200 



Subsurface Geologic Methods 



s is the surface area of a sphere of the same volume as the fragment, and S 

 is the actual area of the object. For a sphere the ratio is 1. For all other 

 solids the ratio has a value less than one") and roundness (a measure of 

 the angularity of the edges and corners) of detrital grains have served in 

 correlating certain strata. In the Rangely oil field of northwestern Colo- 

 rado and adjacent areas, the Entrada and Navajo (Jurassic) sandstones 

 are differentiated from adjacent lithic units by the rounded and frosted 

 character of the quartz grains. 



Rittenhouse ^- has used the degree of roundness of tourmaline and 

 zircon in correlating various strata in the Appalachian Basin. He states: 



In the Appalachian Basin roundness of heavy minerals is extremely 

 valuable as a criterion for differentiating various Mississippian and Pennsylva- 

 nian oil and gas sands, for outlining petrographic provinces, and for inter- 



FiGURE 83. Measurements of pebbles re- 

 quired in determining sphericity, round- 

 ness, and flatness values; a and b are 

 determined from maximum image 

 orientation; c value is normal to a 

 and b. 



preting geologic history. Roundness is particularly significant in the basin 

 because fossils are rare and the heavy-mineral suite is restricted. . . . 



Petti John comments :^^ 



The roundness of a clastic particle sums up its abrasion history. Spher- 

 icity, on the other hand, more largely reflects the conditions of deposition at 

 the moment of accumulation, though to a more limited extent sphericity is 

 modified by the abrasion processes. 



According to Fraser,^^ the absolute size of the grain, nonuniformity 

 in the size of the grain, the proportions of various sizes of grains, and the 

 shape of the grain control porosity of unconsolidated deposits. He fur- 

 ther states: 



Regularities in shape should result in a larger possible range in porosity, 

 as irregular forms may theoretically be packed either more tightly or more 

 loosely than spheres. The degree of rounding generally varies for different 



^" Rittenhouse, Gordon, Grain Roundness — A Valuable Geologic Tool: Am. Assoc, Petroleum Geolo- 

 gists Bull., vol. 30, no. 7, pp. 1192-97, July 1946. 



"2 Pettijohn, F. J., Sedimentary Rocks, p. 53, New York, Harper and Brothers, 1949. 



°* Fraser, H. J., Experimental Study of the Porosity and Permeability of Clastic Sediments: Jour. 

 Geology, vol. 43, no. 8, pp. 910-1010, 1935. 



