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AND DOUGLAS R. REID2. 1Department of Earth and ... sandstones and shales (Morris 1974; Lowe 1989; Loomis et al. 1994). Turbidites, deep-marine fan ...
Journal of Sedimentary Research, 2012, v. 82, 833–840 Current Ripples DOI: 10.2110/jsr.2012.68

TIMING AND RATES OF FLYSCH SEDIMENTATION IN THE STANLEY GROUP, OUACHITA MOUNTAINS, OKLAHOMA AND ARKANSAS, U.S.A.: CONSTRAINTS FROM U-PB ZIRCON AGES OF SUBAQUEOUS ASH-FLOW TUFFS BARRY J. SHAULIS,1 THOMAS J. LAPEN,1 JOHN F. CASEY,1 1

AND

DOUGLAS R. REID2

Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, 77204-5007, U.S.A. 2 Carrizo Oil and Gas, Houston, Texas, 77002, U.S.A. e-mail: [email protected]

ABSTRACT: Uranium-lead geochronology of zircon from five subaqueous ash-flow tuffs distributed throughout the Stanley Group of the Ouachita Mountains in Oklahoma and Arkansas define eruption ages that range from 328.5 ± 2.7 Ma near the base to 320.7 ± 2.5 Ma near the top. Biostratigraphy indicates that the oldest age of the Stanley Group is likely to be between 345.3 and 338 Ma. From these age constraints and stratigraphic positions of the tuff units, calculated deposition rates are 1– 40 m/My in the Lower Tenmile Creek Formation (, 326 to 322.4 ± 2.4 Ma) and increase to 300–1400 m/My for the overlying Moyers and Chickasaw Creek formations (322.4 ± 2.4 to 320.7 ± 2.5 Ma). Ages of zircons representing detrital components in the tuffs indicate that they are derived mainly from Laurentian sources. The flysch units in the Stanley Group were deposited in a remnant ocean basin flanked by the Alleghenian orogen to the east, which supplied west-southwest-flowing submarine fan systems. These fans likely incorporated material cycled through a growing offshore allochthonous prism in front of a northward-encroaching volcanic arc system to the south of the Laurentian margin in the late Mississippian.

INTRODUCTION

Remnant ocean basins flanking orogenic highlands contain some of Earth’s most voluminous and rapidly deposited sedimentary packages (e.g., Graham et al. 1975; Ingersoll et al. 2003 and references therein). The late Paleozoic Ouachita trough is one such remnant ocean basin that was flanked by the southern Appalachian suture to the east, the Laurentian continental margin to the north, and an encroaching arc–trench system to the south. The timing relationships between mountain-building events and the transition from a ‘‘starved basin’’ to classic orogenic flysch deposition in the late Paleozoic Ouachita trough of southeast Laurentia are major unresolved problems (e.g., Morris 1989). The timing, rate, and volume of flysch sedimentation in the Mississippian Stanley Group have important implications for the timing of terrane encroachment and late Paleozoic paleogeographic reconstructions of the region. The Ouachita Orogenic Belt, the result of the collision between Laurentia and Gondwana during the formation of Pangea (Arbenz 1989), stretches from Alabama into Mexico and roughly parallels the ancient coastline of North America. Despite its length, only a small portion of the orogenic belt is exposed, mainly in the Ouachita Mountains of Oklahoma and Arkansas, and the Marathon Basin and Solitario of west Texas (Flawn 1961). The remainder is buried beneath neoautochthonous Cretaceous sedimentary rocks to recent coastal-plain sediments (Flawn 1961; Viele 1989). The exposed Ouachita rocks are Paleozoic in age and show a transition from Cambrian to Devonian deep marine siliciclastics, carbonates, cherts, and novaculite to Mississippian and Pennsylvanian sandstones and shales (Morris 1974; Lowe 1989; Loomis et al. 1994). Turbidites, deep-marine fan deposits, and pyroclastic rocks of the Published Online: November 2012 Copyright E 2012, SEPM (Society for Sedimentary Geology)

