The widespread application of the term Waulsortian to Tournaisian to Viséan carbonate mud mounds in North America without regard to specific characteristics such as size, patterns of aggradation, and composition, contributes little to our understanding of their origin. Our principal objective is to illustrate these characteristics and use them to demonstrate the similarities and differences between mounds in the Ozark region of North America and Waulsortian mounds of Ireland. We use cement stratigraphy to characterize diagenetic history, classify mounds based on sedimentary features and composition, and propose explanations for observed similarities and differences that we attribute to depositional setting and tectonics.

Late Tournaisian to early Viséan (Kinderhookian and Osagean in North America) crinoidal–bryozoan carbonate buildups are widely distributed throughout North America and Europe, as well as Asia and North Africa (Figure 1). Commonly referred to as Waulsortian or Waulsortian-type banks, Tournaisian–Viséan mound structures became prominent after the Devonian mass extinctions and demise of stromatoporoid and tabulate coral reefs (Bridges et al., 1995; Webb, 2002).

First described in Waulsort, Belgium, by Dupont (1863), Waulsortian mounds are defined by the presence of successive phases of polygenetic carbonate mud known as polymuds, that accreted in open spaces and in semienclosed cavities (Wood, 2001) and specific grain-type assemblages that include fenestrate bryozoa and crinoids (Lees and Miller, 1995). The core facies of Waulsortian mounds contain abundant fenestrate bryozoa in a wackestone matrix with both marine-fibrous and sparry calcite cement that fills stromatactis structures (cavity systems) and other porosity, whereas the flanking facies are mostly crinoid-rich packstone. The lithologic characteristics of mounds within the Tournaisian (Kinderhookian and Osagean) Compton, and Pierson limestones, St. Joe Group, in the Ozark region of the North American midcontinent are diverse and display intra- and interbed variability. Furthermore, mounds in the Compton and Pierson limestones do not share several features commonly ascribed to Waulsortian mounds in Northern Europe and elsewhere; most important of these features is well-developed stromatactis structures.

Compton and Pierson mounds have been described as crinoidal bioherms and reefs (Laudon, 1939; Harbaugh, 1957; Huffman, 1958; Troell, 1962; Anglin, 1966; Morris et al., 2013). The terms Waulsortian and Waulsortian-type were applied to these mounds by Manger and Thompson (1982), Gandl (1983), Lane (1984), King (1986), Shelby (1986), King (1990), and Chandler (2001). Morris et al. (2013) differentiated mounds in the Ozarks as Waulsortian-type for mud-cored and crinoidal sand bars for grain-cored.

This work examines mounds in the Compton and Pierson limestones (hereafter called Compton and Pierson mounds; Figure 2) and “classic” Waulsortian mounds in the Feltrim Limestone, Ireland (Figure 3; hereafter called Feltrim mounds). Our objectives are to (1) establish similarities and differences in mound morphology, composition, and diagenetic history and (2) test the hypothesis that mounds formed in similar depositional settings will be similar sized, display similar geometries and patterns of aggradation, group compositionally based on the relative abundance of mud and skeletal content, and have similar cement stratigraphies. Our findings, which are based on field observations and the statistical analysis of petrographic data collected primarily from polarized light and cathodoluminescence (CL) petrography, show that though all mounds have similar cement stratigraphies, Feltrim mounds are larger and aggrade laterally and vertically (Lees, 1994; Lees and Miller, 1995), whereas Compton and Pierson mounds are smaller and stack laterally. Feltrim mounds contain well-developed stromatactis, sheet spar, and polygenetic muds that are uncommon to absent in Compton and Pierson mounds. Compton and Pierson mounds are grainier, a difference attributed to reduced accommodation space and shallower water deposition.

Mounds in the Ozarks occur primarily in the Compton and Pierson limestones of the St. Joe Group, a carbonate-dominated interval that contains, in ascending order, the Compton Limestone followed by the Northview Shale and Pierson Limestone (Thompson and Fellows, 1970; Figure 3). The Compton Limestone (T2) is composed of gray thinly bedded mudstone to packstone that is normally $<1to6m(3.3–20ft)$ thick, except where it contains mounds (Huffman, 1958; Anglin, 1966; Morris et al., 2013; Childress and Grammer, 2015). The Northview Shale (T2) is calcareous shale to silty wackestone that is generally less than 3 m (10 ft) and lacks mounds. The Pierson Limestone (T3) is composed of gray thinly bedded crinoidal wackstone–packstone, $<1to33m(3–110ft)$ thick (Anglin, 1966) that contains single mounds and mound complexes. The St. Joe Group thins to the south and isolated areas of thickening are attributed to mounds (Figure 4).

Overlying the Pierson Limestone is the Boone Group, also known as the Boone Formation (Shelby, 1986; Handford, 1989), that contains the Reeds Spring Limestone and Bentonville limestone (formerly Burlington–Keokuk; Mazzullo et al., 2011; Miller and Boardman II, 2015). Waulsortian-type mounds are described in the Viséan Short Creek Oolite Member of the Bentonville limestone (Harbaugh, 1957; Huffman, 1958, 1959), but were not included in this study, which focuses on Compton and Pierson mounds that are more abundant and accessible.

Early Tournaisian (T2) strata in Ireland are mostly shallow water carbonates and do not host mounds (Strogen et al., 1990). Waulsortian mound facies occur principally in the late Tournaisian Feltrim Limestone in the Dublin Basin and the equivalent age Limerick Limestone in the Shannon Trough (Strogen et al., 1996; Figure 5). Carbonate generation in the late Tournaisian kept pace with subsidence to produce a maximum thickness of $600m(1970ft)$ in the Dublin Basin (Nolan, 1989) and $600to1200m(1969–3937ft)$ in the Shannon Trough (Somerville and Strogen, 1992). In both areas, Waulsortian facies are composed of bryozoan-rich, mudstone–wackestone (Somerville and Strogen, 1992). Across much of the Dublin Basin, the Feltrim Limestone was uplifted, exposed, and underwent karst dissolution prior to deposition of the late Tournaisian, Tober Colleen Formation, a deep-water basinal mudstone, and oolites of the Allenwood Formation in surrounding shelf areas (Hitzman, 1995; Somerville and Waters, 2011). These units were succeeded by the early Viséan basinal limestone of the Lucan Formation, and laterally equivalent bioclastic platform carbonates of the Lane Limestone in north Dublin that contain Waulsortian-type mounds (Somerville et al., 1992a; Strogen et al., 1996).

