Palaeolimnology of the Santa Clara Abajo Formation, Triassic of the Cuyana Basin Argentina, inferred from fish taphonomy

Abstract

The Santa Clara Abajo Formation in the north of Mendoza Province, Argentina, is part of the Triassic Cuyana rift Basin infill and consists of fluvial, deltaic and lacustrine units. The combination of progradational to aggradational stacking pattern plus the identification of a fluctuating profundal facies association suggests the Santa Clara Abajo palaeolake was a balanced-fill lake system. Lake-centre sediments (finely laminated mudrock facies) preserve pseudobeaconiid actinopterygian fish represented primarily by bones and scales, generally undeformed and concordant with bedding planes. We propose that variation of articulation degree of these fish combined with sediment features provides insights into palaeolimnological changes in this single lake system over time, primarily water temperature. The fish fossils are sorted into three taphonomic modes: Mode A, isolated and dispersed scales; Mode B, associated but dispersed scales, loosely to well sorted, and low to moderate density; and Mode C, scale patches and articulated fishes. We conclude these modes represent, respectively, warm, fluctuating warm-cold and cold water conditions. Stratigraphical analysis of the taphonomic modes for the balanced-fill lake model revealed an up-section pattern of fluctuating water temperatures (cold and warm), to a stable-cold pattern, to a stable-warm pattern and to fluctuating thermal conditions (cold and warm) again. This pattern is linked to lake-level fluctuations resulting in lacustrine intervals that reflect hydrological closure to opening to closure again. □ Actinopterygii, lake-basin-type model, Mendoza, Santa Clara sub-basin, Triassic.

Macro- And Micromorphology of Scales from an Endemic South American Actinopterygian Family (Pseudobeaconiidae, Triassic, Cuyana Basin)

“…The Santa Clara Arriba Formation is mainly represented by thin clayey to tuffaceous finegrained sandstones that alternate with laminated mudstones and siltstones (Harrington, 1971). The palaeoenvironment of this unit has been interpreted as a low gradient deltaic-lacustrine system (Benavente et al, 2018;2020).…”

“…New fossil material of Pseudobeaconiidae was collected in fieldwork during 2015-2016 at the Santa Clara Abajo Formation. These specimens are deposited in the IANIGLA collection and they only have been studied from a taphonomic perspective before 6 (Benavente et al, 2020). Moreover, the morphological information about scales of family Pseudobeaconiidae is not complete, especially the one related to micromorphology, which has remained unknown until now.…”

14th Symposium on Mesozoic Terrestrial Ecosystems and Biota

“…In both basins, there are lacustrine sequences assigned to each different category of lake basin type. In the Cuyana Basin, the Agua de la Zorra and Cacheuta Formations (Upper Triassic) have been described as overfilled lakes, the Cerro de las Cabras Formation (Middle Triassic) as underfilled, and Santa Clara Abajo Formation (Middle Triassic) as balanced fill lake (Benavente et al, 2020(Benavente et al, , 2021Pedernera et al, 2021). On the other hand, the Los Rastros Formation (Upper Triassic, Ischigualasto-Villa Uni on Basin) has been described as an overfilled lake .…”

“…Many proxies can be studied from a lake basin to unravel parts of the ecosystem, one of them is the paleontological content. Fish remains and their preservation can provide information regarding conditions of the water column they inhabited (Benavente et al, 2020). Evolutionary questions such as speciation patterns and their causes can also be studied (McCune, 2004).…”