Himalayan-Bengal Model for Flysch Dispersal in the Appalachian-Ouachita System

STEPHAN A. GRAHAM WILLIAM R. DICKINSON } Geology Department, Stanford University, Stanford, California 94305 RAYMOND V. INGERSOLL

ABSTRACT INTRODUCTION still actively consuming oceanic lithosphere off Sumatra. We suggest that the Ouachita The relation of the modern Bengal subsea The tectonic relations of the late orogeny, during which the Ouachita system fan to the Cenozoic Himalayan suture belt Paleozoic Ouachita orogenic belt of the was thrust northward over the southern and the analogous relation of the Car- south-central United States are uncertain, flank of the continent, was caused by the boniferous Ouachita flysch to a presumed both to the Appalachian belt along regional close approach of an analogous arc-trench Paleozoic Appalachian suture belt suggest a strike to the east and to the continental system, whose remains lie buried now in the guiding principle of synorogenic sedimenta- margin across regional strike to the south. subsurface beneath Texas. Figure 1 shows tion. Most sediment shed from orogenic Plate tectonic interpretations of the Ap- the gross tectonic trends of the two regions highlands formed by continental collisions palachian and Cordilleran systems near the at the same scale. pours longitudinally through deltaic com- edges of the continent and of the Caribbean With available data, our interpretation is plexes into remnant ocean basins as turbi- region south of it have not dealt with the speculative but is compatible with current dites that are subsequently deformed and Ouachita question. Recent interpretations knowledge of the Ouachita and southern incorporated into the orogenic belts as col- of Ouachita tectonics rely upon a supposed Appalachian regions. If valid, our view has lision sutures lengthen. analogy with active continental margins, implications concerning the tectonic India first encountered a southern marked by either a marginal magmatic arc framework of the Caribbean region just Eurasian subduction zone near the end of of Andean type (Keller and Cebull, 1973) prior to the Mesozoic development of the Paleocene time. Northward movement of or a fringing island arc of Japanese type fac- Gulf of Mexico and the Antilles. From a India since Oligocene time choked the sub- ing away from the continent (Morris, broader standpoint, the concept that volu- duction zone, stifled the associated mag- 1974a, 1974b). In our view, however, the minous turbidites may be fed longitudinally matic arc, and created a suture complex of general lack of appropriate volcano- into narrowing ocean basins that close se- deformed Cretaceous flysch and younger plutonic complexes or their derivative quentially between colliding continents Tertiary molasse. Strata derived from the sedimentary sequences within the Ouachita may offer a general explanation for the resulting orogen include continental clastic region encourages the consideration of al- common occurrence of synorogenic flysch wedges shed southward toward India and ternative analogies. derived from the same orogenic belt into voluminous turbidites fed longitudinally We suggest a different interpretation by which it is incorporated by later deforma- through the Ganges-Brahmaputra Delta drawing a tectonic analogy between the tion (see also Dewey and Burke, 1974). into the Bay of Bengal. The eastern flank of Cenozoic Himalayas and the Bengal fan, on We first describe the Himalayan-Bengal the Bengal subsea fan is being subducted the one hand, and the Carboniferous Ap- events, discuss the Appalachian-Ouachita now beneath the still-active eastern exten- palachians and the Ouachita flysch on the events by analogy, and then comment on sion of the subduction zone. other. The Himalayas were formed by the the general implications of our interpreta- The sequential, north-to-south welding collision of India with Eurasia, and the final tion. For each of the two regions, we sum- of Europe and Africa to North America development of the southern Appalachian marize in order the general relations of (a) formed the complex Appalachian- system was marked by the collision of Af- oceanic closure by plate consumption be- Caledonide-Mauritanide suture belt, from rica with North America (Watkins, 1972). neath continental margin arcs, (b) de- which Taconic, Acadian, and Alleghanian The immense Bengal fan of turbidites and velopment of a suture belt along the orogen clastic wedges were shed toward the North associated oceanic strata is composed between colliding continents, (c) transverse American craton. Turbidites of the Car- mainly of detritus fed into the remaining dispersal of clastic wedges shed from the boniferous Ouachita flysch were fed lon- ocean basin east of India by the longitudi- collision orogen across an adjacent conti- gitudinally, as sediment supplied through nal drainage of the Himalayas and related nental block, (d) longitudinal dispersal of the Alleghanian clastic wedge, into a rem- highlands. We suggest that the Carbonifer- turbidites into a remnant ocean basin along nant ocean basin lying south of North ous clastic rocks, largely turbidites, of the tectonic strike from the collision orogen, America. The Ouachita system was then Ouachitas are composed mainly of detritus and (e) deformation of the turbidite flysch thrust northward across the continental fed into a remnant ocean basin south of by continued subduction along one flank of edge during arc-continent collision that North America by the longitudinal drain- the remnant ocean. Our arguments are progressed from east to west. Key words: age of highlands formed along the trend of based upon the implications of the tectonic historical geology, areal geology, tectonics, an Appalachian suture between Africa and setting of the modern Bengal fan, as de- sedimentation, flysch, Cenozoic, Car- North America. The flank of the Bengal fan scribed by Curray and Moore (1971). boniferous, Himalayas, Bengal fan, Ap- adjacent to southeast Asia has been partly However, we make no attempt to evaluate palachians, Ouachitas. subducted by an arc-trench system that is details they gathered subsequently on the

Geological Society of America Bulletin, v. 86, p. 273-286, 6 figs., March 1975, Doc. no. 50301.

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evolution of the Bay of Bengal and adjacent parts of the Indian Ocean floor (Moore and others, 1975; Curray and Moore, 1974), al- though our thinking has benefited from the general thrust of their oral presentations (see below). Powell and Conaghan (1973) also presented a detailed analysis of the structural evolution of the Himalayan ranges, to which the reader is referred for additional structural and stratigraphic in- formation. Our evaluation of published in- formation pertaining to the Ouachita re- gion is guided especially by the interpreta- tions of Morris (1974a, 1974b), which we supplemented by reconnaissance visits only.