Mississippian Stanley Group represent a mixture of sediment from Laurentia and an influx of tuffaceous material from an encroaching arc terrane to the south of the Ouachita trough (Morris 1989). Deposition in the Mississippian and Pennsylvanian was followed by telescopic collapse of the Ouachita continental margin, crustal thickening, and low-grade metamorphism that culminated with the termination of the Ouachita orogeny in the Late Permian to Early Triassic (Denison et al. 1977). The general timing constraints of orogenic activity have been bracketed by the depositional age of thick orogenic flysch units (e.g., Stanley Group through Atoka Formation) and the age of the youngest deformed rocks and timing of metamorphism (Denison et al. 1977; Arbenz 1989; Viele and Thomas 1989). In previous models for the development of the Ouachita orogeny, collision began in the Ouachita and Marathon basins at approximately 320 Ma based on the age of the Stanley Group of the Ouachitas in Arkansas and Oklahoma and the Tesnus Formation (Stanley Group equivalent) in the Marathon region (Denison et al. 1977; Arbenz 1989; Ethington et al. 1989; McBride 1989). The upper age constraints of orogenic activity are based on Late Permian–Early Triassic K-Ar ages of schist and phyllite samples from well cores and cuttings and a few outcrop samples from various locations along the Ouachita Orogen (Denison et al. 1977; time scale of Walker and Geissman 2009). We present new U-Pb data of zircon from five tuffaceous units that have previously allowed regional correlation of the Stanley Group within the Ouachita Orogen (Table 1; Figs. 1, 2). These data help define absolute ages for the tuffs, constrain depositional rates and the tectonic environment of the Stanley Group, and identify non-volcaniclastic sedimentary sources. The Stanley tuffs show north-to-south thickening and coarsening, consistent with volcanic sources supplying ash-flow tuffs

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TABLE 1.— For each of the five tuffs in the Stanley Group, the 206Pb/238U zircon age of the youngest zircon found, the youngest peak age as defined by the AgePick program and the weighted average of the bin used in the AgePick program (Gehrels 2007) is given.

Tuff Chickasaw Creek upper Mud Creek lower Mud Creek Hatton Beaver’s Bend

Youngest Age (Ma)

Peak Age (Ma)

6 6 6 6 6

322 322 323 323 322

307.2 316.2 311.6 314.7 318.6

7.1 11.2 9.0 6.6 8.3

Bin Age (Ma, wtd. avg.) 320.7 324.8 322.4 326.1 328.5

6 6 6 6 6

2.5 2.1 2.4 3.7 2.7

from an encroaching arc terrane to the south (Ingersoll et al. 1995; Johnson 1968; Niem 1976, 1977). Sandstone and shale units in the upper part of the Stanley Group contain dominantly ‘‘to-the-west’’ paleocurrent indicators similar to the Pennsylvanian Jackfork Group and Atoka Formation; the lower part of the Stanley Group displays a more prominent S-SE-derived component (Morris 1974; Niem 1976). The new U-Pb zircon age data confirm that, in the late Mississippian, the Ouachita remnant ocean basin experienced rapidly increasing sedimentation rates and flysch deposition from an advancing submarine fan complex built longitudinally along the basin axis from the developing Alleghenian Orogen to the east (Graham et al. 1975). Sedimentary Units The Paleozoic assemblage in the Ouachita Mountains is over 15,000 m in thickness (Morris 1974, 1989). Over 13,700 m of these units comprise the Mississippian Stanley Group and the Pennsylvanian Jackfork Group, Johns Valley, and Atoka formations, and only , 1,600 m consisting of the deep marine ‘‘starved basin’’ Cambrian to Devonian sediments (Cline 1960; Morris 1974; Niem 1976). Sedimentation rates during the Cambrian to Devonian were relatively low (, 190 m/My) (Morris 1974; Lowe 1985) and were punctuated by periods of coarser clastic input (e.g., Ordovician Crystal Mountain and Blakely formations and Silurian Blaylock Formation) (Reid 2003). The contact between the Devonian Arkansas Novaculite and the Stanley Group is considered to be unconformable (Miser and Purdue 1929). A hiatus in sedimentation is known to have

