Ordovician of the Sauk Megasequence in the Ozark Region of Northern Arkansas and Parts of Missouri and Adjacent States
Published:January 01, 2012
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Raymond L. Ethington, John E. Repetski, James R. Derby, 2012. "Ordovician of the Sauk Megasequence in the Ozark Region of Northern Arkansas and Parts of Missouri and Adjacent States", Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia, James Derby, Richard Fritz, Susan Longacre, William Morgan, Charles Sternbach
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Exposures of Ordovician rocks of the Sauk megasequence in Missouri and northern Arkansas comprise Ibexian and lower Whiterockian carbonates with interspersed sandstones. Subjacent Cambrian strata are exposed in Missouri but confined to the subsurface in Arkansas. The Sauk-Tippecanoe boundary in this region is at the base of the St. Peter Sandstone. Ulrich and associates divided the Arkansas section into formations early in the 20th century, principally based on sparse collections of fossil invertebrates. In contrast, the distribution of invertebrate faunas and modern studies of conodonts will be emphasized throughout this chapter. Early workers considered many of the stratigraphic units to be separated by unconformities, but modern analysis calls into question the unconformable nature of some of their boundaries. The physical similarity of the several dolomites and sandstones, complex facies relations, and lack of continuous exposures make identification of individual formations difficult in isolated outcrops.
The oldest formation that crops out in the region is the Jefferson City Dolomite, which may be present in outcrops along incised river valleys near the Missouri-Arkansas border. Rare fossil gastropods, bivalves, brachiopods, conodonts, and trilobites permit correlation of the Cotter through Powell Dolomites with Ibexian strata elsewhere in Laurentia. Conodonts in the Black Rock Limestone Member of the Smithville Formation and the upper part of the Powell Dolomite confirm regional relationships that have been suggested for these units; those of the Black Rock Limestone Member are consistent with deposition under more open marine conditions than existed when older and younger units were forming. Brachiopods and conodonts from the overlying Everton Formation assist in interpreting complex facies within that formation and its correlation to equivalent rocks elsewhere. The youngest cono-donts in the Everton Formation provide an age limit for the Sauk-Tippecanoe unconformity near the southern extremity of the great American carbonate bank. The correlation to coeval strata in the Ouachita Mountains of central Arkansas and in the Arbuckle Mountains of Oklahoma and to rocks penetrated in wells drilled in the Reelfoot rift basin has been improved greatly in recent years by integration of biostratigraphic data with lithologic information.
Ordovician strata deposited on the great American carbonate bank (GACB) (Figure 1) crop out in Arkansas (Figures 2, 3) between the border, with Missouri to the north and the Devonian and Carboniferous rocks of the Boston Mountains on the south (Figure 4). Exposures of St. Peter Sandstone and overlying Middle and Upper Ordovician carbonate units occupy the surface across much of this region (Figure 3). Scattered outcrops of Or-dovician units continue northward into Missouri on the east side of the Ozark dome (Figure 2). The Ordovician outcrops also turn northward into Missouri at their western extremity as part of the broad Ozark plateau that encircles the Ozark dome.
Rocks older than the St. Peter Sandstone crop out in two subregions where younger Ordovician rocks have been stripped away (Figure 3). Lower Ordovician rocks are displayed in valleys of the Buffalo and White Rivers in a western area that extends from central Boone County to near Calico Rock in western Izard County. Farther east, regional erosion removed younger Or-dovician rocks down through the St. Peter Sandstone in a triangular area that encompasses most of Fulton, Randolph, Sharp, and Lawrence Counties, thereby exposing Lower Ordovician strata. Strata older than mid-Ibexian (Stairsian) are restricted to the subsurface in northern Arkansas, but they crop out in a broad region in the Ozark dome in Missouri (Palmer et al., 2012). The Ordovician units continue southward beneath the Boston Mountains, where subsurface information suggests that the GACB carbonates grade into slope and basinal detrital sediments and cherts of the Ouachita facies (Viele and Thomas, 1989). Late Paleozoic orogenic activity eventually transposed the deep-water deposits over the marginal part of the carbonate bank via longdistance thrust faulting, thereby making the exact location of the original boundary between them obscure.
Outcrops of GACB rocks terminate abruptly on the east, where they are overlain by Cretaceous and Ceno-zoic deposits in the Mississippi Embayment (Figure 3). Sauk strata thicken abruptly in the subsurface of northeastern Arkansas into anearly Paleozoic rift zone, which was named the Reelfoot basin by Schwalb (1969), and now is identified as the Reelfoot rift basin. Sedimentation in the rift began with Lower or Middle Cambrian coarse clastics (Houseknecht, 1989), but younger rocks include shelf to outer-shelf carbonates, basinal shaly carbonates, shales, and possible turbidites. Recent investigations in the New Madrid seismic zone indicate that the structure and stratigraphy of the rift and adjacent areas are more complex than previously believed, but studies like those already mentioned show that the relatively thin shallow-water Sauk sequences on the Ozark and Kentucky-Tennessee shelves thicken considerably and locally pass into deep-water facies in the Reelfoot rift basin (Schwalb, 1982; Collins et al., 1992).
Stratigraphic studies in northern Arkansas are hampered by complex facies relations, by thick cover of soil and vegetation that causes outcrops to be localized and to exhibit thin sequences, and by marked physical similarity of several of the recognized formations. Calcareous fossils are sparse as a result of pervasive dolomitization. For example, Cullison (1944) reported and illustrated Or-dovician fossils from southern Missouri and northern Arkansas, but most of them are moldic specimens in chert fragments that were obtained from weathered residuum. Their original stratigraphic position must be interpreted in many cases. Although not abundant, fossils allow the correlation of the Ozark section with Ordovician rocks elsewhere in Laurentia (Figure 1).
History of Stratigraphic Studies in Northern Arkansas
Owen (1858) mentioned lower Paleozoic rocks in the southern mid-continent in his report of a reconnaissance of the region. Early work on Lower Ordovician rocks of northern Arkansas was limited to a western region where attention was focused by lead-zinc ores then being exploited there. Formations named but not described in detail in these studies were accepted in later work to the east, where two additional units were established. Despite the presence of interbedded sandstones within this Ordovician succession, early workers focused on limestone and dolomite intervals, which led to the recognition of all of the exposed rocks beneath the St. Peter Sandstone as the Magnesian Limestone (Branner, 1929, in Wise et al., 1975; Hedden, 1976).
Adams (1904, p. 18–19) substituted Yellville Formation for Magnesian Limestone, and that name was used for a short time. Adams (1904) assumed subsurface presence in Arkansas of Cambrian strata correlative to those already known in southeastern Missouri because a well in the vicinity of Batesville, Independence County, penetrated more than 1500 ft (460 m) of sedimentary rocks without reaching Precambrian basement. He therefore restricted the name Yellville to just the Ordovician rocks beneath the St. Peter Sandstone. In the same publication, Ulrich (1904) recommended that the Yell-ville be divided into a lower unit in which fossils are rare and an upper one with a more abundant and diverse fauna that he believed most likely to demonstrate equivalence with the Shakopee Formation of the upper Mississippi Valley region.