Figure 1. Main tectonic elements of Himalayan-Bengal (A) and Appalachian-Ouachita (B) regions. HIMALAYAN-BENGAL REGION Symbols: A. 1, Himalayan suture belt between India and Eurasia; 2, Main Boundary Fault (thrust) of Indian Ocean Floor Himalayan foothills; 3, Quaternary alluvium of Indus (I), Ganges (G), and Brahmaputra (B) river sys- tems; arrows denote channel trends on subsea Bengal fan with head near Ganges-Brahmaputra Delta The Indian Ocean basin has formed since (GBD); 4, Indoburman-Sunda subduction zone, dashed where inactive; 5, melanges and deformed the end of the Paleozoic Era, and mainly flyschoid rocks of Indoburman ranges (IBR), Andaman (A)-Nicobar (N) insular ridge, and Mentawai since the beginning of the Creta- (M) Islands off Sumatra (S); 6, schematic margin of extensional Andaman basin (AB); dashed line is ceous, from the breakup of Gondwana and 1,000-m isobath of continental slope off India. the dispersal of the continental fragments B. 1, Complex belt of multiple Paleozoic suturing between North American and West African era- derived from it. Their initial configuration tons shown joined (E is Europe) prior to Mesozoic opening of the Atlantic Ocean approximately along the stitched line; 2, fronts of foreland fold-thrust belts of Appalachian Valley and Ridge province within intact Gondwana (Fig. 2A) has been (American side) and Mauritanides (African side); 3, coarse Carboniferous clastics, terrestrial and lit- inferred within close limits from the com- toral, of Appalachian (A) and Dlinois (I) basins; 4, frontal thrusts and folds of Ouachita system; 5, puter fitting of continental margins (Sproll outcrop and known subcrop of Ouachita system; 6, schematic extensional margin of Mesozoic(?) Gulf and Dietz, 1969; Smith and Hallam, 1970; of Mexico (present position of Cuba shown in dotted outline). Dietz and Sproll, 1970). Paleomagnetic data compiled by McElhinny (1970) are compatible with the information from that obscures detailed relations. It is unclear compatible with the assembly shown, al- JOIDES Legs 22, 26, and 27 that the sea whether the crustal segment that first met though Tarling (1972) used similar data to floor of the Wharton Basin west of Aus- Eurasia was firmly attached to India at the support a slightly different reconstruction. tralia is nowhere significantly older than time or was a detached microcontinental A recent analysis of marine magnetic Cretaceous (Scientific staff, 1972, 1973a, fragment now caught up within the anomalies in the Indian Ocean indicates 1973b). Recently identified magnetic Himalayan orogenic belt. that a reconstruction close to the assembly anomalies in the Wharton Basin suggest The initial separation of Australia and shown is probably correct (Sclater and that western Australia initially separated Antarctica occurred about 45 m.y. B.P. (see Fisher, 1974). There is thus a prominent from some appendage of the Indian conti- Fig. 2C), and spreading resumed south of gap of about 1,000 km inferred between nental block in Early Cretaceous time India — but in a new pattern — about 35 peninsular India and Australia on the origi- (Markl, 1974). m.y. B.P. in Oligocene time. Since then, pre- nal margin of Gondwana. In an oral discus- The evolution of the Indian Ocean basin sumably, the driving of India against and sion of the evolution of the Indian Ocean at during the past 75 m.y. has been outlined beneath the Himalayan suture (Fig. 2D) has the 1972 Conference on Geosynclinal by McKenzie and Sclater (1971). By 75 (1) choked the subduction zone at the Sedimentation at the University of Wiscon- m.y. B.P. (Fig. 2B), India, Africa, and southern margin of Eurasia where the sin, J. C. Curray and D. G. Moore sug- Australia-Antarctica were already three once-intervening oceanic lithosphere was gested that the apparent gap in Gondwa- discrete continental blocks separated by re- consumed, (2) stifled the associated mag- na's margin may have been filled by con- gions of oceanic crust through which vari- matic arc along the edge of Eurasia in the tinental crust representing an extension ous microcontinents were scattered. From region of continental collision, and (3) de- of India beyond the present Himalayan 75 to 55 m.y. B.P. (Figs. 2B, 2C), India formed the northern margin of India during border of peninsular India. McElhinny and moved rapidly northward during an partial subduction beneath Tibet and uplift Embleton (1974) recently made the same episode of fast spreading from an ancestral in the Himalayas. Figure 3 depicts sequen- suggestion, reinforcing the paleomagnetic rise in the central Indian Ocean. At 55 m.y. tially the geologic events related to the de- arguments in favor of the Smirh-Hallam re- B.P., near the end of the Paleocene Epoch, velopment of the suture belt as described construction shown in Figure 2A. Accord- spreading slowed markedly or stopped. below. Our discussion here is highly ingly, we have depicted by dashed lines in McKenzie and Sclater (1971) suggested that generalized; the reader is referred to Curray Figure 2 an assumed extension of peninsu- this behavior was probably a signal of the and Moore (1974) for interpretations that lar India that filled all or part of the appar- initial encounter between India and incorporate the most recent oceanographic ent gap in the initial margin of Gondwana. Eurasia. Presumably, the encounter in- information. A combination of subduction beneath Tibet volved peninsular India's appendage, now and multiple thrusting in the Himalayas has obscured by partial subduction and defor- Himalayan Suture Belt obscured an indeterminate portion of the mation beneath and within the Himalayan original continental margin of India (Craw- orogenic belt. Powell and Conaghan (1973) The crustal suture formed by continental ford, 1971). Gansser (1966) estimated 500 called special attention to previously unrec- collision between India and Eurasia lies km of crustal shortening in the Himalayas. ognized metamorphism of the edge of the along a complex tectonic zone, the Indus The suggestion of Curray and Moore is also Indian subcontinent as an additional factor Line, marked by an elongate, semicontinu-

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oceanic lithosphere that once lay between A . B , A India and Eurasia was consumed by a trench along the southern margin of D1A { |§ VJN /» Eurasia. In this view, the thrust contact ) ARASI J.,''INDIA") ( AUSTyH along the southern margin of the Indus flysch was formed at the time that crustal fj. lis elements of India were lodged against the f AFRICA *¥ANT. JM subduction zone and driven partly beneath Tibet. The counterthrust was formed when parts of the subduction mélange were AFRICAH ll j ANT pressed upward and outward from the su- f SOUTH^, ture belt during severe contractions that ac- ( AMERICA companied crustal collision (see also Dewey and Burke, 1973). The molasse, at one time undifferentiated from the marine flysch, contains coal, marl, C EURASIA ! D freshwater mollusks, plant fossils, and abundant current-bedding indicative of i Jr nonmarine deposition (Tewari, 1964). In Ladakh, where the molasse rests uncon- formably on the Ladakh granite, Tewari (1964) concluded that deposition was post-Eocene, and possibly post—middle Miocene. Near Kailas, a conglomerate of unknown age rests unconformably on the Kailas granite (Gansser, 1964) and is over- lain along a north-directed thrust by the Indus flysch in a tectonic setting analogous to that of the sandstone and shale in the molasse of Ladakh. The younger molasse is nearly flat-lying and undeformed except Figure 2. Diagrammatic evolution of the Indian Ocean (India shown as peninsular India with near the thrust contact with the highly de- about 500-km width added arbitrarily to Himalayan border; see text). A. Smith-Hallam reconstruc- formed Indus flysch. Similar nonmarine tion of Gondwana prior to breakup. B. McKenzie-Sclater reconstruction of Indian Ocean region at 75 beds occurring in the Thakkhola region of m.y. B.P. C. Early Cenozoic relations in Indian Ocean region. Rise crests and transforms shown were Nepal have been interpreted as Miocene- active mainly prior to 55 m.y. B.P.; Australia separated from Antarctica about 45 m.y. B.P. and reached position shown about 35 m.y. B.P.; latitudes of India and Australia shown for 35 m.y. B.P. Pliocene intramontane deposits (Bordet and from McKenzie and Sclater (1971); latitude of Eurasia for early Cenozoic estimated from others, 1967). We interpret the molasse of paleomagnetic data (Irving, 1964); relative longitudes of India and Eurasia approximately as depicted the suture belt as terrestrial deposits formed by Le Pichon (1968); barbed line indicates subduction zone of arc-trench system along southern mar- in local depressions when the initial stages gin of Eurasia. D. Late Cenozoic relations in Indian Ocean region (present geography). Rise crests and of continental collision had closed the transforms shown have been active since about 35 m.y. B.P.; open bars denote Chagos-Laccadive ocean between Eurasia and India. This oc- (C-L) and Ninety-East (90-E) Ridges along old fracture zone trends; barbed line denotes curred, however, before continued defor- Indoburman-Sunda subduction zone trending into suture belt between India and Eurasia. mation had raised the Himalayan ranges as broad, unbroken highlands and masked the ous outcrop belt of deformed Upper Cre- regard the rocks between the Main Central line of juncture between the two continen- taceous Indus flysch and associated ophiolit- Thrust and the suture belt along the Indus tal margins. ic rocks (Gansser, 1966; Dewey and Bird, Line as a crustal sliver representing the dis- Mesozoic and Tertiary granitic plutons 1970). Where best known in the west- tal northern edge of India itself, rather than are abundant in the trans-Himalayan central Himalayas, the suture belt also con- as a separate entity. ranges north of the suture belt. Radiometric tains younger molasse (Tewari, 1964). The Indus flysch is composed of shale dates by the Rb/Sr method on 22 samples Within the Himalayas south of the Indus and sandstone of mainly Late Cretaceous from the Hindu Kush in the western Line but still north of the Main Boundary age, and it is tectonically intermixed with Himalayan region cluster in the following Fault (Fig. 1), the Main Central Thrust (a) ultramafic rocks of the ophiolite suite; six approximate age ranges (Desio and forms the tectonic boundary between (b) sequences of red limestone, radiolarian others, 1964): 210 to 215, -185, 130 to Paleozoic and Mesozoic strata with Gond- chert, and slate — probably representing 135, 85 to 95, -30, and 15 to 20 m.y. B.P. wanic affinities to the south and Tethyan oceanic pelagites; and (c) exotic blocks of Farther east in the Karakorum Range, simi- affinities to the north (Gansser, 1966). Al- Permian to Jurassic rocks. We suppose this lar dates include two of about 50 m.y. B.P. though both of these sequences rest on assemblage to be mélange. The southern for small isolated plutons and six from the broadly similar crystalline basements, the boundary of the mélange belt is a south- main axial batholith ranging from nearly stratal contrast across the Main Central directed thrust, and a north-directed coun- 25 m.y B.P. to less than 10 m.y. B.P., below Thrust indicates severe structural telescop- terthrust typically separates the Indus flysch which results with the Rb/Sr technique used ing and may even reflect the juxtaposition from younger molasse lying to the north are inconclusive. The Ladakh and Kailas of two microcontinents, both once external (Tewari, 1964). We interpret the mélange granites lie south of the Miocene to Eurasia. Powell and Conaghan (1973), belt containing the Indus flysch and its Karakorum axial batholith and were not who recently discussed the details of the lithologic associates as a subduction com- dated by Desio and others (1964), but the structural evolution of the Himalayas in a plex composed of materials accreted tecton- Deosai pluton dated near 50 m.y. B.P. lies time frame close to the one we outline here, ically to the southern flank of Eurasia as the along the same structural trend. The

