McNair Scholars Journal Volume 18 | Issue 1 Article 16 2014 The orC dell and Engadine Formations at Seul Choix Point, Upper Michigan: Implications for Silurian (Llandovery-Wenlock) environments Brittany Ward Grand Valley State University Follow this and additional works at: https://scholarworks.gvsu.edu/mcnair Recommended Citation Ward, Brittany (2014) "The orC dell and Engadine Formations at Seul Choix Point, Upper Michigan: Implications for Silurian (Llandovery-Wenlock) environments," McNair Scholars Journal: Vol. 18 : Iss. 1 , Article 16. Available at: https://scholarworks.gvsu.edu/mcnair/vol18/iss1/16 Copyright © 2014 by the authors. McNair Scholars Journal is reproduced electronically by ScholarWorks@GVSU. https://scholarworks.gvsu.edu/ mcnair?utm_source=scholarworks.gvsu.edu%2Fmcnair%2Fvol18%2Fiss1%2F16&utm_medium=PDF&utm_campaign=PDFCoverPages The Cordell and Engadine Formations at Seul Choix Point, Upper Michigan: Implications for Silurian (Llandovery-Wenlock) environments INTRODUCTION rock and thin section analysis, taxonomic faunal classification, a faunal abundance Reconstructing earth history is important survey, faunal relationship mapping, for both intellectual and pragmatic reasons. isotopic age analyses, and measurement The concept of actualism states that natural of cephalopod orientations. These data laws that operate in the universe, have are then used to reconstruct Silurian always operated and apply everywhere. paleoenvironments and test existing Therefore, studying the past gives us insight hypotheses of ocean currents and sea level into why Earth exists in its current state present at deposition. Finally, we use the and what it could be like in the future. isotopic analyses to correlate the rocks with Reconstructing past ocean environments is better dated rocks in eastern Wisconsin. critical to the discovery of oil and natural gas because knowing the depositional Geologic Setting environment of rocks can lead us to predict if the formation may be a valuable oil Seul Choix Point is a small peninsula reservoir. In particular, Silurian rocks are on the northern shore of Lake Michigan highly prized in the oil and natural gas located in Schoolcraft County, Michigan (Figure 2). The point is composed of Brittany Ward industry, hosting nearly a tenth of known reserves (Klemme and Ulmishek, 1991). Silurian carbonates of both the Cordell and McNair Scholar Engadine Formations (Figure 3). Rocks at Rocks and sediments are the physical Seul Choix point are gently folded into an evidence we have to reconstruct earth’s asymmetrical anticlinal structure. Rocks past, and spectacular examples are the on the southwestern and southeastern Silurian rocks at Seul Choix Point shores of the point dip at angles between 2 (pronounced Sish-swa by locals), Upper and 8 degrees to the southwest, south, and Michigan (Figure 1). Rocks crop out southeast. Beds on the northeastern shore along the shore as exposed bedding plane of the point are nearly horizontal or slightly surfaces and extend offshore into Lake dipping to the east and northeast. Michigan. Due to wave erosion, the rocks are swept of most sediments allowing for The Cordell Formation is Llandovery an unobstructed view of bedding plane to early Wenlock in age; the Engadine surfaces. The horizontal bedding surfaces Formation is probably Wenlock in age and allow for a unique way to study these rocks, the numerical ages of these formations in as one can view in situ fossils and their eastern Wisconsin have been estimated to spatial relationships in map view. Walking be ~434 to 428 Ma and ~428 to 421 Ma, on top of the bedding surfaces allows us to respectively (McLaughlin et al., 2013). view these fossils as if walking across the This is based on correlation of oxygen and carbon isotope data from a global type Pat Colgan epicontinental sea floor present during the Silurian. section developed in the Baltic Sea region Faculty Mentor (Salt and Thomas, 2012). In Michigan Previous studies of the Cordell and the Cordell Formation is primarily a Engadine Formations include numerous thinly bedded dolomite and is grouped localities throughout Upper Michigan and together with the Schoolcraft Formation eastern Wisconsin and provide detailed into the Manistique Group (Ehlers and stratigraphic descriptions of unit lithology Kesling, 1957; Catacosinos, Harrison, and fossil fauna (Ehlers and Kesling, 1957; Reynolds, Westjohn, and Wollensack, Ehlers, 1973; Johnson, Kesling, Lilienthal, 2000; 2001). The formation extends and Sorensen, 1979; McLaughlin, Mikulic, from Michigan’s Upper Peninsula down Kluessendorf, 2013). Existing studies of through eastern Wisconsin and has been rocks at Seul Choix Point provide a general described in northeastern Illinois (Ehlers, description of the