Late Mississippian Thrombolite Bioherms from the Pitkin Formation of Northern Arkansas

Late Mississippian thrombolite bioherms from the Pitkin Formation of northern Arkansas G. E. WEBB* Department of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019 ABSTRACT type I mounds was first suggested by Tehan shale intervals. Its maximum exposed thickness (1976), Warmath (1977a, 1977b), and Tehan reaches >120 m in Stone County, north-central Two types of bioherms occur in the Pitkin and Warmath (1977). Further contributions to Arkansas (Gordon, 1965), but it averages 60 m Formation (Chesterian) of northern Arkan- the understanding of these mounds were made farther east in its outcrop belt (Easton, 1942) sas. They are composed of a complex series by Pinkley (1983) and Manger and others and from 7 to 9 m in northeastern Oklahoma of ovoid thrombolite heads intergrown with (1984). (Huffman, 1958). various other algal and bryozoan biolithites The Pitkin mounds are particularly interesting The Pitkin Formation was first formally des- and are distinguished from each other on the in that they allow relationships between cryptal- ignated upper Chesterian in age by Easton basis of geometry and faunal and floral con- gal structures and a wide variety of skeletal or- (1943). It was deposited during the Kladogna- tent. Type I mounds have sharp lateral mar- ganisms to be observed. The domination of their thus-Cavusgnathus naviculus and the younger gins and interfinge:r with horizontally bedded, frameworks by cryptalgal structures also further Adetognathus unicornis conodont zones (Lane, contemporaneous, flanking strata owing to substantiates the important role of cyanobacteria 1967) and the equivalent Cravenoceras richard- having expanded :md contracted in size dur- in post-Early Ordovician reef construction (see sonianum and younger Cravenoceras involutum ing upward accretion. They are interpreted as Pratt, 1982a). Finally, Pitkin mounds may indi- goniatite assemblages (Gordon, 1965). Eumor- having had synoptic relief of 3 m or less dur- cate the beginning of the rise to prominence of phoceras bisulcatum has also been reported ing growth. Type II mounds occur slightly bioherms of the calcareous algae that dominate from the formation (McCaleb and others, 1964). higher stratigraphically, to the east of type I later, Pennsylvanian mounds. Pitkin goniatite stratigraphy was summarized by mounds, and formed in deeper water, farther Saunders and others (1977), who correlated the out on the Ozark shelf. They are interpreted STRATIGRAPHY AND AGE lower part of the formation in Arkansas and as having had synoptic relief of 6 m or more Oklahoma to the top of the El zone of the during growth. Their margins are similar to The Pitkin Formation crops out along the Pendleian Stage of the British Namurian Series those in type I mounds, but owing to higher southern edge of the Ozark uplift from north- and the upper part of the formation, in north- synoptic relief, they interfinger with sedi- central Arkansas to northeastern Oklahoma. It central Arkansas only, to the E2 zone of the ments derived from erosion higher on the is part of a conformable sequence of Chesterian Arnsbergian Stage, also of the British Namurian mound. sediments deposited on the Ozark shelf. The Series. shelf deepened to the south and east where the INTRODUCTION Pitkin Formation thickens and picks up black Carbonate buildups of late Paleozoic age have been the focal point of numerous studies in North America. Upper Mississippian bioherms, however, have proven to be scarce. The paucity of data relating to Chesterian bioherms has thus left a considerable gap in the understanding of reef paleoecology between the better known Lower Mississippian Waulsortian mounds and the later Lower Pennsylvanian phylloid algal mounds. The purpose of this paper is to describe two distinctive types of Chesterian bioherms from the Pitkin Formation of northern Arkan- sas. Pitkin bioherms have previously been the subject of several studies by students at the Uni- • Type I Mounds .—.—.—.—i—. versity of Arkansas. The basic interpretation of O 25km • Type II Mounds *Present address: Department of Geology and Min- eralogy, University of Queensland, St. Lucia, Queens- Figure 1. Known type I and type II Pitkin mound exposures in northwestern Arkansas land 4067 Australia. (modified from Tehan and Warmath, 1977). Geological Society of America Bulletin, v. 99, p. 686-698, 14 figs., 1 table, November 1987. 