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Ladakh granite has since been dated by the MID-CRETACEOUS TO MID-EOCENE MID-EOCENE to MID-OLIGOCENE same method as 45 m.y. B.P. (Powell and t ¡ v c e A rc - > Conaghan, 1973, p. 5). We interpret the I 0- granitic plutons of the Karakorum and re- lated ranges as the exposed roots of the magmatic arc that lay along the southern margin of Eurasia while the nearby subduc- ? /Ill' . tion zone was active. The foregoing data are compatible with the following inferences: (1) During the rapid northward drift of India from 75 to 7. «H 55 m.y. B.P., the flysch and associated rocks I» of the mélange belt were deformed in the • subduction zone of an arc-trench system that had been active along the southern margin of Eurasia through much of MID-OLIGOCENE to MID-MIOCENE MID-MIOCENE TO MID-PLIOCENE Mesozoic time. (2) Following the supposed ¿HkÎc^KjÇ-.-,,. initial encounter of India with Eurasia, subduction stalled as spreading in the In- dian Ocean slowed from 55 to 35 m.y. B.P. (3) With renewed spreading in the Indian Ocean after 35 m.y. B.P., further subduc- tion is reflected by major plutons such as the Karakorum axial batholith, but the forced juxtaposition of continental margins later stifled subduction and fostered the crustal deformation that was recorded by overlapping of tectonic elements along thrusts and accompanied by uplift of the su- ture belt to its impressive late Cenozoic ele- vations. As roots of the magmatic arc, such MID-PLIOCENE TO HOLOCENE Sediment transport directions as the Ladakh and Kailas granites, were ex- -L HIMALAYAN posed by erosion, terrestrial molasse was SUTURE BELT Terrestrial and Littoral sediments

deposited in vestigial depressions along the IIHIIIIIH Shallow-marine shelf sediments suture belt and subsequently deformed. BM89 Deep-marine (lysch sediments The suture belt and related tectonic ele- I | Uplifted melanges and deformed flysch ments cannot be traced in detail on availa- ble geologic maps into the eastern teo'î&l Inferred magmatic arcs Himalayas, but a structurally complex belt A A Active volcanic arcs including exposures of ophiolitic rocks, * " » Thrust fault systems granitic plutons, Mesozoic flysch, and Ter- tiary molasse is reported north of Mount « * « Subduction zones (dashed where inactive) Everest as far as Lhasa (Gansser, 1964). — Suture belts The suture presumably wraps around the eastern Himalayan syntaxis and trends into Figure 3. Sketches showing the inferred evolution of the Himalayan suture belt and the region the Indoburman Ranges (Gansser, 1966; along strike. Places shown include: ANI, Andaman-Nicobar islands; B, Bombay; Bal, Baluchistan; Krishnan, 1968). Similarly, the suture ap- BD, Bangladesh; C, Calcutta; CAB, central Andaman basin; CBL, central Burmese lowland; D, parently wraps around the western Delhi; HK, Hindu Kush; IBR, Indoburman ranges; K, Kailas; Kk, Karakorum; L, Ladakh; Lh, Lhasa; Himalayan syntaxis and is expressed as the 90E, Ninety-East Ridge; S, Sumatra; SP, Shan Plateau. Line off coast of peninsular India is present Quetta Line, an ophiolite-bearing zone of 1,000-m isobath. Central Andaman basin began to open in middle Miocene time (see text), but early intense deformation separating the mark- phases of opening are not shown here. edly contrasting facies belts of Sind and Baluchistan (Gansser, 1966; Krishnan, prior to collision. Faunal and lithologic evi- during early Eocene time because terrestrial 1968). dence suggests that an unrestricted seaway vertebrates with Mongolian and North existed from Afghanistan to Burma in Cre- American affinities were well established in Himalayan Clastic Wedges taceous time, separating the Indian subcon- India by the middle Eocene Epoch (Sahni tinent from Eurasia; however, convergence and Kumar, 1972). The Cretaceous to Holocene strata south apparently reduced considerably the width Extensive sedimentation in the sub- of the Himalayas provide a sedimentary of the seaway during Paleocene time (Sahni Himalayan belt followed closely in time record of events along a continental margin and Kumar, 1974). Overlying middle upon the middle Tertiary rejuvenation of involved in crustal collision. Cretaceous Eocene strata (Kirthar Series), which in- major spreading in the Indian Ocean. The through lower Eocene strata in the sub- clude a regressive sequence of nonmarine Oligocene to middle Miocene Murree For- Himalaya or foothill belt are shallow-water deposits in the north and marine beds in the mation apparently represents thin deltaic marine sequences of shale, sandstone, and south (Sahni and Kumar, 1974), apparently deposits derived partly from a growing limestone (Gansser, 1964). They apparently signal the initial encounter of India with Himalayan source and partly from the In- represent shelf sedimentation near the Eurasia. A nonmarine connection between dian Shield to the south, but it is succeeded northern edge of the Indian subcontinent India and Eurasia likely was established gradationally by the continental Siwaliks,

