Mississippian Pyroclastic Flow and Ash-Fall Deposits in the Deep-Marine Ouachita Flysch Basin, Oklahoma and Arkansas

Mississippian pyroclastic flow and ash-fall deposits in the deep-marine Ouachita flysch basin, Oklahoma and Arkansas

ALAN R. NIEM Department of Geology, Oregon State University, Corvallis, Oregon 97331

ABSTRACT in rocks of many different ages and have subduction of an oceanic part of the North been thoroughly investigated (Fisher, 1966; American plate beneath a southern conti- Two pumiceous vitric-crystal tuffs, the Murai, 1961; Ross and Smith, 1961; Smith, nent (Wickham and others, 1976), resulted Hatton Tuff Lentil and Beavers Bend tuff, 1960). However, such deposits in sequences in a trench-arc complex within the occur in the deep-marine Mississippian of marine rocks are rarely recognized. With Ouachita geosyncline. Resultant tectonic is- Stanley Group. These widespread rhyoda- increased sophistication of oceanographic lands consisting of sedimentary and citic tuffs range in thickness from 7 to 40 m studies and detailed stratigraphie analyses, metamorphic rocks supplied great quan- and are separated by tens of metres of non- submarine pyroclastic flow deposits have tities of fine- to medium-grained quartzose tuffaceous quartzose and feldspathic turbi- been reported from the Permian in Alaska and feldspathic sands to the rapidly subsid- dite sandstone and shale. The tuffs consist by Bond (1970, 1973); from the Cretaceous ing Ouachita trench in the north. These of varying proportions of ash-sized em- to Paleogene in the Philippines by Fernan- sands were transported by turbidity cur- bayed quartz crystals, plagioclase crystals dez (1969); from the Eocene in Washington rents into the deep basin, frequently inter- (oligoclase to andesine), relict shards, vol- by Fiske (1963); from the Oligocene on rupting normal accumulation of pelagic canic dust, and altered flattened pumice Rhodes Island, Greece, by Mutti (1965); mud, and produced more than 6,700 m of fragments. and from the Miocene in Japan by Fiske Carboniferous flysch. Some of these turbi- Each pyroclastic unit consists of two or and Matsuda (1964). These tuffs were de- dite sequences have been interpreted as a more tuff lithologies, including a thick posited by a combination of volcanic erup- series of coalescing submarine fans (Picha lower unstratified pumiceous vitric-crystal tive and sedimentary marine processes. The and Niem, 1974; Niem, 1976; Morris, tuff with a density-graded crystal-rich base interaction of volcanic and sedimentary 1974a). Trace fossils indicate that the Car- overlain by thin-bedded pumiceous tuff and processes in the formation of submarine boniferous flysch was deposited at bathyal an upper massive fine-grained siliceous pyroclastic deposits generally has been depths (Chamberlain, 1971). Several tuffs vitric tuff. overlooked by volcanologists and sedimen- were introduced into this deep-water basin These tuffs were probably formed by tologists. The well-exposed tuffs in the during deposition of the Stanley flysch se- highly explosive eruptions of vesiculating deep-marine Mississippian Stanley Group quence in Late Mississippian time. acidic magma from a vent or fissure that flysch of the Ouachita Mountains are one produced incandescent avalanches of of the few examples where evidence of the STANLEY GROUP pyroclastic debris and accompanying ash nature of these submarine processes can be clouds. The hot turbulent suspensions were unraveled. The dominant rock types of the 4,000- rapidly quenched by sea water to form m-thick Late Mississippian Stanley Group steam-inflated density slurries that flowed GEOLOGIC SETTING are dark-gray deep-marine shales with sub- into the Ouachita basin. Pyroclastic flows ordinate nontuffaceous, fine-grained quartz created thick, density-graded, pumiceous The Ouachita Mountains of southeastern wackes and feldspathic sandstones (Hill, vitric-crystal tuff. Numerous smaller den- Oklahoma and southwestern Arkansas 1967). Dark siliceous shales and tuffs are sity slurries following the main flow in consist mainly of highly deformed and minor units that are invaluable for mapping rapid succession deposited the overlying faulted Mississippian and Pennsylvanian and correlation purposes. Harlton (1938) bedded pumiceous tuff. Toward the end of flysch sequences (Stanley Group, Jackfork divided the Stanley into three formations: each volcanic eruption, continuous settling Group, Johns Valley Formation, and Atoka the Tenmile Creek, Moyers, and Chickasaw of fine ash formed thick, fine-grained upper Formation). Isoclinally folded and thrusted Creek (from oldest to youngest; Fig. 2). The vitric tuff. lower Paleozoic strata (Cambrian through Tenmile Creek Formation accounts for ap- Isopach maps of tuff thicknesses, an iso- Devonian) are exposed in the cores of sev- proximately three-fourths of the total pleth map of pumice sizes, logarithmic plots eral anticlinoria in the southern and central thickness of the Stanley Group. of crystal size versus distance, paleocurrent Ouachitas (Fig. 1). Cline (1960, 1966) in- indicators, and Late Mississippian paleo- terpreted the relatively thin sequence of TUFF DEPOSITS geography suggest a southern volcanic lower Paleozoic shale, chert, argillaceous source that may have been part of a mag- limestone, novaculite, and minor gray- Within the basal 500 m of the Tenmile matic arc formed at a continental margin wacke as having been deposited in a deep- Creek Formation in the southern Ouachitas during plate convergence between the marine "starved" basin in which fine are four major widespread tuffs (the Bea- North American plate and a southern con- biogenic and terrigenous detritus accumu- vers Bend tuff, Hatton Tuff Lentil, and tinental^) plate. lated very slowly. lower and upper Mud Creek crystal tuffs; By Carboniferous time, extensive Fig. 2), which range from 7 to 40 m thick, INTRODUCTION tectonism and subsidence, perhaps as a re- and three local thin tuff beds (15 cm to 2 m sult of subduction of an oceanic plate be- thick; Niem, 1971a). A 2- to 3-m-thick Subaerial pyroclastic flows and welded neath a depressed North American conti- bedded crystal tuff occurs at the base of the ash-flow tuffs are world-wide in occurrence nental plate (Morris, 1974b, p. 277) or Chickasaw Creek Formation in the upper

Geological Society of America Bulletin, v. 88, p. 49-61, 16 figs., January 1977, Doc. no. 70105.

