The Following Pages Refer Chiefly to What Is Known As the Coal Region of Arkansas

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 2, PP. 225-242, PL. 8 FEBRUARY 26, 1891 THE GEOTECTONIC AND PHYSIOGRAPHIC GEOLOGY OF WESTERN ARKANSAS.* BY ARTHUR WINSLOW. (Read before the Society August 19, 1890.) CONTENTS. Page. Introduction_____________________________________________________________ 225 The Area discussed_________________ ____._____________________________ 225 Historical Sketch____________________________________________________ 226 Inception of the present W o rk __________________________________________226 Age of the Rocks of W estern Arkansas_____________________________________ 227 The Geotectonic Geology__________________________________________________ 228 Character and Distribution of the Flexures_____________________________ 228 Dates of Elevation and Folding____________ ___________________________ 231 Cause of Elevation and Folding _____ ________________________ 231 The Physiographic Geology______________________________________________ 234 The general Surface__________________________________________________ 234 M ountains___________________________________________________________ 235 Ridges and Mesas__________________ __________________________________ 235 Valleys_____________________________________________________________ 238 General Character_________________________________________________ 238 Monoclinal V alleys_______________________________________________ 238 Anticlinal Valleys________________________________________________ 238 Synclinal V alleys____________________________ ____________________ 239 Valleys in horizontal Strata------------------------- --------------------------------- 239 The Prairies of Western Arkansas_________________________________ ___ 240 The River System ______________________________ _______________________ 241 I ntroduction . The Area discussed.—The following pages refer chiefly to what is known as the coal region of Arkansas. This is in an area situated in the western part of the state, tributary to the Arkansas river. It extends from the Bos ton mountains on the north to beyond the Poteau and Petit Jean mountains on the south, thus covering an area about 100 miles long in an east-and-west direction by 50 miles broad in a north-and-south direction. * Published with the approval of Dr. John C. Branner, State Geologist of Arkansas. XXXIV—Bun. Geol. Soo. Am., Vol. 2, 1890. (2 2 5 ) 226 A. WINSLOW— GEOLOGY OF WESTERN ARKANSAS. Historical Sketeh.—The geotectonic geology of western Arkansas received little or no attention prior to the inauguration of the geological survey of the state now in progress. Shumard, in the report of Marcy’s Red river expedi tion, made in 1854, makes no reference to the subject. In a paper, published in 1854, by Professor J. A. Warder, of Cincinnati, on a Geological Recon- noissance of the Arkansas River, an attempt is made to locate axes of flexure; and some of the conclusions have been confirmed by recent work of the state survey. Marcou touches upon the subject in the report of the Thirty-fifth Parallel survey. One of the most noticeable points in Marcou’s interpreta tion of the geology of western Arkansas is the separation of the contorted slates about Little Rock, which he terms “ metamorphic slates,” from the overlying sandstones near that place, which he classes as Carboniferous, and which latter, he states, lie horizontally and unconformably upon the former. Beyond this to the westward, all of the various sandstones and shales ex posed in the Ozark mountains and the Petit Jean mountains, as well as in the bluffs at Ozark and Van Buren, are included by him in the Carbonif erous. This distinction between “ metamorphic slates ” and Carboniferous shales and sandstones led to some confusion in fixing the age of the uplifts. The horizontal position of the sandstones at Little Rock seemed to Mar cou to preclude the idea that the force necessary for the metamorphosis and upheaval of the underlying “ slates ” could have been exerted posterior to the deposition of the sandstones, while at other localities rocks recognized as Carboniferous had evidently been subjected to such upthrow. He thus adopted an hypothesis of two distinct periods of disturbance, one anterior and the other posterior to the formation of the Carboniferous rocks. Thus the hills at Little Rock, at Hot Springs and at Sulphur Springs are stated to be dislocations anterior to the Carboniferous. In the reports of the first geological survey of Arkansas, which was prose cuted between the years 1857 and 1860, no attempt is made to delineate geologic structure, and the positions of the anticlines and synclines is referred to only in the most disconnected and incidental manner along with the gen eral descriptions of counties. Failure to consider these features led to most faulty conclusions in the attempt to correlate the rocks of this region with each other, as well as with those of other states. Inception of the present Work.