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shed dominantly from the north as a con- sition of strandline sandstone and coal zone at the front of the Andaman-Nicobar formable sequence extending into the Pleis- measures as the sea retreated (Bhandari and insular ridge (Curray and Moore, 1971, tocene. Three major divisions of the others, 1973). A regional unconformity at 1974). On the west, sedimentary basins Siwaliks (lower, middle, upper) probably the Oligocene-Miocene boundary perhaps along the east coast of India contain late represent time-transgressive facies rather corresponds with the major uplift of the Mesozoic and Tertiary paralic and than time-stratigraphic units (Glennie and Himalayas to the north, and a regional shallow-marine strata that record progres- Ziegler, 1964). The upper Siwaliks are paleoslope from north to south has per- sive downwarping of the continental mar- coarse alluvial fan deposits close to the sisted from early Miocene time to the pres- gin, together with recurrent development of source, whereas the middle and lower ent (Bhandari and others, 1973). Lower local grabens, beginning near the end of Siwaliks contain more distal river-channel Miocene strata (Surma Group) reflect Jurassic time (Sastri and others, 1973). The and flood-plain deposits. Tandon (1971) fluvial, deltaic, brackish, and marine envi- continental margin presumably formed by recorded variable but generally southerly ronments in facies succession from north to rifting during the breakup of Gondwana, paleocurrent directions in the upper south (Raju, 1968) and thicken toward the and the sedimentary basins along the coast Siwaliks, southerly directions in the middle south and southeast (Bhandari and others, and beneath the shelf offshore then evolved Siwaliks, and southeasterly directions in the 1973). Middle Miocene through Pleis- by subsidence concurrent with the forma- lower Siwaliks. More recently, Parkash and tocene strata (Tipam and Moran Groups) tion of adjacent oceanic crust now beneath others (1974) noted both east-west and are fluvial sandstone and claystone that the Bengal fan. north-south dispersal trends in the upper reflect a geography similar to the present. Siwaliks, southerly directions in the middle The upper parts of this sequence resemble Indoburman-Sunda Subduction Siwaliks, and southwesterly directions in the Siwaliks (Krishnan, 1968), and they the lower Siwaliks. Siwalik physiography thicken toward the Himalayas to the The outer sedimentary arc of the Sunda was thus similar to that of the present northwest (Bhandari and others, 1973). arc-trench system continues northward Ganges flood plain and its piedmont from the Mentawai Islands off Sumatra to tributaries, except that the depositional sys- Bengal Fan Turbidites pass first along the Nicobar and Andaman tem has shifted southward with time as the Islands and then into the Arakan Yoma Himalayan uplift has continued. Some fold- The Bengal fan described by Curray and highlands of the Indoburman ranges west ing and faulting of the Siwaliks preceded Moore (1971) is a huge deep-sea fan built of the central Burmese lowland or trough overthrusting along the Main Boundary into the Bay of Bengal from the north by now occupied by the Irrawaddy River (Van Fault, although deposition probably also turbidity currents that issue from the front Bemmelen, 1949; Weeks and others, 1967; continued during thrusting (Johnson and of the Ganges-Brahmaputra Delta. Both the Rodolfo, 1969b). South of Burma, the insu- Vondra, 1972), and the warping of modern deltaic platform and the fan are apparently lar ridge that marks this tectonic feature river terraces suggests continued activity the largest depositional features of their stands east of the Java trench and its filled (Gansser, 1964). kind in the world today. The detritus fed to northern extension. However, it is west of In the Sind area to the west, but still east them is derived by longitudinal drainage of the volcanic arc, which lies along the Bari- of the suture belt along the Quetta Line, the world's highest mountain ranges in the san Range in Sumatra and then along the Permian-Carboniferous through Eocene Himalayan region. The area of the fan is Weh-Barren-Narcondam trend of volcanic marine units are predominantly shelf lime- roughly 3 X 106 km2, the maximum thick- islands and seamounts in the Andaman Sea. stone (Krishnan, 1968). In adjacent outcrop ness of sediment is perhaps 15 km beneath General analysis of arc-trench tectonics belts farther to the east, Kirthar, Murree, the delta-fan interface (Moore and others, suggests that such partly submerged insular and Siwalik sediments in the north pass 1971), the average thickness is about 7.5 ridges standing above the inner walls of southward into a marine section that was km (Naini and Leyden, 1973), and the total trenches are composed of structurally com- evidently deposited in the head of a shallow volume of sediment is thus of the order of plex rock masses uplifted following or dur- gulf lying west of peninsular India. Marine 20 x 106 km3. From correlations of mag- ing deformation in the subduction zones as- Miocene limestone, sandstone, and shale netic anomalies and the results of deep-sea sociated with the adjacent trenches (Dickin- (Nari and Gaj Series) gradually filled this drilling, the oceanic crust beneath the fan is son, 1973). Descriptions of exposures (dis- gulf from the north and were followed by inferred to be Cretaceous to Paleocene in cussed below) along the Mentawai- continental beds (Manchhar Series) roughly age (Sclater and Fisher, 1974). The layering Nicobar-Andaman trend and in the In- equivalent to the Siwaliks but with a within the sediment is complex, as dis- doburman ranges along strike include ref- younger base (Krishnan, 1968). The gulf cussed by Moore and others (1975), and we erence to complex thrusting, glide sheets, succession was later folded during Pliocene offer here no detailed interpretation of the ophiolitic complexes, exotic tectonic inclu- and younger deformation. age and composition of the sediment. The sions, foliated claystone, and related fea- In the Assam-Bangladesh area to the east, sea-floor morphology in the Bay of Bengal tures characteristic of mélanges and as- but still west of the suture belt along the In- and the immense total bathymetric relief of sociated rock masses. Continuous subbot- doburman ranges, Cretaceous through at least 3 km on the sloping fan surface tom profiling by seismic reflection indicates Eocene strata (Disang Series and Jaintia (Curray and Moore, 1971) suggest to us that the western foot of the Andaman- Group) are predominantly shallow-marine that the bulk of the sediment is fan sedi- Nicobar insular ridge is thrust westward and paralic sediments that thicken south- ment derived from the erosion of highlands over strata of the sea floor in the Bay of ward toward the ancestral Bay of Bengal raised along the Himalayan suture during Bengal (Weeks and others, 1967). Recent (Raju, 1968; Bhandari and others, 1973). deformation induced by crustal collision of work by Hamilton (1974) emphasizes the Uplift during Oligocene time apparently India with Eurasia. The fan surface slopes structural continuity of the Mentawai- coincided with resumption of spreading in southward for 1,000 km, confined on the Nicobar-Andaman trend that links subduc- the Indian Ocean and formed a paleoslope west by the continental slope off India and tion complexes of the Sunda arc-trench sys- from northeast to southwest, with regres- Ceylon and confined on the east by the tem in Java and Sumatra with deformed sion and erosion in the northeast near the Andaman-Nicobar island chain. The east rocks of the Indoburman ranges along tec- Indoburman ranges (Raju, 1968). High- flank of the fan fills the northern extension tonic strike to the northwest. Curray and energy coastal environments followed by of the Java trench, and deformed fan sedi- Moore (1974) further noted that the folds brackish lagoonal conditions caused depo- ments there dip eastward into a subduction and faults detected in the deformed Bengal

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fan sediments of the Andaman-Nicobar in- infer that the Indoburman flysch was de- strike from the suture belt. The Bay of sular ridge are geometrically similar to posited as turbidites in deep water at the Bengal and the Bengal fan lie in front of the structures exposed subaerially in the In- head of an ancestral Bay of Bengal and that arc trend, but the central Andaman basin doburman ranges. it was deformed as closure of that deep lies behind the arc trend at a place where In the Andaman and Nicobar islands basin allowed the Assam-Bangladesh shelf extension has detached the arc structure (Karunakaran and others, 1964, 1968), the to approach closer to Burma. from a continental mainland. oldest rocks exposed are upper Mesozoic The deformed Indoburman flysch is radiolarian chert and metagraywacke as- bounded along the eastern flank of the APPALACHIAN-OUACHITA REGION sociated with ophiolitic volcanic rocks and Arakan Yoma highlands by an east-dipping mafic to ultramafic intrusives including ser- thrust system along which the deformed Paleozoic Ocean Closure pentinite. The structurally complex ophio- flysch is tectonically overlain by an ophiolit- litic rocks are overlain by Eocene conglom- ic slab of serpentinite-peridotite and Wilson (1966) suggested, on the basis of erate and sandstone. These pass upward diorite-gabbro (Brunnschweiler, 1966). The contrasting lower Paleozoic faunal realms conformably into the Andaman flysch, ophiolitic rocks are structurally overlain by within the Appalachian orogenic belt, that which is composed of graded sandstone and an orderly Tertiary sequence beginning a pseudo-Atlantic Ocean lay off present- shale beds of Eocene to Oligocene age. with lower Eocene or Paleocene marine day eastern North America prior to the late Paleocurrent directions in the flysch are strata older than the upper Eocene flysch Paleozoic assembly of Laurasia. The idea is consistently south-southwest. This se- exposed immediately to the west of the especially attractive in terms of plate tec- quence is cut by abundant shear zones sug- thrust zone beneath the ophiolitic rocks. tonic models for orogeny, because it per- gestive of thrusting or gliding mainly toward The orderly sequence forms the base of a mits the inference that the Caledonides of the west. The Andaman flysch was strong- thick Tertiary succession, complete except Europe and the Mauritanides of Africa, as ly deformed prior to the deposition of for local disconformities, beneath the cen- well as the Appalachian orogenic belt, were shallow-marine Miocene strata that are tral Burmese lowland (Tainsch, 1950). We related to plate consumption by subduction less deformed but overlain unconformably infer that the Indoburman flysch of the during closure of the old ocean. The open- by Quaternary deposits. We infer that the Arakan Yoma was thrust by subduction ing of the present Atlantic along the general Andaman flysch was derived from south- beneath an arc-trench gap (the ancestral trend of the resulting juncture, but not east Asia and deposited by turbidity cur- Burmese lowland) that lay at the flank of a along precisely the same line as the old su- rents on the sea floor within and near an in- marginal magmatic arc. Igneous activity in ture, is viewed as a later event. The early active trench during the time of slow the arc is recorded by extensive Eocene- Paleozoic ocean has been termed Iapetus spreading in the Indian Ocean. The under- Oligocene plutonism and metamorphism east of Greenland in the region where it lying ophiolitic rocks we interpret as a sub- along the Mogok belt at the edge of the separated Europe from North America duction complex related to the consump- Shan Plateau east of the Burmese lowland (Harland and Gayer, 1972). Farther south, tion of oceanic crust during the earlier (Searle and Haq, 1964). Some plutons in paleomagnetic data suggest that arms of the period of fast spreading, and we relate the the Mogok belt are apparently as young as early Paleozoic ocean separated Africa, pre-Miocene deformation of the Andaman Miocene (Maung Thien, 1973), and in the then a part of Gondwana, from both North flysch to the resumption of spreading in the northern Indoburman ranges, the thick America and Europe (Hailwood and Tarl- Indian Ocean during Oligocene time. De- Miocene-Pliocene Chindwin molasse is lo- ing, 1973). formation of the unconformably overlying cally thrust westward over deformed flysch The manner and timing of ocean closure Miocene strata was apparently concurrent (Brunnschweiler, 1966). Late Cenozoic is still uncertain in detail, but the sequence with the extensional opening of the central arc—volcanic rocks lie west of the site of the of orogenic events in space and time along Andaman basin in the Andaman Sea to the earlier Cenozoic arc along a trend within the Appalachian-Caledonide belt indicates east since middle Miocene time (Rodolfo, the central Burmese lowland (Maung that the suturing of other continental 1969b). Thien, 1973) and in line with the volcanic masses to the eastern margin of North The Indoburman ranges northeast of the islands of the Andaman Sea. America progressed from north to south in Bay of Bengal (Fig. 3) are composed largely With the uplift of the Indoburman present geographic terms (for example, P. of an immense thickness of strongly de- ranges, the Burmese lowland became a nar- E. Schenk in Zietz and Zen, 1973; see also formed flysch, mainly lower Tertiary but row trough that was fed a thick succession Fig. 4). Respective times of inferred sutur- partly Upper Cretaceous (Brunnschweiler, of Oligocene, Miocene, and Pliocene clastic ing have been estimated crudely by examin- 1966). Ophiolitic basalt and serpentinite, strata from the north. A longitudinal facies ing polar wandering paths for the circum- together with chert and fine-grained lime- change from continental and paralic de- Atlantic continents as plotted on Bullard- stone, are tectonically intermixed with Cre- posits in the north to marine strata in the type reconstructions of the Atlantic region taceous phases of the flysch, but they occur south is characteristic of all horizons for Permian-Triassic time (prior to the only as clasts or exotic blocks among (Tainsch, 1950; Maung Thien, 1973). The opening of the present Atlantic). Eocene rocks. A folded unconformity be- lowland is now occupied by the Irrawaddy Paleomagnetic data suggest that polar paths tween Eocene and Cretaceous strata is River, which has built a rapidly growing for the two regions containing the cratonic prominent near the coast and recalls similar delta into the northern end of the Andaman interiors of North America and the Baltic relations along the Andaman-Nicobar insu- Sea (Rodolfo, 1969b). Turbidity currents Shield converged in Silurian time; further- lar ridge. Numerous mélange belts ("exotic carry sediment from the delta to the floor of more, the joint path of the two regions, here facies") occur as dipping layers inclined the central Andaman basin (Rodolfo, assumed to have been connected, then con- steeply to the east subparallel to bedding 1969a), which lies east of the volcanic trend verged with the polar path for the British and to the axial surfaces of isoclinal folds. beside the Andaman-Nicobar insular ridge. Isles in the Devonian Period (Briden and During Oligocene time, when spreading re- The longitudinal drainage of orogenic high- others, 1973). Finally, the North American sumed in the Indian Ocean, the Indobur- lands associated with the Himalayan suture and African or Gondwanan polar paths man flysch was uplifted to form a persistent is thus currently supplying sediment to converged in Carboniferous time (Hail- highland barrier that since has separated deep-water basins lying in tectonic settings wood and Tarling, 1973). General times of the sedimentary basins of Assam- in two different positions with respect to Paleozoic suturing of continental masses in Bangladesh and the Burmese lowland. We the magmatic arc still active along tectonic the Atlantic region, as inferred from

