SILICA DIAGENESIS, POROSITY EVOLUTION, AND MECHANICAL BEHAVIOR IN SILICEOUS MUDSTONES, MOWRY SHALE (CRETACEOUS), ROCKY MOUNTAINS, USA

Quartz in mudstones of the Mowry Shale (Cretaceous, Rocky Mountain region) takes several forms: extrabasinal detrital silt (some with transported overgrowths), replacement of skeletal debris, minor overgrowth on detrital quartz and replaced radiolaria, pore-filling in the intragranular pores of allochems, and authigenic microquartz that is dispersed through the clay-size matrix. The matrix-dispersed microquartz has CL character similar to the overgrowths with which it is intergrown, supporting the interpretation that it is authigenic and porefilling. Radiolaria are widely observed in these mudrocks but are poorly preserved by partial mineral replacement (quartz, dolomite, calcite, pyrite). We interpret dissolved radiolaria, and possibly, now-cryptic diatoms, as the likely source of silica for the overgrowths and microquartz, through a reaction pathway that entailed the formation of opal-CT lepispheres that subsequently recrystallized to microquartz. A newly devised variation of the IGV (intergranular volume) concept, a parameter IGV(m), is applied for partitioning the causes of porosity decline in mudstones. All of the samples examined contain authigenic microquartz, which averages around 40% of the rock volume. The large volume of microquartz, together with the volumes of surviving intergranular pores and pore-filling solid hydrocarbon, constrain the porosity to around 50% at the time of initial lepisphere formation. Despite the exceptional abundance of cement, overall porosity decline is compaction-dominated. The presence of significant volumes of cement has implications for mechanical rock properties in the Mowry Shale, which is expected to manifest brittle behavior where cements are most prominently developed.