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The Decorah Structure, Northeastern Iowa: Geology and Evidence for Formation by Meteorite Impact

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The Decorah Structure, Northeastern Iowa: Geology and Evidence for Formation by Meteorite Impact The Decorah Structure, northeastern Iowa: Geology, formation by meteorite impact The Decorah structure, northeastern Iowa: Geology and evidence for formation by meteorite impact Bevan M. French1,†, Robert M. McKay2,§, Huaibao P. Liu2,†, Derek E.G. Briggs3,†, and Brian J. Witzke4,† 1Department of Paleobiology, Smithsonian Institution, Washington, D.C. 20013-7012, USA 2Iowa Geological Survey, IIHR—Hydroscience & Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, Iowa, 52242, USA 3Department of Geology and Geophysics, and Yale Peabody Museum of Natural History, Yale University, New Haven, Connecticut 06520, USA 4Department of Earth and Environmental Sciences, University of Iowa, 121 Trowbridge Hall, Iowa City, Iowa 52242, USA ABSTRACT pact spike”) triggered by collisions in the as- the impacting projectile) can also provide un- teroid belt at ca. 470 Ma. ambiguous identifications of impact structures The Decorah structure, recently discov- (Tagle and Hecht, 2006; French and Koeberl, ered in northeastern Iowa, now appears as INTRODUCTION 2010; Koeberl, 2014). an almost entirely subsurface, deeply eroded Since the 1960s, the number of established circular basin 5.6 km in diameter and ~200 m During the past few decades, impacts of large terrestrial meteorite structures has increased deep, that truncates a near-horizontal series extraterrestrial objects onto the Earth’s surface steadily at a rate of a few new structures per year of Upper Cambrian to Lower Ordovician have become recognized and generally accepted (Grieve, 1998), chiefly because the recognition platform sediments. Initial analysis of geo- as an important geological process (Grieve, of these shock-metamorphic features (generally logical and well-drilling data indicated char- 1991, 1997, 1998, 2001; French, 1998, 2004; shatter cones and PDFs in quartz) has made it acteristics suggestive of meteorite impact: Lowman, 2002; Jourdan and Reimold, 2012; possible to identify impact structures that are a circular outline, a shallow basin shape, Osinski and Pierazzo, 2013). It has also been old, poorly exposed, deeply eroded, buried, or discordance with the surrounding geology, recognized that these rare but highly energetic even subjected to post-impact metamorphism. and a filling of anomalous sediments: (1) the events can produce major and often widespread At present, more than 190 preserved impact organic-rich Winneshiek Shale, which hosts geological effects, including the near-instanta- structures have been definitely identified (Earth a distinctive fossil Lagerstätte, (2) an under- neous formation of large geological structures, Impact Database, 2016), and model calculations lying breccia composed of fragments from the generation of large igneous bodies, the suggest that at least several hundred preserved the surrounding lithologies, and (3) a poorly creation of economic mineral and petroleum impact structures remain to be discovered on the known series of sediments that includes shale deposits, the deposition of regional and even land areas of the earth (Trefil and Raup, 1990; and possible breccia. Quartz grains in drill global ejecta layers, and (in at least one case) Grieve, 1991; Stewart, 2011; Hergarten and samples of the breccia unit contain abundant the production of a major biological extinction Kenkmann, 2015). distinctive shock-deformation features in (at the Cretaceous-Paleogene boundary, 66 Ma; This growing population of recognized im- ~1% of the individual quartz grains, chiefly Schulte et al., 2010). pact structures, and the increasing diversity and planar fractures (cleavage) and planar defor- This recognition of terrestrial impact craters complexity displayed by individual structures, mation features (PDFs). These features pro- and their effects has been based mainly on dis- have made it possible for current research ef- vide convincing evidence that the Decorah tinctive and permanent petrographic and geo- forts to expand beyond the simple identification structure originated by meteorite impact, chemical effects produced in target rocks and of new structures and to explore more general and current models of meteorite crater for- minerals by the extreme and highly transient impact-related problems: the mechanics and mation indicate that it formed as a complex conditions of pressure, temperature, stress, and complexities of large crater formation, the na- impact crater originally ~6 km in diameter. strain generated by the intense shock waves ture of impact-produced rock deformation, the The subsurface characteristics of the lower uniquely created by hypervelocity impact events establishment