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 presence < 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. Further, mound coalesced structures. Stratigraphically, they sion, to as little as 2.5 m, at maximum contrac- facies and associated carbonate facies are very

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 688 G. E. WEBB

site for deposition of bioclastic and intraclastic rocks that were mostly derived from higher up on the mound. Thin, laterally discontinuous calcareous shales were also locally deposited on the mound flanks, but oolitic grainstones do not occur. The interfingering relationship is most evident on the southern flanks of the mounds, which exhibit more grainstones and fewer wackestones and packstones than do the northern flanks. The core of the mound at locality MS 474 (Fig. 5) consists of crinoid-algal-cryptalgal bio- lithite that was partially covered from the south by calcareous shale. The mound facies then extends over the shale and is in turn overlain by a tongue of intraclastic grainstone. The mound facies finally extends over this grainstone as well. To the north, the mound strata extend over bioclastic grainstones, are penetrated by a tongue of intraclastic and bioclastic rocks, and then are overlain by more mound boundstones. The en- BI tire exposed mound complex is overlain by black shale. The prominent north-to-south M M ^^ asymmetry of the mound is enhanced by unusu- ally abundant Multithecopora boundstone on the northern flank.

CG.BI Mound Fabric

Pitkin mound framework is produced by the a M syndepositional cementation of massive, stacked Is* N thrombolite heads. The term "thrombolite" was 5 introduced by Aitken (1967) for cryptalgal M i) BI structures (that is, structures presumed to have B been created by cyanobacteria) that lack internal MU T ^ T lamination and are characterized by a macro- scopic clotted fabric (Fig. 6). The thrombolite Figure 3. A. Quarry face containing the southern flank of a type I mound at locality MS framework of the Pitkin mounds is intergrown 457. The scale stick is slightly <1.5 m (5.0 ft) in length. Numbers 1, 2, and 5 are located on with, ami/or passes laterally into, various types mound strata (1 is obscured by shadows at the lower right), whereas number 3 is located on an of locally developed calcareous algal or meta- impinging tongue of bioclastic grainstone. B. Diagram, to scale, of the same mound. Symbols zoan biolithites. It also locally grades into are as follows. M - mound strata, CG = coated-grain grainstone, BI = bioclastic grainstone, N = stromatolites as laminae become clear. The nodular grainstone, MU = mudstone, H = Hale Formation shales, T = talus, and a = position of complex interactions of thrombolite, metazoan stromatolites with dessication cracks. skeletons, and associated deposits give the mounds a wide range of distinctive fabrics (Fig. 7). difficult to distinguish in the field due to the thin over their tops. Nowhere are black shales Cryptalgal Framework. Individual throm- prevalence of bioclastic debris in the former and observed interfingering with the carbonate facies bolite heads in the Pitkin mounds assume irregu- to pervasive recryslallization and dolomitization. of the mound complex, and boundaries between lar, rounded, typically ovoid shapes and may Facies boundaries are also not as sharp as those the two lithologies are very sharp. Phosphorite have locally overhanging margins. They range in in type II mounds. beds within the shale occur only away from and diameter from 15 cm to >1 m and are com- The larger of the type II mounds (MS 474) between the mounds and do not extend over posed of a meshwork of thrombolite crusts, had relief above the sea floor of at least 6 m, but their tops. The mounds cause some local arching spar-filled voids, internal sediments, and various the full extent of the relief cannot be determined and thickening within the formation as a whole, skeletal metazoans. These thrombolite heads are owing to the covered base. Flanking bioclastic despite the covering of shale (Fig. 4). As in type typically grown together, but where spaces and intraclastic rocks interfinger with, and I mounds, the type II mound cores expanded occur between them, they are filled with coarse gently slope off, the mound core and occur and contracted during accretion. As a result of bioclastic material, carbonate mud, terrigenous abundantly as pockets within the core. The the higher relief, however, the lateral extensions mud, or a combination of the three. mounds and their ilanking deposits are overlain rest on down-sloping, flanking deposits. During The Pitkin mounds also contain well-devel- by black shales, which lap onto the mounds and contraction of the core, the flanks became the oped hemispherical to digitate stromatolites.

TABLE I. COMPARISON OF TYPE I AND TYPE II MOUND FLORAS AND FAUNAS

Flora and fauna Type I mounds Type II mounds

Thrombolite VC-B VC-B Stromatolite P-L C-B Encrusting bryozoans VC-B VC-B Fenestrate bryozoans VC-B VC-B Hexagonellid bryozoans S-I C-L Ramose bryozoans R-I S-I Encrusting foraminifers VC-L VC-L Asphaltina sp. C-L C-L Phylloid alga A P-I P-L Phylloid alga B S-I C-L Phylloid alga C S-I P-L Calcareous alga D R-I R-I Rhodolithic alga C-L C-L "Spicular" alga VC-B VC-B "Bladed" alga R-I R-I Gimmella A P-L C-L Girvanella B S-I C-L Sponges P-I P-L Crinoids C-I VC-L Brachiopods C-I C-I Parvaxon minutum Webb A C-L Leonardophyllum arkansanum Webb P-I A Cawnosirotion variabilis Easton P-L A Multilhecopora sp. A C-L Annelid tubes P-I S-l Nostociles sp. A R-I Gastropods S-I S-L Figure 4. Type II mound exposure at MS 472. Note the arching of the overlying units. Bivalves R-I R-I

