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Framework studies of the Colorado Front Range region

Task Objectives

The long-term goal of the task is to provide comprehensive geoscience data and interpretations that will allow federal, state, and local land-management entities to make informed land-use decisions in central Colorado. Addressing these issues requires preparation of state-of-the-art digital geologic maps that will be used for a variety of purposes, including: (1) identification and study of Cenozoic igneous systems in order to provide new insights on the origin of hydrothermal mineral deposits, (2) urban and rural planning and maintenance of infrastructure (i.e., construction of state and federal highways, management of National Forests and National Parks, and municipal planning), (3) assessment of mineral resources, including sand and gravel, (4) fracture and shear-zone analysis, critical for assessment of both ground-water circulation and hydrothermal-fluid flow, (5) hazard analysis, including the evaluation and mitigation of abandoned mined lands, (6) understanding changing climatic conditions through study of surficial deposits, (7) assessing the geologic effects of wild fires, a critical question in light of the widespread beetle kill in the western part of the study area, and (8) increasing our understanding of the crustal processes that created the Proterozoic basement of the Front Range by targeted isotopic and structural studies. The core products are four 30' x 60' quadrangles which form a north-to-south set: Fort Collins, Estes Park, Denver West, and Bailey. In addition, seven separate 7-1/2' quadrangles, the eastern halves of the Vail and Leadville 30' x 60' quadrangles, and a variety of topical research papers have been published or are currently being prepared. Scoping studies of the North Park and Medicine Bow Mountains region will be undertaken by J.S. Cole during Fiscal Year 2009 in anticipation of future project support.

Statement of Work

This is the final year of the project and will be used primarily for completion of ongoing maps and written products. The primary goals for 2009 are to: (1) submit for publication the Fort Collins and Bailey 30' x 60' (1:100,000-scale) quadrangles, (2) work with editors toward final publication of the Estes Park 30' x 60' quadrangle (received Director's Approval during 2008), (3) complete isotopic studies (U-Pb and Nd-Sm) of approximately 50 Proterozoic igneous and metamorphic rock samples from the Front Range region and prepare papers for outside publication on the magmatic and metamorphic history of the region, (4) submit a paper for outside publication on the regional metamorphic history of the northern Front Range based on field, geochemical, and petrographic relationships, (5) complete mapping of poorly understood areas in the eastern half of the Vail 30' x 60' quadrangle and submit map for review, (7) complete mapping of a ~300 km² area in and near Leadville, along the northern Rio Grande rift, for incorporation into the Leadville 1:100,000 quadrangle, (8) submit for publication five 7-1/2' quadrangles (listed below), and (9) begin scoping exercises in the Gunnison and Pikes Peak 1:100,000-scale quadrangles in anticipation of the next mapping-project cycle in central Colorado.

The Bailey, Denver West, Estes Park, and Fort Collins quadrangles form a continuous, ~20,000 km² block from the Wyoming border to about 20 km north of Colorado Springs and encompass both large tracts of National Forest land and areas of rapid urban growth. All new mapping embraces a substantially refined and enhanced understanding of surficial deposits. Mapping is also coordinated with mineral and groundwater studies (undertaken by the Minerals and Crustal Imaging Teams) to emphasize new information on fracture patterns.

Separate 7-1/2' quadrangles within the Fort Collins, Estes Park, Denver West, and Bailey 30' x 60' quadrangle that will be submitted for publication are: Sand Creek Pass, Bowen Mountain, Fraser, Elkhorn, and Sulphur Mountain. In addition, scoping of the North Park and Medicine Bow Mountains for a future regional geologic synthesis of this region will be undertaken in subtask 2.1.

Highlights & Key Findings

2009 is intended as the final "write-up year" for the project. The Denver West 30' x 60' quadrangle has been published and the Estes Park 30' x 60' quadrangle has received Director's Approval and is undergoing final revision. Field mapping for the Fort Collins and Bailey 30' x 60' quadrangles has been completed and both maps are being prepared for review. Several topical studies have provided insights into a variety of scientific issues and have been published or are currently being completed and written as either USGS reports or outside publications:

1. Wayne Premo has determined about 50 new U-Pb SHRIMP zircon ages and corresponding Nd isotopic signatures from samples of Paleoproterozoic basement in the central Colorado Front Range; these data delineate an early, immature, oceanic arc complex that formed circa 1780 to 1750 Ma and was derived from sources contaminated by older crustal material, some of which was Archean; erosion of the arc terrane led to the formation of basins between circa 1750 and 1715 Ma that overlapped the early arc terrane; deformation of these supracrustal rocks was accompanied, in part, by widespread dioritic to monzogranitic plutonism from circa 1720 to 1705 Ma, culminating in high-grade metamorphism and the formation of migmatites and monzonitic to granitic crustal melts between circa 1700 and 1690 Ma, considerably younger than previously suggested,
2. Ralph Shroba has completely reinterpreted and written what we feel is the most in-depth summary to date of the Quaternary history of the central Rocky Mountain region, included in the text for the Denver West quadrangle,
3. detailed mapping in the Fort Collins quadrangle has identified an extensive paleo-felsenmeer (ancient periglacial deposits)as low as about 3,000 m, the result of significantly lower late Pleistocene temperatures during the Pinedale glacial period, and
4. in concert with ongoing isotopic studies in the Fort Collins quadrangle, new mapping has refined our knowledge of the extensive ~1,700 Ma Rawah granite batholith and the ~1,400 Ma Log Cabin granite batholith.

In addition to progress on the four current 30' x 60' quadrangles, mapping is complete for five as-yet-unpublished 7-1/2'quadranlges (Sand Creek Pass, Bowen Mountain, Fraser, Elkhorn, and Sulphur Mountain). New mapping is currently underway and will be completed during 2009 in the eastern halves of the Leadville and Vail 1:100,000 quadrangles.

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