Paleoenvironment and Taphonomy of the Chan? ares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions

ROGERS, R. R.; ARCUCCI, A. B.; ABDALA, F.; SERENO, P. C.; FORSTER, C. A.; MAY, C. L.

PALAIOS

SEPM-SOC SEDIMENTARY GEOLOGY

Lugar: Lawrence; Año: 2001 vol. 16 p. 461 - 481

0883-1351

The enigmatic concretionary exposures that typify the Chan? ares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization.Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ashflow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chan? ares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recordedby the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidencedby intercalated bentonite beds. Volcanism played an important role in the generationand preservation of the Chan? ares Formation?s exceptional tetrapod fossil record. This is especially true of the classic Los Chan? ares locality, where more than 100 individualsrepresenting a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chan? ares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counterintuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chan? ares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led tocatastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonateconcretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused  into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record.