Isotopic and Petrographic Evidence for Carbonate Diagenesis in Nonmarine Sandstones, Green River Basin, Wyoming

Abstract

ABSTRACT Petrographic and mineralogic analyses have been made on carbonate minerals in nonmarine rocks of the northern Green River Basin. Ratios of stable carbon and oxygen isotopes were measured in near-surface waters, in organic matter of shales, and in carbonate minerals of sandstones and shales. Normal hydrostatic pressures in the basin indicate an open hydrochemical system above 1,500 m, whereas overpressuring below 3,000 m implies a relatively closed system. Reductions in porosity and permeability with depth not only lead to overpressuring but also lead to the isotopic modification of pore water by reducing the water/rock ratio. In sandstones, 13C and 18O of calcite cements generally become less negative with increasing depth. The increase of 13C in deeper cements indicates that pore waters receive less 12C from soil CO2 and more 13C from the ubiquitous grains of detrital limestone. Most of the carbon in the calcite cement of the deep sandstones is locally derived from these grains. Similarly, increased concentrations of 18O in deeper cements require equilibration with 18O-enriched pore fluids, which evolve by less input of 16O from meteoric waters and a greater input of 18O from silicate and arbonate diagenesis at elevated temperatures. With increasing depth in the overpressured zone, measured 18Ocalcite in sandstone cements and in bulk shales converge from opposite directions toward a value of about - 13 PDB. This suggests that some portion of these calcites continually recrystallizes during burial and approaches isotopic equilibrium with pore waters, which are similar in the shales and in the restricted hydrochemical system of the sandstones. Shifts in the isotopic composition of bulk dolomite in sandstones and shales occur because authigenic dolomite is precipitated as overgrowths on detrital grains of dolomite. The 13C values suggest that the carbon in the authigenic overgrowths comes from either detrital limestone or dolomite.