Purdue (1907) introduced the name Everton for limestone intervals immediately beneath the St. Peter Sandstone in northwestern Arkansas. Ulrich (1911, p. 667) reinterpreted the lowest part of Adams’ Yellville as a southward continuation of the Jefferson City Dolomite of central Missouri and redefined Yellville to consist of “all of the rocks between the top of the Ozarkian Jefferson City Dolomite and the base of the Everton Limestone.” Purdue and Miser (1916) credited Ulrich (1911) for division of the retained Yellville into a lower dolomite sequence that he named Cotter Dolomite and an overlying magnesian limestone unit, the Powell Dolomite. Much later, Hedden (1976) recommended resurrecting Yellville as the name for a group comprising the Jefferson City, Cotter, and Powell Dolomites this is the original concept of Yellville raised from formation to group status.
McKnight (1935) was unable to assign any exposed rocks in the Arkansas lead-zinc district to the Jefferson City Dolomite. Wise et al. (1975) did not find rocks that they believed to represent the Jefferson City Dolomite in northeastern Arkansas, whereas Hedden (1976) stated that exposures of it are present there along the channels of creeks in the vicinity of the Missouri-Arkansas border.
Verification of outcrops of Jefferson City Dolomite in northern Arkansas requires a means of consistently distinguishing that unit from the others within a succession of physically similar formations.
Study of the rocks of the GACB in northern Arkansas in the 20th century focused on recognizing the regional distribution of the major lithologic units and on their fossil content. Whittington (1968) correlated the interval beneath the Everton Formation in the Ozark region (Cotter through Black Rock units), with the upper Arbuckle Group (Kindblade and West Spring Creek Formations) and the lower Simpson Group (basal Joins Formation) of Oklahoma and with Zones G through J in the Lower Ordovician of northeastern Utah and southeastern Idaho (Ross, 1951) and in western Utah and Nevada (Hintze, 1951, 1973). Detailed sedimentologic studies are lacking for the lithologically monotonous lower units (Cotter, Powell dolomites), but Young et al. (1972) briefly summarized the variety of carbonate rock types exhibited in each of them and inferred that they were deposited in very shallow water nearshore environments. Suhm (1973) presented detailed discussions of facies relationships in the western outcrops of the Everton Formation, and Minke (1969) interpreted their depositional environment from information provided by petrologic analysis.
Acting on a recommendation from Ulrich (Purdue and Miser, 1916) established the Cotter Dolomite for exposures near Cotter in western Baxter County, Arkansas (Figures 2, 3). It includes ledges of fine-grained argillaceous dolomite for which the term “cotton rock” has been applied because of their chalky appearance when weathered. Alternating with them are intervals of more coarsely crystalline dolomite that project in exposures between recessive beds of cotton rock. Thin beds of sandstone and of greenish shaly material also are present. Purdue and Miser (1916) stated that the Cotter ranges in thickness from 245 to 455 ft (75–120 m) in its type area, and Wise et al. (1975) suggested comparable thicknesses for exposures in northeastern Arkansas. Young et al. (1972) concluded that the Cotter Dolomite was deposited in fluctuating supratidal and intertidal settings.
Cullison (1944) recognized the Cotter Dolomite in southern Missouri. He defined a Theodosia Formation (with two members: lower Lutie and upper Blackjack Knob) for strata that underlie the Cotter Dolomite, but he believed that these units are not exposed in Arkansas. The Theodosia Formation has not been widely recognized subsequently, and Hedden (1976) subsumed it in an expanded concept of the Cotter Dolomite. He added an upper Bull Shoals Member that corresponds to the Cotter Dolomite as originally defined. The Cotter Dolomite (but not its members) has been identified widely in Missouri, where it overlies the Jefferson City Dolomite. The lithologic similarity of the two units makes recognition of their boundary difficult, leading to the commonly used designation of Jefferson City-Cotter in central Missouri (Thompson, 1991).
Purdue and Miser (1916) considered the contact between the Cotter Dolomite and the overlying Powell Dolomite to be an unconformity marked by chert breccia. McKnight (1935) reassigned the cherty horizon to the upper Cotter Dolomite and declared the formations to be conformable, although he suggested the possibility of “slight emergence” (p. 26) during deposition of the lower Powell Dolomite. Hedden (1976) concluded that the Cotter and Powell Dolomites inter-finger in northeastern Arkansas, and treated them as transitional units. He reasoned that an unconformity between the Cotter and the Powell Dolomites, if such exists in northwestern Arkansas, disappears in the region between their western and eastern outcrops.
Purdue and Miser (1916) reported fossil nautiloids and gastropods from the Cotter Dolomite. They emphasized the presence at many levels of silicified hemispherical features that they identified as Cryptozoon minnesotense and interpreted as fossil plants; these would be reported today as domal stromatolites. Collectively, sparse mollusks from the Cotter Dolomite indicated to them an Early Ordovician age and equivalence with part of the Shakopee Formation of the upper Mississippi Valley region and the Beekmantown Limestone of upstate New York.
Ulrich and Cooper (1938) described three species of brachiopods, Syntrophopsis alabamensis, S. minor, and Polytoechia inaequistriata from the Cotter Dolomite near Yellville in Marion County. Cullison (1944) found two of these species, P. inaequistriata and S. arkansasensis, in the Black Ledge Member of the Powell Dolomite, and S. minor was reported by Ulrich and Cooper (1938) from both the Smithville and the Cotter units, which provide support for the contention of Hedden (1980) that the formations may be partly equivalent temporally.
Yochelson and Bridge (1957) reported Ceratopea ankylosa and possibly C. tennesseensis from the Cotter Dolomite. The occurrence of these gastropod operculae suggests correlation of the Cotter Dolomite with part of the El Paso Limestone of west Texas, intervals within the Kindblade Formation of southern Oklahoma, the Mascot Dolomite of eastern Tennessee, and the Rock-dale Run Formation in Maryland, as well as with the uppermost Honeycut Formation of Texas (Derby et al., 1991, figure 3; Miller et al., 2012, figure 8). These cor-relationsare supportedbyconodonts from Cotterequiv-alents in southwestern Missouri (collections of Ethington), suggesting that a lower part of this formation could be as old as the low-diversity conodont interval (Ibexian, Stairsian Stage; Tremadocian; Zone D of Ross, 1951; Hintze, 1951) described by Ethington and Clark (1982).
The Powell Dolomite consists mostly of fine-grained dolomite, although it was described originally as limestone. The base of the Powell Dolomite commonly is placed beneath the chert breccia that was believed to be evidence of unconformity with the Cotter Dolomite.
Distinguishing the Powell Dolomite from the Cotter Dolomite is difficult because of isolated exposures; criteria for recognition include assertions that cotton rock is more abundant and stromatolites are absent or rare in the Powell Dolomite as compared with the Cotter Dolomite. Reported thicknesses of the Powell Dolomite range markedly from less than 100ft to as much as 164 ft (a few tens to as much as 50 m). Hedden (1976) reported that thicknesses of the Powell Dolomite in the subsurface range widely from a low of 75 ft (23 m) to as much as 500 ft (152 m).
A distinct 5- to 10-ft (2- to 3-m)-thick marker bed in the lower Powell Dolomite of northwestern Arkansas is termed the Black Ledge because of the dark color it displays in weathered outcrops. The Black Ledge has not been reported in Powell outcrops in northeastern Arkansas. Hedden (1976) considered this thin unit to be a tongue of the Smithville Formation of northeastern Arkansas that interfingers with typical Powell Dolomite in the western area. Cullison (1944) observed that most of the fossils he obtained from the Powell Dolomite were found in the Black Ledge.
