Petrology of Quaternary Fluvial Low-Magnesian Calcite Coated Grains from Central Texas

Abstract

ABSTRACT Coated grains formed and accumulated as fluvial deposits, 4 to 5 m thick and up to 16 km wide, parallel to contemporary streams in central Texas. Grain size and sedimentary structure data indicate deposition by bedload ephemeral streams. The source of the calcium carbonate was spring water from the Edwards aquifer which today has a Mg/Ca ratio of 0.5 and is supersaturated with respect to calcite. Ooids are most common in medium- to very coarse-grained sands and range in size from 0.21 mm to 1.5 cm. Cortices consist of alternating fine- and coarse-grained laminae. Fine-grained laminae are 0.5 to 20 µm thick, consist of clay minerals and organic matter, and extend into the interstices of radially oriented calcite crystals of the coarse-grained laminae. Coarse-grained laminae are 4 to 100 µm thick and consist of radially oriented, elongate crystals of low-magnesian calcite 2 to 3 µm wide and 10 to 20 µm long. Rhombohedral terminations of individual crystals occur on ooid outer surfaces and in cortical interiors between coarse- and fine-grained laminae. A primary low-magnesian calcite mineralogy is indicated by the presence of rhombohedral terminations and he absence of replacement fabrics. The stout nature of the original crystals was responsible for the development and preservation of the radial cortical structure. Pisoids most commonly occur in gravels and range in size from 0.67 mm to 12 cm. Nuclei are most commonly terrigenous pebbles and show evidence of boring in the form of dark irregular filaments, approximately 7 µm in diameter, which contain organic matter. Cortices are biogenic in origin and consist of porous zones and zones of dense laminae. Dense laminae are a few tenths of a millimeter thick and are composed of aphanocrystalline low-magnesian calcite. Porous zones are a few tenths of a millimeter to several centimeters thick and consist of domed accretionary bodies. Accretionary bodies range from a millimeter to several centimeters in area and consist of radially oriented, branched filaments 10 to 20 µm in diameter transecting alternating laminae of finely to very finely crystalline calcite.