Regional stratigraphy of folded belt- Zagros Thrust and evolution of fore-ecosystem h is depended on it

Author Medi Alavai
Published At 15 October 2005


Regional stratigraphy of folded-thrust Zagros belt is revised during late neo-Proterozoic to Phanerozoic along last surveys. That includes four big lithologic groups that each groups includes several big sedimentary sequences (megasequence) which is characterized with a uncontinous border and it results from changes in sedimentary-tectonic position.


Regional stratigraphy of folded belt- Zagros Thrust and evolution of fore-ecosystem  is depended on it Abstract: Regional stratigraphy of folded-thrust Zagros belt is revised during late neo-Proterozoic to Phanerozoic along last surveys. That includes four big lithologic groups that each groups includes several big sedimentary sequences (megasequence) which is characterized with a uncontinous border and it results from changes in sedimentary-tectonic position. First lithologic group, that it has a chronologic area of late to Devonian neo Proterozoic and it includes two megasequences: Late Proterozoic to middle Cambrian rocks include an evaporate megasequence, siliciclastic with some carbonate interlayers which is deposited in a explosive basin of pull a part which results from strike-slip tectonism. These sediments are pertaining to megasequences which is related to middle to upper Cambrian which includes siliciclastic deposits and carbonates in shallow marine regions related to over continental bed sequences. Silt and sandstone rocks are with an origin of volcanic related to Ordovician, Silurian and Devonian which covers former sediments, related to local remnants from erosional stratigraphical units which have created big unconformities extensively. Second lithologic group includes two big megasequences pertaining to Permian and Triassic which includes siliciclastic as transgressive basal which is deposited on evaporate carbonates in shallow marine of epi-pangean. Third lithologic group includes four megasequences composed of shallow to deep carbonates with evaporate and detrital sediments which is deposited during early Jurassic to Toronian. Fourth group includes siliciclastic deposits and carbonate sediments with a trend of NW-SE which is deposited during late cretaceous to recent in Zagros fore- ecosystem which is regarded as a major part of Zagros orogeny. This group include three megasequences of IX, X, XI with lateral and vertical Facieses change which is indicative of special tectonic events in the area. Megasequence of IX includes transgressive and regressive carbonate and siliciclastic sediments in upper Toronian to middle Maastrichtian which is indicative of ophiolite fragments obduction of ophiolitic fragments and Zagros orogeny and megasequence of X includes Paleocene to upper Eocene carbonates and siliciclastic rocks which is first of that, due to decrease in tectonic activities as transgressive in frontier of orogeny prism in Zagros orogeny and then with increase in tectonic activities, due to operation of thrust sheets are as regressive. Megasequence of and I includes carbonate deposits which is deposited in a short term from. Oligocene to lower Miocene after a serious activity course of thrust faults in late Eocene in deformed prism and also it includes a sequence from siliciclastic sediments from lower Miocene to recent which is in direction of NE and to SW ax coarsening upward which includes some products resulted from erosion of Zagros thrust sheets. Introduction: Folded- thrust Zagros belt (fig 1) which extends forward into west and south Iran for a distance about 2,000 km from SE of turkey, north Syria and Iraq with many hydrocarbon fields is one of most important oil reservoirs in thrust – fold belt in the world. This belt which is folded- thrust results from changes in pre-ecosystem structural system where currently Persian gulf and continental basins are from mesopotamide (Liz and Falki, 1925, Porster, 1973, Cassler, 1973, Baltz And Porser, 1990) and subthrusts in the fore- ecosystem and it mainly includes continental and bed sequences. This belt is evoluted as a layered prism from structural aspect from thrust plates which are composed of are Neo-Proterozoic and Phanerozoic that its thickness is 7 to 12km in south western part in the wedge (Alavi, 1991-1994). Oldest systematic studies in Zagros stratigraphy is done by James And Wind (1965) which surrounds only Mesozoic and Cenozoic units. In new surveys, some valuable stratigraphical information was represented, especially we believe that a big part of folded- thrust stratigraphical in a pro-foreland as contemporary continental to developed orogeny that its changes depends on Zagros orogeny with structural and tectonic events. Fore land basins which were introduced and represented