Understanding the sediment system of the Maverick Basin, Texas during the Late Cretaceous to Early Eocene through modelling of Apparent Organic Temperatures vs Carbonate Clumped Isotope Temperatures

The Cretaceous Eagle Ford Shale of the Maverick Basin, Texas has been exploited extensively for shale oil and gas and has boosted the US economy. A unique feature of this basin is the missing Cenozoic strata, leaving organic rich carbonates exposed at outcrop. Previous studies suggest two models for this: sediment bypass or extensive exhumation and erosion. Both models have different implications for the thermal and maturation histories of the Eagle Ford Group. Here, we carried out a comparison of inorganic and organic palaeotemperature proxies in unravelling the thermal history, by applying Carbonate clumped isotope paleothermometer and RockEval to Eagle Ford Outcrop samples. Carbonate clumped isotope temperatures (113 3.3C) record recrystallisation in a low water-rock system and therefore are a minimum estimate of the maximum burial temperatures. However, there is a discrepancy between theses temperatures and those from organic proxies. Tmax suggests that the organic matter is immature (<436C) and unlikely to have reached temperatures that carbonate clumped isotope suggest with an apparent maximum temperature of ~45C. This agrees with immaturity of the organic matter but is significantly cooler than carbonate clumped isotope recorded temperature. The discrepancies observed can be explained by the different kinetics between organic matter transformation and calcite recrystallisation. These processes respond to temperatures over different timescales and provide different snapshots of thermal burial histories. Organic proxies alone would result in an accurate understanding of maturation, but a misunderstanding of basin evolution and therefore does not allow us to test the models for missing Cenozoic strata. Therefore, we turned to numerical forward modelling to which model best explains the missing strata. The combination of two different temperature proxies and forward modelling has identified that the most favourable scenario is the exhumation and erosion of ~2800 m of Cenozoic sediments during the Early Eocene, which corresponds to the Latest phase of Laramide Compression. This amount of sediment is significantly less than previously suggested by literature (~6400m), indicating that within the Maverick Basin the Laramide Orogeny had far less of an impact than previously suggested.