1963/05/01 by Carel Otte, James M. Parks
Agricultural and Biological Sciences · Earth and Planetary Sciences · #Biocrusts and Microbial Ecology #Geology and Paleoclimatology Research #Aeolian processes and effects
paper · doi:10.1086/626909
Discrete and interconnected masses of dark crystalline calcite make up more than one-half of the Wolfcamp bioherms and the lower one-third of the Virgil bioherms in the Sacramento Mountains. The remainder of the rock consists of light calcite mud with stromatolitic algal deposits, platy algae, and pockets of skeletal calcarenite. Large calcite crystals have a fibrous radiate appearance. The dark masses occur in four distinctive shapes: (1) irregular globular elements from 1 to 3 cm., (2) irregularly horizontal or ribbon-like tabular bodies up to 3 cm. thick and 15 cm. long, (3) nearly vertical finger-like elements up to 6 cm. long and 2 cm. wide, and (4) three-dimensional networks extending over large areas of vertical outcrop and locally forming nearly 90 per cent of the rock. The contacts between the dark calcite masses and the surrounding sediments are very sharp, prominently displaying botryoidal upper surfaces and rounded configurations suggesting organic growth. These crystalline masses are the fossil remains of a framework-building colonial organism similar in size and shape to Stromatactis of earlier workers. Detailed fabric studies of oriented thin sections and polished surfaces show that the calcite mosaics consist of fan-shaped arrays of radially oriented elongate crystals. In some arrays closely spaced lineations extend across several crystals parallel to the radial pattern. The larger growth forms are composites of many adjoining and superposed arrays, locally separated by thin concentric layers of calcite mud and pellets. Crystals in a single mosaic are of uniform size and shape. The dark crystal mosaics can be distinguished from void space crystallization and grain-growth fabrics, which are also present. Indigenous organic matter causes the dark coloring of the crystalline calcite, which contains almost two times as much organic carbon as the calcite mud. Terrigenous clay and quartz are at least five times as abundant in the calcite muds as in the crystalline material. The Stromatactis-like organism was a firm-bodied but uncalcified colonial form of unknown biologic affinity. It grew above adjacent sediments in various growth forms in response to local environmental differences. We infer that, after burial, calcite was precipitated as microvoid fillings with the fabric controlled by the cellular tissue of the dead organism. "Stromatactis" functioned as both a sediment-binding and a framework-building organism in the construction of the bioherms and may be quite widespread in other late Paleozoic bioherms of the western United States.