paleofloras from Western Gondwana (Argentina) Adriana Cecilia Mancuso ⁎ Ianigla, CCT-CONICET-Mendoza, Adrián Ruiz Leal s/n — Parque Gral. San Martín (5500) Mendoza C.C.330, Argentina

a r t i c l e

i n f o

Keywords: Plant taphonomy Lake Middle Triassic Argentina

a b s t r a c t During the earliest Triassic several rift basins developed along the western margin of Gondwana associated with the pre-breakup of Pangea. They were filled by exclusively non-marine sediments including alluvial, fluvial, and lacustrine deposits. In the Ischigualasto–Villa Unión Basin, the lacustrine-deltaic succession is placed in the Los Rastros Formation and consists of several coarsening-upward cycles of black shale, siltstone, and sandstone. The paleontologic content of the succession includes abundant floral remains (related to the Dicroidium-type flora), invertebrates (conchostracans, insects), and vertebrates (fishes, a temnospondyl amphibian, ichnites). At the Cerro Puntudo area in the Cuyana Basin, the lacustrine succession forms the upper part of the Cerro Puntudo Formation and consists of limestone, stromatolitic limestone, mudstone, sandstone, and tuff. The paleontologic content includes scarce floral remains and rhizoliths; invertebrates are represented exclusively by traces (associated with ichnofacies of Skolithos and Scoyenia), and vertebrates by a fragment of the pelvic girdle of a basal arcosaur. The taphonomic analysis performed in the two Triassic lacustrine successions allows recognition of two different taphonomic histories for the plant remains. The Los Rastros lake preserved both autochthonous (originated in the littoral zone) and allochthonous (originated in the upstream fluvial system) elements. The offshore lacustrine area was dominated by autochthonous well-preserved elements and allochthonous plant debris and wood, which formed time-averaged accumulations. The delta deposits are characterized by allochthonous elements with varied preservational conditions, usually showing evidence of mechanical degradation and accumulation within a short time. Autochthonous and allochthonous material were preserved in the Los Rastros Lake by means of anoxic conditions in the offshore lacustrine area and high sedimentation rates in the delta. In contrast, the Cerro Puntudo Lake preserved only autochthonous elements (originated in the littoral zone), including rhizoliths and foliar material, which formed autochthonous and parautochthonous accumulations at the littoral zone in spite of aerobic conditions. This was the result of tuffaceous material that enhanced preservation. Thus, these very different lacustrine environmental contexts are showing different preservational modes. The fossil assemblages allowed the reconstruction of the original communities from this part of Gondwana. Thus, the Los Rastros lake margins were characterized by shrubs and small trees of Ginkgoales and Corystospermales, and herbaceous members of the Sphenophyta. The sphenophytes were also the dominant floral component along the river margins whereas the Corystospermales, Cycadales, Pteridophyta, and conifers formed the woodland upstream probably related to the floodplains of a trunk fluvial system. The littoral zone of the Cerro Puntudo Lake was dominated mainly by herbaceous sphenophytes and lycopsids. These fossil assemblages characterize the paleoflora associated with lacustrine systems. A fuller understanding of the processes that generate these assemblages is essential for comparisons with other continental paleobotanical records in the Middle Triassic of Gondwana (e.g., Australia, South Africa). © 2009 Elsevier B.V. All rights reserved.

1. Introduction A plant–fossil assemblage is an accumulation of diverse plant parts, sourced from one or more species and individuals, and found associated with sedimentary deposits under specific conditions. Distinct environments can preserve different plant associations ⁎ Fax: +54 261 524 4201. E-mail address: [email protected]. 0037-0738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2009.05.017

depending on their proximity to the original vegetation, type of spatial arrangement, burial mechanisms, shape of the remains, and their hydrologic properties, among other factors. These assemblages are unique in that plants develop a variety of organs which are composed of different chemical constituents (Gastaldo, 1994). These organs are introduced individually into a depositional environment and can have different transport, deposition, and preservation potential (Spicer, 1991; Gastaldo, 1994; Nichols et al., 2000). Thus, taphonomic studies are critical to the correct interpretation of

150

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

151

Table 1 Summary of the Los Rastros sedimentary facies associations (modified from Mancuso and Marsicano, 2008). Facies Lithology associations

Sedimentary structure

Geometry

Fossil content

Interpretation

LR-A

Horizontal laminated

Tabular

Massive or laminated

Tabular

Plants, conchostracans, bivalves, insect, fish Plants, conchostracans, insect, fish and trace fossils

Offshore lacustrine depositional environment and prodelta with distal turbidity currents

Massive rhythmic

Tabular

Plants

Deltaic front with progradation of the mouth-bar

Current ripple cross-lamination, horizontal lamination, planar to trough cross-bedded Tabular to trough cross-stratification, low-scale ripple cross-lamination at the top, lags of mud intraclasts Massive, planar lamination to ripple cross-lamination

Tabular to Plants and fish plane-convex

LR-B

LR-C

Dark gray to black carbonaceous claystone Very dusky purple iron mudstone and very fine-grained sandstone Green siltstones and gray claystone Fine- to coarse-grained sandstone Medium- to coarse-grained sandstone Black mudstone and very finegrained sandstone, coal layers

reconstructed plant paleocommunities, as a good floral reconstruction depends mainly on the understanding of both the depositional environment and the specific taphonomic processes operating in that environment (Gastaldo et al., 1987). Plant taphonomic studies had their first large boom in the 1980s (e.g., Ferguson, 1985; Gastaldo et al., 1987; Spicer and Wolfe, 1987; Spicer, 1989), followed by a progressive increase throughout the following years (e.g., Spicer, 1991; Martín-Closas, 1995; Gastaldo et al., 1996; Nichols et al., 2000; Martín-Closas and Gomez, 2004; Gastaldo et al., 2005; Martín-Closas and Galtier, 2005; Hably and Szakmány, 2006). Lacustrine systems are sensitive environments that preserve rapid shifts in sedimentary facies (Talbot and Allen, 1996) with high preservation potential at fine temporal resolution (e.g. Wilson, 1988; Olsen, 1990; Behrensmeyer and Hook, 1992). In Argentina, the Triassic lacustrine successions are well developed in the extensional basins located in western South America. Two of these successions are known as Los Rastros and Cerro Puntudo Formations, which are part of the continental fill of the Ischigualasto–Villa Unión and Cuyana Basins, respectively (Fig. 1). Processes leading to plant fossilization in lacustrine environments have not been well studied (Martín-Closas, 1995). The present work is the first comparative analysis of the paleofloral content of two very different Triassic rift lacustrine successions in Western Gondwana. This analysis identifies the taphonomic factors that were important in the formation of these assemblages. These successions represent two very different lacustrine environmental contexts, which should have different characteristics of preservation resulting in different preservational modes. The aim of this work is to test this hypothesis using the understanding of the lacustrine depositional environment and the taphonomic processes in operation. 2. Geologic setting Associated with the pre-breakup of Pangea during the earliest Triassic, several rift extensional basins developed along the western margin of Gondwana (Uliana and Biddle, 1988; Uliana et al., 1989). In central-western Argentina (Mendoza, San Juan and La Rioja provinces), two large Triassic depocenters include the Ischigualasto–Villa Unión and Cuyana Basins (Fig. 1A). They are filled exclusively by nonmarine sediments including alluvial, fluvial, and lacustrine deposits. The sedimentary fill of these basins records their tectonic co-evolution (e.g. Spalletti, 2001). The deposits of these Triassic rift basins are the

