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ABSTRACT

Facies analysis utilizing a conodont biostratigraphic framework is a powerful tool for evaluating genetic relationships of Osagean–basal Meramecian strata within the Ozark region (Arkansas–Missouri–Oklahoma) of the southern midcontinent. This investigation builds upon previous work cited herein, and suggests that some lithostratigraphic divisions, although useful in differentiating strata in a localized setting, may not be suitable for regional correlations within the Boone Group. High-resolution conodont biostratigraphy demonstrates the diachronous nature of lithostratigraphic divisions within the Boone Group, with both the Reeds Spring Formation and Bentonville Formation (Burlington–Keokuk) clearly becoming younger as they are traced from southwestern Missouri into northern Arkansas and northeastern Oklahoma. Subsequent facies analysis shows that the Reeds Spring Formation represents deposition within outer ramp through proximal middle ramp settings (low to moderate energy), whereas the Bentonville Formation (Burlington–Keokuk) records deposition within proximal middle ramp to inner ramp settings (moderate to high energy). Integration of facies analysis and conodont biostratigraphy-based relative chronostratigraphy provides the basis for construction of four time-slice maps illustrating the distribution of time-correlative facies belts. Together, these time-slice maps deliver a clearer representation of the evolution of Boone Group carbonate ramp deposition during the Osagean, which was characterized by overall shallowing-upward and progradation to south and southwest.

INTRODUCTION

This study examines the relationship between relative (biostratigraphy-based) chronostratigraphy and lithostratigraphy of the Boone Group (Osagean–basal Meramecian) in parts of the Mississippian outcrop belt of southwestern Missouri, southeastern Kansas, northwestern Arkansas, and northeastern Oklahoma (Figure 1). Within the study area, previous workers have interpreted the depositional setting and platform profile as a broad carbonate shelf (Lane and De Keyser, 1980; Gutschick and Sandberg, 1983). In contrast, recent interpretations of these strata suggest deposition within a carbonate ramp to distally steepened rampsetting (Mazzullo et al., 2011; Price and Grammer, 2015). This work contributes to understanding of the Boone Group and tests the model of diachronous lithostratigraphic boundaries interpreted by Thompson (1986) and Boardman et al. (2013). Together with petrographic analyses, conodont biostratigraphy establishes relative temporal relationships between proximal through distal depositional facies across a carbonate ramp setting. By sampling new sections, the interpretations presented herein augment the extensive work on Mississippian conodonts by Thompson and Goebel (1968), Thompson and Fellows (1970), and Boardman et al. (2013). Along with sampling of the Bentonville type locality as a tie-in with Boardman et al. (2013), this study contributes descriptions of three new surface exposures to the previous volume of work and extends regional biostratigraphic coverage (Table 1).

Figure 1.

Generalized map of the Mississippian outcrop belt (gray shading) in the western Ozarks, southern midcontinent, U.S.A. showing outcrop localities that provided lithostratigraphic descriptions and biostratigraphic data used in this study.

Figure 1.

Generalized map of the Mississippian outcrop belt (gray shading) in the western Ozarks, southern midcontinent, U.S.A. showing outcrop localities that provided lithostratigraphic descriptions and biostratigraphic data used in this study.

Table 1.

Outcrops utilized in the biostratigraphic reconstruction of the upper Osagean depositional regimes. Localities are numbered, and numbers correspond to the outcrops identified in Figure 1. County locations are provided, as well as the referenced study in which biostratigraphic data were obtained.

 LOCATIONSCOUNTY, STATEREPORT REFERENCED
1Bentonville type localityBenton County, ArkansasThis report
2Bentonville co-type localityBoone County, ArkansasThis report
3AOmaha A localityBoone County, ArkansasThis report
3BOmaha B localityBoone County, ArkansasThis report
4Reeds Spring type localityStone County, MissouriThompson and Fellows (1970)
5Elsey type localityStone County, MissouriRobertson (1967)
6Turners StationGreene County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
7Grand Falls Chert type localityJasper County, MissouriRobertson (1967)
8Lake SpringfieldGreene County, MissouriThompson and Fellows (1970)
9Brown QuarryGreene County, MissouriThompson and Fellows (1970)
10Shoal CreekNewton County, MissouriThompson and Fellows (1970)
11Kirshmann Jeffries QuarryJasper County, MissouriThompson and Fellows (1970)
12Baird Mountain QuarryTaney County, MissouriThompson and Fellows (1970)
13Roaring River State ParkBarry County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
14ChesapeakeLawrence County, MissouriThompson and Fellows (1970)
15Lanagan QuarryMcDonald County, MissouriThompson and Fellows (1970)
16Madry QuarryBarry County, MissouriThompson and Fellows (1970)
17Chestnut RidgeChristian County, MissouriThompson and Fellows (1970)
18Marble FallsNewton County, ArkansasThompson and Fellows (1970)
19St. JoeSearcy County, ArkansasThompson and Fellows (1970)
20LeatherwoodCarroll County, ArkansasThompson and Fellows (1970)
21PowellMcDonald County, MissouriThompson and Fellows (1970)
22Tanner BranchMcDonald County, MissouriThompson and Fellows (1970)
23ScraperCherokee County, OklahomaThompson and Fellows (1970)
24Tahlequah NorthCherokee County, OklahomaThompson and Fellows (1970); Boardman et al. (2013, as “No Head Hollow Reference Locality”)
25Type Walls FerryIndependence County, ArkansasThompson and Fellows (1970)
26Glen Creek SouthIndependence County, ArkansasThompson and Fellows (1970)
27Walls Ferry Dam NorthIndependence County, ArkansasThompson and Fellows (1970)
28Sellers CreekSearcy County, ArkansasThompson and Fellows (1970)
29P. LocalityCherokee County, KansasThompson and Goebel (1968)
30Q. LocalityCherokee County, KansasThompson and Goebel (1968)
 LOCATIONSCOUNTY, STATEREPORT REFERENCED
1Bentonville type localityBenton County, ArkansasThis report
2Bentonville co-type localityBoone County, ArkansasThis report
3AOmaha A localityBoone County, ArkansasThis report
3BOmaha B localityBoone County, ArkansasThis report
4Reeds Spring type localityStone County, MissouriThompson and Fellows (1970)
5Elsey type localityStone County, MissouriRobertson (1967)
6Turners StationGreene County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
7Grand Falls Chert type localityJasper County, MissouriRobertson (1967)
8Lake SpringfieldGreene County, MissouriThompson and Fellows (1970)
9Brown QuarryGreene County, MissouriThompson and Fellows (1970)
10Shoal CreekNewton County, MissouriThompson and Fellows (1970)
11Kirshmann Jeffries QuarryJasper County, MissouriThompson and Fellows (1970)
12Baird Mountain QuarryTaney County, MissouriThompson and Fellows (1970)
13Roaring River State ParkBarry County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
14ChesapeakeLawrence County, MissouriThompson and Fellows (1970)
15Lanagan QuarryMcDonald County, MissouriThompson and Fellows (1970)
16Madry QuarryBarry County, MissouriThompson and Fellows (1970)
17Chestnut RidgeChristian County, MissouriThompson and Fellows (1970)
18Marble FallsNewton County, ArkansasThompson and Fellows (1970)
19St. JoeSearcy County, ArkansasThompson and Fellows (1970)
20LeatherwoodCarroll County, ArkansasThompson and Fellows (1970)
21PowellMcDonald County, MissouriThompson and Fellows (1970)
22Tanner BranchMcDonald County, MissouriThompson and Fellows (1970)
23ScraperCherokee County, OklahomaThompson and Fellows (1970)
24Tahlequah NorthCherokee County, OklahomaThompson and Fellows (1970); Boardman et al. (2013, as “No Head Hollow Reference Locality”)
25Type Walls FerryIndependence County, ArkansasThompson and Fellows (1970)
26Glen Creek SouthIndependence County, ArkansasThompson and Fellows (1970)
27Walls Ferry Dam NorthIndependence County, ArkansasThompson and Fellows (1970)
28Sellers CreekSearcy County, ArkansasThompson and Fellows (1970)
29P. LocalityCherokee County, KansasThompson and Goebel (1968)
30Q. LocalityCherokee County, KansasThompson and Goebel (1968)

LITHOSTRATIGRAPHIC NOMENCLATURE

Following the revised lithostratigraphic nomenclature proposed by Mazzullo et al. (2013), the principal study interval includes, in ascending order, the Reeds Spring Formation, Bentonville Formation, and Ritchey formation of the revised Boone Group (Figure 2). The top of the Bentonville Formation is marked by its Short Creek Oolite Member. For purpose of uniformity within the following discussion, the revised nomenclature as defined by Mazzullo et al. (2013) is used. Where needed, however, reference to previously applied nomenclature is given as well, including variable ranking of the terms “Burlington–Keokuk,” “Reeds Spring,” “Elsey,” and “Boone” (e.g., Huffman, 1958; Thompson and Fellows, 1970; Thompson, 1986). Underlying the Boone Group, the St. Joe Group includes, in ascending order, the Compton formation, Northview Formation, and Pierson Formation, applying the revised nomenclature of Mazzullo et al. (2013) and providing historical nomenclature where needed.

