High-Frequency Sequences and Parasequence Stacking Patterns Within the Cretaceous Blackhawk Formation, Book Cliffs, Central Utah

John A. Howell, Diane L. Kamola, Stephen S. Flint

High quality outcrop exposures and well data from the Blackhawk Formation, Book Cliffs of Central Utah offer an ideal opportunity to test and further develop the concepts of high-frequency sequences and parasequence stacking patterns. The Blackhawk Formation represents the highstand systems tract of a 3rd order sequence and comprises 6 basinward-stepping lithostratigraphic members, each bounded by a major marine flooding event.

These 6 lithostratigraphic members contain 24 parasequences and 8 sequence boundaries which subdivide the Blackhawk Formation into 8 high frequency sequences. There is no apparent relationship between the number of parasequences and the occurrence of sequence boundaries, i.e., there is no periodicity to sequence boundary occurrence. High-frequency sequences can contain as many as 7 parasequences or as few as 1 to 2 parasequences.

Each of the 3 lower lithostratigraphic members shows an initial progradational parasequence stacking pattern which changes to an aggradational stacking pattern upsection. In contrast, the upper member shows a strongly progradational stacking pattern throughout. The initial parasequence in each member usually shows the greatest extent of shoreline progradation.

High-frequency sequence boundaries within the Blackhawk exhibit both extensive and limited incision. The extent of facies downshift associated with these sequence boundaries varies and is not predicted by relative position within the 3rd order highstand. Documentation of stratal patterns, coupled with the frequency and degree of incision at sequence boundaries provides important data for understanding the driving mechanism behind high-frequency sequence development.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995