Correlation Between Sections of Ellenburger Group
in making samples and residues available for study made the project possible.
The work of investigation has been car ried on under the auspices of the Bureau of Economic Geology of The University of Texas, and the writer wishes to acknowl edge the great value of the assistance and encouragement received from the Director of the Bureau.
Acquisition and Study of Residues from Surface Sections
Ihe insoluble residue characteristics of each of the Ellenburger formations pre sented here are revealed in samples from measured sections in each of the areas mapped in detail by Cloud and Barnes (PI. I). The samples are from six com posite sections, representing the total meas ureable thickness for each of the six areas, and from one incomplete section. The sec tions are designated as follows:
1. Gorman Falls —Tanyard 2. Cherokee Creek 3. Warren Springs—Moore Hollow 4. Backbone Mountain 5. Johnson City 6. Threadgill Creek 7. Llano River
Sampling the sections was done in parl by Cloud, Barnes, and assistants and in part by the writer, assisted by L. E. War ren. All the sampling was based on meas urements by Cloud and Barnes. Prepara tion of all residues was under the supervi sion of the writer.
In sampling the Ellenburger, fresh chips were broken from beds at as short vertical intervals as exposures would allow. Chips from a 5-foot interval were combined to form one sample. Different types of rock occurring in a 5-foot interval were repre sented in their proper proportion so far as sight judgment could allow. Each sample was crushed in a small jaw crusher and a thorough mixing of fragments from each chip obtained. A uniform portion of each crushed sample, averaging about 30 grams of the material, was used in making resi dues. The fines were not screened from the crushed material, but the rock flour result ing from the crushing was removed by washing and decanting before adding the acid. Solution was carried out in 400-ml. pyrex beakers, using approximately 12 percent hydrochloric acid, or approxi mately 10 percent acetic acid. After comp
lete digestion of the soluble rock the clay-size particles were removed from the residue by washing and decanting. The re maining residue was dried on an electric hot plate and bottled.
This method of collecting and preparing surface samples for study was followed be cause experience in using residues from the Ellenburger as an aid in stratigraphic study has shown that the mass character istic of the residues is the most informa tive. Few, if any, residual materials are individually diagnostic of a formation. Hence in sampling for residue study, as much as possible of the vertical extent of the formation must be included. Trench
sampling would be ideal but is impractical. Spot sampling at intervals of a few feet will not catch enough of the mass char acteristics.
The residues were examined under a binocular microscope, using a magnifica tion of 15.6 diameters for the most part. Light source was a fluorescent lamp with a daylight-type bulb. The data considered were almost entirely qualitative, being quantitative only to the extent of sight comparison of quantities of materials.
Residues obtained from the Ellenburger by treating with hydrochloric acid are made up of four broad types of material— chert, non-clastic quartz, elastics, and ac cessory constituents. Each of these types occurs in a variety of forms and combina tions, thus producing the qualitative varia tions that are significant in stratigraphic interpretation. Chert is the most significant material, occurring in a wide variety of textures, structures, and degrees of trans parency. The distinction between chert and quartz is somewhat arbitrary, since all types of chert are varieties of the mineral quartz. In describing the residues the term quartz is reserved for material with the fracture, luster, and crystal habit of the mineral. By assigning symbols, either colored or drawn, to represent the various materials in residues, the succession found in a section can be presented on a log strip. Correlation between sections can best be accomplished by matching plotted logs.
Samples from limestone beds in the Ellenburger may be dissolved in acetic acid, in which the dolomite is insoluble. Samples from the limestone portions of several sections, both surface and well sec