Controls on the formation of microbially induced sedimentary structures and biotic recovery in the Lower Triassic of Arctic Canada Paul B. Wignall1,†, David P.G. Bond2, Stephen E. Grasby3,4, Sara B. Pruss5, and Jeffrey Peakall1 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 2Department of Geography, Geology and Environment, University of Hull, Hull HU6 7RX, UK 3Geological Survey of Canada, 3303 33rd Street N.W., Calgary, Alberta T2L 2A7, Canada 4Department of Geoscience, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta T2N 1N4, Canada 5Department of Geosciences, Smith College, Northampton, Massachusetts 01063, USA ABS TRACT the intense burrowing in the earliest Triassic preservation potential, because the mats were contradicts the notion that bioturbation was either grazed, or they could not develop in the Microbially induced sedimentary struc- severely suppressed at this time due to extinc- first place because of the unstable, burrowed tures (MISS) are reportedly widespread in tion losses at the end of the Permian. The no- sediment surfaces. In contrast, others have ar- the Early Triassic and their occurrence is tion that Early Triassic MISS preservation gued that Early Triassic open marine conditions attributed to either the extinction of marine was caused by the extinction of mat grazers were not unusually stressful (Hofmann et al., grazers (allowing mat preservation) during is not tenable. 2013; Vennin et al., 2015). Instead, the loss of the Permo-Triassic mass extinction or the bioturbators during the PTME is said to have suppression of grazing due to harsh, oxygen- INTRODUCTION been responsible for the preservation of fine poor conditions in its aftermath. Here we lamination (Hofmann et al., 2015), and a reduc- report on the abundant occurrence of MISS The post-mass extinction world of the Early tion in the depth of the sediment mixed layer in the Lower Triassic Blind Fiord Formation Triassic is often considered to represent an un- (Buatois and Mángano, 2011). In this scenario, of the Sverdrup Basin, Arctic Canada. Sedi- usual time in Earth history when marine recov- geochemical evidence for sediment anoxia is at- mentological analysis shows that mid-shelf ery from the Permo-Triassic mass extinction tributed to a lack of irrigation by bioturbation settings were dominated by deposition from (PTME) was delayed by the prevalence of ma- (Hofmann et al., 2015). Davies et al. (2016) has cohesive sand-mud flows that produced het- rine anoxia (Hallam, 1991; Twitchett and Wig- also challenged the uniqueness of supposedly erolithic, rippled sandstone facies that pass nall, 1996; Woods et al., 2007; Wignall et al., anachronistic “Precambrian” MISS and argued down dip into laminated siltstones and ul- 2010, 2016; Grasby et al., 2013, 2016; Pietsch that they are in fact commonplace in shallow timately basinal mudrocks. The absence of et al., 2014), and anachronistic facies better marine Phanerozoic settings; however most of storm beds and any other “event beds” points known from Cambrian and earlier times were their post-Cambrian examples are from peritidal to an unusual climatic regime of humid, quiet well developed; especially carbonate microbial- or fluvial settings. conditions characterized by near continuous ites (e.g., Bagherpour et al., 2017) and microbi- This study aims to provide the first sedimen- run off. Geochemical proxies for oxygenation ally induced sedimentary structures (MISS) in tological study of the type locations of all four (Mo/Al, Th/U, and pyrite framboid analy- sandstones (Wignall and Twitchett, 1999; Pruss Early Triassic substages that are found in the sis) indicate that lower dysoxic conditions et al., 2004, 2005; Baud et al., 2007; Noffke, Sverdrup Basin of Arctic Canada (Ellesmere and prevailed in the basin for much of the Early 2010). In recent years a broad range of sandstone Axel Heiberg islands). The ammonoid-defined Triassic. The resultant lack of bioturbation bedding features have been related to the pres- age of the strata is independently assessed by allowed the development and preservation ence of microbial mats at the time of deposition. constructing a chemostratigraphic record. Hav- of MISS, including wrinkle structures and These comprise textured surfaces, including ing established an age model, sedimentological bubble textures. The microbial mats respon- small-scale wrinkles and more regular, parallel context and redox variations, we then record and sible for these structures are envisaged to rides, that reflect the original microtopography discuss the origin of the abundant MISS that oc- have thrived, on sandy substrates, within the of the mat. They also include impressions of cur in the basin. Finally, we evaluate the broader photic zone, in oxygen-poor conditions. The bubbles trapped beneath the mats, all of these significance of these occurrences in the debates dysoxic history was punctuated by better- features are