ABSTRACT

In the search for geochemical indicators, 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, Anthaconauta, Carbonicula), (2) B--brackish (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) are:

A three-group, six-variable discriminant function permits complete separation of individual freshwater and marine samples, but is less successful in distinguishing the brackish shales as a separate category. Siderites formed 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.