PALEOZOIC CORDILLERAN GEOSYNCLINE 



87 



NORTHWEST SECTION 



NORTHEAST SECTION 







GRINDSTONE- TWELVEMILE CREEKS AREA, CENTRAL OREGON (MERR1AM AND BERTHIAUME) 



Cc, Coffee Creek fm. (Lower Carb.) Cs, Spotted Ridge fm. (Penn.) Pc, Coyote BuTte fm. (Perm.) 



I MILE 



^^K^-mmMM^WMSm, 



Tv 



MlBl 



cm$m*m 



NORTH-SOUTH SECTION NEAR BAKER, OREGON (GILLULY) 



brs, Burnt River schist; Ce, Elkhorn Ridge argillite (Penn. ?) Ccg, Clover Creek greenstone (Perm.) 

 qd, biotite-quartz dionte; sg, silicified gabbro; mg, metagcbbro; gb, gabbro 



, 5 Ml LELS 



Fig. 6.18. Cross sections in central and eastern Oregon. 



The Silurian graywackes in general of southeastern Alaska are com- 

 posed of particles of rock similar to the kinds that form the pebbles and 

 cobbles in the conglomerates with which they are interbedded, and in 

 addition, of a considerable percentage of plagioclase, potassic feldspar, 

 and quartz grains. The conglomerates are largely made of andesite 

 pebbles and boulders, but slate, diorite, rhyolite, and limestone pebbles 

 are abundant, if not dominant, in some conglomerates. One specimen of 

 graywacke of Devonian or Silurian age, for example, consisted of particles 

 of andesite, felsite, plagioclase, granophyre, quartz, spherulitic rhyolite, 

 and orthoclase, with a chloritic and slightly calcareous groundmass. 



The association of the graywackes and conglomerates that Buddington 

 describes is very revealing of their origin. The conglomerates in them- 

 I selves are indicative of a volcanic archipelago and deserve further men- 

 tion. The following is a reume of the Silurian conglomerates according to 

 Buddington. Varieties of conglomerates are as follows: 



1. A conglomerate composed almost wholly of well-rounded andesite or an- 

 desite porphyry cobbles and boulders; the matrix may be calcareous, and 

 lenses of limestones are intercalated but limestone cobbles are sparse. 



2. A conglomerate composed almost wholly of limestone cobbles or boulders 

 in a limestone or andesitic tufflike matrix; this type is rare, but beds 100 

 feet thick have been noted. 



3. Peculiar conglomerates intermediate between 1 and 2, consisting of pebbles 

 and cobbles of andesite and limestone in a greenish tufflike matrix. 



4. A homogeneous-appearing rock composed of fragments of andesite in a 

 matrix of the same material; the structure is that of a conglomerate or 

 water-wom breccia. 



The limestone fragments are usually of a dense-textured limestone 

 typical of the Silurian, and many carry fossils of Silurian age. The fossils 

 are the same as from the overlying limestone. It is, therefore, believed 

 that the limestone conglomerates are intraformational and that the lime- 

 stone fragments are of practically the same age as the volcanic fragments. 

 Vertical movements of the sea bottom, perhaps local, must have accom- 



