A sequence stratigraphic and geochemical investigation of lower to middle Turonian (Cretaceous) strata of the Western Interior Seaway, Utah, Colorado, and western Kansas

This study examines geochemical facies correlation in mostly fine-grained sediments of the Cretaceous Western Interior Seaway (KWIS; Kansas, Colorado, and Utah), and the relationship to tectonism, eustasy, climate, and the resulting development of accommodation space. A conceptual model of circulation and water-column dynamics controlling sedimentation in the KWIS axial basin is presented. A conceptual geochemical facies model that accounts for patterns in the data sets, and incorporates lithologic and stratigraphic information from outcrops and cores, is used to establish a detailed parasequence stratigraphy.; An overall shallowing- and coarsening-upward sequence characterizes the progradational, early to mid-Turonian strata. This trend is punctuated by transgressive episodes, and disconformities associated with relative sea-level fall. Stratigraphic and sedimentologic evidence combined with the results from geodynamic modeling suggest that a forebulge developed in the Late Cenomanian in central CO. Although this forebulge separated the central UT proximal foredeep from the open-marine axial basin, a distinct eustatic signal is visible in parasequence stacking patterns and in sequence boundary development within the foredeep and axial basin.; The relationship between Th/U, Th/K, %TOC and HI values is used to interpret clay mineralogy and redox conditions during deposition. These parameters suggest that marine algal enrichment is associated with laminated intervals deposited under dysoxic to anoxic conditions. Under dysoxic to oxic conditions, sediments were bioturbated, and more refractory, terrestrial organic matter was preserved.; Organic petrographic-palynological methods were used to examine organic matter in Cenomanian through middle Turonian strata in Utah and Colorado. The stacking of organic facies is suggestive of a sea-level control. During the Middle Turonian stillstand (lowstand), when the combined effects of eustasy and tectonism provided no net change in the availability of accommodation space, climatically-driven cycles reflected in sedimentary and geochemical facies variation appear in the stratal record. Intercalated marine-dominated and non-marine-dominated organic facies in these deposits match stacking patterns exhibited in the geochemical facies. In this stillstand setting high river discharge led to eutrophication in estuaries and in back barrier lagoons established basinward of the forebulge. These environments were subject to episodic marine algal blooms during subsequent periods of lower river discharge.