Sedimentary organic matter from a cored Early Triassic succession, Georgetown (Idaho, USA)

Elke Schneebeli-Hermann,Marc Leu,Torsten Vennemann,Hugo Bucher,Borhan Bagherpour

doi:10.1186/s13358-020-00205-9

Elke Schneebeli-Hermann, Marc Leu + Show 3 more

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https://doi.org/10.1186/s13358-020-00205-9

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Abstract

The plant fossil record from Lower Triassic sedimentary successions of the Western USA is extremely meager. In this study, samples from a drill core taken near Georgetown, Idaho, were analyzed for their palynological content as well as their stable carbon isotope composition. The concentration of palynomorphs is generally low. The lowermost part of the drilled succession represents Dinwoody/Woodside Formation and contains spore and pollen assemblages with Permian and Early Triassic affinity. Representatives of lycophytes (Densoisporites spp., Lundbladisporites spp.) were found in the overlying Meekoceras Limestone, in agreement with middle Smithian assemblages elsewhere. Ammonoids and conodonts are extremely rare, but confirm a middle Smithian age. Bulk organic and carbonate carbon isotope composition provide a stratigraphic framework. Carbonate carbon isotope compositions are compatible with the Smithian–Spathian global trend, with a middle Smithian shift towards lower δ13C values followed by a late Smithian shift towards higher values. Bulk organic carbon isotope compositions have been influenced by changes in the constitution of organic matter. A comparison with other paired carbon isotope datasets from the same basin is difficult due to lithostratigraphic inconsistencies (Hot Springs, ID) or biochemical mediated disturbance of isotope signals (Mineral Mountains, UT).

Highlights

In the western United States, plant fossils from sediments straddling the Permian–Triassic boundary are scarce (McKee 1954), reconstructing the vegetation history during this period is impossible for this region
The scarcity of late Permian–Early Triassic plant macro- and microfossils contrasts with the rich plant fossil records from the Late Triassic Chinle Formation in the southern
Even though the data is scares, spore and pollen grains prove the presence of different plant groups in Western United States, which have further implications for palaeopyhtogeographic and climatic considerations

Summary

Introduction

In the western United States, plant fossils from sediments straddling the Permian–Triassic boundary are scarce (McKee 1954), reconstructing the vegetation history during this period is impossible for this region. Younger Permian fossiliferous deposits are not known from the Western United States (Collinson et al 1976; Wardlaw et al 1995). Besides the rich plant fossil record of the Late Triassic Chinle Formation, the oldest Mesozoic plant fossils have been described from the Lower–Middle Triassic Moenkopi Group. This formation stretches from SE Utah, New Mexico, Arizona and interfingers with the Thaynes

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