Internal Structures and Fabrics of Upper Cambrian Microbial Buildups (Mason County, Central Texas)

Abstract

A three-phase growth model was developed for Upper Cambrian microbial buildups, uniquely outcropping in central Texas, by integrating overall buildup geometries and their relationships with coeval interbuildup strata. Several lateral and vertical core transects, complemented with analysis of several thin sections, improved the understanding of the buildup growth and their internal structures and microbial fabrics. Nucleating on sheets of flat-pebbles, Phase 1 growth results in 3-4 m high buildups defined by their distinct and early cemented outer margin, a thick thrombolitic rind. Buildup interiors exhibit amalgamated microbial heads with poorly preserved internal structure enveloped by cm-thin thrombolitic rinds. The buildups grew in high-energy conditions coevally, but without interacting, with the accumulation of interbuildup oolitic-bioclastic grainstones. Phase 1 terminated with onlapped terrigenous and calcareous silts (35-40% CaCO3). The overlying Phase 2 growth, produces buildups up to 8 m thick characterized by a mutual interaction with interbuildup high-energy oolitic bioclastic grainstones and packstones and, with few exceptions, lacks an external rind. Internally, Phase 2 growth consists of vertically aggrading and laterally expanding stromatolitic columns, each exhibiting a cm-thin thrombolitic rind that directly interacts with intercolumn bioclastic grainstones. Phase 3 of growth develops a well-defined thrombolitic 2-3 m thick rind, crowning the top of Phase 2. These large buildups are onlapped by mixed (as low as 16% of CaCO3) silts. Thrombolitic fabrics evidence direct precipitation of carbonate induced by microbial colonies. This fabric dominantly consists of original microbial calcite micrite as clots with fossil fragments in an extremely tight framework that hindered diagenetic effects. Cavities not infilled by microbial calcite contain dolomitized sediments. Stromatolitic fabrics show alternating laminations of microbially precipitated calcite and amalgamated trapped grainstones. Ferroan dolomite replacement and subsequent oxidation of these grains are observed in thin section. Pervasive diagenesis of the second phase was induced due to a higher porosity-permeability and an interfingering nature with coeval sediments, shown in cores and outcrops. A diverse microbial community, including Epiphyton, Renalcis, Girvanella, and Tarthinia, is readily observed in thrombolitic fabrics with fewer discernable stromatolitic microbe species.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017