Tabular to lenticular

Woody plants, amphibian, Distributary fluvial system with tetrapod and invertebrate traces delta swamps within the deltaic plain

Tabular

Plants

most important of western Gondwana due to their stratigraphic continuity and associated paleontologic record. In particular, their paleofloral content is comparable with Australia and South Africa, and together represent most of the paleobotanical record in continental Triassic Gondwana. This work includes the taphonomic analysis of two lacustrine successions that represent the first lacustrine episode in each basin (Bellosi et al., 2001). They were developed in two isolated basins during the middle Triassic (Uliana and Biddle, 1988), although there are no absolute age data for the studied sequences. Sedimentologic studies were performed in these successions by Mancuso and Marsicano (2008) and Krapovickas et al. (2008) with a facies summary below. 2.1. Ischigualasto–Villa Unión Basin The Ischigualasto–Villa Unión Basin is an elongated rift basin with a NNW–SSE orientation (Fig. 1A). At present, it is developed along the border between the San Juan and La Rioja provinces. Lithostratigraphically, the basin fill is divided into several units. The Talampaya and Tarjados Formations constitute the base of the succession, and are unconformably overlain by the Agua de la Peña Group. The latter is subdivided into four units (sensu Mancuso, 2005a,b) which include, from base to top, the Chañares, Los Rastros, Ischigualasto, and Los Colorados Formations (Fig. 1D). The Los Rastros lacustrine succession, with a thickness between 160 and to 600 m, is dominated by an alternation of laminated dark gray to black carbonaceous claystone and tabular fine- to coarsegrained sandstone, which characterize coarsening- and thickeningupward cyclic deposits (Mancuso 2003, 2005a,b; Mancuso and Marsicano, 2008). The following sedimentologic characteristics are summarized in Table 1. Each cycle begins with horizontally laminated, dark gray to black carbonaceous claystone with interbedded massive or laminated very dusky purple iron mudstone and very fine-grained sandstone (Facies association LR-A) (Table 1; Figs. 1B, 2A), interpreted as offshore lacustrine and prodelta facies (Mancuso, 2003, 2005a; Mancuso and Marsicano, 2008). The lacustrine deposits grade upward into a coarsening- and thickening-upward succession dominated by green siltstone, grey claystone, fine- to coarse-grained sandstone, and rare gravelly sandstone (Facies association LR-B) (Table 1; Fig. 1B, 2A). These facies, interpreted as delta front deposits, represent a shallowing upward trend, which is consistent with a prograding distributary

Fig. 1. (A) Map of west-central Argentina showing the Triassic Ischigualasto–Villa Unión and Cuyana Basins (modified from Stipanicic and Marsicano, 2002b). (B) Schematic stratigraphic section of an ideal cycle defined for the Los Rastros Formation with common fossil occurrences. Scale in 25 m intervals. (C) Schematic stratigraphic section of the Cerro Puntudo Formation. F, Sf, Sm, Sg, and G represent mudrock, fine sandstone, medium sandstone, coarse sandstone, and gravel, respectively. Scale in 50 m intervals. (D) Triassic lithostratigraphy and chronostratigraphy in the Ischigualasto–Villa Unión Basin and Cerro Puntudo depocenter (modified from Mancuso and Marsicano, 2008).

152 A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159 Fig. 2. (A) Exposures of the Los Rastros Formation in the north of the basin (GPS coordinates: 29°37'47.19”S, 68°24'14.05”W). (B) Exposures of the Cerro Puntudo Formation (GPS coordinates: 30°56'56.16”S, 69°17'25.79”W). (C–F) Fossil content of Los Rastros Formation. (C) Crustaceous conchostracans. (D) Blattopteran wings. (E) Actinopterygian. (F) Temnospondyl amphibian. (G–I) Fossil content of Cerro Puntudo Formation. (G) Stromatolites. (H) Invertebrate traces. (I) Fragment of a pelvic girdle of a basal arcosaur.

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

mouth-bar complex (Mancuso, 2003, 2005a; Mancuso and Marsicano, 2008). Finally, an intercalation of gray to black mudstone and very fine-grained sandstone with medium- to-coarse grained sandstone (Facies association LR-C) dominate the succession (Table 1; Fig. 1B, 2A). These were deposited by a distributary fluvial system in a deltaic platform setting with delta plain swamps (Mancuso, 2003, 2005a; Mancuso and Marsicano, 2008). 2.1.1. Paleontology of Los Rastros Formation The paleontologic content is mainly represented by both floral and faunal remains. An abundant macrofloristic assemblage (e.g., Frenguelli, 1948; Stipanicic and Bonaparte, 1979), related to the Dicroidium-type flora, was described from the thick, lacustrine black shales close to the base of the sequence together with a rich palynologic association (e.g., Herbst, 1965, 1970; Yrigoyen and Stover, 1970; Herbst, 1972; Zavattieri and Melchor, 1999, Ottone et al., 2005; Ottone and Mancuso, 2006). The fauna includes both invertebrate and vertebrate remains. The invertebrate record (Fig. 2C and D) consists of different conchostracans (clam shrimps), insects, and bivalves (e.g., Frenguelli, 1945; Gallego, 1992, 1997, 1999; Martins-Neto et al., 2005, 2006, Mancuso et al., 2007), and invertebrate trace fossils (Melchor et al., 2003; Melchor, 2004). The vertebrate record (Fig. 2E and F) includes skeletal remains of both fishes and a temnospondyl amphibian (Cabrera, 1944; Contreras et al., 1997; Mancuso, 2002, 2003; López-Arbarello et al., 2006), as well as trace fossils (von Huene, 1931; Heim, 1949; Marsicano et al., 2004, 2007).

153

Two units are recognized in the Cerro Puntudo area, the Cerro Puntudo and El Relincho Formations (Mombrú, 1973; Krapovickas et al., 2008). In general, the Cerro Puntudo is divided into two units, a lower unit dominated by coarse conglomeratic sediment with red to brown reddish color (Facies association CP-A, CP-B. CP-C), and an upper unit characterized by finer detrital sediment with carbonate, siliciclastic, and pyroclastic deposits with brown reddish to gray or greenish gray color (Facies association CP-D) (Table 2; Fig. 1C, 2B). The upper unit is a lacustrine succession of proximally 40 m thickness and consists of limestone, stromatolitic limestone, mudstone, sandstone, and tuff (López Gamundi and Astini, 2004; Krapovickas et al., 2008). This unit is characterized by gray massive limestone, stromatolitic limestone, brown reddish calcareous sandstone, brown reddish calcareous siltstone, gray marlstone, and light green tuff and tuffite. The succession is well stratified with beds of 0.01–0.2 m thickness and, in some cases, exhibits ripple cross-lamination (Fig. 1C). The sedimentological characteristics are summarized in Table 2. 2.2.1. Paleontology of Cerro Puntudo Formation The paleontologic content includes a scarce palynoflora of Pteridophyta and Lycophyta, megafloral remains, rhizoliths, and stromatolites (Mancuso et al., 2006). Invertebrates are represented exclusively by traces (associated with ichnofacies of Skolithos and Scoyenia) (Krapovickas et al., 2006,) and vertebrates by the fragment of a pelvic girdle of a basal arcosaur (Mancuso et al., 2006) (Fig. 2G–I). 3. Taphonomy

2.2. Cuyana Basin

3.1. Recognized taphonomic processes

The Cuyana Basin is the largest basin in central western Argentina, including several depocenters, and its exposures are developed over 500 km mainly in the Mendoza and San Juan provinces (Kokogian and Mancilla, 1989; Kokogian et al., 2001) (Fig. 1A). In the western Precordillera of San Juan Province, the most northerly outcrops are reported as part of the Cerro Puntudo area (Mombrú, 1973; Strelkov and Alvarez, 1984; Sessarego, 1988; López Gamundi and Astini, 2004; Krapovickas et al., 2008) (Fig. 1A).

