Re: What is Saprolite?

I asked Phil Schoeneberger, Research Scientists at the NSSC to assist me with some of the background to the identification and definition of saprolite at used in the soil survey program and Soil Taxonomy. Below are Phil’s detailed definitions and descriptions. /s/ Ken Scheffe

Ken,

Here are some thoughts in response to John Galbraith’s Soil Tax Forum query regarding saprolite.

1) Definition: saprolite is defined in the National Soil Survey Handbook – Part 629 – Glossary of landform and geologic terms (NRCS, 2017). The current definition below is the same content as in NASIS Data Dictionary, and matches the content of the recently revised Glossary of Soil Science Terms, SSSA. It has been defined this way since 1993. It was derived from a slightly less detailed definition found in an SCS glossary (Hawley & Parsons, 1984). The term itself has been used since the early days of the US Soil Survey (e.g., used by Hugh Hammond Bennet in an early 1930’s document), etc.

saprolite - Soft, friable, isovolumetrically weathered bedrock that retains the fabric and structure of the parent rock (Colman and Dethier, 1986) and exhibiting extensive inter-crystal and intra-crystal weathering. In pedology, saprolite was formerly applied to any unconsolidated residual material underlying the soil and grading to hard bedrock below. Compare - grus, residuum. SW & HP NRCS, 2017: National Soil Survey Handbook – Part 629), current NASIS Data Dictionary, and Soil Science Society America, 2017

2) Related terms (graphics, saprock):
Saprolite doesn’t exist in natural systems by itself. It’s important to simultaneously consider the natural residual continuum of which saprolite can be a part. These concepts were presented in landmark work done by Pavitch et al. (Figs 2, 17, Table 1; 1986) and in Wald et al. (2011). It has been subsequently condensed and graphically modified as shown here (see attached .pptx file). Note the new term “saprock” that has extensive relevance to site index and ecosystem functions in the Sierra Nevada Mountains and elsewhere.

3) Parent Material - Kind: Saprolite is overtly defined and used in NASIS, Field Book, etc. as a Kind (type) of Parent Material. When the term is used elsewhere, it is assumed to mean the same thing. If different criteria, constraints, or other details are brought into play for a different context, a new, separate definition should be provided that identifies the new context and presents a different definition (e.g.: “saprolite [Soil Taxonomy] - … “). Without context qualifiers for alternate uses, competing definitions invariably causes confusion. This has been a chronic problem and can be readily minimized.

4) Saprolite origins: (highly weathered rock formed in acid / felsic crystalline rock vs. other rock types). There are two long standing traditions regarding the scope (rock origins) of saprolite. In the SE USA, it is commonly defined (restricted) in literature and field studies as being derived from acid (or felsic) crystalline rocks: igneous (e.g. granite) and metamorphic rock (e.g. phyllite). Other bedrock types are excluded and relegated to the generic phrase “highly weathered rock”. However, elsewhere the term is applied in a broader bedrock context that includes igneous, sedimentary, and metamorphic rocks:

saprolite – A soft, earthy, typically clay rich, thoroughly decomposed rock, formed in place by chemical weathering of igneous, sedimentary, and metamorphic rocks. It often forms a layer or cover as much as 100 m in thickness, esp. in humid and tropical or subtropical climates; the color is commonly some shade of red or brown, but it may be white or gray. Saprolite is characterized by preservation of structures that were present in the unweathered rock. The term was proposed by Becker (1895). Cf: geest; laterite. Also spelled sathrolith. (Glossary of Geology - Neudorf et al, 2005)

It seems both expedient and reasonable for NCSS to use the broader context (which is why no specific rock type is mentioned in the existing NRCS/ SSSA definition), which lends itself to broader, national applications (Wald et al., 2011).

5) Competing terms and uses (e.g. densic, pararock, etc.). With the advent of pararock (mid 1990’s), and densic qualifiers (materials, fragments, Soil Taxonomy criteria, etc.) and other applications were introduced. Confusion developed as to how these new ideas and uses related to saprolite. For example, saprolite straddles both C and Cr horizons, whereas the new terms don’t (e.g. densic). A draft “decision flowchart” to guide field identification of materials was made approx. 2001. NSSC discussions at the time resulted in the exclusion of saprolite from this chart, as it overlaps with several of the primary chart constituents (e.g. Cr). While informally circulated, to my knowledge the chart was not formally adopted. The chart has continued to change but remains informal (see attached draft decision flowchart: “Substrata Material Classification Decision Tree”). [I will point out that the flowchart contains significant errors and should not be used in its past or present forms (e.g. The use of “Cemented” as a high level criteria: weathered rock materials are rarely cemented (as understood in pedology & geology)]. At the time that the initial flowchart was developed, a simultaneous desire arose amongst some that advocated dropping saprolite from Official Series Descriptions (OSD’s) and from general use (replace it with one of more of the new terms). I will simply point out that there is a huge amount of work published on saprolite. Further, it is still widely used in OSD’s and in NASIS. I believe strongly that saprolite is an important, viable and necessary concept and term (see topics 2-4 above) of wide use, utility, and relevancy.

There is more to the overall issue, both specific details and general, related items. But this gives a summary and touches on the related dominoes.

Phil S.

References:
Colman, S.M. and Dethier, D.P. (ed.) 1986. Rates of chemical weathering of rocks and minerals. Academic Press, Inc., Orlando, FL.

Hawley, John, and Parsons, Roger. 1984. Glossary of selected soil science terms for western soil surveys. West National Technical Center, SCS, Portland, OR.

Neuendorf, K.K.E., Mehl, J.P., Jr., Jackson, J. 2005. Glossary of geology, 5th Ed. American Geological Institute, Alexandria, VA. ISBN 0-922152-76-4

NRCS. 2017. Glossary of landform and geologic terms. In: National Soil Survey handbook – Part 629, NRCS, Lincoln, NE (also on-line version).

Pavitch, M.J. Leo, G.W., Obermeier, S.F. and Estabrook, J.R. 1989. Investigations of the characteristics, origin, and residence time of the upland residual mantle of the piedmont of Fairfax County, Virginia. USGS Prof. Paper 1352. Reston, VA.

Soil Science Society of America (revised, accepted, 2017). Glossary of Soil Science Terms. SSSA, Madison, WI

Wald, J.A, Graham, R.C., and Schoeneberger, P.J. 2011. Distribution and properties of soft weathered bedrock at ≤ 1m depth in the contiguous United States. Publications from USDA-ARS / UNL Faculty. Paper 1131. http://digitalcommons.unl.edu/usdaarsfacpub/1131