Deposition and taphonomy of the Hound Island Late Triassic vertebrate fauna: Fossil preservation within subaqueous gravity flows

Abstract

Late Triassic marine vertebrates occur in the Hound Island Volcanics exposed on the eastern shore of Hound Island, southeastern Alaska. This fossil assemblage derived from slope deposits within an intra-arc basin of a volcanic island arc complex. It is associated with the allochthonous Alexander Terrane prior to its northern translation from tropical latitudes to its current latitude. Fossils occur within a 13-m-thick succession of interbedded calcareous shale and volcaniclastic-rich bioclastic limestone. The limestone layers are skeletal (vertebrate and invertebrate) packstones to wackestones and are interpreted as episodic thin-bedded turbidite and debris flow deposits that resulted from redeposition of slope sediments. Forty-seven vertebrate fossils were collected that consist of complete and fragmented cranial and postcranial elements belonging to Eosauropterygia, Thalattosauria, and Ichthyosauria, including the genus Shonisaurus. Bones are frequently disassociated and show predepositional abrasion and breakage. Another less common taphonomic mode is observed, characterized by several large associated elements, representing a single individual, found parallel to bedding and draped by shale. Rare Osteichthyes and Chondrichthyes teeth are also present. Bivalve and conodont fossils indicate a middle Norian age for the assemblage.