40 



Subsurface Geologic Methods 



electric logs. A single criterion suggesting unconformity may only com- 

 plicate the problem, as almost all such criteria can be explained by fault- 

 ing or some sedimentary process or variation. The more associated criteria 

 than can be established for a given horizon, the greater the probability 



TABLE 2 

 Criteria for Recognition of Unconformities 



Criterion 



Associated with unconformities 

 of indicated origin 



Submarine 



Basal conglomerate - 



Basal black shale 



Desert varnish 



Residual (weathered) chert 



Silicified erosion surface 



Phosphatized erosion surface 



Caliche 



Duricrust 



Porous zones in limestone 



Asphaltic and oil-stained zones 



Buried soil profiles 



Lag gravels (pebble bands) ... 



Glauconite zones 



Iron-oxide zones 



Interbedded conglomerate 



Clastic zones in nonclastics 



Abrupt change in heavy-mineral assemblages. 



Radioactive mineral zones 



Porous zones in general 



Sharp differences in lithology 



Abrupt change in chemical composition 



X-ray patterns in vyrell cuttings 



Concretionary and pisolitic zones 



Abrupt change in fauna 



Gaps in evolutionary development 



Algal biscuits 



Bone and tooth conglomerates 



Undulatory surface of contact 



Edgewise conglomerate 



Phosphatic pellets or nodules 



Manganiferous zones 



Pyritiferous zones 



Corrosion surfaces 



Borings of littoral marine organisms 



Lateral spreading of coral reefs 



XX 

 XX 



X 

 XX 

 XX 



X 



? 



XX 

 X 



? 



XX 

 X 

 XX 

 XX 

 XX 

 XX 

 X 

 XX 

 XX 

 XX 

 XX 

 XX 

 XX 

 XX 



of its being a surface of unconformity. Pyrite crystals by themselves 

 may suggest unconformity, but they might also be a normal dissemination 

 through a conformable series. If they are associated with glauconite and 

 phosphatic pellets, then a marine unconformity is probably present. 



Krumbein ^^ has tabulated criteria cited by various workers for recog- 

 nition of unconformities. Pettijohn ^^ relisted Krumbein's criteria with 



^^ Krumbein, W. C, Criteria for the Subsurf'Sce Recognition of Unconformities: 

 troleum Geologists Bull., vol. 26, no. 1, pp. 36-62, Jan. 1942. 



''Pettijohn, F. J., Sedimentary Rocks, p. 147, New York, Harper & Brothers, 1949. 



Assoc. Pe- 