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Taconic orogeny. This orogenic event probably records either the collapse of a fringing island arc against the continental margin by closure of an intervening mar- ginal sea (Dewey and Bird, 1971; Bird and others, 1971) or the collision of an intra- oceanic island arc (Rodgers, 1971) with the continental margin following consumption of an indeterminate width of intervening oceanic lithosphere. Chappie (1973) has in- terpreted the sequence of diachronous events described as the Taconic orogeny (Rodgers, 1971) to be the result of abortive or partial subduction of the edge of North America beneath the flank of an approach- ing arc-trench system. For the southern Ap- palachians in particular, Odom and Ful- lagar (1973) inferred that an offshore island arc (Piedmont) was not a part of North America during early Paleozoic time but in- Figure 4. Bullard reconstruction of the closed North Atlantic with approximate Paleozoic margins stead was facing North America and con- of Appalachian-Caledonide-Mauritanide suture belt indicated. Patterns show schematically inferred suming intervening oceanic lithosphere by times of final ocean closure along Appalachian belt (1—4) and of climactic deformation along Ouachita subduction beneath the near flank of the orogenic belt (5-6): 1, Middle to Late Silurian time in Scandinavia (Gale and Roberts, 1972); 2, Late arc. They concluded that the arc collided Silurian to Early Devonian in Britain (Dewey, 1969); 3, Middle Devonian in the northern Ap- with the continental edge in Middle Or- palachians (Bird and Dewey, 1970; Schenk, 1971); 4, Late Devonian to Carboniferous in the southern dovician time to form an arc-continent su- Appalachians (Hatcher, 1972); 5, Middle Carboniferous time in the Ouachita Mountains (Ham and ture along the Brevard zone. Whatever the Wilson, 1967); 6, Late Carboniferous to Early Permian in the Marathon region (Ham and Wilson, 1967). details of the arc-continent interaction that produced the Taconic orogeny, the events paleomagnetic data, thus are broadly com- Bird, 1971; Bird and others, 1971). Tec- did not mark terminal suturing within the patible with the specific timings suggested tonic overprints from successive episodes of orogenic belt (for example, Chappie, 1973; by various authors (indicated in Fig. 4) for deformation thus mark the orogenic ter- Odom and Fullagar, 1973). segments of the Appalachian- ranes, and disruption of the orogenic belt The Ordovician clastics shed from Caledonide-Mauritanide belt on the basis by the later opening of the Atlantic has left sources uplifted or brought into position at of local geologic relations. large parts of it hidden beneath the modern the edge of the continent by the Taconic The positions of suture belts formed dur- continental shelves. In the south (see Fig. orogeny in the central Appalachians include ing Paleozoic ocean closure are difficult to IB), the region of possible sutures extends turbidites (Martinsburg) that were depos- specify because they involved interaction from the Brevard zone between the Blue ited in a deep-water trough with longitu- between North America and several differ- Ridge and the Piedmont on the American dinal paleocurrents trending parallel to the ent crustal entities. Most important were side (Watkins, 1971) to an ophiolitic zone continental margin (McBride, 1962). The Europe (Eurasia) and Africa (Gondwana). within the Mauritanides on the African side depositional site may have been a structural Europe was sutured to North America (Sougy, 1969). Estimates of the times of up- furrow formed by depression of the conti- along the northern Appalachian- lift of highlands formed by crustal collisions nental edge as it was drawn down against Caledonide belt before Africa joined North along the complex suture belt can be gained the flank of a colliding arc-trench system America along the southern best from the ages of exogeosynclinal (for example, Neill, 1975). Yeakel (1962) Appalachian-Mauritanide belt (McKerrow wedges and other clastic sediments shed has shown that overlying clastics of Silurian and Zeigler, 1972; see also Fig. 4). Also westward from the orogenic belt toward (Tuscarora) age in the central Appalachians present along the Appalachian belt is an as- the North American craton. Figure 5 de- form a simple clastic wedge with fluvial semblage of rocks representing an elongate picts sequentially the late Paleozoic paleocurrent trends leading transversely island-arc system that was cored by latest geologic events in North America related to away from the orogenic belt toward the Precambrian and Cambrian volcanic and the final development of the suture belt in craton. To the southeast, the Tuscarora plutonic rocks (about 500 to 750 m.y. B.P.) the southern Appalachian-Mauritanide overlaps deformed older beds that were ex- and that probably stood within the ocean region. posed along the eroded flanks of uplands basin (Rodgers, 1971). This latter terrane is raised by the Taconic orogeny; to the represented well in the Avalon belt of New- Appalachian Clastic Wedges northwest, it thins and interfingers through foundland (Hughes and Bruckner, 1971), deltaic facies with epicontinental marine the Carolina slate belt (Fullagar, 1971), and The Cambrian and Early Ordovician shelf deposits of the continental interior. basement beneath nearly flat-lying continental shelf along the eastern edge of A younger (Devonian) clastic wedge, Paleozoic strata in Florida (Bass, 1969). the North American craton is recorded by a which is not prominent south of the central Various subduction zones and magmatic carbonate-bank assemblage that extends Appalachians (Colton, 1970), reflects ter- arcs associated with plate consumption from Newfoundland to Alabama and minal suturing of the northern Appa- may have stood on both sides of and within grades eastward to lutite deposited in lachians associated with the Acadian the ocean during a long history of closure deeper water (Rodgers, 1968). The top of orogeny (Bird and Dewey, 1970). Devonian spanning more than 100 m.y. Across any this assemblage everywhere grades upward continental deposits of the Old Red land- transect of the orogenic belt, arc-continent and eastward into Middle and Upper Or- mass formed by the joining of Europe to collisions may have preceded final dovician shale and graywacke extending North America began to accumulate by continent-continent collision (Dewey and westward from uplands related to the Late Silurian to Early Devonian time in

