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Geology and plate tectonics interpretation of the of the Mesozoic radiolarite- complex in the Neyriz region, southern

A. HALLAM Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford OX1 3 PR, England

ABSTRACT rocks either intrusive (Wells, 1969) or em- tween this controversy and that over the placed by high-angle reverse faulting very similar complex in the , with one New investigations in the Neyriz region (Haynes and McQuillan, 1974). Alternative group of workers favoring a nappe in- of Iran provide support for the hypothesis plate tectonics models have been put for- terpretation (Allemann and Peters, 1972; that Mesozoic radiolarite and ophiolite ward either accepting (Takin, 1972) or re- Glennie and others, 1973) and the other have been thrust southwestward as nappe jecting (Haynes and McQuillan, 1974) the group rejecting it (Morton, 1959; Wilson, complexes over the autochthonous Cre- nappe interpretation. There is general 1969, 1973; Moody, 1974). taceous carbonate platform deposits di- agreement that the whole complex of rocks On the basis of field observations at rectly south of the Zagros Crush Zone. A is overlain unconformably by late Maas- selected key localities in the Neyriz region Late wildflysch sequence is ex- trichtian and younger deposits, hence pro- and laboratory examination of samples, cluded from the nappe complex and put in viding an upper age limit for the tectonic new stratigraphic and sedimentological the autochthonous unit. Different types of movements. A striking parallel exists be- data are here presented and a revised in- radiolarite are distinguished, and a variety of olistoliths are recognized within these rocks. The structural and stratigraphic sequence in the Neyriz region closely resembles that of the Oman Moun- tains. A model is proposed in which a rifted and subsequently aseismic continental mar- gin of Arabia persisted through much of the Mesozoic Era. In Late Cretaceous time the pattern of plate motion changed, and Arabia was driven toward central Iran, with the consequent progressive destruction of the intervening ocean and emplacement of nappes from the northeast. Key words: Iran, plate tectonics, stratigraphy, radiolarite-ophiolite complex.

INTRODUCTION

In recent years the region immediately west of Neyriz, about 100 to 150 km east of Shiraz (Fig. 1), has been recognized as a source of information of critical importance to the interpretation of the geologic evolu- tion of southern Iran. A well-exposed series of Cretaceous limestone, radiolarite and turbidite beds, and ultramafic rocks, all of ^-frNeyriz? unknown or disputed age, hold the key to understanding major structural events over a much larger region embracing the Zagros 15 30 km. suture zone. I The structural relationships of the "Col- oured Series" (or "Coloured Mélange"), as -Cretaceous Tertiary the radiolarite complex has been termed, of Bangestan and the ultramafic rocks with each other and Khami groups and with the Cretaceous limestone have „ SHIRAZ Coloured Series Cretaceous Hmestorws provoked sharp controversy. Gray (1950) of Crush Zone and Ricou (1968a, 1968b, 1971) argued for .Neyriz the presence of nappes; other geologists 0 300km Ultramafics Precambrian metamarphics of Stable Block firmly deny their existence and favor a normal autochthonous sequence of lime- Figure 1. Geological sketch map of the Neyriz region, based on the map of Gray (1950) and the Oil stone and radiolarite, with the ultramafic Consortium 1:1,000,000 map of southwest Iran (Tehran, 1969).

Geological Society of America Bulletin, v. 87, p. 47-52, 3 figs., January 1976, Doc. no. 60106.

