Shelf Storm-Deposited Sandstones, Upper Mancos Shale, San Juan Basin, New Mexico: ABSTRACT

Abstract

Hummocky cross-stratified and amalgamated sandstone beds in the upper Mancos Shale of the San Juan basin record sedimentation on a storm-dominated shelf. Interbedded silty shale, siltstone, and sandstone form coarsening-upward cycles in Mancos shelf deposits. Abundance of cycles suggests storms were frequent, and preservation potential of shelf storm deposits is high. Mancos storm beds typically have an erosional base with tool marks, and planar laminations overlain by hummocky cross-stratification (HCS). HCS comprises 20-90% of the primary sedimentary structures in these beds. Wave-ripple lamination commonly overlies HCS in <= 2-m thick beds high in the section. Less commonly, wave ripples overlie planar lamination in some thinner beds (5-15 cm) low in the section. In comparison, planar lamination commonly overlies HCS in storm sequences. Paleoflow trends are northeast to southwest, normal to presumed regional paleoshoreline. Distal to proximal changes in the beds are seen in vertical section. Increases in sand to shale ratio, amagamated beds, abundance of sole marks, average grain size (from 0.05 to 0.125 mm), and wavelength of HCS are seen upsection. Bioturbation of beds decreases upsection. These trends reflect progressive shoaling of the Mancos shelf. Upward-coarsening interbedded silty shale, siltstone, and sandstone intervals comprise the bulk of upper Mancos shelf deposits. A hummocky storm bed 30-100 cm thick caps each cycle, and contact with the overlying interval is sharp. Contact with the overlying Point Lookout shoreface sandstone is abrupt. The cycles indicate that episodic storm sedimentation characterized the Santonian-Campanian shelf in the study area. End_of_Article - Last_Page 289------------