Origin and evolution of weber and tensleep formation waters in the greater green river and Uinta-Piceance basins, Northern rocky mountain area, U.S.A.

Abstract

Laramide structures exercise strong control on fluid flow and differences in pore-water chemistry between the Weber Sandstone in the Uinta-Piceance Basin and the Weber in the adjacent Uinta Mountain and White River uplifts. Similar relationships between structure and variations in pore-water chemistry exist for the Permo-Pennsylvanian Weber and Tensleep sandstones in the Greater Green River Basin and the Weber in the Uinta Mountain Uplift. Stable-isotope and chemical data indicate that Weber pore water in the Uinta Mountain and White River uplifts and Tensleep pore water along the eastern margin of the Greater Green River Basin are young, fresh to saline, Ca SO 4 and Na Cl, meteoric waters which probably entered these Laramide structures subsequent to Mio-Pliocene epeirogenic uplift of the area. In contrast Weber pore waters in the Uinta-Piceance and central Greater Green River basins are Na Ca Cl and Na Cl SO 4 HCO 3 brines, respectively. These brines are separated from the meteoric waters in the uplifts by major Laramide thrust faults which mark the flanks of the uplifted blocks. Although the origin and evolution of the Na Cl SO 4 HCO 3 brine in the Greater Green River Basin are uncertain, stable-isotope and chemical data indicate that the Na Ca Cl brine in the Uinta-Piceance Basin originated as evaporated seawater during precipitation of the Pennsylvanian Eagle Valley Evaporites in the Eagle Basin which lies immediately east of the Uinta-Piceance Basin. This brine migrated out of the Eagle Basin, which is now part of the White River Uplift, prior to the Laramide orogeny and diagenetically altered the Weber Sandstone. The presence of ancient brines in the Weber Sandstone in both the Uinta-Piceance and Greater Green River basins indicates that there is no hydraulic continuity between the Weber Sandstone in the uplifts and the basins. Their presence also suggests that fluid flow in the deeper strata is stagnant and independent of fluid flow in the uplifts. Although these brines are now restricted to the post-Laramide basins, they probably were much more widespread throughout the Utah and Wyoming thrust belt and its foreland prior to Laramide tectonism. Much of the diagenetic alteration of the Weber Sandstone probably took place in the presence of these brines.