Martin et al. (1961) reported outcrops of the Powell Dolomite in Saint Genevieve and Cape Girardeau Counties in Missouri and believed them to be present in the subsurface in southwestern Missouri. Thompson (1991) suggested that some of the Powell Dolomite reported in Missouri may be correlative with the upper Cotter Dolomite of Arkansas, noting that identification of the Powell Dolomite in Missouri commonly has been based solely on insoluble residue contents. According to Thompson (1991), the Black Ledge unit of Arkansas has not been recognized in any of the purported Missouri occurrences of the Powell Dolomite.
Invertebrate fossils are scarce and poorly preserved in the Powell Dolomite, but a variety of gastropods, nautiloids, brachiopods, and trilobites have been reported. They confirm an Early Ordovician age, but precise correlation with sections in other regions is lacking. Cullison (1944) reported the brachiopods Diparelasma silicum, Syntrophopsis arkansasensis, S. grandis, and Poly-toechia inaequistriata from the Black Ledge; D. silicum has been found in an exposure of the Powell Dolomite near Marble Hill in Bollinger County, Missouri (Ulrich and Cooper, 1938). Syntrophopsis arkansasensis and P. inaequistriata also have been found in the underlying Cotter Dolomite, and S. arkansasensis is known from the Smithville Formation at Smithville, Lawrence County, Arkansas.
Conodonts in U.S. Geological Survey collections from high in the Powell Dolomite at Kyles Landing in Newton County, from near Berryville in Carroll County, and from near Calico Rock in Izard County are dominated by advanced morphotypes of Diaphorodus delicatus. This species is part of an association including Colaptoconus quadraplicatus, Eucharodus parallelus, and Tropodus comptus, whose collective range spans much of the middle and upper Lower Ordovician (Ibexian; Tulean and Blackhillsian Stages; upper Tremadocian and lower Floian). These species occur in the type area of the Jefferson City Dolomite in Missouri (Branson and Mehl, 1933; Ethington and Repetski, 1984), in the Shakopee Dolomite in the upper Mississippi Valley (Furnish, 1938), and in the Kindblade and lower and middle West Spring Creek Formations in Oklahoma (Brand, 1976; Felton, 1979). They were adapted to longstanding environmental conditions that produced shallow-water dolomites in northern Arkansas, and they remained the dominant conodont population in the region as long as those conditions persisted. Because they share long stratigraphic ranges, they do not permit high-resolution zonation of the rocks in which they are found. The occurrence near Kyles Landing of advanced morphotypes of D. delicatus indicates that the upper Powell Dolomite is quite high in the Ibexian, and they provide approximate temporal control for the age of the Powell-Everton boundary.
Smithville Formation and its Black Rock Limestone Member
Ulrich (1911) expanded his study of lower Paleozoic rocks in northern Arkansas to outcrops in Lawrence County near the village of Smithville, Arkansas, where he recognized two lithologic units that he believed tobe absent farther west. The lower of these, 40 ft (12 m) of dolomite restricted to Lawrence County, was deemed to be important because of the presence of graptolites. This unit also contained molluskan operculae for which he advanced the name Ceratopea in an earlier section of the chapter. Ulrich (1911) stated that the higher of these units contains diverse fossils including sponges and bryozoans as well as brachiopods and graptolites, and he made passing reference that these fossils also occur in outcropsin Christian County, southwestern Missouri. Branner (1929) introduced the names Smithville Formation and Black Rock Limestone Member for these units on the geologic map of Arkansas and credited UIrich with authorship. They probably were adopted from Ulrich’s unpublished notes, which McKnight (1935) mentioned as the source of information plotted on the map.
Smithville and Black Rock equivalents in southeastern Missouri were suggested by Heller (1943) and Martin et al. (1961), but definitive evidence for these assertions was not provided until Fix (1975) presented a detailed discussion of outcrops in Bollinger and Cape Girardeau Counties, Missouri. Repetski and Weary (1993) reported conodonts indicating rocks as young as the Black Rock Limestone Member in deep wells drilled in the Reelfoot rift basin in extreme eastern Arkansas and westernmost Tennessee (Figure 5).
Ulrich (1911) distinguished the Smithville Formation from the Black Rock Limestone Member by their respective fossil assemblages instead of by contrasting lithologies. McKnight (1935) stated that Ulrich believed the Black Rock Limestone Member to be conformable with the overlying Everton but argued instead that these units are eastern facies within the lower part of the Everton Formation. Caplan (1954) also believed the Smithville Formation to be equivalent to part of the lower Everton Formation and suggested a Chazyan age. He was uncertain whether the Black Rock Limestone Member is conformable with the Smithville Formation, which he believed to be Chazyan. At that time, he considered the Black Rock Limestone Member also to be equivalent to part of the Everton Formation, but Caplan (1954) reinterpreted it later as a facies of the Powell Dolomite.
Wise et al. (1975) reported the Smithville Formation to consist of 500 ft (150 m) of finely crystalline buff dolomite overlain by 100 ft (30 m) of Black Rock Limestone Member. Fossils are not abundant in the dolomites, but they mentioned ostracods, gastropods, cephalopods, graptolites, and trilobites, most identified at the generic level. Ceratopea unguis, which they found at many outcrops, was considered by them to be sufficient evidence for identifying the Smithville Formation in the absence of other criteria. Like Ulrich (1911), Wise et al. (1975) considered the Black Rock fossils to represent a distinct biofacies from those of the Smithville Formation; they treated the Black Rock as a local physical facies and a member of the Smithville Formation.
Hedden (1976) credited Ernest Glick with the addition of Black Rock and Smithville to a revised geologic map of Arkansas (Haley et al., 1976). Hedden’s (1976) massive study of the Smithville Formation and Black Rock and their relationship to the Powell Dolomite was based on analysis of cores from numerous drilled holes and scattered exposures. He concluded (p. 56) that the Smithville Formation is “a complexly shaped lithosome which is intertongued with lithofacies of the Powell Dolomite... and with the Black Rock lithosome.” Furthermore, he accepted the Smithville as a local aspect of the Powell Dolomite instead of a separate formation, and reasoned that intertonguing of the Smithville with the Black Rock justifies considering the latter also to be a local development within the Powell Dolomite.
As part of a study of outcrops in southeastern Missouri, McLeod (1979) agreed that the Smithville and Black Rock are intertonguing facies. McLeod (1979) and Hedden (1976, 1980) both reasoned that the li-thologies and faunal elements displayed by the Powell Dolomite of western Arkansas and by the Smithville Formation are suggestive of very shallow nearshore marine environments, whereas those of the Black Rock indicate an offshore marine environment above wave base. Hedden (1976) reported the aggregate thickness of these units to be about 475 ft (∼145 m) throughout eastern Arkansas, with the Black Rock thickening eastward from 101 ft (31 m) near the village of Smithville to 309 ft (94 m) at the village of Black Rock, both in Lawrence County. Conversely, the Smithville Formation measures 369 ft (112 m) on the west versus 167 ft (51 m) on the east. Hedden (1976) suggested that progressive subsidence had occurred in the Reelfoot Rift at the time that Smithville-Black Rock strata were being deposited in a westwardly transgressing epeiric sea. As a consequence, a westerly thinning wedge of the Black Rock was formed above an easterly thinning wedge of Smithville Formation, resulting in systematic reciprocal thicknesses of these two units.