Figure 2.

Stratigraphic nomenclature of the Mississippian outcrop belt western Ozarks, U.S.A., adapted from Mazzullo et al. (2013). To the right are historically applied nomenclatural terms of southwestern Missouri, northeastern Oklahoma, and northern Arkansas.

Figure 2.

Stratigraphic nomenclature of the Mississippian outcrop belt western Ozarks, U.S.A., adapted from Mazzullo et al. (2013). To the right are historically applied nomenclatural terms of southwestern Missouri, northeastern Oklahoma, and northern Arkansas.

PREVIOUS CONODONT WORK

In terms of conodont biostratigraphy, less attention has been given to the upper Osagean and basal Meramecian section along the southwestern flank of the Ozark uplift relative to equivalent rocks of the Upper Mississippi Valley (Branson and Mehl, 1941; Rexroad and Collinson, 1965). Within the study area, two important early reports were published for southeastern Kansas (Thompson and Goebel, 1968) and southwestern Missouri (Thompson and Fellows, 1970), both of which include sections utilized in this report. Boardman et al. (2013) discussed Kinderhookian through basal Meramecian conodonts within the study area as an ongoing regional research project of which this report is a part.

Below the interval of interest, basal Kinderhookian strata are dominated by conodonts belonging to the genus Siphonodella. During the middle Kinderhookian, species of Protognathodus and Gnathodus became more common. Of specific interest are the elements Gnathodus delicatus (Branson and Mehl), G. typicus (Cooper), and G. punctatus (Cooper; Boardman et al., 2013), gnathodids, which, in addition to their importance in defining Kinderhookian conodont zones, gave rise to Osagean species (Thompson, 1979; Lane et al., 1980). In Osagean strata, the siphonodellids are absent, and the diversity and abundance of Gnathodus species increase significantly (Pinney, 1962; Thompson, 1979; Lane et al., 1980). As a result, gnathodids are important for defining biozones, especially within upper Osagean sections (Thompson, 1979; Lane et al., 1980). Because of typically sparse faunal recoveries per kilogram of rock processed, upper Osagean stratigraphy is difficult to resolve using conodonts (e.g., Youngquist et al., 1950). This study contributes an additional 260 complete elements to refinement of upper Osagean stratigraphy.

Established upper Osagean biozones (Figure 3) include the Gnathodus texanusTaphrognathus varians Zone and Taphrognathus variansApatognathus Zone of Collinson et al. (1971) and the texanus Zone of Lane et al. (1980) and Lane and Brenckle (2005). Boardman et al. (2013) proposed the division of these long-ranging zones into the lower “texanus” Zone, middle “texanus”–pseudosemglaber Zone, and upper “texanus”–Gnathodus n. sp. 15 aff. punctatus Zone based primarily upon observed ranges of recognized species of Gnathodus, as well as 15 potential new species, including many that were otherwise assigned to G. texanus (Roundy). Bed-by-bed (decimeter-scale [4-inch scale]) sampling provided a high-resolution chronostratigraphic framework for subdivision of the Boone Group and further demonstrated the diachronous nature of established lithostratigraphic units.

Figure 3.

Comparison of conodont zones of this report, Thompson (1967), Collinson et al. (1971), Lane and Brenckle (2005), and Boardman et al. (2013). This study follows the revised zonal scheme proposed by Boardman et al. (2013) but utilizes an undivided anchoralislatus Zone.

Figure 3.

Comparison of conodont zones of this report, Thompson (1967), Collinson et al. (1971), Lane and Brenckle (2005), and Boardman et al. (2013). This study follows the revised zonal scheme proposed by Boardman et al. (2013) but utilizes an undivided anchoralislatus Zone.

The relevant Osagean conodont zones discussed below include, from oldest to youngest, the anchoralislatus Zone and bulbosus Zone of Lane and Brenckle (2005), and Boardman et al. (2013), and the lower “texanus” Zone, middle “texanus”–pseudosemiglaber Zone, and upper “texanus”–Gnathodus n. sp. 15 aff. punctatus Zone proposed by Boardman et al. (2013) (Figure 3). Boardman et al. (2013) divided the anchoralislatus Zone into three zones, similar to Chauff (1981) and roughly equivalent to the subzones defined by Lane and Brenckle (2005), with the revised anchoralislatus Zone as the oldest of the three. The anchoralislatus Zone was not encountered within sections examined for this study, but occurs within sections described by previous workers. Consequently, we hesitate to apply the subdivision proposed by Boardman et al. (2013) and utilize the anchoralislatus Zone proposed by Lane and Brenckle (2005). Although the anchoralislatus Zone was not encountered within sections examined in this investigation, it is included in this discussion because of its regional and global significance.

GEOLOGIC SETTING

The Mississippian outcrops in the western Ozarks are dominated by shallow water carbonates deposited to the south and east of the Transcontinental arch along a broad carbonate platform extending across the central and southwestern United States (see Gutschick and Sandberg, 1983, their figure 1). Lane and De Keyser (1980) and Gutschick and Sandberg (1983) proposed a shelf-slope model to explain observed distributions of Early Mississippian depositional facies in North America. The shelf model has been interpreted in recent studies as a ramp or distally steepened ramp (Mazzullo et al., 2011; Childress and Grammer, 2015; Price and Grammer, 2015). Carbonate facies in ramp settings respond rapidly to changes in water depth and energy, with the greatest impact on low-gradient ramps (Burchette and Wright, 1992). The result is simultaneous deposition of distinct coeval facies and progradation, aggradation, or retrogradation in response to changing water depth and energy.

METHODS

Three road-cut exposures along U.S. Highway 65 north of Harrison, Arkansas (Boone County), near the community of Omaha, were sampled for high-resolution conodont recovery and petrography (locations 2, 3A, and 3B in Figure 1 and Table 1). The Bentonville co-type locality (location 2) is 19.3 km (12 mi) north of Harrison, the Omaha A locality (location 3A) is approximately 21.7 km (13.5 mi) north of Harrison, and the Omaha B locality (location 3B) is approximately 22.4 km (13.9 mi) north of Harrison. Sampling at all three locations was done on a bed-by-bed basis for beds less than 30 cm (1 ft) thick. For thicker beds, the base and top of each bed was sampled. Additional samples were taken within these thicker beds at 30 cm (1 ft) spacing, when needed, to ensure high-resolution facies description and conodont biostratigraphy. The uppermost section of the Bentonville Formation, as well as the contact between the Reeds Spring Formation and overlying Bentonville Formation were sampled specifically to biostratigraphically constrain the age of the Bentonville Formation.

A fourth section, the Bentonville type locality (location 1) along U.S. Highway 71 approximately 5 km (3 mi) north of the town of Bentonville in Benton County, Arkansas, was sampled at a lower frequency for conodont recovery and serves as a tie-in to Boardman et al. (2013).

For each of the three sections in Boone County, individual beds were measured, photographed, described, and then sampled. Samples weighing 5 kilograms (11 lbs) were collected and bagged. Three kilograms (6.6 lbs) were removed for acid digestion and conodont recovery and 2 kilogram (4.4 lbs) pieces of adequate dimensions (~12.7 cm × 7.6 cm [5 in × 3 in]) for acetate peels. Acetate peels were prepared for a representative sample of each conodont sampling location to determine grain size, allochem type and abundance, mud content, diagenetic products, and sedimentary structures. Two kilogram (4.4 lbs) samples for acid digestion were collected from the fourth section, which is described in detail by Price (2014). Sample mineralogy dictated the rock digestion method used for conodont element recovery. The primary solvent used for limestone was dilute formic acid (10% by volume).