This section includes only the taphonomic processes that were identified in the Los Rastros and Cerro Puntudo plant record. They are analyzed in the framework of a lacustrine environment. The decay processes in plants vary with the nature of the organ, the taxon, and the type of environment in which the plant grew and debris accumulated. Decay is a biologic process and, thus, the rate of decay, in lacustrine environment, depends on factors such as temperature, pH, and nutrient status of the water body (Ferguson, 1985; Spicer, 1991;

Table 2 Summary of the Cerro Puntudo sedimentary facies associations (modified from Krapovickas et al., 2008). Facies Lithology associations CP-A

CP-B

CP-C

CP-D

Sedimentary structure

Red to reddish brown coarse conglomeratic Not observed with sandstone matrix, subrounded clasts 1–10 cm Red medium- to coarse-grained sandstone Massive or cross-stratification

Geometry

Fossil content

Interpretation

Tabular to lenticular

–

Proximal to medial alluvial fan deposits

Tabular to lenticular Tabular to lenticular Tabular to lenticular Lenticular

–

Red to reddish brown medium conglomeratic Reddish brown medium- to coarse-grained sandstone, scattered clasts Reddish brown medium- to fine-grained sandstone Reddish brown and white tuffaceous siltstone

Normal gradation, tabular cross-stratification, lags Massive or tabular cross-stratification Massive or tabular cross-stratification Horizontal lamination

Gray limestone

–

Gray limestone

Massive and stromatolitic structure Massive to fine horizontal lamination, liquefaction Massive, or horizontal lamination Tabular or ripple cross-lamination Horizontal stratification Tabular Massive to laminated Tabular

Reddish brown calcareous siltstone Reddish brown calcareous sandstone Gray marlstone Light green tuff and tuffite

Tabular

Lenticular to lentiform Tabular to lenticular Tabular

–

Braided fluvial system deposits associated with distal alluvial fans

Invertebrate vertical trace Skolithos – Fluvial system deposits associated with more distal alluvial fan with lacustrine interbeds Rhizoliths and Invertebrate trace Scoyenia Rhizoliths Rhizoliths Plant debris and rhizoliths Rhizoliths – Plant debris, rhizoliths, and vertebrate

Lacustrine system deposits

154

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

Gastaldo, 1994). These factors, together with inhibitory components of plants, affect the rate of decomposition. Therefore, leaves with relatively high levels of biologically important minerals, which are usually limited in oligotrophic lakes, will be degraded selectively (Ferguson, 1985). In contrast, there is a great probability that all taxa will be represented to some extent in eutrophic lakes (Spicer, 1991). In general, herbivory is recognized by leaves with rounded holes and margins, and biologic degradation by skeletonization (MartínClosas, 1995; Scott et al., 2004; Labandeira, 2006). Leaf tissue with angular tears and breaks constitutes evidence of mechanical damage during water transport (Spicer and Wolfe, 1987; Spicer, 1991; Gastaldo, 1994; Martín-Closas, 1995). Leaves affected by herbivory or biologic degradation are more fragile during water transport and, thus, biologically affected leaves are less common in environments where this process dominates (Spicer, 1991; Gastaldo, 1994; MartínClosas, 1995). Transport is the main biostratinomical process analyzed, including wind, water, and mass flow. The plant material may evidence a complex transport history before final burial, because of its capacity of redistribution (Gastaldo et al., 1987; Gastaldo, 1994). Wind transport is the most selective process in sorting debris according to weight, form, and density. Thus, wind-transported assemblages are less diverse in taxa, form, size, and organ composition, and often consist mainly of leaves and winged plant seed from a plant community close to the site of deposition (Ferguson, 1985; Spicer, 1991; Martín-Closas, 1995). Water transport is a poor sorting process, and the water transported assemblages are systematically highly diverse, with respect to type and size of plant organs (Ferguson, 1985; Gastaldo et al., 1987; Spicer, 1991; Martín-Closas, 1995). Laboratory experiments demonstrate that there is considerable difference in floating times among different organs and plant taxa (Ferguson, 1985; Spicer, 1991; Nichols et al., 2000). Transport of debris within bedload is the most common process that produces

leaves with angular tears and breaks (Spicer and Wolfe, 1987; Gastaldo et al., 1987; Spicer, 1991; Gastaldo, 1994; Martín-Closas, 1995). Finally, subaqueos mass-flow transport caused through the action of unstable sediment along the delta front and lake margins, influenced by a steep margin in the basin, normally is directed toward the prodelta or offshore (Mancuso and Marsicano, 2008). In general, this process results in tabular beds with planar bases. Assemblages transported by mass flow include poorly sorted remains, usually with broken delicate structures (Martín-Closas, 1995). An assemblage that is preserved in the absence of transport is suggested by autochthonous aquatic plants, characterized by the presence of rhizoliths that are associated with vegetative and/or reproductive remains of the same species in a single bed. These remains show anatomical connection between plant organs and an absence of breakage (Martín-Closas, 1995). Biologic or biochemical degradation is another process that depends on the depositional conditions such as oxidation–reduction and burial rate (Gastaldo, 1994). Particularly in lacustrine environments, plants remain entombed under aerobic bottom water conditions show different preservational states depending on the sedimentation rate. For example, leaves are degraded rapidly because the environment in which they accumulated has a low sedimentation rate. In contrast, plant remains accumulated under dysaerobic conditions are preserved independent of the sedimentation rate due to the inhibition of biologic degradation (Spicer, 1991; Martín-Closas, 1995). 3.2. Taphofacies Taphofacies are based on particular combinations of taphonomic and sedimentologic parameters that are easily recognized in the field and which characterize the Los Rastros and Cerro Puntudo fossil assemblages. Thus, the sampling of plant horizons and registration of plant–fossil attributes were conducted according to currently

Table 3 List of taphonomic attributes of plant taphofacies as defined by remains collected from the Los Rastros and Cerro Puntudo successions. Taphofacies Taphonomic attribute

LR-1 (12)

LR-2 (7)

LR-3 (15)

LR-4 (8)

LR-5 (1)

CP-1 (5)

CP-2 (3)

CP-3 (1)

Remains

Debris, leaves, fructifications, seed, wood

Debris, wood, leaves

Debris, wood

Wood

Rhizoliths

Debris

Debris, rhizoliths

Taxa

Dicroidium, Cordaicarpus, Neocalamites, Baiera, Xylopteris Indet.

Neocalamites, Baiera

Neocalamites Indet.

Neocalamites Indet.

Indet.

Indet.