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existed during this time (King 1961; Noble 1993). This hiatus is found regionally across parts of New Mexico, Texas, Oklahoma, and Arkansas (Noble 1993) with areas of nondeposition identified in the Marathon basin and in several locations within the Ouachita Mountains (Miser and Purdue 1929; Noble 1993; Viele and Thomas 1989). However, in some areas of the Ouachita Mountains (e.g., Potato Hills, Black Knob Ridge, and Broken Bow uplift) the contact between the Arkansas Novaculite and the Stanley Group is gradational and considered to be conformable (Goldstein and Hendricks 1962; Viele and Thomas 1989). It has been suggested that this hiatus may represent a temporary cessation in sedimentation as opposed to an erosional event (King 1961; Noble 1993). The Mississippian and younger sandstone and shale units mostly represent an influx of clastic sediment that was deposited as west-building marginal and axial fan systems in a remnant-ocean setting (Johnson 1968; Graham et al. 1975; Morris 1989). This remnant ocean basin was situated between the Laurentian continental margin to the north-northwest (present-day coordinates), Gondwana and associated volcanic arc terranes (e.g., Llanoria) built over a south-dipping subduction zone to the south-southeast and the Alleghanian Orogen to the northeast (e.g., Ingersoll et al. 2003 and references therein). The Mississippian Stanley Group is approximately 3,000 m thick (Morris 1989) and is composed principally of shale and a few thick sandstone units. It is divided into three formations: Tenmile Creek, Moyers, and Chickasaw Creek (and the Hot Springs Sandstone in Arkansas). In Oklahoma, the Stanley Group lies unconformably above the Arkansas Novaculite (King 1961). In parts of Arkansas, the base of the Stanley Group is preceded by the 70-m-thick Hot Springs Sandstone, which lies unconformably above the Arkansas Novaculite. The contact between the Hot Springs Sandstone and the Stanley Group is conformable (Miser and Purdue 1929). The age of the Stanley Group has been debated for as long as geologists have been working in the Ouachita Region (e.g., Taff 1902; Miser and Purdue 1929; Hartlon 1934; White 1937; Hass 1950; Mose 1969; Morris 1989). Reasons include: 1) the Stanley Group is intensely folded and faulted, which makes correlations amongst units difficult, and 2) the Stanley Group is known for its paucity of invertebrate fossils (Mose 1969; Morris 1989). For these reasons, age determinations have varied over time from Ordovician, Silurian, Mississippian, and Pennsylvanian (Hass 1950). The currently accepted age of the Stanley Group is Mississippian based mainly on conodont biostratigraphic correlations in the upper

FIG. 1.—Generalized map showing the location of tuff samples: (1) Beaver’s Bend and Hatton tuffs, (2) lower and upper Mud Creek tuffs, and (3) Chickasaw Creek tuff; and the extent of exposures of the Stanley Group in the Ouachita Mountains of Oklahoma and Arkansas. Map modified from Gleason et al. (1995), Reid (2003), and Stoeser et al. (2005).

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sections of the Arkansas Novaculite and in the lower 500 m of the Stanley Group (Ethington et al. 1989). The Chickasaw Creek Formation of the uppermost Stanley Group is very sparsely populated with nondiagnostic conodont fossils, though a few exotic blocks of Mississippian limestone have been found (Gordon and Stone 1969, 1977; Morris 1971). Conformably overlying the Stanley Group is the 2100-m-thick Jackfork Group. The unit is composed predominantly of sandstones that were deposited as turbidites with interbedded shales (Briggs and Roeder 1975; Morris 1989). Marine fossils found in several locations are suggestive of an early Pennsylvanian age for the Jackfork Group (Gordon and Stone 1977). Unlike the other Paleozoic rock units in the Ouachita Mountains, the Stanley Group contains a number of major and minor tuff units (Fig. 2). In the lower 500 m of the Stanley Group, four major tuffs can be found: the Beaver’s Bend Tuff, the Hatton Tuff, and the lower and upper Mud Creek Tuffs (Niem 1976, 1977). A fifth major tuff, the Chickasaw Creek tuff, can be found in the upper 200 m of the Stanley Group. Mose (1969) obtained an Rb-Sr isochron age of 310 6 15 Ma (l87Rb 5 1.39 3 10211y21) for the Hatton Tuff. This is the only previously reported radiometric age from a tuff in the Stanley Group. Despite the apparent Pennsylvanian age of the tuffs, most investigators have continued to regard the Stanley Group as Mississippian in age for reasons stated previously.