Photographed area is ~25 m across. Symbols: M = mound strata, S = black shale, O = Note: VC = very common B = important mound builder overlying oolitic grainstones. C = common L = local mound builder P = present I = mound inhabitant S = scarce R = rare A = absent They are much less common than are the typical extent, to environmental differences due to their thrombolite-dominated textures but may be lo- relative positions on the Ozark shelf. A compari- cally abundant in type II mounds. In some cases, son of the major mound faunas for the two 1957; Pray, 1958; Pratt, 1982b). Those in the thrombolite textures pass vertically into well- mound types is shown in Table 1. Pitkin mounds were confined to a purely acces- laminated stromatolites, and some stromatolites Bryozoans. Bryozoans are the most abundant sory role and do not appear to be responsible for in both mound types exhibit desiccation fea- metazoans in the Pitkin mounds; however, they the formation of the biolithites in which they tures, including mud cracks. do not approach the volumetric significance of occur. Pitkin fenestrate bryozoans may have Secondary Mound Builders and Inhabi- thrombolite crusts. Fenestrate forms are the formed local current baffles but were more im- tants. Although the mound framework is most widespread, the genus Archimedes being portant as sediment trappers, whereby broken predominantly composed of thrombolite heads, dominant. Broken fronds and columns of fronds covered sediment surfaces and were en- various other biota have accessory roles in the Archimedes are distributed throughout the crusted by cyanobacterial mats, thus stabilizing accretion of the Pitkin mounds. Although the mounds, but complete individuals in growth po- the substratum. Elias and Condra (1957) con- two mound types have similar associated faunas, sition are uncommon. Fragments of other fenes- cluded that most fenestrate bryozoans lived at some major differences do exist. Because the trate bryozoans are distributed sparsely through depths of -25 to 50 m or in relatively quiet basic environments represented by the two the mounds. waters. Because Pitkin mounds were deposited mound types appear to be similar, it is proposed Opposing views exist concerning the impor- shallower than 25 m, scattered occurrences of that faunal differences reflect a difference in tance of fenestrate bryozoans in the formation of specimens in growth position suggest either (a) stratigraphic position and, only to a lesser Mississippian bioherms (for example, Parkinson, the presence of relatively sheltered areas on the

Figure 5. Sketch diagram of type II mound at locality MS 474. Abundant crinoids in growth position are indicated by the letter c, and Multithecopora boundstones are indicated by the letter t.

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 690 G. E. WEBB

mounds from which broken skeletons could be It is also a common nucleus in the abundant sponsible: for the formation of the thrombolites. transported and deposited elsewhere on the algal-coated grains and oncolites of off-mound At least two forms of Girvanella and a number mound surfaces or (b) that the distribution may strata. of calcareous algae can be discerned. Rhodo- have been more even, but colonies failed to be Algae and Cyanobacteria. Pitkin mounds phaecophyta are abundantly represented by a preserved in grow:h position in higher energy contain a variety of calcareous and calcified possible rhodolithic alga of unknown affinities areas. McKinney (1979) stated that the rarity of algae and cyanobacteria other than those re- (Fig. 8H). It forms irregular, self-encrusting Archimedes attachments may indicate that such structures were weakly calcified. The chances of preservation of individuals in growth position thus decreases in more agitated areas, skewing their occurrence toward the more sheltered areas. Several species of encrusting bryozoans are abundant in the mounds. A large percentage are in growth position, but some are disoriented. They appear to be distributed rather equally throughout the mo unds, although local concentrations do occur. The encrusting bryozoans typically occur as thin crusts that may extend laterally for > 15 cm and may be stacked vertically into considerable series, thus binding large quantities of sediment (Fig. 8A). Others are massive and create hemispherical colonies as much as 30 cm in diameter. Fragments of ramose bryozoans occur sparsely throughout both mound types but are much more abundant in type II mounds. Nowhere are they sufficiently abundant to be considered important mound builders. A distinctive hexagonellid bryozoan, which resembles Glyptopora and Prismapora, also occurs in both mound types, its greatest abundance being in type II mounds. There, it may occur in locali zed concentrations sufficient to bind considerable sediment, although individual colonies rarely exceed 8 cm in diameter.

Figure 6. Thrombolite macrotexture. Thrombolite clots (t) appear dark against Figure 7. A, B. Vertically oriented slab and schematic diagram showing anastomosing lighter internal sediments. Large clot on the thrombolite crusts (sample MS 463-22). C, D. Vertically oriented slab and schematic diagram right expanded at one point, becoming mush- showing irregular thrombolite crusts associated with encrusting bryozoans (sample 464-19). room shaped. It was subsequently encrusted Scale bar is 3 cm in length. Key: black = thrombolite crust, stipple = internal sediment, diagonal by bryozoans (b). Scale bar is 1 cm in length, lines = macrofossils, white = spar.

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 6

Figure 8. A. Stacked, encrusting bryozoans (sample MS 464-15). Scale bar is 1 mm in length. B. Rhodolithic alga of unknown affinities (sample MS 474-20). Scale bar is 1 mm in length. C. Polished slab of phylloid alga A boundstone (sample MS 474-23). Scale bar is 1 cm in length. D. Same specimen as in C, in thin section. Note the complete loss of original internal structure. Scale bar is 1 cm in length.

masses that have poorly preserved vesicular in type II mounds, is typically interlayered with be well over 1 cm in diameter but are typically structure. It is abundant in both mound types mats of Girvanella and a problematic organism, smaller. They bear similarities to Ortonella ker- and is locally an important sediment binder. Al- encruster C. Encruster C (Fig. 9D) is abundant shopensis as illustrated by Mamet and Roux though typically occurring with encrusting for- in type II mounds and scarce in type I mounds (1975, p. 173). Another less abundant, but dis- aminifers in laminate cryptalgal structures, it and is typically, but not invariably, associated tinctive, filament arrangement consists of one or may also occur in thrombolite textures. with phylloid alga B. It consists of irregular lay- more rows of parallel to subparallel filaments A phylloid alga ("phylloid alga A") is also an ers of self-encrusting vesicles -0.1 mm in diame- that resemble a string of beads in transverse sec- important localized sediment binder in the ter, but no finer detail is preserved. In general tion. "Calcareous alga D" is uncommon in both mounds (Figs. 8C and 8D). It is most abundant appearance, encruster C is similar to the possible mound types and consists of a rounded, possibly in type II mounds, occurring as large, irregular foraminifer Aphralysial sp., illustrated from the hollow, mass of parallel to subparallel filaments plates > 1 mm in thickness and typically extend- Pitkin Formation by Brenckle (1977), but pos- that approaches 1 cm in diameter. It is similar to ing laterally for several centimetres. The internal sesses much larger vesicles. the genera Garwoodia and Hedstromia. structure of the thalli is not preserved, owing to Other calcified algae are widespread in the Recognizable cyanobacteria are represented recrystallization, but the alga may be Archaeo- Pitkin mounds, but well-preserved individuals in the Pitkin mounds by two types of Girvanella lithophyllum, which has a similar growth habit are uncommon. The most abundant of these that are distinguished on the basis of tubule size. and has been illustrated from Pitkin grainstones forms in both mound types are herein designated Girvanella A (Fig. 10B) occurs in mats of by Nodine-Zeller (1977). Another type of phyl- "spicular algae," owing to their resemblance to loosely twisted, nonbranching tubules -0.025 loid alga ("phylloid alga B") occurs as fairly clusters of spicules. Several different types of mm in diameter. They are typically associated regular, self-encrusting layers that are slightly algae may be included in this group, as the size with the various phylloid algae and generally thinner than are the plates of phylloid alga A and arrangement of the filaments vary consider- occur encased in a sparry matrix that may itself (Fig. 9A). Where not badly recrystallized, inter- ably. Typical examples consist of straight, thin represent a recrystallized calcareous alga. They nal structure consists of rounded to polygonal filaments radiating from a central point (Fig. also occur in oncolites in the surrounding grain- tubes arranged perpendicular to the layers (Figs. 10A). The nature of branching of the filaments stones. Girvanella B consists of smaller tubules, 9B and 9C). Phylloid alga B, also most abundant has not been observed. Individual clusters may -0.008 mm in diameter, that occur in two