Mode of fossilization

Carbonaceous compression, Carbonaceous impression, mummification, compression, impression, permineralization silicification Low/moderate Low Compaction, abrasion, Breakage breakage

Debris, fructifications, leaves, wood Dicroidium, Xylopteris, Taeniopteris Neocalamites Indet. Carbonaceous compression, impression

Carbonaceous compression

Impression

Marks

Carbonaceous Impression compression, charcoal

High Compaction, abrasion, rounding, breakage Without preferential orientation Concordant

High Compaction

High –

High Compaction

High Compaction, oxidation

Without preferential orientation Concordant

–

Without preferential orientation Concordant

Loosele/ densele –

Without preferential orientation Concordant, Discordant Loosele/ densele Loosely sorted Moderate/ high Light brown tuffaceous mudstones

Relative abundance Post-mortem modification

High Compaction, abrasion, rounding, breakage

Spatial pattern

Orientation Without preferential orientation

Preferential orientation

Concordant

Concordant

Biofabric

Crosscutting Packing

Without preferential orientation Concordant

Loosely-packed

Dispersed

Loosele/densele

Sorting

Poorly sorted

Loosely sorted

Loosely sorted

Denselypacked Poorly sorted

Density

Low/moderate

Low

Moderate/high

High

Denselypacked Poorly sorted High

Gray- black claystone

Dusky purple fine sandstone

Gray medium sandstone

Black fine sandstone

Dark gray claystone

Host sediment

The number beside the taphofacies name indicates the number of flora assemblages included in each.

Discordant

Moderate/ high Reddish brown fine sandstone

Loosele/ densele Loosely sorted Moderate/ high Light green tuff

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

accepted methodologies for continental environments (e.g., Spicer and Wolfe, 1987; Spicer, 1991; Gastaldo, 1994; Gastaldo et al., 2005). These taphofacies were clearly distinct in the preservation of plant fossils both within and between facies and reflect their postmortem history (Brett and Baird, 1986; Speyer and Brett, 1986). Each taphofacies is identified by the formation name (i.e., LR for the taphofacies of the Los Rastros Formation and CP for the Cerro Puntudo Formation) and a successive number. Table 3 lists the taphofacies with the taphonomic parameters that were recognized and compiled to systematize the collection of taphonomic data from the fossils. In this table, the number in parentheses next to the taphofacies name indicates the number of assemblages included in each taphofacies. 3.2.1. LR-1 taphofacies This taphofacies includes a high diversity of remains, including plant debris, fructifications, seeds, wood, and leaves (Table 3; Fig. 3A), and is associated with offshore settle-out. It represents a mixture of well-preserved and broken elements, which exhibit evidence of both wind and water transport. The plant material was apparently transported by wind, by floating in the stream, or by wave action along the lake margins. It resided in suspension for a variable time according to its buoyancy, and this might explain the sorting of the remains in the fossil assemblages. While some elements are well preserved, others show post-mortem modifications that suggest mechanical damage which took place during redistribution. An absence of benthic organisms and undisturbed lamination suggest this taphofacies was deposited under dysaerobic conditions, and was covered by sediment that settled from suspension and/or turbidity currents (Mancuso and Marsicano, 2008). 3.2.2. LR-2 taphofacies This taphofacies consists of plant debris, wood, and leaves (Table 3; Fig. 3B), which are found suspended in the iron-beds. Plant remains show evidence of both water and mass-flow transport (see Table 3) (Mancuso and Marsicano, 2008). Thus, the remains were transported by water towards the delta front and then reworked by mass flows towards the prodelta setting. The final site to which the remains were transported had dysaerobic conditions; however, the mass flows represent aerobic conditions with a high sedimentation rate. 3.2.3. LR-3 taphofacies This taphofacies contains plant debris, fructifications, leaves, and wood (Table 3; Fig. 3D–E). It shows the general case taphonomic history for plants in deltas (Spicer, 1991), and the remains are associated with different deltaic facies. The plant detritus was transported by water from a fluvial trunk and deltaic plain, and was deposited according to its hydraulic potential (associated with size fraction and inherent density) as its hydraulic properties are similar to those of associated inorganic particles (Gastaldo et al., 1987). Thus, it can be deposited from the deltaic plain to the distal deltaic front, and show mechanical damage produced during bed-load transport and redistribution (Mancuso and Marsicano, 2008). 3.2.4. LR-4 taphofacies This taphofacies consists of a very high concentration of plant debris and wood (Table 3; Fig. 3F). The remains show the characteristic mechanical damage produced by long-term transport and redistribution (Gastaldo et al., 1987). The material was accumulated under dysaerobic conditions within interdistributary bays on the deltaic plain (Mancuso and Marsicano, 2008). 3.2.5. LR-5 taphofacies This taphofacies contains exclusively well-preserved Neocalamites stems in a single level (Table 3; Fig. 3C), which suggests, along with the taphonomic characteristics listed in Table 3, a short transport time and distance from the source vegetation. The material was trans-

155

ported by mass flow from lake margins near the delta towards the dysaerobic lake bottom. This dense accumulation was an exceptional event, as only one level was recorded (Mancuso and Marsicano, 2008). 3.2.6. CP-1 taphofacies This taphofacies is formed by abundant thin tubes (not more than 0.5 cm diameter), that are interpreted as rhizoliths (Table 3; Fig. 3G–I). These structures are dominantly vertical, and cross cut several beds (approximately 15–20 cm length). The characters of their exteriors are irregular, and branching is uncommon. The infilling of the tubes was passive and in some cases concentric. The dimensions and density suggest that the rhizoliths belonged to herbaceous plants (Retallack et al., 1990; Rodríguez-Aranda and Calvo, 1998; Owen et al., 2008). The sediment surrounding these structures is reddish-brown clay to sandstone indicating oxidizing conditions. This is also evidence that their development was associated with phreatic conditions in the subaqueous lake margin (Rodríguez-Aranda and Calvo, 1998). 3.2.7. CP-2 taphofacies This taphofacies consists of plant debris (Table 3; Fig. 3J) that is very low in diversity and exhibits light charcoalification, probably as a result of burial in pyroclastic material. The remains are loosely sorted, and some anatomy is preserved in the plants, suggesting short water (flooding) transport. Pyroclastic burial is generally rapid and enhances the preservation of foliar material. 3.2.8. CP-3 taphofacies This taphofacies includes plant detritus and rhizoliths (Table 3; Fig. 3K), associated together in the same bed. The foliar material is partially oxidized and forms low-diversity assemblages. The rootlets are similar to those described for CP-1. Both the preservation of anatomically-connected vegetative material and the root impressions of hydrophytic plants suggest an absence of transport. The oxidizing conditions indicated by the sediments require high sedimentation rates to produce the observed preservational conditions of the remains. In this case, the foliar remains show signs of oxidation that suggest a moderate sedimentation rate of a mixed sediment (pyroclastic and siliciclastic) nature. This taphofacies is also associated with a subaqueous lake margin. 4. Comparison between the paleoflora of Los Rastros and Cerro Puntudo Formations Despite the fact that the overall paleontologic content of the Los Rastros and Cerro Puntudo Formations are very different, the plant fossil records show some similarities (Table 4), and both were associated with CSD (Dictyophyllum castellanosii, Johnstonia stelzneriana, Saportaea dichotoma) Assemblage Biozone of Spalletti et al. (1999, 2008). On the one hand, the Los Rastros Formation preserves an abundant palynoflora that consists of fairly well preserved miospores and Chlorococcales (green algae). Miospores include bisaccate pollen grains of corystosperm and conifer affinities, monosulcate pollen grains of Cycadales and Ginkgoales, and spores of pteridophytes, lycophytes, and sphenophytes (Ottone et al., 2005; Ottone and Mancuso, 2006). This can be compared with the Ipswich microflora from Australia of Dolby and Balme (1976; Zavattieri and Batten, 1996; Zavattieri and Melchor, 1999; Ottone et al., 2005; Ottone and Mancuso, 2006). The preserved megaflora is very abundant including remains of ginkgophytes, sphenophytes, corystosperms, cycads, and pteridophyta taxa, and can be related to the Dicroidiumtype flora (Stipanicic and Bonaparte, 1979; Ottone et al., 2005; Ottone and Mancuso, 2006; Mancuso and Marsicano, 2008). On the other hand, the Cerro Puntudo Formation preserves very scarce palynomorphs that include regularly preserved spores of lycophytes and sphenophytes, and inaperturate pollen grains of araucarian affinity (Mancuso et al., 2006; Mancuso, 2007). The