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Scandinavia and Britain, by Early to Middle Devonian time in maritime Canada, and by Middle to Late Devonian in New York and Pennsylvania (Friend, 1969). Pelletier (1958) has shown that the Mississippian (Pocono) upper part of the Acadian clastic wedge in the central Appalachians also dis- plays fluvial paleocurrent trends leading fij^ ACADIAN Ip-^A

from thick continental deposits near high- LASTIC lands on the southeast toward thinner * \C C [DEEPV J /fc^3 WEDGE \ SEA shoreline facies gradational to marine de- posits on the northwest. Widespread marine deposition was not resumed in the northern Appalachians, al- PRE-STANLEY A A ^ STANLEY L though thick Carboniferous continental strata deposited in faulted basins and lo- ^•^AFRICA^^vxxXX?^^ cally strongly deformed are prominent in maritime Canada (Belt, 1968) and New England (Quinn and Moore, 1968). In the southern Appalachians, however, Missis- sippian strata include shallow-marine sand- stone, shale, and limestone with highly var- iable paleocurrent trends suggestive of shelf deposition (Schlee, 1963; Englund, 1968; Colton, 1970) in a part of the orogenic belt not yet fully sutured. A prominent Pennsyl- ? or vanian clastic wedge (which does not ex- tend north of the central Appalachians) re- (y K A A cords the Alleghanian orogeny during JACKFORK ^ ATOKA ~ ^ which the suture in the southern Ap- palachians was presumably completed. LEGEND Mid-Carboniferous plutons (about 300 II11111II Clastic wedges in Appalachian basin m.y. B.P.) that form a distinct trend in the southern Appalachian Piedmont (Fullagar, Deep-marine Ouachita flysch 1971) but are not prominent in the north- 1 I Uplifted Ouachita system. ern Appalachians may also reflect later clo- [X><>3 Sutured Appalachian-Mauritanide sure of the orogenic belt in the south. At the orogenic belt northern extremity of Carboniferous Thrust fault systems marine-shelf deposition in the central Ap- Subduction zones palachians, alluvial-plain deposits (Mauch + -»•-»- Line of Mesozoic Atlantic rifting Chunk) form a continuous section linking • Carboniferous peripheral basins the Mississippian (Pocono) upper part of • Permian peripheral basins the Acadian clastic wedge to the Pennsylva- * Sediment tronsport directions nian (Pottsville) lower part of the Allegha- OUACHITA OROGENY Y./^ nian clastic wedge (Meckel, 1970). Figure 5. Interpretive sketches of southeastern North America showing inferred Carboniferous In the southern Appalachians, regression evolution of Ouachita flysch and Ouachita system. Time frame of each sketch identified by reference associated with deposition of continental to subdivisions of Ouachita flysch (see text for ages). Features shown include: A-A, Anadarko- and littoral deposits in the Pottsville clastic Ardmore basin; A, Arkoma basin; B, Black Warrior basin; F, Fort Worth basin; I, Illinois basin; K, wedge derived from the east began near the Kerr basin; M, Marathon region; V, Val Verde basin. Mississippian-Pennsylvanian boundary. Complex intertonguing of marine and allu- belt and that the dispersal direction in the circuit through the Illinois basin in the in- vial facies developed as seaward prograda- distal facies was toward the southwest sub- terior of the craton (Potter and Pryor, tion advanced toward the west and north- parallel to the orogenic belt (Fig. 5). From 1961). In the Black Warrior basin at the west. Offshore shale and carbonate, the nature of the sedimentary deposits, we southern end of the Appalachian basin, beach-barrier sandstone, and delta-plain infer important bypassing of sediment Pennsylvanian clastics thicken from north deposits with coal are encountered along toward the still undeformed southern or to south on the outcrop (Ferm and others, traverses from northwest to southeast Ouachita margin of the craton. We thus 1967; Colton, 1970) and from northeast to across an elongate depocenter, the Ap- compare the Pottsville dispersal system southwest in the subsurface (Thomas, palachian basin, that lay between the up- geometrically to the Siwalik-Ganges disper- 1972). These have been interpreted as lifted orogenic belt and the craton (En- sal of Himalayan sediment to the head of fluvial and deltaic deposits spread north- glund, 1968; Ferm, 1970; Smith and the Bengal fan. ward from an orogenic highland along re- others, 1971; Ferm and others, 1972). At the northern end of the Appalachian gional strike from the slightly younger Paleocurrent studies (Schlee, 1963; Chen basin, sediment was contributed from the Ouachita belt to the west (Ferm and others, and Goodell, 1964; Meckel, 1967; Jones, craton to the north as well as from the 1967). In view of the uncertain struc- 1972) indicate that fluvial transport in the orogenic belt (Meckel, 1967). Sediment tural relations near the tectonic juncture be- proximal facies was toward the northwest from the northern Appalachians may also tween the Appalachian and Ouachita sys- away from rising highlands in the orogenic have reached the Ouachita region by a wide tems (for example, Thomas, 1973), this

