Fish taphonomy in lacustrine environment: Los Rastros and Cacheuta formations (Middle Triassic, Argentina)

MANCUSO, ADRIANA CECILIA

Mendoza

Congreso; 18 International Sedimentological Congress; 2010

Two important lacustrine successions were housed in Triassic extensional basins at Center-Western Argentina. The Los Rastros Formation (Ischigualasto –Villa Unión Basin) is a lacustrine-deltaic succession that consists of several coarsening-upward cycles of black shales, siltstones and sandstones. The Cacheuta Formation (Cuyana Basin) is a lacustrine succession dominated by black shales and silstones. The fish records from these two lacustrine sequences were evaluated within environmental context and contrasted. The Los Rastros fishes are found as isolated scales and disarticulated cranial bones (fragmentary and/or complete), clumps of scales, body fragments (articulated scales), and complete specimens, occasionally without heads. The Cacheuta fishes are found as isolated scales and disarticulated cranial bones, clumps and stringer of scales, body fragments (articulated scales and/or fins) and complete specimens. The taphonomic features allow us to define three taphofacies for Los Rastros fishes (LR-A to LR-C) and four taphofacies for Cacheuta fishes (Ca-A to Ca-D). A Principal Component Analysis was performed with the taphonomic features, supporting the seven taphofacies defined. The LR-A taphofacies, characterized by scales and bones isolated and dispersed or in clumps, and well sorted, is found in open lake facies. The LR-B taphofacies, characterized by densely packed, partially articulated remains, and poorly sorted, is related to distal turbidity current facies. The LR-C taphofacies, characterized by partially to totally articulated remains, densely to loosely packed, and poorly sorted, is found in mouth-bar deltaic facies. The Ca-A taphofacies, characterized by scales and bones isolated and dispersed and well sorted, is found in open lake facies. The Ca-B taphofacies, dominated by scales and bones associated but dispersed, forming clumps and stringer, concordant to stratification, loosely-packed, and well sorted, is related to underflows facies. The Ca-C taphofacies, characterized by scales and bones associated but dispersed, forming clumps with bioclast cutting the stratification, densely-packed, loosely sorted, is found in distal turbidity current facies. The Ca-D taphofacies, dominated by articulated fish, concordant to stratification, dispersed, and poorly sorted, is related to open lake facies. Thus, the fish features reveal their taphonomic history. The isolated and dispersed remains found in open lake facies reached the bottom after suffering flotation-decay in both Los Rastros and Cacheuta lakes (LR-A and Ca-A). The articulated fish found in open lake facies of Cacheuta lake (Ca-D) reached the bottom intact without suffering flotation-decay, predation or scavenging. The scales and bones associated but dispersed found in Cacheuta underflows facies (Ca-B) reached the bottom after suffering flotation-decay and rework by laminar flow. The scales and bones associated but dispersed found in turbidity current facies reached the bottom after suffering flotation-decay and rework by turbulent flow in both Los Rastros and Cacheuta lakes (LR-B and Ca-C). Finally, the articulated remains found in Los Rastros mouth-bar deltaic facies (LR-C) reached the delta intact without suffering flotation-decay, predation or scavenging. The Cacheuta Lake preserved abundant fish remains, in contrast to Los Rastros Lake. In both cases, flotation-decay process lead mainly to fish disarticulation. Only in Cacheuta Lake articulated fish occur in open lake facies, whereas in Los Rastros Lake these occur in delta facies. The fish assemblage studied allows us to suggest that Los Rastros fish were allochthonous to the lake and probably inhabiting the affluent fluvial system, whereas the Cacheuta fish represent an abundant autochthonous lacustrine community of primarily pelagic and nektonic organisms.