Nautiloid cephalopods from the Rickard Hill facies of the Saugerties Member of the Schoharie Formation, eastern New York, USA (late Emsian, Devonian): A case study in taphonomy from glacial erratics

[摘要] Glacial erratics belonging to the Rickard Hill facies (RHf) of the Saugerties Member of the Schoharie Formation (late Emsian, Devonian) occur scattered throughout the Lower Hudson Valley of New York and northern New Jersey Piedmont. These glacial erratics are most similar to facies exposed ~200 km north in the Helderberg Mountains region of eastern New York. The RHf glacial erratics contain a concentrated assemblage of well-preserved nautiloid cephalopods dominated by large orthoconic and coiled forms. These nautiloids are exposed along bedding planes as a result of a complex sequence of physical and chemical weathering during transport within the Hudson-Champlain Lobe of the Laurentide Ice Sheet and deposition within acidic soils of regional ground moraines. Weathering also reveals taphonomic details within body chambers and phragmocones of these nautiloids that are not readily observable in outcrop exposures of the RHf. Some nautiloids display similar orientations on bedding surfaces while others contain casts and molds of invertebrates including trilobites, brachiopods, and corals that are preserved within and adjacent to body chambers and phragmocones. Comparison of glacial erratic samples to the bedrock source indicates that the large RHf nautiloids represent a post-mortem lag assemblage occurring in the same locality and depositional environment inhabited by the living animals. This post-mortem lag deposit occurs at the boundary between third order eustatic sea level cycles Emsian 5 and Eifelian 1 and accumulated as part of a shallowing upward cycle bounded below and above by the sub-Aquetuck and sub-Edgecliff unconformities. Nautiloids and other invertebrate fauna were concentrated during multiple exhumation and reburial events where localized wave base was capable of eroding into the shallow shelf platform in this area of eastern New York.

[学科分类] 地质学

[关键词] Martin A. Becker;Harry M. Maisch IV;Rebecca A. Chamberlain;John A. Chamberlain Jr.;Christi G. Kline;Clint F. Mautz;Rickard Hill facies;late Emsian;glacial erratics;nautiloid taphonomy