rocks and fossils, as 1973; Luczaj, 2013; McLaughlin et al., well as a classification of a diverse marine 2013). The Engadine Formation is also invertebrate fauna (Ehlers and Kesling, primarily a massively-bedded dolomite 1957; Ehlers, 1973). and extends to Wisconsin and Illinois and also through southern Ontario, western This study is a sedimentologic and New York, and to northern Ohio (Ehlers, paleoenvironmental analysis of the Cordell 1973; Johnson, 1980). Correlative strata of and Engadine Formations exposed at Seul both formations are present as far away as Choix Point, Michigan. This study includes 51 Volume 18, 2014 eastern Iowa and New York at Niagara Falls identify beyond the genus (Luczaj, 2013). were analyzed in two different bedding (Figure 2). The marine fossils are also common genera plane surfaces of the Cordell Formation. in the Lower Paleozoic and are therefore Transect lines ranged from 20 m to 43 m in Lithology & Paleontology not diagnostic of precise ages. Graptolites length. Specimens were recorded until 100 The Cordell Formation consists of both have been employed to define ages in specimens were counted in each bed. thick (>10 cm) and thin (<10 cm) wavy Silurian rocks; however, this has not been bedded dolostone. Some of the beds are done in Michigan (Melchin, Sadler, and rich in chert lenses and nodules and marine Cramer, 2012). Because biostratigraphy The Cordell Formation is cut by numerous fauna including corals, stromatoporids and is not easily used, recent studies have been nearly vertical joints; this divides the cephalopods. The Cordell Formation varies conducted using stable carbon isotope bedding plane surfaces into areas ranging from 41 to 46 meters thick (Ehlers, 1973). chemostratigraphy for dating Silurian from a few to tens of square meters. We The dolostone contains numerous silicified rocks (Salt and Thomas, 2012). These selected one of these blocks near transect 1 and un-silicified fossils and abundant lenses studies have refined age uncertainties from to create a map of in situ fossil distribution. and nodules of chert. Corals are the most ±2 Ma to be accurate within ±0.01 Ma. We laid out a one-by-one square meter abundant fossils found within the Cordell McLaughlin et al. (2013) used carbon grid and each square was examined, and Formation (Ehlers, 1973; Johnson et al., isotope analyses applied to drill cores from fossil occurrences were mapped onto a 1979; Luczaj, 2013). Sheboygan, Wisconsin to develop a carbon scaled grid sheet. The total percentages of isotope curve. They compared this curve faunal occurrences were tabulated, and Overlying the Cordell Formation is the to a standard global isotope curve (Cramer percentages were calculated as in stretch younger Engadine Formation, which et al., 2011). From their correlations they lines 1 and 2. consists of massively bedded dolostone that developed more precise age estimates for ranges from 61 to 76 meters thick. The Silurian rocks in eastern Wisconsin. Carbon Isotope Analysis Engadine Formation contains few body fossils, but brachiopods and corals may be METHODS A cobalt-tungsten-corundum drill bit was used to powder a small subsample of each found in some beds. Clastic materials such Mapping and Rock Classification as quartz and almandine grains are present thin section blank or outcrop sample, in some beds (Ehlers, 1973). A satellite image from Google Earth producing about 10 to 20 mg of powdered and field surveys were used to map the sample for stable isotope analysis. We The contact of the Cordell and Engadine distribution of Cordell and Engadine sampled areas of fine micrite from Formation is commonly marked by Formations of the Seul Choix Point area. outcrops, hand samples, and thin section mudcracks and hardgrounds, and carbon Contacts of the formations and points blanks. Samples were analyzed at the Iowa isotope analysis shows a significant increase State Stable Isotope Laboratory. Samples 13 where data were collected were located in δ C values. Both observations suggest with a hand-held global position system were analyzed using acid digestion in a that a disconformity is present between receiver and plotted as points in Google GasBench and then delivered to a Finnigan the Cordell and Engadine formations MAT Delta Plus XL mass spectrometer Earth software. Orientated rock samples, 18 13 (McLaughlin et al, 2013; Johnson and as well as non-oriented hand samples from where δ O and δ C (in parts per ml) Campbell, 1980). A previous study the Cordell and Engadine Formations were determined and compared to the suggests the nature of this contact indicates were collected to create thin sections. Vienna Pee Dee Belemnite (VPDB) a sea level fall (Johnson and Campbell, The thin sections and sample slabs were standard. A total