OUTCROP LITHOSTRATIGRAPHY

Exposed within the Omaha A section are 7.4 meters (24.4 ft) of the Reeds Spring Formation (lower contact not exposed) overlain by 9 meters (9.6 ft) of the basal Bentonville Formation (Figure 4A). To the north, the Omaha B section includes 7.8 meters (25.6 ft) of the Reeds Spring Formation (lower contact not exposed) overlain by 10.9 meters (35.8 ft) of the Bentonville Formation (Figure 4B). The Bentonville co-type section includes, in ascending order, 17.7 meters (58 ft) of the Bentonville Formation (lower contact not exposed), 1.5 meters (5 ft) of Short Creek Oolite Member and 4.3 meters (14 ft) of the basal Ritchey formation (upper contact not exposed; Figure 5). The contact between the Short Creek Oolite Member and the overlying Ritchey formation marks the boundary between the Osagean and Meramecian series and is regarded as unconformable (Thompson, 1986; Boardman et al., 2013; Mazzullo et al., 2013). The Bentonville type section (Figure 6) exposes 9.1 meters (30 ft) of Reeds Spring Formation (Pineville tripolite facies) and 15.2 meters (50 ft) of Bentonville Formation. Neither the base of the Reeds Spring Formation nor the top of the Bentonville Formation are exposed at the Bentonville type locality.

Figure 4.

Stratigraphic measured sections for the (A) Omaha A and (B) Omaha B localities in Boone County, Arkansas. At both locations, the contact between the Reeds Spring and Bentonville formations is marked by a decrease in chert and increase in bedding thickness (Robertson, 1967; Mazzullo et al., 2011; 2013). Conodonts recovered are displayed with numbers corresponding to sample location. The conodonts recovered indicate a higher abundance of the Gnathodus texanus element relative to a marked decrease in abundance of G. bulbosus. The conodonts recovered place this section in the lower “texanus” Zone (Boardman et al., 2013). Conodonts shown for the Omaha A locality (A), samples 1–3 include intraspecific variations of G. bulbosus. For the Omaha B locality (B), conodonts shown include (1) Gnathodus sp., (2) G. bulbosus, and (3)–(5) Gnathodus sp. aff. texanus.

Figure 4.

Stratigraphic measured sections for the (A) Omaha A and (B) Omaha B localities in Boone County, Arkansas. At both locations, the contact between the Reeds Spring and Bentonville formations is marked by a decrease in chert and increase in bedding thickness (Robertson, 1967; Mazzullo et al., 2011; 2013). Conodonts recovered are displayed with numbers corresponding to sample location. The conodonts recovered indicate a higher abundance of the Gnathodus texanus element relative to a marked decrease in abundance of G. bulbosus. The conodonts recovered place this section in the lower “texanus” Zone (Boardman et al., 2013). Conodonts shown for the Omaha A locality (A), samples 1–3 include intraspecific variations of G. bulbosus. For the Omaha B locality (B), conodonts shown include (1) Gnathodus sp., (2) G. bulbosus, and (3)–(5) Gnathodus sp. aff. texanus.

Figure 5.

Stratigraphic measured section for the Bentonville co-type locality in Boone County, Arkansas, which includes the upper Bentonville Formation, Short Creek Oolite Member, and lower part of the Ritchey formation. Conodonts recovered are displayed with numbers corresponding to sample location. Conodonts shown for the Bentonville co-type locality include (1) Gnathodus sp. aff. texanus, (2) Gnathodus n. sp. 14 (Boardman et al., 2013), (3) Gnathodus sp. aff. bulbosus (Miller, 2016), (4)–(6) G. pseudosemiglaber, (7a) Gnathodus sp. aff. texanus, (7b) Gnathodus n. sp. B aff. pseudosemiglaber (Miller, 2016), (8) and (9) G. pseudosemiglaber, (10) Gnathodus sp. aff. pseudosemiglaber (Miller, 2016), (11)–(13) Gnathodus sp. aff. texanus, (14) G. linguiformis, (15) G. pseudosemiglaber, (16) Taphrognathus varians, (17), 18) and (19) Gnathodus n. sp. A aff. texanus (Miller, 2016).

Figure 5.

Stratigraphic measured section for the Bentonville co-type locality in Boone County, Arkansas, which includes the upper Bentonville Formation, Short Creek Oolite Member, and lower part of the Ritchey formation. Conodonts recovered are displayed with numbers corresponding to sample location. Conodonts shown for the Bentonville co-type locality include (1) Gnathodus sp. aff. texanus, (2) Gnathodus n. sp. 14 (Boardman et al., 2013), (3) Gnathodus sp. aff. bulbosus (Miller, 2016), (4)–(6) G. pseudosemiglaber, (7a) Gnathodus sp. aff. texanus, (7b) Gnathodus n. sp. B aff. pseudosemiglaber (Miller, 2016), (8) and (9) G. pseudosemiglaber, (10) Gnathodus sp. aff. pseudosemiglaber (Miller, 2016), (11)–(13) Gnathodus sp. aff. texanus, (14) G. linguiformis, (15) G. pseudosemiglaber, (16) Taphrognathus varians, (17), 18) and (19) Gnathodus n. sp. A aff. texanus (Miller, 2016).

Figure 6.

Conodonts recovered from the Bentonville type locality. The outcrop pictured above has sampled locations indicated by the black stars. (1) Gnathodus bulbosus and an unidentifiable gnathodid from limestone lenses in the Pineville tripolite. (2) The contact between the Pineville Tripolite and the Bentonville Formation recovered G. bulbosus below the contact, and Gnathodus sp. aff. pseudosemiglaber above the contact within the Bentonville Formation. (3) Gnathodus pseudosemiglaber (3a) and Taphrognathus varians (3b, c) were recovered above the basal Bentonville Formation. (4) At the top of the outcrop a specimen with affinity to G. pseudosemiglaber and possibly G. linguiformis was recovered. Conodonts are numbered corresponding to specimen identification.

Figure 6.

Conodonts recovered from the Bentonville type locality. The outcrop pictured above has sampled locations indicated by the black stars. (1) Gnathodus bulbosus and an unidentifiable gnathodid from limestone lenses in the Pineville tripolite. (2) The contact between the Pineville Tripolite and the Bentonville Formation recovered G. bulbosus below the contact, and Gnathodus sp. aff. pseudosemiglaber above the contact within the Bentonville Formation. (3) Gnathodus pseudosemiglaber (3a) and Taphrognathus varians (3b, c) were recovered above the basal Bentonville Formation. (4) At the top of the outcrop a specimen with affinity to G. pseudosemiglaber and possibly G. linguiformis was recovered. Conodonts are numbered corresponding to specimen identification.

OUTCROP CONODONT BIOSTRATIGRAPHY

Recovery

The conodont collection compiled during this investigation contains elements representing the upper Reeds Spring Formation, Bentonville Formation, and lower Ritchey formation in the northern Arkansas field area. A total of 345 elements were recovered from 148 kilograms (326 lbs) of collected rock. Most residues yielded few complete specimens, and the ones that were intact often were poorly preserved. Two hundred and sixty (260) complete specimens were recovered, including 9 identified lingonodinids, 65 “spathognathodids,” 50 gnathodids, and 136 indeterminate. The additional 85 elements were fragments only. Low yields are expected from higher-energy carbonate facies, with better recoveries from more offshore carbonate facies. Recovered conodonts were catalogued and delivered to the Paleontological Repository, Department of Earth and Environmental Sciences, University of Iowa for storage and curation. A summary of conodont recovery and conodont systematics is available in Miller (2016).

Omaha A Locality

Within the Omaha A section, both the Reeds Spring Formation and Bentonville Formation are placed within the bulbosus Zone, based upon a conodont recovery consisting exclusively of specimens assignable to Gnathodus bulbosus (Thompson) (Figure 4A; Thompson and Fellows, 1970; Boardman et al., 2013).

