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Tectonically Controlled Nearshore Deposition: Cozzette Sandstone, Previous HitBookNext Hit Cliffs, Colorado, USA


The Previous HitBookNext Hit Cliffs of eastern Utah and western Colorado have been pivotal in the development of outcrop-based nearshore sequence Previous HitstratigraphicNext Hit concepts. Prior studies in the area, and more generally in the Cretaceous western interior foreland basin, have concluded that nearshore accumulation is controlled by the interaction between oscillatory eustatic change and longer-term patterns of flexural subsidence. New outcrop and subsurface evidence reported here from the eastern Previous HitBookTop Cliffs suggests that 3D tectonic tilting at length scales of up to 50 km and timescales of <200 kyr also strongly influenced sedimentation. Documented patterns of accumulation are inconsistent with those expected from interactions of eustasy and regional flexure alone. The upper Campanian Cozzette Sandstone Member of the Mount Garfield Formation consists of twelve lithofacies arranged into six lithofacies assemblages, inferred to have been deposited in shallow marine, marginal marine, and nonmarine settings. Shallow marine facies are organized into six wedge-shaped units <40 m thick, bounded by flooding surfaces, and separated into three larger-scale cycles. These successions are interpreted as shoreface foreshore-swamp-parasequences, and are erosionally overlain by fluvial and estuarine deposits. The proposed interpretation is based on high-resolution correlations made from digital video and photographs acquired during a helicopter survey. Our interpretation differs significantly from published cross sections, in which the Cozzette is inferred to consist of at least two sheet-like shallow marine cycles. Facies variations, stratigraphic thickness trends, and geometrical relationships reveal that three basinward-landward cycles of syndepositional-postdepositional tectonic tilting in the southern Piceance controlled accumulation in the Cozzette. Differential subsidence to the southeast led to the development of a northeast-trending clinoform rollover. Subsequent tilting to the north resulted in renewed accommodation creation, and a reorientation of the rollover to an eastward trend. Following deposition of six parasequences, tilting towards the northeast resulted in the development of a sequence boundary. The main conclusion of our study, that nearshore sedimentation was modulated by local tectonism at short timescales, casts doubt on the generally accepted eustatic paradigm at a location that was among the most important in the development of sequence stratigraphic concepts.