Compton and Pierson mounds formed in a relatively shallow marine setting (Burlington Shelf) associated with the Transcontinental Arch, a positive feature during the Mississippian that extended across central North America (Figure 6; Lane, 1984). To the east of the arch, this setting produced mounds in the Ozarks, as well as the Chappel Limestone, west–central Texas, and the Lake Valley Formation, southern New Mexico. To the west of the arch, mounds are observed in the Lodgepole Formation of Montana (Smith, 1982). Compton and Pierson mounds were restricted to what Lane (1978, 1984) called a shelf-edge (Burlington Shelf), now interpreted as a ramp (Morris et al., 2013), or distally steepened ramp (Childress and Grammer, 2015). This gently sloping, shallow-water setting persisted during the Tournaisian–Viséan, with the Compton and Pierson formations (Figure 6) hosting most carbonate mounds.

In Ireland, late Tournaisian Waulsortian mounds occur in the Feltrim Limestone, Dublin Basin, Limerick Limestone, and Shannon Trough (Somerville et al., 1992b), whereas Waulsortian-type mounds are observed in early Viséan platform carbonates (Somerville et al., 1992a; Strogen et al., 1996). Though the North American and northern European depocenters were geographically closer and at a similar latitude during the late Tournaisian to early Viséan (Figure 1), subsidence and carbonate deposition rates in Irish basins far exceeded rates on the Ozark ramp. As a result, the Dublin Basin and Shannon Trough contain over 600 and $900m(1970and2950ft)$⁠, respectively, of late Tournaisian limestone (Somerville, 2003) compared to less than $30m(100ft)$ for the St. Joe Group (Figures 4, 5). In the Dublin Basin and Shannon Trough depocenters, Tournaisian mounds developed in a deeper water outer (distal) ramp setting, which began in southern Ireland and prograded northeastward (Somerville, 2003). In contrast, Viséan mounds formed in a shallower water outer shelf-margin setting (Somerville et al., 1992a; Somerville, 2003).

Tournaisian to early Viséan carbonate mounds are widely distributed across North America and Europe, including some identified in the subsurface in petroleum-producing areas such as the Cherokee Platform, Oklahoma; Appalachian Basin, Tennessee and Kentucky; Williston Basin, Montana; Illinois Basin, Illinois; and Hardeman Basin, Texas (Figure 1; Bolton et al., 1982; Davis et al., 1989; Lees and Miller, 1995; Webb, 2002; Lasemi et al., 2003; Godwin et al., 2013). Mounds outcropping in the Ozarks have been extensively studied, and the interpretation of their origin has changed with increased understanding of carbonate depositional systems. Originally called crinoidal bioherms, Laudon (1939), Harbaugh (1957), and Troell (1962) proposed that Ozark mounds formed from the accumulation of crinoid fragments in shallow-water environments. Pray (1958) suggested that fenestrate bryozoans provided a framework for mud-rich bioherms in New Mexico; Troell (1962) and Anglin (1966) proposed that mudstone–wackestone mounds in the Ozarks formed from bryozoan-entrapped sediments. Likewise, the origin and distribution of European Waulsortian mounds and banks are not well explained, although upwelling of deep-marine waters or methane seeps are cited as possible factors in their genesis (Lees and Miller, 1995). Waulsortian mounds in Ireland have cores that are dominantly peloidal lime mudstone and wackestone, whereas Viséan mud mounds exhibit more biotic diversity including colonial corals, green algae, and cephalopods (nautiloids and goniatites; Somerville, 2003).

Carbonate mud is a fundamental constituent of European (Irish) Waulsortian banks, whose origin is attributed to microbial action (Lees and Miller, 1995). These polygenetic muds or polymuds, formed in open areas and accumulated in cavities to form highly structured deposits that display successive geopetal relationships (Lees, 1964; Lees and Miller, 1985, 1995; Devuyst and Lees, 2001; Wood, 2001). Cavity filling carbonate mud is scarce in Pierson and Compton mounds and when present, forms geopetals with calcite-cemented, stromatactis-like features (Unrast, 2012).

Waulsortian buildups of Europe are dominated by sparry calcite and calcite-filled features called stromatactis. Stromatactis cavities typically have an irregular, digitate roof and a flat base. These cavities contain inclusion-rich radiaxial-fibrous calcite and late-equant, sparry calcite cements (Lees and Miller, 1985, 1995; Miller, 1986; Somerville et al., 1992b; Gregg et al., 2001). Commonly, fenestrate bryozoan fronds appear to support the cavity roof (Somerville, 2003). Stromatactis cavities were not recognized in Pierson mounds, but Compton mounds contain scarce sparry calcite-filled cavities identified as stromatactis (Manger and Thompson, 1982; Gandl, 1983; Morris et al., 2013).

Three mounds in Ireland and four in the Ozark region, U.S.A., were selected for detailed study. The Ozark mounds included two localities each in the Compton and Pierson limestones (Figure 2). The Compton Limestone mounds are located in southern Missouri along road cuts north of Jane in McDonald County, and east of Carr Lane in Stone County (Figure 2). Representative mounds in the Pierson Limestone are located east of Lake Eucha in Delaware County, Oklahoma, and east of Siloam Springs in Benton County, Arkansas (Figure 2). Nine Irish Waulsortian mounds were examined, and three were studied in detail: Feltrim Quarry, County Dublin; Hill 707, County Galway; and Mullawornia Quarry, County Longford (Figure 5). Fieldwork included describing the size, geometry, and depositional features of mounds, establishing lithologic and facies relationships, and sampling.