Omaha B Locality

At the Omaha B locality, recoveries from the available Bentonville Formation yielded specimens attributed to Gnathodus texanus, Gnathodus sp. aff. pseudosemiglaber, and fewer specimens of G. bulbosus. Here, the Bentonville Formation falls entirely within the lower “texanus” Zone (Figure 4B) (Boardman et al., 2013). However, poor preservation of conodont elements make zonal designation difficult, and the final assignment was based primarily on the increased abundance of specimens attributed to G. texanus relative to specimens of Gnathodus sp. aff. pseudosemiglaber.

Bentonville Co-Type Locality

At the Bentonville Co-Type section, the lower “texanus” Zone occurs within the lower 4.6 meters (15 ft) of the Bentonville Formation and includes Gnathodus texanus and G. bulbosus. Based on the absence of G. bulbosus and the predominance of G. pseudosemiglaber (Thompson and Fellows, 1970) and Gnathodus sp. aff. pseudosemiglaber, the overlying 14.6 meters (48 ft) of the Bentonville Formation falls within the middle “texanus” Zone (Miller, 2016). Also included within this zone were specimens of G. texanus, Gnathodus sp. aff. texanus, G. linguiformis (Branson and Mehl), and Gnathodus n. sp. B aff. pseudosemiglaber (Miller, 2016). The upper “texanus” Zone is restricted to the overlying Ritchey formation (Figure 5; Boardman et al., 2013). Absence of Taphrognathus varians (Branson and Mehl) within the middle “texanus” Zone, along with an abundance of specimens attributed to G. texanus support assignment to upper Osagean (Rexroad and Collinson, 1965; Thompson and Goebel, 1968). The overlying Short Creek Oolite Member yielded no conodonts. Present within this section is the Short Creek Oolite Member of the Bentonville Formation, above which is the Ritchey formation. Recoveries from the Ritchey formation at this location included G. pseudosemiglaber, G. linguiformis, Gnathodus sp. aff. texanus, and T. varians. Boardman et al. (2013) placed the Ritchey formation within their middle “texanus” Zone based on the recovery of Gnathodus n. sp. 15 aff. punctatus. Although this new species was not recovered during this study, it has been recovered from the Ritchey formation at multiple locations in Missouri, Arkansas, and Oklahoma as reported by Godwin et al. (2019). The Ritchey formation is therefore tentatively placed within the middle “texanus” Zone.

Bentonville Type Locality

At the Bentonville type locality (Figure 6), limestone lenses within the Pineville tripolite of the Reeds Spring Formation yielded Gnathodus bulbosus and unidentified gnathodids. The Pineville tripolite section is therefore placed within the bulbosus Zone. Fauna of the lower “texanus” Zone occur within the basal 9 cm (0.3 ft) the Bentonville Formation. Above the basal Bentonville Formation, G. pseudosemiglaber, G. linguiformis, and Taphrognathus varians were recovered within the remaining section of the Bentonville Formation at this location, which is placed in the middle “texanus” Zone.

Comparison and Integration with Previous Conodont Investigations

In southwestern Missouri, north of the Boone County, Arkansas, localities, the bulbosus Zone occurs in the Reeds Spring Formation at Roaring River State Park (location 13) and then in progressively higher stratigraphic positions within the Bentonville Formation northward toward Springfield, Missouri, including the Turner Station (location 6) and Chestnut Ridge localities (location 17; see Boardman et al., 2013, their figure 15). The bulbosus Zone was not recognized at the Omaha B locality, and both the Reeds Spring and Bentonville formations yielded taxa consistent with the lower “texanus” Zone. In contrast, the bulbosus Zone was recognized within the entirety of the exposed Reeds Spring and basal Bentonville Formations at the Omaha A locality. Farther south, at the Bentonville co-type locality, the bulbosus Zone is absent within the exposed section of Bentonville Formation (Reeds Spring Formation is not exposed here), which yields taxa of the lower “texanus” Zone and middle “texanus” Zone (Figure 5). Although the Omaha A and Omaha B sections are close to each other, separated by only 0.8 km (0.5 mi), the faunal differences between them are both striking and anomalous. The reason for the discrepancy is unclear, but probable reasons are sparse recovery of identifiable specimens from these strata and difficulty in identifying morphologically similar species of Gnathodus (Lane et al., 1980; Boardman et al., 2013). Examined within the context of previously published conodont biostratigraphic data by Thompson and Fellows (1970) and Boardman et al. (2013), the study interval appears to become progressively younger in a southward direction, with recognized conodont zones occupying lithostratigraphically lower positions.

DEPOSITIONAL FACIES

Four depositional facies, interpreted within a ramp to distally steepened ramp model, were recognized within the study interval and include, in order of increasing depositional energy: outer ramp facies, distal middle ramp facies, proximal middle ramp facies, and inner ramp facies (Figure 7).

Figure 7.

Three-dimensional block diagram representing the depositional environment interpreted from this study. The facies observed represent carbonate deposition on a distally steepened ramp. Approximate depth of fair-weather wave base is estimated to be 20–40 m. The facies succession consists of a peloid dominated bask shoal lagoon environment, which formed adjacent to an ooid shoal bar. A massive grainstone facies represents deposition within an inner ramp setting above fair-weather wave base. A transitional inner to middle ramp environment of grainstones and mud lean packstones with interbedded chert is represented by the Elsey Formation. With decreasing skeletal content, distally, the Reeds Spring Formation represents deposition within a middle to outer ramp environment.

Figure 7.

Three-dimensional block diagram representing the depositional environment interpreted from this study. The facies observed represent carbonate deposition on a distally steepened ramp. Approximate depth of fair-weather wave base is estimated to be 20–40 m. The facies succession consists of a peloid dominated bask shoal lagoon environment, which formed adjacent to an ooid shoal bar. A massive grainstone facies represents deposition within an inner ramp setting above fair-weather wave base. A transitional inner to middle ramp environment of grainstones and mud lean packstones with interbedded chert is represented by the Elsey Formation. With decreasing skeletal content, distally, the Reeds Spring Formation represents deposition within a middle to outer ramp environment.

Outer ramp through proximal middle ramp facies are represented by lithologic variations within the Reeds Spring Formation (including the Elsey Formation of previous usage). Outer ramp facies were deposited within the lowest energy conditions below storm wave base and are dominated by argillaceous lime mudstone lacking current features and containing abundant (greater than 40%) anastomosing networks of gray to black multigenerational chert with Chondrites and Planolites burrows (Mazzullo et al., 2013). Within the distal middle ramp facies, as compared with the outer ramp facies, chert is less abundant and tempestite-derived bioclastic wackestone and packstone beds are more common. Proximal middle ramp facies are represented by the Reeds Spring Formation at the Bentonville co-type, Omaha A, and Omaha B sections, and lithologically resembles the Elsey Formation described by Robertson (1967). The abandonment of the Elsey as a formational designation was proposed by Mazzullo et al. (2013) who interpreted the Elsey as a shallower water gradational depositional phase between the Reeds Spring and Bentonville formations. For this study, these rocks are interpreted as proximal middle ramp facies, replacing the term “Elsey facies” of Miller (2016) to describe the proximal middle ramp phase of the Reeds Spring Formation. Mississippian strata similar to the Reeds Spring Formation and consisting of interbedded packstone and nodular chert were documented along the western flanks of the Transcontinental arch and interpreted as recording deposition in middle to outer ramp settings within a water depth of 25–40 m (82–131 ft) and assuming a storm wave base depth of 40 m (131 ft; Elrick and Read, 1991).

Inner ramp facies, representing deposition above fair-weather wave base, include strata of the Bentonville Formation below the Short Creek Oolite Member (Mazzullo et al., 2013; Price, 2014). These rocks are predominantly cross-stratified, fine- to very coarse-grained, crinoidal and skeletal packstones–grainstones. Very fine- to fine-grained, micrite–lean packstone and skeletal wackestone with light varicolored chert associated with finer-grained beds occur less frequently within the Bentonville Formation, but potentially represent a transition to proximal middle ramp facies. The oolitic grainstone shoals of the Short Creek Oolite Member represent the culmination of high-energy deposition within the inner ramp setting and were succeeded in some areas by bioclastic–peloidal wackestone–packstone–grainstone of a back shoal lagoonal environment (Miller, 2016).

