1. Using rock strength analysis to determine the spatial distribution of erosion and evolution of knickpoints in a stream channel within the Sacramento Mountains, NM
Garber, Kacey L., Tranel, Lisa M., Ph.D., and Rothschild, Tyler J.
Department of Geography & Geology, Illinois State University, Campus Box 4400, Normal, IL
ABSTRACT
In the Sacramento Mountains, rainfall and snowmelt from high elevations supply water to ephemeral streams. The mountains are bound on the western flank by a steep escarpment formed by normal faults associated with extension. We are investigating the spatial distribution of erosion in an arid climate stream channel to better understand where erosion is focused and what controls knickpoint evolution. Knickpoints (sharp changes in slope) within longitudinal profiles are formed by resistant bedrock units that are difficult to erode or by slip and offset during faulting. If knickpoints are created by uplift or faulting, they may occur anywhere along the stream channel regardless of the strength of bedrock material. However, if the knickpoint results from the bedrock’s resistance to erosion, there will be a strong correlation between the position of the knickpoint and rock strength. Based on this, we hypothesize that the rock strength of bedrock with knickpoints will be greater than the rock strength of bedrock units without knickpoints. To test this, we used the Schmidt Hammer and Selby rock strength classification to determine the rock mass strength (RMS) of rock units parallel to the channel in Dog Canyon. We then compared RMS results to changes in slope along a longitudinal channel profile created in GIS identifying geologic units and knickpoint features. We also compared RMS values to the average slopes for each bedrock unit. Initial results suggest that the knickpoints occur in the three strongest rock units tested, which were all carbonates. Dolostone had the highest RMS values on average. RMS appears to generally decrease with increasing elevation. Also, RMS and average slopes of bedrock units show a weak positive relationship. Knickpoints were located near formation contacts and between weaker rock units. These results suggest that knickpoints may be controlled by rock strength and their position along the stream profile. Therefore, the height and extent of these knickpoints may be influenced by the thickness of the bedrock units. Future work on this issue will involve analyzing the spatial distribution of erosion with detrital apatite minerals from samples collected in Dog Canyon.
#1: RMS=83
#2: RMS=85
#3: RMS=87
Figure 2: Table summary of RMS values at our 11 field sites where we assessed overall rock strength based on 7 parameters and Selby’s classification scheme. All units test had either moderate or strong ratings. The dolostone units had the highest RMS values on average based on our field tests. We did not test any Mississippian units because they are covered with Quaternary sediments.
Figure 5: Longitudinal profile showing the geologic units plotted with elevation. Note the three areas in the profile where elevation changes abruptly. Vertical lines between different colors represent the positions of the rock units at the surface. R=
Figure 1: Entrance to Dog Canyon, Oliver Lee Memorial State Park, NM
Figure 6: RMS rating plotted against elevation for each of the 11 field test sites. Note that the RMS appears to generally decrease as elevation increases.
BACKGROUND
The Sacramento Mountains lie on the eastern edge of the Basin and Range province in south-central New Mexico, extending about 80 miles north-south. They are bound on the western flank by a steep escarpment formed by normal faults associated with extension. Past research demonstrates that uplift in this area began in late Tertiary time and is still continuing presently. The bedrock strata range in age from Precambrian to Cretaceous and are primarily marine shelf deposits. The strata have a total thickness of nearly 8,000 feet. There are also Tertiary intrusive sills and dikes present in this area. We focus our research on the thick and well-exposed Paleozoic sedimentary strata within a stream channel in Dog Canyon at Oliver Lee Memorial State Park, located about 10 miles south of Alamogordo, NM off of U.S. Highway 54. The ages of the Paleozoic strata of interest range from Ordovician to Permian. These thick Paleozoic carbonate deposits make up the majority of this area’s stratigraphic section.
