Thermal History of the Denver Basin "Hot Spot"

W. Crawford Elliott, Mary Roden-tice, and Debra K. Higley

The Denver Basin Hot Spot is an area of anomalous thermal maturity within the Denver Basin. It is located on the basin axis north of Denver, and it is also on the northeast extension of the trend defined by the plutons of the Colorado Mineral Belt into the Denver Basin. While deep burial is thought to have formed at least some of the thermal maturation within the Hot Spot, deep burial can not explain the occurrence of the highest levels of thermal maturation (VRo ~1.50%) nor the present high geothermal gradients (35°C/km - 50°C/km). An additional source of heat is necessary to produce these anomalous levels of thermal maturation.

Apatite fission-track ages on apatite and zircon separated from Cretaceous bentonites, previously published K/Ar ages of illite/smectite (I/S) and vitrinite reflectance measurements together indicate that secondary heating events occurred at different times at different places within the Hot Spot. At Lyons, CO, the narrow apatite fission-track length distribution and the concordance of the apatite fission track ages with the K/Ar ages of I/S indicate this area of the Hot Spot was heated by Early Tertiary plutons as well as burial. The highest vitrinite values (VRo ~1.50%) measured from the Muddy J Sandstone are associated with the present geothermal gradients, and are thought to have formed by secondary heating within the past 20 m.y. ago. The thermal history of the Hot Spot is complet, and the processes responsible for the anomalous thermal maturity are elucidated by combined use of fission track, K/Ar and vitrinite reflectance measurements.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California