Rocky Mountain Geology 37.2 - Denver Basin I

Robert G. Raynolds and Kirk R. Johnson

A 2,256 ft (688 m) continuous core was drilled in Kiowa, Colorado in early 1999 as part of a research effort coordinated by the Denver Museum of Nature & Science. The 2.5 inch (6 cm) diameter core sampled the uppermost Pierre Shale, Fox Hills Sandstone, Laramie Formation, and the overlying synorogenic strata (known as the Denver and Dawson Formations) that fill the Denver Basin. Core recovery was 93 percent and the core is archived at the U.S. Geological Survey core research facility in Lakewood, Colorado.

KEY WORDS: stratigraphic coring, Denver Basin, synorogenic, continental drilling.

Robert G. Raynolds

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 CretaceousTertiary 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.

KEY WORDS: Denver Basin, stratigraphy, Laramide orogeny, sequence stratigraphy, synorogenic.

Douglas J. Nichols and R. Farley Fleming

Palynological analysis provides data for determinations of ages and paleoenvironments of Maastrichtian, Paleocene, and Eocene strata in the Denver Basin. These strata were sampled on the surface at natural and artificial outcrops (construction sites) and in the subsurface via auger, drill core, and well cuttings. Extensive artificial outcrops existed temporarily during construction of the Denver International Airport. The Kiowa core, drilled as part of the Denver Museum of Nature & Science s Denver Basin Project, is the principal palynostratigraphic reference section for the basin. It is supplemented by data from the Castle Pines core. Palynostratigraphic zonations of the Kiowa and Castle Pines cores are correlated with magnetostratigraphic zonations in the same cores. Regionally known palynostratigraphic biozones are present in the basin, as follows: Aquilapollenites striatus Interval Zone (Maastrichtian) in the Fox Hills and Laramie Formations; Wodehouseia spinata Assemblage Zone (uppermost Maastrichtian) in the lower part of the D1 sequence; Zones P1 through P3 (lower Paleocene) in the upper part of the D1 sequence; Zone P6 (uppermost Paleocene) locally at the base of the D2 sequence; and Zone E (lowermost Eocene) in the D2 sequence, above the basin-wide paleosol unit. The KT boundary is bracketed by samples from the Kiowa and Castle Pines cores and is located within centimeters at the West Bijou Site. The PaleoceneEocene boundary is determined to be within the paleosol unit of the Denver Basin, near the base of the D2 sequence. These and other palynological age determinations supplement independent paleobotanical and vertebrate paleontological research in the Denver Basin.

KEY WORDS: palynology, palynostratigraphy, Maastrichtian, Paleocene, Eocene, CretaceousTertiary boundary, Denver Basin.

Only a few radiometric ages are available to help constrain the depositional history of Laramide synorogenic strata of the Denver Basin. This paper provides an evaluation of previous radiometric work in the basin and presents analyses of six recently sampled tuffs and tonsteins. Although tuffs and tonsteins are known to exist in Upper Cretaceous parts of the strata, ages for these are either yet to be determined or are unreliable. The few reliable ages, based on ⁴⁰Ar³⁹Ar, conventional K-Ar, and fission-track dating, fall into two groups: early Paleocene and early Eocene. A pronounced hiatus, related either to erosion or nondeposition, can be shown to exist between ~6364 Ma and 54 Ma.  Some anomalous results will require additional fieldwork and analysis to resolve.

KEY WORDS: Denver Basin stratigraphy, Paleocene, Eocene, ⁴⁰Ar³⁹Ar dating, geochronology, tuffs, tonsteins.

Michael D. Wilson

To assess provenance variations during Late CretaceousTertiary uplift of the Colorado Front Range, detailed petrographic analysis was performed on core samples from the Kiowa Cored Well, Elbert County, Colorado. Forty-two samples from five stratigraphic intervals were used in the analysis. Samples range in depth from 91.5 to 2,242 ft (27.9 to 683.4 m).

