The two formations and eight members that constitute the Upper Cambrian in the Llano uplift of central Texas are described or redefined, and their lithic characters in 19 measured sections are graphically summarized. Standard reference to them is thus furnished.
The Riley formation comprises the basal Paleozoic strata of the Llano uplift. Its initial sediments were deposited on a submerged pre-Cambrian terrane having a known topographic relief as great as 800 feet. Its thickness normally averages about 680 feet but ranges from probably less than 200 to about 800 feet. At most places it is subequally divisible between the Hickory sandstone member below and the Cap Mountain limestone member above, with the thin but widespread, glauconitic Lion Mountain sandstone member capping and completing the sequence.
The Wilberns formation includes five named members between the Riley formation and rocks of the Lower Ordovician Ellenburger group. It normally averages about 580 feet thick and ranges from 540 to 610 feet thick, but in the southeastern corner of the Llano uplift truncation of the upper beds has reduced it to 360 feet. The thin but widespread, nonglauconitic Welge sandstone member introduces the sequence. Above it is the Morgan Creek limestone member, grading to the succeeding argillaceous beds of the Point Peak shale. At the top of the sequence are the San Saba limestone and Pedernales dolomite members. These two are essentially equivalent and gradational facies, with the Pedernales normally overlying the San Saba.
