Interfluve sequence boundaries in the Grassy Member, Book Cliffs, Utah: criteria for recognition and implications for subsurface correlation

Abstract

Abstract The Late Campanian Grassy Member of the Blackhawk Formation, Mesaverde Group, Utah consists of two unconformity-bounded fourth-order sequences within a third-order highstand sequence set. Several orders of bounding surface define the internal architecture of each sequence and component parasequences. Incised valley fill deposits, dominated by braided channel-fills updip and laterally accreted fluvial and estuarine channel-fills down dip, overlie the fourth-order sequence boundaries, truncating shoreface and shelf facies of underlying sequences. Incised valleys are separated by extensive interfluves with evidence of subaerial exposure and minor tributary channel systems. The interfluve gullies are interpreted to have transported very little bedload during lowstand and acted primarily as rainwater run-off or bypass channels lateral to the main incised valley system. They are filled with bioturbated fine-grained sandstone and siltstone and capped by marine shale. Interfluve areas provide subtle but crucial evidence for base level falls, including rare examples of roots penetrating formerly offshore marine shale, partially eroded hematitic hardpan sediments and sand or silt filled gullies containing coal and shell debris. Interfluve sequence boundaries have powerful predictive application in respect to recognition of along-strike incised valley fills. Recognition of these surfaces is commonly hampered by removal of subaerial indicators as a result of subsequent transgressive erosion. There is indirect stratigraphic evidence of an emerging salt or basement fault driven structural high, orthogonal to the Late Campanian palaeoshoreline, which controlled the locus of valley incision and associated interfluves in two successive high frequency sequences.