Carbonate samples from each field location were slabbed and photographed. Approximately 100 thin sections from the Ozarks and 35 from Ireland were evaluated petrographically using polarized light microscopy combined with standard staining techniques (Friedman, 1959). The lithology, relative abundance of major and minor allochems, cements, and accessory minerals were determined for each thin section, which was then classified according to Dunham’s (1962) carbonate classification scheme. This was followed by CL petrography using a CITL CL8200 MK5-1 Optical Cathodoluminescence System mounted on an Olympus BX 51 microscope equipped with 4× and 10× long focal distance objective lenses, and a “Q Imaging” 5-megapixel, cooled, low-light, digital camera system. Mineralogical evaluation was augmented by x-ray diffractometry using a Philips Analytical PW 1830 x-ray diffractometer. Samples were x-rayed across a range of 2θ angles from 10° to 55° with a step size of 0.025° scanned at 0.5 s/step.

Statistical data on the abundances of grains and carbonate mud in core and flanking facies were collected by counting 400 random points on each thin section. The percentages of allochems and detrital matrix were treated as the dependent variables, whereas core and flanking facies were treated as independent variables. The procedures of statistical analysis were based on Freund and Wilson (2003) and the dataset processed using Statistical Analysis Software (SAS; Unrast, 2012).

A group of small mounds in the lower Compton Limestone crops out on the east side of U.S. Route 71 near the town of Jane in McDonald County, Missouri (Figure 2). The road-cut is less than one-half mile (0.8 km) north of the intersection of U.S. Route 71 and Missouri Highway 90 at coordinates 36°32’46.40”N latitude and 94°19’34.87”W longitude and contains a larger primary mound and an adjacent smaller mound situated below and to the left (Figure 7A). Additional smaller mounds are observed in the Compton Limestone immediately to the southeast and northwest along the Highway 71 outcrops. The core of the larger mound is $4.2m(14ft)$ high and $15.5m(51ft)$ wide (Table 1). Both of these asymmetrical mound cores are mostly light gray massive bryozoan–crinoid mudstone–wackestone. The bed hosting these mounds contains syndepositional carbonate breccia (Morris et al., 2013) that is compositionally similar to the mound core facies and may be remnants of mound blocks broken in transport (Childress and Grammer, 2015). Flanking beds above these mounds are thinly laminated wackestone–packstone that dips to the northwest and southeast with dip angles from $18°$ to $30°$⁠. A more diverse biota, represented by rugose and tabulate corals and crinoid fragments, occurs near the top of the larger mound core. The beds overlying the flanking facies and mound core are medium-gray, noncherty mudstone with no apparent macro-invertebrate fossils.

The 2nd group of Compton Limestone mounds is a complex in Stone County, Missouri, at $36°31’50.38”N$ and $93°27’51.27”W$⁠. The outcrop is a road-cut on the north side of Highway 86 approximately $10.6km(7mi)$ east of the intersection of Highway 86 and Highway 39 near the small town of Carr Lane (Figure 2). This mound complex was studied by Gandl (1983), Lasemi et al. (2003), and Morris et al. (2013) and consists of a large asymmetrical mound core with a smaller symmetrical buildup at the base (Figure 7B, C). The asymmetrical mound core is steeper dipping to the east and forms a mass $3m(9ft)$ high and $28.5m(94ft)$ wide (Table 1). The wackestone mound core is brecciated near the margin with clasts ranging from $0.25to1cm(0.1–0.4in)$ in diameter. One of the smaller buildups, which is intact, is $0.5m(1.6ft)$ high and has thin flanking facies of $0.3m(1ft)$ of yellow weathered laminated mudstone (Figure 7B). Flanking facies are less developed and more difficult to identify here as compared to other Ozark mounds. The nonmound laminated beds overlying the mound core are light-gray wackestone with 1 mm size crinoid fragments. These beds dip gently away from the apex to the west and east at angles of about $10–15°$ (Table 1).

Cores of Compton mounds are $>50%$ mud, $30–40%$ skeletal, and $10–20%$ cement. They classify as mudstone near the center and wackestone toward the base and top (Figure 8A). Bioclasts are approximately $50%$ bryozoans and $20–35%$ fragmented crinoids $(0.5–3mm)$⁠. Other skeletal fragments include brachiopod shells $(0.7–3mm)$ and spines, gastropods (0.5–1 mm), ostracodes $(2–8mm)$⁠, and foraminifers $(≈5mm)$⁠. Geopetal structures within brachiopods in the Compton mound–core in McDonald County range from $<1°to45°$⁠, whereas those for brachiopods in Stone County range from $15°$ to $30°$ (Table 1). Flanking facies are approximately $40–60%$ crinoids (Figure 8B), and $30%$ bryozoans with concavo-convex and sutured contacts between bioclasts. Other features include dissolution seams, late-stage stylolites and euhedral dolomite rhombs near the mound core base and irregular or cubic pyrite scattered across core and flanking facies (Figure 8C, D).

Five stages of calcite (Table 2) precipitated in vugs, geopetal structures, and stromatactis-like cavities were delineated using CL petrography. In Compton mound samples, larger brachiopod fragments and fractures are edged with fine fibrous to bladed calcite cement (Stage 1) followed by coarse equant calcite overgrowths. Under CL, calcite overgrowths and finer grained calcite cements appear non-CL black (Stage 2 cement) with bright yellow rims (Stage 3 cement; Figure 8E, F). Dissolution incisions (Kaufman et al., 1988) occur in the bright and banded equant calcite. Subsequent to the bright rims is moderate CL Stage 4 cement that is multiple dull light and dark bands called cyclic zonation (Emery and Dickson, 1989; Figure 8E, F). The last stage is very dull equant calcite (Stage 5) that occurs in the center of vugs and stromatactis-like features (Figure 8E, F).

Mound cores in the Compton Limestone are calcite-rich and contain stromatactis-like features with radiaxial-fibrous calcite lining cavity walls and radiating from bryozoan fronds followed by equant calcite cement (Figure 9A). In contrast, flanking facies are mud-rich and devoid of cavity-filling cements (Figure 9B). Stromatactis-like features contain calcite cement Stages 1, 3, 4 and 5, but Stage 2 cement is consistently missing (Figure 9A, C–E). Vugs, fractures, and the interior of fossils are filled with equant, fine- to medium-size $(0.3–3mm)$ calcite cements (Stages 2 through 5; Figure 9F–G). In addition to calcite cement (Figure 10A–C), stromatactis-like features contain limited numbers of small allochems, especially bryozoan fragments. Rarely, light-colored mud and the overlying sparry calcite cement form geopetals (Figure 10C).