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suggestion is difficult to evaluate. Pottsville as the clastic fill of an initially deep trough effects extended progressively northward paleocurrents on the outcrop in the same or elongate oceanic basin, and we accept with time. In the upper Atoka, deposition in area indicate generally westward movement this view. Morris (1974b) has provided shallow-marine waters and on adjacent of sediment (Schlee, 1963; Metzger, 1965). summary descriptions of each of the four coastal plains became dominant as deltaic We infer tentatively that an orogenic syn- stratigraphic divisions of the Ouachita and nearshore clastic accumulations pro- taxis lying beyond the exposed end of the flysch: graded southward from the continental southern Appalachian belt blocked axial 1. The Stanley Group (~4 km) of Mis- margin to the north, as reflected by south- dispersal of sediment moving southward sissippian (Osagean to Chesterian) age is erly paleocurrent trends (Briggs and Cline, along the trend of the Appalachian basin mainly dark shale with some intercalated 1967; Morris, 1974b; Stone, 1969). The and diverted sediment westward toward the silicic tuffs, but it includes turbidite sand- Atoka is overlain unconformably by mid- Ouachita region. An analogous dextral di- stone beds that increase in number, thicken Pennsylvanian (approximately Des- version of sediment dispersal occurs now at individually, and become more feldspathic moinesian) coal-bearing strata of the Krebs the eastern end of the Himalayas, where the toward the south in outcrop sections. Tuff Group along the northern flank of the Ganges drainage along the foot of the and tuffaceous sandstone are also more Ouachita Mountains. mountains is blocked by the Indoburman prominent in southern outcrops. Paleocur- The continental margin that lay north of ranges in Assam-Bangladesh and is turned rent studies (Johnson, 1968; Morris, the Paleozoic ocean floor upon which the toward the deltaic complex at the head of 1974b) of turbidites indicate northwesterly Ouachita flysch was deposited probably the Bay of Bengal. flow in eastern outcrops and westerly flow formed by continental rifting near the in western outcrops, with westerly direc- Precambrian-Cambrian time boundary. Carboniferous Ouachita Flysch tions everywhere dominant in the upper The elongate Anadarko-Ardmore basin Stanley. Locally, basal quartzose sandstone (Ham and Wilson, 1967) extends nearly Carboniferous turbidites, in aggregate and slumped limestone blocks reflect minor 500 km into the continent from the north- 7.5 to 12.5 km thick (Cline, 1970; Haley sediment transport off the shelf to the westernmost bulge in the Ouachita system. and Stone, 1973; Morris, 1974b), are ex- north. The basin is apparently an aulacogen that posed in the Ouachita Mountains for 350 2. The Jackfork Group (~3 km) of formed as a gash in the continental margin km along strike. The exposed rocks form Pennsylvanian (approximately early Mor- along the failed arm of a triple junction as- but a small segment of a largely buried fold rowan) age consists of shale and quartzose sociated with the continental separation belt, the Ouachita system, which extends turbidite sandstone with westerly (for example, Hoffman and others, 1974). for 2,000 km from the southern Ap- paleocurrent trends (Briggs and Cline, Though less than 100 km wide, this basin is palachians to the Mexican border (Flawn 1967; Morris, 1974b). Presumed slump floored by 2 to 3 km of Cambrian volcanic and others, 1961). The flyschoid nature of structures are again important locally along rocks, which do not appear on the cratonic the Ouachita Carboniferous rocks was doc- the northern fringe of exposures, and tur- platforms to either side, and has a Paleozoic umented by Cline (1970), and we refer to bidite beds appear more proximal toward sedimentary fill 10 to 12 km thick. Except the sequence here as the Ouachita flysch. the southeast. for their greater thickness, the sedimentary The volume of the Ouachita flysch within 3. The Johns Valley Shale (<1 km) of deposits are much like those of the adjacent the Ouachita Mountains is of the order of 3 Pennsylvanian (approximately late Mor- cratonic cover. Contrasting sediment thick- X 106 km3, or about one-seventh of the rowan) age contains only distal turbidite nesses within the Arbuckle Mountains se- volume of the Bengal fan within about sandstone, but it is known for large exotic quence (Ham, 1969) suggest rapid thermal one-seventh of the length of the Ouachita blocks of shallow-marine limestone and subsidence within the aborted rift soon system. sandstone. Shideler (1970) successfully cor- after its initiation. In less than 100 m.y. The Ouachita flysch was deposited con- related individual exotic blocks with a from Late Cambrian to Late Ordovician formably on a continuous sequence of Or- number of specific pre-Pennsylvanian for- time, 3 to 4 km of mainly shallow-water dovician to Mississippian strata less than 2 mations exposed in the Paleozoic shelf se- carbonate strata accumulated, yet less than km thick and exposed as the highly de- quences of the Arbuckle and Ozark regions a kilometer of beds were deposited in the formed core of the Ouachita Mountains. north of the Ouachita system in Oklahoma succeeding 100 m.y. and more up to mid- These strata are largely graptolitic shale, and Arkansas, respectively. The exotic Mississippian time. Evidence for progres- chert, and mainly thin-bedded sandstone blocks are generally supposed to have been sive early Paleozoic subsidence of a gradu- interpreted generally as deep-marine de- derived from an unstable continental slope ally cooling new sea floor south of the con- posits of a sediment-starved sea floor and to have been transported southward tinental margin may be preserved also in (Cline, 1970; Haley and Stone, 1973; Mor- into deeper water as clasts within sub- the lithology of the strata underlying the ris, 1974b). No depositional base to the se- marine mass flows. Ouachita flysch. Shale in strata older than quence has been observed. Thrusts have 4. The Atoka Formation (~5+ km) of Middle Ordovician age in the Ouachita core carried the Ouachita core northward into Pennsylvanian (approximately Atokan) age sequence are commonly calcareous, but near juxtaposition with age-equivalent but records transition upward to shallow-water pre-Stanley shale younger than Middle Or- carbonate-rich shelf sections (Ham, 1959; deposits. Atoka sandstones apparently con- dovician age are associated with bedded Frezon and Glick, 1959). We infer that the tain more lithic grains than those of the cherts (Morris, 1974b); the contrast may lower and middle Paleozoic sequence of the Jackfork (Morris, 1974b). The lower Atoka reflect the time of subsidence through the Ouachita core was deposited in a deep is shale and turbidite sandstone beds with carbonate compensation depth. oceanic basin that lay south of a shelf edge westerly paleocurrent trends, and proximal The sources for the Ouachita flysch sug- at the southern margin of Paleozoic North turbidites are more prominent on the out- gested by Morris (1971a) are generally America. The Carboniferous Ouachita crop toward the east (Briggs and Cline, compatible with our inference of a subsea flysch was also deposited in deep water and 1967; Morris, 1974b). The lower Atoka fan system derived mainly from the Ap- contrasts lithologically with thinner shelf thins northward and grades into more mas- palachian suture belt to the east and south- equivalents in the Arkoma basin north of sive sandstone units of shelf and littoral en- east. Morris inferred a dispersal system for the present Ouachita Mountains (Cline, vironments (Stone, 1969). As the sequence the more quartzose sand from the northern 1970; Chamberlain, 1971). Morris (1971a, accumulated, the shelf subsided by growth Appalachians and the Canadian Shield 1974b) regards the Carboniferous sequence faulting (Koinm and Dickey, 1967) whose across the Illinois basin of the craton, and a

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dispersal system for the more lithic sand Paleozoic strata is grossly similar to that in between the Andaman-Nicobar segment of directly from the region around the south- the Ouachita Mountains, although only the Indoburman-Sunda subduction zone ern end of the Appalachian basin. Thomas roughly half as thick; the time of main de- with the eastern continental margin of (1972) and Stone (1968) also suggested a formation was also slightly later, near the peninsular India and thus crumpling of the link between the Ouachita flysch and clas- Pennsylvanian-Permian boundary (Ham Bengal fan and thrusting of its sediment tics derived in part from south of the pres- and Wilson, 1967). The extension or termi- westward over shelf sequences now along ent Appalachians. The tuffs and feldspathic nation of the Ouachita system within Mex- the coastal zone. sands of the Stanley Group may reflect an ico remains unclear (Lopez-Ramos, 1969). Most published hypotheses for polarity additional source south of North America. In the Marathon region, pre-Car- of subduction in the northern Appalachians boniferous strata of the overthrust (Bird and Dewey, 1970), the southern Ap- Ouachita Orogenic Events Ouachita system were once widely inter- palachians (Hatcher, 1972), and the preted as mainly shallow-water deposits, Ouachitas (Keller and Cebull, 1973) all rely The youngest beds of the Ouachita flysch but Thomson and McBride (1964) showed on arc-trench systems built near the edge of are of Middle Pennsylvanian (Atokan) age. that the depositional features of most of the a Paleozoic North American plate and fac- Shortly after deposition, the whole section sequence are indicative of deep-water con- ing away from the continent. However, a was folded, together with the underlying ditions. The Carboniferous flysch includes recent analysis of relations in Newfound- core sequence, and carried northward at two packets of terrigenous turbidites with land indicates that the opposite polarity, least 75 km in complex thrust sheets that generally westerly paleocurrents separated with an arc-trench system facing North rode across the coeval shelf section on the by a sequence of carbonate turbidites America and having an east-dipping sub- Paleozoic continental margin. Viele (1973) (Dimple Limestone) with southeasterly duction zone, was the more likely suggested that the thrusting began during paleocurrents. The carbonate flysch was de- configuration there prior to continental col- flysch deposition, and he has documented rived from a shelf region along the edge of lision (Strong and others, 1974). In the the overall geometry of a complex stack of the adjacent continental platform (Thom- Piedmont, many fundamental relations folded nappes in which both the Car- son and Thomasson, 1969). The underlying must yet be deciphered before any model boniferous flysch and the underlying core terrigenous flysch (Tesnus Formation) con- can be defended in detail (Glover and sequence are involved. Middle Pennsylva- tains quartz-rich turbidites with westerly Sinha, 1973), but Dewey and Burke (1973) nian (Desmoinesian) strata in the Arkoma and northwesterly paleocurrents, and the inferred an east-dipping subduction zone basin are folded in the subsurface in sym- overlying terrigenous flysch (Haymond there in Carboniferous time. This zone was pathy with Ouachita structures; equivalent Formation) contains somewhat less quartz- presumably related to an arc-trench system and younger Pennsylvanian strata farther ose sandstones with westerly and southwest- along the edge of Africa south of the South west contain chert-pebble conglomerate erly paleocurrents (McBride, 1970). The Atlas fault. Paleozoic tectonic trends have probably derived from rising Ouachita turbidites grade upward into shallow-water yet to be traced from West Africa or the highlands (Ham and Wilson, 1967). Strong molasse deposits at the top of the Marathon Piedmont into the circum-Caribbean region Pennsylvanian deformation of the south- sequence. with any confidence. eastern end of the Anadarko-Ardmore au- Closely related to the Ouachita fold belt In any case, we conclude that geologic re- lacogen nearest the Ouachita orogenic belt are a series of basins, distributed around the lations within the Ouachita region favor the was accompanied by deposition of approx- curving periphery of the Ouachita system, concept of orogeny owing to collision with imately 5 km of clastics along the trend of with late Paleozoic sections 2.5 to 5 km a north-facing arc over the concept of that basin. thick resting depositionally on undoubted orogeny associated with a south-facing arc Most of the remainder of the Ouachita continental basement or its cratonic cover built along or near the continental margin. system is concealed by overlapping Cre- (Ham and Wilson, 1967). Each of these Paleozoic shelf sequences north of the taceous strata of the Gulf Coast lowlands. peripheral basins is marked by rapid Ouachita Mountains betray no evidence of In the subsurface to the southeast (Thomas, downwarping and filling that began shortly orogenic activity to the south until the 1972), slate and chert that apparently rep- before or concurrently with climactic de- sharp onset of the Ouachita orogeny in the resent the Ouachita core sequence can be formation in the adjacent segment of the Pennsylvanian Period, and it main effects traced toward the southern end of the Ap- Ouachita system. For example, in the Fort were complete prior to Permian time. This palachian belt. In the subsurface to the Worth basin of central Texas, the thickest style of orogenic development (Dickinson, southwest (Flawn and others, 1961), a fill is of Pennsylvanian age (Weaver, 1956; 1971) is compatible with the concept of sinuous deformed belt of the Carboniferous Turner, 1957), whereas the thickest fill is of crustal collision, strictly limited in time, fol- Ouachita flysch and the older Paleozoic Permian age in the Val Verde basin of West lowing consumption of offshore oceanic core sequence can be traced discontinu- Texas (Vertrees and others, 1959). It seems lithosphere beneath an arc-trench system ously in well cores to exposures in the clear that the major episodes of subsidence facing the continent on the edge of a sepa- Marathon region of West Texas. In the and deposition in the peripheral basins rate plate, but it is difficult to reconcile with Texas subsurface, as on the outcrop, the were related to the tectonic depression of an long-continued arc-trench activity along the flyschoid rocks of the Ouachita system are elongate region adjacent to the Ouachita continental margin itself. In either case, the thrust northwestward over coeval shelf orogenic belt. The string of peripheral ba- roots of the arc must now lie in the Texas strata. Additional major thrusts, however, sins thus defines a pericratonic foreland belt subsurface as the buried former Paleozoic also bring metasedimentary rocks from the lying between the late Paleozoic orogen and sediment source called Llanoria (Denison southeast across the Ouachita sedimentary the era ton proper. and others, 1969). units in the subsurface. Subcrops of post- Our tectonic analogy suggests that the If the arc faced the continent, the main orogenic(P) late Paleozoic strata apparently Ouachita orogeny reflects the crustal colli- suture belt must lie somewhere within the rest depositionally on rocks of the Ouachita sion of a north-facing arc-trench system, Ouachita system. The only alpine serpen- system southeast of the main outcrop belt with a south-dipping subduction zone, tinite known is exposed as small tectonic (Vernon, 1972; Meyerhoff, 1973; Woods against the southern margin of North slivers within the Ouachita core sequence and Addington, 1973). In the Marathon America. This conclusion is highly specula- (Sterling and Stone, 1961; Wicklein and exposures farther west, the overthrust sec- tive, because the analogy with the Indian Carpenter, 1973). If these are scraps of tion of Carboniferous flysch and underlying Ocean would involve some future collision oceanic lithosphere, they could be frag-