The spatial distribution of the Smithville Formation and Black Rock Limestone Member is consistent with a model Harris (1973) proposed for the origin of the widespread dolomites of the Sauk megasequence in central and eastern United States including those of northern Arkansas. Harris (1973) postulated that the shallow and gently shelving Sauk sea did not permit thorough mixing of water, and strong evaporation in tropical latitudes resulted in a marked horizontal salinity gradient normal to the shoreline. He argued that these conditions caused dolomite to be deposited across a broad region in a belt of high salinity separated by a narrow transitional interval from a belt farther seaward of normal marine salinity where limestone accumulated. Tectonic and/or eustatic factors caused these environments and their respective deposits to migrate, thereby producing a spatial relation between dolomite and limestone like that proposed for the Smithville and its Black Rock Limestone Member.
Yochelson and Wise (1972) reported C. unguis to be the most abundant fossil in the type area of the Smithville Formation. Ceratopea unguis is followed in order of relative abundance in their collections by nau-tiloids, high- and low-spired gastropods, bivalves, and brachiopods. Rohr et al. (2004) summarized the known occurrences of C. unguis, which indicate that this species is endemic to Laurentia where it occurs in rocks of latest Ibexian age (Blackhillsian, Cassinian Stages). In addition to its presence in the Smithville Formation in Arkansas and Missouri, they noted that it has been recorded from the Nunatami and Wandel Valley Formations of north Greenland, the Aguathuna Formation of western Newfoundland, the Providence Island Dolomite of New York, the Rockdale Run Formation in Virginia, the West Spring Creek Formation in southern Oklahoma, the Skoki Formation in British Columbia, and the “Cherty Unit” of the Northwest Territories.
Ulrich (1911) reported graptolites identified as Phyllo-graptus illicifolius, P. angustifolius, Didymograptus bifidus, and D. amplus, accompanied by orthid brachiopods and fragmented trilobites from a locality 3 mi (5 km) northwest of Smithville. He considered this occurrence to be in the lower part of the Yellville Formation as he conceived it, but today it would be assigned to the Smithville Formation. Decker (1944) found D. bifidus at a locality near Black Rock, Arkansas, and D. protobifidus at another locality near Smithville, both in Lawrence County. Berry (1970) reported more graptolites from the Smithville Formation including Didymograptus artus and a new species, D. smithvillensis. He concluded that they most likely indicate a stratigraphic position near the boundary between Lower and Middle Ordovician as it was recognized at that time. In contrast to this, Hedden (1976, p. 165) declared that “graptolites do not occur in the Smithville Dolomite but are restricted to the Black Rock Lithosome.” This disagreement perhaps can be attributed to difficulty in confidently assigning isolated outcrops of dolomite in northern Arkansas to named formations.
In their monographic treatment of Upper Cambrian and Lower Ordovician brachiopods, Ulrich and Cooper (1938) listed 12 species from the Black Rock and Smithville units, all of them new; eight were restricted to the Black Rock, two to the Smithville, and two were found in both units. They found a diversity of brachiopods in the Black Rock at the quarry near Black Rock (Lawrence County) that includes Syntrophopsis magna, Polytoechia alabamensis, Tritoechia transversa, T. incurva, Diparelasma mesleri, D.?magnum, D. extensum, and Syntrophia torynifera. They also reported the presence of S. minor and of S. cf. S. arkansasensis in the Smithville Formation at a locality about 1 mi (∼1.6 km) northeast of Smithville, Arkansas, and in the Cotter Dolomite at two localities along the railroad 1.5 and 2.0 mi (2.4 and 3.2 km) east of the depot at north Yellville, Marion County. Because the specimens from near Smithville town clearly are in the type area of the Smithville Formation, Derby (letter to Ross, 1973) suggested that the rocks near Yellville identified as Cotter Dolomite are equivalent to part of the Smithville Formation or are misidentified as to formation.
Derby (same 1973 letter) identified Diparelasma typ-icum along with many other species of brachiopods and the trilobites Goniotelina brevis and G. brighti in samples from the Verkler quarry in Lawrence County, Arkansas. These brachiopod and trilobite species from the Black Rock and Smithville units strongly indicate a correlation with strata in the upper half of the West Spring Creek Formation of Oklahoma (see Derby et al., 1991, their figure 14).
The Black Rock Limestone Member facies expands in thickness eastward into the Reelfoot rift basin (see below). Evidence from ostracods, brachiopods, and tri-lobites indicates that the Black Rock and a unit termed the “super-Black Rock” thicken and transition into limestone facies in the Reelfoot rift basin (Figure 6A). Derby (1982) reported on fossil invertebrates found in insoluble residues of samples from two wells drilled in the Reelfoot rift basin. Initial study of the wells by M. and E. McCracken of the Missouri Division of Geology and Water Resources (unpublishednotes; accessed by Derby, 1968) identified Everton dolomite, a limestone unit, and a supra-Black Rock unit that contains lower Whiterockian faunas in both wells (see Figures 6A, 7).A20-ft (6-m)-intervallogged as Black Rock Limestone Member in the Tenn-Ark 1 Ruby Martin well in Craig-head County, Arkansas, yielded silicified specimens of the brachiopods Pomatotrema and Diparelasma at 2347 ft (716 m). Another core interval contained the trilobite Dimeropygiella at 2964 ft (904 m). A thicker interval interpreted to include the Black Rock Limestone Member in the Ken-Ten 1 Sanger in Fulton County, Kentucky, contained brachiopods, including Diparelasma and Poma-totrema as well as a specimen provisionally identified with Syntrophopsis. In the same report, Derby also identified a species of Diparelasma that was found in the U.S. Geological Survey 1 Fort Pillow well, Lauderdale County, Tennessee. He interpreted this assemblage of brachiopods together with the few trilobites to be representative of Zone Jin the succession of Laurentian Lower and Middle Ordovician biostratigraphic units that was recognized at that time (Hintze, 1951; Ross, 1951). He believed these occurrences to indicate that the super-Black Rock unit in these wells is equivalent to the Everton Formation in the Arkansas outcrops and that no hiatus exists between the Smithville-Black Rock and the Everton in the thick sequences in the Reelfoot rift basin.
Wise et al. (1975) mentioned a sparse conodont fauna from the Smithville Formation including E. parallelus (then identified as Drepanodus subarcuatus) and C. quad-raplicatus (identified as Scolopodus quadraplicatus). These long-ranging Lower Ordovician conodonts occur in the Powell Dolomite in northwestern Arkansas. Other known occurrences are in the Cotter Dolomite of Missouri, the Shakopee Formation of Iowa and Wisconsin, the Kind-blade and lower West Spring Creek Formations in Oklahoma, and the Fillmore Formation in the eastern Great Basin.
McLeod (1979) listed conodonts from the Black Rock (identified by Repetski; U.S. Geological Survey collections 8807-CO and 8808-CO), and Ethington and Repetski (1986) reported conodonts from measured sections near the villages of Smithville and Black Rock. These sections demonstrate that at least 10 m (33 ft) of dolomite typical of the Smithville lie above the Black Rock limestone, reinforcing the concept of Wise et al. (1975) and McLeod (1979) that the Black Rock is a member of the Smith-ville Formation.