Mounds in the Pierson Limestone were studied at two locations: Benton County, Arkansas, and Delaware County, Oklahoma (Figure 11). The Pierson mounds in Benton County (⁠$36°10’16.66”N$ and $94°23’26.05”W$⁠) occur along the south side of U.S. Route 412 approximately 4 miles (6.4 km) east of Siloam Springs (Figure 2). Here two prominent mounds occur within the upper Pierson Limestone (Figure 11A). The mound core of the west mound is approximately $3.9m(13ft)$ high and $8.5m(28ft)$ wide, whereas the mound core of the east mound is approximately $4.1m(14ft)$ high and $8m(26ft)$ wide (Figure 11B). The cores of both mounds display two growth periods. The early growth in both mounds is symmetrical, whereas the overlying 2nd growth phase is asymmetrical in the west mound (Table 1; Figure 11B), but symmetrical in the east mound. These mounds have been described previously, including Chandler (2001) who proposed that they are transported blocks and Morris et al. (2013), who described two periods of accumulation: an early phase mound core and a later capping grainstone that is saturated with oil at the apex of the east mound. The cores of both the east and west mounds are dominantly packstone containing mostly crinoid and bryozoan fragments. Tabulate corals occur near the base of the core of the east mound, but are not recognized in mound–core facies of other Pierson and Compton mounds.

The flanking beds of dark-gray thinly laminated crinoidal packstone and greenish-gray fossiliferous shale are thicker on the west side of both mounds as compared to the east. Flanking beds of the asymmetrical west mound dip $55°$ to the east and $30°$ to the west (Table 1). In contrast, flanking beds of the more symmetrical east mound dip $30°$ to the west and $36°$ to the west (Table 1). Overlying the flanking beds is the gently dipping Reeds Spring Limestone, which is composed of interbedded dark mudstone and chert.

The Pierson mound cores in Benton County, Arkansas are $50%$ skeletal, $30%$ cement, and $20%$ mud. Skeletal grains are $40%$ crinoids, $30%$ bryozoans, and $30%$ others including brachiopods, ostracodes, and foraminifers. The early growth phases are dominantly crinoidal packstone (Figure 12A) that contain sections of brachiopod shells $(≈2%$ of all allochems) with bladed calcite cement followed by equant cement (Figure 12B–D), whereas the late growth phases are dominantly bryozoan–crinoid grainstone. Geopetals in brachiopod shells have contact angles ranging from $2°to50°$ (Figure 12B, C). Abraded crinoid fragments range in size from $0.5mm$ to $4mm$ within the core facies, but increase in size up to 6 mm in the flanking beds (Figure 12E). The contacts between these larger bioclasts in the flanking beds are frequently concavo-convex and sutured.

Benton County Pierson mounds contain cement Stages 1–4. Stage 1 bladed calcite cement lines cavities and the margins of larger skeletal fragments including brachiopod shells (Figure 12B, C) and is followed by equant cement of Stages 2–4 (Figure 12B–D). However, the most abundant calcite cement in both the core and flanking facies is non-CL Stage 2 syntaxial overgrowths on crinoid fragments (Figure 12D). Bright yellow CL Stage 3 equant cement occurs on rims of overgrowths and within microfractures, but does not completely enclose Stage 2 and is followed by Stage 4 dull calcite that fills geopetals and vugs and has as many as nine alternating light and dark bands (Figure 12D). Stage 5 cement was not observed in the Benton County Pierson mounds.

The Pierson mound complex in Delaware County, Oklahoma, contains two mounds exposed in a valley wall along the north side of Spavinaw Creek $8km(5mi)$ southeast of Jay, Oklahoma. The outcrop has latitude and longitude coordinates of $36°20’14.32”N$ and $94°45’02.68”W$ (Figure 2) and is visible to the north from Delaware County road E047 (Figure 11D). The Pierson mound complex overlies tabular bedded submound Pierson Limestone, which is approximately $6m(20ft)$ thick and forms a resistant ledge above the Northview Shale. The Pierson Limestone is gently folded and dips between $5–7°$ to the west and $7–9°$ to the east. The two mounds are less resistant to weathering than the submound limestone or the lateral nonmound facies, which onlap the mound complex.

This asymmetrical mound complex is approximately $10m(33ft)$ high and $79m(259ft)$ wide with dips of $14°$ east and $10°$ west. Based on the east and west dipping tabular submound beds, the complex is a gently folded anticline (Figure 11D). Mound–core facies are parallel bedded, slightly cherty crinoidal packstone with alternating $0.5cm(0.2in)$ thick green shale beds (Figure 11C). Thin gray crinoid packstone beds in the mound core are $5–10cm$ (2–4 in) thick and weather light gray to cream, creating a sharp color contrast with the darker gray submound Pierson beds and overlying Reeds Spring Limestone (Figure 11C, D). Crinoid fragments in the core range from $0.5to1.5cm(0.2–0.6in)$ in diameter and decrease in size toward the edge of the mound. Both mound core and flanking facies are less resistant to weathering than the tabular sub mound limestone. Mound–core facies transition to flanking facies, which are muddier and contain fewer thin packstone beds. Adjacent to the mound complex are beds of resistant dark-gray-crinoidal wackestone that feather into the mound. Overlying the Pierson Limestone are the bedded cherts and dark carbonate mudstone of the Reeds Spring Limestone (Figure 11D).

The Pierson mounds in Delaware County have cores that are approximately $76%$ skeletal, $17%$ mud, and $7%$ cement, whereas flanking facies are $64%$ skeletal, $28%$ mud, and $8%$ cement. Approximately $60%$ of the skeletal fragments in the core and flanking facies are crinoids; $16%$ are bryozoans. Both the mound–core and flanking facies are composed of packstones and wackestones containing larger abraded crinoid and bryozoan fragments (Figure 13A, B), and finer grained skeletal debris composed of bryozoan and unidentifiable skeletal fragments. Flanking facies contain smaller crinoid fragments $(0.5–3mm)$ than those in mound–core facies $(4–32mm)$⁠. Bryozoa are still an important allochem and larger longitudinal sections are observed (Figure 13C, D). Bioclasts in both mound–core and flanking facies have concavo-convex contacts; some grains are partially replaced by chert.

