Brentwood Limestone of Madison County, Arkansas

Journal of the Arkansas Academy of Science Volume 7 Article 19 1955 Brentwood Limestone of Madison County, Arkansas Vance O. Cook University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/jaas Part of the Geology Commons, and the Sedimentology Commons Recommended Citation Cook, Vance O. (1955) "Brentwood Limestone of Madison County, Arkansas," Journal of the Arkansas Academy of Science: Vol. 7 , Article 19. Available at: http://scholarworks.uark.edu/jaas/vol7/iss1/19 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact , . Journal of the Arkansas Academy of Science, Vol. 7 [1955], Art. 19 " THE BRENTWOOD LIMESTONE OF MADISON COUNTY, ARKANSAS VANCE 0. COOK University of Arkansas INTRODUCTION The Brentwood limestone, the lower limestone member of the Bloyd formation, is of lower Pennsylvanian age, and a part of the Morrow Group. It is exposed in a wide belt across South Central Madison County, trending generally in an eastwest direction (Figure 1). Most of the outcrops are found on small hills on the northern edge of the Boston Mountains. The area included approximately 160 square miles. GEOGRAPHY The topography of the area varies from moderately rough to extremely rugged. Most of the hills are the outlying regions of the Boston Mountains, and are not as high as the Boston Mountains. The maximum relief in southern Madison County ranges from 800 to 900 feet. BLOYD FORMATION The Bloyd formation was named by A. H. Purdue (2) from Bloyd Mountain in (Table I) conWashington County, where it is well exposed. The Bloyd formation " sists of the Brentwood limestone member, the middle shale or coal-bearing shale," which in some areas contains the Baldwin coal, the Kessler limestone, and the upper shale member. However, throughout Madison County, all of the beds above the Brentwood member are missing. Brentwood limestone is the most persistent member of the Bloyd formation in Madison County. It occurs on the mountains on the north side of the Drakes Creek Fault in a belt trending generally east-west, and extending from the Washington County line to the vicinity of Kingston in eastern Madison County. Inthe vicinity of Kingston the Brentwood belt disappears and probably is absent to the east and north. Thickness . The Brentwood member is characterized by a great variation in thickness. Sections measured within relatively small areas show great changes in thickness. One section near Aurora in Sect. 11, T. 15N. , R. 26W. , consists of 93 feet of Brentwood displayed in a massive bluff overlooking the highway. Only four miles to the northeast another section consists of only five feet of sandy limestone. In general, the thickness of the Brentwood member varies from 20 to 30 feet. Lithology. The Brentwood consists of one, two, or three layers of calcitic limestone, interstratif ied with beds ofshale. The limestone beds vary from litho- graphic to coarse crystalline. A thin section study of some of the Brentwood shows an interesting grain size relationship. The limestone consists of three different sizes of grain. The larger grains are fossil fragments, measuring about 0.85 millimeters. Between the larger grains, occurring as interstitial material, numerous oolites measuring about 0.25 millimeters are found. These two types of grains are cemented by a very fine grained calcite. The crystals measure at most 0.067 millimeters. The smallest of these cementing grains was 0.0071 millimeters, and the average size was 0.026 millimeters. In places the Brentwood is oolitic. Some of the oolites are elongated and appear to have formed around a nucleus of bryozoan branches. These oolites are microscopic in size. The limestone is predominantly blue-gray, but the color is often variable. In some places it is light red, while in others it is light gray. Sedimentation. The beds of the Brentwood were deposited during Bloyd time in a synclinal basin. Apparently they were deposited on a sea floor that was moving alternately up and down, as evidenced by the thin beds of alternating limestones and shales. The lower shale was formed by the deposition of much mud on a slowly subsiding sea floor, after which the sea became free from muddy sediNOTE: Research Paper No. 1108, Journal Series, University of Arkansas. Published by Arkansas Academy of Science, 1955 64 64 • Journal of the Arkansas Academy of Science, Vol. 7 [1955], Art. 19 BRENTWOOD LIMESTONE OF MADISON COUNTY 65 limestone, most of merits and elements of marine life were deposited to form the the limestone seems to have as its origin the piling up of remains of the marine organisms. Ripple marks and cross bedding found at various places suggest that the deposition of some of the Brentwood beds occurred in shallow water. An angular unconformity above the Bloyd indicates that the land was uplifted Bloyd time and the surface was eroded before the deposition of the at the end of Atoka formation. During this erosion, the upper part of the formation was removed in many places, leaving only the Brentwood. In places the entire formation was removed, or never was deposited, and the Atoka formation was deposited on the Hale formation. Before deposition of the Atoka, the Bloyd beds were tilted, forming a conspicuous angular unconformity. The general direction of thinning of the Brentwood in Madison County is to the northeast. Near the Madison-Newton County line it is very thin and is probably absent over a large area east of this line. This suggests two possibilities: (1) That the Brentwood first was deposited and later removed by erosion in this area, or (2) that the Brentwood never was deposited in that area, which may represent an ancient land mass present during the time of deposition of the Brentwood. Some field evidence has been found that supports the second possibility to a certain extent. In the area south and east around Huntsville, very near where the Brentwood seems to disappear, many sandy phases of the limestone are found, including pure sandstone channels in the limestone. These suggest the possibility that an ancient shoreline is represented here, with the land mass lying to the north and east of Huntsville. Weather ing. Normally the Brentwood weathers to a smooth, rounded surface. Two exceptions to this weathering were observed during field work. In the first, the lower bed of limestone, which is sandy, has weathered to a distinct honeycombed patte (...truncated)