In an investigation of Lower Ordovician rocks in the subsurface of the Black Warrior Basin (Mississippi), Reelfoot rift basin, and eastern Arkoma basin (Arkansas), Alberstadt and Repetski (1989) reported the presence of limestones characterized by abundant sponges, conodonts, and algae including Nuia, Sphaerocodium, and Girvanella. They observed that sponge-algal deposits occur along the southern margins of the Laurentian platform from the southern Appalachian region to the Great Basin and contrasted them with the shallow-water deposits, mostly dolomites, which are the norm for most of the Lower Ordovician of the North American craton. The conodonts in the sponge-algal rocks are dominated by species believed to have inhabited relatively cool water environments, and Alberstadt and Repetski (1989) suggested that the sponge-algal rocks represent incursion of cooler oceanic water onto the shallow margin of the craton that occupied subtropical latitudes. Many of these species are present in the upper Smithville Formation and its Black Rock Limestone Member including Juanognathus variabilis, Oepikodus communis, Protopanderodus gradatus, Protopanderodus leonardii, Reuterodus and in us, and Scolopodus emarginatus. The presence of oceanic forms including O. evae and Bergstroemognathus extensus exclusively in the Black Rock Limestone Member and of shallow-water Diaphorodus delicatus only in the Smithville Dolomite was cited as evidence that the Black Rock is “a small exposed segment of the more extensive limestone-dolostone transition” along the margin of the Ordovi-cian shelf (Alberstadt and Repetski, 1989, p. 240).
These conodonts are present in the upper Ibexian (upper Floian; R. and inus Biozone) strata elsewhere in North America including high in the El Paso Group of western Texas and the uppermost Fillmore and Wah Wah Formations of western Utah, as well as in the San Juan Formation in Precordilleran Argentina. They confirm Derby’s (1973, 1982) correlation of rocks containing the Black Rock fauna with the upper, but not uppermost, West Spring Creek Formation in southern Oklahoma and with Zone J in the Wah Wah Formation in Utah (high Ibexian Blackhillsian Stage; upper Floian; see Ross, 1951; Hintze, 1973, for Zone J).
The name Everton was introduced by Purdue (1907) for limestone sandwiched between the St. Peter Sandstone above and an older sandstone in the Missouri-Arkansas border region. Purdue (1907), who credited Ulrich with the name, reported (p. 252) that “he named the limestone Everton, from the town of Everton, Boone county, Arkansas.” Purdue (1907) also discussed and illustrated significant relief at the top of the Everton Formation in northwestern Arkansas where it is penetrated deeply by the St. Peter Sandstone in features that he interpreted as caves. Ulrich (1911) mentioned the Everton Formation briefly in his attempt to reorganize the lower Paleozoic rocks of North America, and his correlation chart shows the Everton Formation enclosed between unconformities that separate it from underlying Yellville and overlying units identified variously as St. Peter Sandstone or (incorrectly) as Joachim Formation. Most subsequent reports followed Ulrich (1911) in considering the Everton Formation to rest unconformably on the Powell Dolomite. However, Hedden (1976) dissented, remarking that evidence for an unconformity beneath the Everton Formation is not conclusive suggesting that the Everton is conformable above the Black Rock Limestone Member in northeastern Arkansas. Suhm (1974) recommended that the Everton Formation be treated as a Chazyan substage that is recognizable in the southern mid-continent.
Purdue and Miser (1916) subdivided the Everton Formation into three units in the vicinity of Eureka Springs Carroll County and Harrison, Boone County, Arkansas: a basal carbonate sequence they designated the Sneeds Limestone, which is overlain by an eastwardly wedging sandstone they called the Kings River Sandstone, followed in turn by a succession of interbedded limestones and sandstones that they identified collectively as Everton Limestone. This succession was interpreted as a near-shore deposit laid down along the coast of a shallow transgressing sea. Purdue and Miser (1916) misidentified sandstone that crops out along the valley of the Buffalo River in the southern part of the Harrison Quadrangle (Boone County) as St. Peter, which led them to interpret dolomite above the sandstone as the Joachim Formation. They presumed that Joachim Formation is succeeded by about 50 ft (∼15 m) of limestone, which they named the Jasper Limestone. Their misinterpretation may have led Croneis (1930) to report that the Jasper Limestone in Newton County, Arkansas, occupies the stratigraphic position of the contact between the Joachim and Plattin Formations farther to the east in Izard County. Croneis (1930) also concluded that the Ordovician rocks of the Ozark region in Arkansas offer little probability of containing significant amounts of petroleum or natural gas, an opinion that was seconded subsequently by Caplan (1954, 1960 in Fix, 1975).
McKnight (1935, p. 38) discussed the regional range in lithologic characters displayed by the Everton Formation. He noted that it consists of alternating units of limestone, dolomite, and sandstone but observed that these are not arranged systematically so that “the section at any one locality is in general not duplicated elsewhere, except in a very rough way.” McKnight (1935) lamented that this extreme lithologic variation from place to place renders regional interpretation difficult. His particular frustration resulted from the presence within the Everton Formation of intervals of sandstone that are indistinguishable from each other and from the overlying St. Peter Sandstone. McKnight (1935) included among these the Kings River Sandstone of Purdue and Miser (1916), which he showed to thin eastward and to be absent or nearly so in the Yellville Quadrangle (Marion County) and the Calico Rock Sandstone, described and named by Giles (1930) for outcrops in Izard County. He also proposed the name Newton Sandstone for outcrops in the southeastern part of the Harrison Quadrangle that had been misidentified previously as St. Peter by Ulrich (in Purdue and Miser, 1916). McKnight (1935) reinterpreted the outcrops identified as Joachim and Jasper in these earlier reports to be members of the Everton Formation.
Lantz (1950) described 482 ft (147 m) of Ordovician strata in the Arkla Gas 1 Barton drilled on the Cecil anticline in Franklin County, Arkansas (Figure 2; loc 1). Pertinent to this report, Lantz (1950) described 100 ft (30 m) of St. Peter Sandstone and 240 ft (73 m) of Everton Formation from cable tool samples, which include 17 ft (9 m) of black shale as petroleum source rocks. Suhm (1997, figures 12,16) apparently assigned the bulk of this Everton sandstone sequence to the Newton Sandstone Member of the Everton Formation. The Arkla 1 Barton was plugged back and produced gas at the rate of 7 mmcf/day from the Pennsylvanian (Morrowan) Hale Formation.
Exposures of Everton Formation have been identified along the eastern flank of the Ozark dome from Scott County, Missouri, to near Festus in Jefferson County, about 25 mi (40 km) south of St. Louis (Martin et al., 1961) (Figure 1). The southernmost of these exposures exceeds 400 ft (122 m) in thickness, but the formation thins to the north and is not believed to extend far beyond the Festus outcrops. Golden (1969) obtained conodonts that confirm correlation of these Missouri strata with the Everton Formation of Arkansas.
Thompson (1991) reported that as much as 85 ft (26 m) of the Everton Formation has been found in wells drilled on the north flank of the Ozark Dome in Boone and adjacent counties of central Missouri. He considered marked variation in thickness of the Everton Formation in Missouri to be the result of relief on an eroded surface on which it was deposited and/or erosion prior to St. Peter deposition at its top. Templeton and Willman (1963) mentioned the presence of the Everton Formation in the subsurface on the western side of the Illinois Basin but did not report any details other than a measurement of about 100 ft (∼30 m) in a well in Monroe County, Illinois, and that thickness declines northward from there. Rexroad et al. (1982) identified 22 ft (6.28 m) of dolomite beneath the St. Peter Sandstone in Posey County in southwestern Indiana as the Everton Formation on the basis of conodonts recovered from a core.
