Results and Discussion Figure 8. Box-and-whisker summary data plot. Gully density generated by measuring the sum of the lengths of the escarpments bounding individual gullies within each basin divided by the area of each basin. Red lines are median values for each set. Drainage basins are assigned to the Pancho Rico Formation (mudstones) and Paso Robles Formation (sandstones and conglomerates) sets if >60% of the drainage basin is underlain by that lithology. Data that didn’t fall into either of these categories was labelled a “Mixed Basin.”

Figure 9. Top-Relative uplift rate model using the record of defor-mation of the initial horizontal, now tilted, Paso Robles surface that characterizes the mesa top of this region. Bottom-Basin averaged gully density in different quartiles of the surface deflection.

Figure 10. Basin averaged gully density in different quartiles of the depth of incision below the modelled Paso Robles surface.

Figure 7. Top Left-Aerial view of bedrock incising gullies. Bottom Left-Slope map of bedrock incising gullies. Bottom Right-5m contour map of bedrock incising gullies within the Gabilan Mesa.

The Paso Robles Formation is preserved in the most northern parts of our study areas as a thin stratigraphic cap on top of the Pancho Rico For-mation as illustrated in the top image of Figure 9. By mapping out its extent (Figure 4) and extracting the elvations from these locations, we can model a surface that represents the uneroded geometry of this formation. We model the surface as an inclined plane. By assuming that the surface represents a stratigraphic unit that was horizontal at the time of deposition we can use the model of the tilted surface as a proxy for relative uplift rate. We expect that erosion rates would increase to match the gradient in uplift rate recorded by the tilting of this surface. Similarly we compute the depth of incision beneath this modelled Paso Robles surface as another proxy for erosion rate.

There is no consistent correlation between relative erosion rate and the con-centration of gullies in the Pancho Rico Formation.

While the concentration of gullies increases when moving from low to moderate relative uplift rates, this trend does not continue through the highest fraction of uplift rates where instead we see fewer gullies. Moreover, there are no significant variations in gully density across the observed variations in incision depth. However, some areas in the high uplift/erosion regions were heavily vegetated, impeding our mapping of gullies.

Gully formation is often associated with severe land degradation, posing local and global environmental problems. Here we assess the role that un-derlying rock type and erosion rates play in determining the concentration of bedrock-incising gullies in the Gabilan Mesa, in the central Coast Ranges of California. Specifically, we propose that the concentration of gullies is greater within mudstones than within neighboring sandstones and conglomerates. Mudstones of the Pancho Rico Formation break down during wetting and drying, through a process known as slaking, which we propose encourages the headward erosion of gullies. We mapped the con-centration of bedrock-incising gullies over a 700 km2 area using 0.3 -m -resolution satellite data, or 1 -m -resolution elevation data. We quantified the density of gullies by calculating the total length of gullies in a given area, using both a 3 km diameter moving window and the area of 77 manu-ally selected drainage basins. Measures of gully density within drainage basins show increased concentrations in basins that are predominantly un-derlain by the mud-rich Pancho Rico Formation. This is highlighted by maps of gully density measured with a moving window, which show abrupt increases in gully density across Pancho Rico Formation lithologic con-tacts. We expand on these results by investigating controls on gully density within a particular rock type. While direct measurements of erosion rates are unavailable, we use the depth of incision into the initially horizontal (now tilted) Paso Robles Formation surface that characterizes the mesa top of this region as a proxy for relative erosion rate. While the concentration of gullies increases when moving from low to moderate relative erosion rate, this trend does not persist through the highest fraction of uplift and erosion rates. We speculate this may arise due to the difficulty of mapping gullies in satellite imagery from the highest relative erosion rate regions, which tend to be more heavily vegetated. These findings indicate that varia-tions in underlying lithology and perhaps erosional history can trigger variations in erosional processes.

Abstract

Gabilan Mesa at a Glance

Figure 1. South facing gullies actively cutting through hillslopes in the Gabilan Mesa. Photo courtesy of Juan C. Fernandez-Diaz.

How are gullies distributed?

Figure 2. Gully formation as a result from headward erosion.

What do these gullies look like?

Gully density is greater within basins that are pre-dominantly underlain by mudstones than within basins underlain by sand-stones and conglomerates.

How Are Gullies Formed?

Headward erosion

Elevation

Figure 5. Gully density throughout the Gabilan Mesa. Density of gullies quantified by calculating the total length of gullies in a 3 km diameter moving window. Here the length of a mapped gully is calculated as the distance around the bounding escarpment (see Figure 7), hence this quantity is not directly analogous to drainage den-sity as it contains some information about variability in the aspect ratio of gullies. Also shown is the extent of the 77 manually selected drainage basins.

5 kilometers

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5 kilometersUSGS FaultMapped GulliesPaso RoblesPancho Rico

Figure 6. Local lithology overlying Gabilan Mesa. Geologic map provided by the United States Geologic Survey and California Geologic Survey, Saucedo et al., 2000.

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Pancho Rico Normalized Surface Deflection 0-25% 25-50% 50-%75 >75%

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Figure 4. Hillshade showing showing region over which gullies were mapped in the Gabilan Mesa. Elevation data from USGS NED.

Mesa SurfaceUSGS FaultMapped Gullies

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Does Rock Type Matter?

Figure 3. Left-Mudstones before drying and wetting cycle. Right-The same samples after one drying and wetting cycle.

Mudstones often fracture due to swelling of hydrated clays in a process known as slaking. This makes these rocks vulnerable to erosion, and perhaps the formation of bedrock incising gullies.

Assessing How Rock Type and Relative Erosion Rate Control the Concentration of Bedrock Incising Gullies; Gabilan Mesa, Central Coast Range, California

Miguel Frias (1,2), Sam Johnstone(3), George Hilley (3)

1Summer Undergraduate Research in Geoscience and Engineering 2University of Wisconsin - Madison, Department of Geological Engineering

3Stanford University, Department of Environmental and Geological Sciences

Acknowledgements and ReferencesSpecial thanks to the Summer Undergraudate Research in Geoscience and Engineering Program at Stanford University.

Saucedo, George J., Bedford, David R., Raines, Gary L., Miller, Robert J., and Wentworth, Carl M., GIS Data for the Geologic Map of California, California Geological Survey, CD 2000-07.

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10 Source: NCALM SEED Grant to S. Johnstone

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping,Aerogrid, IGN, IGP, swisstopo, and the GIS User Community

Bedrock floored gullies are actively retreating into soil-mantled hillslopes.

Bedrock is rarely observed outside of gullies.

Mixed Basins Pancho Rico Fm.(mudstone)

Paso Robles Fm.(sandstone & conglomerate)

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