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Exceptionally Well-Preserved Fossils in a Middle Ordovician Impact Crater

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Exceptionally Well-Preserved Fossils in a Middle Ordovician Impact Crater Review focus Journal of the Geological Society Published online September 24, 2018 https://doi.org/10.1144/jgs2018-101 | Vol. 175 | 2018 | pp. 865–874 The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater Derek E.G. Briggs1,2*, Huaibao P. Liu3, Robert M. McKay3 & Brian J. Witzke4 1 Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA 2 Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA 3 Iowa Geological Survey, IIHR – Hydroscience & Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, IA 52242, USA 4 Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USA D.E.G.B., 0000-0003-0649-6417 * Correspondence: [email protected] Abstract: The Winneshiek Shale (Middle Ordovician, Darriwilian) was deposited in a meteorite crater, the Decorah impact structure, in NE Iowa. This crater is 5.6 km in diameter and penetrates Cambrian and Ordovician cratonic strata. It was probably situated close to land in an embayment connected to the epicontinental sea; typical shelly marine taxa are absent. The Konservat-Lagerstätte within the Winneshiek Shale is important because it represents an interval when exceptional preservation is rare. The biota includes the earliest eurypterid, a giant form, as well as a new basal chelicerate and the earliest ceratiocarid phyllocarid. Conodonts, some of giant size, occur as bedding plane assemblages. Bromalites and rarer elements, including a linguloid brachiopod and a probable jawless fish, are also present. Similar fossils occur in the coeval Ames impact structure in Oklahoma, demonstrating that meteorite craters represent a novel and under-recognized setting for Konservat- Lagerstätten. Received 18 May 2018; revised 7 August 2018; accepted 7 August 2018 The Winneshiek Konservat-Lagerstätte was discovered in 2005 in fossils, including arthropod appendages (Fig. 3h, i)(Nowak et al. the Winneshiek Shale, which directly underlies the Ordovician St 2018) and filamentous algae (Nowak et al. 2017). Peter Sandstone, during mapping by the Iowa Geological Survey (Liu et al. 2005)(Fig. 1). The only exposure, which represents the The meteorite crater and the age of the Winneshiek Shale top of the Winneshiek Shale, is in the valley of the Upper Iowa River near Decorah in Iowa. Collections from this locality yielded The Winneshiek Shale, an unusual greenish brown to dark grey conodonts, linguloid brachiopods, large fragments of eurypterid organic-rich shale up to 26 m thick, was noted in cores and well cuticles and phyllocarid crustaceans (Liu et al. 2005). A coal-like cuttings some years prior to the discovery of the exposure near layer had been reported in local newspapers in the 1920s, which Decorah (Young et al. 2005). It overlies an unnamed unit dominated could only have been a concentration of eurypterid cuticles, perhaps by breccia (Fig. 1c), the two reaching a combined thickness of to encourage speculators. H. Paul Liu recognized the exceptional c. 200 m. Occurrences of the shale in drill core and in outcrop nature of these fossils and he and colleagues published a preliminary delimit a circle with a diameter of c. 5.6 km (Liu et al. 2009) account of the discovery in 2006 (Liu et al. 2006). A collaborative (Fig. 1b). This outline was also detected in electromagnetic and grant to the authors funded the construction of a temporary dam in gravity data from airborne geophysical surveys. The circular 2010, which isolated the exposure of the Winneshiek Shale on the structure and its sedimentary fill are concealed by the overlying St north bank of the Upper Iowa River near Decorah (Fig. 2a–c). This Peter Sandstone and Quaternary alluvium, except at one small allowed access to the shale and a sequence of large samples, locality in the Upper Iowa River Valley near Decorah, where the top representing c. 4 m of the section, was extracted by hand with the of the shale is at the land surface. The circular depositional basin is aid of earth-moving equipment (Fig. 2b, c). The shale was split at characterized by evidence of local deformation near the perimeter the Iowa Geological Survey sample repository over the next three (Fig. 2e) and its unusual stratigraphy. The Winneshiek Shale and years and yielded >5000 macrofossil specimens. underlying breccia are found only within the circular structure, The biota of the Winneshiek Shale is characterized by exceptional where the units are completely different from the regional preservation combined with a restricted diversity, both of which stratigraphic sequence (Liu et al. 2009; French et al. 2018). All reflect the unusual depositional setting. The dominant macro- these attributes are consistent with deposition within a meteorite fossils are arthropods: the giant