CHRONOSTRATIGRAPHICALLY CONSTRAINED FACIES DISTRIBUTION AND CARBONATE RAMP EVOLUTION

Conodont biostratigraphic data recovered during the course of this investigation, combined with that of previous workers (i.e., Thompson and Goebel, 1968; Thompson and Fellows, 1970; Boardman et al., 2013), provide the basis for a relative chronostratigraphic framework. Subsequent integration of depositional facies analysis described previously provides further testing of the hypothesis that lithostratigraphic boundaries are time-transgressive and that upper Osagean strata consist of a series of time-correlative ramp facies (Thompson, 1986; Boardman et al., 2013).

Four relative time slice maps (Figure 8), each corresponding to refined conodont biozones of Boardman et al. (2013), were constructed through the integration of biostratigraphic data and depositional facies interpretations and those of previous investigations focusing on both the St. Joe Group and Boone Group (Thompson and Goebel, 1968; Thompson and Fellows, 1970; Boardman et al., 2013; Mazzullo et al., 2013; Figure 1; Table 1). Data from 22 previously sampled Reeds Spring localities and 3 previously sampled Bentonville Formation (Burlington–Keokuk) sections were incorporated into these facies maps (Figure 8). Locations shown on each map are those from which elements of that specific biozone were recovered. Therefore, each map illustrates the distribution of interpreted depositional facies within a specific conodont biozone and demonstrates the coeval deposition of rocks otherwise mapped within different lithostratigraphic divisions. The four time-slice maps shown in Figure 8 are the anchoralislatus Zone (Figure 8A), bulbosus Zone (Figure 8B), lower “texanus” Zone (Figure 8C), and middle “texanus” Zone (Figure 8D). The upper “texanus” Zone was not mapped due to inadequate biostratigraphic control.

Figure 8.

Distribution of facies by specific biozone demonstrating a basinward shift in more proximal facies with time. (A) anchoralislatus Zone. Map generated from conodont data from Thompson and Fellows (1970). (B) bulbosus Zone. Ellipsoid around no. 15, McDonald County, Missouri indicates the biozone in the Pierson Limestone. Data from Thompson and Fellows (1970) and Miller (2016). (C) Lower “texanus” Zone map generated from conodont data from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). (D) Middle “texanus” Zone. Map generated from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). Line on Figure 8C represents orientation of cross section shown in Figure 9.

Figure 8.

Distribution of facies by specific biozone demonstrating a basinward shift in more proximal facies with time. (A) anchoralislatus Zone. Map generated from conodont data from Thompson and Fellows (1970). (B) bulbosus Zone. Ellipsoid around no. 15, McDonald County, Missouri indicates the biozone in the Pierson Limestone. Data from Thompson and Fellows (1970) and Miller (2016). (C) Lower “texanus” Zone map generated from conodont data from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). (D) Middle “texanus” Zone. Map generated from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). Line on Figure 8C represents orientation of cross section shown in Figure 9.

AnchoralisLatus Zone

Distribution of time-equivalent depositional facies, based upon the reported occurrence of taxa of the anchoralislatus Zone, is shown in Figure 8A. Although the anchoralislatus Zone, representing the middle Osagean prior to the bulbosus Zone, was not identified within the four sampled outcrops, it was identified in all three of the proximal facies by Thompson and Fellows (1970) and Boardman et al. (2013). Furthermore, the anchoralislatus Zone is important because it is recognized globally and is the zone used by Lane and De Keyser (1980) and Gutschick and Sandberg (1983) to construct the widely referenced model of Early Mississippian deposition in North America. Inner ramp facies of the Bentonville Formation (Burlington–Keokuk) cropping out at the Turners Station locality (location 6) in southeastern Greene County, Missouri, yielded a faunal assemblage assigned to the anchoralislatus Zone (Thompson and Fellows, 1970). Approximately 7.6 kilometers (4.6 mi) south of the Turners Station locality, the Lake Springfield locality (location 8) yielded the anchoralislatus Zone in the proximal middle ramp facies of the Reeds Spring Formation (Elsey of previous usage). The inferred contact between inner ramp and the proximal middle ramp facies was placed between these two localities. In southern Christian County, Missouri, at the Chestnut Ridge locality (location 17), the anchoralislatus Zone faunal assemblage occurs above and below the contact between the proximal middle ramp and outer middle ramp facies of the Reeds Spring Formation (Elsey and Reeds Spring of Thompson and Fellows, 1970). The middle ramp facies is observed to the south and west of the Chestnut Ridge locality and extends southward into northern Arkansas. The southernmost appearance of the anchoralislatus Zone is within the St. Joe Group in southeastern Searcy County, Arkansas, at the Sellers Creek locality (Location 28; Thompson and Fellows, 1970). Here, the St. Joe Group has thinned to a 0.8 meter (2.5 ft) section and is overlain by outer ramp facies of the Reeds Spring Formation (irregularly bedded argillaceous limestone with >50% chert interbeds; Thompson and Fellows, 1970).

Bulbosus Zone

Distribution of time-equivalent depositional facies, based upon the reported occurrence of taxa of the bulbosus Zone, is shown in Figure 8B. Thompson and Fellows (1970) proposed the bulbosus Zone as a defined interval of relative time between the underlying anchoralislatus Zone and overlying Gnathodus texanusTaphrognathus varians Zone (lower “texanus” and middle “texanus” Zones in this study). At the time of this definition (Collinson et al., 1971), a lack of complete exposures made it difficult to determine the distribution of the bulbosus Zone and interpret corresponding depositional facies. New road-cut exposures associated with the improvement of U.S. Highway 65 through Boone County, Arkansas, were critical in evaluating the viability of the carbonate ramp depositional model (Miller, 2016) and extending biostratigraphic coverage. It is also important to note that the in the Upper Mississippi Valley, the bulbosus Zone is not recognized due to its removal by erosion (Collinson et al., 1971).

The bulbosus Zone (late–middle Osagean) records facies migration to the south-southwest after anchoralislatus time. At the Lake Springfield locality (Location 8 in Greene County, Missouri), Thompson and Fellows (1970) reported Gnathodus bulbosus in beds above those containing specimens of G. antetexanus Rexroad and Scott (anchoralislatus Zone) within a section of stacked skeletal grainstone (inner ramp facies) of the Bentonville Formation (Burlington Formation of Thompson and Fellows, 1970). South of the Lake Springfield locality, in northern Boone County, Arkansas, at the Omaha A locality (location 3), proximal middle ramp facies transitions to inner ramp facies also contain abundant specimens of G. bulbosus. Consequently, the contact between the inner ramp facies and the proximal middle ramp facies likely falls near the Arkansas–Missouri state line.

Middle ramp facies sampled in southern Barry County, Missouri, at the Roaring River State Park locality (location 13), is dark, fine crystalline mudstone with grainstone lenses (Thompson and Fellows, 1970). South of Barry County, Missouri, at the Leatherwood locality (location 20) in Carroll County, Arkansas, the middle ramp facies consisting of nodular limestone and anastomosing beds of bluish chert yielded G. bulbosus (bulbosus Zone) (Thompson and Fellows, 1970). Westward in McDonald County, Missouri (locations 15, 21, and 22), the bulbosus Zone is observed in the middle ramp facies (Reeds Spring Limestone), as well as the underlying Pierson Formation. South of McDonald County, in northern Benton County, Arkansas, at the Bentonville type locality (location 1), G. bulbosus was recovered from the Pineville tripolite facies of the Reeds Spring Formation.

Lower “texanus” Zone

Distribution of time-equivalent depositional facies, based upon the reported occurrence of taxa of the lower “texanus” Zone, is shown in Figure 8C. Transition from the bulbosus Zone to the lower “texanus” Zone is marked by an increase in specimens assigned to Gnathodus texanus and decrease in the abundance of G. bulbosus (Boardman et al., 2013). Conodonts corresponding to the lower “texanus” Zone were recovered by Thompson and Fellows (1970) in the inner ramp facies of the Bentonville Formation (Keokuk Formation of previous authors) 15 meters (50 ft) below contact with the oolitic grainstone facies at the Brown Quarry locality (location 9) in Greene County, Missouri. South of Greene County, at the Bentonville co-type and Omaha B localities (locations 2 and 3) in Boone County, Arkansas, the lower “texanus” Zone is found in the inner ramp facies of the Bentonville Formation. In west-central McDonald County, Missouri, at the Lanagan Quarry locality (location 15), middle ramp facies overlying the Pierson Formation contain only G. texanus specimens (Thompson and Fellows, 1970). In southern McDonald County, Missouri, at the Tanner Branch locality (location 22), middle ramp facies yielded conodonts assigned to the lower “texanus” Zone (Thompson and Fellows, 1970). These sections represent the most southerly mappable extent of the middle ramp facies during lower “texanus” time (Figure 8C).