156

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

Fig. 3. Taphofacies associated with the Los Rastros and Cerro Puntudo successions. (A) Well-preserved equisetalian remains included in Taphofacies LR-1. (B) Large Neocalamites stem included in the turbidity current deposit from Taphofacies LR-2, including the rose diagram of preferential orientation. (C) Agglomerate of Neocalamites stems included in the turbidity current deposits of Taphofacies LR-5. (D–E) Different preservational stages of plant detritus associated with Taphofacies LR-3. (F) Laminated dark mudrock shows abundant organic matter included in Taphofacies LR-4. (G–I) Cylindrical structures associated with rhizoliths of Taphofacies CP-1, superficial and lateral view. (J) Charcoalified plant detritus associated with Taphofacies CP-2. (K) Surface with plant detritus and rhizoliths included in Taphofacies CP-3. Arrows indicate rhizoliths.

megaflora consists of abundant rhizoliths associated with hydrophytic plants growing in the littoral zone of the lake, and rare, undeterminate foliar material (Mancuso et al., 2006; Mancuso, 2007). The plant record of these two units is similar in the presence of lycophytes, sphenophytes, and conifers, whereas the ginkgophytes, corystosperms, cycads, and pteridophyta are only present in Los Rastros Formation (Table 4). The restricted plant record in the Cerro Puntudo Formation suggests some special environmental conditions,

because, at the same time, an abundant and diverse flora was growing in other areas of Cuyana Basin (Stipanicic and Archangelsky, 2002a; Zavattieri, 2002). The difference in the plant-fossil content might be a consequence of environmental, depositional, and taphonomic conditions involved in the origin of both sites. In general, the plant remains may be divided into autochthonous, parautochthonous, and allochthonous elements, in relation to their growth position in and around the lake (Gastaldo et al., 1987; Martín-

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159 Table 4 Summary of megaflora and microflora recorded in Los Rastros and Cerro Puntudo Formations.

Chlorococcales Corystospermales Coniferales Cycadales Ginkgoales Pteridophyta Lycophyta Sphenophyta

Los Rastros Formation

Cerro Puntudo Formation

Microflora

Microflora

X X X X X X X X

Megaflora

Megaflora

X X X X X X

X X

X? X?

Closas, 1995). Autochthonous remains originated in the littoral zone. The parautochthonous remains originated in the deltaic plain, while the allochthonous remains originated in the upstream floodplain of the trunk fluvial system and were transported into the lake system. The assemblages may be also divided into autochthonous, parautochthonous, and allochthonous, based on distance of translocation of remains. According to this, the autochthonous assemblages are formed by aquatic plants (grown in the littoral zone) with no transport. The parautochthonous assemblages include material that was transported only a short distance, while the allochthonous assemblages consist of remains transported a long distance. The Los Rastros megafloral remains show signs of the effects of wind, water, and mass-flow transport processes (Fig. 4). For example, associated well-preserved and broken elements suggest both wind and water transport (e.g., LR-1); loosely sorted and fragmented material indicate that two consecutive kinds of transport (water and mass flow transport) may have occurred (e.g., LR-2). Thus, the Los Rastros plants were exposed to the three kinds of transport mechanisms, which enhanced their transport from the original growth setting to the final depositional environment. Because of these conditions, autochthonous assemblages are very rare in this unit; the plant detritus was, at least, transported for short distances before its deposition and formed parautochthonous assemblages. Consequently, the Los Rastros Lake allowed for the preservation of all plant detritus (autochthonous, parautochthonous, and allochthonous) in the delta and lake sediments as parautochthonous and allochthonous assemblages. In contrast, the Cerro Puntudo floral material shows loose sorting, an association with rhizoliths, and some anatomical connections that indicate an absence of transport or, at least, very limited water transport. Thus, the Cerro Puntudo assemblage consists of particularly autochthonous and parautochthonous assemblages. A combination of these two study cases is the La Cerdanya Lake (NE Spain), where plant detritus was preserved and exhibits characters including detritus without transport, located in a zone of autochthonous plant remains, and detritus transported by wind, water, and mass-flow, located in zones of transported plant remains (Martín-Closas, 1995). The Los Rastros succession is characterized by both kinds of depositional conditions. In general, the offshore lacustrine facies is dominated by dysaerobic conditions with a very low sedimentation rate, whereas the deltaic facies are dominated by aerobic conditions and a high sedimentation rate. In particular, the distal turbidity currents that form prodelta facies were characterized by aerobic conditions and a high sedimentation rate, while the deltaic swamps and interdistributary bay facies were characterized by dysaerobic conditions and a very low sedimentation rate. The Los Rastros plant remains are preserved at a low density in the offshore lacustrine facies and a high density in the deltaic swamps or interdistributary bay facies. But, in both facies, the remains form time-averaged assemblages. This is because the sedimentation rate is very low and preservation is mainly attributable to dysaerobic conditions. In contrast, the plants are preserved with moderate to low density in

157

the distal turbidity currents and high to moderate density in the delta front and plain. The preservation in these facies is mainly associated with aerobic conditions and a high sedimentation rate. The Cerro Puntudo succession is characterized by aerobic conditions with a low to high sedimentation rate. In particular, the lacustrine margin facies are dominated by a low sedimentation rate, but in the uppermost unit, they are dominated by high pyroclastic sedimentation. The Cerro Puntudo plants are preserved exclusively as rhizoliths in the aerobic subaqueous lacustrine margin facies that represent preservation under a low sedimentation rate. In contrast, the plants are preserved as foliar remains and rhizoliths where the sedimentation rate is high due to the high supply of pyroclastic material. The La Cerdanya case (NE Spain) shows a similar situation. In the deltaic zone with a high sedimentation rate the plant detritus was preserved even under aerobic conditions, while plant remains were preserved only in dysaerobic bottoms, independently from the sedimentation rate in the lake zone (Martín-Closas, 1995). 5. Summary and biotic reconstructions The Los Rastros offshore lacustrine area is dominated by autochthonous, well-preserved elements, and, to a lesser extent, allochthonous debris and wood, which form time-averaged

Fig. 4. Paleobiologic reconstruction of the Los Rastros (A) and Cerro Puntudo (B) plant communities and taphonomic pathways that lead to the recorded fossil occurrences.

