Abstract

The Sussex Sandstone at House Creek and the older Shannon Sandstone at Hartzog Draw fields, have been interpreted as thicker, relatively sandy parts within extensive far-offshore shelf sandstone and shale complexes. The complexes were deposited on broad depositional shelves in the early Campanian seaway and are now enclosed in the marine Cody Shale. Exact limits of the complexes are uncertain. Shale and thin-bedded sandstone are relatively widespread within the complexes. Mappable sandy bodies in the field areas are about 30 to 50 km long and perhaps up to 25 km wide. Thicker pans containing cross-bedded sandsone form elongate bodies trending about 30°NW at Hartzog Draw and House Creek. Maximum thickness ranges from about 15 m at House Creek to 26 km at Hartzog Draw. Overall shape of the thicker parts is plano-convex upward in cross-section and ellipsoidal in plan; thinning is relatively rapid on the northeastern flanks, which are also relatively straight. General slopes on the flanks of the complexes, estimated from isopach maps, were very slight; relatively steep slopes on the northeastern flanks were probably a fraction of one degree. Lithological variation within the thicker parts of the complexes also displays asymmetry; sandstone on the southwestern flanks is finer grained, relatively shaley, thinner bedded and grades laterally into the more extensive shaly sandstone. In the field sequences, shaley sandstone separates silty and sandy Cody Shale below from cross-bedded sandstone above. Lower parts of the cross-bedded sandstone are relatively variable, forming the lower part of a sanding-upward sequence. The cross-bedded sandstone is capped by thin, bioturbated, silty sandstone or burrowed sandstone. The transition zone in the lower part is much thicker than that in the upper, resulting in vertical asymmetry. Lateral asymmetry at House Creek field is expressed statistically with wireline log data. Cross-bedding vectors from oriented cores at Hartzog Draw indicate a southerly transport direction; hence steeper slopes and relatively well developed cross-bedded sandstone appear to characterize the left sides of the complexes when viewed down current.