Schneebeli‑Hermann et al. Swiss J Palaeontol (2020) 139:5 https://doi.org/10.1186/s13358‑020‑00205‑9 Swiss Journal of Palaeontology RESEARCH ARTICLE Open Access Sedimentary organic matter from a cored Early Triassic succession, Georgetown (Idaho, USA) Elke Schneebeli‑Hermann1* , Borhan Bagherpour1,2* , Torsten Vennemann3, Marc Leu1 and Hugo Bucher1 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 afnity. Representatives of lycophytes (Densoisporites spp., Lundbladisporites spp.) were found in the overlying Meekoceras Limestone, in agreement with middle Smithian assemblages elsewhere. Ammo‑ noids and conodonts are extremely rare, but confrm 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 infuenced by changes in the constitution of organic matter. A comparison with other paired carbon isotope datasets from the same basin is difcult due to lithostratigraphic inconsistencies (Hot Springs, ID) or biochemical mediated disturbance of isotope signals (Mineral Mountains, UT). Keywords: Palynology, Particulate organic matter, Stable carbon isotopes Introduction part of this area (New Mexico, Arizona) (Ash 1972; In the western United States, plant fossils from sediments Litwin 1985; Litwin et al. 1991; Lindström et al. 2016). straddling the Permian–Triassic boundary are scarce Approaching the Permian–Triassic boundary from (McKee 1954), thus reconstructing the vegetation his- the Palaeozoic, the youngest Palaeozoic plant macrofos- tory during this period is impossible for this region. Te sils are known from the mid-Permian Blaine Formation scarcity of late Permian–Early Triassic plant macro- and in Texas (DiMichele et al. 2004). From Roadian–Word- microfossils contrasts with the rich plant fossil records ian deposits of the Park City and Phosphoria formations from the Late Triassic Chinle Formation in the southern in NE Utah a number of acritarchs have been described (Jacobson et al. 1982). Younger Permian fossiliferous deposits are not known from the Western United States (Collinson et al. 1976; Wardlaw et al. 1995). Besides the Editorial handling: Susanne Feist‑Burkhardt. rich plant fossil record of the Late Triassic Chinle For- *Correspondence: [email protected]; [email protected] mation, the oldest Mesozoic plant fossils have been 1 Paleontological Institute and Museum, University of Zurich, Karl Schmid‑Str. 4, 8006 Zurich, Switzerland described from the Lower–Middle Triassic Moenkopi 2 Department of Earth Sciences, Faculty of Sciences, Shiraz University, Group. Tis formation stretches from SE Utah, New Shiraz, Iran Mexico, Arizona and interfngers with the Taynes Full list of author information is available at the end of the article © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. 5 Page 2 of 16 E. Schneebeli‑Hermann et al. Group towards the NW (Idaho) and the Chugwater in Morales 1987). However, their identifcation was ques- Group to the N (Montana) (Kummel 1954, 1957; Stew- tioned (Litwin and Ash in Morales 1987). art et al. 1972; Lucas et al. 2007) (Fig. 1). Since the early During the Early Triassic the climate was suggested twentieth century poorly preserved plant remains, to have been globally infuenced by the Pangean mega- mostly impressions and a few petrifed stem fossils, were monsoon (Parrish 1993; Miller and Baranyi 2019). Te known from the upper part of the Moenkopi Group (late Sonoma basin was situated on the western Pangean mar- Spathian–Anisian) (McKee 1954; Morales 1987) includ- gin close to the equator. Continent interiors were arid to ing sphenophytes and plant remains resembling the semi-arid, but not necessarily stable throughout the Early Palaeozoic conifer genus Walchia spp. (Gregory 1917; Triassic, various studies present evidence for changes in McKee 1954). Ash and Morales (1993) described Anisian temperature and precipitation (Preto et al. 2010). Te plant assemblages from the Holbrook Member of the formation of red beds during the Early Triassic indicates Moenkopi Group of Arizona. Tese belong to the old- at