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terpretation given of the structural evolu- suggests a sudden change to a deeper water two blocks yielded the alga Permocalculus tion. . The identification of Globotruncana and the foraminiferan Choffatella, which Southwest of the Zagros Crush Zone, paraventricosa by F. C. Dilley (1973, per- indicated a late Early Cretaceous age. Thus which is composed of severely deformed sonal commun.) clearly established a mid- early Mesozoic macrofossils found in the Cretaceous limestone belonging geologi- Turonian to early Senonian age for these obviously exotic blocks of limestone cannot cally to central Iran (Stocklin, 1968), three younger rocks. be used to date the time of deposition of the northwest-trending zones are distinguished Gray's geologic map indicates that the matrix rock. (Fig. 1). Farthest to the southwest is a series Sarvak is overlain by what he termed the Samples of matrix rock collected for this of shallow-water limestone beds of the au- "Radiolarite-Ophiolite Nappe." The lowest investigation have yielded abundant oligos- tochthonous Zagros fold belt. The Col- division of this higher unit can be readily teginids that indicate an Albian through oured Series lies northeast of the limestone seen in the Rafiabad-Arsinjan region, but Senonian age; Ricou (1970) recorded the zone, and the ultramafic rocks occur next to stratigraphic concordance with the Servak, presence of Globotruncana, thus restricting the crush zone. Apparent irregularities in although confirmed elsewhere by Ricou the range effectively to Senonian. Quite this zonation are due partly to Tertiary (1970), is not evident here because of local probably the bulk of the olistoliths are also high-angle faults and partly to a continua- high-angle faults. Cretaceous in age. The breccia beds, there- tion of the Tertiary Zagros folds into the A regular bedded sequence cannot be dis- fore, appear to succeed the Sarvak strati- Coloured Series, so that domes of mid- cerned. Instead there is a chaotic jumble of graphically, and the local presence of high- Cretaceous limestone, such as the mountain irregularly oriented gray and brownish angle fault contacts is a consequence of late known as Kuh-e-Dalnashin (Fig. 1), struc- limestone blocks, which range in size from Tertiary tectonism. Gray's (1950) interpre- turally underlie the radiolarite complex. pebbles or small boulders to masses tens of tation of an overthrust contact must be re- Because he found and Jurassic fos- metres across, that are set in a soft- jected as not in accord with the evidence, sils in the radiolarite complex, Gray (1950) weathering matrix of fine-grained marly and the breccia beds should be specifically interpreted Kuh-e-Dalnashin and related and siliceous limestone varying in color excluded from the radiolarite proper. structures as tectonic windows where the from pale gray to green and purple, that is, nappe cover of the radiolarite complex had a wildflysch facies. Although the matrix Southeastern Region been stripped off by erosion. rocks may contain , they are de- Wells (1969) claimed that the pre- decidedly not typical radiolarian . This Another critical locality for the nappe in- Cretaceous came from exotic lime- zone of rocks extends southeastward to terpretation is in the Pichakun Mountains, stone blocks within radiolarite and that Pichakun, but in the absence of regular where Ricou (1968a) described an easterly more reliable evidence of microfauna in situ bedding, reliable estimates of thickness are dipping sequence of radiolarite and other indicated a Senonian age for the radiolarite hard to obtain; Ricou (1970) estimated a rocks that he claimed ranges in age from near the Zagros Crush Zone throughout thickness of 250 m in the southeast. Cretaceous down to Triassic; this sequence southern and western Iran; hence there The limestone blocks are clearly exotic is structurally but not stratigraphically un- was, in fact, a normal stratigraphic succes- and can accurately be described as olis- derlain by Upper Cretaceous limestone be- sion near Neyriz. Ricou (1968a, 1968b, toliths. Some, including the largest blocks, longing to the autochthon. Ricou's "Bar 1971) accepted Gray's (1950) interpreta- are pale gray and fine grained or sparry Er" section (loc. C, Fig. 1) is as follows: tion in part and brought forward in support with pellet and ooid allochems but virtually 4. Conglomeratic limestone () 100 m more paleontological evidence from the no recognizable macrofossils. The smaller 3. Varicolored radiolarite Pichakun Mountains in the southeast. blocks include limestone turbidites with (U. Jurassic to L. Cretaceous) 230 m Haynes and McQuillan (1974), while re- graded and convolute bedding; the coarser 2. Siliceous limestone (M. Jurassic) 120 m jecting the nappe interpretation, accepted particles consist of ooids and pellets of simi- 1. Black marl (U. Triassic) 100 m the fact that some of the radiolarite might lar size. Unit 1 is dated by corals, brachiopods, be pre-Cretaceous in age. At locality B (Fig. 1) between Kuh-e- and the bivalve Megalodon; unit 2 by the Investigation was concentrated in one Dalnashin and Lake Tashk, Gray (1950) Trocholina and Proto- area west and south of Kuh-e-Dalnashin found limestone containing undoubted peneroplis; and unit 4 by a number of and in another area in and around the Triassic and Liassic