Polarized light and CL petrography revealed that both the core and flanking facies of Delaware County mounds contain very little calcite cement and only Stages 2–4 were identified. Early equant calcite cement fills larger longitudinal sections of bryozoan zooecia (Figure 13C, D), but most cement is Stage 2 syntaxial overgrowth that does not completely enclose crinoid fragments. These non-CL overgrowths are partially rimmed by bright CL Stage 3 cement, which is followed by the alternating light and dark dull bands of Stage 4 (Figure 13F, G). Dissolution incisions removed Stage 3 and banded Stage 4 equant calcite cement (Figure 13G), generating scalloped boundaries as described by Kaufman et al. (1988).

Nine field localities in Ireland were chosen to examine late Tournaisian to early Viséan mounds (Figure 5). The late Tournaisian Waulsortian buildups within the Feltrim Formation are mostly massive dark mudstone with calcite-cemented features including stromatactis (Figure 10D), sheet spar, fractures, and vugs filled with dog-tooth calcite. Skeletal material includes bryozoans, crinoids, brachiopods, and goniatites. Narrow $2–3cm(0.8–1.2in)$ wide vertical or horizontal fissures, filled completely with carbonate mud or mud followed by calcite cement, occur in the mound facies. Fractures and breccias were observed in the outer mound–core facies.

The dimensions of Irish mounds commonly range from $20to100m(66to328ft)$ thick (Somerville, 2003) and partially exposed cores we sampled range from $3to25m(10–82ft)$ thick and $15to77m(50–255ft)$ wide (Figure 14). Late Tournaisian mounds commonly coalesce and when stacked, form complexes that exceed 900 m (2953 ft) in thickness in the Shannon Trough (Somerville, 2003). Waulsortian bank complexes approaching $1000m(3280ft)$ in thickness with a lateral spread of more than $30,000km2(11,583mi2)$ occur in southern Ireland (Lees and Miller, 1995). Viséan mounds are typically $10–50m(33–164ft)$ thick with diameters of $200–500m(656–1640ft$⁠; Somerville, 2003). Three late Tournaisian mounds were chosen for detailed petrographic and statistical analyses: Feltrim Quarry near Malahide, County Dublin (⁠$53°26’11.87”N,6°11’28.79”W$⁠), Hill 707 near Loughrea, County Galway (⁠$53°09’05”N,8°32’17”W$⁠), and Mullawornia Quarry near Ballymahon, County Longford (⁠$53°34’51.87”N,7°48’14.70”W$⁠; Figure 5).

Core facies were sampled in the County Dublin and Longford mounds. Both cores are mud-rich with the Dublin mound being $45%$ mud, $43%$ cement, and $12%$ skeletal grains, and Longford 50% mud, $36%$ cement, and $14%$ skeletal. Dominant skeletal grains are bryozoans and crinoids, with bryozoans making up $54%$ of the bioclasts in the Dublin mound and $47%$ of the Longford core. Crinoids range from $10$$to35$ mm in length and are approximately $10%$ of the bioclasts followed by brachiopod and goniatites; both mound cores classify as mudstones to wackestones. Brecciation is observed in the County Dublin core, but not Longford. Flanking facies are mud-rich with Galway 44% mud, 34% skeletal, and $22%$ cement and Longford $57%$ mud, $30%$ skeletal, and $13%$ cement. Crinoids and bryozoans are abundant in flanking facies with Longford mounds containing $55%$ crinoids and $35%$ bryozoans. Galway flanking facies are $20%$ crinoids and $54%$ bryozoans. Crinoid fragments in the flanking facies range from $10mmto35mm$⁠. Both core and flanking facies contain cavities lined with radial-fibrous cement and filled with equant cement (Figure 15A, D, G).

Five stages of calcite cementation were identified in core and flanking facies using CL petrography. Vugs, fractures, fissures, and stromatactis structures (Figure 10D) are filled with geopetal muds and early radial-fibrous calcite cement (Stage 1) followed by late-equant calcite cement (Stages 2–5; Figure 15A–I). Stage 2 non-CL calcite cement (Figure 15F, H) is succeeded by bright CL Stage 3 and moderate CL-banded Stage 4 cements with dissolution incisions (Figure 15F–H). Stage 5 dull CL equant calcite fills fractures and the centers of vugs and cavities (Figure 15B, C, H, I).

Ternary diagrams were used to graphically compare the relative abundances of micrite (mud), cement (sparry or equant and fibrous), and skeletal fragments, such as crinoids, bryozoans, and other bioclasts, in mound facies (Figure 16A–C). The mound–core facies of Feltrim mounds are more than $80%$ mud, whereas core facies of Pierson mounds are less than $50%$ mud (Figure 16B). In addition, Compton mounds and the Feltrim mounds have bioclast-dominated flanking facies that contrast the mud-rich core facies. The flanking facies of all mounds are crinoid-rich, whereas the mound–core facies of Compton mounds and Feltrim mounds are bryozoan-rich (Figure 16A). Mound cores of the Delaware County Pierson mounds are crinoid-rich and the bryozoan fraction reduced. The cores of Feltrim Waulsortian mounds contain more than 30% calcite cement. Compton mound cores contain $10–20%$$10%–20%$ calcite cement, whereas Pierson mound cores are 5–30% (Figure 16C).

Basic statistical analysis of the point count data yielded results that corroborate trends evident from ternary diagrams. The cores of Feltrim mounds contain less skeletal material than the flanking facies, and less skeletal material than the cores or flanking facies of Compton and Pierson mounds. Pierson mound cores have the least carbonate mud, but more mud in the flanking facies compared to other sampled mounds. Tables containing data on sample size and standard deviations are shown in Unrast (2012). Standard deviations of the abundance of skeletal grains and mud are higher for Ozark mounds as a result of the larger sample size, which increased the confidence level.