statistical robustness of the ages. The following is a description of how each age in this report was obtained. The youngest zircon age was determined by picking the youngest zircon age with , 30% age discordance. Because these ages are represented by a single zircon analysis, they are not considered to be statistically relevant and do not represent a geologically meaningful age. The peak age is derived from the AgePick program developed by George Gehrels (2007) at the Arizona LaserChron facility at the University of Arizona. The relative age probability plots are derived by summing the Gaussian distribution of each U-Pb zircon age and its associated error (Silverman 1986; Sircombe 1999) from within an age bin defined by the AgePick program. For samples with multiple age populations, one or more peak(s) may be defined. The bin age is derived by the AgePick program (Gehrels 2007) during the construction of the relative age probability plots. For each peak, a bin is derived and the number of U-Pb ages within each bin is displayed. In many cases the number of U-Pb ages within the bin is greater than the number of U-Pb ages used to define the peak age. In defining the Bin Age for this report, we have taken the weighted average of all the U-Pb ages from the bin and have allowed for rejection. The weighted average ages tend to be older or equivalent to the peak age given by the AgePick program (Gehrels 2007). For the purposes of this study, the ‘‘bin age’’ is interpreted to represent the maximum depositional age of each tuff.

METHODS

The tuffs in the Stanley Group contain clasts that are not related to the volcanic sources of the tuffaceous material, as evidenced by the presence of Proterozoic and older aged zircons (Fig. 2). Recycled components in the tuffs are also evidenced by shale rip-up clasts observed in some of the tuffs (e.g., Niem 1977). For these reasons, the tuffs were analyzed in a manner similar to a provenance study of detrital zircons such that 120 zircon grains were analyzed from each sample. The samples were processed using standard sample processing methods (e.g., rock crushing, sieving, and heavy liquids). The heavy-mineral separates were then mounted in 1 inch epoxy rounds and polished to expose the zircon cores. For each sample, the zircons were analyzed in a random order regardless of color, shape, size, etc. in order to avoid any biasing of the data. Samples were analyzed by laser ablation ICPMS using a Varian 810 Quadrupole ICP-MS coupled with a PhotonMachines Analyte.193 excimer laser. Machine operating conditions, analytical methods, and data reduction techniques are outlined in Shaulis et al. (2010). Internal standard FC5z (equivalent to 1099 6 1 Ma for FC1, Duluth Complex; Paces and Miller 1993) was used to correct instrumental fractionation and drift for the Chickasaw Creek, upper Mud Creek, and Beaver’s Bend tuffs. Peixe (564 6 4 Ma; G. Gehrels, personal communication) and Plesˇovice (337 6 0.4 Ma; Sla´ma et al. 2008) zircon standards were used to correct instrumental fractionation and drift for the lower Mud Creek and Hatton tuffs, respectively. Standard zircons analyzed as unknown grains yielded the following weighted average ages with stated uncertainties that include both internal random and external systematic uncertainties (Shaulis et al. 2010): Plesˇovice yielded a 206 Pb/238U weighted average age of 334.4 6 2.8 Ma (n 5 101); FC5z yielded a 206Pb/238U weighted average age of 1093 6 12 Ma (n 5 40); Peixe yielded a 206Pb/238U weighted average age of 564.2 6 4.7 Ma (n 5 15); and Stettin yielded a 207Pb/206Pb weighted average age of 1566 6 17 Ma (n 5 14; ID TIMS age 1565 6 8 Ma; van Wyck et al. 1994). Given that the tuffs contain extraneous zircons indicative of reworking (see discussion), the U/Pb zircons ages reported here are based on the youngest age peak ($ 3 grains) on a population density plot. We also indicate the youngest U/Pb zircon age as well as a weighted average age (Table 1). Dickinson and Gehrels (2009) provide a detailed discussion about the different methods for reporting detrital zircon ages and the