686 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/5/686/3998411/i0016-7606-99-5-686.pdf by guest on 27 September 2021 LATE MISSISSIPPIAN THROMBOLITE BIOHERMS, ARKANSAS 687 tion, in one example (MS 461). Surrounding strata are horizontally bedded; do not dip off of the mounds except in rare, small-scale instances; and may show no indication of a mound's pres- ence < 1 m from a mound extension. Although no mounds were observed in plan view, the constant bilateral symmetry of all observed transections suggests that they are roughly equi- dimensional in outline. The mounds are completely enclosed in a series of interbedded bioclastic and coated-grain grainstones, mudstones, and nodular limestones. Coarse crinoidal grainstones, oolitic grainstones, and cryptalgal-coated-grain sandstones are pre- dominant. The mounds do not cause any overall thickening of the formation. The boundary between the mound facies and its surrounding facies is extremely sharp in cases in which grainstones and bedded mudstones are involved (Fig. 2). It becomes less well defined in conjunc- tion with nodular limestones owing to the sim- Figure 2. West flank of a type I mound at locality MS 463. Scale stick is slightly <1.5 m ilar weathering texture of the two facies. (5.0 ft) in length. Note the 30° truncation of the horizontally bedded, nonmound strata at the Nodular limestones typically flank the mounds left and the sharp boundary between them and the imbedded mound facies at the right. at maximum extension, and thin calcareous shale re-entrants may penetrate the mounds at constrictions. Small (1 m or less in greatest di- mension), irregular pockets of the surrounding INTERMOUND LITHOLOGY occur within the lower 20 m of the formation facies may occur within the mound facies, but and may themselves be 15 to 20 m thick and no channel-like structures have been observed The Pitkin Formation is dominated by mas- extend laterally for >50 m. Although their bases dissecting the mounds. sive, oolitic and bioclastic grainstones, which have not been observed, the mounds appear to The mound at locality MS 457 is typical of may contain locally abundant intraclasts, onco- originate at or very near the base of the type I mounds (Figs. 3A and 3B). The base of lites, and smaller cryptalgal-coated grains. Skele- formation. the mound is covered. The lowest exposed tal packstones and wackestones are scarce. Type II mounds are represented by two mound extension (point 1 on Figs. 3A and 3B) Mudstones are rare in western exposures but are known examples that crop out in Searcy County rests on mudstone but contracts as the rocks more common to the east in north-central Ar- (Fig. 1). Their exact stratigraphic position is become more bioclastic. A local increase in the kansas. Nodular limestones, which are charac- obscure owing to the structural complexity of shale content produced nodular grainstones im- terized by thin, laterally discontinuous, wavey the area and lack of complete Pitkin exposures. mediately overlying this extension. The shale beds or discrete nodules of coarse bioclastic Sandstones, possibly of the overlying Imo For- content diminishes away from the mound, yield- grainstones (more rarely wackestones) con- mation, appear to crop out above the Pitkin ing more massive bioclastic grainstones. The tained within black shale, occur in the western limestones at the mound exposures and suggest mound undergoes two more expansions and Pitkin exposures. Black shales with associated that the mounds occur near the top of the Pitkin contractions and is then covered by a bed of phosphorite beds occur in the thicker eastern Formation, but the association of the mounds coated-grain grainstone just below the Missis- sections. The distribution of the various facies is with a thick black shale unit may, instead, indi- sippian-Pennsylvanian unconformity at the top of very irregular and the formation is characterized cate that they occur near the middle of the for- the quarry. Although the mound shows evidence by abrupt lateral and vertical facies changes. mation (-30 m above the base), where sub- of subaerial exposure (for example, stromatolites Nageotte (1981) applied the term "facies mo- stantial black shales are abundant elsewhere in with dessication cracks) at point a (Fig. 3B), saic," as proposed by Laporte (1967), to the area. In any event, the type II mounds occur neither the mound nor the surrounding sequence describe the distribution of western Pitkin facies. stratigraphically higher than the type I mounds. shows evidence of major disconformity. Other The largest of the type II mounds has an ex- type I mounds (MS 461 and MS 463) do con- PITKIN MOUNDS posed thickness of 6 m with a covered base and tain thin calcareous black shales re-entrants at extends laterally for >50 m. some constrictions. These shales were deposited The Pitkin Formation exhibits two types of throughout the area and may represent diastems. bioherms, herein termed "mud-mounds" as they Mound Geometry Type II Mounds. The geometry of type II are dominantly composed of lime mudstone and mounds is more obscure owing to the fact that contain relatively few metazoan frame builders. Type I Mounds. The margins of type I only two type II mound exposures are known Type I mounds are the most abundant and occur mounds expand and contract symmetrically on and only the tops are exposed, leaving the lower in Washington and Madison Counties (Fig. 1) opposing flanks, causing lateral dimensions to portions and the lateral, contemporaneous, off- as isolated, clustered, and in at least one case, vary from as much as 46 m, at maximum exten- mound strata entirely unknown.

Load more