by Gilz and Diselz (1996) are considerable for many earth science scientists because these basins a good location for production, immigration and aggregation of hydrocarbon materials that they include some surveyed issues in stratigraphy, sedimentology, structural and geodynamic. Pre-foreland basin of Zagros (which is a special type for pre-foreland basins. For more information  should refer to Veylet et al., 1993, Johnson and Biomont, 1995), as a part of folded- thrust belt of Zagros with its economical nature and this problem is yet remained. Geological surveys in Iran, in Zagros region is near to one before century with exploration of first oil field is begun in the area and some surveys which is done by some of oil geologists in this area and it results in better recognition of geology and contemporary sedimentation to tectonic in this oil zone. For example, Liz, 1950, Falcon, 1958 And 1974, Donington, 1958 And 1976, James and Wind, 1965, Mina et al., 1967, Camenkay, 1970, Zabu and Kheradpir, 1978 and …) In these surveys, thrust-folded belt of Zagros is as pro-foreland  and so there is no imply to more details about stratigraphy and extent in sedimentary zone and relation between sedimentary evolution and tectonic events which results in formation of this orogenic belt. It is prepared in frame of geological maps in last years in internal and external parts in this belt (for example, Alavi, 1994 and 1991, Alavi And Mahdavi, 1994, Ehsan Bakhsh, 1996, Nazari and Shahidi, 1998, Talebian, 1999, Karimi Bavand Pour, 1999, Mahjal And Fergocen, 2000) has represented a better recognition for evolution of this orogenic belt and its depending pro-foreland . Tectonic position Zagros orogenic belt (fig 1) results from three tectonic events as follows: 1.      Subduction of neo Tethys oceanic lithosphere under iran continental lithosphere during early to late cretaceous. 2.      Subduction of oceanic fragments of NeoTethys (NeoTethys ophiolites) on inactive margin of African- Arabian plate during late cretaceous (Toronian to Campanian). 3.      Collision of continental lithosphere of African-Arabian plates to Iran plate during late cretaceous, after that (Alavi, 1994). 4.      This orogenic belt is limited from north by sinistral strike-slip fault to eastern Anatolia (EAF) and to south east by Oman line (falcon 1969) (Oman line is regarded as a transform fault remnant from NeoTethys ocean). This orogeny includes three parallel belts as follows: 1.      Urumieh- dokhtar volcanic association (UDMA): which is a volume volcanic arc and depended on subduction and it is composed of intrusive and extrusive igneous rocks with a toleitic- calc alkaline and alkaline composition rich from potassium (with volcanoclastic sequences) along active margin of Iran lithosphere. 2.      Felsic shape belt of Zagros (Sanandaj- Sirjan zone) (ZIZ): Sanandaj-Sirjan zone, as we implied by Alavi, 1994 after Stocklin, 1998 and 1977 is described and it includes a region composed of thrust faults which has numerous plates from stratigraphical units in metamorphic type from Phanerozoic is deformed and non-deformed from collision location in north east toward south west of internal parts of Arabian craton. 3.      Folded- thrust Zagros belt (ZFTB): That external part is little deformed in Zagros orogenic belt and it includes asset from folded and fault rocks with a thickness of 4 to 7m from Mesozoic to Paleozoic sequences which have been covered by Cenozoic carbonate and siliciclastic rock with a thickness of 3 to 5km. This complex is from Proterozoic on seriously metamorphic bed rock. Result of strike – slip faults is late NeoProterozoic to Cambrian (Brown and Jackson, 1960, Mohr 1819, Agar 1987 and Hosseini, 1988). Current deformation of Zagros frontier is in level of south western limit in this belt (ZDF) and Zagros deformation has not been propagated in south western borders (change frontiers). Stratigraphy Over underground and ground surface wells stratigraphic columns are surveyed in most points of Zagros belt points. Based on these studies and other reports from stratigraphy, sedimentology, petrography, field studies and experimental studies have been mentioned for each stratigraphic unit (not necessary for all of formations). Late Neoprotozoic statigraphy and Phanerozoic folded-Zagros thrust belt is based on tectonic-sedimentary characteristics into four megasequences. Each group includes several megasequences with unconformity orders which is indicative of a unconformity boundary  a separated sedimentary cycle  it includes several stratigraphic units as follows (fig 2). Pull a part basins and bed rocks as over continental from late NeoProterozoic to Devonian? Oldest rock group in Devonian belt is related to Neoprotozoic to Devonian? Which includes non-marine and marine sediments composed