Examination of Everton outcrops in Marion County, Arkansas, led Minke (1969) to identify five depositional facies within the Everton Formation, including the previously recognized Jasper Member and Newton Sandstone plus three contrasting dolomite intervals. He attributed the entire complex to sedimentation on tidal flats and in adjacent shallow subtidal environments, with the whole depositional system prograding westward toward more open marine environments.
A regional study by Suhm (1974) demonstrated inter-tonguing relationships between sandstone and carbonate units within the Everton Formation in 25 closely spaced outcrop sections along the Buffalo and White Rivers and in eight sections described previously by Glick and Frezon (1953). These 33 sections are distributed from Ponca in northwestern Newton County to near Calico Rock in southeastern Izard County, a distance of about 65 mi (∼105 km). The thickness of the Everton Formation in this region ranges from 350 to 650 ft (105-200 m).
Suhm (1974) recognized a basal Sneeds Dolomite Member that extends throughout the length of his traverse (Figure 4). The Sneeds Dolomite is overlain in the western part of the traverse by an interval of cal-carenite and calicilutite that he identified as Member B, which is followed in turn by the Newton Sandstone, then a sequence of dolomite and sandstone ledges (his Member C), and at the top, the Jasper Member. Suhm (1974) concluded that Member B and the Newton Sandstone interfinger in the central part of his traverse with a thick dolomite unit for which no member name was provided. Suhm (1974) reported that the Jasper Member extends from its type area in Newton County, Arkansas, on the west to near Calico Rock, Arkansas, in western Izard County, where it disappears against the unconformity beneath the St. Peter Sandstone. The Sneeds Dolomite is overlain toward the eastern end of Suhm’s restored cross section by Member A consisting of limestone and sandstone intervals, followed upward by the Calico Rock Sandstone Member. The Calico Rock Sandstone and Member C are separated in this region by an eastward tongue of the unnamed dolomite member (which he correlated with the upper part of Member B) and the Newton Sandstone Member of the western measured sections. Suhm (1997) concluded subsequently that the Jasper Member of northwestern Arkansas is transitional into dolomite underlying the St. Peter farther to the east where it is difficult to distinguish from the dolomites of his Member C. He suggested that the Jasper can be recognized, albeit with difficulty, in the vicinity of the Reelfoot rift basin and in southeastern Missouri.
Suhm (1997) noted the presence of fossils in the Everton Formation but did not use them for correlating with Ordovician rocks elsewhere in central North America. Suhm (1997, figure 7) cited evidence from a plexus of wells to support lithologic correlation of the units he identified in the Everton Formation in Arkansas with sequences known from outcrops in northeastern Oklahoma and in the Arbuckle Mountains of southern Oklahoma as well as in the subsurface between them (Figure 4). He focused on temporal and spatial relations of the several sandstone units in these several regions that he believed to reflect pulses of sand introduced periodically to the region from sources generally to the north. Suhm (1997) equated the Sneeds Dolomite and his members A and B of the Everton Formation with uppermost West Spring Creek and Joins Formations of the Arbuckle region, and he correlated the Newton Sandstone of Arkansas with the Burgen of northeastern Oklahoma and with sandstone low in the Oil Creek Formation. Member C of the Everton Formation was correlated with the lower Tyner of northeastern Oklahoma and with the Oil Creek Formation of the Arbuckle region. The Jasper Member of the Everton Formation was assumed to be equivalent to sandstone in the lower McLish Formation of Oklahoma and to a level low in the middle Tyner in intervening regions. Suhm (1997) interpreted the St. Peter Sandstone of northern Arkansas to be equivalent to the part of the McLish Formation above the basal sandstone and to the middle part of the middle Tyner. He concluded that the lower part of the Everton Formation, together with strata in Oklahoma that he considered to be correlative, are transgressive initial deposits of the Tippecanoe megasequence laid down in the early Whiterockian.
Early investigators stated that the Everton Formation, like other sub-St. Peter Ordovician units in northern Arkansas, had a sparse fossil content, but they reported finding ostracods, gastropods, trilobites, bivalves, nautiloids, and scattered pelmatozoan fragments. Although these fossils did not provide them a strong basis for correlation, they believed them to be sufficient to demonstrate an age after Beekmantown but before Chazyan time of deposition. Cooper (1956) observed that fossils are rare in the Everton Formation of Missouri and Arkansas but named a new brachiopod species,Onychoplecia matutina, from the Jasper Memberata locality near Pindall, Searcy County, Arkansas. He acknowledged that it does not serve for precise correlation of the Jasper Member but reasoned (p. 117) that its occurrence below the St. Peter Sandstone suggests “a possible link with the Lenoir of the Appalachians.”
Following a comprehensive review of known Ever-ton fossils, Derby (1973) concluded that the Everton Formation is of Whiterockian age and older than Chazyan. He interpreted the lower part of the Everton Formation to be correlative with Middle Ordovician beds in the uppermost West Spring Creek Formation and the lower part of the overlying Joins Formation in Oklahoma. He reported that the upper Everton Formation has an invertebrate fauna comparable to the fossils of the upper Joins and Oil Creek Formations in Oklahoma and those found in the lower Tyner at outcrops in northeastern Oklahoma. He interpreted the unconformity between the Everton Formation and the St. Peter Sandstone in northern Arkansasto correspond temporally to a disconformable boundary between the Oil Creek and McLish Formations in Oklahoma.
Conodont investigations in recent years have produced evidence bearing on the correlation of the Everton Formation with lower Middle Ordovician successions elsewhere in the United States. Golden (1969) obtained conodonts at 11 Everton localities between southeastern Carroll County and Calico Rockinwestern Izard County, Arkansas, and from two places in eastern Missouri. He found that the species in the lower parts of his sections differ from those found higher. Golden (1969) found an as yet unnamed species of Dischidog-nathus close to the bottom of the Everton Formation at a locality near Eros in Marion County, Arkansas. Mills (1980) reported this species from a lower Whiterockian sample collected 56 ft (17 m) below the top of the West Spring Creek Formation in the Interstate Highway 35 sectionin the Arbuckle Mountains, southern Oklahoma.
This species occurs in the upper part of the Rockdale Run Formation in western Maryland (Repetski and Harris in Brezinski et al., 1999, their figure 3H), and Repetski (unpublished data) has found it in the Providence Island Dolomite in western Vermont. These latter three occurrences are within the lower Whiterockian Tripodus laevis to Histiodella altifrons Zones.
A sample from basal Everton at Kyles Landing, Newton County (U.S. Geological Survey collection) contains conodonts similar to forms Harris and Harris (1965), Potter (1975), and McHargue (1981) found in the Tri-podus laevis Zone and low in the Histiodella altifrons Zone near the top of the West Spring Creek Formation. Placement in either zone would establish the Stratigraphic position of these Everton samples to be low in the Whiterockian Series and in the Dapingian Stage.
U.S. Geological Survey samples from the Rodabaugh quarry near Poughkeepsie, Sharp County, Arkansas, also indicate early Whiterockian age for the lowest part of the Everton Formation. They include the unnamed Dischidognathus species previously mentioned (Repetski, internal U.S. Geological Survey fossil report O-94-13). Everton strata above the lowest 20 to 26 ft (6–8 m) in this quarry contain upper Whiterockian co-nodonts of the Histiodella sinuosa to Histiodella holoden-tata Zones (Ethington and Repetski, 1986). These cono-dont data are consistent with Derby (1973) and Suhm (1997) who correlated the base of the Everton Formation with a position near the top of the West Spring Creek Formation in southern Oklahoma. Furthermore, the purported unconformable contact between the Everton and the Powell or Smithville has not been demonstrated, but if one exists, the interruption of sedimentation would be slight because no conodont zones are missing in this interval.