RESULTS

Zircons analyzed from the five tuff units in the Stanley Group yielded a total of 439 U-Pb ages (, 30% discordant) ranging from , 320 Ma to older than 2000 Ma (Fig. 2). The Beaver’s Bend tuff yielded 96 U-Pb ages with significant age peaks ca. 320 Ma and between 900–1600 Ma. The Hatton Tuff yielded 94 U-Pb ages with significant age peaks ca. 320 Ma, 450 Ma, and between 1000 and 1500 Ma. The lower Mud Creek Tuff yielded 69 U-Pb ages with significant age peaks ca. 320 Ma, 350 Ma, and between 900 and 1500 Ma. The upper Mud Creek Tuff yielded 102 U-Pb ages with age peaks ca. 320 Ma, 450 Ma, and between 900 and 1700 Ma. The Chickasaw Creek Tuff yielded 78 U-Pb ages with age peaks ca. 320 Ma, 500 Ma, and 1000 Ma. From the lowermost to uppermost tuffs analyzed, the weighted average ages for the youngest age populations are 328.5 6 2.7 Ma (all uncertainties are at the 2s level) for the Beaver’s Bend tuff, 326.1 6 3.7 Ma for the Hatton tuff, 322.4 6 2.4 Ma for the lower Mud Creek tuff, 324.8 6 2.1 Ma for the upper Mud Creek Tuff, and 320.7 6 2.5 Ma for the Chickasaw Creek Tuff (Fig. 2). The upper Mud Creek tuff is slightly older than, but within error of, the lower Mud Creek tuff. DISCUSSION

U-Pb Ages The U-Pb zircon age spectra for each of the five tuffs contains a significant age peak at , 320–330 Ma consisting of 20–50% of all zircons analyzed in each tuff. This peak is representative of the tuff age as discussed above. In addition to the Carboniferous-age zircons, each tuff also contains a range of older zircons from , 450 Ma to older than 2000 Ma. Significant age peaks in the tuffs indicate sources with ages ca. 450 Ma, 500 Ma, and 900–1700 Ma. The 400–500 Ma age zircons are likely associated with volcanic and plutonic igneous activity during the Taconic Orogeny along the eastern margin of the continent (Thomas et al. 2004). Another possible source is the Southern Oklahoma Aulacogen, which was magmatically active ca. 530 Ma (Hogan and Gilbert 1998). UPb zircon ages between 900 and 1300 Ma are likely ultimately derived from the Grenville Orogen, which extends along the eastern and southern margin of Laurentia (Tollo et al. 2004) and are fertile sources of zircons

