PETROGRAPHIC ANALYSIS OF ORIGIN AND EVOLUTION OF POROSITY IN LATE CRETACEOUS SEEP CARBONATE

A belt of Late Cretaceous authigenic carbonate mounds extends from South Dakota to New Mexico and represents carbonate formed from effluence of hydrocarbon to the sediment-water interface during the time of the Western Interior Seaway (Late Campanian). Petrographic studies of these exposures in Pueblo County, Colorado, known as the Tepee Buttes, reveal complicated carbonate structures typical of seep carbonates. Closer inspection reveals a high degree of recrystallization and a variety of void and pore spaces both open and infilled. The types of voids and pores as well as their history of formation and infill can reveal much about the depositional and diagenetic history of these complicated rocks. Pore spaces present in hand sample and thin section are herein classified into three categories: boring holes created by organisms when rock was exposed at or near the sediment-water interface, conduit holes created as hydrocarbon fluids escaped through the sediment-water interface, and dissolution cavities which are a later diagenetic feature containing void spaces and dissolved bivalve shells. The three different types of pore spaces make up roughly 47% of the total rock, with boring and conduit holes making up roughly 15% of total pore spaces and dissolution cavities occupying the remaining 85% of pore spaces. Closer examination via SEM, reveals a difference in magnesium to calcium ratios of replaced bivalve shells (0.00) versus calcitic in-filling of other pore spaces (0.02 - 0.11). Cross-cutting relationships seen in the three different categories of pore spaces seem to indicate a relative timing of events with conduits forming concurrently with soft matrix formation (Stage 1), boring holes appearing after the hardening of matrix rock (Stage 2), and dissolution cavities occurring during several periods of diagenesis (Stages 3+) that occurred throughout the recession of the Western Interior Seaway and during later exposure to non-marine conditions. SEM results and size of blocky calcite crystals within dissolution spaces show that void space crystallization preceded and was distinct from bivalve shell recrystallization. Despite the complex overprinting, existence of visible trace fossils points to an ecosystem that is similar to those surrounding authigenic carbonate seep sites in the present day.