Description
The San Diego area has abundant sedimentary rocks exposed at the surface. Fourteen samples of relatively unweathered sedimentary rocks were collected in order to measure their bulk densities, which is useful in gravity surveys. Bulk densities were obtained using a wax-coated, water-immersion technique. The sample ages range from Quaternary to Cretaceous and the lithologies consist of sandstone and mudstone. The sandstone grain size ranges from very fine to coarse, the roundness from angular to subrounded, and the sorting from poor to well. Most are cemented by calcite, but iron oxides and/or iron oxyhydroxides cement a few and the cement in the others is unknown. The densities of the samples range from 1.75 to 2.35 gm/cc. This range in density is probably due to differences in porosity. On the basis of this assumption, the calculated porosity ranges from 16% to 56%. Samples with angular grains are denser than those with subangular grains, but these are less dense than rocks with subrounded grains. Density shows a general increase from well to poorly sorted rocks. This is typical because the poorer sorting allows smaller grains to fill pore spaces between the larger grains, therefore increasing the density. The density increases slightly from rocks with unknown cement to calcite-cemented rocks. Normally, with increasing geologic time, density increases because of diagenesis. However, the rocks do not show any correlation between density and age. Theoretically, there is no correlation between grain size and density and these samples show none.