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 692 G. E. WEBB

Figure 9. A. Vertically stacked, recrystallized sheets of phylloid alga B (sample MS 474-14). Scale ban- is 0.5 mm in length. B. C. Tangential and longitudinal sections, respectively, of phylloid alga B, showing relict cellular structure (samples MS 474-13 and MS 474-14, respectively). Scale bar(s) is (are) 0.1 mm in length. D. Encruster C (sample MS 474-14). Scale bar is 0.1 mm in length.

growth habits. The usual growth habit is tightly The coated objects of micrite tubes may rep- densities of columns reaching 100 individuals in packed, subparaM tubules that tend to pass into resent an alga or vascular plant that has left no 930 cm2 of horizontal surface. The largest such micritic texture. This growth habit is typical of other trace upon death. Scoffin (1970) noted "crinoid thicket" at locality MS 474 is -6.2 m the occurrence of Girvanella B in oncolites. The that one important line of evidence for the exist- across. No such crinoid thickets occur in type I second growth habit consists of "columns" of ence of plants, which have themselves failed to mounds. Articulated crinoid calices are rare; tubules that are tightly, but irregularly, twisted be preserved, is the preservation of associated none were observed within the mounds al- together and that may be arranged perpendicu- epiphytes. Pitkin mounds contain abundant en- though a few were recovered from the flanking larly to the encrusted surface (Fig. 10C). This crusting foraminifers that appear to have en- deposit!!. Blastoids, although common in parts of growth habit is typically associated with Girva- crusted objects similar in size to those of the the formation away from the mounds, have not nella A and the various phylloid algae. Another micrite tubes (Fig. 10F). The abundance of the been observed within the mounds. structure, interpreted as a blue-green alga, re- micrite tubes suggests that the responsible orga- It is likely that crinoid calices and the upper- sembles Rectangulina (see Mamet and Roux, nism could have had significant local effects on most lengths of column were disarticulated after 1975, p. 169; 1978, p. 85). This genus was illus- mound sedimentation. death in the high-energy conditions above the trated from the F'itkin Formation by Brenckle Encrusting Foraminifers. A variety of fora- mound surface, thus providing the abundant, (1977). Pitkin mound examples consist of minifers encrusts Pitkin thrombolite and stroma- scattered debris to the mounds. Only the lower straight, parallel to subparallel tubes that appear tolite surfaces as well as the various other sections of column, which were encrusted by angular or "blade-like." They occur sparsely in mound biota. They may be locally abundant or and contained within the thrombolite matrix both mound types but are common in the flank- rather sparse but occur throughout the mounds. prior to death, were preserved in growth posi- ing grainstones. They are also abundant in flanking grainstones. tion. Crinoids were not abundant enough on the A final group of structures that owe their ex- Where grouped on particular surfaces, foramin- mounds to provide the only source for the crino- istence to blue-green algae are herein termed ifers may lend a laminar texture to thrombo- zoan debris in the flanking grainstones of the "micrite tubes." These structures, which are in- lites. Many genera are represented, but the most type I mounds, but the crinoid thickets of type II variably associated with thrombolite textures, abundant forms resemble the calcivertellids, il- mounds may have had a local effect on mound appear to be the result of the coating of larger lustrated from the Pitkin Formation by Brenckle sedimentation. tubular objects by thrombolite-producing algal (1977), and the palaeo-nubecularids, pseudo- Corals. Pitkin mounds contain several species mats. The coated objects were not preserved, glomospirids, and calcitornellids illustrated by of rugose and tabulate corals (Webb, 1987). leaving a hollow micritic sheath that is later filled Groves (1983) from the Morrowan rocks of the Each mound type has its own particular fauna, with spar cement (Figs. 10D and 10E). The na- Ozark uplift. Free-living benthonic foraminifers but corals serve as frame builders only in type II ture and thickness of the coating is highly vari- are rare within the mounds but are abundant in mounds. Type I mound cores contain rare, scat- able and the tubes range widely in internal the flanking grainstones. tered packets of the solitary rugosan, Leonardo- diameter, few exceeding 1 mm. Micrite tubes Crinozoans. Crinoids are common mound phyllum arkansanum Webb. The large, den- occur singly and :!n groups and typically do not constituents, and scattered holdfasts are found in droid rugosan, Caninostroiion variabilis Easton, contain internal sediments. Their orientations growth position in both types of mounds. Type is abundant in growth position near the flanks of vary considerably, and they may be abundant. II mound cores contain 1-m-long, 1-cm-wide some type I mounds but not in the mound cores. They are distributed throughout both mound sections of column standing vertically, with at- The solitary rugosan Barytichisma ozarkana types but are not associated with stromatolites. tached cirri, in the thrombolite matrix, the Webb and the tabulate Michelinia meekana

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 8

Figure 10. A. Portion of a "spicular alga," similar in appearance to Ortonella (sample MS 474-30). Scale bar is 2.5 mm in length. B. Girvanella A tubules within a recrystailized calcareous alga (lithosample MS 474-14). Scale bar is 0.1 mm in length. C. Irregular columns of Girvanella B oriented normal to the encrusted surface (sample MS 474-14). Scale bar is 0.1 mm in length. D. Longitudinal section through a "micrite tube" contained within thrombolite fabric (sample MS 461-12). Scale bar is 1 mm in length. E. Dense microtexture of "micrite tube" (sample MS 457-44). Scale bar is 0.1 mm in length. F. Epibiontic foraminifer which may have encrusted the same organism as was responsible for "micrite tubes" (sample MS 472-24). Scale bar is 0.2 mm in length. G. Reticulate macrotexture produced by sponge (sample 474-19). Scale bar is 1 cm in length. H. Asphaltina, apparently in growth position within a thrombolite crust (sample MS 457-44). Scale bar is 1 mm in length.

