The coal-bearing rocks of Arkansas were separated by the Geological Survey of Arkansas into three divisions, as follows: " Coal-bearing rocks in Arkansas, according to the Geological Survey of Arkansas. Feet. Western and upper division, including Huntington and other coal beds.. 3,000± 500 250 These divisions were not clearly defined, and the rocks below the coal-bearing formations were not discussed. The correlation of these groups into which the coal measures of Arkansas and Indian Territory have been subdivided by various surveys is set forth in the following table: Stratigraphic relations in the Arkansas-Indian Territory coal field as exhibited in reports already published and in the present report. In the course of the present investigation the formations mapped in the Indian Territory coal field were traced from the ArkansasIndian Territory boundary eastward into Arkansas with a considerable degree of success. The Hartshorne sandstone had already been located on the boundary line at three localities, Bonanza, Hackett, and the south slope of Poteau Mountain, from which points its outcrop was traced without a single break in continuity eastward along the southern margin for 75 miles to the eastern end of the field. Its a Ann. Rept. Geol. Survey Arkansas for 1888, vol. 3, pp. 10-11. C c Taff, J. A., op. cit., Pl. XXXVII. continuity along the north side of the field is broken at a number of places and its recognition here depended on stratigraphic evidences other than continuous exposures. Nevertheless, the formation has been identified with a considerable degree of certainty in several disconnected areas within the field, along its northern border, and its character over the whole field is fairly well known. The McAlester shale formation extends from Indian Territory into Arkansas, where it comprises the whole of the coal-bearing series. Some of the lower beds of the Savanna formation are probably also present in Arkansas, though it has not been possible to identify them with certainty at any point. The McAlester formation as represented in Arkansas contains a group of sandstone beds near its middle portion. These were traced over a large part of the field and their stratigraphic position determined. On the geologic map herewith presented this group of sandstones is differentiated from the more shaly parts of the McAlester formation, subdividing it into three members called Spadra shale, Fort Smith formation, and Paris shale, which can be recognized throughout the field. The economic value of such a subdivision will not fail to be appreciated, since the boundaries of the formations mapped will serve as contour lines indicating approximately the depth of the Hartshorne coal horizon. The rocks that underlie the Hartshorne formation in Arkansas have received little attention. They comprise a succession of shale and sandstone known to exceed 5,000 feet in thickness near the southern edge, but they are probably not more than 2,000 feet thick near the northern boundary of the field. In the southern part this group consists mainly of shale, but along the northern boundary the sandstone beds are much more important, making up possibly one-third of the section. The subdivisions of the coal-bearing rocks of Arkansas, as differentiated and described in this report, are briefly tabulated as follows: The general relations and lithologic features of these subdivisions are set forth in Pl. I. ATOKA FORMATION. The rocks belonging to this formation outcrop in the surrounding regions both north and south of the coal field and also occur in a number of limited areas included in it, the best known of which is that in the Backbone Ridge between Hackett and Bonanza. They have also been penetrated at a number of places by deep drill holes through the overlying rocks. South of the coal field the rocks of the Atoka formation are tilted at high angles, and extensive sections of them can be measured, but on the north their attitude is more commonly horizontal and only the upper members are exposed. In the latter region these rocks correspond in stratigraphic position with the Winslow formation as mapped by Purdue in the Winslow quadrangle. In general the Atoka formation consists of shales with occasional sandstone beds, which in the northern part make up possibly as much as one-third of the section, while farther south the proportion of sandstone is very much less. The thickness of these rocks north FIG. 3.-Graphic sections of the upper part of the Atoka formation. A, north of Van Buren; B, Massard Prairie; C, Jenny Lind; D, south of Huntington; E, Denning; F, Dardanelle. of the coal field has been estimated to be approximately 1,500 feet, but near Mansfield, in the southern part, over 3,000 feet are exposed and 2,000 more have been pierced by deep drilling. The shales vary from fine-grained, black or blue clay stones to gray argillaceous sandstones. Generally the more sandy beds are harder, and where they outcrop form ridges and other prominent topographic features. Although the rocks of this formation have not been investigated in close detail, the observations made and the records of deep wells drilled through them show that the sandstone members are lenticular bodies, having no general distribution over the field. In many places where they have been examined the sandstones are very irregularly bedded, and in some places finger out into shale beds. A characteristic exposure of one of these beds is shown in Pl. IV, A. |