The Relationship Between Geology and Soil

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The Relationship Between Geology and Soil
Development in Austin, Texas
Rob Forkner
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Map courtesy of Sue Hvorka, BEG
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Mt. Bonnell Fault. View from the top of Mt.
Bonnell
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Outcrop of the Glen Rose Limestone at Mt. Bonnell
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Skimpy soil at Mt. Bonnell
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Outcrops of the Buda Limestone at Shoal Creek
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Soil heave and its effect on a sidewalk at Shoal
Creek
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VS.
Inceptisol
Vertisol
Images from USDA
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Shrink/Swell factor in Austin Soils
Shrink/Swell in soil is a function of
seasonality (wet/dry) and amount of illitic clay
within the soil matrix.
Map from STATSGO, TX statewide Soil data.
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Geology vs. Soils. Relationship???
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How can we use spatial analysis to determine a
more quantitative relationship between soils and
bedrock?

• Rasterize data so that spatial analyst can work
  with it.
• Reclassify data into groups of similar
  erodability.
• 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
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Rasterized geological map with geological units
grouped into formations.
Reclassified geological map with formations
classified into erodability groups. More
resistant rocks have lower erodability.
Limestone1, Marl2 Alluvium3
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Slopes on Austin West DEM reclassified into 4
groups with 20 degree increments. Steeper slopes
are colder. 1-20 deg1 21-40 deg2, etc.
Slope calculation applied to Austin West DEM
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Annual precip for Austin, 2001. Map from
NRCS/USDA Brown area gets 1-1.5 inches more
precip. per year. For erosion weighting, Blue
area 1, brown area 2
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The Combination
Geology- Limestone 1 Marl 2 Alluvium
3 Slope- 0-20 deg. 1 21-40 deg. 2 41-60
deg. 3 60-80 deg.
4 Precipitation- Most of the map 1 (appx.
31) with a small area in the southeast being
2 (appx. 33)
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VIOLA! Sum of Raster Data
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.
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Whats Next?

• Add a erodability parameter for vegetation cover
  and soil
• Be more quantifiable with parameters for rock