74 Subsurface Geologic Methods 



The thickness of conglomerates and breccias ranges from inches to 

 thousands of feet. Extreme variations may occur in relatively short dis- 

 tances horizontally. These rocks may grade in all directions into time 

 equivalent though lithologically dissimilar strata. They frequently rep- 

 resent the basal phase of many formations and thus serve as criteria for 

 recognizing unconformable relationships. Intraformational, coarse elas- 

 tics have also been observed in many major lithologic units. 



A systematic analysis of the coarse elastics should include such studies 

 as (1) type of pebbles, (2) ratio of pebble types, (3) degree of sorting, 

 (4) fabric patterns, (5) alteration of pebbles, (6) type and distribution 

 of bindents, (7) thickness trends and variations, (8) shape of pebbles 

 (roundness, sphericity, flatness), and (9) relationship to adjacent de- 

 posits. The results of these investigations should be recorded whenever 

 possible in graphic form (histograms, percentage curves, composition 

 triangles, and fabric diagrams) . 



Sandstones 



Sands and sandstones represent the medium-grained clastic sediments 

 and are composed of grains of various rocks and minerals ranging from 

 1/16 to 2 millimeters in diameter. 



Lithologists frequently assume sandstones to be composed primarily 

 of quartz. This concept should be discouraged, as detailed examinations 

 reveal the composition of sandstones to be extremely diverse and complex 

 as exemplified by Pettijohn:^ 



Rough inspection of 50 thin sections of sandstone chosen at random from 

 the University of Chicago collection shows than 45 percent are graywackes 

 and subgraywackes, 35 percent orthoquartzites, and 20 percent are arkoses. 



A graywacke, according to Pettijohn, is composed of large very 

 angular grains, mainly quartz, feldspar, and rock fragments (chiefly chert, 

 phyllite, and slate). The grains are set in a prominent-to-predominant 

 "clay" matrix which was, on low-grade metamorphism, converted to a mix- 

 ture of chlorite and sericite and partially replaced by carbonate. 



An orthoquartzite is a sedimentary quartzite developed as a result 

 of excessive silicification without the impress of metamorphism. 



An arkose or arkosic sandstone, according to the Committee on Sedi- 

 mentation, contains 25 percent or more of feldspar derived from the dis- 

 integration of acid igneous rock of granitoid texture. 



The terms "graywacke" and "arkose" have recently received con- 

 siderable attention in studies of the relationship between tectonism and 

 sedimentation. Arkoses are considered to be typically developed in in- 

 tracratonic basins, whereas graywackes accumulate dominantly within 

 geosynclinal downwarps. The contrasting features of these two rock types 

 are given by Pettijohn:'* 



' Pettijohn, F. J., Sedimentary Rocks, p. 229, New York, Harper and Bros. 1949. 

 ' Pettijohn, F. J., op. cit., p. 261. 