macrofossils, and much foraminifera. Pichakun Mountains. of his nappe interpretation depends upon H. C. Jenkyns and I examined units 1, 2, these data. Accordingly, a close examina- and 3, and we differ from Ricou as follows: Northwestern Region tion was made at this locality. All the units compose a bedded chert se- Gray's illustrated section (1950, Fig. 2) quence. The colors are predominantly pur- Directly east of the village of Rafiabad, shows a regular bedded sequence of plish and gray green, and the chert occurs which lies several kilometres south of Arsin- radiolarite and limestone disturbed by as beds ranging in thickness from 2 to 20 jan (loc. A, Fig. 1), limestone of the Sarvak faulting. There is no evidence of a regular cm, with partings of less siliceous, shaly Formation (the top formation of the Dages- sequence. The general aspect of the geology material. Fine millimetre-scale laminations tan Group) is in fault contact with rocks closely resembles that farther west but with are common. Unit 2 consists of a series of mapped by Gray (1950) as part of the Col- an even greater concentration of partly silicified limestone turbidite beds as oured Series. The Sarvak has been dated by irregularly oriented limestone olistoliths, much as 1 m thick, some of which are microfossils as Cenomanian in age, locally many of which exceed 10 m in size; locally, graded and interstratified with bedded ranging up into the Turonian stage (James the volume of exotic blocks exceeds the chert. The limestone particles are predom- and Wynd, 1965). The section studied was volume of matrix. Some of the limestone is inantly pellets with a maximum size of composed of several tens of metres of mas- pelletal, with breccia horizons including about 2 mm. sive, carbonate-platform pelsparite, oospa- chert. Some are also fine grained and others Although microfossils within unit 2 are rite, biosparite, and biomicrite that contain have features such as graded bedding. The likely to have been transported from a shal- echinoids and pectinid bivalves and that dip complex abuts the Sarvak limestone of lower marine regime (apart from radiolaria, about 10°S. These beds are overlain with- Dalnashin Mountain at what appears to be which are too poorly preserved to be out transition by a few metres of gray, a high-angle fault. identifiable), their presence as discrete par- marly, thin-bedded micrite and microspar- No recognizable macrofossils were found ticles rather than in lithoclasts means that ite with planktonic foraminifera, which in the olistoliths. Thin sections of one or they are almost as reliable for age determi-

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nations as in situ material (compare Glen- radiolaria. Isolated masses of similar chert radiolarites bear the characteristics of nie and others, 1973). In thin section, it was have also been seen within the ophiolite deep-sea deposits and are now juxtaposed not possible to confirm the presence of complex east of Lake Bakhtegan (also against a thick Mesozoic sequence laid Trocholina or Protopeneroplis (which known as Lake Neyriz). down on a shallow carbonate platform taken together would indicate an age be- The hill is capped by massively bedded without the intervention of a series of beds tween and exclusive of Liassic and Aptian gray limestone at least 100 m thick that is of transitional facies. Furthermore, they lie rather than more specifically Middle Juras- structurally concordant with and overlying along the line of strike southeast from the sic), but some algal fragments were found the radiolarites. This limestone occurs in a autochthonous Sarvak of Dalnashin and that were evidently derived from a shallow number of huge concordant masses hun- must therefore be part of a thrust sheet. marine platform. According to G. F. Elliot dreds of metres long, extending for at least The oil company geologists' claim (James (1974, oral commun.), in the Middle East, several kilometres along the strike north- and Wynd, 1965; Wells, 1969) that all the Thaumatoporella parvovesiculifera occurs westward from the lake shore. It contains radiolarites in the belt southwest of the in rocks from the to the Senonian abundant megalodontid bivalves as much Zagros Crush Zone are Late Cretaceous in stages, but it is most abundant in Middle as 10 cm long together with oncolites and age must be rejected. It may be that the and Upper Jurassic and Senonian rocks. must have come from an Upper Triassic de- error has arisen because different types of The preservation of the Pichakun specimens posit laid down in very shallow water, as is radiolarite have not been distinguished and suggests a Late Jurassic age rather than a well understood for Alpine occurrences their tectonic setting with respect to the Senonian age, a dating that is also sup- (Bernoulli and Jenkyns, 1974). From a dis- Zagros Crush Zone not clearly established. ported by the presence of Pycnoporidium. tance the limestone masses look as though Wells refers, for instance, to deposits in Thus unit 2 can be attributed to the Upper they could be a part of a small allochthon- Luristan. Jurassic with minor uncertainty, substan- ous thrust slice, but close to the outcrop H. C. Jenkyns and I have examined rocks tially supporting Ricou's (1968a) age de- they are seen to be disposed in too irregular mapped by the oil companies as radiolarite termination. a