NSF- REU 2007

FIELD-BASED STUDIES OF LANDSCAPE EVOLUTION IN WESTERN COLORADO

Student Research Activities

We will offer 3 out of the 4 student projects described below each year. Field work (~ 6.5 weeks) will be followed by 1.5-weeks of data synthesis including Geographic Information Systems (GIS) work.

• Holocene Arroyo Histories
• Pleistocene History of the Colorado and Gunnison Rivers
• Long-term Fluvial Histories and Volcanic Stratigraphy of Grand Mesa (not offered in 2007)
• Late Quaternary Neotectonics
• Selected References

• Holocene Alluvial Histories and the Arroyo Problem

  The origins of arroyos in the southwestern U.S., sometimes referred to as the arroyo problem, have long intrigued geologists (Bryan, 1925; Cooke and Reeves, 1976). Historic arroyo entrenchment occurred across much of the southwestern US between 1880 and 1920 (Hereford, 1984, 2002; Webb, 1985; Graf and others, 1987), and multiple cycles of arroyo entrenchment and infilling have occurred during the Holocene (Love, 1983; Patton and Boison, 1986). Possible causes of these arroyo-forming events include changes in stream hydrology and sediment transport linked changes in climate, land-use activities such as grazing, biological changes (e.g. invasion by tamarisk), or other as yet unidentified causes (Graf and others, 1987). Arroyo development can occur as a result of autocyclic processes such as knickpoint migration caused by a downvalley drop in base level.

  Arroyos are widespread but virtually unstudied in the Grand Valley of western Colorado. They typically occur as channels that have incised thick (up to 20 m) fill terraces or alluvial-fan complexes. Preliminary studies of the arroyo histories on the Uncompahgre Plateau indicates that arroyo filling commenced ~10,000 yr B.P. (Scott and others, 2001; Aslan unpublished data). This early Holocene phase of arroyo infilling may have been triggered by a decrease in runoff relative to high rates of sediment delivery at the late Pleistocene-Holocene transition. Studies of arroyo fill sedimentology and stratigraphy, and the timing of arroyo cutting and filling will be used to evaluate existing hypotheses (cf, Cooke and Reeves, 1976; Hereford, 1984, 2002; Graf and others, 1987; Hall, 1990; Waters and Haynes, 2001). For example, regional synchroneity of arroyo filling and cutting would imply that climatic factors played a major role in the development of the arroyos. Local examples of the effect of arroyo entrenchment and infilling on agricultural and developed land along with analysis of the potential effects of global warming on arroyo processes will also be emphasized so that students appreciate the importance of scientific investigation to real-life issues.

  The major unresolved questions regarding the arroyo problem and Holocene alluvial histories are as follows:

  • What major factors control arroyo development? To what degree are these features formed by changes in climate and hydrology, land use, biological factors, or other geomorphic controls?

   

  • How are arroyos in the region similar or different from one another? Is there synchroneity among regional arroyo fill and cut cycles? If so, can these cycles be related to specific Holocene climatic events?
  • Did arroyo development influence the departure of prehistoric Fremont Indians from western Colorado?
    
    

  Specific tasks that student participants will complete include:

  • Examination of historical photographs, aerial photographs, and topographic maps to evaluate historic arroyo incision.

   

  • Geological mapping of Holocene valley bottoms and arroyos using existing aerial photographs and topographic maps. Work will begin on the Uncompahgre Plateau and will progress to studies of arroyos along the northern perimeter of the Grand Valley (Book Cliffs region) and along the flanks of Grand Mesa in subsequent years.

   

  • Stratigraphic and sedimentological studies of arroyo fill sequences and comparisons with sediments along the modern arroyo floors.

   

  • Measuring rates of arroyo incision and filling and, possibly fire frequency, using 14C dating of charcoal in the arroyo fills.
    

