Sedimentary Cycles in Green River Formation (Eocene): Modification of Walther's Law: ABSTRACT

Abstract

Along Raven Ridge in northeastern Utah, the Parachute Creek Member of the Green River Formation contains contemporaneous sedimentary cycles that range in environment from fluvial through deep lacustrine. End_Page 722------------------------------ Fluvial cycles consist of basal channels, channel-filling sandstone, and silty claystone floodplain deposits capped by soil(?) horizons. Deep lacustrine cycles are alternations of oil shale and thin carbonate rock. Complex nearshore cycles contain a basal terrigenous sequence, grading upward from mudstone to sandstone, and an upper carbonate sequence of oomicrite and oosparite capped by discontinuous algal mats and stromatolites. Cycles in other paleogeographic settings are modifications or combinations of these three basic types. Through correlations of groups of cycles, fluctuations in the extend of Lake Uinta and resultant changes in local base level are interpreted to have produced the cycles. Walther's Law, which relates lateral facies changes to similar vertical sequences, cannot be applied to the Parachute Creek if only lithofacies are considered. For example, fluvial disconformities are equivalent to terrigenous lacustrine rocks, but none of the cycles contains both. Thus, Walther's Law must be modified to consider events and history, rather than lithofacies. Then the fluvial disconformities and terrigenous lacustrine rocks are seen to represent a single event, a fall in base level, and should not occur in vertical sequence. This modification of Walther's Law does not violate its original intent. Rather, the applicability of this useful principle is broadened by removing the general requirement of regionally extensive lithofacies. Furthermore, attention is focused on history and causes, rather than on products. End_of_Article - Last_Page 723------------