106 University of California Publications in Geological Sciences 



apparent that downwarping was greater in the western sector, nearer the Sierra 

 Nevada. It was also in this region that the marginal east-west faults appear to have 

 been more active during deposition, and there has apparently been renewed move- 

 ment on them. As previously noted, a coarse sedimentary breccia is buttressed 

 against an old scarp in the area from Bull Ranch Creek westward into California. 

 To northward, the fault is clearly overlapped by younger sediments. Movement 

 continued into the early part of Coal Valley deposition because subangular blocks 

 of Coal Valley basalt occur in the lower member at the mouth of Bull Ranch Creek, 

 and also at the northwest corner of the prominent river-bluff section. More recent 

 movement on this fault is obviously indicated, for it brings the upper and lower 

 members into juxtaposition. 



One of the puzzling features of the geology of the area is that Kate Peak andesite 

 is not exposed on the flanks of Peavine Mountain, yet the thick section in the 

 Carson Range dips northward toward it. One might argue that the Peavine block 

 was an elevated tract and that the Kate Peak did not accumulate there. This seems 

 unlikely, however, because Kate Peak andesite occurs north of the river at three 

 sites (map 1), and in each instance it is separated from Alta andesite by a major 

 fault. Furthermore, a thick Kate Peak section is exposed along the west margin of 

 Peavine Mountain in the valley of Dry Creek on the California-Nevada border. 

 These relations suggest that Kate Peak andesite probably covered at least parts 

 of the Peavine block, that during the late phases of Kate Peak eruption the Peavine 

 block was elevated by faulting, following which Kate Peak andesite was eroded 

 from the area. The fault presumably corresponds to the one directly north of Verdi 

 against which the 300-foot breccia rests, and on which renewed movement accounts 

 for the basalt-bearing breccia beds early in Coal Valley deposition. 



Aside from the major northwesterly trending fault that was active early in 

 deposition, there are two younger sets. As shown on the map, a northwesterly 

 trending set slices the basin into a prominent pattern. Locally, it is cut by a 

 northerly to northeasterly trending set that is more prominently developed to the 

 west, in California. 



Summary 



1. The Coal Valley formation, which contains the Verdi flora and mammalian 

 fossils, rests unconformably on Kate Peak andesite or on metavolcanics on the 

 south side of the basin, but on Alta andesite or granodiorite to northward. 



2. The structural setting of the basin appears to have been a downwarp bordered 

 by a low fault scarp on the north which was active in the early phases of deposition, 

 but was soon overlapped by the accumulating sediments (fig. 2). 



3. The section is approximately 3,000 feet thick and contains three mappable 

 units, all of which may rest on an old regolith on the margins of the basin: (1) a 

 lower, dark-colored conglomerate-breccia-sandstone member, (2) an augite olivine 

 basalt, and (3) an upper, light-colored, pebble conglomerate-sandstone-shale- 

 diatomite member. The flora is near the middle of the upper member, and mam- 

 malian fossils lie several hundred feet above and below it. 



4. The presence of hydrothermally altered Kate Peak andesite cobbles in the 

 Coal Valley formation indicates metasomatism occurred before deposition of the 



