88 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 6.19. Map showing coincidence of 

 Permian volcanic trough (stippled margins) 

 and zone of Sierran intrustives (lines). Dots 

 indicate location of Carboniferous, Perm- 

 ian, and Triassic areas referred to the 

 text. Pennsylvanian and Permian basins 

 combined isopached. Zone of Sierran in- 

 trusives includes nearly all satellites and 

 palingenetic areas. 



panied the volcanism and resulted in contemporaneus erosion and sub- 

 marine slumping of slightly compacted fine lime mud. A part of the 

 volcanic material, at least, must have been erupted from central vol- 

 canoes, which were built up above the surface of the ocean and were 

 thus subjected to erosion. 



Although recognizing unsolved elements in the problem of the origin 

 of the graywackes, conglomerates, and limy argillaceous beds, Budding- 

 ton visualizes a sedimentary environment as follows: the great lens- 

 shaped beds of conglomerate may be local deposits made by torrential 

 streams, and the graywacke may be in part the more finely comminuted 

 peripheral marine equivalent. The calcareous shale and argillaceous limy 

 beds which are locally intercalcated with the clean, thick-bedded lime- 

 stone may be in part the more distant offshore equivalent of the con- 

 glomerate and graywacke. 



The limestone is in part dense white on fresh surfaces, and massive 

 with only rare, if any, evidence of stratification. Beds as thick as 2000 feet 

 have been observed. In part it is interbedded with thin-layered limestone, 

 nodular and shaly limestone, calcareous shaly argillite, dense platy si- 

 liceous layers, green-gray shale, and sparse buff-weathering sandstone. 

 The massive limestone seems to be due to rapid deposition, and where 

 clean the site of accumulation was sufficiently distant from land so not to 

 have received any clastic material. Volcanic activity has been thought of 

 as contributing to the deposition of the limestone, through the activity of 

 magmatic waters or meteoric waters draining from a volcanic terrane or 

 by the warming of the marine water, but the chemistry and oceanography 

 of the problem have not been worked out. 



Schofield ( 1941 ) discussed the problem of granitoid pebbles and 

 cobbles in the conglomerates of several periods, especially the Triassic. 

 Buddington refers to them also. In one locality, the Britannia map area 

 of British Columbia, an arkose is described as composed of irregular 

 grains of quartz, plagioclase, orthoclase, and sericite schist. The lack of 

 rounding of the grains, the freshness of the plagioclase, and the consider- 

 able thickness of the unstratified beds, prove that the material accumu- 

 lated rapidly and was transported only a short distance from a source of 

 granitoid plutonic rocks. Buddington failed to trace the granitoid elastics 