fashion for this to be likely. They are evi- in the Kermanshah region of Luristan and On the other hand, Ricou's (1968a) dently olistoliths of huge size that have slid beds of very similar type in Fars Province, claims of a Triassic age for unit 1 must be into a basin in which radiolarites were 20 km northwest of Darab. Both groups of strongly contested. The beds in question are being deposited. Conspicuous as these fea- rocks are varicolored and range from red not marl but chert, and they contain hori- tures are they have provoked little interest and mauve to yellow and gray; they are zons of obvious olistoliths in jumbled, ran- among or have escaped the attention of thin-bedded siliceous marly limestone domly oriented and detached blocks as previous field workers, although Gray rather than true chert and hence are unlike much as 10 m across. Some of these are (1950) did note on this map the presence of the Pichakun Series and the Bakhtegan breccioid, with fragments of black, partly Megalodon limestone at one locality. beds. They resemble the Pichakun Series in silicified oolite; others are pelletal. Several To summarize, the following groups of possessing fine laminations but differ in masses of colonial coral resembling rock are distinguished from southwest to lacking prominent turbiditic interlayers. Thecosmilia were discovered among the northeast: (1) shallow-water limestone of The abundant radiolaria are much better olistoliths. The presence of other shallow- the Sarvak Formation (Cenomanian- preserved than in the Pichakun and Bakh- water fossils such as recorded by Ricou Turonian); (2) thin-bedded marly and tegan deposits (although still not datable) would occasion no surprise in this exotic cherty limestone with numerous limestone and occur in association with calcific mi- carbonate platform assemblage, but it olistoliths ranging in age from Triassic to crospheres and rarer planktonic forami- would be astonishing to find such material Cretaceous, here informally named the nifera including Globotruncana. Other fos- in situ within a radiolarian chert sequence. Dalnashin beds (Senonian); (3) varicolored, sils are lacking. A further important differ- A few kilometres north and east of the thin-bedded radiolarian chert with interca- ence is that the radiolarite around Kerman- eastern end of Ricou's Bar Er section, lations of limestone turbidite; olistoliths in- shah and near Darab apparently occurs in beyond an area of low ground with no ex- cluding Triassic material near base (this is stratigraphic sequence with normal posures, one passes into a zone of Ricou's Pichakun Series [Jurassic- shallow-water Cretaceous limestone and is ultramafic rocks and associated radiolarian Cretaceous]); (4) red chert lacking turbidite not directly adjacent to ophiolite. chert. The former are briefly described by deposits but with intercalated basalt and It is concluded that the deposits in ques- Haynes and McQuillan (1974) and attrib- huge olistoliths of Triassic limestone, here tion are indeed Senonian in age and signify uted to an ophiolite complex, although the termed the "Bakhtegan beds" (age not es- a widespread deepening of the sea within full ophiolite sequence has not been recog- tablished); and (5) ophiolite complex, here the northeastern part of the Zagros zone nized. Xenoliths occur within the igneous termed the "Neyriz ." A more following the shallow-water conditions of rocks, including a huge isolated mass of comprehensive stratigraphic study would the Sarvak. This is in accord with Wells's marble that is probably derived from the be required before formal groups or forma- (1969) view, and my principal argument extensive area of Precambrian rocks north- tions could be proposed. with him is that he has overgeneralized in east of the crush zone. Attention is confined including the Neyriz region, which has here to the sedimentary rocks. more complicated geological relationships Locality D (Fig. 1) is a hill where several INTERPRETATION than he assumed. hundred metres of radiolarites dip approx- No further evidence has turned up in the imately 40°W. They are adjacent to the Presence of Nappes Pichakun area to substantiate or refute ophiolites farther east, but the contact is Ricou's (1968a) claim of a succession of not exposed. The radiolarites are brick red, Although parts of the interpretations of nappes, as opposed to high-angle reverse are highly siliceous with a blocky fracture, Gray (1950) and Ricou (1968a) are re- faults, to account for repetitions of the se- and lack any trace of turbiditic interlayers jected, I accept the existence of allochthon- quence. Although the contact zone was not or laminations, unlike the radiolarite dis- ous structural units emplaced as nappes seen, the Bakhtegan beds are probably cussed earlier. There are, however, several that traveled from the northeast. thrust over the Pichakun Series. The rejec- horizons of interbedded basalt as much as 1 The best evidence for nappes comes from tion by Ricou (1970) and Haynes and Mc- m thick; some of these contain small clasts the Pichakun Series. The age of the Quillan (1974) of Wells's view that the of pink and white cherty microsparitic radiolarian chert sequence ranges down Neyriz ophiolites have been intruded into dolomite that is unfossiliferous except for from the Cretaceous into the Jurassic. The the surrounding rocks is here supported.

