DEPOSITIONAL PROCESSES AND ARCHITECTURE OF MUDSTONE BEDSETS WITHIN AN OFFSHORE PARASEQUENCE CLINOTHEM, GRASSY MEMBER, BOOK CLIFFS, UTAH

Abstract:

The stratigraphic relationship between sandstone-dominated strata deposited in shoreface environments and coeval mudstone-dominated strata that occur in offshore transition and offshore environments is commonly interpreted as a simple down-dip transition. This relationship is critically tested at parasequence scale in the Campanian Blackhawk Formation, Book Cliffs, Utah. Sixteen logged sections through age-equivalent down-dip mudstones have been directly tied to up-dip sand-dominated strata of the uppermost parasequence, GPS3, of the Grassy Member by mapping key stratal surfaces along near-continuous exposures for over 10 km. The down-dip GPS3 succession in the study area includes six of seven identified 2–8-m-thick units of thin-bedded bioturbated medium to coarse mudstones that coarsen upwards into thin bedded coarse mudstone and fine-grained muddy sandstone, bounded by the lowest energy facies. Detailed sedimentological logging and thin-section analysis identified turbidite, hyperpycnite, and debrite beds that range in thickness from ∼2 to 150 mm, with only 15% of the mudstone deposited by suspension fall-out. These units are interpreted as bedsets, denoted as ‘a’ to ‘g’, which stack to form a recognizable upward-coarsening parasequence. The mudstone-dominated bedsets all thin and feather-out at their down-dip transition to offshore mudstone. Bedsets a through c can be correlated in outcrop to sand prone equivalents up-dip. However, bedsets d to g are top truncated by a composite surface reflecting amalgamation of a sequence boundary and ravinement surface DSB1-FS with no up-dip preserved equivalents. The mudstone-dominated bedsets are arranged into muddy subaqueous clinoforms, with mud and sand supplied through the up-dip shoreface and potentially with contributions from the Bitter Creek delta across strike to the north.