SANDSTONE PROVENANCE IN THE JURASSIC SUNDANCE SEAWAY, WYOMING, USA

The Jurassic Lak, Redwater, Windy Hill, Preuss, Stump, and Morrison formations of Wyoming and adjacent states were deposited in near the southern terminus of an elongate retroarc foreland basin with a north-south axis. At least three potential sediment sources may have been present near this terminus, wind-blown sediment coming from a granitic source to the east, a southern source of unknown properties, and a western source consisting of a fold and thrust belt and possibly a more distant volcanic arc. Thin sections from fifty-four sandstone samples from these formations were point-counted (200 points each), plotted on QFL, QmFLt, QpLvLs, and QmPK ternary diagrams to diagnose provenance. Although most of the sandstones contain over 60% monocrystalline quartz, the formations can be divided into several groups based on their composition, suggesting that they had different sources. The Lak was deposited on the eastern flank of the basin in desert mudflats, and it is has characteristically elevated amounts of plagioclase (10–15%) and few lithic grains (<2%), which distinguishes it from all other formations and points to a craton interior continental block provenance. The northward-prograding marine Redwater and coastal Windy Hill are typically poor in feldspar (usually <3%, uncommonly up to 15%) but are commonly rich in chert (usually 10-20% and rarely over 30%) and other sedimentary rock fragments (5–15%). Most of these samples plot in the quartzose recycled orogenic field, suggesting a fold-and-thrust-belt source. Preuss and Stump samples, which come from the westernmost part of the outcrop area, are generally poor in rock fragments, but some have elevated feldspar (up to 30%). Although these units occur closest to the fold and thrust belt, and to the more distant volcanic arc, they fall within the craton interior continental block provenance. Finally, the fluvial Morrison Formation, which overlies the entire succession across the region, also lies within the craton interior continental block provenance. Although more samples of Preuss, Stump, and Morrison formations are needed to better characterize their variability, sandstones of the Jurassic Sundance Seaway confirm the presence of distinct sediment sources.