A study of the mineralogy and lithology of cuttings from the U.S. Bureau of Reclamation MESA 6-2 Drillhole, Imperial County, California, including comparisons with MESA 6-1 Drillhole

Abstract

The Mesa 6-2 drillhole penetrates 6,000 feet of sediments in Imperial County, California. The cuttings material from the upper part of the drillhole is chiefly unconsolidated mud and silt. Soft siltstone fragments occur at about 1,400 feet and increase in amount down to 2,400 feet. Some pebbles are found above 2,400 feet, but the pebble-rich horizons are less distinctive than the pebble zone in the Mesa 6-1 drillhole. Below 2,400 feet, cuttings ape composed of about two-thirds siltstone and one-third sandstones, ranging from very fine to very coarse sand, plus loose Sand grains. Although below 2,400 feet there is no systematic change in color of siltstones, grain size, or cementation with depth, horizons composed predominantly of loose sand are more common at deeper levels. Fragments of igneous and metamorphic rocks are less common than in the Mesa 6-1 drillhole. Quartz, calcite, K-feldspar, plagioclase (albite), illite, and mixed layer clays are identified by X-ray diffractograms of whole-rock samples throughout the hole. Chlorite occurs in all samples from below 2,100 feet, and probably also occurs at shallower depths. In most siltstones, montmorillonite occurs only down to the interval 2,200-2,300 feet, but in the buff siltstone it is found to the bottom of the drillhole. Kaolinite occurs at least down to 4,700 feet. Dolomite is found down to at least 5,970 feet, but is generally absent from horizons composed mostly of loose sand. Pyrite occurs in many samples. No zeolites, ankerite, or amorphous sulfur were detected. Theme is no horizon that may be used fop conclusive correlation with the Mesa 6-1 drillhole.