Results indicate that minor but significant amounts of low-grade, metasedimentary debris occur in the Fox Hills Sandstone and Pierre Shale samples and require a relatively distal, westerly source. Subsequent to a strong volcanic pulse during deposition of the Upper Cretaceous Laramie Formation and lowermost D1 sequence, a gradual unroofing sequence is recorded in the overlying Upper Cretaceousmiddle Paleocene D1 synorogenic deposits. The volcanics encountered in the Laramie and lowermost D1 sequence are primarily silicic types.

An influx of chert detritus to sandstones of the lower D1 sequence is interpreted to reflect unroofing of post-Lyons Sandstone, chert-rich, Mesozoic sandstone. This was followed by overlapping depositional pulses of feldspathic and then chert- and dolomite-rich sandstone. The first of these is interpreted to have coincided with unroofing of the arkosic Fountain Formation and the second with the removal of the underlying lower Paleozoic carbonates and clastics. The shallowest occurrence of components having a definite sedimentary origin is at a depth of 976.2 ft (297.5 m) in the upper D1 sequence. The shallowest sample containing quartz that exhibits significant rounding, indicating derivation from sedimentary sources, occurs at a depth of 1,061 ft (323.4 m).

Abundances of feldspathic components exhibit a dramatic increase in the upper D1 sequence and continue into the lower Eocene D2 sequence sandstones, where they comprise the bulk of the framework minerals. A second major pulse of volcanic debris occurs in the uppermost D1 sequence. The D2 sequence was derived almost exclusively from plutonic materials, with no evidence indicating input from high-grade metamorphics (feldspathic gneiss and amphibolite) that tend to dominate the Front Range foothills north of the Castle Rock area.

KEY WORDS:  Front Range, Denver Basin, provenance, Kiowa Cored Well, synorogenic deposits, framework mineralogy, petrographic analysis.

Shari A. Kelley

Fission-track (FT) thermochronology of detrital apatite and zircon from synorogenic sedimentary rocks in the Kiowa Cored Well and the Castle Pines drillhole in the Denver Basin can be used to document a generally predictable unroofing sequence for the southern part of the Colorado Front Range. First, Phanerozoic sedimentary rocks that covered the Front Range in late Mesozoic time contributed recycled sedimentary and far-traveled volcanic zircon and apatite to the oldest sediments as Laramide deformation began. Next, two significant pulses of volcanic grains are recorded in both the detrital zircon and apatite FT age populations in the Kiowa Cored Well as volcanoes along the Colorado Mineral Belt dominated the landscape. The volcanic contribution is not as significant in the Castle Pines well, which is more proximal to the mountain front compared to the Kiowa Cored Well. Higher in the stratigraphic sequence, a mix of volcanic grains, recycled grains from the Phanerozoic cover, and Proterozoic basement grains is present, with the percentage of basement grains in the mixture increasing up section. Finally, in the youngest part of the sequence, metamict zircon grains from the basement and apatite grains, derived from the Proterozoic basement with 50 to 70 Ma apatite fission-track cooling ages derived from below the base of the apatite partial annealing zone in the Front Range, dominate the age populations. Complexities in the form of pulses of volcanic or basement components are superimposed on the simple pattern. This study demonstrates the power of analyzing both apatite and zircon when examining detrital grains derived from a basement-cored uplift where basement resided at shallow levels of the crust (<4 km) prior to deformation.

KEY WORDS: Denver Basin, detrital fission-track geochronology.

Timothy M. Farnham and Mary J. Kraus

Paleogene strata in the Denver Basin contain an interval about 10 m thick and distinguished by bright red, purple, and yellow-brown colors. The bright-colored interval has been regarded as a paleosol marking an unconformity in the stratigraphic record. The interval consists of mudrocks that show morphologic features indicating ancient soil development. The mudrocks showing paleosol modification are interbedded with coarser-grained deposits that show little, if any, paleosol development. Thus, the interval can be subdivided into a series of vertically stacked alluvial paleosols indicating that pedogenesis was coeval with deposition of the parent material. This interpretation contrasts with that of workers who have considered the paleosol interval to represent a long-lived (millions of years) episode of landscape stability.

