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

Abstract

Other| October 01, 2001 Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions RAYMOND R. ROGERS; RAYMOND R. ROGERS 1Department of Geology, Macalester College, St. Paul, MN 55105 Search for other works by this author on: GSW Google Scholar ANDREA B. ARCUCCI; ANDREA B. ARCUCCI 2Museo de Paleontologia, Instituto de Anthropologia, Universidad Nacional de La Rioja, Avenida Ortiz de Ocampo 1700, 5300 La Rioja, Argentina Search for other works by this author on: GSW Google Scholar FERNANDO ABDALA; FERNANDO ABDALA 3Laboratorio de Paleontologia, Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Avenida Ipiranga 6681, Porto Alegre (90619-900), Brazil Search for other works by this author on: GSW Google Scholar PAUL C. SERENO; PAUL C. SERENO 4Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637 Search for other works by this author on: GSW Google Scholar CATHERINE A. FORSTER; CATHERINE A. FORSTER 5Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794 Search for other works by this author on: GSW Google Scholar CATHLEEN L. MAY CATHLEEN L. MAY 6Institute for Environmental Education, Geological Society of America, 3300 Penrose Place, Boulder, CO 80301 Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (5): 461–481. https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Article history accepted: 30 Mar 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation RAYMOND R. ROGERS, ANDREA B. ARCUCCI, FERNANDO ABDALA, PAUL C. SERENO, CATHERINE A. FORSTER, CATHLEEN L. MAY; Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions. PALAIOS 2001;; 16 (5): 461–481. doi: https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The enigmatic concretionary exposures that typify the Chañares 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. Ash- flow 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 Chañares 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 recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds.Volcanism played an important role in the generation and preservation of the Chañares Formation's exceptional tetrapod fossil record. This is especially true of the classic Los Chañares locality, where more than 100 individuals representing 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 Chañares 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 Chañares 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 to catastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.