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MAP LOCATION

// C retaceous

Jackfork and Atoka Groups

Stanley Group

Pre -Carboni ferous

Stanley tuffs: Hatton & Beavers Bend pumiceous vitric - crystaI

Lower & upper Mud Creek crystal tuffs Figure 1. Geologic map of southern Ouachita Mountains (modified after Honess, 1923; Miser and Purdue, 1929).

part of the Stanley Group and is widespread z throughout the central Ouachitas (Laudon, z 1959; Niem, 1971a; Morris, 1974a). ÜJ JACKFORK GROUP 0. The Hatton and Beavers Bend tuffs are predominantly altered pumiceous vitric- CHICKASAW CREEK* tuff crystal to vitric tuffs following the classification of Pettijohn (1975, p. 306; see MOYERS Fig. 3) and are rhyodacitic in composition Q. (Niem, 1971a). The lower and upper Mud 3 Creek tuffs and the Chickasaw Creek tuff o are crystal tuffs. All of the tuffs contain a tr. very fine grained, dense, siliceous, vitric o TENMILE CREEK upper part that is useful for determining >- z LÜ Figure 2. Stratigraphy of stratigraphic up direction and aids in un- the Stanley Group. raveling the complex structure of the area. < 1 — Z The pyroclastic units are separated from CL < each other by tens to hundreds or, in the Q_ 1- case of the Chickasaw Creek tuff, by cn CO upper Mud Creek tuff thousands of metres of deep-marine CO lower Mud Creek tuff quartzose and feldspathic turbidite sandstones and shales. This study deals with CO CO the two lowest tuffs of the Stanley Group, Hatton Tuff the Beavers Bend tuff and the Hatton Tuff. Ü Beavers Bend tuff The Hatton Tuff Lentil, the thickest and most widespread of the tuffs in the Stanley ARKANSAS NOVACULITE Group, was the first to be recognized and mapped in the Ouachitas (Miser, 1920; (upper part) Honess, 1923; Miser and Purdue, 1929). This pyroclastic unit ranges in thickness *Briggs (1973; Briggs and others, 1975) includes Chickasaw from 20 m in southwestern Arkansas to 40 Creek Fm. in the Pennsylvanian Jackfork Group. m in Oklahoma and crops out over 9,300

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/ Crystal Lithic \ / Tuff Tuff \

Crystals Rock Fragments

Crystal- Vitric - Crystal Vilric— Crystal Vitric Tuff Tuff ^ Tuff Tuff » • • • ¿V TCD Figure 3. Classification diagrams for Hatton Figure 4. Cliff exposure of Hatton Tuff (A) and underlying Beavers Bend tuff (B). Lower limb of and Beavers Bend tuffs. Solid circle = Hatton isoclinal fold, Rattlesnake Bluff, Beavers Bend State Park, Oklahoma. unstratified pumiceous vitric-crystal tuff; plus symbol = Hatton bedded pumiceous tuff; open circle = Hatton fine-grained vitric tuff; solid square = Beavers Bend unstratified pumiceous vitric-crystal tuff; open square = Beavers Bend F F" T îéfll * * '»>' - fine-grained vitric tuff.

km2 (Niem, 1971a). It occurs 310 to 80 m above the Devonian to Lower Mississippian Arkansas Novaculite (Fig. 2). The Hatton Tuff, which has been dated by whole-rock Rb-Sr radiometric analysis, yields an iso- chron age of 306 ± 8 m.y. or Late Missis- sippian (Mose, 1969a) or 310 ± 15 m.y. (XRb87 = 1.39 X 10-" yr) or Pennsylvanian (Mose, 1969b). Conodonts collected by «F V'.r."; Hass (1950, p. 1578) in the Stanley shales directly above and below the tuff, however, confirm the Late Mississippian age (Meramecian). The isoclinally folded outcrop pattern of the tuff closely follows the % (¡¿i Arkansas Novaculite-Stanley contact around the Broken Bow-Benton uplift, which forms the core of the southern Ouachitas (Fig. 1). The Hatton Tuff also has been recognized 145 km northeast of the Broken Bow—Benton uplift in the Hot springs area of Arkansas by Danilchik and Figure 5. Unstratified pumiceous vitric-crystal tuff, Hatton Tuff. Dark fragments are flattened al- Haley (1964) and by Niem (1968). The tuff tered pumice clasts aligned subparallel to bedding. crops out as a cliff (Fig. 4) or a low resistant ridge partly covered with large gray-green in Oklahoma) and less widespread (less parallel to bedding) and consists domi- spheroidal boulders and is characterized by than 2,000 km2) than the overlying Hatton nantly of flattened and altered dark-green abundant altered flattened dark-green Tuff. The unit lies 15 to 25 m below the to weathered dusky yellow pumice in a pumice lapilli and blocks in a very light Hatton Tuff Lentil and parallels the out- light-green, fine-ash matrix of crystals and gray ash matrix of crystals and relict shards crop pattern of the Hatton Tuff in Ok- altered shards. (Fig. 5). lahoma but pinches out toward the Arkan- Both Hatton and Beavers Bend ruffs are The Beavers Bend tuff was first recog- sas border (Niem, 1971a). It lies from 150 composed of two or more major tuff nized by Hill (1967) at Beavers Bend State to 310 m above the Arkansas Novaculite. lithologies, including thick, unstratified Park near Broken Bow, Oklahoma. This In outcrop, the Beavers Bend tuff displays a pumiceous vitric-crystal tuff with a crystal- pyroclastic unit is much thinner (2 to 12 m distinctive cleaved appearance (parting rich base; thin-bedded pumiceous tuff; and