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ments of the oceanic substratum that lay offshore arc, would then be a potential tur- contiguous with the edge of the adjacent bidite fill of the Andaman basin, rather than North American continent through the the Bengal fan as we suggest. Moreover, the whole Paleozoic Era and need not mark a analog for the Oklahoma-Arkansas shelf true suture. Because intact lateral grada- would be the Southeast Asian mainland tions are commonly inferred between the rather than the east coast of peninsular Ouachita flysch and the shelf equivalents to India, and the geometric similarities implied the north, the most likely location of the su- by Figure 1 would dissolve. In our view, the ture belt probably now lies along or to the only evidence that favors this interpretation south of the metamorphic front in the sub- is the presence of a few thin tuffs and surface of the Ouachita system, rather than feldspathic sandstones that are reportedly anywhere on the outcrop. However, de- more prominent southward in the Stanley tailed facies relations are not clear in strata (Mississippian) division of the Ouachita older than the Atoka Formation, and any flysch. Although this evidence is concrete, palinspastic restoration of the Ouachita we do not regard it as conclusive and be- system is uncertain. Moreover, if the over- lieve that it is outweighed by other consid- thrust mass of the Ouachitas is viewed as a erations we have discussed. Figure 6. Conceptual diagram to illustrate structurally telescoped subduction complex progressive incorporation of synorogenic flysch within an orogenic suture belt by sequential clo- beneath which the edge of the continent LONGITUDINAL SEDIMENT sure of remnant ocean basin. was tucked, then the fundamental suture DISPERSAL may be masked beneath the piles of nappes orogenic belt where each progressive stage that form the exposed structural levels of Others have also emphasized the impor- of orogenic suturing occurred at a slightly the orogen. This possibility is enhanced by tance of longitudinal drainage of orogenic earlier time. Although we do not pretend to the presence within the Ouachita flysch of a highlands (Eisbacher and others, 1974). We have solved the puzzle of characteristically variety of disturbed bedding types (Morris, hold that the Himalayan-Bengal and longitudinal paleocurrents for all 1971b) of the sort common elsewhere in Appalachian-Ouachita analogy illustrates synorogenic flysch sequences, we hope we subduction complexes. In addition, the an important principle of sedimentation in have offered an approach that will lead to a exotic-bearing Johns Valley Shale, although relation to tectonics. At any given time, satisfactory explanation of many common regarded now as olistostromal, is similar in existing oceans are the chief repository of occurrences. general character to lithologic belts that in sediment, and orogenic highlands are the other orogens are interpreted as melanges main sediment sources. It follows that the ACKNOWLEDGMENTS indicative of differential tectonic movement ocean floors closest to the orogenic high- within subduction zones. lands have the best access to sediment. Discussions and correspondence with Regardless of where the suture is located, Ocean floors immediately adjacent to Kevin Burke, P. J. Coney, J. R. Curray, J. F. the peripheral basins strung along the edge orogenic highlands include those that lie off Dewey, Warren Hamilton, J. C. Ingle, G. of the craton north and west of the active continental margins marked by arc- D. Johnson, D. G. Moore, W. M. Neill, and Ouachita system are regarded here as indi- trench systems, but sediment deposited in D. R. Seeley were helpful in developing the cating depression of the continental margin this setting will be returned quickly to sub- ideas expressed in this article, and the man- by partial subduction just before the collid- duction zones at the continental margins uscript was improved by the critique of ing arc-trench system was stifled. As in and converted to melanges or metamorphic Robert Scholten. We are especially indebted other foreland belts, some proportion of the terranes. The ocean floors best sited to re- to R. C. Morris for extended discussions subsidence in these basins can be attributed ceive thick continuous sections of clastic and for guidance during field reconnais- to the downwarping of an isostatic moat in sediment are thus the remnant oceans that sance of the Ouachita flysch and the Black front of the tectonic load of nappes and lie off the ends of sequentially developing Warrior basin by Graham. G. W. Viele overthrust sheets advancing northward suture belts between colliding continents. guided Dickinson on a valuable field trip along the flank of the adjacent orogen Because colliding continental margins can- through the Ouachita core sequence during (Price and Mountjoy, 1970). Hamilton not be aligned perfectly for simultaneous and following which B. R. Haley and C. G. (1973) recently discussed similar down- contact along their full length, sequential Stone also gave graciously of their hard- bowing of subducted slabs of lithosphere in collisions, accompanied by repeated ad- won insights into the difficult geology of response to the tectonic load imposed by a justments in plate motions, must be the that complex region. growing wedge of subduction complexes, general rule. Sequential collision thus af- and Coney (1973) has discussed general fords a general model for the successive de- plate tectonic relations of foreland fold- position of longitudinally transported REFERENCES CITED thrust belts. The interplay between subduc- flysch sequences, which in turn are succes- Bass, M. N., 1969, Petrography and ages of crys- tion tectonics related directly to plate con- sively deformed and incorporated into su- talline basement rocks of Florida, in ture belts along the same orogenic belt that sumption and gravity tectonics due to McBirney, A. 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MANUSCRIPT RECEIVED BY THE SOCIETY MARCH Thomas, W. A., 1972, Regional Paleozoic 64-73. 15, 1974 stratigraphy in Mississippi between Viele, G. W., 1973, Structure and tectonic his- REVISED MANUSCRIPT RECEIVED JULY 29,1974

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