In northeastern Cherokee County, Oklahoma, at the Scraper locality (location 23), the lower “texanus” Zone occurs in Reeds Spring Formation containing 90% chert (Thompson and Fellows, 1970) and is assigned to the outer ramp facies. At the P locality of Thompson and Goebel (1968), southeastern Cherokee County, Kansas (location 29 of this report), the base of the Osagean section appears to correspond to the lower “texanus” Zone as the lower cherty section yielded multiple G. texanus specimens. This is the westernmost extent of the outcrop belt before the Osagean strata dip into the subsurface.

Middle “texanus” Zone

Distribution of time-equivalent depositional facies, based upon the reported occurrence of taxa of the middle “texanus” Zone, is shown in Figure 8D. The middle “texanus” Zone is recognized in the upper section of the Lake Springfield locality (location 8) (Thompson and Fellows, 1970), where it underlies the oolitic grainstone facies and contains an abundance of Gnathodus texanus relative to Taphrognathus varians, a form that makes its first appearance. The middle “texanus” Zone occurs in the upper inner ramp facies at the co-type locality (location 2) in northern Boone County, Arkansas, and the P and Q localities (locations 29 and 30) of Thompson and Goebel (1968) in Cherokee County, Kansas. In eastern Cherokee County, Oklahoma, the Tahlequah North locality (location 24) contains the middle “texanus” Zone in middle ramp facies (Reeds Spring Limestone, Thompson and Goebel, 1968). Inner ramp facies likely extend farther southwestward into northeastern Oklahoma, but lack surface exposure before dipping into the subsurface.

Time-Slice Map Stacking and Inferred Carbonate Ramp Setting

Vertical stacking of the conodont-based time-slice maps confirms that interpreted depositional facies, which essentially represent defined lithostratigraphic units, are time-transgressive (Figure 9). Stacking of the time-slice maps also demonstrates the northeast-to-southwest progradational nature of these strata, which agrees with previous interpretations by Mazzullo et al. (2011) and Boardman et al. (2013).

Figure 9.

Generalized interpretation of facies by biozone. Each biozone records a transition from proximal to more distal facies. The progression of facies basinward includes inner ramp facies (light blue), proximal middle ramp facies (light yellow), distal middle ramp facies (light gray), and outer ramp facies (dark gray). The position of outcrops, with corresponding biostratigraphic-constrained facies, support a ramp depositional model and basinward progradation of more proximal facies with time. The position of this cross section is shown on Figure 8C.

Figure 9.

Generalized interpretation of facies by biozone. Each biozone records a transition from proximal to more distal facies. The progression of facies basinward includes inner ramp facies (light blue), proximal middle ramp facies (light yellow), distal middle ramp facies (light gray), and outer ramp facies (dark gray). The position of outcrops, with corresponding biostratigraphic-constrained facies, support a ramp depositional model and basinward progradation of more proximal facies with time. The position of this cross section is shown on Figure 8C.

The composite section of Thompson and Fellows (1970) in Greene County, Missouri, contains 62.7 m (207 ft) of grainy inner ramp facies lithostratigraphically recognized as the Burlington–Keokuk Formation. The basal grainstone corresponds to the anchoralislatus Zone and the uppermost oolitic grainstone to the middle “texanus” Zone. The time-transgressive nature of these proximal facies demonstrates that during the middle to late Osagean, the Greene County area remained in a proximal position on a southerly dipping carbonate ramp.

The biostratigraphic equivalents to each biozone recorded in the proximal area of Greene County have down-dip proximal middle ramp and distal middle ramp facies equivalents. The middle to upper Osagean section contains four biostratigraphically constrained prograding wedges with distinct intrawedge lithologies that reflect the facies architecture on a distally steepened ramp. Inner ramp facies, proximal middle ramp facies, distal middle ramp facies, and outer ramp facies can occur in different stacking patterns depending on the relative location (proximal or distal) on the carbonate ramp. The observed facies migrated basinward with time, suggesting a long regression occurred during the late Osagean.

The observed lithofacies support a distally steepened ramp model in which skeletal sand shoals become cherty argillaceous facies toward the distally steepened outer ramp setting (Figure 9). Proximal inner ramp deposits include ooid shoal bars and back-shoal peloid- and oncolite-rich lagoonal packstones. Most of the inner ramp is dominated by massive crinoidal grainstones deposited above fair-weather wave base. Crinoidal debris and mud were transported down slope from the inner ramp settings into middle to outer ramp settings. Proximal middle ramp facies are crinoidal grainstones and mud-lean packstones with nodules and discontinuous beds of chert. Skeletal content decreases moving distally from the inner ramp setting to more mud- and chert-rich middle and outer ramp settings (Figure 9).

Sequence Stratigraphy

The Boone Group was interpreted by Handford (1995) as the highstand systems tract of his Burlington Sequence, with the underlying St. Joe Group as the transgressive systems tract. Handford, as well as other previous workers (e.g., Thompson, 1986; Boardman et al., 2013), recognized the time-transgressive nature of the lithostratigraphic boundary between the Reeds Spring Formation and overlying Bentonville Formation (Burlington–Keokuk) and that the two formations represent coeval deposition of distal (Reeds Spring) and proximal (Bentonville) facies within a basinward prograding system. The boundary between the Reeds Spring Formation and underlying St. Joe Group is locally unconformable, but this unconformity resulted from syndepositional forebulge uplift related to incipient Ouachita tectonism and is not considered a sequence boundary (Mazzullo et al., 2013; Mazzullo et al., 2016).

SUMMARY AND CONCLUSIONS

The results of this study provide evidence for a number of critical conclusions regarding the regional architecture of Osagean Boone Group strata and support the assertion that the application of traditional lithostratigraphy alone is inadequate for modern interpretations concerning the depositional character of genetically related carbonate strata:

  1. Conodont recoveries from the Osagean Boone Group support the revised conodont zonal scheme of Boardman et al. (2013). These data show that the long-ranging Gnathodus texanusTaphrognathus varians Zone of Collinson et al. (1971) and texanus Zone of Lane and Brenckle (2005) may be refined using high-frequency sampling. Of specific note are the observed ranges of Gnathodus bulbosus and G. pseudosemiglaber within the upper Osagean section, which facilitate the definitions of the bulbosus Zone, lower “texanus” Zone, and middle “texanus” Zone of Boardman et al. (2013). The upper “texanuspunctatus Zone assigned to the basal Meramecian (Boardman et al., 2013) requires additional biostratigraphic analysis before a zonal definition can be designated with confidence.

  2. Higher-resolution conodont biozones provide a relative chronostratigraphic context in which to evaluate the Mississippian strata and subsequently demonstrate the diachronous nature of component lithostratigraphic divisions of both the Boone Group.

  3. Facies analysis suggest that the Bentonville Formation (including its Short Creek Oolite Member) was deposited within a high-energy inner ramp to moderate-energy proximal middle ramp setting, whereas the Reeds Spring Formation (including the Elsey Formation of previous usage) includes lithologic variations consistent with deposition in moderate-energy proximal middle ramp through low-energy outer ramp settings.

  4. Integration of relative chronostratigraphy (conodont biozones) and facies analysis, regardless of lithostratigraphic boundaries, allow the construction of time slice maps illustrating the spatial relationships between depositional facies that define a basinward transition from high-energy skeletal sand shoals of the inner ramp facies, through low- to moderate-energy middle ramp facies of wackestone–packstone, to low-energy outer ramp facies of cherty argillaceous lime mudstone.

  5. Together, these time slices illustrate the evolution of carbonate deposition along a ramp to distally steepened ramp characterized by northeast-to-southwest progradation of time-equivalent carbonate wedges comprised different defined lithostratigraphic units.