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ZONATION OF PUMICEOUS VITRIC-CRYSTAL TUFFS

EXPLANATION

&« CRYSTAIS J CLAY RIP-UPS EXPLANATION

PUMICE I API LLI FLUTE MARKS I ->Soft sediment LAMINATED AND CROSS-BEDDED ASH £ 1 deformed clay Figure 6. Idealized succession of tuff lithologies in Stanley pumiceous vitric-crystal tuffs. Fine - g rained vitric tuff

massive fine-grained siliceous vitric tuff that o• «=• Massive pumiceous is less commonly laminated or rarely vitric-crystal tuff cross-bedded. Figure 6 illustrates the generalized stratigraphic succession of these Laminated tuff lithologies. Some or all of these vitric tuff lithologic types may be present and locally repeated in each of the tuffs. Locally, the Bedded pumiceous Hatton contains two cycles of the vitric-crystal tuff generalized stratigraphic succession (sections 2, 3, and 4 in Fig. 7), and the Beavers J J J Crystal- enriched J ^ Bend contains the massive pumiceous lower tuff part and a vitric upper portion, although it lacks the bedded pumice lithology (Fig. 8). Shale Massive Unstratified Pumiceous Vitric-Crystal Tuff Load casts Stratigraphy and Lithology. The basal unstratified pumiceous vitric-crystal zone is ° Pumice clasts thick, composing one-half to two-thirds of each tuff (1 to 6 m in the Beavers Bend tuff; in the Hatton there are two zones, 3 to 20 m). The zone is widespread, covering approximately 8,000 km2 in the Hatton Tuff and less than 2,000 km2 in the Beavers Bend Figure 7. Correlation of measured section of Hatton Tuff showing the variety of tuff lithologies. tuff. The bottom contact is sharp with the underlying shale or quartz wacke and locally contains flute marks and load casts. nantly euhedral and broken angular are present in minor amounts. Relict

The upper contact is gradational with the plagioclase (An30 to An34) in a hematite- shards, shreds of altered pumice, green overlying fine-grained vitric or bedded stained dark detrital clay matrix. Locally, tourmaline, zircon, and mica flakes are pumiceous tuff. The basal metre is enriched crystals are imbricated and aligned parallel rare. (as much as 60 per cent) in poorly sorted to bedding in the Beavers Bend tuff. Em- Pumice and shards gradually increase in (fine to coarse ash-sized) crystals that in the bayed volcanic quartz grains, small lami- abundance from the crystal-rich base to the Beavers Bend tuff occur in two to four 1- to nated shale and siltstone rip-ups, quartzite thick pumiceous upper part of the zone. 7-cm graded beds. Crystals are predomi- pebbles, and quartz-mica schist fragments This change is a form of graded bedding,

defined by an upward decrease in density of B1 B2 B3 B4 pyroclastic debris. Dense coarse ash-sized crystals grade upward into lapilli and block-sized less dense pumice. Fiske (1969) B4. _ « noted that this type of density grading is > common in submarine pyroclastic sequences. Shale rip-ups (as much as 33 cm long by 10 cm wide) occur locally in the middle and upper parts of the massive zone. / INDEX MAP On bedding planes, the flattened pumice is elliptical in shape, is rounded to sub- / rounded, and ranges from 4 to 30 cm in / diameter (lapilli and blocks according to / the classification of Fisher, 1961). In cross ' CP y• y y y v • • lm section, the long-tube pumice clasts are flat- / / tened and elongated parallel to the plane of / stratification, forming a pseudo-eutaxitic / texture similar to welded ash-flow tuffs V y s/ J y y ^, / (Fig. 5). Large phenocrysts (2 to 13 mm) of s y y euhedral plagioclase and embayed volcanic / quartz are scattered in the pumice clasts. / The glassy pumice in all zones of the Bea- / EXPLANATION vers Bend and Hatton tuffs has been / diagenetically altered to dark-green cela- / donite (an iron-rich 1M muscovite iden- •s y y y Fine-grained vitric tuff tified by x-ray powder photograph). Pumice • • • / [ clasts are supported in a light blue-gray / • • y y matrix of fine to coarse ash—sized sickle- Massive pumiceous vitric — shaped and bubble-wall shards (Fig. 9), crystal tuff volcanic dust (now cryptocrystalline sili- • o / ceous intergrowth of chert, hematite, limo- s• y • s• C rystal - enriched tuff nite, and clay), and minor angular silt-sized quartz and plagioclase crystals. The albite 7 twinned plagioclase is oligoclase and an- Flute casts desine (An™ to An ) and is partly altered to :M Figure 8. Correlation of measured sections of Beavers Bend tuff showing the variety of tuff sericite. Some untwinned varieties show lithologies. peristeritic exsolution texture. Relict shards and pumice, although altered, are abundant (as much as to 90 percent in some samples) and well preserved in thin section (Fig. 9). Altered shards consist of an intergrowth of polycrystalline quartz and minor feldspar (Fig. 10) and less commonly calcite, zeo- lites(?), and (or) celadonite. The devitrified glass shards lack the microscopic axiolitic texture that is common in many welded ash-flow tuffs (Ross and Smith, 1961, p. 37). Origin. The abundance of delicate shards, pumice, and angular euhedral and broken plagioclase and embayed unstrained quartz crystals with little or no admixture of other foreign minerals suggests that the pyroclastic material was freshly erupted and rapidly deposited. The general homo- geneity, poor size sorting, wide areal extent and thickness, density grading, large shale rip-ups, flute marks, and lack of stratification suggest that the massive pumiceous zones within the two tuffs were deposited by subaqueous pyroclastic flows. These same characteristics have been attributed by Fiske (1963), Fiske and Matsuda (1964), and Bond (1973), respectively, to marine Figure 9. Photomicrograph of fibrous celadonite-replaced pumice (A) with incorporated quartz tuffs formed by pyroclastic flows in Wash- phenocryst in a matrix of bubble-wall and sickle-shaped shards, Hatton Tuff.

