Ellenburger Group Research Articles

STRATIGRAPHY OF THE UPPER CAMBRIAN, LLANO UPLIFT, TEXAS

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.

Geology of North-Central Texas

Because of the possible economic importance of the arched Ellenburger group, the identification of its subdivisions by insoluble residues and general character of well cuttings has been attempted with encouraging results. To facilitate correlation of the Pennsylvanian and lower Permian strata, classification by series is proposed, based on unconformities and faunal changes of wide regional importance. Rearrangement of certain formations and groups is also advocated. Former stratigraphic names are retained where possible even though redefined or changed in rank. Cross sections and maps reveal the Concho arch as an imposing northwest trending structural feature, denuded in the Llano uplift area. Uplift and erosion of Ordovician beds along this axis evidently began during pre-Mississippian time. Beds from Barnett to Strawn in age show marked thinning and considerable truncation as this regional feature is approached. Attention is also given to the evidence of intermittent growth of the Ouachita-Marathon Mountains, the Electra and Muenster arches. The Bend flexure and other large structural features of this region are described. Regional conditions affecting oil and gas migration and accumulation are discussed. These include the progressive development of structural trends and local folds, overlap of structurally high reservoirs by apparent source beds, differential pressures resulting from wedge-shaped overburden, distribution of sedimentary material, and the development of local sandstone or limestone reservoirs and stratigraphic traps. A compaction-hydraulic theory of oil migration is favored as the main cause of movement of oil into and laterally within the reservoir toward areas of less overburden and pressure.