Girty occur in dark, calcareous shales that pene- in areas that contain incursions of calcareous have bound sediment to some extent, and the trate the mounds and in the nodular limestones shale, where it may occur in growth position. sponges themselves may be encrusted by other that may occur at the extremities of mound Despite its small size, it may locally bind sedi- organisms such as bryozoans and calcareous lobes but not within the mound cores. The tabu- ment, owing to its habit of growing upon other algae. lates M. tenuicula Moore and Jeffords and members of its species, thus creating thin veneers Miscellaneous Biota. Brachiopods were Sutherlandia sp. do occur within the mound of adjoined individuals. The small rugosan Am- common inhabitants of both mound types and cores and may be locally abundant enough to plexus sp. typically occurs with P. minutum but occur scattered throughout the mound litholo- contribute to coral-algal boundstones. is more scarce. gies. A majority of them are unbroken and con- Corals are abundant in type II mounds. The Sponges. Sponges occur as scattered constitu- tain geopetal fillings. Productid brachiopods tabulate coral Multithecopora sp. occurs scat- ents in both mound types but may be locally occur in growth position, the most delicate tered throughout the mounds and forms coral abundant in type II mounds. They have a very spines being preserved intact in type I mounds. boundstone veneers in the upper parts of their distinctive structure (Fig. 10G) but lack spicules. Composita-like forms seem to be the most northern flanks. The most abundant coral in Most sponges are barrel shaped, as much as 15 abundant brachiopods in all Pitkin mounds, and type II mounds is the small, solitary rugosan cm in diameter, but tabular, encrusting examples large, unbroken orthetids are abundant in type II Parvaxon minutum Webb. It occurs scattered or with the same internal structure also occur. mounds. Brachiopods do little more than con- concentrated throughout the mounds, generally Sponge aggregations in type II mounds may tribute sediment to the mounds, but some pro-

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 694 G. E. WEBB

Figure 11. Vertically oriented thin sections of thrombolite crusts. A. Small thrombolite "clots" (dark) with lighter internal sediments (sample MS 463-18). Scale bar is 3 mm in length. B. Larger thrombolite "clots" with sparry voids, fossil debris, and internal sediments (sample MS 463-22). Scale bsir is S mm in length. C. Irregular, lacey meshwork of thrombolite crusts exhibiting spongiform microtexture and incorporated "micrite tubes" (sample MS 463-34). Scale bar is 1 mm in length. D. Spongiform microtexture (sample MS 460-6). Scale bar is 0.1 mm in

vide substrates for the various encrusting in type II mounds. Bivalves are rare in both gal crusts. As in other algal-bound deposits (see organisms. mound types. The rarity of burrowing forms Wolf, 1965, and Pratt and James, 1982), Pitkin Encrusting, Spirorbis-like annelid tubes with may be due to the scarcity of unconsolidated thrombolite crusts are generally darker in thin tangentially fibrous microstructure are locally muds on the mound surfaces. Cephalopods ap- section than are the surrounding matrix and in- abundant in type I mounds but scarce in type II pear to be absent from the mounds although the ternal sediments. mounds. They occur both on metazoan skele- flanking deposits contain a varied, if scarce, nau- Pitkin thrombolite frameworks contain many tons and on thrombolite surfaces. tiloid fauna. Ostracods and disarticulated trilo- thrombolite-encrusted skeletal metazoans. The Asphaltina, which consists of a colony of in- bites are locally abundant within the mounds. encrusting thrombolites generally coalesce with tertwining, cylindrical tubes of unknown affini- Shark teeth in both mound types indicate the nearby reticulate frameworks or, where skeletal ties, is an abundant encruster in the Pitkin presence of a large vertebrate fauna. organisms are very abundant, with each other. mounds (Fig. 10H). It typically occurs on hard Thrombolites may be encrusted by the various substrates such as bryozoans, calcareous algae, Petrology of the Mounds encrusting mound biota and may contain dis- brachiopods, and thrombolite surfaces. It is also crete layers of encrusting foraminifers and an- abundant in the flanking grainstones. Another Thrombolite Crusts. Individual thrombolite nelid worm tubes. mound inhabitant of unknown affinities is the crusts in Pitkin mounds range from aggregations Three textures are characteristic of Pitkin small encrusting organism Nostocites, which is of small, irregular "clots," which typically mea- thrombolite crusts. These are (1) spongiform rare in type II mounds and absent in type I sure from 1 to 3 mm across (Fig. 11 A), to ag- texture, which Pratt (1982b) characterized as mounds. It consists of a single layer of loosely gregates of larger, more massive "clots," which consisting of often indistinct, silt-sized micrite packed, barrel-shaped cells (see Groves, 1983). measure from 1 to 3 cm across (Fig. 1 IB). Irreg- pelloids separated by interparticle and tiny fene- The mounds pi ayed host to a variety of other ular, lacey meshworks also occur (Fig. 11C). stral pores; (2) vermiform texture, which con- organisms. Gastropods, although rare as a Clots are generally arranged into reticulate sists of micrite that contains irregular, roughly group, occur in small, local concentrations. They frameworks, and laminar areas typically result tubular fenestrae; and (3) massive texture, which are typically small, low-spired forms, but beller- from horizontal, thrombolite-encrusted fene- consisti! of undifferentiated, dense micrite. The ophontid gastropods may be locally abundant strate bryozoan fronds, not independent cryptal- most abundant texture in Pitkin cryptalgal struc-

