--> --> Trace Fossil Origin for Pervasive Pyritized Films in the Estuarine Ingersoll Shale, Cretaceous Eutaw Formation, Eastern Alabama

GCAGS

Datapages, Inc.Print this page

Trace Fossil Origin for Pervasive Pyritized Films in the Estuarine Ingersoll Shale, Cretaceous Eutaw Formation, Eastern Alabama

Abstract

The Ingersoll shale, a thin carbonaceous clay lens in the Cretaceous Eutaw Formation, eastern Alabama, is a fossil lagerstätte that contains a diverse fossil flora, abundant amber clasts with inclusions, and fossil feathers. This clay lens, which was deposited in a restricted estuarine tidal channel, also contains common but enigmatic pyritized structures. In order to establish their origins, these structures were subjected to macroscopic, x-radiographic, petrographic, and scanning electron microscopic (SEM) analyses. The pyritized structures are pervasive, occurring on bedding surfaces in densities ranging from 400–1400 specimens/m2. Most structures appear as irregular, narrow, elongate, flattened pyrite films with lengths ranging from 0.6 to 3.0 cm, diameters ranging from 1.0 to 3.5 mm, and thicknesses of ≤ 1 mm. Axial traces of the structures are straight to tortuous. Commonly, structures narrow and widen irregularly along their axes and locally appear to branch. X-radiographs indicate that some of the structures are oblique to bedding but confirm that they are relatively short and discontinuous features. Thin-section and SEM–EDS (SEM energy dispersive spectroscopic) analyses indicate that the structures are composed almost entirely of pyrite; detrital silt and clay common in host sediment are rare to absent. Pyrite most commonly occurs as framboids that range from 10 to 15 μm in diameter, comprise 1–2 μm pyrite crystallites, and appear to be surrounded by a matrix of less organized crystallites of comparable size. Larger, nonframboidal pyrite octahedra also occur locally. Because they contain no vestiges of plant ultrastructure, cut across identifiable unmineralized leaves, and are commonly oblique to bedding planes, plant remains can be excluded as precursors to the pyritized structures. Rather, these features are best explained by very early pyritization of nearly pure, discontinuous mucus films or strands that were secreted by unknown organisms moving along or through the sediment and that provided highly favorable microenvironments for sulfate-reducing bacteria.