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(Moecher and Samson 2006). Zircons with U-Pb ages between 1350 and 1550 Ma are likely ultimately sourced from the mid-continent Granite– Rhyolite province that borders the Ouachita region (Whitmeyer and Karlstrom 2007) and 1600–1700 Ma zircons are likely ultimately sourced from the mid-continent orogens in Yavapai–Mazatzal region (Whitmeyer and Karlstrom 2007). Paleozoic strata in the central and southern Appalachians and the Grand Canyon all contain 400–500 Ma zircons ( Thomas et al. 2004; Park et al. 2010; Gehrels et al. 2011). Older Proterozoic zircons (900–1900 Ma) and Archean zircons (2500 + Ma) are found in Paleozoic strata across North America (e.g., Thomas et al. 2004; Park et al. 2010; Gehrels et al. 2011; Hadlari et al. 2012), and similar patterns have recently been shown in Paleozoic strata from the Ouachita Mountains (Shaulis 2010). Considering the U-Pb zircon ages of the tuffs in the Stanley Group, which are contemporaneous with deposition, it is clear that the tuffs were mixed with detritus dominantly from Laurentian sources. It is very likely that these pre-Carboniferous grains are recycled from eroding, and previously mixed sedimentary deposits and are not directly derived from their respective source terranes in the Mississippian. Given that there is evidence of significant detrital components to the tuffs, it is possible that the younger tuffs contain some recycled zircons from the older tuffs, which could slightly influence the weighted average ages of the youngest age populations. The lower Mud Creek Tuff, on the other hand, has significantly fewer older recycled grains, and thus the weighted average age is more likely to represents its true depositional age. These data from the Stanley Group are used to constrain 1) the absolute age range of the Group, 2) rates of deposition, and 3) the depositional setting. Age of the Stanley Group The U-Pb ages of the tuff units range from 328.5 6 2.7 near the base to 320.7 6 2.5 Ma near the top of the Stanley Group. The four oldest tuffs are contained within the Tenmile Creek Formation, which is within 500 m of basal contact of the Stanley Group (Fig. 2). Since a well-known regional hiatus exists at the Devonian–Mississippian boundary (Noble 1993), it is plausible that the base of the Tenmile Creek formation is at or near the lower Mississippian boundary. Since there are no absolute age constraints at this boundary, we rely on the biostratigraphic data for the age of the lower Tenmile Creek. Hass (1950) found the conodont Gnathodus texanus (345.3–338 Ma; Ogg et al. 2008) near the base of the Stanley Group, which suggests that the lower Tenmile Creek formation is mid-Mississippian. Hass (1950) also found the conodont Gnathodus bilineatus below and within about a meter above the Hatton Tuff (326.1 6 3.7 Ma U-Pb), but the subspecies of Gnathodus bilineatus is not specified. Gnathodus bilineatus bilineatus has an age range of 337–332 Ma, and Gnathodus bilineatus bollandensis has an age range of 323–318.1 Ma (Ogg et al. 2008). Even considering that the sediments within the Stanley Group (and other Mississippian–Pennsylvanian units) have been reworked (Gordon and Stone 1977), the total age ranges of both conodonts (337 to 318 Ma) are consistent with the newly established ages

of the tuffs. Based on the occurrence of Gnathodus texanus near the bottom of the Stanley Group, the maximum age of the Tenmile Creek Formation is likely to be between 345 and 338 Ma. The Chickasaw Creek tuff (320.7 6 2.5 Ma) constrains the age of the upper Stanley Group. Biostratigraphic data from the overlying Jackfork Group and Johns Valley formation are also in agreement with a late Mississippian age for the entire Stanley Group turbidite flysch sequence. In the lower Jackfork Group, plant and marine fossils from exotic blocks of the late Mississippian Pitkin Limestone provide maximum age constraints (White 1937; Gordon and Stone 1977). Additionally, marine fossils from the mid- to upper Jackfork and the lower Johns Valley formation are early Pennsylvanian in age (Gordon and Stone 1977; Ethington et al. 1989). Based on the U-Pb geochronology of the tuffs and the existing biostratigraphy, the main volume of the Stanley Group is younger than 330 Ma and older than 320.7 6 2.5 Ma (Fig. 2). The base of the Stanley Group could be as old as 359 Ma (Devonian–Mississippian boundary), but is likely variable because the basal contact with the Devonian Arkansas Novaculite is locally disconformable (King 1961; Noble 1993). The presence of Gnathodus texanus near the base of the Tenmile Creek Formation suggests a maximum age for the Stanley Group as Visean, between 345.3 and 338 Ma (Fig. 2). Rates of Deposition Pre-Mississippian sedimentary strata in the Ouachita Mountains are relatively thin (approximately 1,600 m thick) compared to the much thicker Mississippian–Pennsylvanian units (Niem 1976). Sedimentation rates for the Cambrian–Devonian units ranged from , 5–190 m/My, but accumulated at an average rate of , 25 m/My (Morris 1974). The , 300-m-thick Devonian Arkansas Novaculite had an average sedimentation rate of , 5 m/My prior to a period of nondeposition that lasted for , 10 My near the Devonian– Mississippian boundary (Noble 1993). Previously, sedimentation rates of 100–1350 m/My had been reported for the Mississippian–Pennsylvanian Stanley–Jackfork–Johns Valley–Atoka sequence, with an average deposition rate of 200 m/My for the Stanley Group (Coleman 2000; Morris 1974). The new U-Pb ages of the five tuffs in the Stanley Group record the transition from slower (pre-flysch?) sedimentation to rapid flysch sedimentation during the late Mississippian (Fig. 3). The transition to flysch sedimentation in the late Mississippian can be demonstrated using the stratigraphic positioning of marine fossils (Gnathodus texanus (345.3–338 Ma), Gnathodus bilineatus bilineatus (337–332 Ma), and Gnathodus bilineatus bollandensis (323–318.1 Ma; Ogg et al. 2008)) and the five tuff units (328–320 Ma) (Fig. 3). The conodont Gnathodus texanus is found within a few meters of the base of the Stanley Group, and the ages of the four lowermost tuff units, found in the lower 500 m of the Stanley Group, indicate deposition rates (1–40 m/ My, Fig. 3) remained low until , 323 Ma. The Chickasaw Creek tuff is found within 250 m of the top of the Stanley Group and is separated from the lower Mud Creek tuff by over 2500 m of sediment. The difference in age between the lower and upper Mud Creek tuff is , 3 Ma, indicating

