100 University of California Publications in Geological Sciences 



rock types — such as andesite pebble conglomerate, coarse andesitic sandstone, and 

 diatoniite — occur throughout. Owing to poor exposures in much of the area and 

 to the numerous faults which bring different members — yet similar rocks — into 

 juxtaposition, consistent separation of the units was found difficult. 



Lower member. — Wherever the lower member is well exposed resting on the old 

 regolith, it consists typically of poorly indurated, dark conglomerate and coarse 

 sandstone made up largely of andesitic debris. On the north side of the basin the 

 well-rounded clasts are reworked Alta andesite, but quartzite, gneiss, and 

 granodiorite pebbles occur occasionally. The clasts are of variable size, depending 

 chiefly on position with respect to the bordering valleys from which streams were 

 carrying sediment into the basin. In general, they range from small to large 

 cobbles, but boulders were noted locally. Coarse andesitic sandstones are associated 

 with these conglomerates, and locally thin diatomaceous lenses a few inches to a 

 foot or two thick occur in the section. 



Dark-colored conglomerate of the lower member is well exposed on the flanks of 

 the Carson Range south of the Truckee River, where it rests unconformably on 

 Kate Peak andesite. Commencing in the area about 2 miles east of Fleish, the 

 conglomerate is represented chiefly by small cobbles, but they rapidly increase in 

 size to eastward. In sections 26 and 27 large cobbles and boulders up to a foot long 

 are abundant. The clasts are largely Kate Peak andesite, but a few pebbles of 

 Alta andesite as well as metamorphic and granitic clasts can be found in any large 

 sample. The section alternates with coarse, gray to brown or (locally) yellow- 

 brown thick sandstones, and thin tuffaceous sandy shales that are largely masked 

 by the cobble-littered slopes. The conglomerate appears to have been deposited by 

 comparatively large streams; locally the cobbles are somewhat flattened and dis- 

 play a crude imbricate structure. To eastward, in the valley of Hunter Creek, the 

 conglomerate rapidly thins and grades laterally into finer lighter-colored sedi- 

 ments of the upper member which there rest on a thick regolith that is developed on 

 Kate Peak andesite. 



Sedimentary breccias are associated with the lower member in two areas. Along 

 the irrigation canal adjacent to the Truckee River south of Lawton, a number of 

 breccia beds are intercalated with thick, coarse sandstone and conglomerates. The 

 most conspicuous breccia is 150 feet thick, but others below it range from 3 to 4 

 feet up to 10 or 12 feet thick. The breccias are monolithologic, consisting of angular 

 blocks of Kate Peak lava, some of which are 6 to 8 feet long, embedded in a coarse 

 matrix of Kate Peak debris. These breccias do not crop out in the hills to south- 

 ward, probably because they represent mudflows that were deposited in the low- 

 lands away from the bordering hills. 



The thickest breccia section occurs north of the Truckee River from the mouth 

 of Bull Ranch Creek westward into California, where it is terminated by a major 

 north-south fault just off the map. The clasts are angular to subangular, and range 

 in size from 1 or 2 feet up to block 5 feet long, all of which are in a poorly bedded, 

 poorly indurated matrix of Alta andesite debris. In the lower part of the section 

 on the Dry Creek road just north of Crystal Peak cemetery, blocks of granodiorite 

 are also in the basal part of the section, which here rests unconformably on Kate 

 Peak mudflow-breccia. The breccia has a maximum thickness of approximately 300 