of a thick evaporate layer with sedimentary and volcanic beds in the basements which are covered with several unconformities with siliciclastic and carbonate sediments (fluvial sediments) (fig 2) and it includes two big Gondwana sedimentary sequences: Late Neoprotozoic megasequences to Cambrian (megasequence I) which is created from aggregation of sediments in a expansive sedimentary basin, depended on a strike- slip fault of najd, from early to late Precambrian in north east of. Related maps to middle and upper Cambrian (Mila Fm.) are in shape of mainly regressive and transgressive sequences and carbonate sequence (megasequence II). With a distinct discontinuity from lower sedimentary sequence is separated which includes orthoquartzite and quartz andesite which is deposited on continental bed. Sedimentary-tectonic position is uncharacterized from late Cambrian to before Permian. Presence of some layers with volcanic origin in younger stratigraphic units (for example in Zardkuh) is indicative of its relation to develop paleotethys ocean which is located in Alborz belt, located in north Iran in before Paleozoic (Alavi, 1966). Two sedimentary units are before Permian and they have main role in structural and stratigraphical evolution. First unit is evaporate unit in late Neoprotozoic Cambrian (Hormuz series) which is currently as an important separating fault in evolution of folded belt Zagros structure and a diapirism structure pattern of salt is effective in the region (for example cont, 1958, falcon, 1969, Talbot and Joroisi, 1984, Alavi, 1994, Talbot and Alavi, 1996). Second unit is Silurian graptolite shales (Gahkom Fm.) which is an origin for oil in Permian- Triassic hydrocarbon reservoirs (Bordnav and Boroud, 1990). Beds on Permian to Triassic Pangaea Late NeoProtozoic to Devonian layers with a unconformity is composed of sandstone and a conglomerate covering of a bed on Pangaea, pertaining to Permian to Triassic (Zabou and Khradpir 1978, Sharif, 1982, Ghavidel Sivaki, 1988). Permian-Triassic sediments are deposited in a shallow and equatorial sea and it includes two megasequences of III and IV which are separated by an unconformity in Permian-Triassic border (fig 2). These two big megasequences play role for biggest reservoirs for gas in the world. Permian sedimentary sequence III are deposited consistently with sea transgression from north east on Pangaea bed, Pangaea platform and it includes basal red conglomerate, arkose-bearing and quartzite sandstone. Upper Permian sediments are with dolomite layers and evaporate deposits (in south west) and it is accompanied with full energetic limestone to north east and it is extensive laterally eastward and north eastwards which is metamorphic currently and carbonate rocks and clastic rocks, full of fusulinide in felsic shape Zagros zone (Sanandaj-Sirjan) by Alavi 1997 and it is as Mahdavi, 1995 in central Iran in the map. Megasequence IV is with transgressive sediments with north eastern part includes interlayers of evaporate and dolomite with calcareous layers of shallow seas but it is with low energy. An equivalent metamorphism is recognized with late Triassic unconformity in fels shape zone of Zagros, this IV sequence is separated from upper deposits related to Jurassic continental terraces up to cretaceous. This unconformity, with extensional activities with extrusive-intrusive igneous and metamorphic rocks is recognized in felsic shape zone of Zagros by Tous Alavi, 1994. In folded-thrust fold belt, this unconformity is very distinct and it is accompanied with erosion and decrease in Lorestan and Khuzestan in thickness of dolomite and evaporates from Triassic are about 250m rather than other areas. This unconformity is attributed to opening NeoTethys ocean. Basin which is mainly to north east during continental margin. Jurassic to late cretaceous continental terrace sequence in NeoTethys Jurassic bed to upper Toronian (based on James, 1965, Khardpir, 1975, Khalili, 1976 And Shakib 1990) are composed of several megasequences (V to VIII) which are aggregated in a shallow terrace environment and it covers former sequence and former sequence (Permian-Triassic bed) (fig2). This lithologic group is deposited through north, north east towards NeoTethys sea in an equatorial weather. Sedimentary sequences from V to VIII, bed rock and cap rock of oil reservoirs are formed and they are separated by erosional unconformity and small gaps are separated from each other. Lowest unit of this group, Neyriz Fm. is of early Jurassic age and it includes argillic and detrital sandstone with sedimentary origin towards northeastern which is likely due to deep operation resulted from continental depression in time of opening of NeoTethys ocean. Covering units are as some sequences of V to VIII and they have change in some