r FIG. 2.—Stratigraphic column of the Stanley Group. A) The age ranges of conodont biozones based on Ogg et al. (2008): Gnathodus texanus (345.3–338 Ma), Gnathodus bilineatus bilineatus (337–332 Ma), and Gnathodus bollandensis (323–318.1 Ma), as well as the relevant age markers for the base and top of the Stanley Group. B) The stratigraphic column of the Stanley Group (after Niem 1976) showing the approximate stratigraphic location of tuffs within the Stanley Group, plus the approximate location of conodonts found in relation to the tuffs. C) Population-density plots from the AgePick program (Gehrels 2007) for each of the five tuffaceous members of the Stanley Group. The youngest age peak found for each tuff were: 322 Ma for the Beaver’s Bend Tuff, 323 Ma for the Hatton Tuff, 323 Ma for the lower Mud Creek Tuff, 322 Ma for the upper Mud Creek Tuff, and 322 Ma for the Chickasaw Creek Tuff. D) Weighted average ages of the five tuffs: Beaver’s Bend Tuff (328.5 6 2.7 Ma), Hatton Tuff (326.1 6 3.7 Ma), lower Mud Creek Tuff (322.4 6 2.4 Ma), upper Mud Creek Tuff (324.8 6 2.1 Ma), and Chickasaw Creek Tuff (320.7 6 2.5). The weighted average ages are considered to be the depositional age of each tuff.

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FIG. 3.—Sedimentation rates in the Stanley Group based on the minimum and maximum ages from the calculated weighted average 206 Pb/238U age for each tuff: Beaver’s Bend, Hatton, lower and upper Mud Creek, and Chickasaw Creek, and the beginning and end of the biozone for Gnathodus texanus (A) and (B), respectively (Ogg et al. 2008). An average sedimentation rate of , 800 m/My is calculated from the average age of the lower and upper Mud Creek tuffs and the Chickasaw Creek tuff.

