Axelrod: The Pliocene Verdi Flora of Western Nevada 105 



POST-COAL VALLEY ROCKS 



The rocks that accumulate in the region following deposition of the Coal Valley 

 formation comprise terrace deposits and basalt. Several terraces are in the Truckee 

 River valley, and some of them lie 800 feet above the present river. The higher 

 ones are covered with Kate Peak debris on the south side of the basin, but Alta 

 andesite on the north. The deposits range from 10 or 15 feet up to 30 or 40 feet 

 thick. These older terrace deposits can be traced onto the bordering slopes of 

 Peavine Mountain and the Carson Range north and south of the river, where they 

 merge into old erosion surfaces. The material making up the low terrace deposits 

 near the Truckee River differs from that on the high terraces in one important 

 respect: granitic and metamorphic clasts occur in abundance only on the low 

 terraces. This clearly indicates that the exposure of wide areas of basement rock 

 in the nearby region to westward is a comparatively recent event. Since many of 

 the contacts between the present broad exposure of the basement and younger 

 rocks are faults, the inference is that they are quite recent, probably very late 

 Pleistocene. 



Dark-gray to black basalt that weathers reddish-brown, and on fresh fracture 

 reveals a dull vitreous surface, is present on the high terrace and bordering erosion 

 surface south of the Truckee River in section 21. V. P. Gianella has suggested that 

 it may be equivalent to the Lousetown andesite-basalt that covers a wide area in the 

 Carson Range to southward. Similar rocks appear to have been mapped by 

 Lindgren (1897) over a broad region in the Truckee quadrangle to the southwest. 

 At several localities in the area north and northwest of the basalt bluff near the 

 flora, basalt has intruded and baked the Coal Valley diatomite brick red (Gianella, 

 1948). It may be a part of the same magma that contributed to the flows on the 

 high terrace south of the river ; in any event, it is of post-Coal Valley age. 



Structure 



The regional relations of the Coal Valley-Kate Peak unconformity have an im- 

 portant bearing on interpreting the structure of the Verdi basin. In the area to the 

 east, near Virginia City, the Coal Valley formation is interbedded with the higher 

 parts of the Kate Peak andesite (Gianella, 1930; Thompson, 1956 ). 1 In the Verdi 

 basin the Coal Valley rests with angular discordance on the volcanics ; although the 

 lower parts of the formation locally contain a few thin Kate Peak tuffs, they appear 

 to represent the last phases of that volcanic episode. In the region near Boca, 10 

 miles southeast, Daley and Poole (1949) found an angular discordance of 15° to 

 20° between the Kate Peak andesite and the Coal Valley sediments. It is apparent 

 that in proceeding eastward from the tectonically active Sierra Nevada, the 

 angular unconformity between the Coal Valley and Kate Peak andesite disappears 

 in the conformable section. 



The unconformity apparently was owing both to folding and faulting. Dis- 

 cordance between the Coal Valley formation and the underlying volcanics shows 

 that warping followed Kate Peak vulcanism. Furthermore, since younger sedi- 

 ments lap onto the Kate Peak and Alta andesites in proceeding farther east, it is 



1 These sediments were mapped as the Truckee formation by Thompson. 