A hypothesis that all mounds have equal mean amounts of skeletal fragments and carbonate mud was tested using analysis of variance (ANOVA) and the results shown in the F-value table (Table 3). Larger F-values indicate increased variance about the mean abundance of the item of interest. For our data, larger F-values for skeletal grains and mud in mound–core facies, as well as skeletal grains and mud in mound–core plus flanking facies, indicate that the sampled mounds are not statistically similar at a significance level p-value = 0.0001. Therefore, the hypothesis that the mean abundance of skeletal grains and carbonate mud are the same across the sampled mounds is rejected. In contrast, the low F-value for flanking facies supports the inference that there is no significant difference in the mean abundance of skeletal grains and mud in flanking beds at a p-value = 0.0001.

Three post hoc comparison tests, Fisher LSD (least significant difference) test, Duncan multiple range test, and Tukey’s HSD (honestly significant difference) were used to analyze the apparent discrepancies between the mean abundance of mud and skeletal grains in mound–core, flanking, and mound–core plus flanking facies. These results group Compton mounds with Feltrim mounds for mean values of mud in their mound–core facies and group all mounds based on mud in their flanking facies. Similarities in skeletal grain content in flanking facies for Feltrim and various Compton and Pierson mounds are also evident (Table 4). Lesser similarity is evident with respect to mound–core plus flanking facies and these results, as well as statistical raw data and supporting information, are reported in Unrast (2012).

The geometry of individual Compton and Pierson mounds is similar to Feltrim mounds, but aggradation style and size are distinctly different. The growth forms of the Waulsortian banks are classified as tabular, knoll, and sheet (Lees and Miller, 1995). Pierson mounds in Delaware County and Compton mounds in McDonald County, which are completely exposed in cross-section, are asymmetrical structures above flat-lying substrates, characteristic of knoll-form buildups (Lees and Miller, 1995; Devuyst and Lees, 2001). The base and submound strata are not exposed for the Stone County Compton and Benton County Pierson mounds, but their visible portions appear knoll form. Waulsortian mounds, which can be as much as $100m(328ft)$ thick (Somerville, 2003), are noticeably larger than Compton and Pierson mounds that range from $3to10m(10–33ft)$ in height and $8to79m(26–260ft)$ in width. However, Harbaugh (1957) and Anglin (1966) recognized larger Pierson mounds in Cherokee and Delaware counties, Oklahoma, that ranged from $6to20m(20–66ft)$ thick and $15to300m(49–980ft)$ long. Unfortunately, most of these larger Pierson mounds are inundated by lakes and no longer accessible.

In addition to individual mound size, aggradation style separates Compton and Pierson mounds from Feltrim mounds. Smaller, laterally aggraded Compton and Pierson mounds are confined to formations less than $30m(100ft)$ thick. In contrast, the Feltrim Limestone exceeds $600m(1970ft)$ in the Dublin Basin and Feltrim Waulsortian mounds coalesce and stack vertically to reach thickness exceeding $500m(1635ft)$⁠. In the nearby Shannon Trough, mound complexes in the Limerick Limestone, Shannon Trough, reach thicknesses exceeding $900m(2950ft$⁠; Somerville, 2003). These differences in size and aggradation style are attributed to much lower subsidence rates and reduced accommodation for the Compton and Pierson formations compared to the Feltrim and Limerick limestones. If accommodation were driven dominantly by eustatic changes in sea level, it is expected that the shallow areas on both the east and west sides of the continent would have been similarly affected. We propose that the lower rate of subsidence and reduced accommodation in the Ozark region during the Tournaisian is the result of localized uplift that did not impact other areas along the Burlington Shelf. For example, the coeval Tournaisian–Visean Lake City Formation in New Mexico is over $100m(330ft)$ thick and contains mounds of similar height (Giles, 1998).

Polymuds are a primary characteristic of Irish Waulsortian banks (Lees and Miller, 1995). However, cavity lining muds are rare or indistinct in Compton and Pierson mounds, with only Compton mound cores containing stromatactis-like features with thin carbonate muds (Figures 9A–C, 10). Stromatactis-like features in Compton mounds are bordered by bryozoan fronds, similar to bryozoan roofed cavities in Feltrim mounds that filled with geopetal sediments and calcite cement (Somerville, 2003).

Stromatactis is not a feature of all Waulsortian rocks (Lees and Miller, 1995); however, a major difference between Compton and Pierson mounds and Feltrim mounds is the amount and type of calcite cement observed. Irish Waulsortian mounds contain abundant cemented cavities, including sheet-spar and stromatactis (Lees and Miller, 1995; Somerville, 2003), resulting in rocks containing $35%$ or more calcite cement (Unrast, 2012). True stromatactis with polymuds are prominent in Feltrim mounds where they generate geopetals and contribute to distinct calcite-cemented fabrics (Figure 10D–F). Feltrim mounds are dominated by radiaxial-fibrous cements, which are subordinant to absent in Compton and Pierson mounds. Cores of Compton mounds and Benton County Pierson mounds contain fibrous and bladed calcite cement, but successive stage equant calcite cement is dominant.

Compton and Waulsortian mound cores are more similar with respect to mud content than skeletal content, whereas Pierson mound cores are not similar to mound cores of either Compton or Feltrim mounds. Pierson mounds in Delaware County are anomalous and rarely group with the other mounds (Table 4). Based on post hoc comparisons, the flanking facies of all mounds contain similar amounts of mud and display some degree of similarity in skeletal fragments content. Compton and Feltrim mound–cores with similar mud and skeletal content formed in deeper water depositional settings. Mud to skeletal grains ratios in cores of Pierson mounds reflect shallower water with the bedded crinoidal accumulations in the Delaware County Pierson mounds representing shallow water, high-energy current deposition.

Ternary diagrams (Figure 16) were used to classify mound–core facies as either mud-dominated or skeletal-dominated. Compton and Feltrim mounds are bryozoan-rich and mud-dominated, whereas Pierson mounds are crinoid-rich and skeletal-dominated.

A change in mechanical energy from the core to flanking facies is evident. Cores of Feltrim and Compton mounds contain abundant bryozoans (Figure 16A) and mud (Figure 16B), whereas grainier flanking facies contain more disarticulated crinoid fragments (Figure 16B). Pierson mounds are crinoid-rich (Figure 16A) and cores of Benton County Pierson mounds interfinger with grainier flanking beds. In contrast, Delaware County Pierson mounds transition from skeletal-rich mound–core facies to more mud-rich flanking facies (Figure 16B, C).

