Depositional environment and taphonomy of marine vertebrate biofacies in the lower Cretaceous (Albian) thermopolis shale, South-Central Montana
Lash, Catherine Eileen
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In south-central Montana, west of the Pryor Mountain Range, the Lower Cretaceous Thermopolis Formation contains an unusual abundance of marine vertebrate fossils which comprises the majority of the coarse-grained material in an otherwise fine-grained, marine deposit. The fossil zone lacks invertebrates and contains predominantly marine vertebrate skeletal, tooth, and coprolitic material, including; plesiosaur, marine crocodile, shark, freshwater turtle, ray, saw fish, and boney fish. Four lithofacies associations (depositional packages) represent fluctuations of relative sea level within the Western Interior Cretaceous Seaway during Albian time and correspond to the four designated members of the Thermopolis Formation ('rusty beds", Lower Thermopolis, Sandy, and Upper Thermopolis Members). Deposition of the Thermopolis Formation in south-central Montana occurred within a persistent marine basin even during the lowstand event. Two distinct marine vertebrate bioclastic assemblages occur near the middle of the Thermopolis Formation, at the base of the Upper Thermopolis Member, within a 5.6 m thick zone deposited above a the lowstand event. The first fossil assemblage consists of relatively large, isolated and articulated skeletal elements that are generally dispersed throughout the fossil zone within a structureless claystone/mudstone matrix. This assemblage records a low energy, offshore environment (condensed section) with skeletal accumulation primarily driven by suspension settling of biological components and represents an autochthonous to parautochthonous assemblage. The second fossil assemblage consists of relatively small, fragmented bioclasts that are highly concentrated within very thin, cross-laminated litharenite and sandy volcaniclastic ash lenses. These lenses represent high energy event bed deposition resulting from storm-induced flows that entrained and transported previously accumulated fossil material from a nearshore to an offshore environment and represent an allochthonous assemblage. The coarse-grained bioclasts of the second assemblage are transported and therefore do not represent a correlative transgressive lag as previously thought. The second fossil assemblage occurs within the base of the marine vertebrate fossil zone and is surrounded by the first fossil assemblage; both fossil assemblages are time-averaged and occur within a condensed section. Therefore, syndeposition of these two fossil assemblages occurred within the same offshore depositional environment and represent only a change in depositional energy, not a change in overall depositional setting.