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

MANCUSO, ADRIANA CECILIA

SEDIMENTARY GEOLOGY

Elsevier

Año: 2009 vol. 222 p. 149 - 159

0037-0738

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 paleontological 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 paleontological 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 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. On the one hand, 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 was 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 analyzed 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 show a characteristic paleoflora associated to lacustrine systems. The full understanding of the processes related to their generation is essential to better compare with other records, as Australian and South African, which together with Argentinian record represent most of the paleobotanical fossil content in continental Middle Triassic Gondwana.