446 abstracts: geology 



beds ranging in age from lower Carboniferous to Triassic. They have 

 been cut, deformed, and altered by a series of instrusives including the 

 highest of the laccolithic masses and the upward cutting stocklike and 

 dikelike masses which extend northeastward diagonally across the area 

 to the andesite flows which cover the sediments at the northeast. These 

 sediments rim around the instrusives and dip off from them to the 

 northwest, north, northeast, east, and southeast. 



The sedimentary rocks of the Park City district are separable into six 

 divisions. The lowest comprises quartzites and limestones, each mapped 

 separately. The quartzites are unfossiliferous but undoubtedly repre- 

 sent the Weber quartzite (of Pennsylvanian age). Some of the lime- 

 stones, however, are of Pennsylvanian age, and others have yielded 

 lower Mississippian (Madison) fossils. Overlying the quartzite is the 

 Park City formation, consisting of limestone and sandstone. The Park 

 City formation has not furnished very abundant or well-preserved fos- 

 sils, but the fauna obtained from it is characteristic and has provisionally 

 been referred to the Permian. Next above is the Woodside shale, un- 

 fossiliferous in the Park City district, but in Idaho it has yielded a fauna 

 which is quite different from that of the Park City formation and closely 

 related to that of the overlying Thaynes formation. The Thaynes has 

 been recognized in Idaho and Wyoming and includes the well-known 

 Meekoceras zone, the typical lower Triassic of North America. These 

 characteristic cephalopods have not yet been definitely recognized in 

 the Thaynes formation in this district, but Meekoceras types have been 

 found at the same horizon elsewhere. The next formation is the Ankareh 

 shale, whose fauna does not differ conspicuously from that of the Thaynes, 

 which lies below. Overlying the Ankareh shale is the basal part of the 

 Nugget sandstone, whose age has not been definitely determined, but is 

 either Jurassic or Triassic and which in the Park City district is repre- 

 sented by beds 515 feet thick. A few miles to the northwest of the 

 district the Nugget sandstone is thicker and is overlain by the Twin 

 Creek limestone of Jurassic age. 



The ores of the Park City district occur as lode deposits and as bedded 

 deposits in sedimentary and intrusive country rocks. The two types of 

 deposits are commonly associated thruout the region, tho some lodes 

 and veins occur alone.. 



The facts indicate that between early Triassic and early Tertiary time 

 dioritic intrusives invaded this area, metamorphosed the sediments, and 

 induced the deposition of rich lead-silver ores in certain members of the 

 calcareous formations; that after these intrusives had cooled to at least 