tures is spongiform (Fig. 11D). Thrombolite brecciation in the Pitkin mounds. This feature ble to trace surfaces of accretion through crusts that appear to have an amorphous struc- is complicated by later, spar-filled, diagenetic thrombolites and thus show that even thrombo- ture are less abundant. They typically appear cracks that truncate thrombolite crusts, internal lite frameworks that are not associated with dense but are commonly mottled to some extent sediments, and the cements within the sparry metazoan skeletons were capable of undergoing and may contain fine skeletal debris. The coat- voids. Breccia clast margins are in some cases accretion on vertical surfaces. This differs from ings of "micrite tubes" are generally of this type. concave boring margins. the conclusion of Pratt and James (1982) that Dense thrombolite crusts also occur at the mar- Internal Sediments. These vary considerably Early Ordovician thrombolite crusts had only gins of thrombolite masses that progressively in the Pitkin mounds. They typically consist of slight relief above the sediment during growth. pass into spongiform textures inwardly. Vermi- light-colored carbonate mudstone or siltstone The encrusted surfaces in Pitkin mounds indi- form textures are rare. (Figs. 12A and 12B) but may be entirely com- cate that single thrombolite accretion surfaces Although spongiform texture is found only in posed of detrital pellets. Either may contain scat- could have vertical relief of several centimetres cryptalgal rocks, its origin is uncertain. Wolf tered, small, abraided fossil fragments or (Fig. 13). This relief is partially responsible for (1965) attributed it to the precipitation of au- complete, articulated ostracods. In some in- the sharp boundaries of the mounds with the tochthonous micrite by small algal colonies, thus stances, two or more generations of internal surrounding grainstones. The growing edge of forming peloids, and noted that these peloids sediments are preserved that vary considerably the mound grew out over the surrounding facies could be mixed with detrital "pellets" that were in grain size and composition (Fig. 12C). Inter- by lateral accretion on vertical to overhanging incorporated into the algal mat. He was careful nal sediment surfaces are typically horizontal, or thrombolite surfaces. Although fine sediments to distinguish his "pelletoid" structure from nearly so, but rarely, in type II mounds, they were easily entrapped on these surfaces, larger what are strictly accumulations of allochthonous may slope as much as 20° and contain low-angle allochems were not and thus did not make the pellets. The inclusion of various detrital grains in cross-laminae (Fig. 12D). Many internal sedi- boundary between the coarse grainstones and the algal-bound deposits allows a high degree of ments appear to be preferentially recrystallized finer mound muds more gradational. textural variability. The prevalence of spongi- to microspar. Growth rates for ancient cryptalgal structures form texture in Pitkin thrombolites may be are not well understood. Modern stromatolites somewhat atypical for thrombolites as a whole, Thrombolite Growth in Bermuda were found to have accretion rates as Pratt (1982b) noted that spongiform textures ranging from 0 to 3 mm per day (Gebelein, are not common in deeper water mud-mounds. Pratt (1982b) outlined two major types of 1969). This yields a maximum accretion rate of They have long been noted, however, from thrombolite accretion, each of which is inter- >1 m per year although variables such as sedi- shallow-water cryptalgal deposits (for example, preted to produce distinctive textures in cryptal- ment availability and water energy combine to Schwarzacher, 1961; Wolf, 1965; Alberstadt gal biolithites. The two frameworks are laminar keep the actual rate much lower. The occur- and others, 1974; Pratt and James, 1982). The and reticulate. Their distribution is controlled by rence of metre-long crinoid columnals in growth passing of amorphous into spongiform textures the distribution of algal mats, the rate of sedi- position within thrombolite matrix in type II suggests that the latter, in this instance, may be mentation, and the degree of winnowing of un- mounds suggests that these thrombolites were partially due to the effect of winnowing on ir- consolidated sediment. Laminar frameworks are capable of accreting at least 1 m during the life regularly cemented material beneath a better in- interpreted to occur in cases in which laterally span of a single crinoid. The carbonate mud durated crust. Similar well-indurated crusts, extensive algal mats are smothered by heavy necessary for such a quick accretion rate was which become progressively less well indurated influxes of sediments. New algal mats then produced on the mounds by the various biota, downward, were described from the Holocene establish themselves on the sediment surface. and very little occurs in most of the flanking by Bathurst (1980). Repetition of the process results in vertically strata. Spar-filled Voids. A distinctive feature of the stacked, preferentially cemented, algal-bound Many bryozoan and crinozoan fragments in Pitkin mound lithologies is the abundance of layers alternating with layers of unbound sedi- the mounds exhibit thrombolite coatings on all spar-filled voids, which range in size from the ment. Later winnowing may then remove the sides, suggesting that they were originally en- very small sparry areas associated with spongi- unconsolidated sediment, leaving a series of lam- crusted in growth position. The encrustation, form microtexture to large, stromatactis-like, inar crusts separated by cavities. These cavities breakage, and transport of such fragments pro- spar-filled cavities. Spar-filled voids larger than may be filled to varying degrees with one or vides a possible mechanism for the dispersal of 6 cm are not abundant in the Pitkin mounds but more generations of internal sediment or may be pieces of algal mat over the mound surface, thus are best developed in type II mounds. The more filled entirely by spar cement. Reticulate frame- re-establishing them on areas that had been typical spar-filled voids average centimetres or works are interpreted to form in a similar smothered by sedimentation. less in diameter but may represent well over 50% manner, except that the algal mats are irregu- In order for cryptalgal structures to serve as a of the volume of the rock in small areas. The larly distributed and may branch or coalesce framework for the Pitkin mounds, they had to voids may be supported by thrombolite crusts, vertically, to form a branching and anastomos- be rapidly lithified by synsedimentary cementa- metazoan skeletons, or combinations of the two ing framework (see Pratt, 1982b, Fig. 16). Pitkin tion. The importance of winnowing in the crea- and may be filled to varying degrees by internal mounds are dominated by reticulate frameworks tion of thrombolite textures has been mentioned, sediments. Various, syndepositional, fibrous and that are locally modified owing to the encrusta- and such winnowing clearly could not have bladed rim cements occur in the voids and un- tion of skeletal metazoans by thrombolite crusts. taken place prior to at least partial lithification common, recrystallized, relict aragonite fan ce- The Pitkin mounds contain extensive evi- of the algal-bound crusts. The importance of ments, which still exhibit sweeping extinction, dence that thrombolite algal mats were capable synsedimentary cements to the formation of may occur. Large void areas that are not filled not only of upward accretion, but also of lateral, cryptalgal structures that had relief above the sea by rim cements are typically filled with coarse, and perhaps even downward, accretion under floor was emphasized by Pratt (1982b) and blocky spar. small overhangs. The occurrence of encrusting Pratt and James (1982). Other spar-filled voids are associated with lo- foraminifers, encrusting annelid tubes, and var- Several lines of evidence exist for the calized areas of synsedimentary fractures and ious other encrusting organisms makes it possi- synsedimentary lithification of Pitkin thrombo-