    Pleistocene Histories of the Colorado and Gunnison Rivers

    Fluvial strath terraces provide important records of river incision, rock uplift, and climatic perturbations (Hancock and Anderson, 2002). The Grand Valley of western Colorado contains a spectacular sequence of poorly studied Colorado and Gunnison River terraces that record the Pleistocene evolution of the region. Preliminary mapping and data compilation shows a minimum of 6 Pleistocene Colorado and Gunnison River terraces and an unknown number of Pliocene to Miocene terraces of the Gunnison River (Carrara 1999a, b; Hood et al., 2002; Scott and others 2002b). Relief between the highest (~2500 m a.s.l.) and therefore oldest terrace remnant and the modern rivers is ~1100 m. Gunnison River terrace remnants in Cactus Park relate to the abandonment of Unaweep Canyon, the cause of which is the subject of much recent debate (Cole and Young, 1983; Scott et al., 2002a; Garhart and others, 2003).

    The major unresolved questions regarding Pleistocene histories of the Colorado and Gunnison Rivers are as follows:

  • Do strath terraces reflect glacial-interglacial cycles, climatically-driven erosional isostasy, rock uplift caused by late Cenozoic tectonic activity, or a combination of these factors?

  • Was the Gunnison River the dominant fluvial system (vs the Colorado River) during periods of deglaciation and meltwater discharge?

  • How does the incision history of the upper Colorado River relate to development of the Grand Canyon and the lower Colorado River system?

    Specific tasks that student participants will complete in the field include:

  • Geological mapping of terraces. A key objective will be to determine the number and distribution of Colorado and Gunnison River terraces. During the 3-year period, work will generally advance downvalley. Year 1 will focus on the Gunnison River terraces including those located in Cactus Park, which represents a Plio-Pleistocene course of the Gunnison River along the NE flank of the Uncompahgre Plateau, and relates to the origin of Unaweep Canyon. Years 2 and 3 will focus on Colorado River terraces in the Grand Valley and bedrock canyons (Ruby-Horsethief and Westwater) located downstream. Terrace mapping within the bedrock canyons will be accomplished via several raft trips. These areas are inaccessible and have not been studied although aerial photographs show numerous terrace remnants.

  • Evaluation of gravel provenance. Pebble counts will be used to document gravel composition and determine provenance. Gunnison River gravels are dominated by intermediate volcanics that are derived from the San Juan Mountains whereas Colorado River gravels are chiefly comprised of sedimentary rocks (including oil shale), phaneritic igneous clasts, and metamorphic rock types, which are derived from the central Rocky Mountains.

  • Stratigraphic studies. Correlations of terraces will be accomplished through a combination of soil descriptions and evaluation of topographic position and gravel composition. Older terraces typically have stage III-IV carbonate accumulation whereas younger terraces have stage I-II carbonate accumulation. Gravel composition will also be used to correlate specific terrace remnants. Relief measurements between terrace straths (erosional contact between gravels and underlying bedrock) and the modern rivers will also be used to correlate terrace remnants.

  • Construction of longitudinal profiles. Long profiles of terrace straths will be constructed and compared to modern stream profiles. Differences in profile gradients will be used to evaluate possible changes in climate, tectonic activity, and base level related to reorganization of drainage patterns (e.g., incision of the Grand Canyon, major stream piracy events).

  • Calculation of incision rates. Comparisons between terrace long profiles and those of the modern rivers will be used to determine the amount of bedrock incision recorded by each terrace. Age estimates of the terraces come from either use of 1) the 600 ka Lava Creek B ash or by 2) extrapolation of long-term incision rates. The Lava Creek B ash is present locally and can be used to constrain the ages of terraces (i.e., some are younger and some are older and 1 terrace correlates with the ash). The long-term incision rate of ~16 cm/Ky (Baker and others, 2002) and the relief between terrace straths and the modern rivers can be used to approximate terrace ages. Based on these age estimates, bedrock incision rates will be calculated. A key objective will be to determine whether or not incision rates increased significantly with the advent of Pleistocene ice ages. If so, this would indicate that climatically driven erosional isostasy and/or meltwater discharge has strongly influenced bedrock incision.

