by Jim Cole, Research Geologist [email]
Last updated: November 18, 2010
This multi-year investigation will obtain evidence from various disciplines to understand the geologic history of this region, in particular the events of the Cenozoic Era (last 65 million years). These mountains and valleys of northern Colorado and southern Wyoming reflect the combination of geologic processes that have formed mountain uplifts and eroded them away several times during this period of time.
The project area encompasses the lowlands of North Park and Middle Park in north-central Colorado, the southern Saratoga and Laramie River valleys of Wyoming, and the surrounding highlands of the Front Range, Medicine Bow Mountains, the Rabbit Ears Range, and the Park Range-Sierra Madre.
Geologically, the project is defined by the Paleocene-Eocene structural basin that underlies the modern lowlands between the Fraser valley on the south and the North Park valley on the north. To avoid confusion, we refer to this older geologic structure as the Colorado Headwaters Basin (CHB). The name recognizes that this area includes the headwaters of 5 major rivers: the Colorado, Big Thompson, Cache la Poudre, Laramie, and North Platte rivers.
The project has three main topical focuses, as detailed in following pages. A fourth objective of the project is to compile a regional geologic database that will include a wide variety of geologic, structural, paleontologic, geochronologic, geophysical, and related data sets.
The eastern Rocky Mountains of northern Colorado and southern Wyoming consist of Proterozoic-basement-cored uplifts and flanking sedimentary basins that formed during the protracted Laramide orogeny from Late Cretaceous to early Eocene time (about 67 to 50 Ma (Dickinson and others, 1988)). Recent detailed work in the Denver Basin shows that synorogenic sediments comprise overlapping alluvial-fan complexes related to early pulses of uplift (Raynolds and others, 2007); significant gaps in these deposits denote times of relative stability, denudation, and deep soil formation. Deposits in the Colorado Headwaters Basin consist of volcaniclastic and arkosic fluvial strata of the Paleocene and Eocene Middle Park and Coalmont Formations ( >8,000 ft thick) that similarly record uplift and erosion of surrounding ranges, but with a distinct timing and sequence.
Post-early Eocene deformation in this area produced northwest-trending folds in the sedimentary formations of the Colorado Headwaters Basin, as well as steep reverse faults with significant vertical displacement. This deformation took place in the waning stages of the Laramide orogeny (Tweto, 1957, 1975,; Izett, 1968).
Late Oligocene calc-alkaline volcanic activity in the Never Summer Mountains and Rabbit Ears Range was accompanied by major, local normal faulting. Regional uplift and erosion of the volcanic highlands (northern Rio Grande rift signal?) led to deposition of the Oligocene-Miocene North Park Formation here and northward into Wyoming (Montagne, 1957; Cole and others, 2008). Northwest-trending folds in the North Park deposits are not explained by any existing regional tectonic models.
Post-Miocene time is marked in this area by erosion, drainage integration, and locally significant stream capture and headwater diversion. Significant young faulting is not evident, and yet the persistent height of the Rockies in this area (significant areas in excess of 10,000 ft) and the rapid, deep incision of most major drainages indicates young uplift.
This project addresses questions of fundamental crustal structure and deformation kinematics that have been controversial for decades. The area is particularly well suited to investigating the timing and styles of uplift because so many of the synorogenic sedimentary sections remain intact (including the Paleocene-Eocene, Oligocene, and Oligocene-Miocene sections) and because each section is deformed in somewhat different ways. In addition, the older sediments have the potential to provide critical paleobotanical data regarding the Paleocene-Eocene Thermal Maximum (PETM) because pollen, leaves, and coal deposits are present in the section.
Landscape evolution during late Oligocene-Pliocene has been the subject of considerable research and debate for decades as well (Steven, 1960; Izett, 1968; Mears, 1993). This project will assess the contributions of volcanism (Never Summer Mountains and Rabbit Ears Range) and extensional faulting to the altitude history of the region and its impacts on drainage modification and integration.
Oil and gas exploration in the 1950's and 1960's did not produce significant discoveries, but rising market prices have stimulated renewed activity in 2008-2010 (more than six oil wells drilled since January 2008, some with significant production). Natural gas resources are also known in the basin.
More than two-thirds of the lands in this project area are public, including parts of Rocky Mountain National Park, large areas of national forest (with included wilderness areas), Bureau of Land Management acreage, Colorado State Forest, Arapaho National Wildlife Refuge, and smaller State and county wildlife management tracts.
The Arapaho National Wildlife Refuge covers nearly 35 square miles in the center of North Park along the Illinois River drainage. The meandering course of the river, along with numerous oxbow lakes, and natural and enhanced deflation-basin lakes, provide diverse habitat for hundreds of species of birds. Elk, moose, and deer are endemic, as are beaver, coyote, red fox, weasel, porcupine, and numerous smaller mammals, amphibians, and snakes. The habitats in this refuge owe some of their diversity to the erosional and depositional cycles of the river system in response to climate fluctuations during the Ice Ages of the last 1.8 million years.