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ington, Japan, and Alaska. However, these randomly oriented (Fig. 9), whereas glass was formed as the denser quartz and tuffs, unlike the Stanley tuffs, are predomi- shards in welded ash-flow tuffs are flattened feldspar crystals with higher settling veloc- nantly composed of angular nonvesicular and aligned due to combined effects of re- ities concentrated near the base and the lapilli and blocks of dacite and pumice with tained heat and compaction (Smith, 1960, front of the flow and settled out first. The minor ash-sized shards and crystal matrix. p. 33-34). lighter but larger, more buoyant pumice The Ouachita massive pumiceous tuffs The presence of density grading, flute lapilli and finer shard-rich ash remaining in more closely resemble in texture, composi- marks, and mudstone rip-ups and the lack the upper and rear portions of the flow set- tion, and thickness submarine welded ash- of evidence for welding in the massive tled later to form the massive pumiceous flow deposits that have been described pumiceous tuff suggest that these pyroclas- upper parts. Middleton and Hampton within the flyschlike Oligocene Vati Group tic units were emplaced by density flows of (1973) noted such a concentration of on the island of Rhodes, Greece, by Mutti quenched shard-rich ash and pumice that coarser and denser debris in the head of ex- (1965) and in the Cretaceous to Paleogene was too cool when emplaced to produce the perimentally produced turbidity currents shallow-water deposits of Siargao Island, characteristic textures and zonation of and suggested that this concentration is due the Philippines, by Fernandez (1969, p. 29). welded ash-flow tuffs. to the divergent nature of flow within the These deposits, like the massive Stanley The density-flow mechanism of these head and the higher settling velocities of the pumiceous tuffs, are composed predomi- pyroclastic units is probably analogous to coarser and denser debris. Alternatively, nantly of shard-rich rhyodacitic ash and that of a high-density turbidity current de- Walker (1972) suggested that crystal en- contain randomly distributed subparallel scribed by Middleton and Hampton (1973) richment observed in the basal part of sub- streaks of flattened pumice (similar to the in which pyroclastic debris in the flows was aerial ash flows may be attributed to prefer- eutaxitic texture of subaerial welded ash- supported by fluid turbulence, dispersive ential comminution and subsequent loss of flow tuffs described by Smith, 1960, and pressure related to shear stress between lighter pumice relative to abrasion-resistant Ross and Smith, 1961). There is, however, grains, and possible upward flow, in this crystals during flowage. no evidence of heat retention or welding in case, of escaping steam and water. The In addition, some sorting in the Hatton the massive Stanley pumiceous vitric-crystal mechanism presented here differs from the and Beavers Bend massive pumiceous tuffs tuffs, such as columnar jointing, vapor Middleton and Hampton flows in scale and may have occurred through density separa- phase minerals, or welding zonation attrib- from the Ouachita flows because they were tion of different pyroclastic components uted to subaerial welded tuffs (Smith, 1960; probably directly and continuously fed by during settling directly from the eruptive Steiner, 1960; Walker, 1970). The pumice an eruptive source. A steady eruptive sup- column. Such an eruptive sorting mech- in the Ouachita tuffs probably was flat- ply of pyroclastic debris would have per- anism was inferred by Hay (1959) for the tened and stretched by later compaction mitted continuous movement of the flow to 1902 nuée ardente eruption of Soufrière on under a thick sequence of marine muds and produce a relatively thick (6 to 15 m) and St. Vincent (Lesser Antilles), which formed turbidite sands or perhaps by shearing dur- widespread (more than 8,000 km2) layer of a crystal-enriched basal ash. ing the Pennsylvania!! Ouachita orogeny. massive pumiceous tuff. The type of eruption that produced these Some pumice lapilli are stretched parallel to The density sorting of pyroclastic debris pyroclastic flows is inferred to have been a cleavage planes. In addition, relict glass in the massive pumiceous zone in both the nuée ardente-like eruption, similar to that shards in the massive pumiceous tuff are Hatton and Beavers Bend tuffs probably described by LaCroix (1904) and Perret (1935) or probably on the scale of an ash- flow eruption (Smith, 1960). Rapid eruption of great volumes of vesiculating acidic magma from a vent or fissure produced a dense, hot, turbulent gas-charged incandescent avalanche of fine pyroclastic debris (glowing avalanche) and an associated ash cloud (nuée ardente). It is hypothesized that as this glowing avalanche flowed downslope into the ocean, ash rapidly in- termixed with sea water and formed steam-inflated slurries of partly cooled or quenched pyroclastic debris, escaping steam, and sea water. Some buoyant pumice floated, and finer ash remained in suspension in a cloud in the water column above the flow. LaCroix (1904) postulated that the nuée ardente from the eruption of Mont Pelée in 1902 on the island of Mar- tinique (Lesser Antilles) flowed several miles over the sea floor, as indicated by the simultaneous breakage and "singing" of submerged telegraph lines. Alternatively, the massive pumiceous vitric-crystal tuffs may have been produced Figure 10. Photomicrograph of devitrified glass shard in Hatton Tuff. Note minute parallel elon- from large volumes of vesiculating acidic gate quartz crystals normal to shard boundaries and the coarse interlocking crystal mosaic in shard magma erupted under water. Pyroclastic center. Shard and angular quartz grain (A) are set in a cryptocrystalline siliceous matrix. Crossed debris may have settled rapidly from a shal- nicols. low submarine eruptive column on unstable

