ABSTRACT
Clay mineral data, potassium-argon (K-Ar) ages, and mathematical model simulations indicate the important kinetic factors controlling the smectite to illite transformation (“illitization”) in Upper Cretaceous Mowry and Niobrara core and outcrop bentonites from the Denver basin. The data also provide additional information on the thermal history of the basin. Data show that both the percentages of illite layers and the K-Ar ages of mixed layer illite-smectite (I/S) increase with increasing depths of burial, and this trend was simulated mathematically employing a very simplified burial model with a fifth-order overall kinetic expression for the formation of illite. For most of the basin, the data is compatible with I/S having been formed in response to increased temperature from progressive burial, because the oldest K-Ar ages of I/S are from the deepest buried I/S along the basin axis. In addition, these data qualitatively suggest that I/S even as small as <0.1 μm retains radiogenic argon under Denver basin burial temperatures.
I/S separated from drill core bentonites from the Wattenberg gas field, astride the basin axis and known to have been subjected to an anomalously high temperature history, are the most illitic and among the oldest measured from core in this study. The ages of illitization (about 60 Ma) are coincident with the Laramide time of structural development of the basin and uplift of the Front Range. The reconstructions of the burial history, and the modelling of the timing and extent of illitization, suggest that the area of the Wattenberg gas field was an upper Cretaceous depocenter, an area of relatively deeper burial along the basin axis. However, the data can not rule out the possibility that these sediments at and near the Wattenberg gas field were heated not only by burial but also by igneous and related hydrothermal activity associated with the Colorado mineral belt during the Laramide uplift (70–45 Ma) of the Front Range. Likewise, the anomalously illitic I/S in outcrop bentonites along the Front Range adjacent to the Wattenberg gas field may have been formed in response either to deep burial and/or by Laramide-age secondary heating superimposed on burial.