—When, therefore, in the autumn of the year 1887,1 entered the coal region of Arkansas, preparatory to its study, it was with very vague anticipations; yet these anticipations were not suffi ciently vague to preclude surprise, and, as my journey proceeded from point to point, surprise succeeded surprise. The thickness of the rock column which developed itself exceeded all expectations, as did also the number of the beds constituting this column. Instead of horizontal or gently dipping strata such as I expected to find, steep dips seemed to predominate, and the RELATION BETWEEN STRUCTURE AND CONFIGURATION. 2 2 7 directions of these dips varied rapidly. While slowly meander- rr ing through the country on horseback, with only an occasional jf ! extended view from some peak, each topographic feature pre- ; * sented itself separately, and the differences in size, in outline, in profile and in trend, within comparatively small areas, added to the confusion of conception. It soon became evident that the problems which this country presented could be solved only through detailed work in which delineation of the topography _ must play an important part. Thus we came to the construc- j tion of the detailed topographic maps which are now being I engraved, and which will appear in a forthcoming report of , report of the Arkansas geological survey. t The small topographic map accompanying this paper, plate 1 8, is condensed from these detailed maps. It expresses only faintly, however, much that is shown to be of geological signifi cance on the large sheets. As the construction of these sheets progressed, order began to develop itself; the relations of dis connected parts gradually became apparent; until now, as completed, the maps themselves stand as the solutions of vari ous problems for the solving of which they were originally intended to be only one of the mean. To some of the results of this work I now wish to call attention. A g e o f t h e R o c k s o f W e s t e r n A r k a n s a s . From the time of Owen’s work, preceding the year 1860, the ! belief has been prevalent that the coal beds and associated rocks “ of western Arkansas are of sub-conglomerate or lower Carbon- | iferous age. Without analyzing the reasons for this conclusion 5 here, it suffices to say that our present knowledge of the flora of —from Generalized Cross-Section Poteau Mountains to Fayetteville. these coal beds, as well as the results of independent strati- graphic studies, lead to the conclusion that the coal beds of the F ig u r e 1 Arkansas river and the associated rocks belong well up in the Carboniferous system. The complete display of the evidence leading to this conclusion is reserved for a future publication, ' since this question is not properly an essential part of this paper; but in general terms the line of argument is as follows : In northwestern Arkansas, in the vicinity of Fayetteville, A r chimedes limestones and other rocks of probable lower Carbon Scale, horizontal and vertical, 12 miles = inch.1 Looking westward. Showing structure and thickness of the rocks exposed. iferous age are well developed. Thence southward, across the Boston mountains, several cross-sections have been constructed with the results of proving the existence of barren strata over 1,000 feet in thickness intervening between these limestones and u. 228 A. WINSLOW----GEOLOGY OF WESTERN ARKANSAS. the coals of the valley of Arkansas river. The results of one of these sec tions are embodied in the generalized section, figure 1, and the total thickness of the formations exposed within the area described is there approximately shown. Further, a study, by Mr. G. D. Harris, of the flora of certain coal beds on the Arkansas river, in the vicinity of Russellville, led to the con clusion that they represent horizons above the conglomerate. On strati graphie grounds these latter beds are placed many hundreds of feet beneath the uppermost beds represented farther west in the state : hence we have reason for assigning to these latter beds a position well up in the Coal Measures. T h e G e o t e c t o n ic G e o l o g y . Character and Distribution of the Flexures.—In illustration of the char acter of the flexures of this area, figures 1, 2, 3, 4, 5 and 6 are presented. Figure 1 is a generalized cross-section through the Boston mountains and Scale, horizontal and vertical, 1.6 miles = 1 inch. Looking westward. Showing the steeper dips northward. the Arkansas river valley. It illustrates the diminution in intensity of flex ing from south to north. This section is constructed along a line

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