The Denver Basin: A Rocky Mountain Foreland Basin
editBasin Tectonic Summary
The Denver Basin is a foreland basin in north-central Colorado located at the foot of the Colorado Front Range, a subrange of the Rocky Mountains. This Front Range foreland basin was created as a result of Front Range uplift during the Laramide Orogeny which initiated during the late Cretaceous time and ended in the early-mid Eocene. Basin infill can be broken down in to two groups, a synorogenic and a post-orogenic infill group. The Denver Basin is an area of geologic significance as it provides a valuable record of Rocky Mountain uplift and denudation. Additionally, information gained from stratigraphic studies within the basin has provided constraints for North American floral/faunal distributions as well environmental variations in the area through time. The Denver Basin is of socioeconomic importance as it provides abundant natural resources for the millions of people that inhabit north-central Colorado.
Denver Basin Physiography
The Denver Basin occupies a significant portion of north-central Colorado, stretching from Pikes Peak to the Colorado-Wyoming border area (Fig. 1). General boundaries of the basin are the Front Range to the west, the Apishapa Arch to the south, the Greeley Arch to the north, and where basin strata onlap onto the Great Plains to the east (Raynolds 2002). While the northern, western, and southern limits of the basin are well defined by geologic structures, the eastern limit is not, thus the eastern boundary of the basin is relatively poorly constrained.
Denver Basin Geologic History
Before uplift associated with the Laramide Orogeny began, the area currently occupied by the Denver Basin was a part of a significantly larger Cordilleran foreland basin. This Cordilleran foreland was largely occupied by the Western Interior Seaway (WIS) that cut through the North American continent during the Cretaceous period. Once uplift associated with the Laramide initiated in the late Cretaceous as the WIS began to retreat, the former Cordilleran foreland basin was partitioned in to multiple smaller basins, the Denver Basin being one (Lawton 2008). As a Laramide sedimentary basin, the Denver Basin is further classified as a Perimeter Basin, that is its located on the eastern fringe of the Laramide zone of deformation (Dickinson et al. 1988). The Front Range began to uplift relative to the subsiding Denver Basin in the late Cretaceous time along the Golden and Rampart Range thrust faults that run parallel to the western edge of the basin (Raynolds 2002). Orogenic infill was deposited in the adjacent foreland basin in two, unconformity-bound sequences during uplift and subsequent denudation (Raynolds 2002). Orogenic uplift ceased as the Laramide concluded in the early-mid Eocene. Formation (i.e. subsidence) of the Denver Basin concluded coeval with the end of Laramide uplift, allowing the basin to stabilize (Raynolds 2002). In the late Eocene, the entire area experienced uplift and regional tilting to the east, a process that continues today. The cause of this epeirogenic uplift has been a matter of debate with one group arguing for a tectonic driver and the other for a climatic driver (Steven et al. 1997). In the climate-driven camp, workers argue that uplift is a regional response to continued erosion and beveling of the Front Range after Laramide thrusting ceased, this unloading, they argue, caused an isostatic uplift of both mountain and foreland (e.g. Burbank 1992). Whereas the tectonic-driven camp argues that uplift has largely been a result of regional doming due to mantle processes since the end of the Laramide (McMillan 2006).
Denver Basin Stratigraphy
Denver Basin strata can be divided in to three broad groups: pre-orogenic, synorogenic, and post-orogenic strata. The age, thickness, and composition of each of these groups are summarized in figure 2. Denver Basin stratigraphy has been largely characterized via sedimentary cores and geophysical well logs recovered from within the basin (e.g. Robson and Banta 1993; Raynolds and Johnson 2002). While basin framework units (i.e. pre-orogenic strata) aren't the focus here, they do have socioeconomic impacts on the Denver Basin that will be discussed below. Figures 3a and 3b show what these units look like relative to one another in cross section.
Synorogenic Infill
Synorogenic strata of the Denver Basin are grouped in to two unconformity-bound sequences known as the D1 and D2 sequences in accordance with the nomenclature set forth by Raynolds (2002). The D1 sequence lies unconformably below the D2 sequence and is composed of late Cretaceous to early Eocene material whereas D2 material is early to mid-Eocene in age. D1 units include the basal Arapahoe Formation, the Denver Formation, and the Dawson Formation. D2 material is composed of a basal paleosol series which is overlain by arkosic sandstones and mudstones. The D1 sequence contains a larger component of both recycled sedimentary material and volcanics while the the D2 sequence contains more plutonically-sourced sediments (Wilson et al. 2002). This change in material between the two sequences is thought to reflect the beginning of Laramide uplift followed by the progressive unroofing/erosion of uplifted material (D1) that ultimately came to include the plutonic cores of the Front Range (D2) (Raynolds 2002). Both sequences were deposited in an alluvial environment that graded from a coastal plain of the WIS in the Cretaceous to a tropical rainforest during the Paleogene (Ellis et al. 2003).