least seasonal precipitation (van Houten 1973; Parrish est known Mesozoic megafora in North America. Te 1993). During the Triassic, the strength of the monsoonal fora is composed of fungi, pith casts of Neocalamites sp., systems may have enabled circulation patterns that are petrifed fern trunks, and conifer wood resembling Arau- drawing moisture from the equatorial Panthalassa onto carioxylon arizonicum (Ash and Morales 1993). From the western coast of Pangea (Parrish 1993). Tis provided the late Early Triassic (?) Wupatki Member only spheno- conditions for red bed formation, and rich plant commu- phyte stems are reported (Morales 1987). Benz (1980; in nities during the Late Triassic. Morales 1987) was the frst to document palynomorphs Despite the dearth of plant fossils, the lithologies of from late Spathian to Anisian coprolites of the Moenkopi the western United States are well known for their Early Group in Arizona. Te spores and pollen grains were Triassic marine fauna including ammonoids, conodonts, poorly preserved and were assignable to Gnetales, Cor- bivalves, and gastropods (Kummel 1943, 1954; McKee daitales, seed ferns, sphenophytes and ferns (Benz 1980 1954; Müller 1956; Ware et al. 2011; Brayard et al. 2013). A B 43° K WASHINGTON MONTANA NORTH DAKOTA MINNESOTA J CARIBOU COUNTY WISCONSIN Tr MICHIGAN OREGON SOUTH DAKOTA Soda IDAHO ? Springs WYOMING IOWA CP OHIO NEBRASKA INDIANA ILLINOIS George NC~ town NEVADA BEAR LAKE MISSOURI COUNTY UTAH ? UNCOM ~~CO KANSAS 89 FRONT RANGE P A HIGHLAND GRE HI TENNESSEEMontpelier COLORADO 30 G CALIFORNIA HLA ND OKLAHOMA Paris ARKANSAS ? Dayton Ne ALABAMA O~SDMISSISSIPPI ? Preston NEW MEXICO Bear Hot Springs Lake MOGOLLO 91 N HIGHLAN Weston FRANKLIN COUNTY D 42° ARIZONA 112° 111° TEXAS 30 0 LOUISIANA 40 Georgetown Core, Idaho Hot Springs, Idaho (Caravaca et al., 2018) Mineral Mountains, Utah (Thomazo et al., 2016) Fig. 1 a Map of the Western USA (based on Google maps). The Sonoma basin is indicated after Caravaca et al. (2018), direction of the Smithian transgression after Vennin et al. (2015) and Brayard et al. (2013). Extent of the Moenkopie and Chugwater groups (pale area) as well as sediment transport directions after Stewart et al. (1972). b Geological map of the Bear Lake area based on Lewis et al. (2012) (Geological map of Idaho 1:750,000) Early Triassic organic matter Georgetown, Idaho, USA Page 3 of 16 5 More recently, both fsh and ichthyosaurs have also been Fifty-four samples were selected from the Georgetown described (Romano et al. 2012, 2017; Scheyer et al. 2014) core. Tey were cleaned, crushed and weighed (15 g on and exceptionally diverse Early Triassic marine fauna, average) and subsequently treated with concentrated known as the Paris biota (Brayard et al. 2017), is found hydrochloric and hydrofuoric acid as described by Trav- approximately 20 km South of Georgetown. erse (2007). Te residues were sieved over a 11-µm mesh Tis study documents palynomorphs partly from the screen. For palynofacies analysis organic matter particles Dinwoody/Woodside Formation and from the lower part were counted on unoxidized slides to a minimum of 250 of the Taynes Group recovered from samples of a drill counts. Particle counts included the categories of trans- core from a scientifc borehole near Georgetown in the lucent and opaque wood (charcoal), but also degraded Bear Lake district. With respect to the scarcity of land woody particles (pseudoamorphous particles partly with plant fossils so far recovered in this region, these fndings angular shape and/or remaining internal structure). Cuti- are illustrated and discussed. Paired carbon isotope com- cles, inertinite (used for particles describable as “worn positions including particulate organic matter data are and transported oxidized or carbonized wood” Tyson discussed. 1995, p 351), membranes, spores and bisaccate pol- len grains were also counted (Tyson 1995; Batten 1996). Geological setting Aquatic particles include acritarchs, Leiospheridia