Mississippian Barnett Formation, Fort Worth Basin, Texas: Bulk geochemical inferences and Mo-TOC constraints on the severity of hydrographic restriction

摘要

Chemostratigraphic evidence froth the Barnett Formation (Texas, USA) elucidates the restricted nature of the depositional environment in the Fort Worth Basin during the early progression of the late Paleozoic 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 silted 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 8 x 10(3) years and potentially as long as 2 x 10(4) years. An integrated assessment of elemental concentrations, the degree of pyritization, 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 (i.e., organogenic 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 that limited the microbial consumption rate of organic carbon.