Mississippian Barnett Formation: Bulk Geochemical Constraints from the Severity of Hydrographic Restriction and the Biogeochemical Cycling and Fate of Iron

Harry Rowe¹, Robert Loucks², Stephen Ruppel², and Susan Rimmer^1
1Earth and Environmental Sciences, University of Kentucky, Lexington, KY
²Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Chemostratigraphic evidence from the Barnett Formation (Texas) elucidates the restricted nature of the depositional environment in the Fort Worth Basin during the early progression of the Ouachita Orogeny. In accord with recent lithostratigraphic and petrographic studies, stratigraphic bulk geochemical analyses reveal that the environment of deposition was anoxic to euxinic, sediment-starved, with relatively high rates of organic matter accumulation. Using an environmental proxy developed from the sediment geochemistry of modern anoxic silled basins, the stratigraphic concentrations of total organic carbon (TOC) and molybdenum (Mo) in the Barnett Formation reveal a high degree of subpycnoclinal water mass restriction and an extended timescale of deep-water renewal of at least 8000 years and potentially as long as 20,000 years. An integrated assessment of elemental concentrations and TOC-S-Fe relationships reveals that severe Fe limitation controlled pyrite formation and the large excess of sulfide in the overlying water column. Mineralogical and elemental constraints suggest that, following sulfate reduction, less easily bio-extractable Fe(III) was bio-reduced under methanogenic conditions, consequently liberating Fe(II) which was subsequently incorporated into pore-water-formed dolomite. The preserved Mo-TOC, TOC-S-Fe, and Fe-bearing mineral relationships collectively indicate that the protracted turnover rate and persistent sediment starvation resulted in a biologically inhospitable environment, limiting the microbial consumption rate of organic carbon. These coupled inferences complement paleoenvironmental interpretations of the Barnett Formation based on core descriptions and petrography, and, more generally, help establish the framework for defining the extremes of water mass evolution and biogeochemical cycling in isolated paleomarine depositional systems.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas