Organic dolomite from Point Fermin, California

Petrography and stable carbon isotope data indicate that small dolomite rhombohedra that occur with tar as cement in some California Miocene sands were partially derived from the alteration of the tar or other organic matter. The dolomite cement occurs in blue-schist turbidite sandstones that are exposed in sea cliffs at Point Fermin, Los Angeles County, Calif. The euhedral crystals are attached to sand grains and are also apparently embedded in the tar cement. The dolomite crystals have several types of nuclei: (1) rhombohedral to spherical tar globules similar to the tar surrounding the euhedra, (2) gaseous bubbles, and (3) combinations of these. . . .