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There is little doubt that they represent one Hawasina. Glennie and others (1973) ex- tional environments based on analogies or more thrust slices, as with other ophio- cluded it from the latter and supported Wil- with Mesozoic radiolarite in the Mediter- lite complexes in mountain belts. son (1969) in treating it as autochthonous. ranean region and modern oceanic deposits The structural-stratigraphic sequence of Another comparison is with the Gurpi (compare Bernoulli and Jenkyns, 1974): the rocks relevant to this study is illustrated Formation, which is extensively developed 1. Marly siliceous varicolored limestone diagrammatically in Figure 2. in the Iranian Zagros ranges as a marly with turbiditic laminae. Deposited in rela- Senonian formation resting unconformably tively deep water on the northeastern mar- ROCK GROUPS OMAN on older Cretaceous limestone. gins of the Arabian continent. EQUIVALENTS IN NEYRIZ REGION 3. The zone of radiolarian , lime- 2. Varicolored chert with laminae and Tartxir and younger Maastrichtian and stone and turbidites, and olistoliths, beds of turbiditic (or allodapic) limestone younger which lies directly to the east, composes the derived from a carbonate platform. Possi- Neyriz ophiolites Semail Hawasina Complex, an allochthonous unit bly deposited on continental rise, but Upper Hawasina consisting of several thin imbricated nap- farther from the continent than 1. Bakhtegan beds (with Oman Exotics) pes. Exotic blocks of and Triassic 3. Red chert totally depleted in CaC03 shallow-water limestone occur at the base and lacking any lamination or turbiditic and top of the structural sequence. Those at beds — intercalations of basalt. Intimate Pichakun Series Lower Hawasina the top are huge in size, as much as 1,000 m association with ophiolite although no thick, and may form whole mountains. stratigraphic contact seen. Deposited below Dalnashin beds Muti Formation They include megalodontid and stromatoli- CaC03 compensation depth on ocean floor. tic limestones and are known as the Oman The olistoliths within the Pichakun and Exotics. Basalt flows intercalated in the Sarvak Formation Wasia Group Bakhtegan beds confirm the evidence of cherts are only found in the upper part of turbidites in signifying the former existence Figure 2. Comparison of stratigraphie- the sequence. The more reliable microfossil of marked differences of relief on the sea structural units of the Neyriz region and the determinations in the Hawasini give dates bed and movement along fault scarps, Oman Mountains. ranging from Triassic to Cretaceous. which caused large blocks of shallow-water The Hawasina beds obviously compare limestone to be dislodged and carried sea- Comparison with the Oman Mountains closely with the Pichakun and Bakhtegan ward into deeper water. The olistoliths low beds. Especially noteworthy are the exotic in the Pichakun sequence signify exposure A number of close comparisons can be blocks of Triassic limestone at the base of of rocks of the Arabian Platform down to made between the Neyriz region and the the Pichakun Series and the similarly aged Triassic at least, probably at some time Oman Mountains approximately 600 km blocks of huge size at the top of the radiola- within the Jurassic. The source of the to the south-southeast (Fig. 2). The similar rite structural sequence north of Lake enormous Triassic olistoliths in the Bakh- dispute about whether allochthonous units Bakhtegan that recall the Oman Exotics. tegan beds is more enigmatic. are present in the latter region hinges Furthermore, only this upper unit, the There appear to be several possibilities. primarily on the stratigraphie evidence Bakhtegan beds, contains intercalated First, the limestone might have been depo- from microfossils. As a result of a very basalts. sited close to sea level within the oceanic comprehensive study by a team of Shell 4. Thrust over the Hawasina Complex is regime on a submarine rise. This interpreta- geologists, who covered an extensive ter- a large nappe of ultrabasic and basic igne- tion was favored for the very similar Oman rain in great detail and took great care to ous rocks, the Semail, in which a full Exotics by Glennie and others (1973), who exclude fossils contained within derived ophiolite sequence has been recognized inferred deposition above the CaC03 com- lithoclasts, a persuasive case for nappe em- (Reinhardt, 1969; Allemann and Peters, pensation depth more or less on the axis of placement from the east has been put for- 1972). This is obviously equivalent to the a spreading ridge. One can note in support ward (Glennie and others, 1973). less-extensively exposed and less-studied that the matrix rocks are of deep oceanic 1. In the west is a zone of undisputed au- Neyriz ophiolite. type. The interpretation suffers, however, tochthonous rocks forming the margin of 5. All the previously mentioned rock from the fact that the megalodontid lime- the Arabian Shield, with the Mesozoic se- units are unconformably overlain by late stone, with large, thick-shelled benthonic quence predominantly shallow-water car- Maastrichtian shallow-water limestone, as fossils and algae, was almost bonate. The Cenomanian Wasia Group with the Tarbur Formation in the Neyriz certainly deposited in very shallow water, consists of platform limestone with abun- region. and the only sediments on modern oceanic dant benthonic fossils, and it is evidently Evidently the geologic development of ridges are pelagic oozes. Second, it could equivalent to the Sarvak Formation. the two regions was very similar, and any have formed the more southerly, offshore 2. Unconformably overlying the Wasia overall synthesis should link them in a con- part of a shelf zone fringing what is now and older beds is the Muti Formation, tinuous belt. central Iran. Where, in that case, has the which consists of radiolaria- (and Globo- remainder of this continental margin gone, truncana-) bearing marl with lenses of A Model for the Mesozoic Evolution and where are the turbidites that might limestone conglomerate and quartz turbi- of the Neyriz Region have been expected to occur in association? dite. The limestone clasts include Albian to In fact, the Bakhtegan beds are structurally Cenomanian and Permian material. The Acceptance of the nappe interpretation overlain by an ophiolite nappe, presumably formation is dated as Coniacian to Campa- implies a telescoping of structural units signifying a slice of oceanic floor, and im- nian in age and is reckoned to intertongue from northeast to southwest, with the mediately to the north are the Precambrian with marls in the interior of Oman farther higher units having once been more distant and younger rocks of the central Iranian west. There is evidently some resemblance from the Arabian continent, of which the continental mass. A further difficulty is that of the Muti to the Dalnashin beds, and it is Zagros ranges form the northeastern mar- the Triassic of central Iran is in terrigenous therefore interesting to note that there has gin. Among those rocks that have hitherto clastic facies (Stocklin, 1968). been disagreement between various been grouped together as radiolarite rocks A third possibility is that the limestone geologists about whether to include the of the "Coloured Series," I distinguish the olistoliths are derived from the southwest Muti with the structurally overlying following three types and propose deposi- continental margin of the Mesozoic ocean.

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The fades of the olistoliths is consistent given to an extensive and thick marly se- distance offshore to the north and depos- with a southerly origin and so is the struc- quence of Senonian age that rests nonse- ited in a more extensive zone of subsidence tural position beneath the ophiolite. The quentially on the Sarvak in the northeastern on its southern side. These changes seem to awkward point is that the Bakhtegan beds part of the Zagros ranges. Southwestward mark the southward spread of the older are structurally the highest unit of the it passes into limestone, and northeastward, fault zone, now concealed by the over- radiolarite complex and hence the most dis- in Luristan, coarser clastic rocks enter the thrusting, that bordered the Arabian conti- tal from the continent. There is no reason to succession (James and Wynd, 1965). I sug- nent. The faulting on the oceanic side of the assume, however, that the shelf edge was gest that the Dalnashin beds could be a Arabian continental margin clearly heralds uniformly straight, and one could perhaps local breccioid development of the Gurpi, the subsequent nappe emplacement from envisage a promontory extending north- and that the group 1 radiolarites should the northeast. Perhaps, therefore, it signifies ward bringing the carbonate platform adja- perhaps be attributed to this formation. It is the first stage of continental collision before cent to a zone of deep ocean as is the case noteworthy that Glennie and others (1973) compressive forces built up sufficient today, for example, with the Bahama Bank inferred a northeasterly source for the energy to provoke low-angle thrusting. and its surrounds, including Tongue of the coarse clastic rocks within the equivalent In early Maastrichtian time, collision of Ocean. Marginal faulting could then have deposits in the Oman, namely the Muti continental blocks and nappe emplacement caused huge blocks to tumble or slide Formation. took place, to be followed by regional up- downslope for several kilometres. Perhaps The association of large exotic blocks, lift, erosion, and renewed shallow-water the least implausible explanation, however, turbidites, and rapid lateral changes of carbonate deposition in late Maastrichtian is that the shallow-water carbonate was de- facies points to a regime of strong differen- time. A tentative model of the Mesozoic posited on oceanic islands or seamounts tial relief and tectonic activity along high- evolution is portrayed schematically in Fig- lying a considerable distance from land but angle faults, either dip or strike slip, as dis- ure 3. The diagram for Coniacian- not necessarily related to a spreading ridge. cussed extensively, for instance, for the time shows a newly established Another major problem is the emplace- Mesozoic Era of the Mediterranean region normal fault with southerly downthrow. ment of the olistoliths. The difference in by Bernoulli and Jenkyns (1974). One can This inferred fault could just as well be por- depth of deposition of the limestone blocks also infer an earlier Jurassic phase of high- trayed as a reverse or high-angle thrust and the matrix cherts could well have been angle faulting, causing olistoliths to tumble fault, signifying initiation of compression several thousand metres. Yet the lime- into the radiolarites at the base of the from the north. stone-chert contact appears to be broadly Pichakun Series. The evidence presented in The post-Cretaceous history is as sum- concordant, and there is no evidence of this paper is indeed consistent with the in- marized by Haynes and McQuillan (1974). stratal contortion of the cherts, as might be ference of a tensional regime on the north- The Arabian and Iranian continents con- expected if the blocks had tumbled in sud- eastern margins of the Arabian continent tinued to close on each other, leading even- denly as a result of fault activity on the persisting from the Jurassic and perhaps the tually to extensive folding and reverse fault- seamount margins. The possibility cannot Triassic into Late Cretaceous time, to be ing in Pliocene time, accompanied by the be discounted, therefore, that, after de- succeeded abruptly by intensive compres- formation of the crush zone. tachment, the olistoliths slid downslope for sion and nappe formation in early Maa- The evolutionary model outlined above some distance before coming to rest, with- strichtian time. In terms of plate tectonics accords well with the more comprehensive out causing significant contortion of the this could imply sea-floor spreading for story worked out by Glennie and others radiolarian ooze. The alternative possibility much or most of the Mesozoic Era, between (1973) from much more abundant data for is that the ooze was too homogeneous or what is now Central Iran and Arabia, with the Oman. Comparable and substantially isotropic in character to reveal contortions the intermittent collapse of fault blocks contemporaneous events can be inferred and that the regular bedding observable along the adjacent continental margin, along the westward continuation of the today is a result of secondary, diagenetic as during the late Era in the Zagros suture zone through and unmixing of silica from the subordinate Red Sea region (Hutchinson and Engels, into Greece (Dewey and others, 1973). clay that now forms the shaly partings of 1970; Lowell and Genik, 1972). Near the In southwest Turkey, for instance, an au- the chert layers. end of Cretaceous time, the pattern of plate tochthonous carbonate sequence in the The abundance and size of the olistoliths motion changed suddenly and Arabia was south is overlain by a so-called Intermediate in the Dalnashin beds signifies close prox- driven toward Iran, causing the intervening Complex consisting of imbricated wedges imity to one or more fault scarps that were ocean to be progressively destroyed and a and small thrust sheets, in turn overthrust active in early to mid-Senonian time, and series of nappes including part of the ocean from the north by the "Peridotite Nappe" which must, locally, have caused exposure floor to be thrust southwestward over the (Bernoulli and others, 1974). The Inter- of Triassic limestone of the Arabian conti- continental margin. mediate Complex consists of a Permian to nental margin. That the source lay approx- Very little information is available on the Upper Jurassic carbonate platform se- imately to the northeast is indicated by two pre-Cretaceous history. Whether sea-floor quence overlain by Cretaceous pelagic lime- sets of facts. First, there is an abundance of spreading began in the Jurassic Period or stone passing up into Senonian wildflysch, blocks of limestone turbidite, a type of rock earlier, as proposed for the Oman by with large olistoliths in a matrix of shale that does not occur in the carbonate plat- Glennie and others (1973) and accepted by and turbidite. Thrust over the wildflysch is form Mesozoic sequence of the Zagros but Dewey and others (1973), cannot yet be de- the Diabase Nappe, which consists of can reasonably be inferred to have been de- termined. A dating of the Bakhtegan chert radiolarite, pelagic limestone and pillow posited in deeper water along the continen- would obviously be of great value but has basalt of various ages ranging up to Upper tal slope farther northeast. Second, the lat- so far not proved possible. From Cenoma- Cretaceous. Emplacement of the Diabase eral equivalents of the Dalnashin beds a few nian time onward, however, a much fuller Nappe is dated as Late Cretaceous to early kilometres to the southwest at Khaneh Kat picture can be worked out. The northeast- Tertiary. The Senonian wildflysch unit re- directly south of Lake Bakhtegan are some ern margins of a widespread carbonate calls the Dalnashin beds, and the Diabase 260 m oiGlobigerina marl (Gray, 1950). In platform began to undergo tectonic distur- Nappe recalls the Pichakun, Bakhtegan, other words the matrix rock is similar, but bance in early Turonian time that extended and Hawasina beds. coarse clastic rocks are absent. into Senonian time. Large quantities of clas- Clearly an event of great geological The name "Gurpi Formation" has been tics were shed from a horst uplifted a short significance took place in the Middle