158

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159

accumulations. The taphonomic characteristics observed in the studied fossils are a consequence of transport, deposition, and burial. In general, the autochthonous elements are present in larger proportions than allochthonous elements and with better preservation (Mancuso, 2003, 2005a; Mancuso and Marsicano, 2008). The Los Rastros delta is characterized by abundant allochthonous elements, with variable preservation according to their hydrologic features that determined their transport, usually showing evidence of mechanical degradation and accumulations over a shorter period of time (Mancuso, 2003, 2005a; Mancuso and Marsicano, 2008). The Cerro Puntudo Lake preserved only autochthonous elements including rhizoliths and foliar material; the former occurred in the littoral zone as vertical cylindrical structures with concentric infill. The foliar material is exclusively recorded at the top of the sequence where the pyroclastic material is the most dominant. The plant material forms autochthonous and parautochthonous assemblages at subaqueous lake margin facies, and foliar material and rhizoliths are found associated in the same horizons (Mancuso et al., 2006). In conclusion, the Los Rastros Lake preserved autochthonous and allochthonous material as a consequence of its anoxic conditions in the offshore lacustrine area and the high sedimentation rate in the delta (Mancuso, 2005a; Mancuso and Marsicano, 2008). In contrast, the Cerro Puntudo Lake preserved only autochthonous material in the subaqueous lake margin area, in spite of aerobic conditions. This was produced by the emplacement of tuff that enhanced the preservation (Mancuso et al., 2006). According to the paleoecologic studies of the Triassic Gondwana floristic associations by Anderson and Anderson (1995, 1998), the Los Rastros and Cerro Puntudo plant fossil assemblages, along with associated palynoflora, allow for the reconstruction of possible original communities from this part of Gondwana during the Middle Triassic (Fig. 4). Thus, the Los Rastros lake margins were characterized by shrubs and small trees of Ginkgoales and Corystospermales, and herbaceous sphenophytes. The Sphenophyta are also the dominant floral component along the river margins while the Corystospermales, Cycadales, Pteridophyta, and conifers formed the woodland upstream probably within the floodplains of an affluent fluvial system (Mancuso, 2005a; Mancuso and Marsicano, 2008). The littoral zone in the Cerro Puntudo Lake was mainly dominated by herbaceous sphenophytes and lycopsids (Mancuso et al., 2006). The Los Rastros and Cerro Puntudo lakes developed under the subtropical to temperate seasonal climate that was common in Gondwana during the Triassic. Although the paleoflora associated with each lake responded to a similar climate and environment, they developed distinct flora. The large, deep lake of Los Rastros preserved more extensive flora that included an evergreen forest, whereas the small, shallow lake of Cerro Puntudo preserved a more reduced flora associated with the lake margin only.

Acknowledgments I acknowledge Concha Arena (Universidad de Zaragoza), David Ferguson (University of Vienna), Robert Gastaldo (Colby College), Edith Taylor (University of Kansas), Elizabeth Gierlowski-Kordesch (Ohio University), Wolfgang Volkheimer (CCT-CONICET, Mendoza, Argentina), Guillermo Ottone (Universidad de Buenos Aires) and Tomás Heredia for the critical reading of the manuscript and their pertinent comments. Also, I especially thank Alberto Caselli, Claudia Marsicano, Guillermo Ottone, Silvia Barredo (Universidad de Buenos Aires), Andrea Arcucci (Universidad Nacional de San Luis), Wolfgang Volkheimer (CCT-CONICET, Mendoza, Argentina), and Tomás Heredia for their support during field work. Field research was supported by the PIP CONICET 0535/98 (A. Arcucci), PIP CONICET 5222/05 (W. Volkheimer), PIP CONICET 5120/ 05 (G. Ottone), Agencia No. PICT 32236, and the Jurassic Foundation

(A.C. Mancuso). Additional financial support was provided by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). References Anderson, J.M., Anderson, H.M., 1995. The Molteno Formation: window onto Late Triassic flora diversity. In: Pant, D.D. (Ed.), Proceedings of the International Conference on Global Environment and Diversification of Plant through Geological Time: Society of Indian Plant Taxonomists, Allahabad, India, pp. 27–40. Anderson, J.M., Anderson, H.M., 1998. Late Triassic ecosystems of the Molteno/Lower Elliot biome of Southern Africa. Palaeontology 41, 387–421. Behrensmeyer, A.K., Hook, R.W., 1992. Paleoenvironmental contexts and taphonomic modes in the terrestrial fossil record. In: Behrensmeyer, A.K., Damuth, J.D., DiMichele, W.A., Potts, R., Sues, H.D., Wing, S.L. (Eds.), Terrestrial Ecosystems Through Time: Evolutionary Paleoecology of Terrestrial Plants and Animals. University of Chicago Press, pp. 15–136. Bellosi, E.S., Jalfin, G.A., Bossi, G.E., Boggetti, D., Chebli, P., Murruaga, C., 2001. Facies y sedimentación. In: Artabe, A.E., Morel, E.M., Zamuner, A.B. (Eds.), El Sistema Triásico en la Argentina, Fundación Museo de La Plata “Francisco Pascasio Moreno”, La Plata, pp. 103–129. Brett, C.E., Baird, G.C., 1986. Comparative taphonomy: a key to paleoenvironmental interpretation based on fossil preservation. Palaios 1, 207–227. Cabrera, A., 1944. Dos nuevos peces ganoideos del triásico de Argentina. Notas del Museo de La Plata, 9, Paleontología 81, 569–576. Contreras, V.H., Bracco, A., Sill, W., 1997. Primer registro de restos de tetrápodos en la Formación Los Rastros (Triásico Medio), Parque Provincial Ischigualasto, San Juan, Argentina. Ameghiniana 34, 534. Dolby, J.H., Balme, B.E., 1976. Triassic palynology of the Canarvon Basin, Western Australia. Review of Palaebotany and Palynology 22, 105–168. Ferguson, D.K., 1985. The origin of leaf-assemblages — new light on an old problem. Review of Palaebotany and Palynology 46, 117–188. Frenguelli, J., 1945. Moluscos continentales en el Paleozoico Superior y en Triásico de la Argentina. Notas del museo de La Plata, Paleontología 10 (83), 180–204. Frenguelli, J., 1948. Estratigrafía y edad del llamado Rético en la Argentina. Anales de la Sociedad Argentina de Estudios Geográficos, Sociedad Argentina de Estratigrafía Geológica, GAEA 8, 159–310. Gallego, O.F., 1992. Conchóstracos Triásicos de Mendoza y San Juan, Argentina. Ameghiniana 29, 159–175. Gallego, O.F., 1997. Hallazgos de insectos Triásicos en la Argentina. Ameghiniana 34, 511–516. Gallego, O.F., 1999. Estudio Sistemático de las Faunas de Conchostracos Triásicos de la República Argentina. Ph.D. Thesis, Universidad Nacional de Córdoba, Córdoba, Argentina. Gastaldo, R.A., 1994. The genesis and sedimentation of phytoclasts with examples from coastal environments. In: Traverse, A. (Ed.), Sedimentation of Organic Particles. Cambridge University Press, pp. 103–127. Gastaldo, R.A., Douglass, D.P., McCarroll, S.M., 1987. Origin, characteristics, and provenance of plant macrodetritus in a Holocene crevasse splay, Mobile Delta, Alabama. Palaios 2, 229–240. Gastaldo, R.A., Ferguson, D.K., Walther, H., Rabold, J.M., 1996. Criteria to distinguish parautochthonous leaves in Tertiary alluvial channel-fills. Review of Palaeobotany and Palynology 91, 1–21. Gastaldo, R.A., Adendorff, R., Bamford, M., Lavandería, C.C., Neveling, J., Sims, H., 2005. Taohonomic trends of macroflora assemblages across the Permian–Triassic Boundary, Karoo Basin, South Africa. Palaios 20, 479–497. Hably, L., Szakmány, G., 2006. Prospects of flora and vegetation reconstructions dependent on sediment petrological conditions, exemplified by eight Egerian floras of Hungary. Géobios 39, 385–393. Heim, A., 1949. Estudio geológico del carbón “retico” y del valle de la Peña (provincias de San Juan y La Rioja). Boletín v. 69, Ministerio de Industria y Comercio de la Nación. Herbst, R., 1965. Algunos esporomorfos del Triásico de Argentina. Ameghiniana 4, 141–155. Herbst, R., 1970. Estudio palinológico de la Cuenca Ischigualasto–Villa Unión, (Triásico), provincias de San Juan–La Rioja. I. Introducción. II. Monoaperturados. Ameghiniana 7, 83–97. Herbst, R., 1972. Estudio palinológico de la Cuenca Ischigualasto–Villa Unión (Triásico), provincias. San Juan–La Rioja. III. Esporas triletes. Ameghiniana 9, 280–288. Kokogian, D.A., Mancilla, O., 1989. Análisis estratigráfico secuencial de la Cuenca Cuyana. In: Chebli, G.A., Spalletti, L.A. (Eds.), Las Cuencas Sedimentarias Argentinas. Tucumán, Universidad Nacional de Tucumán: Series de Correlación Geológica, 6, pp. 169–202. Kokogian, D.A., Spalletti, L.A., Morel, E.M., Artabe, A.E., Martínez, R.N., Alcober, O.A., Milana, J.P., Zavattieri, A.M., 2001. Estratigrafía del Triásico argentino. In: Artabe, A.E., Morel, E.M., Zamuner, A.B. (Eds.), El Sistema Triásico en la Argentina, Fundación Museo de La Plata “Francisco Pascasio Moreno”, La Plata, pp. 23–54. Krapovickas, V., Mángano, M.G., Mancuso, A., Volkheimer, W., 2006. Controles Paleoecológicos y Paleoambientales de una icnofauna continental Triásica, Formación Cerro Puntudo, Provincia de San Juan. 9°Congreso Argentino de Paleontología y Bioestratigrafía. Córdoba, Argentina. Krapovickas, V., Mángano, M.G., Mancuso, A.C., Marsicano, C.A., Volkheimer, W., 2008. Icnofaunas triásicas en abanicos aluviales distales: evidencias de la Formación Cerro Puntudo, Cuenca Cuyana, Argentina. Ameghiniana 45, 463–472. Labandeira, C.C., 2006. The tour phases of plant-arthropod associations in deep time. Geologica Acta 4, 409–438.

