Plio-Pleistocene Geomorphic Evolution

Science Center Objects

The Southern Rocky Mountains encompass the highest elevation within the conterminous United Sates, having fifty-three 14,000-ft peaks. Two national parks (Great Sand Dunes and Rocky Mountain), the San Luis Valley National Heritage Area, the Rio Grande Del Norte National Monument, and several wildlife preserves are also in this area. This region has experienced multiple episodes of tectonic activity, extending from the Proterozoic to Pleistocene.

A Task of the Cenozoic Landscape Evolution of the Southern Rocky Mountains Project.

 

A long-held paradigm has been that the current physiographic expression is a vestige of the Laramide orogeny, occurring between approximately 70 and 40 Ma. However, ongoing studies are revealing that much of the topographic relief and physiography are a result of Neogene uplift and extension. The Rio Grande rift has been accepted as a result of this intracontinental extension, but the extent of regional rifting and uplift is poorly understood. Features throughout central Colorado from the San Luis Basin to North Park suggest that neotectonic activity is ongoing, creating the backbone of North America, rearranging drainage systems, and creating previously unrecognized seismic hazards. As a result of Plio-Pleistocene uplift, past climatic patterns have created large glacial systems and allowed the integration of once-closed basins into three of the countries' largest fluvial systems: Rio Grande (3,051 km), Platte River (2,358 km), and Colorado River (2,334 km). We will reevaluate previous studies and include new mapping to assess timing and rates of late Neogene uplift, intra-continental climatic fluctuations over the Pleistocene, fluvial integration of once-closed basins, and potential seismic hazards.

Mt. Elbert and surrounding mountains

View from Mt. Elbert (14,439 ft/4,401 m) looking southwest towards Castle Peak (14,278 ft/4,352 m) across the Sawatch Range, Colorado. (Credit: Cal Ruleman, USGS. Public domain.)

Glacial terrain

View looking north of heavily glaciated terrain from Mount Massive (14,429 ft/4,398 m) towards the Holy Cross Wilderness, northern Sawatch Range, Colorado. (Credit: Cal Ruleman, USGS. Public domain.)

Rio Grande Gorge

Current investigations involve understanding the paleoclimatic and neotectonic influences on timing and rates of evolution of the Rio Grande gorge, south-central Colorado and north-central New Mexico. (Credit: Cal Ruleman, USGS. Public domain.)

Objectives

The primary objective is to understand the Plio-Pleistocene tectonic and climatic effects on inter-montane basins within the Southern Rocky Mountains. These topical studies directly relate to the structural and kinematic evolution of the region and their controls on hydrology and seismic hazards. These surficial studies will also support the structural and geophysical subsurface studies within the basin. They will involve field, laboratory, geophysical, and remote-sensing investigations in order to understand the spatial and temporal geomorphic evolution of the region.

Specific objectives include:

  • Compilation of existing surficial geologic mapping and assimilation into a common nomenclature to gain a refined understanding of spatial relationships of Quaternary deposits and the geomorphic evolution of the basin.
  • New 1:24,000-scale and 1:50,000-scale geologic mapping for 1:100,000-scale geologic compilation of the San Luis Basin.
  • Assessment of previously developed regional kinematic models for extensional tectonism and how they compare throughout the region from basin to basin.
  • Determining the difference between the morphology of various basaltic flow margins, fluvial erosion along the Rio Grande, and previously interpreted active fault boundaries.
  • Determining the timing, cyclicity, and effects of Pleistocene glaciations within this intracratonic region and how they relate to the global terrestrial and marine records. This will directly relate to determining the transition time between pluvial and fluvial geomorphic systems.
Worker using instruments to monitor rates of movement of geologic structures

Monitoring rates of movement and causes of initiation of large sakungen structures in the northern Sawatch Range Colorado with Jeff Coe and Jonathan Godt (pictured) of the USGS Geologic Hazards Science Center. (Credit: Cal Ruleman, USGS. Public domain.)

 

Geologist next to outwash deposits

Jim Paces analyzing Quaternary sediments of the Santa Fe Group capped by glacial outwash deposits for timing of transition from low-energy basin sedimentation to coarser, high-energy glacial deposition related to paleoclimate change. (Credit: Cal Ruleman, USGS. Public domain.)

 

Workers standing in front of sediment deposit

Sampling of Quaternary sediments for optically stimulated luminescence dating for paleoclimatic studies within the northern Rio Grande rift, northern New Mexico with National Association of Geoscience Teachers intern Sydney Gunnarson and Harrison Gray (USGS). (Credit: Cal Ruleman, USGS. Public domain.)

 

Worker taking samples

Sampling of Pleistocene glacial deposits to understand magnitude, timing, and rates of deglaciation for paleoclimatic studies of central Colorado. Marc Caffee of the Purdue University Rare Isotope Measurement (PRIME) Laboratory pictured on the flanks of Mount Massive, northern Sawatch Range Colorado. (Credit: Cal Ruleman, USGS. Public domain.)

Methodology

Geologists in a trench analyizing the stratigraphy

Analyzing trench stratigraphy for paleoseismic investigations along the northern Sangre de Cristo fault zone, San Luis Valley, Colorado. (Credit: P. Lucha, University of Zaragoza, Zaragoza, Spain)

To understand the geology of the San Luis Basin, we plan to compile and reinterpret existing geologic mapping, conduct new focused mapping in areas that need more control, and provide a modern surficial geochronologic framework for the area using the following methods.

Geologic Mapping

  • Compile detailed mapping at 1:100,000 scale in the Alamosa, parts of the Wheeler Peak, Chama and Blanca Peak sheets to understand San Luis Basin surface geology.
  • Conduct new 1:24,000-scale surficial geologic mapping where Quaternary geology will help solve paleoseismic, hydrologic, structural, and/or stratigraphic problems.
  • Establish location and temporal evolution of active faults within the San Luis Basin.
  • Compile detailed maps of Quaternary faults in the San Luis Basin/Taos Plateau to assess their paleoseismic history including slip rate, recurrence intervals, and maximum credible earthquake values.

Geochronology

  • Generate Optically Stimulated Luminescence (OSL) ages of eolian, fluvial, and potentially lacustrine deposits to support relative-age interpretations of basin depositional history and relationships with climatic events and possibly paleoseismic activity along intra-basin fault zones.
  • Generate 10Be cosmogenic radionuclide ages of deposits to constrain Pleistocene glacial/interglacial history of the region and subsequent depositional/erosional rates over the Pleistocene.
  • Generate Helium surface exposure ages for fluvially scoured Servilleta Basalt to establish incision history of Rio Grande north of the Chama River to provide quantitative constraints on the history of Rio Grande drainage integration and its' relation to Pleistocene Lake Alamosa and the evolution of the hydrologic system.