Phases of cementation are similar for all mounds. Compton and Pierson mounds exhibit five stages of calcite cement under CL. This CL zoning is similar to the five-zone CL stratigraphy described by Ritter and Goldstein (2012) for intergranular calcite cement in the Burlington–Keokuk Limestone in eastern Kansas and western Missouri and Kaufman et al. (1988) in the Burlington–Keokuk Limestone of Missouri and Illinois. The studied Feltrim mounds contain similar CL Stages 1 through 5; however, additional stages of cement in Waulsortian banks from Ireland and England include ferroan calcite, dolomite, and silica replacement (King, 1984; Miller, 1986; Gillies, 1987; de Brit, 1989; Lees and Miller, 1995; Gregg et al., 2001).

Waulsortian mounds experienced initial phases of silica and carbonate dissolution that predate the earliest stage of marine cement (Miller, 1986). Host matrix and skeletal grains were dissolved, contributing to the development of cavities that filled with early marine cements characteristic of Waulsortian buildups (Lees and Miller, 1995). Compton and Pierson mounds contain only evidence of initial carbonate dissolution phases that resulted in partial and complete dissolution of crinoid fragments, as well as the collapse and etching of bryozoans.

Waulsortian banks are dominated by early marine, radiaxial-fibrous, calcite cements that neomorphosed into cryptofibrous calcite (CFC) in cavities with ample pore-water flows (Gillies, 1987; and Lees and Miller, 1995). Abundant CFC Stage 1 cement filled cavity systems and stromatactis features. Radial-fibrous and CFC Stage 1 cements are absent to rare in Compton and Pierson mounds, but finely crystalline fibrous to bladed calcite cements are observed within cavities and lining articulated brachiopods (Figure 14B–D) and bryozoan fronds in Compton and Benton County Pierson mound cores. Fibrous to bladed calcite cement is rare in stromatactis-like features in Compton and Pierson mounds, and this absence is attributed to pore restriction and inefficient water circulation (Folk, 1974).

Radial-fibrous and fibrous calcite cement is followed by equant calcite cements in all mounds. Equant calcite cement in Compton and Pierson mounds occurs as non-CL Stage 2, bright CL Stage 3, and moderate CL-banded Stage 4. Together, these stages are called OBC zones (Meyers, 1991). King (1984) interpreted OBC cement in Irish Waulsortian mounds as synchronous meteoric cement, whereas a similar cement stratigraphy in British Waulsortian mounds is interpreted as marine diachronous cement (Gillies, 1987; Lees and Miller, 1995). However, because the CL stages of the Compton, Pierson, and Feltrim mounds display a similar OBC sequence of non-CL Stage 2, bright Stage 3 and moderately cyclic-zoned Stage 4, and evidence for subaerial unconformities and meteoric circulation are not obvious, a marine origin is proposed (Meyers, 1991). The origin of the Compton and Pierson cements is complex, but marine and meteoric diagenesis are indicated for Stages 1–3 by carbon and oxygen isotope analyses (Ritter and Goldstein, 2012; Morris et al., 2013; Mohammadi et al., 2015, 2019). On the basis of cross-cutting relationships with sulfide minerals, C, O, and Sr isotope geochemistry, and fluid inclusion analysis, a late diagenetic or “hydrothermal” origin for the later CL cement zones (Stage 4 and 5) is indicated (Kaufman et al., 1988; Ritter and Goldstein, 2012; Mohammadi et al., 2015, 2019).

Although Compton and Pierson mounds formed at widely separated locations from the Feltrim Waulsortian mounds (Figure 1), their remarkably similar CL cement stratigraphies are interpreted as evidence of similar postdepositional modification and the absence of significant localized or regional uplift that disrupted burial. Minor discontinuities in OBC cements in Compton and Pierson mounds are attributed to localized dissolution. Post-OBC phases in Feltrim mounds include dolomite and chert mineralization, whereas in Compton and Pierson mounds Stage 5 cement fills fractures that cut the older banded cements (OBCs; Mohammadi et al., 2019). Additional diagenetic alteration of Pierson mounds includes limited chert mineralization, but dolomitization is rare. Evidence of late-stage petroleum migration occurs in the form of petroleum residue in the crinoidal grainstone at the apex of a Pierson mound in Benton County (Morris et al., 2013) and in a shallow subsurface core that contains more than $30m(100ft)$ of a Pierson mound consisting of oil-stained crinoidal grainstone (Godwin, 2017).

Compton and Pierson mounds were restricted to the Burlington shelf-edge or immediately off-shelf environments (Lane, 1984). Recent interpretations establish the depositional setting as a ramp (Morris et al., 2013) or distally steepened ramp (Childress and Grammer, 2015). Because cores of Irish mounds contain intact fossils, including goniatites that are unknown in Compton and Pierson mounds, Feltrim mounds are interpreted as forming in a distal ramp setting in low-energy, aphotic marine environments at water depths ranging from approximately $150–300m(492–984ft$⁠; Somerville, 2003) to $65–170m(213–558ft$⁠; Histon and Sevastopulo, 1993).

Compton and Feltrim mounds are fenestrate bryozoan-rich and mud-dominated. Fenestrate bryozoans occur at shallow depths, their abundance increases with depth and they are more common in middle to deep (⁠$80–220m[262–722ft]$⁠) shelf environments (Amini et al., 2004). Fenestrate bryozoan fronds have long been regarded as having an important role to play in Mississippian mound construction through their baffling, trapping, and stabilizing of sediment (Pray, 1958; Cuffey, 1977). However, Compton mounds contain more crinoidal skeletal debris than Feltrim mounds, and this more abundant and diverse grain content, coupled with the thinness of the Compton Limestone, supports a shallower water setting for Compton mounds compared to Feltrim and other Waulsortian mounds. Furthermore, the Compton Limestone is conformably succeeded by the Northview Formation (Shoeia, 2012), which is interpreted as tidal flat in the upper part (Childress and Grammer, 2015). Pierson mound cores are crinoidal wackestone–packstones with two distinct and different modes of occurrence. The cores of the Benton County Pierson mounds are mud-rich, devoid of apparent mechanical sedimentary structures, and interpreted as forming below fair-weather wave base in a distal ramp setting unaffected by strong ocean currents. In stark contrast, cores of the Delaware County Pierson mounds contain bedded, current transported crinoidal debris deposited in a high-energy environment above fair-weather wave base. The Pierson is succeeded by the Reeds Spring Formation, a deeper water, chert-rich carbonate that is devoid of mounds.