slopes around the submarine vent, then less concentrated in the pumice-rich layers, pumice size gradually decreases to 4 mm or slumped and flowed down the volcanic smaller in size, and more randomly distrib- less. Generally, the largest pumice lapilli flanks into deeper water. Such an eruption uted through the tuff. and blocks are concentrated in a few beds mechanism was proposed by Fiske (1963, At the base of the bedded zones, well- near the middle of the zone. An isopleth p. 404) for pumiceous pyroclastic flow de- rounded altered pumice fragments in the map of pumice size shows that the largest posits in the Eocene Ohanapecosh Forma- pumice-rich layers average 4 mm in diame- sizes occur in the southern part of the Hat- tion in Washington. ter (lapilli). Pumice diameters in the Hatton ton outcrop area (Fig. 12). Although the water depths in which the Tuff increase upward in the zone to approx- Origin. The rhythmic alternation of eruptions that formed the rhyodacitic ash imately the middle of the zone, where pumice-rich layers with pumice-poor layers of the Hatton and Beavers Bend tuffs can- pumice diameters are as large as 22 cm suggests a regular repetition of eruptive and not be determined precisely, the water was (blocks). In the upper one-third of the zone, depositional events. The similarity of corn- probably relatively shallow. McBirney (1963) calculated on the basis of theoretical considerations that explosive ash formation from basaltic and rhyolitic magmas is un- likely below water depths of 500 m (although unusually water-rich rhyolitic magmas could theoretically produce ash in depths as great as 2,000 m or more).

Bedded Pumiceous Tuff

Stratigraphy and Lithology. The zone of bedded pumiceous tuff is absent in the Beavers Bend tuff but is a distinctive rock type that occurs twice in the Hatton Tuff; each overlies a massive unstratified pumiceous tuff (sections 2, 3, Fig. 7). The bedded pumiceous zones range from 1 to 3 m thick. The bedded unit is composed of 7- to 10-cm-thick pumice-rich layers alternating with 5- to 7-cm-thick pumice-poor vitric-crystal layers (Fig. 11). Celadonite- replaced pumice in the pumice-rich layers makes up as much as 50 percent of the tuff and is supported by a relict shard and crystal ash matrix. The intervening pumice- Figure 11. Blocks of bedded tuff. Dark layers in upper block are pumice rich. Hammer rests on poor layers are composed of abundant fine boulder showing shape of flattened rounded pumice on bedding planes. White rectangular fragments to medium ash-sized altered shards, minor within dark pumice clasts are feldspar crystals. plagioclase and quartz crystals, and rare altered pumice lapilli set in a siliceous matrix of volcanic dust. The gradational change between pumice-poor and pumice-rich layers also suggests a density grading of pyroclastic debris similar to the grading in the underlying massive pumiceous tuff but on a smaller scale. Rare sand-sized siltstone and shale clasts and rounded quartzite and basalt pebbles (less than 1 cm in diameter) occur in some beds. The alternation of dark-green pumice layers and light-gray ash layers imparts a distinctly banded appearance to the tuff (Fig. 11). A few fine parallel laminations occur within some pumice- poor layers. There are as many as 30 to 40 couplets of pumice-rich and pumice-poor layers in each bedded zone. The couplets are continuous and uniform in thickness for 6 to 15 m along an exposure. The bedded zones are correlatable over 2,000 km2. The upper and lower boundaries of the zone are gradational with the underlying massive pumiceous tuff and the overlying fine-grained siliceous vitric tuff as the pumice becomes Figure 12. Isopleth map of maximum sizes of pumice clasts in the Hatton Tuff.