A.C. Mancuso / Sedimentary Geology 222 (2009) 149–159 López-Arbarello, A., Rogers, R., Puerta, P., 2006. Freshwater actinopterygians of the Los Rastros Formation (Triassic), Bermejo Basin, Argentina. Fossil Record 9, 238–258. López Gamundi, O.R., Astini, R., 2004. Alluvial fan–lacustrine association in the fault tip end of a half-graben, northern Triassic Cuyo basin, western Argentina. Journal of South American Earth Sciences 17, 253–265. Mancuso, A.C., 2002. Vertebrate taphonomy in a Triassic lake: tetrapods of the Los Rastros Formation (Ischigualasto–Villa Unión Basin, central-western Argentina). 8° Congreso Argentino de Paleontología y Bioestratigrafía, (Corrientes), Actas p. 35. Mancuso, A.C., 2003. Continental fish taphonomy: a case study in the Triassic of Argentina. Journal of South American Earth Sciences 16, 275–286. Mancuso, A.C., 2005a. Tafonomía en ambientes lacustres: estudio integral de las asociaciones fósiles de las secuencias lacustres del Triásico Medio de la Cuenca de Ischigualasto–Villa Unión (formaciones Chañares, Los Rastros e Ischichuca). Ph.D. Thesis, Universidad de Buenos Aires, Buenos Aires, Argentina, 208 p. Mancuso, A.C., 2005b. Revisión y aportes a la estratigrafía de la sección inferior del Grupo Agua de la Peña (Triásico Medio, Argentina). XVI Congreso Geológico Argentino, La Plata, Argentina: Actas, vol. 3, pp. 415–422. Mancuso, A.C., 2007. Taphonomic analysis in lacustrine environment: two very different Triassic lake paleoflora contexts from Western Gondwana. 4° International Limnogeology Congress, Barcelona, España, pp. 180–181. Mancuso, A.C., Marsicano, C.A., 2008. Paleoenvironments and Taphonomy of a Triassic Lacustrine System (Los Rastros Formation, Central-Western Argentina). Palaios 23, 535–547. Mancuso, A.C., Ottone, E.G., Volkheimer, W., 2006. Tafonomía en ambientes lacustres: el caso particular de una paleoflora triásica de Precordillera. 9°Congreso Argentino de Paleontología y Bioestratigrafía. Córdoba, Argentina. Mancuso, A.C., Gallego, O., Martins-Neto, R.G., 2007. The Triassic Insect Fauna from the Los Rastros Formation (Bermejo Basin), La Rioja Province (Argentina): their context, taphonomy and paleobiology. Ameghiniana 44, 337–348. Marsicano, C.A., Arcucci, A., Mancuso, A.C., Caselli, A., 2004. Middle Triassic tetrapod footprints of southern South America. Ameghiniana 41, 171–184. Marsicano, C., Domnanovich, N., Mancuso, A.C., 2007. Dinosaur origins: evidence from the footprint record. Historical Biology 19, 83–91. Martín-Closas, C., 1995. Plant taphonomy of La Cerdanya Basin (Vallesian, Eastern Pyrenees). Géobios 18, 287–298. Martín-Closas, C., Gomez, B., 2004. Taphonomie des plantes et interpretations paléoécologiques. Une synthèse. Géobios 37, 65–88. Martín-Closas, C., Galtier, J., 2005. Plant taphonomy and paleoecology of Late Pennsylvanian intermontane wetlands in the Graissessac-Lodève Basin (Languedoc, Frances). Palaios 20, 249–265. Martins-Neto, R.G., Mancuso, A.C., Gallego, O., 2005. The Triassic insect fauna from Argentina. Blattoptera from the Los Rastros Formation (Bermejo Basin), La Rioja Province. Ameghiniana 42, 705–723. Martins-Neto, R.G., Gallego, O., Mancuso, A.C., 2006. The Triassic insect fauna from Argentina. Coleoptera from Los Rastros Formation (Bermejo Basin), La Rioja province. Ameghiniana 43, 591–609. Melchor, R.N., 2004. Trace fossil distribution in lacustrine deltas: example from the Triassic rift lake of the Ischigualasto–Villa Unión basin, Argentina. In: McIlroy, D. (Ed.), The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis: Geological Society, London, Special Publications, vol. 228, pp. 335–354. Melchor, R.N., Bellosi, E.S., Genise, J.F., 2003. Invertebrate and vertebrate trace fossils from a Triassic lacustrine delta: the Los Rastros Formation, Ischigualasto Provincial Park, San Juan, Argentina. In: Buatois, L.A., Mángano, M.G. (Eds.), Icnología: Hacia una Convergencia entre Geología y Biología, Asociación Paleontológica Argentina, Publicación Especial, vol. 9, pp. 17–33. Mombrú, C., 1973. Observaciones geológicas en el Valle Calingasta–Tocota, Provincia de San Juan. Informe Inédito YPF. Nichols, G.J., Cripps, J.A., Collinson, M.E., Scott, A.C., 2000. Experiments in waterlogging and sedimentology of charcoal: results and implications. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 43–56. Olsen, P.E., 1990. Tectonic, climatic, and biotic modulation of lacustrine ecosystems — examples from Newark Supergroup of eastern North America. In: Katz, B.J. (Ed.), Lacustrine Basin Exploration-Case Studies and Modern Analogs: AAPG Memoir, vol. 50, pp. 209–224. Ottone, E.G., Mancuso, A.C., 2006. Algas Chlorococcales como indicadores paleoambientales: nuevos datos de la Formación Los Rastros, Triásico Medio a Superior del centro-oeste de Argentina. Revista del Mueso Argentino de Ciencias Naturales Bernardino Rivadavia 8, 209–220. Ottone, E.G., Mancuso, A.C., Resano, M., 2005. Miospores and Chlorococcales from the Los Rastros Formation, Middle Triassic of central- western Argentina. Ameghiniana 42, 347–362.

