GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 218-3
Presentation Time: 2:05 PM


NALIN, Ronald, Geoscience Research Institute, 11060 Campus Street, Loma Linda, CA 92350, OHIARA, Timothy, University of Manchester, School of Earth and Environmental Sciences, Manchester, United Kingdom, PRINCE, Kieron, Texas A&M University, Department of Geology and Geophysics, College Station, TX 77843, NANA YOBO, Lucien, Department of Earth and Atmospheric Sciences, University of Houston, Science and Research Building 1, 3507 Cullen Blvd, Houston, TX 77204-5007, BRAND, Leonard R., Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA 92350 and CHADWICK, Arthur V., Geology, Southwestern Adventist University, 100 Magnolia, Keene, TX 76059

The Notch Peak Formation, a unit of the Cambrian to lower Ordovician great American carbonate bank deposited on the passive margin of Laurentia, contains a laterally continuous stratigraphic contact between two of its members (Hellnmaria and Red Tops). We mapped this contact over most of its area of exposure in Millard County, Utah, for ~70 km in the N-S direction and ~20 km in the E-W direction, and found that it invariably consists of a flat erosional surface. Lithofacies cut by the surface vary, the most common being a widespread bed of columnar stromatolites (up to ~2 m thick) or a ~50 cm thick, fine-grained limestone package, both capping a ~10-15 m thick, coarsening-upward succession from wavy-laminated micritic mudstone to oolitic and bioclastic limestone. The erosional contact is overlain by a thin (2-5 cm) interval of laminated, very fine grainstone to packstone, and then by several meters of relatively high-energy facies (grainstones, intraclastic conglomerates, and laminated fines with common HCS), all containing a siliciclastic fraction of silt-sized grains. No channels, scours, or relief were observed along the surface, except for rare, very low amplitude undulations, of a few centimeters over several meters. Widespread early marine cementation of the substrate before truncation is inferred from petrographic data and lack of differential erosion between laterally adjacent lithofacies below the surface. No ichnological overprint, attachment of build-ups, or overlying basal lag is observed along the surface. δ13C values of carbonate samples, from vertical transects covering several meters above and below the surface, range from -1.36 ‰ to 0.8 ‰, and δ18O values range from -8.15 ‰ to -10.68 ‰. The δ18O isotope ratios do not positively correlate with δ13C suggesting depletion in δ13C cannot be explained by meteoric diagenesis. We favor a submarine and physical rather than subaerial and chemical process for the origin of this surface and suggest its formation was related to a sudden increase in energy conditions over the whole shelf, concomitant with offshore redistribution of fine siliciclastics from the inner detrital belt, aided by vigorous winds and storm systems. The abrasive power of this minute detrital component may have played a role in the beveling of the cemented substrate under significant wave action.