CONSTRUCTING SEQUENCE STRATIGRAPHIC FRAMEWORKS USING MUDROCK GEOCHEMISTRY

Chemostratigraphy is capable of highlighting facies shifts within apparently homogenous mudrock successions with greater confidence than is possible with visual inspection. These facies shifts underscore compositional variation within a stratigraphic unit and can also represent the changing depocenter position within a basin due to changing shoreline position. The principal elements for this approach in this study are: titanium (Ti), zirconium (Zr), silicon (Si), Calcium (Ca), strontium (Sr), phosphorous (P), aluminum (Al), potassium (K), molybdenum (Mo), nickel (Ni), and vanadium (V). Diagenetic alteration concerns may be evaluated when examining each element with paired environmental proxies. Ti and Zr are associated with continentally derived sediment. Ca and Sr are associated with carbonate accumulation. Al and K are associated with feldspars and clays. Mo, Ni, and V can be used as an indication of restriction. However elemental interactions may affect the concentrations of redox sensitive trace metals. Si is found in biogenic quartz, detrital quartz, feldspars, and clays. As such, it is useful to evaluate Si as a ratio between Si/Al. Interpreting these changing chemostratigraphic trends in a sequence stratigraphic context allows workers to resolve high frequency cyclicity and refine sequence stratigraphic frameworks used as a means of correlation across a basin.

Chemostratigraphic profiles can be interpreted within a sequence stratigraphic framework using the following general criteria. The Lowstand Systems Tract (LST) is defined by increasing concentrations of Ti, Zr, Al, and K as well as high levels of Mo and V. The Transgressive Systems Tract (TST) is defined by declining concentrations of Ti and Zr, though Al and K may remain elevated, Mo and V also show a general decline. The Highstand Systems Tract (HST) is defined by increasing concentrations of Ti, Zr, Al, and K, but is distinguished from the LST by low levels of Mo and V. Additionally, by correlating progradational and retrogradational stacking patterns, it is possible to build stratigraphic frameworks that are independent of paleotopography. This allows workers to build correlations across portions of the basin that were periodically disconnected at the time of deposition.