Publication Date
5-1-1979
Series Number
174
Abstract
Identification of specific environments of deposition represented by black shales has been a traditional puzzle for geologists. The present study was undertaken to see whether sedimentary structures, especially biogenic structures, could be used to subdivide the inclusive term "black shale facies." Because of the prevalence of infauna, modern marine muds are typically bioturbate; only where animal life has been excluded, usually by oxygen deficiency, is the depositional lamination preserved. Fine laminae are characteristic of anoxic water in all modern seas and oceans. Two ancient black shale sequences were investigated; one, in the Upper Devonian Sonyea Group of New York, provided samples along a thin time-parallel shale tongue; the other, the Upper Cretaceous Pierre Shale and Fox Hills Formation of the Western Interior, permitted vertical sampling of a thick section in a single area. Similar facies occur in both sequences. The shales can be divided into four biofacies, depending on the amount of biogenic reworking. In the Sonyea Group four facies are encountered from west to east: 1) laminated, 2) bioturbate, 3) mottled, 4) bedded; in the Pierre Shale and Lower Fox Hills these facies occur in the same order upward from the bases of the section. Laminated sediments are thought to indicate anoxic conditions below the mixed layer in a silled basin, probably at depths greater than 150 m. Bioturbate sediments reflect the presence of infauna, living under marginally aerobic conditions in shallower water. The mottled fabric, with larger and more diverse burrows partially obscuring physical sedimentary structures, was produced at still shallower depths. The bedded facies lacks burrows, due to physical' reworking in the shallowest zone; these nearshore, agitated waters were probably less than 20 m deep. This facies pattern defines the bathymetry of the Sonyea Sea: a deep anoxic basin in western New York graded up onto a shelf in the Ithaca area. During the deltaic episodes which alternated with black shale deposition, the shelf water was shallow and fully aerobic, and therefore fossiliferous. Deeper water and marginally aerobic conditions on the shelf were the consequences of a transgression, which shifted anoxic environments eastward from a persistent anaerobic basin. Each shale tongue in the Catskill wedge represents such a transgression. in the Pierre-Fox Hills section the facies pattern resulted from the gradual filling of an originally anoxic basin. In any one locality bottom conditions changed from anaerobic to marginally aerobic to fully oxygenated and agitated as shallowing proceeded. The Middlesex is thin, laterally persistent, and probably isochronous, while the Pierre is quite thick, with time-transgressive members. Lateral facies changes in the Middlesex should therefore indicate contemporaneous paleo-environments within the Late Devonian basin. Vertical changes in the Pierre, on the other hand, would reflect the depositional environment varying through time as the Late Cretaceous basin was filled. The major lithologic facies of the two shales were already mapped, but the more obscure facies changes based on internal fine structure had not yet been examined. As usual, recognition and interpretation of subtle facies changes demands that a good stratigraphic framework be established first. Since the lithostratigraphy of these two shales was known, they could be sampled quickly. Facies were defined in this study by means of the fine structure of the shale fabric, especially the biogenic structures: their sizes, abundances, and morphologies. Then, using similar structures from modern seas as guides, it was possible to infer the conditions of deposition in the ancient seas, especially the factors of water depth and oxygenation. In this way the various environments of deposition in the Devonian and Cretaceous basins were recognized, and some of the basin hydrographies inferred.