SMALL-SCALE VARIATIONS IN BLACK SHALE SEQUENCES OF INDIANA

Contemporaneous sandstone channels of presumed fresh-water origin interrupt beds of an otherwise widespread black shale sequence that overlies the Springfield Coal Member of the Petersburg Formation (Pennsylvanian) in several areas of Indiana. The shales change thickness, even pinching out near the channels, and exhibit mineralogical and geochemical changes over short distances. Shale samples from 52 sites were collected at distances that range from immediately adjacent to as far as 28 miles distant from the channels. Vertical patterns of mineralogy and chemistry were much more prominent than were lateral changes, contrary to expectations if channels were completely fresh water. Eight varieties of black shale were distinguished including a very clay-rich fresh water type. Marine black shales that directly overlie coals were relatively rich in organic carbon (average 14.8 weight %), sulfur (average 3.2%), and contained an enriched suite of trace metals (averages in PPM Mo [225], Ni [230], V [770], Zn [770]). Gray, siderite-bearing shales (thought to represent deposition under brackish conditions) overlie the black shales and contain similar silicate mineralogy but are much lower in carbon (4.2%), sulfur (1.8%), and trace elements (averages in PPM Mo [33], Ni [78], V [175], Zn [148]).

A similar stratigraphic geochemistry was seen in many examples of the fully marine and probably relatively deep New Albany Shale (Devonian and Mississippian) of Indiana. The New Albany Shale contains thin metal-rich and isotopically anomalous units that are laterally widespread and more homogeneous than the Pennsylvanian black shales.