TIGER DIRT

http://www.ces.ncsu.edu/plymouth/septic2/Session%207/7-3b-Leab-Tiger%20Dirt%20b.pdf
“Tiger Dirt” What is it and what does it mean?
Prepared by: Roger Leab, Natural Resource Conservation Service (NRCS) Soil Scientist
April, 2009

Definition: Tiger Dirt: A general term applied to soil horizons that have patterned mottling.

Patterned Mottling: The most typical is reticulate or netlike mottling. The horizontal component of the mottling is usually the more pronounced, giving a striped appearance (Hence “Tiger Dirt”) that has a strong horizontal component, However, the Vertical Component can be Stronger.

What Causes It? It is caused by extended saturation and is usually the result of a perched water table, but it can be an apparent water table.

Where Does It Occur? It most often occurs in soils developed in sedimentary deposits, but occasionally occurs in residual soils.

What Type Sedimentary Deposits? Old colluvial/alluvial capping on high parts of the landscape; old stream terraces; marine, fluvial-marine, and eolian deposits of the Sandhills; marine deposits of the Upper Coastal-plain.

In What Type Residual Soils Does it Occur? Some examples are: Cataula series (thermic); Helena series (thermic); Vance series (thermic); Hard Labor series (thermic); Casville series mesic); and Halifax series (mesic).

“Tiger Dirt” Development: Mottling is generally much more strongly expressed in the soils formed in sediments than in the soils formed in residuum.

“Tiger Dirt” in Old Colluvial/Alluvial Capping. Some examples are: Hiwassee series (thermic); Mattaponi series (thermic); Appomattox series (mesic); Yadkin series (mesic).

"Tiger Dirt” in Old Stream Terrace. Some examples: Masada series (thermic); Dorian series (thermic); Danripple series (mesic); Banister series (mesic)

Marine, fluvial-marine, and eolian deposits of the Sandhills: Ailey series and Fuquay series (Platy Structure).

An example of Marine Deposits of Upper Coastal-plains is the Dothan series.

A good example of mottling in Residual Soils is the Cataula series (thermic).

Why Does the Mottling Pattern Develop? Water stands in the larger pores between soil peds and frequently in old root channels above a layer that does not have the large pores and acts as an aquatard causing the formation of redoximorphic features.

Residual Aquatard: The aquatard is generally a thin layer with platy structure just above the C horizon, or it is a C horizon where most or all of the old rock fractures are filled and sealed with clay.

Sedimentary Aquatard: The aquatard layer is generally a 2C horizon in the sedimentary deposits or occasionally a 2B horizon.

What is a 2C Horizon? It is a layer of different source material than that of the soil horizon above it.

  • Example 1—the soil formed in old alluvium, but the 2C horizon material underneath it is residuum.
  • Example 2—the soil formed in fluvialmarine sediments, but the 2C or 2B horizon is a much older paleosol (a very old soil) which has been truncated and buried under the sediments in which the new soil formed.

An example of “Tiger Dirt” is located at Leesburg, in Wake County, North Caro, near the home of John Kelly who assisted in profiling an alluvial capping of the Appomattox series in which the living roots and old root channels are all in the gray,iron depleted zones.
The redox mottling extends down to the contact with the underlying residuum (over 200 inches).

What Does It Take To Form Redox Features?

  • Organic matter for microbes to feed on, in the saturated zone.
  • Continuous saturation for 15 or more days such that available oxygen is depleted in that zone by the feeding microbes.

So! What Does “Tiger Dirt” Mean for Interpretations? That is a tricky one!

Interpretations: In many cases it means there is a water table (perched or apparent) during significantly wet periods.
In other cases the redox features are relict and pose no present engineering limitation

What is the Answer for Onsite Construction Use? I don’t think we can be sure in many cases without a piezometer or water well study.

  • If the water table was perched and the aquatard is still present and not too deep, why would we think the wetness condition no longer exists?
  • If the water table was apparent and the hydrology of the area has changed such that the present water table is much lower or no long exist, then we

should assume the redox features to be relict.

End

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