Carbonate deposition, physical limnology and environmentally controlled chert formation in Paleocene-Eocene Lake Flagstaff, central Utah

Abstract

Lake Flagstaff (Paleocene-Eocene of central Utah) owed its existence, large size, shallowness and low clastic content to its tectonic setting, but the climate controlled its mostly autochthonous sediments. During the first and most humid of the three lake phases, the lake was fresh, calcareous, large, and highly productive, with abundant vegetation and snails. The first deposits were limestones with gradually increasing detrital dolomite eroded from lake-marginal carbonate mudflats that were undergoing penecontemporaneous dolomitization. Continuous fluctuation in lake level produced many secondary dewatering and pedogenic-paludine fabrics. The lake developed into a playa lake during the very arid second phase, becoming restricted in area and fauna. The water was saline and alkaline, and twice became a gypsum-precipitating brine. Frequent exposure of the lake beds resulted in much calichification and in rapid and complete dolomitization. Under the intermediate climate of the third phase, the lake reexpanded and freshened, snails returned, and many limestones escaped dolomitization. Each phase defines a member of the Flagstaff Formation.Chert, absent in the oldest beds, increases up-section. The lowest chert forms minor small spheres and linings in voids near the top of the lowest member, suggesting limited silica input and post-dolomitization precipitation in areas of greatest groundwater movement. The base of the middle member contains horizons of atextural chert mudcrack fillings and nodules that appear to have been an inorganically precipitated gel. Most of the middle member, however, contains abundant chert nodules that replaced limestone prior to dolomitization. The nodules grew prior to burial (some even prior to mudcrack formation) by the movement of dissolved silica to areas of relatively low pH. In the upper member, chert is even more prevalent: entire thin beds were pervasively but incompletely silicified prior to dolomitization, probably by vertical evaporative pumping of siliceous groundwater moving under mudflats toward the lake.