Pentecopterus decorahensis crater. This inference was elegantly confirmed by the demonstration (Fig. 3) is the earliest described eurypterid (Lamsdell et al. 2015b) that quartz grains observed in drill samples from the breccia show and a diversity of bivalved arthropods includes a phyllocarid, shock-induced phenomena (Liu et al. 2009; McKay et al. 2011), Ceratiocaris winneshiekensis, and ostracods (Fig. 4). Conodont particularly the fractures and deformation features characteristic of elements are the most abundant fossils, many occurring as bedding an impact (French et al. 2018). plane assemblages (Fig. 5), and extrapolation indicates that some are The stratigraphic context and fossils indicate that the Winneshiek from individual animals of very large size (Liu et al. 2017). Rare Shale is Darriwilian (Middle Ordovician) in age, i.e. from 458.4 to specimens interpreted as the head shield of an early armoured 467.3 Ma (Gradstein et al. 2012; Cohen et al. 2013). None of the vertebrate (Liu et al. 2006) are also present (Fig. 5d). Dissolution of the conodonts associated with the exceptionally preserved fossils are organic-rich shale has yielded a variety of microscopic carbonaceous zonal index fossils, but they include Multioistodus subdentatus, © 2018 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/175/6/865/4549063/jgs2018-101.pdf by guest on 30 September 2021 866 D.E.G. Briggs et al. Fig. 1. (a) Location of Decorah in the NE of the State of Iowa, USA. (b) Geological map of Winneshiek and Allamakee counties in the northeastern corner of Iowa State, bounded to the east by the Mississippi River and the State of Wisconsin. From the map Bedrock Geology of Northeast Iowa; Witzke et al. (1998).(c) Geological cross-section through the western part of the Decorah impact structure. The gently dipping Cambrian and Ordovician formations are disrupted by the impact basin, which is filled with breccia, conglomerate and sandstone (unnamed), overlain by the Winneshiek Shale. Much of the impact basin is overlain by the St Peter Sandstone, which varies in thickness due to the erosional contact at its base. Based on Wolter et al. (2011) and French et al. (2018). which is known from Darriwilian strata around Laurentia from Shakopee Formation, the youngest unit penetrated by the crater, Alberta to New York State (Witzke et al. 2011; Liu et al. 2017). The which is separated from the overlying St Peter Sandstone by a major presence of the unusual conodont Archeognathus primus (Fig. 5a, b) unconformity (Fig. 1c). The Shakopee Formation yields conodonts is consistent with this age. The St Peter Sandstone overlies the of Middle Tremadocian age, indicating that the impact occurred no Winneshiek Shale (Fig. 1c) and drill cores through this formation earlier than c. 482 Ma. The impact penetrated a marine embayment in Iowa, Minnesota and Indiana have yielded a diversity of conodonts or estuary in the southern part of the palaeocontinent Laurentia indicating a late Darriwilian age. Characteristic early Darriwilian (Witzke 1990; Liu et al. 2009, 2017; Witzke et al. 2011). The conodonts (the Histiodella–Paraprioniodus–Pteracontiodus– complexity of the crater-fill and the lithological variability of the Fahraeusodus fauna) are absent in the Winneshiek Shale. Thus the breccia suggest that the meteorite entered a significant depth of conodont data constrain the age of the exceptionally preserved fossils, water (French et al. 2018). Cratering models indicate that a which were collected near the top of the Winneshiek Shale, to substantial thickness of younger sediments was present at the time middle–late Darriwilian (Liu et al.2017). Chemostratigraphic data, of the impact, possibly 300–500 m, which was subsequently 13 based on δ Corg data from 36 drill core samples, narrow the age of removed by erosion. This period of erosion may have lasted for the Winneshiek Shale to c.464–467 Ma (Bergström et al. 2018), i.e. 10–20 myr, after which the St Peters Sandstone was deposited, early–middle Darriwilian. Thus the exceptionally preserved fossils burying the crater and the surrounding area (French et al. 2018). are likely to be middle Darriwilian. The age of the Decorah impact structure is not well constrained, The meteorite impact clearly predates deposition of the but it coincides with the break-up of an L-chondrite meteorite parent Winneshiek Shale and the underlying breccia. The Winneshiek body, which resulted in a series of Middle Ordovician impacts Shale provides a minimum age for the impact of c. 464–467 Ma (Bergström et al. 2018)atc. 470 Ma (Korochantseva et al. 2007)or (Bergström et al. 2018). A maximum age is provided by the 468 Ma (Lindskog et al. 2017). A possible link between the asteroid Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/175/6/865/4549063/jgs2018-101.pdf by guest on 30 September 2021 Middle Ordovician Winneshiek biota 867 Fig.
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