TESTING FOR CHANGES IN PROVENANCE IN THE MARINE JURASSIC OF WYOMING AND UTAH

Late Jurassic (Oxfordian) marine and terrestrial rocks preserve deposition in a retroarc foreland basin and adjacent cratonic basin. The strata that filled the cratonic basin include shallow marine deposits of the Redwater Shale, shoreline deposits of the Windy Hill Formation, and coastal plain deposits of the Morrison Formation, which were deposited across most of Wyoming. The foredeep on the western flank of this cratonic basin was itself bounded to the west by a fold and thrust belt in central Utah and Idaho. Deposits of the foredeep are recorded by the Preuss and Stump Formations. This study tests whether the Preuss and Stump Formations have a different sand source than the the Redwater, Windy Hill, and Morrison Formations. It is expected that the Preuss and Stump would have been sourced from the fold and thrust belt to the west, and possibly the volcanic arc beyond it, whereas the Redwater, Windy Hill, and Morrison Formations should reflect a cratonic source. Thin-sections were prepared from 70 sandstones collected from all of these formations across Wyoming and northeasternmost Utah. These were doubly stained for plagioclase and potassium feldspar, point counted to 200 sand-sized grains, and plotted on Q-F-L and Qm-F-Lt provenance plots. As expected, Preuss and Stump sandstones differ in their composition from those of the Redwater, Windy Hill, and Morrison. In particular, the Preuss and Stump have a greater proportion of feldspar, especially plagioclase, and a lesser proportion of lithic grains, notably chert. Both sets of samples are quartz-rich, with the Redwater, Windy Hill, and Morrison plotting in the quartzose to transitional recycled orogen provenance fields, and the Preuss and Stump plotting in the transitional continental to craton interior fields. The increased amount of feldspar in the Preuss and Stump may reflect uplift of immature sandstones in the eastern part of the fold and thrust belt, granitic basement uplift in the westernmost part of the fold and thrust belt, or possibly an arc source to the west of the fold and thrust belt.