72 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



not strictly comparable with the approximately contemporaneous Eureka- 

 Carlin and Antler sequences. 



The basal sediments of the overlap assemblage differ in age throughout 

 north-central Nevada. In the Eureka area the intertonguing Chainman shale 

 and Diamond Peak formation of Late Mississippian age are the earliest orogenic 

 sediments recognized. In the Carlin area the Tonka formation of Dott (1955, 

 pp. 2222-33) and correlative units farther southeast in Pine Valley mapped by 

 J. Fred Smith and Keith Ketner included Lower Mississippian clastic beds 

 that overlap the upper plate of the Roberts Mountains thrust fault, indicating 

 that the thrust reached the Carlin area during Late Devonian or Early 

 Mississippian time. 



Orogenic movements continued along the belt in Pennsylvanian and 

 Permian time, and also throughout the Mesozoic. Examine the structure 

 cross sections of Chapter 17, Figs. 17.3-17.6. 



Walter Sadlick and F. E. Schaeffer (personal communication) recog- 

 nize an angular unconformity at the base of the Chainman formation in 

 western Utah and are calling the disturbance represented by it the 

 Wendover phase of the Antler orogeny. They are of the opinion that 

 this time (early Valmeyer of the early Mississippian) marks the begin- 

 ning of the Antler orogeny. They recognize beveled folds covered by the 

 Chainman, and the axes of the folds trend to the northwest. 



Klamath Mountains and Sierra Nevada 



The Mississippian is made up of two formations in the Klamath Moun- 

 tains, the Bragdon and the Baird (Fig. 6.13). They are probably the most 

 widespread formations in the region. The Bragdon is chiefly shale and 

 slate, generally gray, in contrast to the black shale and slate of the older 

 Kennett formation of Devonian age. Some sandstones are conglomeratic 

 near the base and contain fragments of both the Kennett and Copley 

 formations. Within the Redding quadrangle, a volcanic sequence called 

 the Bass Mountain basalt is present. The Bragdon may exceed 6000 feet 

 in thickness in places. The Bass Mountain volcanic sequence contains 

 many tuff beds. Its position on Bass Mountain, according to Hinds ( 1939), 

 is in the lower part of the Bragdon formation. 



The Baird formation consists largely of sandstone and tuff, but the 

 upper part has calcareous and siliceous slates. It is about 700 feet thick 

 and apparently rests conformably on the Bragdon (Hinds, 1939). 



In the northern Sierra Nevada, the metamorphic Calaveras formation 

 of Carboniferous age is widespread. It consists chiefly of black phyllite 

 with subordinate fine-grained quartzite, limestone, and chert. Associated 

 and in part interbedded with the formation are green amphibolite schists 

 of contemporaneous age. From fossils, found chiefly in the limestone, the 

 Calaveras formation is known to be at least in part of Carboniferous 

 age, but parts of it as mapped may be Devonian and Triassic. Because 

 of the metamorphosed condition of the rocks in which the fossils are 

 found, it has been difficult for paleontologists to determine to what part 

 of the Carboniferous the faunas belong. Groups of Calaveras fossils from 

 the Taylorsville region are more closely related to the Baird, now recog- 

 nized as Mississippian, than to the McCloud limestone, now believed to 

 be Permian. 



The amphibolite schists were originally fine pyroclastics (Knopf, 1929). 

 The bedded rocks are most abundant in the northern Sierra Nevada, 

 but southward become increasingly metamorphosed, and progressively 

 greater areas are occupied by granitic intrusives. In the Tehachapi Moun- 

 tains and the southern Coast Ranges, pre-granitic rocks are present, but 

 highly altered. 



A thick sedimentary deposit, now schist, in southern California, has 

 yielded Mississippian fossils ( Larsen, 1948 ) . The sequence appears to be 

 miogeosynclinal in type and at the same time seemingly out of place in 

 the geosynclinal setting. 



Pennsylvanian Basins 



Of the miogeosyncline the Oquirrh basin is the most striking feature 

 of Pennsylvanian and Permian time. It appears to have been a sharp and 

 small basin in which over 15,000 feet of strata accumulated. The thickest 

 section is in the Provo part of the Wasatch Mountains of central Utah 

 where Baker ( 1947) reports 26,000 feet of beds. The upper 9800 feet is of 

 Permian age. A short distance to the southeast 20,000 feet of beds have 

 been estimated in the Mt. Nebo district (Eardley, 1934), and in the 

 range to the west, the Stansbury, 15,000 feet (Rigby, 1958). The basin 

 has been contoured with a northwest trend and an abrupt northeast 

 margin (Stokes and Heylmun, 1958). This permits the interpretation 



