Outcrop and thin section scale descriptions of Silurian carbonates within the Hiawatha National Forest, Mackinac County,
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ORIGINAL ARTICLE
Outcrop and thin section scale descriptions of Silurian carbonates within the Hiawatha National Forest, Mackinac County, Upper Peninsula, Michigan, USA Jonathan B. Sumrall1 · Erik B. Larson2 Accepted: 12 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Silurian carbonate outcrops within the southern part of the St. Ignace District of the Hiawatha National Forest (Mackinac County, Michigan) were examined to develop a local description of the exposed geology. The bedrock geology consists of the Manistique Group (dominantly the Cordell Formation) and the Engadine Group (Rockview Formation, Rapson Creek Formation, and Bush Bay Formation). Building on observations and differentiation from previous investigations, outcrops were selected to describe the macroscopic characteristics of these formations in attempt to develop a local description that would be more useful compared to confusing regional descriptions for researchers and forest officials. In Mackinac County, the Cordell Formation is a dark-to-light gray, chert-rich dolostone with abundant corals, stromataporoids, brachiopods, and echinoids. The Rockview Formation is a dark gray-to-white, chert-poor dolostone with microbiolite laminations, and brachiopod molds. The Rapson Creek Formation is a light gray-to-white, chert-free dolostone with abundant stromatoporoids. The Bush Bay Formation is light reddish brown to white in color, chert-poor, crystalline dolostone with abundant echinoids, brachiopods, and corals. Keywords Michigan basin · Engadine group · Hiawatha national forest · Silurian · Carbonates
Introduction
Study objectives
Outcrop scale differentiation of the Silurian carbonates of the Michigan Basin in the field has proven to be quite difficult. Regional descriptions are based on core and outcrops outside of the study area (Fig. 1), which vary considerably from the local outcrop characteristics. Some of the problems in identifying these units in the field are their similar descriptions and lack of lateral continuity in terms of thickness (and suspected lithology). The purpose of this paper is to develop a new framework through which to identify these carbonate units in the Mackinac County, Michigan (Fig. 1) and its surrounding area at outcrop, hand sample, and thin section scales.
The objectives of this study were to collect outcrop data about the Silurian strata exposed in the St. Ignace Ranger District of the Hiawatha National Forest in Mackinac County, Michigan to: (1) understand local variations in stratigraphy; and, (2) develop a generalized description of the outcrops, hand samples, and thin section petrography.
* Jonathan B. Sumrall [email protected] 1
Department of Geosciences, Fort Hays State University, 600 Park Street, Hays, KS, USA
Department of Natural Sciences, Shawnee State University, 940 Second Street, Portsmouth, OH, USA
2
Review of the literature Michigan Basin history The Michigan Basin is a nearly circular basin, approximately 500 km in diameter, covering the Lower
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