Testing the strength of these rocks in the field provides insight into where erosion and knickpoint evolution is focused in this arid climate stream channel. Knickpoints, or sharp changes in slope within a stream profile, can be a product of uplift and faulting or a bedrock’s resistance to erosion. They tend to form within resistant bedrock units and between weaker units. Our goal is to examine the control that bedrock lithology has on erosion in Dog Canyon through various field tests and observations.
Figure 3: Table summarizing all data collected at each of the 11 field test sites. R=
Figure 7: Average slope of bedrock units plotted against RMS ratings for each unit. Note that there is a weak positive correlation between the two variables.
METHODS
In the field, we hiked the Dog Canyon trail to observe Ordovician-Pennsylvanian strata and drove up to the top of the canyon near Sunspot, NM to observe thicker Permian strata. Our main objective in the field was to conduct rock mass strength (RMS) tests based on Selby’s classification scheme from These tests were conducted at 11 suitable sites containing 11 different Paleozoic formations where strata were well-exposed and relatively simple to access. We observed the strata and assessed rock strength based on 7 parameters at each formation including intact rock strength, degree of weathering, spacing of joints, joint orientations, width of joints, continuity of joints/degree of infill, and outflow of groundwater. Intact rock strength was assessed using a Schmidt hammer, which measures the rebound strength of a spring loaded mass as it impacts the surface of bedrock or any rigid object. It gives a value from , with 10 being the weakest and 100 being the strongest. Upon returning, we compiled our RMS data into summary tables and assigned overall rock mass strength values based on the 7 parameters assessed in the field. We also created a geologic map of Dog Canyon using ArcGIS that displays the stratigraphic units with each of our test locations and data plotted within them. We used this to create a longitudinal stream profile comparing changes in elevation with the stratigraphic units to assess where knickpoints occur in the profile and then turning to our rock strength data to look for correlations. We also created graphs plotting factors such as elevation, slope, and RMS against each other to observe possible correlations.
RESULTS AND CONCLUSIONS
Our longitudinal profile (figure 5) displays three significant knickpoints. The most significant occurs within the Gobbler Formation (Pennsylvanian limestone) at field stop RS05 where RMS=83. There are two other large knickpoints, one within the Fusselman Formation (Silurian dolostone, RS03, RMS=85) and the other within the El Paso Formation (Ordovician dolostone, RS11, RMS=87). These three knickpoints occur within the three strongest carbonate units that we tested in the field. They hold the highest assigned RMS values. They also occur near formation contacts and between weaker rock units. This finding suggests that the position of knickpoints in the profile are likely controlled by the position and strength of the lithology in the stream channel rather than strictly by faulting and uplift. Therefore, the height and extent of these knickpoints may be influenced by the thickness of the bedrock units. Based on our graphs, we found that RMS appears to generally decrease as elevation increases (figure 6). There is also a weak positive relationship between average slope and RMS ratings for field tested units (figure 7).
Figure 4: Geologic map of Dog Canyon created in ArcGIS (digitized from Pray, 1961).
REFERENCES
Pray, L.C., Geology of the Sacramento Mountains Escarpment, Otero County, New Mexico. State Bureau of Mines and Mineral Resources, New Mexico Institute of Mining & Technology. Bulletin Vol. 35, 144 p.
Selby, M.J., A rock mass strength classification for geomorphic purposes: with tests from Antarctica and New Zealand. Zeitschrift fur Geomorphologie 24 (1),
FUTURE WORK
We collected several large samples in Dog Canyon from multiple Paleozoic formations. We are currently working on processing these samples for detrital apatite minerals. This involves crushing the samples to under 250 micrometers in size using a chipmunk crusher and dry sieves as well as using various cleaning and separation methods to obtain the apatite. These extracted minerals will later be sent to a geochronology center to determine exposure ages for our samples, enabling us to analyze the spatial distribution of erosion in Dog Canyon.
ACKNOWLEDGEMENTS
Funding support for this project was provided by the British Society for Geomorphology.