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 696 G. E. WEBB

Figure 12. Veitically oriented thin sections showing internal sediments. A. Internal sediments in spar-filled voids (sample 463-46). Note the large spar-filled shelter porosity supported by the bryozoan in the center and the unsupported void at the upper left. Scale bar is 5 mm in length. B. Fine-grained internal sediment between thrombolite crusts (sample MS 463-18). Scale bar is 1 mm in length. C. Multiple generations of internal sediments (sample MS 472-22). Note the lower, coarser internal sediment, which is partially recrystallized, and the later generation, finer stratified internal sediment that lies above it. Scale bar is 0.5 mm in length. D. Cross-laminated internal sediment contained within a disarticulated brachiopod shell (talus at locality MS 474). Scale bar is 5 mm in length.

lites. The occurrence of cross-laminae in internal Brecciation in Pitkin mounds could be a result in type II mounds may result from their orienta- sediments provides clear evidence of winnowing of the collapse of areas of thrombolite frame- tion in relation to hydrodynamic forces on the in Pitkin mound: . Borings that occur in Pitkin work weakened by bioerosion. Borings are Ozark shelf. The occurrence of more wacke- thrombolite crust» and in large spar-filled voids abundant in the mounds and make up the mar- stones and packstones on their northern flanks (Fig. 14) indicate the extent to which synsedi- gins of some breccia clasts. Collapse could have may reflect more protected, lower energy condi- mentary cements were deposited. Other spar- been due to high-energy conditions at the tions on their landward sides whereas the grain- filled voids contain ghosts of original aragonite mound surface or to loading from subsequent stones on their southern flanks reflect the higher radial-fan cements, and cement crusts are rarely mound growth. energy, seaward side. overlain by internal sediments. Synsedimentary The interfingering nature of type I mounds brecciation in the mounds, mound intraclasts Discussion with their contemporary strata suggests that containing commuted skeletal grains in flanking some cyclic mechanism, possibly related to oscil- grainstones, and the lack of burrowed textures Type I mounds are interpreted as having lations in sea level, might be in operation. No or evidence of compaction of the mound muds grown more or less continuously and contempo- evidence was found to support this possibility, further indicate rapid lithification of the Pitkin raneously with flanking strata in very shallow however. Pinkley (1983) suggested that the in- thrombolites. Further, many of the bryozoans water with synoptic relief not exceeding 3 m. terfingering is due to the influence of shifting encrusting Pitkin thrombolite surfaces have thin, They were at least occasionally subjected to ex- carbonate sand shoals, independent of sea level. poorly calcified e ncrusting walls, which Schopf posure owing to slight drops in relative sea level. Had sea level been a major factor in producing (1969) attributed to a hard surface of encrusta- Type II mounds are interpreted as having grown the mound expansions and contractions, it tion. The mechanisms involved in the preferen- in deeper water (at least 6 to 10 m) and interfin- would be possible to correlate distant mounds tial lithification of algal-bound sediments are ger with deposits derived mostly from higher on on that basis. No such regional correlation be- poorly understood but are discussed by many the mounds. They likewise underwent exposure tween mounds has been possible. It is possible, authors (for example, Monty, 1976; Riding, on at least one occasion during growth. The however, to correlate mound expansions and 1977; Klappa, 1979; Pratt, 1979). asymmetrical faunal and sediment distributions contractions at the same horizons in beds that

achieving relief of scores of metres, occurred in deeper water, grew to much larger dimensions in many instances, and possess gradational facies boundaries. Lees and Miller (1985) used allo- chem content to delineate four depth-related phases in Waulsortian buildups. Pitkin mounds are most similar to the shallow-water Phase D, on the basis of allochems, but as Lees and Miller (1985) did not indicate the extent to which their components were in growth position, compari- sons are difficult. The mud component of Waul- sortian mounds is similar to that in Pitkin mounds, containing clotted fabrics, supported and unsupported spar-filled cavities, and internal sediments. These features led Pratt (1982b) to propose a thrombolite framework for Waulsor- tian mounds. The Pitkin mounds differ from typical Penn- sylvanian bioherms, in which phylloid algae are the primary frame builder (see Heckel, 1974, and Toomey and others, 1977), in having smaller dimensions, sharper facies boundaries, and only small, localized phylloid algal biolithites.

CONCLUSIONS

The Pitkin Formation contains two types of shallow-water bioherms that have frameworks Figure 13. Vertical thrombolite accretion surface (sample MS 457-1). Reticulate thrombolite dominated by thrombolites. Type I mounds mass at right contains internal sediment, spar-filled voids, brachiopods, and annelid tubes. occur near the base of the formation; have mar- Depression at left contains fine sediment and sparry cement and is floored by a thrombolite- gins which expand and contract, resulting in the coated fenestrate bryozoan frond. Note annelid tubes (a) encrusting the surface and the throm- interfingering of their cores predominantly with bolite coating of the lower one. Also note the overhanging bryozoan (b) near the top. Thin oolitic and bioclastic grainstones; and had syn- section is oriented vertically. Scale bar is 5 mm in length. optic relief of as much as 3 m. Type II mounds differ in that they occur stratigraphically higher than type I mounds; had synoptic relief of at least 6 m; interfinger predominantly with bio- can be traced between mounds that are close to clastic and intraclastic rocks derived from higher each other (for example, MS 461 and MS 463, on the mounds; exhibit asymmetrical margins which are 100 m apart). These mounds were from north to south; and caused local, minor influenced by the same local depositional arching in the formation. Faunal and floral dif- history. ferences are summarized in Table 1. The reported bioherms most similar to the Pitkin thrombolites are dominated by reticu- Pitkin mounds are those described from the Di- late frameworks and encrusted metazoan nantian of Furness, England, by Adams (1983). skeletons. They were rapidly lithified by syn- These structures consist of rigid frameworks sedimentary cementation, and some accretion containing the tabulate coral Syringopora, the surfaces were vertical, having several centime- encruster Aphralysia, thrombolites, and soleno- tres of relief above the substratum. Thrombolite porid algae. They have near-vertical margins growth rates appear to have been very rapid. with sharp facies boundaries, and one example Pitkin thrombolites played host to various skele- exhibits a lateral extension similar to those in the tal metazoans, many of which are preserved in Pitkin mounds (Adams, 1983). Pitkin mounds growth position, or form local biolithites, within differ in their geometries and faunas and in hav- them. The domination of Pitkin mounds by ing their frameworks more fully dominated by thrombolites further demonstrates the important thrombolites. role of cyanobacteria in the formation of some faftfcJKEPitkin mounds differ considerably from the lat e Paleozoic buildups, as suggested by Pratt Figure 14. Borings into spar-filled voids Lower Mississippian Waulsortian mounds from (1982a), and the occurrence of phylloid algal and syndepositionally cemented thrombolite North America and Europe, which have re- biolithites in type II mounds may represent an crusts (sample MS 463-38). Note the geopetal cently been discussed by Lees and others (1985) early attempt at competition on bioherms by a filling of detrital pellets in the lower central and Lees and Miller (1985). Differences include type of alga that becomes much more prominent boring. Scale bar is 0.5 mm in length. that Waulsortian mounds were capable of during the Pennsylvanian.

