ABSTRACT: Application of Porphyrin and Other Biological Marker Geochemistry to Paleoenvironmental Assessment--Pennsylvanian Black Shales, Northern Denver Basin
E. Michael, J. Clayton
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 4000 to 8000 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 (20,000-30,000 and 300-3000 ppm, respectively). The total porphyrin concentration (vanadyl + nickel + car oxyl) correlates with total sulfur content in the rocks (normalized to TOC) and C14-C20 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. The open-marine environment had contained a thicker oxygen-containing water column which allowed selective degradation of porphyrin compounds. Low Eh conditions were undoubtedly present in either the bottom water or sediments of the open-marine setting, but sulfide content must have een low in the photic zone of the water column. High organic productivity is believed to be the principal reason for high organic accumulation in the sediments.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990