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position and microtextures and the grada- pyroclastic turbidites (igniturbidites) on the mum of 30 km from the nearest possible tional contacts between pumice-poor and island of Rhodes, Greece. volcanic source based on outcrop and sub- pumice-rich layers indicate that the pyro- Alternatively, some couplets of pumice- surface data (Niem, 1968). clastic material in both are related. The rich and pumice-poor tuff may represent a Some large pumice lapilli and blocks may well-developed stratification, widespread succession of ash falls settling through have floated to the site of deposition. extent, density grading in each pumiceous water. Historic ash falls, cored in the deep Richards (1964, p. 1157) noted that pumice and nonpumiceous couplet, and the Tyrrhenian Basin in the Mediterranean Sea lapilli in "rafts" floating on the Pacific presence of rare shale chips and rounded off the west coast of Italy, have produced a Ocean are rapidly rounded by constant col- quartzite and basalt pebbles (in some similar stratification of pumice-rich and lisions of particles on a turbulent wave sur- layers) suggest deposition by a series of pumice-poor vitric-crystal layers only a few face. Many flattened large pumice lapilli in small density flows. The high degree of centimetres thick (Norin, 1958). Subaerial the bedded pumiceous tuff are well roundness of the quartzite and volcanic and submarine ash falls, deposited several rounded, suggesting that some of the pebbles also suggests that these clasts were miles from their vents, are typically wide- pumice may have floated. However, some originally abraded in shallow water and spread and well sorted and stratified (Ross rounding may have occurred as a result of were incorporated and transported by den- and Smith, 1961; Murai, 1961; Fiske and abrasion during explosive ejection of the sity flows into the deeper water environ- Matsuda, 1964; Fiske, 1963; Bond, 1973), pumice from the volcanic vent (Knopf, ment of the Ouachita trough. as are the Ouachita bedded pumiceous 1966). The gradational transition of the bedded tuffs. It is hypothesized that some lighter and pumiceous tuff from the underlying massive In an ash fall, fine crystals and shards buoyant pumice probably floated during an unstratified pumiceous tuff also suggests a would quickly become separated from eruption while the abundant fine to close relationship between the two pumice lapilli during settling in air and medium ash —sized shards and minor lithologies. It is postulated that a thick water owing to differences in bulk density quartz and feldspar crystals slowly settled submarine pyroclastic flow may have been and settling velocity. A similar separation out of an eruptive column over the depo- followed by a large number of genetically mechanism has been inferred by Rittmann sitional site to form pumice-poor layers. related surges in rapid succession, and the (1962) to explain alternating ash and Rafts of floating pumice may have drifted resulting deposit is a thick unstratified pumice layers in recent bedded ash-fall tuffs over the depositional site. As the buoyant pumiceous unit overlain by a series of thin on Monte Somma, Italy. The higher long-tube pumice lapilli and blocks became pumice-poor and pumice-rich tuff beds. viscosity of water would further accentuate saturated, they settled with the rain of re- The surges may have been produced by a this density separation. Studies of the size maining fine crystals and shards in the erup- series of explosive pulses during expulsion distribution of pyroclastic material from re- tive column to form gradational pumice- of the mass of pyroclastic debris or by a se- cent ash falls (Wilcox, 1959; Rittmann, rich vitric ash layers. The settling rate of ries of surges produced on the upper part of 1962; Fisher, 1964) suggest that it is un- medium ash-sized shards and fine crystals the pyroclastic flow during flow over an ir- likely that the large, well-rounded pumice in water is extremely slow (2 mm/sec), ac- regular topography. Mutti (1965) proposed lapilli and blocks (up to 22 cm in diameter) cording to experimental studies by Fisher a similar irregular topographic control for in the bedded pumiceous tuff could have (1965). Thus, the development of bedded the origin of a thick Oligocene submarine been blown or transported by winds to the ash-fall pumiceous tuff may have taken ash-flow tuff overlain by a series of thin present depositional site, which is a mini- days, weeks, or months of successive ash eruptions. For example, freshly erupted ash would take six days to settle through 1,000 m of undisturbed water.

Fine-Grained Vitric Tuff

Stratigraphy and Lithology. Massive, locally laminated and rarely cross- laminated, fine-grained silicified vitric tuff is the uppermost zone in the Hatton and Beavers Bend tuffs (Fig. 6). The zone is composed of structureless, hammer-ringing hard, blue-gray, compact, siliceous, very fine tuff. It ranges from 1 to 10 m thick in the southern Ouachitas, although in the Hot Springs, Arkansas, area it is 30 m thick, forming the only rock present in the Hatton Tuff. The vitric zone of both tuffs in Oklahoma thickens uniformly to the south-southeast. The bottom contact is gradational over several centimetres, and the upper grades into the overlying fissile, dark-gray to dark-green Stanley shales. Locally, a 0.6-m-thick vitric tuff bed occurs 15 cm above the main siliceous vitric tuff of the Beavers Bend tuff. In fresh exposures, the fine-grained Figure 13. Rare planar cross-beds in siliceous vitric tuff of Hatton Tuff. siliceous vitric tuff breaks with a conchoi-

