Geologie map and cross section, eastern Ouachita Mountains, Arkansas: Map summary GEORGE W. VIELE Department of Geology, University of Missouri, Columbia, Missouri 65211 SCOPE The U.S. Geodynamics Committee has sponsored preparation and publication of geological sections across the nation's con- tinental margins. Most of the cross sections are at a scale of 1:250,000, without vertical exaggeration, and include the basic data from which they were constructed. The geologic strip map and cross section de- scribed here (Viele, 1979) extends from Mountain View, Arkansas, south to the Chevron No. 1 Cabe well in Nevada County, Arkansas (Fig. I).1 On the map and cross section the areal and vertical distribution of the rocks is portrayed through the use of conventional lithostratigraphic units. In addition, be- neath each tectonic province a stratigraphic column describes the formations, the rock types, and the inferred environments of deposition for that province. For the north- ern one-third of the section the stratigraphic descriptions include all strata above the Precambrian basement, but for the southern two-thirds a gap exists between the lower- most limits of stratigraphic control and the inferred level of the basement. These areas were left blank. The colors of the map and cross section so 1J0 KM (represented by patterns in Figs. 2 and 3 here) reflect a tectonic-stratigraphic classi- Figure 1. Index map of Arkansas showing major tectonic provinces and line of cross fication in which the sedimentary rocks are section. grouped according to the tectonic setting of the basin of deposition (Dickinson, 1974). The terminology is that of plate tectonics; along which the North American craton by use of aerial photographs, and some new the basins are classified by their relations to and a southern plate or microcontinent mapping was done. Thus, except for a few spreading centers, subduction zones, and were joined. Those parts of the cross section areas, the strip map is essentially an en- tectonic sutures. The central idea of the that are germane to this idea are presented largement of the state geological map of map and cross section (discussed more fully here (Figs. 2, 3). Arkansas (Haley and others, 1976). The ex- in the explanatory text) is that the Ouachita ceptions constitute a line of quadrangles folded belt records a late Paleozoic suture DATA BASE along the northern margin of the Benton uplift, for which more detailed mapping Published geological maps were the pri- and structural data are now available. 1 This article provides a summary discussion of mary source of information for the region (Viele, 1966, 1973; Karlo, 1973; Waugh, the text, geologic map, and cross section pub- north of the coastal plain. Within the past 1977; Zick, 1977). lished in color under the same title as MC-28F in the Geological Society of America Map and few years, as part of the preparation of a For the region of the Arkoma Basin and Chart Series. Contribution of Plate Margins new geological map of Arkansas, older for parts of the Ozark uplift and frontal belt Group, U.S. Geodynamics Committee. mapping was rechecked on the ground and of the Ouachitas, scattered wells provide Geological Society of America Bulletin, Part I, v. 90, p. 1096-1099, 3 figs., December 1979, Doc. no. S91204. 1096 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/12/1096/3444206/i0016-7606-90-12-1096.pdf by guest on 29 September 2021 EASTERN OUACHITA MOUNTAINS: MAP SUMMARY 1097 MAUMELLE ZONE BENTON UPLIFT MAZARN BASIN TRAP MOUNTAINS limited subsurface control. Depths to base- Figure 2. Part of cross section showing ment for these regions were taken largely Benton uplift and adjacent regions. Forma- from the Tectonic Map of North America tion descriptions are omitted. See Figure 3 (King, 1968) and from unpublished work stepping down along high-angle faults for key to patterns. Scale of 1:250,000 re- maps held by the Arkansas Geological (Buchanan and Johnson, 1968; Bush and duced here to 1:500,000. Ou = Ordovician Commission (W. M. Caplan, 1977, per- others, 1977), to depths of 4,000 m at the undivided; MDSO = Arkansas Novaculite, sonal commun.). The thicknesses of Paleo- front of the thrust belt. Still farther south, Missouri Mountain Shale, and Big Fork zoic units were taken from published re- the basement descends' to unknown depths Chert; Ms = Stanley Shale; Pj = Jackfork ports (Caplan, 1960). beneath the Benton uplift. Sandstone; IPal = Atoka Formation (lower The region of the Benton uplift and of the The overlying carbonate rocks, a plat- part); Ki = alkalic intrusive rocks. Paleozoic rocks to the south lacks subsur- form facies, thicken southward at least as face control. Published maps (Lyons, 1951; far as the frontal thrust belt and probably Howell and Lyons, 1959; Nicholas and Ro- beyond. Unconformably overlying the plat- quence contains exotic blocks of granite, zendal, 1975) of the Bouguer gravity field form carbonate rocks are Pennsylvanian