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Geochemistry, clay mineralogy, and taphonomy of the fossil-rich butter shales

Hanna F. Cohen, Jacalyn M. Wittmer, Shane K. Butler and Jordan M. Young
Geochemistry, clay mineralogy, and taphonomy of the fossil-rich butter shales
Palaios (August 2018) 33 (8): 393-402


The Cincinnati Arch contains a suite of Upper Ordovician, mixed siliciclastic-carbonate, shallow marine strata, with alternating packages of packstones and shales that contain exceptionally well-preserved articulated fauna. The Waynesville Formation (C5 sequence) in Eastern Indiana includes distinctive clay-rich shales, which are locally referred to as butter shale. These bluish-green claystones are distinct in comparison to other shales because of their geochemistry, clay mineralogy, bulk composition, and well-preserved and biostratigraphically important fauna. This study focuses on an outcrop with exposed butter shale from the Waynesville Formation at Hanna Creek in Brookville, Indiana. Bulk-rock samples were collected and the geochemical and clay mineralogical composition of the claystones were analyzed using x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. On average, the butter shale clay mineralogy is 74% illite, 25% chlorite/kaolinite, and less than 1% smectite. The butter shale lacks expandable smectite, likely from post-depositional alteration of original smectite through illitization and chloritization. The presence of iron in the system as a result of diagenetic processes allowed for the immediate mineralization, and in some cases pyritization, of fossils. Bulk chemical analysis coupled with clay mineral analysis provides an explanation for the Fe substitution into chlorite for the butter shales and supports the interpretation of rapid sediment sealing that allowed for the pristine preservation of articulated fauna and mineralization observed in the butter shale.

ISSN: 0883-1351
Serial Title: Palaios
Serial Volume: 33
Serial Issue: 8
Title: Geochemistry, clay mineralogy, and taphonomy of the fossil-rich butter shales
Affiliation: Western Michigan University, Department of Geosciences, Kalamazoo, MI, United States
Pages: 393-402
Published: 201808
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 45
Accession Number: 2018-082095
Categories: Sedimentary petrologyGeochemistry of rocks, soils, and sediments
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., strat. cols., 5 tables, geol. sketch map
N39°34'60" - N39°34'60", W84°57'00" - W84°57'00"
Secondary Affiliation: State University of New York at Geneseo, USA, United StatesUniversity of North Dakota, USA, United StatesState University of New York at Stony Brook, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 2018
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