dal fracture and forms resistant outcrops. shard-rich ash. Due to the higher viscosity and (or) reworking by weak bottom cur- The vitric tuff weathers to pale-green or of water, a larger amount of fine ash —sized rents. Each couplet of light and dark gray- mottled green blocks. It is composed of shards would remain in suspension longer green laminae probably represents an ash well-sorted fine ash—sized relict bubble- than in air and thus would be carried by fall of fine ash—sized shards, minute pumice wall and sickle-shaped shards (up to 55 currents a greater distance. For example, clasts, and silt-sized crystals, mainly quartz, percent) in a cryptocrystalline chert matrix Fisher (1965, p. 352) calculated from ex- which underwent some density separation (altered volcanic dust) and less abundant perimental studies that pumice shards (0.06 while settling very slowly through the water silt-sized angular quartz (2 to 15 percent) mm in size) would travel 75 km in a current column to a quiet sea floor. The infall rate and plagioclase (2 to 10 percent) crystals, of 15 cm/sec before settling 1,000 m. and amount of settling volcanic debris were iron oxides (13 percent), and rare silt-sized The laminated part of the vitric tuff zone sufficient to mask the normal pelagic shale clasts, carbonized plant fragments, probably represents a succession of ash falls sedimentation of terrigenous clay. Some detrital muscovite, and zircon. Pumice lapilli are rare. Shards are altered to either ERUPTION AND ASH FLOW quartz, calcite, or rare celadonite and are generally smaller (average 0.05 mm long) than shards in the underlying pumiceous vitric-crystal tuff. In general, the crystal and shard size and abundance decrease upward, whereas the amount of cryptocrystalline volcanic ash matrix increases upward (up A to 60 percent). Widespread laminated fine-grained vitric tuff occurs in a 1- to 3-m bed in the middle and locally near the top of the vitric part of the Hatton Tuff (Fig. 7). The alternating light and dark gray-green laminae (2 to 6 mm thick) are defined by slight variations of crystal and shard sizes and clay components. The darker green laminae contain a greater concentration of small discontinu- ous fibrous stringers of contorted microscopic dark iron-stained clay that may be very small, highly compacted, altered pumice clasts. Shards in some laminae are oriented parallel to the bedding. Locally, laminae are microfaulted or planar cross- B laminated (Fig. 13). Origin. The widespread areal extent (greater than 30 by 50 km), uniform thickness, overall size grading from shards to volcanic dust upward, homogeneous tuff lithology, and gradational contact with the compositionally similar but coarser grained underlying massive and bedded pumiceous vitric-crystal tuffs suggest that the fine- grained massive siliceous vitric tuffs proba- SETTLING OF FINEST ASH bly formed from continuous settling of very fine pyroclastic debris from a remnant eruptive cloud that had drifted into the Ouachita trough. The amount of ash necessary to produce 3 to 10 m of structureless vitric tuff over a o O O O 0 O O 2 O o o ö I—! o O 1,550 km area a minimum of 30 km from C o o o e o ® ö the volcanic source as present in the Hatton Tuff would not likely be deposited from one subaerial ash fall into water. Only a thin vitric tuff a few centimetres thick would result from an eruption at such a distance (R. S. Fiske, 1970, personal com- mun.). If the thick vitric tuff was produced from a succession of ash falls, the resulting Figure 14. Schematic diagrams of inferred development of pumiceous vitric-crystal tuffs. A. Early deposit would be laminated, which it is not. in volcanic eruption, large volume of pyroclastics produced pyroclastic flow that deposited thick un- The thick structureless vitric tuffs are, stratified pumiceous vitric-crystal tuff. B. Numerous thin slurries followed the main flow and deposit- more likely, the result of continuous settling ed thin-bedded tuff. C. Toward end of volcanic eruption, settling of fine ash produced fine-grained of a vast submarine cloud of current-wafted vitric tuff.

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laminae contain oriented sickle-shaped events (left) are accompanied by diagrams water, and escaping gases. This slurry, shards and a few planar cross-beds; this tes- of the accumulating deposit (right). being more dense than normal sea water, tifies to some reworking by bottom currents An initial, extremely violent eruption of flowed downslope as a density current (Fig. (Fig. 13). rapidly vesiculating acidic magma from a 14A). Fine ash remained in a large eruptive vent or fissure produced an ash flow that cloud, and buoyant pumice lapilli and HYPOTHETICAL SEQUENCE OF consisted of an incandescent avalanche and blocks floated on the sea surface. The ad- ERUPTION AND DEPOSITION an accompanying overriding ash cloud. vancing dense slurry was supplied with con- Probably little sorting of the pyroclastic tinuously erupted debris as it progressed The repetition of distinctive rock types in debris occurred in the turbulent avalanche. downslope into the Ouachita flysch basin. two widely separated pumiceous tuffs (the The extremely hot suspension either was The flowing mass scoured the muddy sea Hatton and Beavers Bend) suggests a repeti- erupted in shallow water or entered the sea floor and probably differentiated into a tion of eruptive and depositional events in soon after eruption from a subaerial vent, denser crystal-rich head followed by a trail- the Ouachita geosyncline. A hypothetical where it cooled and interacted with sea ing cloud of pumice and finer shard-rich ash sequence of events is illustrated in Figure water to produce a steam-inflated, turbu- in the body of the flow. Surges of crystal- 14. The inferred eruptive and depositional lent, watery slurry of pyroclastic debris, sea enriched material within the base of the head produced thin graded crystal beds in the basal deposit. As momentum of the flow waned, finer shard-rich ash and pumice from the tail of the submarine avalanche settled on the crystal-enriched part and formed a density grading. Eruptive surges in the volume of pyroclastic debris pro-

Figure 15. A. Isopach map of the thickness of Beavers Bend tuff (not a palinspastic reconstruc- tion). Beavers Bend tuff outcrop pattern parallels Hatton Tuff outcrop pattern. Arrow indicates mean paleocurrent direction of 10 flute casts at Beavers Bend State Park. B. Relation of maximum crystal size (average of five largest sizes) to distance for basal crystal-rich layers of Hatton and Beavers Bend tuffs, assuming the source vents were 20 km south of Ouachita outcrops. C. Isopach map of thickness of fine- grained vitric tuff of the Hatton Tuff.

