GENESIS OF A WIDESPREAD EROSIONAL SURFACE ON THE CONTINENTAL SHELF OF LAURENTIA (UPPER CAMBRIAN, NOTCH PEAK FORMATION, UTAH)

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.