The Relationship Between Geology and Soil Development in Austin, Texas Rob Forkner

Map courtesy of Sue Hvorka, BEG

Mt. Bonnell Fault. View from the top of Mt. Bonnell

Outcrop of the Glen Rose Limestone at Mt. Bonnell

Skimpy soil at Mt. Bonnell

Outcrops of the Buda Limestone at Shoal Creek

Soil heave and its effect on a sidewalk at Shoal Creek

VS. InceptisolVertisol Images from USDA

Shrink/Swell factor in Austin Soils Map from STATSGO, TX statewide Soil data. Shrink/Swell in soil is a function of seasonality (wet/dry) and amount of illitic clay within the soil matrix.

Geology vs. Soils…. Relationship???

How can we use spatial analysis to determine a more quantitative relationship between soils and bedrock? 1)Rasterize data so that spatial analyst can work with it. 2)Reclassify data into groups of similar erodability. 3)Assign weights to erodability groups: higher weight = more erodable 4) Add raster layers to determine combined erodability. Method Developed by Dr. Mark Helper, UT DOGS

Rasterized geological map with geological units grouped into formations. Reclassified geological map with formations classified into erodability groups. More resistant rocks have lower erodability. Limestone=1, Marl=2 Alluvium=3

Slope calculation applied to Austin West DEM Slopes on Austin West DEM reclassified into 4 groups with 20 degree increments. Steeper slopes are “colder” deg=1; deg=2, etc.

Annual precip for Austin, Map from NRCS/USDA Brown area gets inches more precip. per year. For erosion weighting, Blue area = 1, brown area = 2

The Combination… Geology- Limestone = 1; Marl = 2; Alluvium = 3 Slope deg. = 1; deg. = 2; deg. = deg. = 4 Precipitation- Most of the map = 1 (appx. 31”); with a small area in the southeast being 2 (appx. 33”)

Calculation of “erodability” as a function of bedrock, precipitation, and slope. darker areas are more readily erodable. In general, rock on the upthrown side of the fault is less erodable than the rock on the downthrown side. VIOLA! Sum of Raster Data

What’s Next? Add a erodability parameter for vegetation cover and soil Be more quantifiable with parameters for rock