Parasequences and Bedsets : Examples from the Book Cliffs and Wasatch Plateau of Eastern Utah

John Howell

The Late Cretaceous strata that crop-out in east central Utah (USA) have been central to the development and testing of the sequence stratigraphic paradigm for almost 40 years. Large continuous cliff sections in the Books Cliffs and Wasatch Plateau are composed of shallow marine and coastal plain strata arranged in to cycles that show an upward shallowing of facies interpreted as parasequence. These outcrops were central to the definition of the parasequence concept.
The parasequence is a fundamental building block of sequence stratigraphy. However, in recent years, several publications have questioned their significance, even suggesting that the concept should be dropped. The key issues include: 1) the misbelief that all sequence stratigraphic components are “scale independent”, 2) the misinterpretation of what is meant by the term flooding surface and 3) a tendency to favour theoretical concepts over practical observations. A parasequence is the deposits of a unique phase of shoreline transgression and subsequent progradation with the majority of the shallow marine deposit being laid down during the regressive portion of the cycle.
The flooding surfaces that bound the parasequences are generated by allogenic rises in “relative sea-level”. As such they are associated with landward dislocation of the shoreline on a scale that is greater than a single feeder system. While superficial similar surfaces, at least in vertical section, are generated by autogenic processes such as lobe switching, these produce smaller scale cycles called bedsets. Much of the ambiguity around the applicability of the parasequence concept has arisen from the historical misinterpretation at this scale.
A revaluation of the Cretaceous Blackhawk and Star Point formations in the Book Cliffs and Wasatch Plateau of Eastern Utah using over 150 logs and 84 km of virtual outcrop data has mapped out 31 progradational events which have an average progradational extent of 9.9
km. The progradational length is strongly controlled by the landward offset during flooding (ave 6.0 km) and parasequences typically only prograde an average of 4.0 km beyond the shoreline break of the underlying parasequence. The number of constituent bedsets is strongly controlled by the depositional process with wave dominated systems having less than their fluvial or tidally influenced counterparts. The average sea-level rise associated with a parasequence boundary is 14.6 m and the average calculated depths to wave base are 22.9. m (FFWB) and 35.7 m (SWB). Typical shoreline trajectories are 0.1 degrees.
While the parasequence may not be ideologically perfect, it is a useful and practical concept which is supported by field observation. If correctly applied the principals are applicable to understanding and predicting facies distributions and as a basis for correlation and modeling subsurface data.