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 698 G. E. WEBB

REFERENCES CITED Parkinson, D., 1957, Lower Carboniferous reefs of northern England: ACKNOWLEDGMENTS American Association of Petroleum Geologists Bulletin, v 41, Adams, A. E„ 1983, Development of algal-foraminiferal-coral reefs in the p. 511-537. Lower Carboniferous of Furness, northwest England: Lethaia, v. 17, Pinkley, V., 1983, A paleoecological investigation of Pitkin lime mud mounds, The research presented in this paper was car- p. 233-249. Durham, Arkansas [M.S. thesis]: Fayetteville, Arkansas, University of Aitken, J. D., 1967, Classification and environmental significance of cryptalgal Arkansas, 85 p. ried out as a partial requirement for a master's limestones and dolomites, with illustrations from the Cambrian and Pratt, B. R., 1979, Early cementation and lithification in intertidal cryptalgal degree at the University of Oklahoma. The wri- Ordovician of southwestern Alberta: Journal of Sedimentary Petrology, structures, Boca Jewfish, Bonaire, Netherlands Antilles: Journal of Sed- v. 37, p. 1163-1178. imentary Petrology, v. 49, p. 379-386. ter is most indebted to P. K. Sutherland, who Alberstadt, L. P., Walker, K. R., and Zurawski, R. P., 1974, Patch reefs in the 1982a, Stromatolite decline—A reconsideration: Geology, v. 10, Carters Limestone (Middle Ordovician) in Tennessee, and vertical p. 512—515. served as thesis director, for all of his time, inspi- zonation in Ordovician reefs: Geological Society of America Bulletin, 1982b, Stromatolitic framework of carbonate mud-mounds: Journal of ration, and advice. Thanks also go to W. L. v, 85, p. 1171-1182. Sedimentary Petrology, v. 52, p. 1203-1227. Bathurst, R.G.C., 1980, Stromatactis—Origin related to submarine-cemented Pratt, B. R., and James, N. P., 1982, Cryptalgal-metazoan bioherms of Early Manger, at the University of Arkansas, who crusts in Paleozoic mud mounds: Geology, v. 8, p. 131-134. Ordovician age in the St. George Group, western Newfoundland: Sedi- Brenckle, P., 1977, Foraminifers and other calcareous microfossils from late mentology, v. 29, p. 543-569. took me to mound localities, pointed out the Chesterian (Mississippian) strata of northern Arkansas, in Sutherland, Pray, L. C., 1958, Fenestrate bryozoan core facies, Mississippian bioherms, basic differences between type I and type II P. K., and Manger, W. L., eds.. Upper Chesterian-Morrowan stratig- southwestern United States: Journal of Sedimentary Petrology, v. 25, raphy and the Mississippian-Pennsylvanian boundary in northeastern p. 261 -273. mounds, and provided much useful discussion. Oklahoma and northwestern Arkansas: Oklahoma Geological Survey Riding, R., 1977, Calcified Plectonema (blue-green algae), a recent example of Guidebook 18, p. 73-87. Girvanella from Aldabra Atoll: Paleontology, v. 20, p. 33-46. B. R. Pratt, at the University of Toronto, pro- Easton, W. H., 1942, The Pitkin Limestone of northern Arkansas: Arkansas Saunders, W. B., Manger, W. L., and Gordon, M., Jr., 1977, Upper Mississip- vided invaluable comments on the manuscript, Geological Survey Bulletin, no. 8, 1 IS p. pian aid Lower and Middle Pennsylvanian ammonoid biostratigraphy 1943, The fauna of the Pitkin Formation of Arkansas: Journal of Pa- of northern Arkansas, in Sutherland, P. K., and Manger, W. L., eds., and J. D. Pigott, at the University of Oklahoma, leontology, v. 19, p. 125-154. Upper Chesterian-Morrowan stratigraphy and the Mississippian- Elias, M. K., and Condra, G. E., 1957, Fenesteila from the Permian of West Pennsylvanian boundary in northeastern Oklahoma and northwestern gave advice on carbonate petrology. N. P. Texas: Geological Society of America Memoir 70,158 p. Arkansas: Oklahoma Geological Survey Guidebook 18, p. 117-137. James, of Queen's University, Ontario, and Gebelein, C. D., 1969, Distribution, morphology, and accretion rate of recent Schopf, T.J.M., 1969, Paleoecology of ectoprocts (bryozoans): Journal of Pa- subtidal algal stromatolites, Bermuda: Journal of Sedimentary Petrol- leontology, v. 43, p. 234-244. J. D. Aitken, of the Geological Survey of ogy, v. 39, p. 49-69. Schwarzachei, W., 1961, Petrology and structure of some lower Carboniferous Gordon, M., Jr., 1964 [1965], Carboniferous cephalopods of Arkansas: U.S. reefs in northwestern Ireland: American Association of Petroleum Canada, also provided useful comments on the Geological Survey Professional Paper 460, 322 p. Geologists Bulletin, v. 45, p. 1481-1503. manuscript. Financial support was provided by Groves, J. R., 1983, Calcareous foraminifers and algae from the type Morrowan Scoffin, T. P., 1970, The trapping and binding of subtidal carbonate sediments (Lower Pennsylvanian) region of northeastern Oklahoma and north- by marine vegetation in Bimini Lagoon, Bahamas: Journal of Sedimen- the University of Oklahoma and by P. K. Suth- western Arkansas: Oklahoma Geological Survey Bulletin 133,65 p. tary Petrology, v. 40, p. 249-273. Heckel, P. H., 1974, Carbonate buildups in the geologic record: A review, in Tehan, R. E, 1976, The sedimentary petrology of the Pitkin (Chesterian) erland through a research grant from the Mobil Laporte, L. F., ed., Reefs in time and space: Society of Economic Paleon- Limestone, Washington and Crawford Counties, Arkansas [M.S. thesis]: Foundation. tologists and Mineralogists Special Publication 18, p. 90-154. Fayetteville, Arkansas, University of Arkansas, 149 p. Huffman, G. G., 1958, Geology of the flanks of the Ozark Uplift, northeastern