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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 49 (1965)

Issue: 3. (March)

First Page: 362

Last Page: 362

Title: Chemical Composition of Siderite Nodules in the Environmental Classification of Shales: ABSTRACT

Author(s): Jon N. Weber, Eugene G. Williams

Article Type: Meeting abstract


In the search for geochemical indicators for the environmental classification of non-fossiliferous, clastic sedimentary rocks, the chemical composition of 45 syngenetic "siderite nodules" from shales of Pennsylvanian age was investigated. Nodules were assigned to three categories on the basis of closely associated fossils: (1) FW--freshwater (Estheria, Leaia, Anthraconauta, Carbonicula), (2) B--brackish, restricted marine or nearshore marine (Lingula, Orbiculoidea, Dunbarella, Aviculopecten), and (3) M--marine (Chonetes, Mesolobus, etc.). Of 11 elements determined, Si, Al, Mg, Ca, Ba, and V are useful as environmental discriminators. Means (and standard deviations) of these elements by category are as follows:


A three-group, six-variable discriminant permits complete separation of individual FW and M samples, but is less successful in distinguishing the brackish and restricted marine shales as a separate category. Siderites forming during sedimentation may prove especially useful for environmental discrimination where variations in the detrital to authigenic clay mineral ratio diminish the value of trace element indicators in the argillaceous fraction of the rock.

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Copyright 1997 American Association of Petroleum Geologists