  • Comparisons with terrace data from elsewhere in the Colorado River system. Information on the numbers and distribution of terraces and their associated bedrock incision rates will be compared with those from elsewhere in the region (cf, Kunk and others, 2002). By determining whether or not incision rates increase or decrease upvalley and downvalley, we will be able to evaluate whether or not the river incision is driven by uplift (tectonic or isostatic) in the mountains or by a drop in base level related to downvalley incision of the Grand Canyon.
    

    Late Cenozoic Fluvial Histories and Grand Mesa Basalt Stratigraphy

    Grand Mesa provides significant insight into the earliest history of the upper Colorado River system. Located between the present-day Colorado and Gunnison Rivers, this "Y-shaped" basalt-capped plateau is a remnant of the land surface as it existed approximately 10 million years ago (Yeend, 1969; Young and Young, 1978, Sinnock, 1981a, 1981b; Cole and Sexton, 1981). Basaltic magma erupted along fissures in the area east of the remnant lava field 9.7 to 10.7 million years ago (Marvin and others, 1966; Kunk and others, 2002) and apparently flowed into shallow, east-west-oriented Miocene valleys. In 1987, the U.S. Bureau of Reclamation (USBR) drilled nine core holes through the basalt sequence on the Palisade lobe; hole locations are shown on Figure 1B. Results from this work show a thickness range for the total sequence from 248 feet (west lobe) to 616 feet (east lobe). The sub-basalt paleotopographic surface slopes from east (10,300 feet) to west (9,749 feet). Strata beneath the basalt consist of Miocene(?) sediments (Weston, 1987).

    Ancient river gravel containing igneous and metamorphic clasts have been reported at the base of the basalt sequence at scatted locations (Nygren, 1935; Larson and others, 1975). The USBR core drilling detected igneous gravel in only one hole (Number 7). The presence of the igneous and metamorphic clasts suggests that through-going rivers, possibly part of the earliest Colorado River system, were present in the Grand Mesa area prior to basaltic volcanism. On the eastern end of Grand Mesa near Mt. Darline, a thick gravel sequence with Oligocene volcanic clasts from the West Elk Mountains was found beneath the basalt sequence. The extent of these gravels has not been defined, nor have they been described.

    Ancient river gravel also rests on the basalt sequence near the western limit of the Palisade lobe near present-day Whitewater Creek. The clasts, which consist of quartz, quartzite, chert, granite, arkose, and petrified wood, were possibly derived from the White River Uplift or the Elk Mountains to the east (Yeend, 1969). Yeend (1969) suggests that this gravel may also have been deposited by the ancestral Colorado River. The general trend of the gravel is towards Unaweep Canyon, which may support the hypothesis that the Colorado River once flowed through this now-abandoned canyon (Lohman, 1961, 1981; Yeend, 1969).

    The major unresolved questions regarding Grand Mesa and its relationship to the landscape evolution of the upper Colorado River system are:

  • How extensive was the original basalt field, and how did its geometry influence positioning of the incipient Colorado and Gunnison Rivers? Did it divert the ancestral Colorado River to its present course?

  • What are the gross stratigraphic relationships of the basalt flow units, and how does this architecture relate to paleo-topography?

  • Was the pre-volcanic topographic surface sloping to the east or to the west, and has there been any post-volcanic structural tilting?

  • What is the distribution, composition, and provenance of the gravels beneath the basalt? Are they evidence for early development of the Colorado River system?

  • What is the distribution, composition, and provenance of river gravels on top the basalt? Are they related to the incipient Colorado River, or to local drainages?

    Specific tasks that will be attempted in the field are as follows:

  • Definition of the gross stratigraphic architecture of the basalt sequence to determine flow directions with regard to the postulated Miocene valleys. Flow directions will be determined by measuring elongated vesicles in the basalt, plus mapping of individual flow units exposed along the perimeter of Grand Mesa.

  • Utilize the USBR core-drilling data to better define the sub-basalt paleo-topographic surface, and the type of Miocene sedimentary rock underlying the basalt. MSC has copies of all of the drilling records, including more than 750 feet of the original core, which is in good condition.