Post-orogenic Infill
Post orogenic stratigraphy in the Denver Basin isn't as well characterized relative to the underlying orogenic infill nor is it as well distributed. This material consists of late Paleogene tuffs, Miocene gravels, and late Cenozoic alluvium. Post-orogenic material isn't well preserved throughout the Denver Basin since accommodation space was no longer being created (i.e. the Denver Basin was no longer subsiding) when these packages were shed into the basin (Raynolds 2002). Although it is not as well preserved as its orogenic counterpart, the post-orogenic infill of the Denver Basin records both basin stabilization and the regional uplift/tilting of the Front Range region discussed above. Furthermore, this more contemporary material has provided valuable constraints on the development of modern basin drainage networks within the basin such as the South Platte and Arkansas Rivers (Raynolds 2002).
Denver Basin Significance
Geologic Significance
Denver Basin studies have yielded a multitude of insights that have had broad implications for multiple fields of study, making the area geologically significant. Most importantly, analysis of basin infill has provided valuable constraints on the timing and nature of tectonism in both the Rocky Mountains and the American Cordillera since the initiation of Laramide deformation in the Cretaceous time. Furthermore, these analyses have also aided in paleoenvironmental reconstructions of the Denver area since the retreat of the WIS began. Lastly, paleontological findings in the area have provided workers with more detailed North American floral and faunal distributions. For example, both Tyrannosaur and Triceratops remains recovered near the towns of Golden and Littleton, respectively, have come to define the southeastern-most extent of these species' geographic ranges (Carpenter and Young 2002).
Socioeconomic Significance
In addition to being an area of geologic importance, the Denver Basin is of societal importance as it is a source of abundant natural resources. The following are a few, but not all of the natural resources provided by the Denver Basin. Both hydrocarbon and coal deposits contained within the pre-Laramide Laramie and Pierre Formations, respectively, have been exploited economically (Clayton and Swetland 1980; Carroll 2009). Perhaps the most important natural resource found within the Denver Basin are the fresh water aquifers that supply freshwater to basin inhabitants, allowing for the sustained regional population growth observed over the past several decades. The Dawson Formation, Denver Formation, Araphaoe Formation, Laramie Formation (pre-Laramide), and Fox Hills Formation (pre-Laramide) are among the units that comprise the Denver Basin freshwater aquifer system (Musgrove et al. 2014).
References
Burbank, D.W., 1992, Causes of recent Himalayan uplift deduced from deposited patterns in the Ganges basin: Nature, v. 357, p. 680-683.
Carpenter, C., and Young, D.B., 2002, Late Cretaceous dinosaurs from the Denver Basin, Colorado: Rocky Mountain Geology , v. 37, no. 2, p. 237-254.
Carroll, C.J., 2009, The Coal Geology and Mining Resources of the Boulder-Weld Coal Field: The Mountain Geologist, v. 46, no. 1, p. 13-25.
Clayton, J.L., and Swetland, P.J., 1980, Petroleum Generation and Migration in Denver Basin: AAPG Bulletin, v. 64, no. 10, p. 1613-1633.
Dickinson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, E.R., McKittrick, M.A., and Olivares, M.D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region: GSA Bulletin, v. 100, p. 1023-1039.
Ellis, B., Johnson, K.R., and Dunn, R.E., 2003, Evidence for an in situ early Paleocene rainforest from Castle Rock, Colorado: Rocky Mountain Geology, v. 38, no. 1, p. 73-100.
Lawton, T.F., 2008, Laramide Sedimentary Basins, in Sedimentary Basins of the World, v. 5, p. 429-450.
McMillan, M.E., Heller, P.L., and Wing, S.L., 2006, History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau: GSA Bulletin, v. 118, no. 3-4, p. 393-405.
Musgrove, M., Beck, J.A.,Paschke, S.S., Bauch, N.J., and Mashbum, S.L., 2014, Quality of Groundwater in the Denver Basin Aquifer System, Colorado, 2003-5: U.S. Geological Survey Scientific Investigations Report 2014-5051, 123 p., https://pubs.usgs.gov/circ/1357/pdf/circ1357.pdf.
Raynolds, R.G., 2002, Upper Cretaceous and Tertiary stratigraphy of the Denver Basin, Colorado: Rocky Mountain Geology, v. 37, no. 2, p. 111-134.
Raynolds, R. G., and Johnson, K.R., 2002, Drilling of the Kiowa Core, Elbert County, Colorado: Rocky Mountain Geology, v. 37, no. 2, and p. 105-109.
Steven, T.A., Evanoff, E., and Yuhas, R.H., 1997, Middle and LAte Cenozoic Tectonic and Geomorphic Development of the Front Range of Colorado: Colorado Front Range Guidebook, Rocky Mountain Association of Geologists, p. 115-124.
Robson, S.G., and Banta, E.R., 1993, Data from Core Analyses, Aquifer Testing, and Geophysical Logging of Denver Basin Bedrock Aquifers at Castle Pines, Colorado: U.S. Geological Survey Open-File Report 93-442, 98 p., https://pubs.usgs.gov/of/1993/0442/report.pdf.
Wilson, M.D., 2002, Petrographic provenance analysis of Kiowa Core sandstone samples, Denver Basin, Colorado: Rocky Mountain Geology, v. 37, no. 2, p. 173-187.