RECOGNITION OF CRYPTIC STRATIGRAPHIC SURFACES IN ORGANIC-RICH MUDROCK SUCCESSIONS BASED ON HIGH-RESOLUTION TOC AND CARBONATE DATA

The recognition of characteristics of mudrock facies that are indicative of changes in sedimentation, in particular cryptic hiatuses, can make an important contribution to development of stratigraphic frameworks seeking to reconstruct relative sea level change. Ideally, such features can be correlated to shoreface architecture to confirm their relationship with systems tracts. However, such correlations can be difficult due to loss of section in shallow portions of a basin, as well as uncertainties in correlation within thick mudstone successions. The Cretaceous strata of the Western Interior offer a unique opportunity to develop such frameworks due to pervasive and wide ranging marker beds, excellent biostratigraphic control, and useful chemostratigraphic markers. In addition, common volcanic ash beds intercalated throughout Cenomanian through Campanian marine strata are not only excellent correlation tools, but radioisotopic dating of these units provides time control, the precision of which has improved significantly in recent years. In past studies of Western Interior mudrocks, certain sedimentologic features (e.g., interbedded skeletal limestones with HCS) have been used to identify cryptic lowstands. This study reports on the development of a high resolution record of weight % TOC and CaCO3 data (coulometric) from two cores of mid-Cenomanian through early Campanian strata of the Western Interior that provides a potentially powerful complementary tool for the recognition of stratigraphic discontinuities and changes in sedimentation in the distal reach of systems tracts. In many cases, distinctive shifts in TOC and/or CaCO3 are associated with biostratigraphically well-constrained hiatuses related to known or proposed lowstands. Other similar shifts lacking sufficient data to recognize a biostratigraphic gap are candidate surfaces. These major and minor discontinuities in the carbon records are correlated westward to time-equivalent shoreface deposits, wherever possible, in order to evaluate correspondence with sequence boundaries, lowstand desposits, or forced regressions. The results suggest that high resolution TOC/CaCO3 data can provide a very useful tool for sequence stratigraphic reconstructions in the distal facies tracts of appropriate depositional systems.