CRETACEOUS SHALES AND THEIR INFLUENCE ON DENUDATION ESTIMATES FOR THE SOUTHWESTERN UNITED STATES

Thermochronologists, as they attempt to constrain the amount of denudation in a region, often assume a constant geothermal gradient throughout the missing section in order to estimate the paleodepth of critical isotherms. In this study, reconstructions of stratigraphic sections in the Grand Canyon of Arizona, the Front Range of Colorado, and the northern Rio Grande rift of New Mexico are presented to illustrate the influence of lithology on the geometry of paleoisotherms. In particular, the influence of thick Cretaceous shales on paleogeothermal gradient estimates is examined. For example, new equilibrium temperature logs measured in the southern Denver Basin indicate that the modern geothermal gradient through the Cretaceous Pierre Shale is on the order of 50°C/km, while the gradient through the Cretaceous Dakota Sandstone is about 20°C/km. The modern heat flow is ~60 mW/m², so the thermal conductivity of the Pierre Shale in this area is about 1.2 W/m-K. The Pierre Shale is about 1200 m thick adjacent to the southern Front Range, and it thickens dramatically to 2400 m in the northern Front Range. The influence of variations in the thickness of the low thermal conductivity Pierre Shale on paleotempertures in the Proterozoic basement is investigated using simple 1-D thermal models. The calculated temperature profiles show that the 110°C isotherm was in the Proterozoic basement in the southern Front Range in late Cretaceous time, as recorded by the preservation of a fossil apatite PAZ on Pikes Peak. In contrast, the base of the PAZ was in the Pierre Shale in the northern Front Range prior to Laramide deformation, consistent with the absence of a preserved PAZ on Longs Peak. The temperature at the top of the basement at the end of Cretaceous time was ~100°C in the southern Front Range and ~135°C in the north.