generally grouped together as MISS on the significance of Early Triassic facies and oxygenated phases, which coincide with the (Noffke et al., 2001; Davies et al., 2016). the nature of recovery from the PTME. loss of MISS. Thus, Permo-Triassic bound- Only the shallowest waters provided better ary and Griesbachian mudrocks from the oxygenated settings and a refuge from the wide- REGIONAL GEOLOGY OF STUDY deepest-water settings have common benthos spread, inimical conditions of the Early Triassic AREA and a well-developed, tiered burrow profile (Wignall et al., 1998; Beatty et al., 2008; Knaust, dominated by Phycosiphon. The presence of 2010; Chen et al., 2011; Song et al., 2013; Pro- Present-day outcrops of Sverdrup Basin strata emse et al. 2013). MISSs are rarely reported lie in the northern-most part of Nunavut, Cana- †[email protected]. from such environments indicating their poor dian High Arctic. The basin extends ∼1000 km GSA Bulletin; May/June 2020; v. 132; no. 5/6; p. 918–930; https://doi.org/10.1130/B35229.1; 10 figures; 1 table; published online 30 August 2019. 918 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/132/5-6/918/4987619/918.pdf by guest on 25 September 2021 Microbial structures Early Triassic Arctic Canada º º 90º 80 70º 85º 120º 110º 100 Crockerland Sp Di Gr Axel Sm Banks Heiberg Island Island Ellesmere Melville Island Green- Island land 80º Fluvial plain siliciclastic Substage Members Shelf siliciclastic Spathian Svartfjeld (320 m) Offshore shelf-slope siliciclastic Smithian Smith Creek (355m) Basin non-siliceous mudrock Dienerian Confederation 0 km 200 Point (260 m) Griesbachian Blind Fiord Fm. º 70 Figure 1. Regional paleogeography of the Sverdrup Basin, Arctic Canada, in the Early Triassic (from Embry and Beauchamp, 2008). Lower Triassic regional stratigraphy is given and thicknesses are those measured at Spath Creek/Cape St. Andrews. Red stars mark the field loca- tions. Fm.—Formation. east-west and 300 km north-south with its dep- basin margins. Stable isotope studies show that began at N80° 54.464′ W89° 11.571′ (NAD83) in ocenter located in northern Ellesmere Island and the Blind Fiord Formation transgression occurred the upper part of the Confederation Point Member Axel Heiberg Island (Fig. 1). The basin margins just prior to the PTME and that the sequence is and continued to a level near the top of the Smith lay to the west, south, and east and opened to the conformable in the basin center (Grasby and Creek Member where a major dolerite sill forms northwest into the Boreal Ocean, although con- Beauchamp, 2008). Shale deposition was most a prominent ridge. Logging was resumed to the nection was limited due to the presence of Crock- extensive in the basal Griesbachian, while the northeast, at the same stratigraphic level (with erland—an island or structural high that formed base of the Smith Creek Member (basal Smithian) some overlap), on the nearby slopes of Cape St. the northern basin rim (Embry, 1988, 1989, 2009; and base of the Svartfjeld Member (basal Spath- Andrews (N80° 55.072′ W89° 14.491′) and con- Fig. 1). Lower Triassic strata belong to the Blind ian) also mark major transgressions. tinued up to the lower beds of the Svartfjeld Mem- Fiord Formation, a thick succession of offshore ber (Fig. 3). In addition to these two locations the mudstone, siltstone, and shelf-slope sandstone Study Sections same levels were also examined (and sampled) at that passes into paralic and terrestrial strata of the Smith Creek and at Diener Creek (Fig. 2). Bjorne Formation (Embry, 1988, 2009; Devaney, The four substages of the Early Triassic were 1991; Embry and Beauchamp, 2008; Midwinter established within the Blind Fiord Formation by METHODS et al., 2017). The Blind Fiord Formation is divid- Tozer (1965, 1967) and defined by their ammo- ed into three members: the Confederation Point noid content. His four type localities (Griesbach Facies and Fossils Member of Griesbachian to Dienerian age con- Creek, Diener Creek, Smith Creek, and Spath sists of heterolithic strata ranging from shale to Creek continuing to Cape St. Andrews; Figs. 1 Sedimentary logging was undertaken through sandstone; the Smith Creek Member of Smithian and 2) were the subject of our study during an the basal 640 m of the Blind Fiord Formation at age consists of fine sandstone and siltstone; and expedition in July 2015. The basal 50 m of Con- Griesbach Creek and Spath Creek to Cape St. the Svartfjeld Member of Spathian age consists of federation Point Member strata, with the excep- Andrews, and facies were also examined and shale (Fig. 1). The Blind Fiord strata are arranged tion of the unexposed basal-most meter, were sampled at Diener and Smith Creeks (Fig. 3). into a series of coarsening up/progradational logged in the western bank of Griesbach Creek Trace and body fossils were identified in the packages punctuated by flooding events that saw on Axel Heiberg Island (Fig.