Upper Cretaceous and Tertiary stratigraphy of the Denver Basin, Colorado

Abstract

The Laramide synorogenic strata of the Denver Basin record the uplift and denudation of the central and southern Front Range of the Rocky Mountains. Synorogenic sedimentation took place in two distinct pulses, the first spanning the Cretaceous–Tertiary boundary and extending into the early Paleocene. The second occurred during the latest Paleocene and early Eocene. Facies patterns reflect proximal to distal fluvial environments. Progressive unconformities mark the western side of the basin, and lignite beds and ponded deposits characterize the distal part of the first pulse. The second pulse preserves a more uniform fluvial succession characterized by alternating arkosic channel sandstones and olive-brown overbank deposits. The lateral and vertical facies changes found in these strata have engendered a complex history of nomenclature. By considering the units as a pair of unconformity-bounded sequences called the D1 sequence and the D2 sequence, a simpler pattern emerges that allows regional facies variability to be understood on a basin-wide scale. Two complete cores are used to calibrate geophysical logs from oil and water wells, providing a subsurface database that is correlated to outcrops. This allows the creation of cross sections that permit isolated outcrops and fossil occurrences to be correlated into an integrated basin-wide framework. The paleontologic record is used together with radiometric dating and magnetostratigraphy to define the time span during which sediment accumulated. The episodic accumulation of sediment is interpreted to reflect episodic displacement along the thrust faults bounding the Front Range. The first episode of sedimentation, defined as the D1 sequence, is interpreted to represent uplift of that part of the Front Range bounded by the Golden and Rampart Range faults. During this period of sedimentation, andesitic volcanic rock that covered much of the Front Range was stripped from the uplift and deposited in the D1 sequence. The second episode of sedimentation, the D2 sequence, is interpreted to involve sediment eroded from the mountains west of the Colorado Springs area, perhaps as a result of uplift of the Pikes Peak area by the Ute Pass fault. Depositional patterns in the basin are a logical response to differential uplift and variations in fluvial geomorphology. Understanding these systems can help quantify depositional patterns and subsurface distribution of bedrock aquifers.