  • Map the base of the basalt sequence around the edges of Flowing Park and Palisade lobes to define the extent of the sub-basalt gravels. Existing oblique aerial photographs will be employed in this effort.

  • Describe the sub-basalt gravels near Mt. Darline.

  • Describe and map the gravels on top of the basalt on the west end of Palisade lobe. Do a reconnaissance evaluation of Flowing Park lobe to see if similar gravels exist.
    

    Structural Geology and Late Quaternary Neotectonics

    One of the outstanding questions regarding landscape evolution in western Colorado is whether or not there has been substantial late Cenozoic rock uplift (Pederson and others, 2002). Previous investigations have suggested that Quaternary uplift of the Uncompahgre Plateau resulted in abandonment of Unaweep Canyon by the Gunnison River (Cater, 1966; Lohman, 1961, 1981; Scott and others, 2002a). However, there has been no documented Quaternary-age movement along faults in the area. A substantial number of suspected Quaternary-age faults are mapped along the Uncompahgre Plateau (Lohman, 1961; Williams, 1964; Witkind, 1976; Howard and others, 1978; Kirkham and Rogers, 1981; Colman, 1985; Lettis and others, 1996). These data have been compiled into a Quaternary-age fault map by the Colorado Geological Survey (CGS) (Widmann and others, 1998, 2003). The data reported in all of these references for Quaternary-age faulting, however, is circumstantial and contradictory, and no unequivocal evidence of late Quaternary-age faulting has been documented in the region. The key problem is that many of these structures have simply not been mapped in detail. The purpose of this proposed research is to construct detailed geological maps of portions of the Uncompahgre Plateau to determine whether or not there has been Quaternary-age fault movement. There has been historic seismic activity in the region and our mapping will also be used to address fundamental questions regarding potential seismic risk associated with these faults.

    Some new, detailed, mapping has been done along portions of potential Quaternary structures. This includes mapping of the Redlands fault (Scott and others, 2001) and current work by the Mesa State geology program (funded by USGS EDMAP, 2004-2005, awarded to R. Livaccari) along the Cactus Park-Bridgeport and Big Dominguez Creek faults. Initial results of our EDMAP research indicate that the Cactus Park-Bridgeport fault is a, subvertical, left-lateral strike slip fault of probable Laramide-age. Our mapping has identified two areas along this Cactus Park-Bridgeport fault that may indicate reactivation during the Quaternary. We have also recognized a possible kinematic link between the Cactus Park-Bridgeport fault and other faults found to the northwest. This kinematic link may be a regional left-lateral strike-slip fault system connected by reverse faults. The second hypothesis to be tested with this proposed work is the viability of this kinematic link. This is important because this region is the northern boundary of the Colorado Plateau. Recognition of the deformation pattern will provide information regarding the relative movement of the Colorado Plateau with respect to the craton.

    The major unresolved questions regarding the structural geology and late Quaternary deformation of the Uncompahgre Plateau are:

  • Is there any evidence of late Quaternary movement on faults of the northern Uncompahgre Plateau? If no evidence of recent faulting is present, could this region have undergone broad uplift related to isostatic unloading?

  • Are the major faults along the northern and northeastern perimeter of the Uncompahgre Fault linked, and do they represent a regional strike-slip fault system?

  • How is this northeastern boundary zone related to the late Cenozoic development of the Colorado Plateau?

    Specific tasks that student participants will complete in the field include:

  • Geological mapping (1:24,000 scale) of faults and related structures with an emphasis on potential Quaternary faults. These faults are listed on the Colorado Geological Survey (CGS) database and we will use this database to target specific fault segments. Mapping will concentrate on two key regions: 1) the Bangs Canyon area, which is located between the mapping completed by Scott and others (2001), and the area currently being mapped by Mesa State College (USGS-EDMAP project), and 2) the Bull Canyon fault region (western part of fault system).

  • Detailed descriptions of kinematic structures (e.g., slickensides, deformation bands) and folds, which will be used to reconstruct stress fields.

  • Compilation of data into a regional-scale structure map of the Uncompahgre Plateau.

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