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Geosciences and Environmental Change Science Center

Cenozoic Landscape Evolution of the Southern Rocky Mountains

Geochronologic Investigations

Contact: Mike Cosca

The present landscape of the Southern Rocky Mountains and surrounding physiographic areas results from time-integrated interaction between the asthenosphere, lithosphere, and hydrosphere. Large-scale phenomena, such as asthenospheric upwelling directly or indirectly affect lithospheric thinning, faulting, basin development, and igneous activity sometimes associated with ore deposits. During the Cenozoic, these phenomena created near-surface regions that have been sculpted by glacial and hydrologic processes, and combined with subsurface geology, control a significant part of the nations' fresh water supply. In addition, the nations' largest molybdenum mines and prospects occur within the southern Rocky Mountain corridor and formed during the last 30 m.y. Establishing precisely when these geologic and geomorphic features developed is critical to characterizing the regional geologic framework that will be used to address future, potentially difficult, decisions such as rights to hydrologic flow and mineral exploitation. Because there are two distinct (Oligocene and Miocene) episodes and styles of tectonism recognized throughout the highly extended western US, including northern New Mexico and central Colorado, it is essential to unravel how these (and possibly other) tectonic styles impacted the geologic landscape evolution of the greater southern Rocky Mountain region. Although some parts of this region are well-studied, others have been virtually ignored. The combination of high-precision geochronology with field mapping, geochemistry, and geomorphology will greatly enhance the geologic characterization of a region that is rich in critical natural resources.

View of remnant volcanoes in background Eroded remnants of small, alkali-rich volcanoes erupted during latest Miocene, Elkhead Mountains, Colorado. (Photo by M. Cosca)
Looking up at columnar jointing View of columnar jointing in late Miocene basalts from the Elkhead Mountains, Colorado. (Photo by M. Cosca)
photo of upper Servilleta lava sequence section View from top of sampled section of upper Servilleta lava sequence looking north toward Ute Mountain. (Photo by M. Cosca)
photo of upper Servilleta lava sequence section View toward northeast from sampled section of upper Servilleta lava sequence. Note landslide blocks in foreground and Sangre de Cristo range in the distance. (Photo by M. Cosca)

Objectives

View of Yampa Valley showing volcanic features Volcanic necks, fissures, and flows of 4.5–6 Ma erupted along the flanks of the Yampa Valley, Colorado and are a consequence of regional extension of the lithosphere. (Photo by M. Cosca)

Cenozoic igneous rocks occur throughout a wide area of the southern Rocky Mountains corridor. One of the primary objectives is to establish a coherent and reliable chronology of igneous activity within the southern Rocky Mountains corridor and surrounding areas from the Oligocene to the present. Regional-scale faulting is often accompanied by marker bed displacement and/or fault gouge, and in some cases, (e.g., volcanic horizons and Krich gouge minerals) these features can be dated directly using radiogenic and stable isotopes to determine when faulting was active. In the southern Rocky Mountain corridor there are major zones of fault displacement with no established age that could be > 70 Ma or as young as a few million years. Moreover, regional fault arrays may have governed the localization of past volcanism; new mapping of regional faulting in the Southern Rocky Mountains combined with geochronology will facilitate geologic interpretations.

The preservation of recessional moraines is an important marker of past climate change, and boulders within them can be dated using cosmogenic isotopes to define when and how long the glacial ice retreated. Additionally, isolated Quaternary volcanism (< 1Ma) occurs from New Mexico to Wyoming, and characterization of this volcanism is important for determining current zones of lithospheric weakness, any relationship to active or past faulting, potential for volcanic hazards, and tracing past magmatism and possible ore mineralization. Precise chronology of when these features developed is necessary for interpreting the geologic and geomorphic evolution of this area and its resources. This task will focus on acquiring and interpreting high-precision geochronologic data in collaboration with the mapping and structural studies of this project.

Objectives will focus on the following:

  1. Establishing the absolute timing of Cenozoic igneous activity and relationship to regional tectonics within the Southern Rocky Mountains.
  2. Determining the timing of significant fault displacements associated with basin-bounding faults and accommodation zones.
  3. Determining the timing of glacial retreat and paleohydrologic conditions in response to past climate change.
  4. Developing novel methodology for analyzing Quaternary (< 2 Ma) basalts.

Additional targets of investigation will be speleothems and paleohydrologic features, such as groundwater discharge mounds. Combining geochronology of these mounds, through U-Th disequilibrium, with 10Be data from glacial stands may provide important information related to paleohydrology within this region.

Methodology

Several geochronologic methods will be employed for this project, but the principle analytical method for this task will be 40Ar/39Ar geochronology. This method is suitable for any mineral or rock that contains potassium. The Denver USGS 40Ar/39Ar laboratory just received a new generation of multi-collector noble gas mass spectrometer capable of providing increased precision on ages of small samples. The Denver USGS 40Ar/39Ar laboratory is also developing high-precision methods for analyzing small samples, including basalts with low potassium content, by progressively heating rock fragments with an infrared laser. Such methods will be applied to all appropriate samples of this project. The increased analytic precision of the newer mass spectrometers will permit higher resolution 40Ar/39Ar and paleomagnetic correlations and increasingly precise determinations of time in lavas and their eruption rates. Other methods of geochronology employed in this task will be U-Th-Pb, U-Th disequilibrium, Sm-Nd, and Rb-Sr. The U-Pb, Sm-Nd and Rb-Sr methods will be applied to assess source isotopic signatures of magmatic systems, and the U-Th disequilibrium method will be applied as a means of dating neotectonic faulting and will be assessed for dating of Quaternary basalts with 40Ar/39Ar geochronology. The acquisition of stable isotope data and additional geochronologic methods such as 10Be, OSL, and (U-Th)/He will be employed as funding permits.

Picture of argon extraction system in lab Low-volume, laser-based, argon extraction system and mass spectrometer for isotopic determination of time in K-bearing rocks and minerals by the 40Ar/39Ar method. (Photo by M. Cosca)
Closeup picture of CO2 sublaser Close up view of a CO2 laser positioned over samples in a custom sample chamber evacuated to 10-10 Torr. (Photo by M. Cosca)

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