EPISODIC DEPOSITION IN THE CRETACEOUS AND PALEOGENE DENVER BASIN: HOW TECTONICS CONTROLS THE CONTINUITY OF THE STRATIGRAPHIC RECORD

A chronological framework for fluvial strata preserved during the latest Cretaceous and early Paleogene in the Denver Basin, Colorado has been established using radiometric dating, paleontology and magnetostratigraphy. This effort has been coordinated by the Denver Museum of Nature and Science under the multidisciplinary Denver Basin Project. In the Denver Basin, Laramide style synorogenic sedimentation spans the K/T boundary and extends into the Eocene. The synorogenic strata occur in two unconformity-bounded sequences which are interpreted to reflect two episodes of uplift and deformation in the adjacent Front Range. The first, termed the D1 sequence, accumulated during the first phase of uplift of the Front Range. This sequence spans the end of the Cretaceous and extends up to about 64 MY. Rates of accumulation in the central part of the basin are on the order of 100m/million years with rates of up to 150 m/million years on the active western margin. The synorogenic strata are comprised of alternating fluvial channel sandstone and overbank mudstone beds. Coal and lignite beds occur in a portion of the basin suggesting low gradients and underfilled basin conditions. Sandstone compositions vary as a function of the unroofing of the Front Range and the eruption and subsequent erosion of an andesitic volcanic terrain. Compositional changes ranging from andesite-rich litharenites to arkoses reflect these changes in the character of the catchments feeding the rivers draining the uplifting Front Range. Fossil plants are common, and vertebrates are locally abundant. Moist, warm and generally well-drained to swampy conditions are indicated. Near the end of this period of accumulation, sedimentation became fine-grained, then ceased for a period of approximately 9 MY during which little sedimentary record is preserved other than a thin aggradational paleosol. Sedimentation resumed and spanned a poorly-defined interval during the early Eocene, approximately 54 MY ago, comprising the unconformity-bounded D2 sequence. During this interval, arkosic fluvial strata indicate the erosion of a granite-rich source terrain much like we see today in the modern Front Range. Fossil plants are less common and vertebrate remains are rare in this sequence, but the record is indicative of warm and well-drained conditions.