THE CHARACTER AND ORIGIN OF INCLINED HETEROLITHIC STRATA DEPOSITED WITHIN SINUOUS PALEO-RIVERS OF THE CAMPANIAN LOWER CASTLEGATE SANDSTONE, UTAH

Outcrop investigation of inclined heterolithic strata (IHS) within the Campanian Lower Castlegate Sandstone near Crescent Junction, Utah, has revealed the complex internal stratal architecture of these deposits. The IHS accretion packages comprise approximately half of the exposed fluvial strata within the local study area (approximately 1 sq. km). Detailed planform mapping of beds and bounding surfaces within the IHS packages as well as adjacent sandstone barforms was undertaken to understand the relationship between river-bend geometry and lithofacies distributions. Measured paleocurrents and bed orientations are consistent with sinuous river bends and lateral accretion of bar deposits, and the paleo-river system is consequently interpreted as a meandering river. Extensive sampling for grain-size measurements was done to understand the relationship between planform geometry, paleo-sediment transport conditions, and stratal properties.

The observed IHS packages exhibit significant variability in grain size and internal structure, and are punctuated by several reactivation surfaces. Facies present within the IHS packages include 1) organic-rich mudstone and shale, 2) very fine- to fine-grained sandstone with mud-draped ripples and abundant plant fragments, and 3) fine-grained, scour-based, cross-stratified or rippled sandstone with rip-up clasts and abundant plant fragments. Paleocurrents measured from ripples are highly variable with both upstream and downstream directions represented. Facies 1 and 2 are interpreted to have been deposited from suspension within back-eddy cells generated by flow separation around a sinuous bend based on the reconstructed planform geometry. The low flow velocities necessary for in-channel mud deposition have been demonstrated within these cells in modern rivers, experimental channels, and numerical flow models. Facies 3 is interpreted as bedload deposits created by dunes and traction ripples within the channel thalweg. The presence of Facies 3 and the reactivation surfaces within the IHS packages suggest intermittent collapse of the flow-separation cell. The transient nature of flow separation results in highly heterogeneous accreted deposits which record both the smallest and largest sediment particles in transport within a given reach.