Three kinds of paleosols are recognized based on the color of the B horizon: red, purple/red, and purple paleosols. Reddening of the B horizon was an important process in developing the paleosols and indicates that the paleosols formed under moderately well-drained and oxidizing conditions in a warm temperate to subtropical climate. The bright red and purple paleosol intervals directly overlie strata dominated by gray colors and organic-rich deposits. The transition in lithology suggests a change in soil drainage conditions possibly caused by a climate change. A similar lithologic change characterizes continental strata in other sedimentary basins in the Rocky Mountain region. The transition is best known in the Bighorn Basin, where it has been attributed to a combination of local tectonic controls and a global climate change that took place from late Paleocene to early Eocene time. We speculate that the lithologic change in the Denver Basin corresponds to that found in the Bighorn Basin and other basins in the Rocky Mountain region. Thus, development of the paleosol intervals in the Denver Basin may have been caused primarily by a global change to a warmer and drier climate and may have taken place at or about the PaleoceneEocene boundary.

KEY WORDS: Paleogene, PaleoceneEocene boundary, Denver Basin, alluvial paleosols, paleoclimate.

Shari A. Kelley and David D. Blackwell

Equilibrium temperature logs and thermal conductivity values were measured in the Kiowa Cored Well to calculate a heat flow value of 56.4 ± 1.0 mW/m² for the site. The temperatures in the Kiowa Cored Well are higher than those for a similar interval in the Castle Pines well, a research well located west of the Kiowa Cored Well. This difference in temperature is to be expected because the synorogenic sediments penetrated by the Kiowa Cored Well are finer-grained and more coal-rich than those in the Castle Pines well, resulting in a lower overall thermal conductivity in the Kiowa Cored Well. Temperature logs from groundwater monitoring wells in the Denver Metropolitan area are complicated by intra-borehole downflow and thus are of limited use in assessing regional temperature variations. In contrast, high-quality temperature logs in oil wells along the southeastern margin of the Denver Basin and on the Las Animas Arch yield estimated heat flow values of ~64 to 70 mW/m², which are similar to values determined in southwestern Kansas. Fluid flow in the Dakota aquifer in southeastern Colorado has little impact on the temperature regime along the southeastern margin of the basin.

KEY WORDS: Denver Basin, heat flow, equilibrium temperatures.

Laura Lapey Woodard, William Sanford , and Robert G. Raynolds

Regulations pertaining to development of groundwater resources of Denver Basin bedrock aquifers are based in part on estimates of specific yield. To examine spatial variation in specific yield, data from the Castle Pines and Kiowa research cores in east-central Colorado were used to develop a west-to-east cross section of the Denver Basin. Specific yield varied within each aquifer according to geology and distance from the sediment source, the Rocky Mountains. Laboratory measurements of specific yield from deep core samples were considerably lower than estimates obtained previously by the State of Colorado from outcrops along the basin margins. Consideration should be given to this disparity in specific yields to ensure that the volume of groundwater stored in the Denver Basin bedrock aquifers is not overestimated.

KEY WORDS: Denver Basin, bedrock aquifers, specific yield, groundwater.

Kenneth Carpenter and D. Bruce Young

Late Cretaceous dinosaurs have been known from the Denver Basin, Colorado, since the mid-1860s. Most of the fossils are scrappy, although several fragmentary skeletons are known. Most recently discovered specimens are the result of salvage work at construction sites in the Denver metropolitan area. Dinosaurs from the Denver Basin include Triceratops, Torosaurus, Edmontosaurus, Thescelosaurus, Edmontonia, Pachycephalosaurus, Tyrannosaurus, a dromaeosaurid, and Ornithomimus. All of these taxa are also known from the Lance, Hell Creek, and Scollard Formations to the north and are known collectively as the Lancian fauna. Thus, these Late Cretaceous dinosaurs from Colorado are the southeastern-most extension of the Lancian fauna. Furthermore, there may be ecological segregation of some taxa based on their facies distribution, with Thescelosaurus and Torosaurus restricted to the wetter lowlands and Ornithomimus and possibly Pachycephalosaurus to the better drained uplands.

KEY WORDS: Dinosauria, Late Cretaceous, Laramie Formation, lower Denver Formation, Theropoda, Ornithopoda, Pachycephalosauria, Neoceratopsia, Ankylosauria.