Tehan, R. E., and Warmath, A. T., 1977, Lime-mud mounds of the Pitkin Oklahoma: Oklahoma Geological Survey Bulletin 77, 281 p. Formation (Chesterian), northwestern Arkansas, in Sutherland, P. K., Klappa, C. F., 1979, Calcified filaments in Quaternary calcretes: Organo- and Manger, W. L., eds., Upper Chesterian-Morrowan stratigraphy and mineral interactions in the subaerial vadose environment: Journal of the Mississippian-Pennsylvanian boundary in northeastern Oklahoma APPENDIX A. LOCALITIES Sedimentary Petrology, v. 49, p. 955-968. and northwestern Arkansas: Oklahoma Geological Survey Guidebook Lane, H. R., 1967, Uppermost Mississippian and Lower Pennsylvanian cono- 18, p. 49-54. donts from the type Morrowan region, Arkansas: Journal of Paleontol- Toomey, D. F., Wilson, J. L., and Rezak, R., 1977, Evolution of Yucca Mound ogy, v. 41, p. 920-942. complex. Late Pennsylvanian phylloid-algal buildup, Sacramento Locality information is given below for Pitkin Laporte, L. F., 1967, Carbonate deposition near mean sea-level and resultant Mountains, New Mexico: American Association of Petroleum Geolo- mounds that are discussed in the text or have yielded fades mosaic: Manlius Formation (Lower Devonian) of New York gists Bulletin, v. 61, p. 2115-2133. specimens for illustrations. The MS prefix indicates State: American Association of Petroleum Geologists Bulletin, v. 51, Warmath, A. T., 1977a, Algal-bryozoan carbonate buildups within the Pitkin p. 73-101. Limestone (Mississippian-Chesterian), northwest Arkansas: Arkansas that the localities are listed in the Carboniferous of the Lees, A., and Miller, J., 1985, Facies variation in Waulsortian buildups, part 2; Acadjmy of Science Proceedings, v. 30,1976, p. 93-94. Southern Midcontinent file at the University of Okla- Mid-Dinantian buildups from Europe and North America: Geological 19773, Sedimentary petrology and lithofacies of the Pitkin Formation homa. See Webb (1984) for more detailed locality Journal, v. 20, p. 159-180. in western Madison and eastern Washington Counties, Arkansas [M.S. Lees, A., Hallet, V., and Hibo, D., 1985, Facies variations in Waulsortian thesis]: Fayetteville, Arkansas, University of Arkansas, 86 p. information. buildups, pan 1; A model from Belgium: Geological Journal, v. 20, Webb, G. E., 1984, Coral fauna and carbonate mound development, Pitkin MS 457—located in the abandoned quarry at p. 133-158. Forrration (Chesterian), North America [M.S. thesis]: Norman, Okla- McCaleb, J. A., Quinn, J. H., and Furnish, W. M., 1964, Girtyoceratidae in the homa University of Oklahoma, 267 p. NEWNWWSWW sec. 4, T 14 N., R. 30 W., Washing- southern midcontinent: Oklahoma Geological Survey Circular 67, 1987, The coral fauna of the Pitkin Formation (Chesterian), northeast- ton County, Arkansas. p. 41. ern Oklahoma and northwestern Arkansas: Journal of Paleontology, MS 460—located in the south-facing bluff of the McKinney, F. K., 1979, Some paleoenvironments of the coiled fenestrate v. 61, p. 462-493. bryozoan Archimedes, in Larwood, G. P., and Abbott, M. P., eds., Wolf, K. H, 1965, Pedogenesis and palaeoenvironment of Devonian algal east-west-trending ridge at N^SE^NW^ sec. 27, Advances in bryozoology: New York, Academic Press, The Systematics limestones of New South Wales: Sedimentology, v. 4, p. 113-178. T. 15 N., R. 29 W, Washington County, Arkansas. Association Special Volume 13, p. 321-335. Mamet, B. L., and Roux, A., 1975, Algues devoniennes et carbonifères de la MS 461-located in the south-facing bluff north Tethys occidentale: Revue de Micropaléontologie, v. 18, p. 134-187. of the White River at NE^NW'A sec. 34, T. 15 N., — 1978, Algues viséenes et namuriennes du Tennessee (Etats-Unis): Revue R. 28 W., Madison County, Arkansas. de Micropaléontologie, v. 21, p. 68-97. Manger, W. L., Pinkley, R., and Webb, G. E., 1984, Late Mississippian lime MS 463—located -90 m west of the mound at MS mud mounds, Pitkin Formation, northern Arkansas [abs.]: American 461, in the same blu ff at NE'ANW'A sec. 34, T. 15 N., Association of Petroleum Geologists Bulletin, v. 68, p. 503. Monty, C.L.V., 1976, The origin and development of cryptalgal fabrics, in R. 28 W., Madison County, Arkansas. Walter, M. R., ed., Stromatolites: Amsterdam, Elsevier, Developments MS 464—exposed in the southeast-facing bluff at in Sedimentology 20, p. 193-249. NWViSE^SW'/i sec. 29, T. 16 N., R. 27 W., Madison Nageotte, A. L., 1981, Lithostratigraphy and depositional environments of the Pitkin Limestone (Chesterian, Mississippian) in portions of Cherokee, County, Arkansas. Muskogee, and Sequoyah Counties, Oklahoma [M.S. thesis]: Norman, MS 472—located on the west side of U.S. Highway Oklahoma, University of Oklahoma, 197 p. 65 at SE^NE'ANEVi sec. 2, T. 13 N., R. 15 W., Searcy Nodine-Zeller, D. E., 1977, Microfauna from Chesterian (Mississippian) and Morrowan (Pennsylvanian) rocks in Washington County, Arkansas, County, Arkansas. and Adair and Muskogee Counties, Oklahoma, in Sutherland, P. K., MS 474—exposed just north of MS 472 at and Manger, W. L., eds.. Upper Chesterian-Morrowan stratigraphy and S^SE'ASE^ sec. 35, T. 14 N., R. 15 W., Searcy the Mississippian-Pennsylvanian boundary in northeastern Oklahoma MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 20,1986 and northeastern Arkansas: Oklahoma Geological Survey Guide- REVISED MANUSCRIPT RECEIVED APRIL 6,1987 County, Arkansas. book 18, p. 89-99. MANUSCRIPT ACCEPTED APRIL 8, 1987

Printed in U.S.A.

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