Abstract

Ground-penetrating radar (GPR) measurements, in conjunction with outcrop sedimentology, were carried out at Murphy Creek reservoir in the Upper Cretaceous Turonian Wall Creek Member of the Frontier Formation in Wyoming. The objectives were to apply GPR to map geometrical details of a top-truncated lowstand delta front and to estimate the volumes of the prograding bar deposits of the delta lobe. Eleven GPR profiles totaling about 4400 m (14,435 ft) were acquired using 50-MHz antennas on a coarsely spaced, two-dimensional grid of lines lying parallel and perpendicular to the average depositional dip. Ground-penetrating radar reflections were detected from within the outcrop to a depth of about 10–15 m (33–49 ft). Four southerly dipping major surfaces identified in the GPR data are correlated with the boundaries of progradational delta-front facies, stacked as distal mouth-bar deposits, in the outcrop. The major boundaries correspond to lithological changes between relatively clean sandstones that are interpreted to have been deposited during floods with high sediment supply, alternating with bioturbated sandstones and mudstones deposited during interflood periods with correspondingly low sedimentation rates. These two lithological units, which also correspond to the two main GPR facies, repeat at least three times with no change in dominant average sand-grain size. Subsequent erosion by transgressive ravinement caused the significantly truncated lowstand delta long after the sandstones were deposited. The bar assemblage volume at successive stages of growth is estimated using measurements from the outcrop and the GPR data. The migrating bars have an estimated average half-length of 650 m (2132 ft); a lower bound on the average volume of the bar is 3.9 × 106 m3 (1.37 × 108 ft3). As the volume of the bars increases, the bar deposits appear to have a landward as well as a basinward component of accretion.

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