chert, and carbonate rock and is entirely cover the area of the Ouachita Mountains (Morrowan to Atokan) clastic sedimentary Ordovician. The upper sequence is proba- in western Arkansas and eastern Okla- strata, deposited in environments ranging bly less than 600 m thick (Bush and others, homa, and recently completed gravity sur- from continental to shallow marine. These 1977), and it accumulated from Late Or- veys cover the eastern Ouachita Mountains strata thicken southward into the Arkoma dovician to Early Mississippian time, about (Hendricks, 1979). Basin, where south-dipping normal faults 120 m.y., implying a very slow rate of dep- The southern part of the cross section ex- offset them, as do younger, northward- osition. tends well into the area of the Gulf Coastal moving thrust faults. The clastic strata Within the Benton uplift the strata are ar- Plain; its representation is based on infor- grade into a thick sequence of deep-water ranged in stacks of multiply folded nappes mation from wells projected into the line of flysch overlain by a peripheral basin with penetrative fabrics. South of the Ben- section from locations farther west. In the molasse, although the facies changes are ton uplift and Mazarn Basin, the intensity area of the Chevron No. 1 Cabe, however, hidden in the subsurface. of the deformation decreases; the most in- the determination of subsurface geology is Still farther south, the section portrays tense zone of deformation lies along the based also on proprietary reflection profiles. the frontal thrust fault belt of the Ouachita northern margin of the uplift. The depth to basement in southern Arkan- Mountains. Here the rocks constitute a Another noteworthy feature of the north- sas is a matter of speculation, but gravity thick sequence of flysch deposited in a rem- ern margin of the uplift is a line of pods of and magnetic surveys suggest 6.4 to 8 km nant ocean by westward-flowing turbidity serpentine as much as 0.8 ha in area (Sterl- (Chevron Oil Company, 1976, personal currents. Also noteworthy within this zone ing and Stone, 1961). The largest of these commun.). are negative gravity anomalies west of the pods crops out just east of the line of strip line of section (Lyons, 1951; Nicholas and maps. The serpentinite pods lie within the DESCRIPTION OF CROSS SECTION Rozendal, 1975) and a belt of broken for- noses of recumbent folds in bedded cherts mation or near-melange, the Maumelle and black slates; drill holes go completely The northern end of the cross section on zone, in the Carboniferous flysch. through the pods and re-enter the sedimen- the Ozark uplift shows Precambrian base- Flanking the zone of broken formation tary sequence. ment dipping southward, at a relatively on the south is the Benton uplift, composed South of the Benton uplift, Carboniferous shallow depth of about 1 km, beneath a of Ordovician to Mississippian strata di- flysch, including a few rhyodacitic tuffs thin cover of lower to middle Paleozoic car- vided on the cross section into a lower se- (Niem, 1977), underlies the surface as far bonate rocks and Carboniferous clastic quence of black shale, micrite, and quartzite south as the Gulf Coastal Plain. In the strata. The southward-dipping Precambrian and an upper sequence of black shale, bed- southernmost exposures at Degray Dam, surface extends beneath the Arkoma Basin, ded chert, and novaculite. The lower se- the Carboniferous beds dip homoclinally Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/12/1096/3444206/i0016-7606-90-12-1096.pdf by guest on 29 September 2021 1098 G. W. VIELE GULF COASTAL PLAIN sw Pjv Pal PJv Pal TKJ Chevron No. 1 Cabe (TD A,276 m, 14,025') -5 km CARBONIFEROUS REMNANT OCEAN STRATA Flysch deposited in deep water between approaching plates. In part trench sediments, incor- porated into the subduction TRIASSIC RIFT VALLEY STRATA complex. South of the Benton Rift valley strata associated with uplift, the flysch lies in the Mesozoic rifting and spreading of position of a forearc basin the Gulf of Mexico LOWER AND MIDDLE PALEOZOIC SLOPE AND CARBONIFEROUS PLATFORM STRATA ¿00000 booooo000000. ABYSSAL STRATA Post-orogenic, shallow-water innnnnTOTO- Deep-water sedimentary rocks in- sediments deposited south of corporated into a subduction the Ouachitas after collision complex. Deposited in an early- and suturing middle Paleozoic spreading ocean. CARBONIFEROUS PERIPHERAL BASIN MOLASSE PRECAMBRIAN BASEMENT Shallow-water clastic rocks of Continental crust Arkoma Basin deposited mostly after collision and suturing. Lower formations grade southward to flysch Figure 3. Southern part of cross section. Formation descriptions are omitted. Scale of 1:250,000 reduced here to 1:500,000. Ms = Stanley Shale; IPj = Jackfork Sandstone; IPjv = Johns Valley Shale; IPal = Atoka Formation (lower part); IPa = Atoka Formation; IPa? = Atoka Formation(?); PIP = Permian to Desmoinesean strata; te = Eagle Mills Formation TKJ = Gulf Coastal Plain strata.