Sandstone Diagenesis in Mixed Siliciclastic-Carbonate Sequences: Quadrant and Tensleep Formations (Pennsylvanian), Northern Rocky Mountains

Abstract

ABSTRACT Sandstones in the Quadrant and Tensleep Formations are well to extremely well sorted and compositionally mature (mean = Q98). Out of 9 cements, 4 have special significance: 1) anhydrite (earliest), 2) dolomite, 3) quartz, and 4) calcite (latest). The distribution of major cement types was strongly influenced by vertical location within sandstone units. Early anhydrite and dolomite are principally responsible for pore space reduction near sandstone-carbonate contacts. These cements were precipitated from pore fluids with compositions controlled by depositional setting and proximity to enclosing carbonate sediments. The later precipitation of quartz overgrowths preferentially occluded pore space toward the middle of sandstone beds. Calcite, sporadically distributed throughout sandstones, represents a late-stage, passive pore-filling event. The middle 50% (thickness) of sandstone bodies exhibits porosities averaging about two times greater than lower and upper portions of sandstone units. Hence, Quadrant and Tensleep sandstones are highly stratified on a relatively small scale as to cement and porosity distribution. Estimates of starting porosity and measurements of intergranular volume show that, regardless of the location within sandstone bodies of the Quadrant, cementation dominates over compaction in terms of intergranular porosity destruction. This relation also holds for the base and top of sandstone units in the Tensleep. Only within the middle portion of sandstones of the Tensleep does compaction equal or dominate over cementation in destruction of pore space.