Quantitative Studies of Compaction in Mississippian Skeletal Limestones, New Mexico

Abstract

ABSTRACT Quantitative study of compaction in Mississippian coarse-grained skeletal grainstones and cement-rich packstones over a region of about 25,000 km2 of southwestern New Mexico has shown that intergranular compaction was a major process of porosity destruction in more than 90 percent of all coarse grainstones and packstones. Having been deposited with at least 42 percent intergranular porosity, these calcarenites now contain about 27 percent intergranular space (now occupied by cement plus mud). They, therefore, have lost about 38 percent of their intergranular porosity by intergranular compaction, the remainder having been lost by cementation. Regionally, compaction intensity is greatest in the north, with southern measured sections on average containing about 32 percent and orthernmost sections about 20 percent intergranular space. Point-count data on grain contacts show that intergranular pressure solution increases with compaction intensity and is most abundant in the north. It was the main process affecting intense intergranular compaction and was the reason for compaction intensity being greatest in the north. Bryozoan deformation and grain rearrangement, although poorly quantified, were also important contributors to intergranular compaction, whereas mechanical breakage was of negligible importance. Of the factors that potentially controlled compaction intensity, chemistry of paleogroundwaters and overburden were probably the most important, whereas bryozoan content, terrigenous clay content, and depositional textures may have been of secondary importance, and tectonic stress was probably of no importance. We propose a model in which a major control on intergranular pressure solution intensity was the degree of saturation with respect to calcite in pre-Pennsylvanian groundwater systems. These groundwaters flowed from north to south, with the main recharge region in the north. Groundwaters near and in the recharge regions in the north more commonly were undersaturated or had lower degrees of saturation with respect to calcite compared to groundwaters in downflow (south) regions. This promoted greater chemical compaction in the north even under low to moderate overburden (tens to hundreds of meters: maximum of about 1000 meters).