Palustrine-lacustrine and alluvial facies of the (Norian) Owl Rock Formation (Chinle Group), Four Corners region, southwestern USA: Implications for Late Triassic paleoclimate

The Upper Triassic (Norian) Owl Rock Formation was deposited in a low-gradi ent floodbasin at a subtropical paleolatitude. The lower part of the format ion consists predominantly of fine-grained siliciclastic lithofacies deposi ted 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 con ditions. The upper part of the formation consists mostly of interbedded fin e-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 subor dinate Limestone facies displays wavy to irregular argillaceous lamination, desiccation cracks, and oscillation ripples, and is vertically and lateral ly gradational with the brecciated facies, The upper Owl Rock Formation rec ords deposition of aggrading sequences of alluvial sediments deposited duri ng base level rise, capped by highstand carbonates deposited in small peren nial and ephemeral carbonate lakes and ponds. Base-level lowstand in an ove rall semiarid climate resulted in extensive pedogenesis of the limestone an d 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.