Fitting the Facies Mosaic Together: Controls on Lateral Heterogeneity of Microbial Reefs

Abstract

The lateral heterogeneity of sedimentary sequences frustrates attempts to predict and project the characteristics of sedimentary rocks through space and stratigraphy. The Cambrian Carrara Fm of Death Valley, USA, has been described as a “facies mosaic” because of its lateral heterogeneity. In the upper Carrara Fm, microbialites interfinger with channel fill, lagoonal deposits, and foreshore to shoreface facies in a mixed carbonate-siliciclastic system. In some locations, microbialites grow into large reefs. Continuous exposure of the Carrara Fm over 2 kilometers at Eagle Mountain allows detailed investigation of lateral heterogeneity within the formation, with implications for understanding and predicting facies distributions in microbialite systems.

We present high-resolution drone imagery, high-resolution stratigraphic sections and geochemical carbon and oxygen isotope analysis of the Carrara Fm. We test facies models that predict where different types of microbialites grow. We also use a new database of carbonate stratigraphy that spans the Precambrian through Cambrian. We have digitized high-resolution bed-scale facies stacking patterns from over 150 formations, for a sum total of 65 vertical kilometers, to test the predictions from the Carrara Formation on a broader set of formations and field locations. The high resolution, comprehensive nature of this database, and its focus on all beds within a formation, rather than only on atypical features, allows us to test hypotheses regarding the spatial and environmental relationship of facies in microbially dominated environments. Some clear patterns emerge: oncoids and thrombolites are temporally and spatially linked, and Cambrian stromatolites consistently occur in shallower, more restricted environments than thrombolites.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019