LINKAGES AMONG CLIMATE, TECTONICS, AND TERRESTRIAL PRODUCTIVITY IN THE LATE CRETACEOUS FORT CRITTENDEN FORMATION, SOUTHEASTERN ARIZONA

Shales of the Campanian Fort Crittenden Formation were evaluated within a lacustrine stratigraphic framework to document the terrestrial response to changing tectonics and paleoclimate during the onset of ice house conditions within the western interior of the U.S. If uplift enhances aridity by deflecting the Pacific Trade winds then the lacustrine strata should reflect balance- to under-filled conditions and nutrient limited productivity. If uplift enhances monsoonal circulation then the lacustrine strata should reflect balance- to over-filled conditions and nutrient-stimulated productivity. We found stacked sequences of wetland, beach, and deep-lake deposits suggesting multiple episodes of lake expansion and contraction in response to either climate or tectonics. Average organic carbon (Corg) contents and clay mineralogy are facies controlled. Prodeltaic sands display ball and pillow-like structures and contain Corg ranging from 0.03 – 1.2 wt. % with an average of 0.2 wt. % (n=9). Beach sands contain bivalve lags with Corg of 0.3 – 1.15 wt. % (average=0.46 wt. %, n=5). Wetland mudstone deposits were characterized by the presence of gastropods, slickensides, mottles, and either root traces or burrows, with Corg of 0.12 – 1.31 wt. % (average=0.43 wt. %, n=16). Mottles suggest pronounced wetting and drying of this facies. Mixed-layer chlorite/smectite (< 10% smectite layers) dominates the clay mineralogy of the wetland facies. Deep-lake deposits contained bivalves, gastropods, ostracods, and fish remains, with Corg of 0.21 – 1.94 wt. % (average=1.03 wt. %, n=15). Discrete illite is the major component of this facies with both minor mixed-layer chlorite/smectite and an ordered illite/smectite with 80% illite that display no systematic trend in mixed-layering or ordering. If the discrete illite and chlorite/smectite are detrital, then they may reflect weathering of the local bedrock. Preliminary interpretations support a freshwater environment indicating balance- to over-filled conditions for the Fort Crittenden Formation.