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Cryptic Sequence Boundaries in an Ancient Offshore Mudstone-Dominated Succession: The Upper Cretaceous Mancos Shale Formation, South-Central Utah


The recognition of sequence boundaries in stratigraphic sections is a critical component of sequence analysis. Due to the limited variability in grain size and sedimentary features, the sequence boundaries in offshore mudstone-dominated successions tend to be cryptic compared to those present in coarser-grained clastic and carbonate successions. In this study, part of the Upper Cretaceous (Early to Middle Turonian) Mancos Shale Formation exposed in south-central Utah was examined through a combination of field work and petrographic methods in order to characterize the cryptic nature of sequence boundaries in this offshore mudstone-dominated succession. The studied succession was deposited in environments ranging from prodelta/lower shoreface to shelf during the regression of the long-term (2nd-order) Greenhorn sea-level cycle. At least four sequence boundaries were identified, which are all very subtle. A given sequence boundary is typically associated with several, if not all, of the following attributes: 1) increases in the relative amount of particles derived from terrestrial input (e.g., detrital grains and terrestrial organic matter); 2) increases in the abundance and thickness of sedimentary structures generated by storm- or river-dominated depositional processes; 3) changes in bioturbation characteristics (i.e., bioturbation intensity and diversity); 4) silty, sandy, fossiliferous, or phosphatic lag deposits up to a few centimeters thick; and 5) sharped-based event beds. Except for the sequence boundary formed in the most distal environment (outer shelf), the remaining three sequence boundaries all show characteristics indicative of basinward shifts in depositional facies. Comparison between proximal and distal successions reveals that the sequence boundary formed in the most proximal environment (inner shelf to prodelta/lower shoreface) is characterized by varying magnitude of facies shift along the depositional dip direction. The sequence boundaries identified in this study were interpreted to be results of short-term sea-level cycles superimposed on the 2nd-order Greenhorn cycle. The recognition of cryptic sequence boundaries based on detailed analysis of the sedimentary facies and petrographic characteristics in this study provides conceptual approaches that can aid the development of high-resolution sequence stratigraphic frameworks in other offshore mudstone-dominated successions.