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Correlation Between Sections of Ellenburger Group
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growth. The sand occurs in laterally dis continuous zones in the carbonate rocks. The amount of sand in the zones varies from widely scattered grains to very abundant. But rarely does the sand be come abundant enough to form sandstone. Some zones are gradational at the top and bottom, while others show an abrupt disap pearance of sand from the section at either the top or bottom of a zone.
Accessory Materials
Accessory materials are those that oc cur sporadically, or more or less continu ously in small amounts, in the Ellenburger residues. These materials are both mineral and organic in nature. Brief mention of each follows.
Pyrite is the commonest accessory. It occurs as interstitial filling and as indi vidual crystals. The crystals range from microscopic to megascopic in size. The pyrite is often associated with waxy shale partings in the rock, occurring as minute crystals embedded in the shale. Some of the mineral called pyrite may actually be marcasite, as distinction between the minerals in small occurrences is difficult.
Mica, commonly muscovite, more rarely biotite, occurs as very small, thin flakes. The flakes occur either free or embedded in the shale or any of the aggregates found in the residues.
Feldspar appears rarely in the residues as minute, light-colored crystals, probably sedimentary in origin. Feldspar also oc curs as detrital grains at certain places in the Ellenburger.
Limonite is more common in residues from surface samples than from well sam ples. Commonly it appears to be replace ments of pyrite.
Anhydrite appears erratically in sub surface sections of the Ellenburger, usually in a clear, crystalline form. The common occurrence of anhydrite in drill ing muds makes it difficult to determine small amounts in place in well samples.
Glauconite occurs in the residues usually as small grains, either free or em bedded in shale or silty aggregates. It oc curs also as interstitial filling and is dolo moldic when thus occurring in dolomite. Glauconite is more common in the lower portion of the Ellenburger than elsewhere.
Millerite, in typical needle-like crystal clusters, occurs very rarely.
Fossil fragments are too rare and non diagnostic to offer faunal evidence with correlative significance. The commonest fragments are sponge spicules, either free or embedded in chert. Both simple, needle like forms and six-pointed, hexactinellid types are found. Rarely small gastropods and fragments of brachiopods in siliceous replacements are found. As noted above, a few conodonts have been obtained from acetic acid residues of limestone samples.
Beekite is a form of chalcedonic chert associated with fossil replacement. The chert forms a series of rough, button shaped bodies made up of roughly con centric rings of chert, partly or wholly covering the fossil surface. The material is frequently observed on the outcrop, and fragments of the beekite bodies appear sporadically in the residues.
Other insoluble minerals and materials than those mentioned above undoubtedly occur in trace amounts in the Ellenburger residues. An exhaustive study and identi fication of all the more rare constituents of the residues has not been attempted by the writer. Goldich and Parmelee (1947) have published detailed analyses of sam ples from portions of Ellenburger surface sections.
References Cited
Brainkrd, A. E., Baldwin, H. L., Jr., and Keyte, I. A. (1933) Pre-Pennsylvanian stratigraphy of Front Range in Colorado: Bull. Amer. Assoc. Petr. Geol., vol. 17, pp. 375-396.
Cloud, P. E., Jr., and Barnes, V. E. (1948) The Ellenburger group of central Texas: Univ. Texas Pub. 4621, June 1, 1946, 473 pp.
•, and Bridge, Josiah (1945) Stratig raphy of the Ellenburger group in central Texas —a progress report: Univ. Texas Pub. 4301, Jan. 1, 1943, pp. 133-161.
Cole, C. T. (1942) Subsurface study of Ellen burger formation in west Texas: Bull. Amer. Assoc. Petr. Geol., vol. 26, pp. 1398-1409.
Crowley, A. J. (1946) Subdivisions of the Ellen burger limestone in the southwestern part of the X.M.A. oil field, north-central Texas: Univ. Texas Pub. 4301, Jan. 1, 1943, pp. 191 195.
—, and Hendricks, Leo (1945) Lower Ordovician and Upper Cambrian subsurface subdivision in north-central Texas: Bull. Amer. Assoc. Petr. Geol., vol. 29, pp. 413-425.
Dake, C. L., and Bridge, Josiah (1932) Faunal correlation of the Ellenburger limestone of Texas: Bull. Geol. Soc. Amer., vol. 43, pp. 725-741.
