ABSTRACT

The Upper Triassic (Norian) Owl Rock Formation was deposited in a low-gradient floodbasin at a subtropical paleolatitude. The lower part of the formation consists predominantly of fine-grained siliciclastic lithofacies deposited by sheetflood and sinuous streams on a muddy floodplain during a period of continuous basin aggradation. Nodular calcretes are increasingly mature higher in the formation, suggesting increasingly episodic depositional conditions. The upper part of the formation consists mostly of interbedded fine-grained siliciclastic facies and laterally continuous ledges of limestone and sandstone. The predominant limestone facies has brecciated to peloidal fabrics, spar-filled circumgranular cracks, and root channeling. The subordinate limestone facies displays wavy to irregular argillaceous lamination, desiccation cracks, and oscillation ripples, and is vertically and laterally gradational with the brecciated facies. The upper Owl Rock Formation records deposition of aggrading sequences of alluvial sediments deposited during base-level rise, capped by highstand carbonates deposited in small perennial and ephemeral carbonate lakes and ponds. Base-level lowstand in an overall semiarid climate resulted in extensive pedogenesis of the limestone and laterally equivalent alluvial facies. Basin wide variations in base level are interpreted as resulting from climatic fluctuations. This depositional model is consistent with an interpreted trend towards aridification on the Colorado Plateau during the Late Triassic as Pangea drifted northward from one climate zone to another.