SPRING MOUNDS WITHIN THE TIPTON AND WILKINS PEAK MEMBERS OF THE EOCENE GREEN RIVER FORMATION: GEOCHEMICAL AND SEDIMENTOLOGICAL INFLUENCES ON LAKE GOSIUTE

Irregular thick-bedded to massive dolomitic mound-like structures occur within the Rife Bed of the Tipton Shale Member and the Wilkins Peak Member of the Eocene Green River Formation. They are located approximately 25 kilometers north of Rock Springs, WY. Three structures occur within 1/2 a kilometer along the strike of the outcrop. They are approximately 12 meters high and 35 meters wide. We are interpreting these structures as spring deposits due to vuggy, honeycomb textures, mound shape, vadose void spaces coated with laminated calcite, and high silicic composition at the mound center compared to surrounding lake sediments. This evidence suggests these mounds formed from a meteoric water source. Present outcrops provide an excellent cross-sectional view showing groundwater interaction with ancient Lake Gosiute during its underfilled stage. The basal contact is conformable with the Farson Sandstone, which may have provided an aquifer for the source of spring water discharge. Lake sediments appear to interfinger with the mounds. This is illustrated by a stromatolite bed that can be traced into the mound. This bed becomes progressively more silicic toward the mound core. The mound deposits range in mineralogy from calcite to dolomite to dominantly quartz at the core. This suggests lithology variation due to spring water-chemistry interaction with the spring mounds. A detailed study of the lithologic, stratigraphic and geochemical trends between the spring mounds and surrounding lake sediments may have implications to theories concerning the origin of carbonates during Wilkins Peak time, especially with respect to the influence of spring-water discharge.