that sedimentation rates increased significantly to 300–1400 m/My. A preferred average rate of , 800 m/My can be calculated between the average age of the lower and upper Mud Creek tuffs (323.6 Ma) and the Chickasaw Creek tuff (320.7 Ma), which is much faster than the rate of 200 m/My suggested by Morris (1974) and Coleman (2000). The rapid flysch sedimentation rate found in the late Mississippian is similar to that of , 900 m/My inferred for the Atoka Formation (Morris 1974; Coleman 2000). The significance of the varying depositional rates between 115 and 400 m/My (Morris 1974; Coleman 2000) of the Pennsylvanian Jackfork Group, Johns Valley, and Atoka formations are difficult to interpret given relatively poor absolute age control on the formations. Based on the biostratigraphic and U-Pb zircon age data from the Mississippian Stanley Group, the onset of significant flysch influx into the Ouachita trough was in the late Mississippian (, 323 Ma). Depositional Setting Closing remnant ocean basins are some of the largest sedimentary depositional systems on the planet and are dominated by the rapid deposition and accumulation of turbidites that are then incorporated into the suture belt during collision (Ingersoll et al. 2003). During collapse of the basin, sediments are eroded from tectonically uplifted regions and then transported longitudinally through foreland basins and deltaic fan systems which feed the turbidite fans (Graham et al. 1975; Ingersoll et al. 1995; Ingersoll et al. 2003). The tuffaceous units studied here are composed dominantly of volcanic material, but the zircon age data indicate a significant contribution of detritus from other sources. For example, there is significant contribution of detritus from pre-900 Ma sources such as the Grenville and Granite– Rhyolite Provinces, likely reflecting a significant recycled component (Fig. 2). Other prominent age peaks at , 450 Ma and , 500 Ma are similar in age to plutonic and volcanic rocks associated with the Laurentian Taconic orogeny (Thomas et al. 2004 and references therein) as well as Cambrian igneous rocks in the Oklahoma Aulacogen (Hogan and Gilbert 1998). Despite the proximity of a magmatic arc during collision, most of the sediment shed from the arc terrane, with the exception of air-fall tuff, is predicted to be trapped within the forearc basins (Ingersoll et al. 1995). This is consistent with the lack of prominent Pan-African age peaks that would reflect Gondwanan sources to the

south (Fig. 2). Thus, the tuffaceous units were likely reworked, allowing for significant incorporation of recycled flysch detritus from the southern Alleghenian orogen to the east (e.g., Graham et al. 1975; Gleason et al. 1995) and from the assembling allochthonous accretionary prism to the south, which progressively incorporated distal to proximal Laurentian margin sedimentary sources as it encroached northward (Morris 1974; Wickham et al. 1976). A remnant-ocean-basin setting for flysch units of the Stanley Group is consistent with easterly-derived detritus (Johnson 1968) dominated by recycled Laurentian sources (Gleason et al. 1995; Shaulis 2010). The rapid influx of detritus into the basin is coeval with the Alleghenian orogeny (, 330–280 Ma). The major influx of sediment in the late Mississippian is likely related to closure and exhumation of the Alleghenian suture to the east and the assembling allochthons to the south of the basin (Graham et al. 1975). The relative volumetric influx from each source is difficult to determine because both of them would have Laurentian-margin reworked sedimentary components. CONCLUSIONS

New U-Pb zircon ages from 328 to 320 Ma tuff units from the Stanley Group indicate that sedimentation rates remained low until the late Mississippian (, 323 Ma), when they increased from , 1–40 m/My to . 300 m/My. The upper age limit of the Stanley Group is 320.7 6 2.5 Ma, slightly younger than the Mississippian–Pennsylvanian boundary. Based on grain size and thickness (Johnson 1968; Niem 1977), the tuffs are likely derived from an encroaching arc terrane to the south. The significant recycled components within the tuffs and mostly easterly derived paleocurrent indicators in sandstone and shale units (Johnson 1968) suggest that much of the flysch was likely derived from the assembling accretionary prism (allochthon) to the southeast and the southern Alleghenian orogen to the east. This is consistent with the Ouachita trough having been a remnant ocean basin to the west of the Alleghenian suture in the late Paleozoic (e.g., Graham et al. 1975). ACKNOWLEDGMENTS

Funding for this project was provided by the American Chemical Society Petroleum Research Fund and the State of Texas Norman Hackerman ARPATP grants to TJL. Funding for the Photon Machines Analyte.193 laser ablation system was provided from NASA Cosmochemistry and NSF

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Instrumentation and Facilities programs to TJL. Yongjun Gao is thanked for maintaining the ICPMS. We would like to thank an anonymous reviewer and the Associate Editor Lynn Soreghan for their helpful comments, and we especially thank Dr. James Gleason for his very detailed review. All reviews significantly improved this manuscript. Supplemental data is available from JSR’s Data Archive: http://www.sepm.org/pages.aspx?pageid5229. REFERENCES CITED

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Received 23 March 2012; accepted 6 July 2012.