Stratigraphic, Structural, and Correlation Considerations 43 



typical of recent soils while being reworked by an advancing sea. Krum- 

 bein ^^ in his excellent article enumerates a number of criteria useful in 

 recognizing ancient soil horizons. 



A concentration of iron oxides may be indicative of old soil profiles. 

 In the Midcontinent, thin beds of red shale in normal sections of the 

 Pennsylvanian are considered to be ancient soil profiles. During the for- 

 mation of lateritic soils, there is a strong concentration of iron, which may 

 show up as a tough, porous "duricrust." Zones of concretions may be 

 found in fossil soil horizons. Caliche, for example, is often concretionary. 

 It has been suggested that variegated shales might be the end products of 

 modifications in long-buried soils. Basal black shales may be formed when 

 soils with a high humus content are incorporated into the first formation 

 of a marine transgressive overlap. 



Certain residual deposits should also be considered under this head- 

 ing. The weathering of limestones with the selective solution of the cal- 

 cium carbonate may leave a residual soil rich in silica, phosphate, or clay. 

 Soils formed from the weathering of cherty limestones might be recognized 

 by masses of broken and cemented chert fragments. Concentrations of clay 

 or calcium phosphates at the top or within a limestone section are good 

 signposts of unconformity. 



Porous Limestone: In areas of limestone bedrock, a considerable por- 

 osity may be developed by the solvent effect of ground water. Solution ac- 

 tion may develop porosity ranging from microvugular to cavernous, de- 

 pending on such factors as precipitation, rate of erosion, and length of 

 exposure to erosion. Although other agents and processes may be respon- 

 sible for limestone porosity, solution channels, caverns, and a decrease in 

 porosity in depth without change in lithology are good evidences of ground- 

 water action and hence of unconformity. Further, the porosity not only is 

 a criterion for unconformity but may also be the locus for commercial oil 

 accumulation. 



Angular Coal Fragments: An interesting proof of a long erosional 

 break in Pennsylvania sedimentation in eastern Kansas has been ad- 

 vanced by John L. Rich.^^ He found angular coal fragments in the lower 

 few feet of a channel sandstone in the base of the Lawrence shale near 

 Ottawa, Kansas. Many fragments have square ends indicating jointing 

 before burial. The coal must have advanced at least to the lignite stage. 

 The source of the coal fragments must be from underlying Weston shale, 

 also Pennsylvanian, which shows marked crumpling below the uncon- 

 formity with the Lawrence shale. If coal of Weston age had become 

 lignite before its burial in Lawrence shale, the unconformity between 

 these two must represent a long time interval. 



Silicified Shell Fragments: Weathering of some Paleozoic lime- 

 stones with subsequent development of residual clay has been observed 



^"Op. cit. 



^^ Rich, J. L., Angular Coal Fragments as Evidence of a Long Time Break in Pennsylvanian Sedimen- 

 tation in Eastern Kansas : Geol. Soc. America BuU., vol. 44, no. 4, pp. 865-870, Aug. 31, 1933, 