duced at the vent resulted in numerous A southern source is also supported by a feldspar crystals increases exponentially smaller density slurries that followed the southerly increase in grain size in the Hat- from north to south over a distance of 50 main flow in rapid succession to form the ton Tuff Lentil toward the south-southeast km. Additionally, an isopach map of the overlying bedded pumiceous tuff (Fig. 14B). (Niem, 1971a, 1976). An isopleth map of siliceous vitric zone of the Hatton Tuff Density sorting in each slurry separated maximum size of pumice clasts in the Hat- shows a thickening trend toward the ash-sized shards and crystals from pumice ton Tuff (Fig. 12) shows a general coarsen- south-southeast (Fig. 15C). The tuff appar- lapilli. Some pumice, rounded by abrasion ing southward. In addition, a logarithmic ently pinches out toward the northwest and during transport in "rafts" on the sea sur- plot of maximum crystal size against dis- is absent in a well-exposed correlated sec- face, eventually may have become saturated tance (Fig. 15B) illustrates that in the tion in the Potato Hills approximately 60 and sunk with the settling ash to form some crystal-rich base of the Hatton Tuff size of km northwest of the Broken Bow—Benton bedded pumice-rich ash-fall layers at a later time. Reworking by bottom currents may have aided in separating shard-rich ash from pumice. Toward the end of the volcanic eruption, continuous settling of finest ash (minute crystals, shards, and volcanic dust) that had remained in suspension in the upper part of the eruptive column formed the thick structureless fine-grained vitric tuff of the upper part of each of the pyroclastic deposits (Fig. 14C). Some reworking by weak bottom currents produced minor cross-beds and laminated vitric tuff. Intermittent eruptions after the main event produced a thin vitric ash-fall layer above the Beavers Bend tuff that is separated by a thin green clay layer formed from normal pelagic clay sedimentation between eruptions. The length of the eruption that produced the zoned tuffs is difficult to estimate. Pyroclastic flows probably were deposited within minutes to hours, whereas the overlying vitric tuff was deposited over a much longer period.

SOURCE AREA

Paleocurrent indicators, grain-size distribution, and geometry of the Hatton and Beavers Bend tuffs suggest derivation from a volcanic area to the south or southeast. Studies of thicknesses and grain-size dis- tributions of recent ash falls and ash flows show that typically pyroclastic deposits thicken and grain size increases exponentially toward eruptive sources (Fisher, 1964, 1966; Hay, 1959; Wilcox, 1959; Van Bemmelen, 1949). The Beavers Bend tuff thickens to the southeast and thins toward the west, north, and east (Fig. 15A; Niem, 1976). A logarithmic plot of maximum crystal size (measured parallel to the c axis) against distance (Fig. 15B) shows that in the crystal- rich base of the Beavers Bend tuff, size of feldspar crystals increases exponentially from north to south over a distance of 30 km. In addition, the number and thickness of thin graded crystal-rich beds in the basal Beavers Bend tuff increase southward over a 25-km distance from two beds to four beds and from an average thickness of 1 to 7 cm. The orientation of flute casts on the base of the tuff indicates flow toward the northwest (mean N42°W, Fig. 15A).

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uplift (Niem, 1971a, 1976). Scant cently by Wickham and others (1976) on Danilchik, W., and Haley, B. R., 1964, Geology paleocurrent data from cross-beds in the the basis of structural and petrologic evi- of the Paleozoic area in the Malvern Quad- upper vitric tuff zone of the Hatton indicate dence. Cline (1970), Hill (1967), and rangle, Garland and Hot Springs Counties, sediment dispersal toward N30°W (mean). Morris (1974b) pictured these southern Arkansas: U.S. Geol. Survey Misc. Inv. Mississippian paleogeography also source areas for the Stanley quartzose and Map 1-405. suggests a southern volcanic source for feldspathic turbidites as a series of tectonic Dickinson, W. R., 1974, Sedimentation within both tuffs. Shallow-marine carbonate plat- islands composed of low-grade metamor- and beside ancient and modern magmatic arcs, in Dott, R. H., and Shaver, R. H., eds., forms existed in Mississippian time in the phic and volcanic rocks in the deep Modern and ancient géosynclinal sedimen- nearby Arbuckle Mountains and the Ozark Ouachita geosyncline. I hypothesize that tation: Soc. Econ. Paleontologists and dome to the northwest and northeast, re- some of these tectonic areas were centers of Mineralogists Spec. Pub. 19, p. 230-239. spectively. No volcanoes, lava flows, or active, very explosive stratovolcanoes Fernandez, H. E., 1969, Notes on the submarine pyroclastic deposits of Late Mississippian formed in a magmatic arc adjacent to a con- ash flow tuff in Siargao Island, Surigao del age are known immediately to the west, tinental margin(?) in Stanley time, perhaps Norte: Philippine Geologist, v. 23, p. 29- north, or east of the Ouachitas. during a period of active plate subduction 36. The only recognized volcanic terrane that as a result of convergence of the North Fisher, R. V., 1961, Proposed classification of volcaniclastic sediments and rocks: Geol. may have been active during Stanley time American plate and a southern continental plate. Soc. America Bull., v. 72, p. 1409-1411. lies south-southwest of the Ouachitas in the 1964, Maximum size, median diameter, and continuation of the Ouachita system in the sorting of tephra: Jour. Geophys. Research, Texas subsurface. This terrace was di- ACKNOWLEDGMENTS v. 69, p. 341-355. vided by Flawn and others (1961) into two 1965, Settling velocity of glass shards: belts separated by the Luling thrust: (1) a This work was supported by a National Deep-Sea Research, v. 12, p. 345—353. highly deformed Paleozoic frontal belt, Science Foundation grant to the late Lewis 1966, Mechanism of deposition from pyro- which includes Stanley shales, and (2) a M. Cline and by grants from the Oklahoma clastic flows: Am. Jour. Sci., v. 264, metamorphic interior belt (Fig. 16). Deep Geological Survey and Exxon Oil Com- p. 350-363. 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