Similarities in skeletal material and mud from post hoc comparison tests are interpreted to support the contention that Compton, Benton County Pierson mounds and Feltrim mounds formed in lower energy environments that occur in moderate to deeper water (Type 1 buildups of Bridges et al., 1995), whereas Delaware County Pierson mounds formed in higher energy environments above the fair-weather wave base (Type 2 buildups of Bridges et al., 1995). The latter are more comparable with Fort Payne buildups of Kentucky (Ausich and Meyer, 1990; Meyer et al., 1995), especially in the abundance of crinoids in the mounds and the presence of interbedded green shales. Mound position on the ramp is inferred from the relative abundances of carbonate mud and skeletal fragments, presence or absence of bedding in the mound core, and variations in the size and shape of crinoid and bryozoan fragments.

Several studies show evidence that some Tournaisian mounds are allochthonous (Somerville et al., 1992b; Giles, 1998; Chandler, 2001; Morris et al., 2013; Childress and Grammer, 2015). Compton mounds in McDonald County are associated with lateral brecciation indicating transportation, possibly as trailing blocks from a nearby slump (Childress and Grammer, 2015) or mobilized blocks whose force brecciated the host bed (Morris et al., 2013). The spatial relationship in which a large mound appears to ride up on a smaller one indicates northwestward transport (Figure 7A). Geopetals in these mounds range from $0°to45°$ relative to flat-lying submound strata (Unrast, 2012; Childress and Grammer, 2015). The Compton mound complex in Stone County contains smaller mounds at the base as well as laterally to the base. Slumping and rotation are not evident and geopetals range from $15°to30°$ (Unrast, 2012). Pierson mounds in Benton County are asymmetric with flanking beds and geopetals dipping up to $55°$⁠; this steepness is attributed to deformation by mobilized mound cores. Compton and Pierson mounds with flanking beds and geopetal structures with dips below $30°$ are interpreted as in place or minimally rotated and moved, whereas others with high-angle geopetals or steeply dipping flanking beds are transported and rotated.

In Ireland, slumped Feltrim mounds contain breccia, soft-sediment deformation, and dewatering structures such as shale injections (Somerville et al., 1992b). In New Mexico, U.S.A., gravity-driven sedimentary features are cited as evidence supporting an allochthonous origin for Tournaisian mounds in the Lake City Formation (Giles, 1998). Compton mounds near Noel in McDonald County, Missouri, contain soft-sediment deformation attributed to mobilization (Morris et al., 2013). These mounds and those in McDonald County described herein appear to have moved counter to the expected southerly transport toward the basin axis. Morris et al. (2013) attribute this reversal to uplift caused by forebulge tectonism associated with Laurussian–Gondwanan convergence.

Mounds were classified using schemes of Wilson (1975), Lees and Miller (1995), and Bridges et al. (1995; Table 1). Feltrim mounds, Compton mounds, and Benton County Pierson mounds are compositionally similar and classify as lime mud accumulation bioherms (Wilson, 1975) and Type 1 (Bridges et al., 1995). Feltrim mounds are true Waulsortian, whereas Compton and Benton County Pierson mounds classify as Waulsortian-type (Lees and Miller, 1995).

The grain-rich Pierson mounds in Delaware County with cores containing abundant crinoid and bryozoan skeletal debris have no affinity to Waulsortian mounds and are classified as Type 2 (Bridges et al., 1995) and not-Waulsortian (Lees and Miller, 1995). Granted, bryozoan debris may result from biodegradation rather than mechanical comminution during transport (Lees and Miller, 1995), but the combination of abundant abraded grains and bedding in the mound core is indicative of mechanical transportation. Mounds in this complex are sediment piles or organic banks (Wilson, 1975) that develop with bioclast transportation and accumulation. Hence, the term “transported bioaccumulation mound” is suggested.

The direct comparison of Waulsortian mounds in the Feltrim Formation of Ireland with mounds in the Compton and Pierson limestones, U.S.A., allow us to state that although individual mounds have similar geometries and cement stratigraphies, in some cases, like compositions, the differences are striking. As a result, three types of mounds are recognized: Waulsortian, Waulsortian-type, and transported bioaccumulation. Waulsortian mounds in the Feltrim Limestone are much larger, compositionally richer in multigenerational mud as well as calcite-cemented features, including stromatactis, and aggrade vertically and laterally. In contrast, Pierson and Compton Waulsortian-type mounds are smaller, grainier, deficient in multigenerational muds, and aggrade laterally. Pierson transported bioaccumulation mounds have no features in common with Waulsortian mounds except geometry.

Compton and Pierson mounds formed in a more proximal setting on a carbonate ramp than Feltrim mounds. The St. Joe Group that contains the Compton and Pierson formations is much thinner than the Feltrim Limestone, a difference that is attributed to lower rates of subsidence and less accommodation. Localized uplift, possibly associated with a forebulge generated by incipient Ouachita tectonism (Morris et al., 2013) limited accommodation and perturbed the southerly dipping ramp to cause slope reversal that facilitated northwestward transport of mobilized Compton mounds.

Waulsortian-type Compton and Pierson mounds are thin, areally limited, and likely too small to be viable reservoirs for petroleum. Pierson crinoidal transported bioaccumulation mounds reach reservoir-scale thicknesses, but their lateral scales are not well constrained.

The authors wish to thank the quarry operators in Ireland for access to pit outcrops and James Higgins, M.D. and Craig Phillips for granting us permission to sample outcrops on the Higgins property in Oklahoma. We are grateful to the Geological Society of America, Oklahoma Geological Foundation, AAPG, and Herb and Shirley Davis for student scholarship support. We also thank Joshua York for his contribution to the construction and editing of figures.