The most commonly occurring conodont species in the upper part of the Everton Formation at most of Golden’s (1969) localities is Paraprioniodus costatus. Lepto-chirognathus quadrata ranks second, followed in turn by Peracontiodus cryptodens and Apteracontiodus sinuosus. He also found smaller numbers of specimens of H. holoden-tata, Drepanoistodus angulensis, and Chosonodina rigbyi. Craig (1986) also reported this distribution of conodonts from the Everton Formation. These conodonts are indicative of the H. holodentata Zone and of mid-Whiterockian (= early Darriwilian) age. They occur elsewhere in the Joins and Oil Creek Formations (Simpson Group) in southern Oklahoma (Harris, 1964a, b; Mound, 1965a, b; McHargue, 1975, 1981; Bauer 2010) and the Lehman Formation (Zone N of Hintze, 1951, 1973) in western Utah (Ethington and Clark, 1982).
Conodonts demonstrate that the Everton Formation spans the sub-Chazyan part of the Whiterockian Series (Dapingian through lower Darriwilian Stages). They support the correlation of the part of the Everton Formation above the Newton Sandstone Member with the Bur-gen through the lower middle Tyner of northeastern Oklahoma as suggested by Suhm (1973, 1997) and by O’Brien and Derby (1997). Bauer (1989) reported three faunal associations in succession within the Burgen-Tyner sequence. The lowest of these, conodont association I (CAI) of Bauer (1989), ranges from the Burgen through the lower part of the middle Tyner, and it includes co-nodonts of the H. holodentata Zone (species of Drepanois-todus, Histiodella, Paraprioniodus, Multioistodus, Oistodus, Ansella) that dominate faunas of the upper Everton Formation in Arkansas including the Jasper Member.
Bauer’s (1989) CAII, whose range extends upward through all but the highest part of the middle Tyner, comprises Pharagmodus polonicus (reported as Phrag-modus harrisi) as well as species of Leptochirognathus, Coleodus, Plectodina, and Erismodus. This mid-Darriwilian conodont fauna is missing in northern Arkansas because of the unconformity between the Everton Formation and the St. Peter Sandstone. The presence of the P. polonicus Zone above the H. holodentata Zone is replicated in sections in western Utah (Ethington and Clark, 1982). Bauer (1987) concluded that P. polonicus is the ancestor of Phragmodus flexuosus in anevolutionary lineage that has been reported from many places in the Great Basin (Harris et al., 1979).
Bauer’s (1989) CAIII extends from the highest middle Tyner into the lower part of the Fite Formation. It includes an assemblage of species of Plectodina, Aphelognathus, Erismodus, and Oulodus, whose lowest occurrences elsewhere are in rocks of Mohawkian (Sandbian) age. Bauer (1989) considered the abrupt replacement of CAII by CAIII in the rocks of northeastern Oklahoma to document a major hiatus in the strat-igraphic record of the Burgen-Tyner-Fite sequence. The position of this hiatus is subsumed in Arkansas in the St. Peter Sandstone and the underlying unconformity.
In a regional lithologic synthesis of subsurface information and outcrop data, Suhm (1997) equated the sandstone of the lower part of the McLish Formation in the Arbuckle Mountains of southeastern Oklahoma with middle Tyner sandstone in northeastern Oklahoma and with the Jasper Member of the Everton Formation in Arkansas. As already shown, conodont evidence supports the correlation of the Jasper Member with lower middle Tyner but does not confirm equivalence of the Jasper and lower McLish. Bauer (1987, 2010) showed that the conodonts found in the upper part of the Everton Formation (his CAI) also occur throughout the Joins and Oil Creek Formations of the Arbuckle outcrops. Biostratigraphic evaluations of fossils from this area indicated that a major unconformity separates the Oil Creek Formation from the overlying McLish Formation (brachiopods by Cooper, 1956; Derby, 1973; conodonts by Branson and Mehl, 1943; Sweet, 1984; ostracods by Harris, 1957; unpublished reports of ostracods by Creath and data of Creath in Derby et al., 1991, their figure 18).
The McLish Formation is slightly more than 400 ft (>122 m) thick in the outcrops along Interstate Highway 35 where Bauer (1987) collected samples for his study. He did not obtain conodonts from the basal sandstone unit of the McLish Formation, which is about 60 ft (∼18 m) thick. Samples collected just above the top of this sandstone (at 66 and 67 ft [∼20 m]) produced conodonts characteristic of the H. holodentata Zone; the next highest sample (at 71 ft [21.5 m]) contained a modest representation of these species plus specimens of Phragmodus flexuosus. Higher samples in Bauer’s (1987) collection from the McLish Formation do not contain representatives of the H. holodentata Zone, and P. flexuosus is associated in them with species of Ansella, Drepanoistodus, Eoplacognathus, Protopanderodus, and Staufferella.
Thus, the conodont succession obtained by Bauer (1987, 2010) from the Ordovician of the Arbuckle region includes abundant representatives of species of the H. holodentata Zone in the Joins and Oil Creek Formations and modest occurrences of them just above the basal sandstone of the McLish Formation. A conodont fauna characterized by P. flexuosus is introduced only 3.3 ft (1 m) higher in the succession and persists above there. Pharagmodus polonicus, the ancestral species of P. flexuosus, has not been found in southern Oklahoma. Bauer (1987) reported that physical evidence does not support attributing the absence from the McLish Formation of beds containing the P. polonicus fauna to faulting, unconformity, or abrupt environmental change. Therefore, he concluded that conodonts representing the H. holodentata Zone in the McLish Formation were introduced there by redeposition of specimens removed from older strata, and that they do not demonstrate the age of the part of the McLish Formation in which they occur. This interpretation supports Derby’s (1973) suggestion that the Everton-St. Peter unconformity can be correlated approximately with a disconformable boundary between the Oil Creek and McLish Formations (Figure 7).
Correlation With Slope Deposits of the Ouachita Mountains
The thick section of black shales alternating with sandstones in the Ouachita Mountains of central Arkansas traditionally has been correlated with rocks elsewhere on the basis of graptolites, which are scarce or lacking in most rocks of the GACB. Recent work demonstrates that conodonts and rare trilobites in thin limestone intervals within the Ouachita succession (Repetski and Ethington, 1977; Ethington and Repetski, 1986; Stitt et al., 1994) permit generalized correlations with the formations of northern Arkansas (Figure 1). The Collier Shale includes rocks as old as the medial Upper Cambrian Elvinia Zone (Hart et al., 1987; Hohensee and Stitt, 1989; Stitt et al., 1994) and as young as the Lower Ordovician Rossodus manitouensis Zone (Repetski and Ethington, 1977; Ethington et al., 1989). No counterpart is exposed in northern Arkansas, but the upper Collier is correlative with at least part of the Elvins Group through the middle of the Gasconade Dolomite of Missouri (Repetski et al., 2000). A few conodonts of the R. mani-touensis Zone recovered from the overlying Crystal Mountain Sandstone establish its age as Early Ordovician (Repetski and Ethington, 1994), but the ages of the base and top of the formation are not well constrained.