159

Owen, R.A., Owen, R.B., Renaut, R.W., Scott, J.J., Jones, B., Ashley, G.M., 2008. Mineralogy and origin of rhizoliths on the margins of saline, alkaline Lake Bogoria, Kenya Rift Valley. Sedimentary Geology 203, 143–163. Retallack, G.J., Duglas, D.P., Bestland, E.A., 1990. Fossil soils and grasses of a middle Miocene East African grassland. Science 247, 1325–1328. Rodríguez-Aranda, J.P., Calvo, J.P., 1998. Trace fossils and rhizoliths as a tool for sedimentological and palaeoenvironmental analysis of ancient continental evaporate successions. Palaeogeography, Palaeoclimatology, Palaeoecology 140, 383–399. Scott, A.C., Anderson, J.M., Anderson, H.M., 2004. Evidence of plant–insect interactions in the Upper Triassic Molteno Formation of South Africa. Journal of the Geological Society 161, 401–410. Sessarego, H.L., 1988. Estratigrafia de las secuencias epiclasticas devónicas a triásicas aflorantes al norte del río San Juan y al oeste de las sierras del Tigre, Provincia de San Juan. Ph.D. Thesis Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires. 308 p. Spalletti, L.A., 2001. Evolución de las cuencas sedimentarias. In: Artabe, A.E., Morel, E.M., Zamuner, A.B. (Eds.), El Sistema Triásico en la Argentina, Fundación Museo de La Plata “Francisco Pascasio Moreno”, La Plata, pp. 81–101. Spalletti, L.A., Artabe, A., Morel, E., Brea, M., 1999. Biozonación paleoflorística y cronoestratigrafía del Triásico argentino. Ameghiniana 36, 419–451l. Spalletti, L.A., Fanning, C.M., Rapela, C.W., 2008. Dating the Triassic continental rift in the southern Andes: the Potrerillos Formation, Cuyo basin, Argentina. Geologica Acta, 6, 267–283. Speyer, S.E., Brett, C.E., 1986. Trilobite taphonomy and Middle Devonian taphofacies. Palaios 1, 312–327. Spicer, R.A., 1989. The formation and interpretation of plant fossil assemblages. Advances in Botany Research 16, 95–191. Spicer, R.A., 1991. Plant taphonomic processes. In: Allison, P.A., Briggs, D.E.G. (Eds.), Taphonomy: Releasing the Data Locked in the Fossil Record. InPlenum, New York, pp. 72–113. Spicer, R.A., Wolfe, J.A., 1987. Plant taphonomy of late Holocene deposits in Trinity (Clair Engle) Lake, northern California. Paleobiology 13, 227–245. Stipanicic, P.N., Bonaparte, J., 1979. Cuenca triásica de Ischigualasto–Villa Unión (Provincia de La Rioja y San Juan). Geología Regional Argentina, Academia Nacional de Ciencias, Córdoba 1, 523–575. Stipanicic, P.N., Archangelsky, S., 2002a. Megafloras. In: Stipanicic, P.N., Marsicano, C. (eds.), Léxico Estratigráfico de la Argentina: TRIÁSICO, Volúmen VIII, Serie ”B” (Didáctica y Complementaria), Asociación Geológica Argentina, Buenos Aires, pp. 309–313. Stipanicic, P.N., Marsicano, C.A., 2002b. Léxico Estratigráfico de la Argentina: TRIÁSICO, Volúmen VIII, Serie ”B” (Didáctica y Complementaria), Asociación Geológica Argentina, Buenos Aires, 370 p. Strelkov, E., Alvarez, L., 1984. Análisis estratigráfico y evolutivo de la cuenca triásica mendocina–sanjuanina. 9° Congreso Geológico Argentino, San Carlos de Bariloche: Actas, vol. 3, pp. 115–130. Talbot, M.R., Allen, P.A., 1996. Lakes. In: Reading, H.G. (Ed.), Sedimentary Environments: Processes, Facies and Stratigraphy. Blackwell Science, pp. 5–36. Uliana, M.A., Biddle, K., 1988. Mesozoic–Cenozoic paleogeographic and geodynamic evolution of southern South America. Revista Brasileira de Geociências 18, 172–190. Uliana, M.A., Biddle, K.T., Cerdan, J., 1989. Mesozoic extension and the formation of Argentine sedimentary basins: AAPG. Memoir 46, 599–614 Tulsa. von Huene, F., 1931. Die fossilien Fährten im Rhät von Ischigualasto in Nordwest– Argentinien. Palaeobiologica 4, 99–112. Wilson, M.V.H., 1988. Reconstruction of ancient lake environments using both autochthonous and allochthonous fossils. Palaeogeography, Palaeoclimatology, Palaeoecology 62, 609–623. Yrigoyen, M.R., Stover, L.E., 1970. La palinología como elemento de correlación del Triásico en la Cuenca Cuyana. 4° Jornadas Geológicas Argentinas, (Mendoza): Actas, vol. 2, pp. 427–447. Zavattieri, A.M., 2002. Microfloras. In: Stipanicic, P.N., Marsicano, C. (eds.), Léxico Estratigráfico de la Argentina: TRIÁSICO, Volúmen VIII, Serie ”B” (Didáctica y Complementaria), Asociación Geológica Argentina, Buenos Aires, 318–321. Zavattieri, A.M., Batten, D.J., 1996. Miospores from Argentinian Triassic deposits and their potential for intercontinental correlation. In: Jansonius, J., Mc Gregor, D.C. (Eds.), Palynology: Principles and Applications, vol. 2. American Association of Stratigraphic Palynologists Foundation, pp. 767–778. Zavattieri, A.M., Melchor, R.N., 1999. Estudio palinológico preliminar de la Formación Ischichuca (Triásico), en su localidad tipo (quebrada de Ischichuca Chica), provincia de La Rioja, Argentina. Asociación Paleontológica Argentina, Publicación Especial 6, 33–38.