of new criteria for impact events, portion of the structure are not well known; (French and Short, 1968; French, 1998; French the effects of the target geologic setting on im- in particular, there is no evidence for the ex- and Koeberl, 2010; Koeberl, 2014). In practice, pact crater development, and the wider geologi- istence of a central uplift, a feature generally one of the most widespread and widely used cri- cal and environmental consequences of impact observed in impact structures of comparable teria for the identification of shock waves and events, However, despite the large number of size. The current estimated age of the Deco- meteorite impact structures has been the multi- presently known impact structures and the so- rah structure (460–483 Ma) suggests that it ple sets of narrow, closely spaced lamellae (pla- phisticated state of current impact research, the may be associated with a group of Middle nar deformation features [PDFs]) developed in field continues to evolve rapidly and unpredict- Ordovician impact craters (a terrestrial “im- quartz (see papers in French and Short [1968]; ably, and the identification of new impact struc- also Stöffler and Langenhorst, 1994; Grieve et tures remains an essential source of new data and of new and often unexpected questions. †[email protected], [email protected], derek al., 1996; Ferrière et al., 2009), although a small [email protected], [email protected]. number of other criteria (e.g., megascopic shat- In this paper, we describe the geology and §Corresponding author: [email protected]. ter cones and unique geochemical signatures of origin of the Decorah structure, now present GSA Bulletin; Month/Month 2018; v. 130; no. X/X; p. 1–25; https://doi.org/10.1130/B31925.1; 18 figures; 2 tables.; published online XX Month 2018. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 130, no. XX/XX 1 © 2018 Geological Society of America French et al. as a small, isolated circular basin (diameter GEOLOGY OF THE DECORAH River meanders across the southern half of the ~5.6 km) in virtually undeformed Upper Cam- STRUCTURE structure where, on average, 20 m of Quaternary brian and Lower Ordovician cratonic strata in alluvium overlies bedrock. The depth to the top northeastern Iowa (Liu et al., 2009; McKay Regional Geology, Target Rocks, and Age of structure varies from 0 m at the river level et al., 2010, 2011). This structure, recognized outcrop of the Winneshiek Shale near the struc- during the investigation of a new Konservat- The Decorah structure is located in Win- ture’s eastern edge to 111 m along the north- Lagerstätte, the Winneshiek Lagerstätte (Liu neshiek County, northeastern Iowa (Fig. 1); its western margin on the upland above the et al., 2006, 2007a), is almost entirely sub- center is located at latitude 43°18′ 49″ and lon- river valley. surface, appearing as an area of localized and gitude 91°46′19″. The structure lies within the Paleozoic bedrock in the region is a gently anomalous sedimentary strata and structural Paleozoic Plateau landform region of Iowa, an tilted sequence of Upper Cambrian through deformation. The basin contains strata unlike area of relatively high topographic relief and Upper Ordovician cratonic sedimentary strata those of the surrounding Cambrian through thin Quaternary deposits lying upon Paleozoic (Table 1), chiefly quartzose and fine-grained Ordovician rocks (McKay et al., 2010, 2011; bedrock (Prior, 1991). The upper surface of the feldspathic arenites, carbonate rocks, and lesser Witzke et al., 2011; Wolter et al., 2011). At structure, currently best defined by the mostly shale (Witzke and McKay, 1987; Witzke and present, two distinct lithologies are well rec- subsurface distribution of the Winneshiek Glenister, 1987; McKay, 1988, 1993; Lud- ognized in the upper portion of the basin- Shale, spans a circular area of ~24.8 km2. About vigson and Bunker, 2005; Runkel et al., 1998, filling sediments: (1) the Winneshiek Shale 10.1 km2 of the structure underlies the town of 2007, 2008; Wolter et al., 2011). The maximum (Wolter et al., 2011), a greenish-gray to black Decorah. The bedrock-entrenched Upper Iowa thickness of Paleozoic strata is estimated from shale, and (2) an underlying polymict brec- cia apparently derived from local lithologies (McKay et al., 2010, 2011). A third series of 91º45´00´´W 91º37´30´´W 91º52´30´´W rock types, shale, and possible sandstone brec- 43º22´30´´N cia, are poorly known from the deeper por- tions of the basin. The areal extent of the basin is presently defined by the known distribution Decorah impact structure of the Winneshiek Shale, the uppermost basin- fill unit. In particular, we report here the results of petrographic and petrofabric examinations of quartz grains from small samples of the sub- River surface polymict breccia underlying the Win- Upper Iowa neshiek Shale. We provide evidence
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