ABSTRACT: Sequence Boundary Variations within the Aberdeen Member, Cretaceous Blackhawk Formation, Utah

KAMOLA, DIANE L., Old Dominion University, Norfolk, VA, and JACQUELINE E. HUNTOON, Michigan Technological University, Houghton, MI

Five parasequences are defined in the Aberdeen Member, Upper Cretaceous Blackhawk Formation, central Utah. Initial impressions are that these five parasequences form a single progradational parasequence set within one depositional sequence. However, detailed field observations identify two sequence boundaries within the Aberdeen Member, each having a different stratigraphic expression. The first (older) sequence boundary is recognized by the presence of a fossiliferous shale interval directly overlying delta front sandstones of the underlying parasequence. This shale occurs at the base of a proximal marine parasequence that represents a depositional environment too shallow for marine shale deposition. In addition, this shale contains a restricted fossil assemblage strongly suggestive f marginal marine settings. When traced in a seaward direction, coal is found in this same stratigraphic horizon (directly overlying distal marine sandstones). The fossiliferous shale is interpreted as a marginal marine deposit and is analogous to a lowstand interfluve as defined by Van Wagoner and co-workers. The sequence boundary is interpreted to occur immediately beneath the shale, and exhibits no local or regional downcutting.

The second (younger) sequence boundary occurs at the top of the Aberdeen Member and contrasts with the older sequence boundary by showing substantial downcutting. At least one marine parasequence has been removed by this relative sea level fall. The vertical succession reflecting this event begins with lower shoreface HCS beds directly overlain by an erosional surface, which is in turn overlain by 9.5 m of channel-fill sandstone. The channel-fill is dominated by trough crossbeds containing numerous mud rip-up clasts. Mud rip-up clasts are abundant in many lowstand channel-fill sandstones.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)