Application of Porphyrin and Other Biological Marker Geochemistry to Paleoenvironmental Assessment--Pennsylvanian Black Shales, Northern Denver Basin: ABSTRACT

Abstract

Thin, Middle Pennsylvanian (Desmoinesian) organic-rich black shales underlie much of the northern Denver basin. During the Desmoinesian, the present-day area of the Nebraska panhandle was a shallow, at times highly saline restricted sea. In contrast, black shale deposition occurred in a marine environment with normal salinity (open marine) in northeastern Colorado. About eight individual black shale beds are present in the Desmoinesian section in both the restricted basin and open-marine settings, separated vertically by carbonate-evaporate rocks in the restricted basin and by predominantly carbonate rocks in the open-marine sequence. Preservation of high amounts of organic matter occurred in black shales deposited in both the restricted basin and open-marine settings. Chloroform-extractable organic matter (bitumen) concentrations are typically in the range of 4,000 to 8,000 ppm, and total organic carbon content (TOC) is as high as 20 wt. %. However, the composition of the bitumen contained in stratigraphically equivalent black shales is variable and is correlative to differences in paleoenvironmental conditions during sedimentation in the restricted and open-marine basins. Total porphyrin concentration (ppm relative to bitumen) is typically 10-100 times greater in the restricted basin compared to the open marine. The total porphyrin concentration (vanadyl + nickel + carboxyl) correlates with totalmore » sulfur content in the rocks (normalized to TOC) and C{sub 14}-C{sub 20} aryl isoprenoid content. The coincidence of high porphyrin and aryl isoprenoid concentration, along with high S/C ratios, is explained by differences in water column chemistry of the restricted basin and open-marine environments during sedimentation. The restricted basin was characterized by a stratified water column with euxinic conditions below the chemocline and a thin overlying oxic water column.« less