LACUSTRINE SEQUENCE STRATIGRAPHIC EXPRESSION WITHIN THE BALANCED FILL TO OVERFILLED LANEY MEMBER OF THE EOCENE GREEN RIVER FORMATION

The complex sequence stratigraphic packaging of the Laney Member records an overall trend toward decreasing basin subsidence, erosion of basin sills, or increasing sediment + water supply. Balanced fill strata of the lower LaClede Bed constitute a sequence set that can be divided into 4 sequences, which is recognized by an exposure surface and basinward shift in facies at its base. Internally, each of these sequences are composed of repeated1-3 m lacustrine parasequences. The 4 sequences thin upward in succession, which we interpret to record net expansion of Lake Gosiute and a resultant decrease in sedimentation rates near the basin center. Near the top of this succession alluvial facies are closely interbedded with profundal muds, suggesting a decrease in basin-floor gradient.

The overlying “buff marker” and upper LaClede and Sand Butte Beds together are interpreted as a single sequence that comprises both balanced fill and overfilled strata. Deposition of this sequence coincided with a major re-organization in the drainage basin, as evidenced by a shift in basin depocenter from west to east across the Rock Spring Uplift. The basal sequence boundary is identified as a major desiccation event, recorded by mudcracks up to 3 m deep that formed in underlying profundal mudstone. Volcaniclastic and intraclastic fluvial strata were deposited above this sequence boundary, followed by a series of retrogradational, balanced-fill lacustrine parasequences that culminate in a maximum expansion surface. This surface marks a threshold above which the basin remained hydrologically open. The basin was subsequently infilled by progradation of volcaniclastic deltaic sediments as lake level remained relatively stable.