Samples from the Mazarn Shale at numerous localities in the Ouachita region contain conodonts that are assumed to be indigenous in their Ouachita setting because their occurrences in other parts of the world indicate that they were adapted to deep- and/or cool-water environments. Admixed with them in varying proportions in many Mazarn samples are species that dominate collections from the shallow- and presumably warm-water carbonate rocks of the GACB (Chludzinski, 1993). Their presence in the Mazarn is attributed to transport from their GACB home into cooler water environments by episodic currents. The warm-water component in most Mazarn samples is dominated by E. parallelus and C. quadraplicatus, long-ranging species that indicate that the part of the Mazarn where they occur is correlative to an interval within the Jefferson City through the lower Powell Dolomites in the southern Ozarks. Unfortunately, no continuous sections that would allow individual collections to be arranged in stratigraphic order are exposed in the Ouachitas, and instead, their relative positions are interpreted by comparison to known successions elsewhere. Repetski and Ethington (1994) reported a few collections that contain O. evae, R. andinus, and B. extensus, species that occur elsewhere in uppermost Ibexian strata. These occurrences show that the Mazarn includes rocks as young as the top of the Powell of northwestern Arkansas and the Black Rock Limestone Member of northeastern Arkansas.
Conodonts have not been recovered from low in the Blakely Sandstone, but Mazarn samples believed to be near the formational boundary include species representing the highest Ibexian. Samples taken near the top of the Blakely Sandstone at several places (Bergström, 1979; Repetski and Ethington, 1994; Krueger, 2002) include Spinodus spinatus, Paroistodus horridus, Periodon aculeatus, and Histiodella holodentata, species indicative of early (but not earliest) Whiterockian age. However, numerous samples from the Blakely Sandstone also contain shallow-shelf species, for example, Leptochirognatus quadratus, Curtognathus spp., and P. costatus, indicating that at least some of this sand could be equivalent to part of the Everton Formation to the north (Ethington et al., 1989).
The lowest conodonts recovered thus far from the Womble Shale (Krueger, 2002) are indicative of the upper Whiterockian Cahabagnathus friendsvillensis Zone. This stratigraphic level is occupied by the unconformity between the Everton Formation and the St. Peter Sandstone in the Ozark region.
Correlation With Sauk Sediments of the Reelfoot Rift Basin
The Reelfoot rift basin, which connects to the Illinois basin, Rough Creek graben, and Rome trough is part of a major system formed during the initial breakup of the supercontinent Rodinia (Thomas, 1991). The thick Sauk succession in the Reelfoot rift basin remains poorly understood since the early reports of exceptionally thick Cambrian–Ordovician strata in the region by Freeman (1953) and Grohskopf (1955). Ross (1975) identified the area as an aulacogen based on stratigraphic data provided by Derby and by information in Schwalb (1969) and in a summary publication by Bond et al. (1971). Keller et al. (2000) and van Arsdale (2009) summarized the regional geology of the Reelfoot rift basin. In general, the oldest rocks are coarse arkosic sandstones of Middle Cambrian age (Houseknecht, 1989), succeeded by carbonates and fine clastics that have been referred to the Lamotte and Bonneterre Formations, followed in turn by carbonates assigned to the Elvins Group in the Ozark region and/or to the Arbuckle (southern Oklahoma) or Knox (Appalachian region) Groups or by dark shaly carbonates of equivalent age. As shown by Palmer et al. (2012), the relatively thin shoal-water carbonates that crop out in the Ozark region thicken abruptly into the Reelfoot rift basin.
Numerous subsequent reports describe sections penetrated by various wells and their contained sparse faunas, but the most complete reference known to the writers is the summary provided by Collins et al. (1992; also see Derby, 1982; Repetski and Weary, 1993) who provide a map of these deep wells (Figure 5). They also present lithologic descriptions of and paleontologic correlation between the carbonate strata in the Dow 1 Wilson well and the older siliciclastic rocks in Dow 1 Garrigan (Figures 6, 8). These data show that the Lower Ordovician carbonates in the Wilson well are approximately 6560 ft (∼2000 m) thick as compared with less than 1520 ft (<400 m) in outcrops in the Ozarks (also see Palmer et al., 2012). More important for deciphering the stratigraphy of the Reelfoot rift basin, these data demonstrate that the lowest 1650 ft (500 m) of carbonates in the Wilson well are equivalent to approximately 4000 ft (∼1200 m) of deep-water siliciclastics in the 1 Garrigan well, and that the Bonneterre dolomite unit in the Garrigan well is more than five times thicker than the same unit in the Wilson well. This stratigraphic evidence combined with increasingly abundant seismic evidence demonstrates that the stratigraphy and structure of the Reelfoot rift basin is not simple and that faulting within the rift was active throughout deposition of the Sauk megasequence (Figure 9).
It is clear that the Reelfoot rift basin provided a long, narrow, deep seaway during deposition of the Sauk megasequence (Figure 5). This trough was occupied by cool water as indicated by the presence of cool-water conodonts of the O. evae Zone in the U.S. Geological Survey 1 Ft. Pillow well, as reported by Repetski (in Derby, 1982; Collins et al., 1992). In addition, Wood and Stephenson (1989) reported cool-water microphyto-plankton in the Bonneterre and Davis Formations from six outcrop localities in the St. Francois Mountains region of southeastern Missouri and from eight subsurface borings in southeastern Missouri and northeastern Aransas, including wells 13 to 15 on Figure 5.
The Reelfoot rift basin continued as a deep seaway and depocenter for open-marine sediments well into the Middle Ordovician, as indicated by the thick Everton-equivalent super-Black Rock limestone section in the Reelfoot rift basin wells (Figures 6A, 9). In contrast, the more inland extensions of the rift such as the Rome trough were inactive by the Late Cambrian.
Rocks that underlie the St. Peter Sandstone in the Ozark region present problems for detailed regional synthesis because of the close physical similarity of some of the established stratigraphic units, the relative scarcity of fossils within them, the complex facies relations displayed by some of them, and the paucity of thick continuous exposures. Correlations with sections outside the Ozark region have been approximate because invertebrate fossils are scarce; they typically are poorly preserved because of pervasive dolomitization. The Cambrian through lowest Ordovician part of the Sauk mega-sequence crops out only in Missouri (see Palmer et al., 2012). Conodonts and invertebrate fossils, particularly brachiopods, found in the upper Powell, Smithville, Black Rock, and Everton units allow the highest part of the succession to be correlated internally within the Ozark region and externally with rocks elsewhere in the GACB and with the thick siliciclastic sequence of central Arkansas. The lower units (Jefferson City, Cotter, and all but the upper Powell) are not fossil rich; very long ranges known elsewhere for the few conodonts recovered from these units in Arkansas and Missouri allow only coarse biostratigraphic assignment. Exposed Sauk rocksinnorthern Arkansas span the interval from the Lower (but not lowest) Ordovician (Tremadocian; middle Ibexian, Cassinian Stage) into the Middle Or-dovician (Darriwilian; lower middle Whiterockian, H. holodentataZone). The youngest conodonts in the Everton Formation provide a lower age limit for the Sauk-Tip-pecanoe boundary in the Ozark region. Sedimentologic and paleontologic evidence from scattered wells permits recognition of equivalents of the exposed Saukian units of the Ozark region into the thicker and structurally more complex sequence of the Reelfoot rift basin and in the Ouachita Mountains of central Arkansas and allows a broader regional interpretation of the history of the development of the GACB.
Figures & Tables
Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia
The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.