Facieseses gradually in south west from shores, lagoons, evaporate, clastic and dolomitic layers and north east of shallow waters, full energetic environments to deep seas, pelagic, clay limestone to marl. It is in various stratigraphic levels with characterized Facieses change and it seems that it a result from reactivation of structural elements before existence and they are mainly with trend of N-S under pan- African- Neoperotozoic. For example, argillites and carbonate rocks are in Fahlian, Gadvan and Darian in middle part of belt with a thickness of about 750m (in Khuzestan province) while it is south eastwards (in fars province) and their thickness reaches to 250 to 300m. These changes are famous all over lineation with a trend of N-S in famous lineation of Qatar-Kazerun that its characteristics has been mentioned by falcon, 1969, Petinson and Takin, 1971 and baker et al., 1993 are explained. Pro-foreland basins, pertaining to late Toronian to Recent Continental terrace sequence is as unconformity to continental and marine sediments pertaining to upper to recent cretaceous which mainly includes siliciclastic deposits, carbonates in shallow depths to deep marine and evaporate depths and they have been covered by north eastern sediments. This Facieses change and thickness is related to pro-foreland  basin with a mend basin with a trend of NW-SE in frontal regions of orogenic regions during Zagros. Pro-foreland  sedimentary sediments includes sedimentary sequences of 1X to X1 as a part of pro-foreland  system in folded-thrust belt in Zagros that it is remnant of erosional thin layers from pro-foreland  deposits by Alavi and Mahdavi, 1994 and Alavi 1997, Localy In Zagros Fels Shape Zone (Sanandaj- Sirjan) is mapped. To south western wards, sediments in this basin are mainly as overlap and laterally during unconformities as thin and they are finally ruined. Siliceous thickness in pro-foreland siliciclastic thickness is measured in pro-foreland from type section of various stratigraphical units, approximately 6480m and marine carbonate sediments in deep basin points about 1940m. through north eastern basin and inside of siliciclastic sediments without any economical hydrocarbon but it is in deep and low energy parts and also shallow regions and full energetic regions of basin towards beach (towards south west), source rock and oil reservoir rock. Evaporate rocks, calcareous rocks in deep regions of the sea, marls and clay limestone, hydrocarbon reservoir cap rock are formed. Pro-foreland megasequences Three megasequences of XI to IX are parallel to general trend of folded thrust belt are extended that it shows each Facieses change sequence as lateral and vertical. Four stratigraphical profiles of the belt (fig 3) is with stratigrapical relation in pro-foreland basin. In this section, latitude profile is balanced in structure of position in primary stratigraphical columns before Zagros position is reconstructed deformation. General stratigraphical relation is between regressive and progressive aggregation in various pro-foreland sedimentary zones is shown in figure 4. Structural interpretation indicates this reality that resulted load from sedimentary aggregation of them in a basin depends on thrust faults in Zagros orogenic belt.  Oldest sedimentary sequence in Zagros Pro-foreland is of late Toronian to middle Maastrichtian in formations of Sourgah-Sarvak – Ilam- Gurpi- Amiran and Tarbur (bon can see appendix index) as a regression and transgression limited to an unconformity (fig 3 and 4). Basal unconformity is seen as a hematite discontinuity which is accompanied with basal conglomerates as locally and it shows a gap from 4 to 15 million years south westwards. This hiatus is as marls from Gurpi of Santonian age (about 84 may) and Sarvak Rocks are seen from Albian to Cenomanian. Northern west of Sarvak Fm. which is limited to central parts that middle and upper parts of Sarvak is ruined and Santonian shales covers Sarvak Fm. in lower part with an iron-oxide weathering surface (likely related to Albian). In Khuzestan, Ilam limestone covers Sarvak lower basement with some iron-oxide surfaces abruptly. In Lorestan, Sarvak Fm. is covered by Sarvak limestone by iron-oxides discontinuities as limonite on shales of Santonian age. In Lorestan and Khuzestan, Pabdeh and Sarvak Fm. are of Paleocene to Eocene with a thickness of 1 to 3 m from shale and silt and then shales in parts of Gurpi Fm. covers by phosphorite and glauconite (fig 3) which is indicative a unconformity surface. Based on paleontology information, presence of hiatus with 10 m.y indicates a sedimentary gap. In north eastern parts of Khuzestan and Lorestan provinces, unconformity as a distinct contacts between Shahbazan dolomite (Paleocene) and carbonates in Tarbur Fm. (upper cretaceous, Maastrichtian). