STRATAL PATTERNS IN THE CRETACEOUS BLACKHAWK FORMATION AND CRITICAL TAPER DEVELOPMENT IN THE SEVIER THRUST BELT

Stratal patterns are seen at the parasequence scale in the Upper Cretaceous Blackhawk Formation, in the Book Cliffs of Utah. The first pattern is defined by comparing the extent of shoreline progradation of individual parasequences. When the extent of shoreline progradation in successive parasequences is compared for each member of the formation, the first parasequence of each member always shows the greatest amount of progradation (with distances up to ~30 km) and initiates a progradational parasequence stacking pattern. Successive parasequences within the same member show progressively less progradation (average progradational distance of youngest parasequences in each member is 2-3 km), to culminate with an aggradational parasequence stacking pattern. A second pattern is seen when the extent of transgression between parasequences within the same member is compared. As with progradational extent, the extent of transgression (which forms the parasequence boundaries) also decreases upward within in each member. The greatest extent of transgression occurs at the member boundaries. Both patterns repeat with each member. These patterns occur in members where parasequences are not interrupted by a sequence boundary (SB) (interpreted as a surface of sea level fall) or when one or multiple sequence boundaries occur within a member. Patterns are hypothesized to reflect the development of the critical taper in the time-equivalent Sevier thrust belt to the west. The stacking pattern of successive parasequences within each member record the building of the critical taper. Member boundaries reflect advancement of the thrust sheet once the critical value is exceeded. Pattern recognition is complicated by presence of SB and associated incised valleys, which erode/replace part of the sedimentary record. However, the patterns persists even in the presence of SBs, suggesting a different (eustatic) mechanism for SB development.