Charles Sandberg
R.C. Gutschick
1991
<p><span>The Late Devonian Michigan Basin was floored by the Middle and Upper Devonian Squaw Bay Limestone, which was deposited during the downwarping that produced the basin within a former Middle Devonian carbonate platform. The Squaw Bay comprises three beds, each having a different conodont fauna. The two upper beds, deposited during the </span><i>transitans</i><span> Zone, have different conodont biofacies that reflect this deepening. The basin was largely filled by the deep-water, anaerobic to dysaerobic, organic-rich, black Antrim Shale, which has a facies relationship with the prodeltaic, greenish gray Ellsworth Shale that prograded into the basin from the west. The Upper Devonian (Frasnian to Famennian) Antrim Shale is divided into four members, from base to top: the Norwood, Paxton, Lachine, and upper members. These members are more or less precisely dated by conodonts. The Norwood was deposited during the </span><i>transitans</i><span> Zone to </span><i>Ancyrognathus triangularis</i><span> Zone, and the Paxton was deposited from that zone probably through the </span><i>linguiformis</i><span> Zone at the end of the Frasnian. The overlying Lachine was deposited during the early Famennian and has yielded faunas of the Upper </span><i>crepida</i><span> and Lower </span><i>rhomboidea</i><span> Zones. Only the lower part of the upper member is exposed, and near Norwood, Michigan, it yielded conodonts of the Lower </span><i>marginifera</i><span> Zone. The widespread Famennian floating plant </span><i>Protosalvinia (Foerstia)</i><span> has not yet been found in outcrops of the Antrim, and should not be expected to occur except in the upper member or highest part of the Lachine Member. Its range in terms of conodont zones is from the Upper </span><i>trachytera</i><span> Zone through the Lower </span><i>expansa</i><span> Zone and possibly into the Middle </span><i>expansa</i><span> Zone. One known subsurface occurrence might be datable as </span><i>rhomboidea</i><span> or Lower </span><i>marginifera</i><span> Zone, depending on gamma ray correlations to outcrops. Black shale deposition ended when the Late Devonian mud delta of the Bedford Shale prograded across the Michigan Basin from the east and then retreated as the regressive Berea Sandstone was being deposited during the major eustatic sea-level fall that ended the Devonian. The Bedford was deposited during the Upper </span><i>expansa</i><span> to Lower </span><i>praesulcata</i><span> Zones, and the Berea was deposited during the Middle to Upper </span><i>praesulcata</i><span> Zones. Both formations contain the spore </span><i>Retispora lepidophyta,</i><span> which is a global indicator of latest Devonian age.</span></p>
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10.1130/SPE256-p155
en
GSA
Upper Devonian biostratigraphy of Michigan Basin
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