  6. Although useful locally for differentiating strata and mapping, lithostratigraphically defined units and their boundaries are inherently time-transgressive. Conodont biostratigraphic data provide temporal boundaries through which time-equivalent and therefore genetically related strata may be correlated regardless of lithology.

ACKNOWLEDGMENTS

This paper is dedicated to the memory of Darwin R. Boardman II, a passionate and enthusiastic geologist whose devotion to his profession was unrivaled. Darwin’s extension of ammonoid and conodont biostratigraphy well beyond classification to integration with depositional processes encouraged geoscientists to examine their work in a regional context. Conodont SEM photography was performed at the Oklahoma State University Microscopy Laboratory in Stillwater, Oklahoma. Funding for portions of this study was provided by participating petroleum industry sponsors of the Mississippian Lime Consortium at the Boone Pickens School of Geology at Oklahoma State University. The authors gratefully acknowledge the valuable constructive comments offered by Dr. Walter Manger and an anonymous reviewer.

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Figures & Tables

Figure 1.

Generalized map of the Mississippian outcrop belt (gray shading) in the western Ozarks, southern midcontinent, U.S.A. showing outcrop localities that provided lithostratigraphic descriptions and biostratigraphic data used in this study.

Figure 1.

Generalized map of the Mississippian outcrop belt (gray shading) in the western Ozarks, southern midcontinent, U.S.A. showing outcrop localities that provided lithostratigraphic descriptions and biostratigraphic data used in this study.

Figure 2.

Stratigraphic nomenclature of the Mississippian outcrop belt western Ozarks, U.S.A., adapted from Mazzullo et al. (2013). To the right are historically applied nomenclatural terms of southwestern Missouri, northeastern Oklahoma, and northern Arkansas.

Figure 2.

Stratigraphic nomenclature of the Mississippian outcrop belt western Ozarks, U.S.A., adapted from Mazzullo et al. (2013). To the right are historically applied nomenclatural terms of southwestern Missouri, northeastern Oklahoma, and northern Arkansas.

Figure 3.

Comparison of conodont zones of this report, Thompson (1967), Collinson et al. (1971), Lane and Brenckle (2005), and Boardman et al. (2013). This study follows the revised zonal scheme proposed by Boardman et al. (2013) but utilizes an undivided anchoralislatus Zone.

Figure 3.

Comparison of conodont zones of this report, Thompson (1967), Collinson et al. (1971), Lane and Brenckle (2005), and Boardman et al. (2013). This study follows the revised zonal scheme proposed by Boardman et al. (2013) but utilizes an undivided anchoralislatus Zone.

Figure 4.

Stratigraphic measured sections for the (A) Omaha A and (B) Omaha B localities in Boone County, Arkansas. At both locations, the contact between the Reeds Spring and Bentonville formations is marked by a decrease in chert and increase in bedding thickness (Robertson, 1967; Mazzullo et al., 2011; 2013). Conodonts recovered are displayed with numbers corresponding to sample location. The conodonts recovered indicate a higher abundance of the Gnathodus texanus element relative to a marked decrease in abundance of G. bulbosus. The conodonts recovered place this section in the lower “texanus” Zone (Boardman et al., 2013). Conodonts shown for the Omaha A locality (A), samples 1–3 include intraspecific variations of G. bulbosus. For the Omaha B locality (B), conodonts shown include (1) Gnathodus sp., (2) G. bulbosus, and (3)–(5) Gnathodus sp. aff. texanus.

Figure 4.

Stratigraphic measured sections for the (A) Omaha A and (B) Omaha B localities in Boone County, Arkansas. At both locations, the contact between the Reeds Spring and Bentonville formations is marked by a decrease in chert and increase in bedding thickness (Robertson, 1967; Mazzullo et al., 2011; 2013). Conodonts recovered are displayed with numbers corresponding to sample location. The conodonts recovered indicate a higher abundance of the Gnathodus texanus element relative to a marked decrease in abundance of G. bulbosus. The conodonts recovered place this section in the lower “texanus” Zone (Boardman et al., 2013). Conodonts shown for the Omaha A locality (A), samples 1–3 include intraspecific variations of G. bulbosus. For the Omaha B locality (B), conodonts shown include (1) Gnathodus sp., (2) G. bulbosus, and (3)–(5) Gnathodus sp. aff. texanus.

Figure 5.

Stratigraphic measured section for the Bentonville co-type locality in Boone County, Arkansas, which includes the upper Bentonville Formation, Short Creek Oolite Member, and lower part of the Ritchey formation. Conodonts recovered are displayed with numbers corresponding to sample location. Conodonts shown for the Bentonville co-type locality include (1) Gnathodus sp. aff. texanus, (2) Gnathodus n. sp. 14 (Boardman et al., 2013), (3) Gnathodus sp. aff. bulbosus (Miller, 2016), (4)–(6) G. pseudosemiglaber, (7a) Gnathodus sp. aff. texanus, (7b) Gnathodus n. sp. B aff. pseudosemiglaber (Miller, 2016), (8) and (9) G. pseudosemiglaber, (10) Gnathodus sp. aff. pseudosemiglaber (Miller, 2016), (11)–(13) Gnathodus sp. aff. texanus, (14) G. linguiformis, (15) G. pseudosemiglaber, (16) Taphrognathus varians, (17), 18) and (19) Gnathodus n. sp. A aff. texanus (Miller, 2016).

Figure 5.

Stratigraphic measured section for the Bentonville co-type locality in Boone County, Arkansas, which includes the upper Bentonville Formation, Short Creek Oolite Member, and lower part of the Ritchey formation. Conodonts recovered are displayed with numbers corresponding to sample location. Conodonts shown for the Bentonville co-type locality include (1) Gnathodus sp. aff. texanus, (2) Gnathodus n. sp. 14 (Boardman et al., 2013), (3) Gnathodus sp. aff. bulbosus (Miller, 2016), (4)–(6) G. pseudosemiglaber, (7a) Gnathodus sp. aff. texanus, (7b) Gnathodus n. sp. B aff. pseudosemiglaber (Miller, 2016), (8) and (9) G. pseudosemiglaber, (10) Gnathodus sp. aff. pseudosemiglaber (Miller, 2016), (11)–(13) Gnathodus sp. aff. texanus, (14) G. linguiformis, (15) G. pseudosemiglaber, (16) Taphrognathus varians, (17), 18) and (19) Gnathodus n. sp. A aff. texanus (Miller, 2016).

Figure 6.

Conodonts recovered from the Bentonville type locality. The outcrop pictured above has sampled locations indicated by the black stars. (1) Gnathodus bulbosus and an unidentifiable gnathodid from limestone lenses in the Pineville tripolite. (2) The contact between the Pineville Tripolite and the Bentonville Formation recovered G. bulbosus below the contact, and Gnathodus sp. aff. pseudosemiglaber above the contact within the Bentonville Formation. (3) Gnathodus pseudosemiglaber (3a) and Taphrognathus varians (3b, c) were recovered above the basal Bentonville Formation. (4) At the top of the outcrop a specimen with affinity to G. pseudosemiglaber and possibly G. linguiformis was recovered. Conodonts are numbered corresponding to specimen identification.

Figure 6.

Conodonts recovered from the Bentonville type locality. The outcrop pictured above has sampled locations indicated by the black stars. (1) Gnathodus bulbosus and an unidentifiable gnathodid from limestone lenses in the Pineville tripolite. (2) The contact between the Pineville Tripolite and the Bentonville Formation recovered G. bulbosus below the contact, and Gnathodus sp. aff. pseudosemiglaber above the contact within the Bentonville Formation. (3) Gnathodus pseudosemiglaber (3a) and Taphrognathus varians (3b, c) were recovered above the basal Bentonville Formation. (4) At the top of the outcrop a specimen with affinity to G. pseudosemiglaber and possibly G. linguiformis was recovered. Conodonts are numbered corresponding to specimen identification.

Figure 7.

Three-dimensional block diagram representing the depositional environment interpreted from this study. The facies observed represent carbonate deposition on a distally steepened ramp. Approximate depth of fair-weather wave base is estimated to be 20–40 m. The facies succession consists of a peloid dominated bask shoal lagoon environment, which formed adjacent to an ooid shoal bar. A massive grainstone facies represents deposition within an inner ramp setting above fair-weather wave base. A transitional inner to middle ramp environment of grainstones and mud lean packstones with interbedded chert is represented by the Elsey Formation. With decreasing skeletal content, distally, the Reeds Spring Formation represents deposition within a middle to outer ramp environment.