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/1/47/3444145/i0016-7606-87-1-47.pdf by guest on 27 September 2021 6 A. HALLAM NE Gray, K. W., 1950, A tectonic window in south-western Iran: Geol. Soc. London Radiol, cherts with basalts and large Quart Jour., v. 105, p. 189-223. exotics ol unknown source Haynes, S. J., and McQuillan, H., 1974, Evolu- tion of the Zagros suture zone, southern Iran: Geol. Soc. America Bull., v. 85, p. 739-744. Hutchinson, R. W., and Engels, G. G., 1970, Tectonic significance of regional geology and evaporite lithofacies in northeastern Ethiopia: Royal Soc. London Philos. Trans., Ser. A., v. 267, p. 313-329. James, G. A., and Wynd, J. G., 1965, Strati- graphic nomenclature of Iranian Oil Con- 1. CENOMANIAN sortium agreement area: Am. Assoc. Pe- troleum Geologists Bull., v. 49, p. 2182-2245. Lowell, J. D., and Genik, G. J., 1972, Sea-floor spreading and structural evolution of the Red Sea: Am. Assoc. Petroleum Geologists Bull., v. 56, p. 247-259. Moody, J. D., 1974, Late Cretaceous nappes in Oman Mountains and their geologic evolu- tion: Discussion: Am. Assoc. Petroleum Geologists Bull., v. 58, p. 889-895. Morton, D. M., 1959, The geology of Oman: 2. CONIACIAN - CAMPAN IAN World Petroleum Cong., 5th, New York, Proc., sec. 1, p. 277-294. Reinhardt, B. M., 1969, On the genesis and em- placement of ophiolites in the Oman Moun- Uplift and erosion of nappe complex tains geosyncline: Schweizer. Mineralog. u. Petrog. Mitt., v. 49, p. 1-30. Ricou, L. E., 1968a, Une coupe à travers les séries à radiolarites des monts Pichakun (Zagros, Iran): Bull. Soc. Géol. , v. 7, p. 478-485. 1968b, Sur la mise en place au crétacé 3. EARLY TO MID MAASTRICHT!AN supérieur d'importantes nappes à radiola- rites et ophiolites dans les Monts Zagros Limestones of Carbonate platform [.- Arenaceous limestone dastics | j Radiolarites (Iran): Acad. Sci. Comptes Rendus, v. 267,

A p. 2272-2275. A A Breccias Marls Ophiolites -*- Thrusts 1970, Comments on radiolarites and ophiol- ite nappes in the Iranian Zagros Moun- Figure 3. Postulated model of the structural evolution of the Neyriz rocks during Late Cretaceous tains: Geol. Mag., v. 107, p. 479^80. time. 1971, Le croissant ophiolitique Peri-Arabe, une ceinture de nappes mise en place au East—eastern Mediterranean region near REFERENCES CITED Crétacé supérieur: Rev. Géographie Phys. et the end of the Cretaceous Period that prob- Géologie Dynam., v. 13, p. 327-350. ably involved a major change in the re- Allemann, F., and Peters, T., 1972, The Stócklin, J., 1968, Structural history and tec- gional tectonic patterns. As plate motions ophiolite-radiolarite belt of the north Oman tonics of Iran: A review: Am. Assoc. Pe- are interlinked across the world, it can be mountains: Eclogae Geol. Helvetiae, v. 65, troleum Geologists Bull., v. 52, p. speculated that Late Cretaceous spreading p. 657-697. 1229-1258. in the South Atlantic caused Africa and Bernoulli, D., and Jenkyns, H. C., 1974, Alpine, Takin, M., 1972, Iranian geology and continen- Mediterranean, and central Atlantic tal drift in the Middle East: Nature, v. 235, Arabia to be driven toward Eurasia, in an Mesozoic facies in relation to the early p. 147-150. early, perhaps the first, stage in closing the evolution of the Tethys: Soc. Econ. Paleon- Wells, A. J., 1969, The crush zone of the Iranian Tethyan Ocean, a process not completed tologists and Mineralogists Spec. Pub. 19, , and its implications: until late Tertiary time. p. 129-160. Geol. Mag., v. 106, p. 385-394. Bernoulli, D., Graciansky, P. C. de, and Monod, Wilson, H. H., 1969, Late Cretaceous eugeosyn- O., 1974, The extension of the Lycian clinal , gravity tectonics, and ACKNOWLEDGMENTS nappes (SW Turkey) into the southeastern ophiolite emplacement in Oman Moun- Aegean Islands: Eclogae Geol. Helvetiae, v. tains, southeast Arabia: Am. Assoc. Pe- I thank the Royal Society for a grant-in- 67, p. 39-90. troleum Geologists Bull., v. 53, p. aid and my field companion H. C. Jenkyns Dewey, J. F., Pitman, W. C., Ill, Ryan, W.B.F., 626-671. for his valuable assistance and his contribu- and Bonnin, J., 1973, Plate tectonics and 1973, Late Cretaceous nappes in Oman tion to many discussions. Considerable the evolution of the Alpine system: Geol. Mountains and their geologic evolution: Soc. America Bull., v. 84, p. 3137-3180. Discussion: Geol. Bull., v. 57, p. logistic and scientific help was provided by Glennie, K. W., Boeuf, M.G.A., Hughes Clarke, 2282-2298. various members of the Geological Survey M. W., Moody-Stuart, M., Pilaar, W.F.H., of Iran and by H. C. McQuillan, P. E. Kent, and Reinhardt, B. M., 1973, Late Cretace- MANUSCRIPT RECEIVED BY THE SOCIETY H. C. Warman, and J. T. O'B. Perry. F. C. ous nappes in Oman Mountains and their NOVEMBER 25, 1974 Dilley and G. F. Elliot identified the mi- geologic evolution: Am. Assoc. Petroleum REVISED MANUSCRIPT RECEIVED MAY 12, 1975 crofossils. Geologists Bull., v. 57, p. 5-27. MANUSCRIPT ACCEPTED MAY 22, 1975

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