Cañon City Uplift

If the desert is holy, it is because it is a forgotten place that allows us to remember the sacred. Perhaps that is why every pilgrimage to the desert is a pilgrimage to the self. There is no place to hide and so we are found.

Terry Tempest Williams writes about the desert as a sacred place to connect with self and the natural world. Her work spans canyons and mountain tops and inspired in me a love of the desert. That love brought me to CC, encouraged me to go into Southwest Studies, and to explore geology. To find the answers to what makes the natural world that I love so much.

Cañon City, located about an hour southwest of Colorado Springs, served as a geologic study area for this project. The Cañon City region experienced uplift that is largely unexplored in the greater scientific community. One team (Marder et al.) has done some work in the area exploring one factor that can cause uplift, isostasy, which will be explored further in this project. The region experienced Laramide era deformation including compressional forces from plate subduction on the west coast of the United States.

Our GY212 class worked in this small water gap in a park in Cañon City to measure geologic units and create a geologic map. The map below shows those units as well as strike and dip measurements (measuring the angle of rock formations) in our study area. One interesting thing to note is the Precambrian units contact with the Fountain and Morrison Formations. The formations are significantly younger, and should not be contacting the Precambrian units. This implies a disconformity of some sort. We attributed this disconformity to a fault that caused the rock units to move in such a way that the Precambrian now contacts the much younger rock.

The team of  Moucha et al . looked at the Cañon City region to explore the possibility of uplift in the region. The team explored dynamic topography in the region, which will be elaborated on more later. The team came up with three hypotheses of topographic change in the area. An eastward tilt, westward tilt, or erosional forces. To explore this further, as a class, we explored surface geology, isostasy, stream steepness and dis/equilibrium, and river terrace erosion. I will sum up what we discovered in each study method by adding to the table below.

Isostatic Anomalies show areas where a concept known as dynamic topography may be occurring. The anomalies show areas where there is either a change in rock density or there is dynamic topography. We know this because isostatic anomaly maps show the areas where after accounting for differences in elevation (the Bouguer and free air anomalies), there is still a measurable anomaly. Rock density differences can be determined by exploring the surface rock type map. If the anomaly is not explained by rock type, it can be assumed it is due to mantle dynamics. Mantle dynamics refers to convecting processes in the mantle of the earth that affect topography on the surface. For further reading on the topic, I recommend  this  paper from Molnar et al. Areas of high isostatic anomaly are less dense and uplifting more than areas of low anomaly which are more dense.

When looking at stream dis/equilibrium in the Cañon City study area our class split up into three area groups and three micro groups within those areas (nine groups total), that looked at stream steepness rates across the study area. We also brought in the work of  Marder et al . which showed a potential northward tilt in the topography. We looked at a north/south transect of the Wet Mountains to investigate the northward tilt possibility. We found no observable north/south difference in stream steepness. Overall the stream steepness data across the study area did not show a consistent overall trend. Different areas supported different hypotheses on micro scales. Stream steepness should display a graded profile which will be elaborated on a bit later, but the stream will be steeper at the headwaters and less steep at the outlet. We see unexpected stream steepness when there has been some sort of change in the topography. This change can be related to dynamic topography or faulting. In our study area there was a bit of support for both tilt directions, but the most interesting findings were faults impacting stream steepness.

This table illustrates the maximum, average, and minimum erosion rates of the river channel from Canon City flowing east along the Arkansas River. The data was collected by measuring differences in elevation between rock units and putting that difference over the amount of time that the erosion could have occurred. All of these were ranges hence the max, min, and mean datapoints. The elevation changes can be pictured by imagining you were standing on the rocks while this erosion happened. The net change you would experience accounting for erosion and isostatic response as of today is shown in the elevation change. There is also a column that shows how much has yet to be compensated given the standard that isostasy accounts for 80% of erosion. This data most likely has some errors with collection as the results do not show an explainable trend.

This is the data from a class wide analysis of river terraces. The terraces can show valley bottoms from various points in time that a river flowed over. Terraces record the histories of where a river once was in layers of sedimentary deposits. If erosion was the main driver of uplift in Canon City we would expect to see the erosional data get steeper from west to east so that the river would have a graded profile. A graded profile is the ideal shape a river would like to be in from headwaters to ocean. The trend we found shows that either erosion is too new or that there is something else changing the topography of the landscape around Canon City. It also seems likely that our data is not particularly accurate and more research should be done into mapping the terraces of the Arkansas.

The uplift in the Cañon City region needs further study. From the data we collected and examined as a class, it seems like multiple things are true. There is both westward and eastward tilt depending on the micro area, and there may be erosional factors that need further investigated.