Axelrod: The Pliocene Verdi Flora of Western Nevada 103 



markings typical of floodplain deposition. The conglomerates are in general lighter 

 colored than those lower in the section. The clasts are similar, but their small size 

 and the more abundant matrix of sand gives the rock an over-all lighter color. The 

 volcanic pebbles are generally small, though small cobbles are present locally. 



Topographic Setting 



The Coal Valley formation was deposited in a structural low, which apparently 

 was brought into existence by downwarping of the Carson block, and by renewed 

 faulting on the front of the Peavine block following erosion of the Kate Peak 

 andesite there (see "Structure" ; also fig. 2) . The bordering scarp on the north side 

 of the basin appears to have been low to judge from the few exposures available 

 for study. The bulk of the formation accumulated on a floodplain within the basin, 

 but owing to continued subsidence the successive members gradually overlapped 

 the marginal fault and bordering low slopes. 



Higher hills appear to have stood to the northwest and southeast of the plant 

 locality only during the early phases of deposition. On the north side of the basin 

 coarse elastics are found near Bull Ranch Creek, where a 300-foot breccia is 

 buttressed against Alta andesite along a fault that appears to mark the old scarp. 

 The occurrence of granodiorite blocks in the section suggests that they probably 

 were derived from a region of moderate relief directly north of the scarp, several 

 miles northwest of the plant locality. On the south side of the basin higher hills 

 were present in the area southwest of Hunter Creek, for the coarsest conglomerates 

 occur there. The deposits derived from these higher areas are, however, at strati- 

 graphic levels well below the Verdi flora. By the time the flora was living, border- 

 ing hills were much lower, to judge the finer nature of the marginal deposits. Thus 

 there is no reason to suppose that high mountains were in the immediate area dur- 

 ing deposition of the sediments containing the Verdi flora ; the hills flanking the 

 basin may have been only a few hundred feet above the Verdi floodplain. Since the 

 extravasation of Kate Peak andesite just prior to the Coal Valley deposition was 

 chiefly by autobrecciation from fissures, rather than by eruptions (Curtis, 1954), 

 high volcanic mountains probably were not in the immediate region. Nonetheless, 

 the presence of occasional andesite lapilli tuff in the Coal Valley section suggests 

 that a few low cones may have dotted the distant hills. 



The existence of coaly beds in the section, the presence of diatomaceous lenses 

 at most stratigraphic levels, and the occurrence of shaly beds filled with aquatic 

 plants, clearly indicate ponded water bodies in the lowlands. Most of these beds 

 are thin and generally discontinuous, suggesting more or less short-lived, small 

 lakes, ponds, and swamps. Since they occur all across the basin, and at nearly all 

 stratigraphic levels, generally low relief on the floodplain is suggested. Deposition 

 in these sites was regularly brought to a close by flooding and filling up of these 

 small basins by coarse sands and gravels deposited chiefly at times of flood, as 

 judged from conspicuous cross-bedding, scour and fill, and other structures in the 

 sediments. Finally, it is recalled that the upper, flow-jointed basalt, which occurs 

 below the flora stratigraphically, has a comparatively wide distribution in the area. 

 This also suggests a subdued lowland, for it is doubtful if such a thin flow would 

 spread very widely if moderate relief were present. 



