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GEOREF RECORD

Enhanced paleoecological and paleoenvironmental interpretations result from analysis of early diagenetic concretions in Pennsylvanian shales

Christopher G. Maples
Enhanced paleoecological and paleoenvironmental interpretations result from analysis of early diagenetic concretions in Pennsylvanian shales
Palaios (October 1986) 1 (5): 512-516

Abstract

Diagenesis commonly is a subtle, yet significant source of taphonomic bias. Failure to recognize diagenetic influences on primary depositional features can lead to spurious paleoecological and paleoenvironmental conclusions. This is exemplified by early diagenetic calcareous concretions and laminated siderite in the lower part of the Dugger Formation (Pennsylvanian: Desmoinesian) in southwestern Indiana. Calcareous concretions are common in organic-rich, black shale, in which they record vital paleoecological and paleoenvironmental information lost in the surrounding deposits. Evidence of early cementation in these concretions includes 1) presence within concretions of thin-shelled or weakly ornamented molluscs (e.g. Placopterina ohioensis, Parallelodon obsoletus, and Nuculopsis girtyi), which are not preserved in surrounding black shales, 2) Preservation of original convexity in bivalves, 3) preservation of woody fragments not preserved in the enclosing black shale, 4) up to 80 percent post-diagenetic compactional drape of the surrounding black shale around the concretions, and 5) preservation of bioturbation horizons that indicate temporally short oxygenation events not preserved in the enclosing black shale. The presence of these feature within the concretions contradicts previous interpretations of very low faunal diversity resulting from totally anoxic environmental conditions. Laminated siderite occurs stratigraphically higher within the Dugger Formation. Evidence for early diagenesis of this laminated siderite is the preservation of uncrushed productid brachiopods in growth position. A fresh-water origin for laminated siderite is supported by the presence of brackish and nonmarine fossils within and immediately above the siderite layer. The preservation of productid brachiopods in growth position at the base of the siderite suggests a sudden fresh-water influx into marine or brackish waters. Here, paleontological evidence supports previous geochemical interpretations of laminated-siderite formation. Recognition and exploitation of early diagenetic lithotypes in other rock units may also provide enhanced insights into faunal diversity, paleoecology, and depositional setting.


ISSN: 0883-1351
Serial Title: Palaios
Serial Volume: 1
Serial Issue: 5
Title: Enhanced paleoecological and paleoenvironmental interpretations result from analysis of early diagenetic concretions in Pennsylvanian shales
Affiliation: Clemson Univ., Dep. Earth Sci., Clemson, SC, United States
Pages: 512-516
Published: 198610
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 23
Accession Number: 1987-032099
Categories: StratigraphySedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
N37°45'00" - N41°45'00", W88°10'00" - W84°45'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 1987
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