Abstract

A new look at old friends—The paleogeography of the ancestral Rocky Mountains of Colorado

By Chuck Kluth, Center of Research Excellence, Colorado School of Mines

New data, combined with earlier data, indicate that previous interpretations of the geometry and timing of the classical Ancestral Rocky Mountains in the Colorado region are in need of revision. N-S stratigraphy of the Fountain Formation along the present Front Range suggests that the Fountain on lapped a broad NW-SE arch that began to develop in early Pennsylvanian time. Interbedded or subjacent marine rocks are preserved as far north as Lyons and as far south as Perry Park, Colorado. The interbedded marine rocks and the preservation of earlier Paleozoic rocks indicate that the Front Range was separated from a narrow, uplifted block in the Colorado Springs area, the Ute Pass Block. The southwestern margin of the Front Range was faulted and had approximately 6 kilometers of structural relief. In contrast, the NE side of the Front Range is now interpreted to have been a NE dip slope with only minor faulting. The presence of Pennsylvanian marine rocks constrains the Front Range to have had its northern plunge end at approximately the Colorado-Wyoming state line.

The San Luis Highland is interpreted to have been a west dipping fault block with approximately 8 kilometers of structural relief on its eastern side and a gentle west dip-slope on the western side. It is interpreted to have been a separate uplift from the adjacent Uncompaghre Uplift to the west, during at least its early history, and possibly its entire history. The San Luis Highland was uplifted in early Pennsylvanian time and shed coarse sediments eastward and northward into the Central Colorado Trough, and more fine grained sediments westward into the Paradox Basin.

The Uncompaghre Uplift is interpreted to have been uplifted in late Pennsylvanian and early Permian time, after the deposition of the middle Pennsylvanian evaporites. The data show that the geometry of the Uncompaghre front, SE of the Utah/Colorado state line, contrasts to the single large fault in Utah, and is a stack of SW directed thrust faulted basement blocks. Distribution of synorogenic sediments derived from the Uncompaghre Uplift was largely by axial river systems. Loading by the sediments caused the underlying salt to move into salt walls that nucleated on basement faults. These basement faults formed between middle-late Mississippian and middle-early Pennsylvanian time. The basement faults are usually interpreted as normal faults, but there is evidence that at least some of them were reverse faults. The development of accommodation space for each minibasin between salt walls ended when the pre-salt and post-salt sections welded together, as the last of the salt moved from beneath the basin. The locus of deposition then moved to the SW, farther away from the Uncompaghre front, and a younger salt wall and minibasin formed. This process was repeated several times, with the result that the salt walls are progressively younger toward the SW. The coarse alluvial fan material was preserved and prograded away from the mountain front only after the locus of deposition moved to the SW and axial rivers no longer redistributed the erosional debris. The new interpretation of the geometry and timing of the Uncompaghre Uplift suggests that the Paradox salts and the Eagle Valley Evaporites were deposited in a continuous basin that existed across the site of the later Uncompaghre Uplift.

The Central Colorado Trough was a NW-SE basin located between the Ancestral Front Range and the San Luis Highland and Uncompaghre Uplift. The Trough appears to have been complexly faulted, and contained crustal slivers that were uplifted in a complicated pattern within the trough. These blocks and slivers included the Ute Pass, Wet Mountain/Hartsel Uplift, possibly the ancestral Sawatch Uplift, and unknown small uplifts known only from lithologies and paleocurrent data from their synorogenic sedimentary packages. Normal block faults and thrust faults are located in the Central Colorado Trough, although the details of their relationships to each other are not yet known.

There appears to have been almost no reactivation of Late Paleozoic Ancestral Rocky Mountain structures during the Late Cretaceous/Early Cenozoic Laramide Orogeny. Most of the younger structures cut across the earlier structures. Structures oriented almost normal to the Laramide regional stress, such as the Uncompaghre and San Luis fronts, the Ute Pass and Gore faults, were reactivated with movements that appear to be orders of magnitude less than the late Paleozoic movement. The Laramide Front Range formed in a N-S orientation that is oblique to the NW-SE orientation of the Ancestral Front Range, which might have been at almost right angles to the regional Laramide stress fields.

Abstract

By Genevieve Young, Colorado Geological Survey

The Colorado Geological Survey is a participant in the Southwest Regional Partnership on Carbon Sequestration whose primary goal is to determine an optimum strategy for minimizing greenhouse gas intensity in the southwestern United States. The Southwest Partnership is a large, diverse group of expert organizations and individuals specializing in carbon sequestration science and engineering, as well as public policy and outreach.

The Southwest Partnership is currently conducting three pilots for Phase II (2005-2009) of the U.S. DOE/NETL Carbon Capture and Storage Program. These include one pilot each in the San Juan Basin, the Paradox Basin, and the Permian Basin. The Colorado Geological Survey is taking a key role in the design, implementation, and analysis of the San Juan Basin Fruitland coal pilot because of expertise in coalbed methane development.

President’s Note--October 2007

By Bill Nesse

Call for Papers

What: Colorado Scientific Society

When: November 15, 2007

Who: Graduate and Undergraduate Students in the Earth Sciences

Abstract Deadline: October 28, 2007

Honorarium: $250 and free membership for 2008

Submit your abstract (~300 words, GSA, AAPG, or similar format) as a Microsoft Word attachment to an e-mail to:

Dr. William Nesse nesse@ctos.com

Please provide your contact information, the name of your research advisor, and whether you are a PhD, Master’s, or Bachelor’s degree candidate. You will be informed in the first week in November if your presentation has been accepted.

Questions? Contact Dr. William Nesse: 970 330 7183 or nesse@ctos.com