Tectonically Controlled Nearshore Deposition: Cozzette Sandstone, Book Cliffs, Colorado, U.S.A

Abstract

Abstract The Book Cliffs of eastern Utah and western Colorado have been pivotal in the development of outcrop-based sequence stratigraphic concepts for nonmarine to shallow marine siliciclastic depositional settings. Prior studies in this area, and more generally in the Cretaceous western interior foreland basin of North America, have concluded that nearshore accumulation is controlled for the most part by the interaction between oscillatory eustatic change and longer-term regional patterns of flexural subsidence. New outcrop and subsurface evidence reported here from the eastern Book Cliffs suggests that three-dimensional tectonic tilting at length scales of up to ∼ 50 km (31 mi) and timescales of less than ∼ 200 kyr also strongly influenced sedimentation. Continental ice sheets are thought to have been small at the time. 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 depositional environments. Shallow marine facies are organized into six wedge-shaped units < 40 m thick, bounded by flooding surfaces, and separated into three larger-scale cycles, which display along-strike and time-equivalent aggradational and progradational stacking. These successions are interpreted as shoreface–foreshore–swamp parasequences, and are erosionally overlain by fluvial and estuarine deposits. Both fluvial and estuarine accumulations are underlain by composite erosional surfaces, belonging either to two distinct incised-valley fills or to one composite fill. The proposed interpretation is based on high-resolution correlations made from digital video and continuous photographs acquired during a helicopter survey of the member. This interpretation differs significantly from published cross sections of the Cozzette, in which the sandstone is inferred to consist of at least two sheet-like shallow marine parasequences, truncated by sequence boundaries. Facies variations, stratigraphic thickness trends, and geometrical relationships reveal that three basinward–landward cycles of syndepositional to postdepositional tectonic tilting in the southern Piceance basin controlled accumulation in the Cozzette Sandstone. 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 reorientation of the rollover to an eastward trend. Following deposition of six shallow marine parasequences, tilting towards the northeast resulted in bypass and the development of a sequence boundary. The main conclusion of our study, that nearshore sedimentation was modulated by local tectonism at timescales of less than hundreds of thousands of years, casts doubt on the generally accepted eustatic paradigm at a location that was among the most important in the development of sequence stratigraphic concepts.