Figure 7.

Three-dimensional block diagram representing the depositional environment interpreted from this study. The facies observed represent carbonate deposition on a distally steepened ramp. Approximate depth of fair-weather wave base is estimated to be 20–40 m. The facies succession consists of a peloid dominated bask shoal lagoon environment, which formed adjacent to an ooid shoal bar. A massive grainstone facies represents deposition within an inner ramp setting above fair-weather wave base. A transitional inner to middle ramp environment of grainstones and mud lean packstones with interbedded chert is represented by the Elsey Formation. With decreasing skeletal content, distally, the Reeds Spring Formation represents deposition within a middle to outer ramp environment.

Figure 8.

Distribution of facies by specific biozone demonstrating a basinward shift in more proximal facies with time. (A) anchoralislatus Zone. Map generated from conodont data from Thompson and Fellows (1970). (B) bulbosus Zone. Ellipsoid around no. 15, McDonald County, Missouri indicates the biozone in the Pierson Limestone. Data from Thompson and Fellows (1970) and Miller (2016). (C) Lower “texanus” Zone map generated from conodont data from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). (D) Middle “texanus” Zone. Map generated from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). Line on Figure 8C represents orientation of cross section shown in Figure 9.

Figure 8.

Distribution of facies by specific biozone demonstrating a basinward shift in more proximal facies with time. (A) anchoralislatus Zone. Map generated from conodont data from Thompson and Fellows (1970). (B) bulbosus Zone. Ellipsoid around no. 15, McDonald County, Missouri indicates the biozone in the Pierson Limestone. Data from Thompson and Fellows (1970) and Miller (2016). (C) Lower “texanus” Zone map generated from conodont data from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). (D) Middle “texanus” Zone. Map generated from Thompson and Fellows (1970), Thompson and Goebel (1968), and Miller (2016). Line on Figure 8C represents orientation of cross section shown in Figure 9.

Figure 9.

Generalized interpretation of facies by biozone. Each biozone records a transition from proximal to more distal facies. The progression of facies basinward includes inner ramp facies (light blue), proximal middle ramp facies (light yellow), distal middle ramp facies (light gray), and outer ramp facies (dark gray). The position of outcrops, with corresponding biostratigraphic-constrained facies, support a ramp depositional model and basinward progradation of more proximal facies with time. The position of this cross section is shown on Figure 8C.

Figure 9.

Generalized interpretation of facies by biozone. Each biozone records a transition from proximal to more distal facies. The progression of facies basinward includes inner ramp facies (light blue), proximal middle ramp facies (light yellow), distal middle ramp facies (light gray), and outer ramp facies (dark gray). The position of outcrops, with corresponding biostratigraphic-constrained facies, support a ramp depositional model and basinward progradation of more proximal facies with time. The position of this cross section is shown on Figure 8C.

Table 1.

Outcrops utilized in the biostratigraphic reconstruction of the upper Osagean depositional regimes. Localities are numbered, and numbers correspond to the outcrops identified in Figure 1. County locations are provided, as well as the referenced study in which biostratigraphic data were obtained.

 LOCATIONSCOUNTY, STATEREPORT REFERENCED
1Bentonville type localityBenton County, ArkansasThis report
2Bentonville co-type localityBoone County, ArkansasThis report
3AOmaha A localityBoone County, ArkansasThis report
3BOmaha B localityBoone County, ArkansasThis report
4Reeds Spring type localityStone County, MissouriThompson and Fellows (1970)
5Elsey type localityStone County, MissouriRobertson (1967)
6Turners StationGreene County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
7Grand Falls Chert type localityJasper County, MissouriRobertson (1967)
8Lake SpringfieldGreene County, MissouriThompson and Fellows (1970)
9Brown QuarryGreene County, MissouriThompson and Fellows (1970)
10Shoal CreekNewton County, MissouriThompson and Fellows (1970)
11Kirshmann Jeffries QuarryJasper County, MissouriThompson and Fellows (1970)
12Baird Mountain QuarryTaney County, MissouriThompson and Fellows (1970)
13Roaring River State ParkBarry County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
14ChesapeakeLawrence County, MissouriThompson and Fellows (1970)
15Lanagan QuarryMcDonald County, MissouriThompson and Fellows (1970)
16Madry QuarryBarry County, MissouriThompson and Fellows (1970)
17Chestnut RidgeChristian County, MissouriThompson and Fellows (1970)
18Marble FallsNewton County, ArkansasThompson and Fellows (1970)
19St. JoeSearcy County, ArkansasThompson and Fellows (1970)
20LeatherwoodCarroll County, ArkansasThompson and Fellows (1970)
21PowellMcDonald County, MissouriThompson and Fellows (1970)
22Tanner BranchMcDonald County, MissouriThompson and Fellows (1970)
23ScraperCherokee County, OklahomaThompson and Fellows (1970)
24Tahlequah NorthCherokee County, OklahomaThompson and Fellows (1970); Boardman et al. (2013, as “No Head Hollow Reference Locality”)
25Type Walls FerryIndependence County, ArkansasThompson and Fellows (1970)
26Glen Creek SouthIndependence County, ArkansasThompson and Fellows (1970)
27Walls Ferry Dam NorthIndependence County, ArkansasThompson and Fellows (1970)
28Sellers CreekSearcy County, ArkansasThompson and Fellows (1970)
29P. LocalityCherokee County, KansasThompson and Goebel (1968)
30Q. LocalityCherokee County, KansasThompson and Goebel (1968)
 LOCATIONSCOUNTY, STATEREPORT REFERENCED
1Bentonville type localityBenton County, ArkansasThis report
2Bentonville co-type localityBoone County, ArkansasThis report
3AOmaha A localityBoone County, ArkansasThis report
3BOmaha B localityBoone County, ArkansasThis report
4Reeds Spring type localityStone County, MissouriThompson and Fellows (1970)
5Elsey type localityStone County, MissouriRobertson (1967)
6Turners StationGreene County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
7Grand Falls Chert type localityJasper County, MissouriRobertson (1967)
8Lake SpringfieldGreene County, MissouriThompson and Fellows (1970)
9Brown QuarryGreene County, MissouriThompson and Fellows (1970)
10Shoal CreekNewton County, MissouriThompson and Fellows (1970)
11Kirshmann Jeffries QuarryJasper County, MissouriThompson and Fellows (1970)
12Baird Mountain QuarryTaney County, MissouriThompson and Fellows (1970)
13Roaring River State ParkBarry County, MissouriThompson and Fellows (1970); Boardman et al. (2013)
14ChesapeakeLawrence County, MissouriThompson and Fellows (1970)
15Lanagan QuarryMcDonald County, MissouriThompson and Fellows (1970)
16Madry QuarryBarry County, MissouriThompson and Fellows (1970)
17Chestnut RidgeChristian County, MissouriThompson and Fellows (1970)
18Marble FallsNewton County, ArkansasThompson and Fellows (1970)
19St. JoeSearcy County, ArkansasThompson and Fellows (1970)
20LeatherwoodCarroll County, ArkansasThompson and Fellows (1970)
21PowellMcDonald County, MissouriThompson and Fellows (1970)
22Tanner BranchMcDonald County, MissouriThompson and Fellows (1970)
23ScraperCherokee County, OklahomaThompson and Fellows (1970)
24Tahlequah NorthCherokee County, OklahomaThompson and Fellows (1970); Boardman et al. (2013, as “No Head Hollow Reference Locality”)
25Type Walls FerryIndependence County, ArkansasThompson and Fellows (1970)
26Glen Creek SouthIndependence County, ArkansasThompson and Fellows (1970)
27Walls Ferry Dam NorthIndependence County, ArkansasThompson and Fellows (1970)
28Sellers CreekSearcy County, ArkansasThompson and Fellows (1970)
29P. LocalityCherokee County, KansasThompson and Goebel (1968)
30Q. LocalityCherokee County, KansasThompson and Goebel (1968)

Contents

GeoRef

References

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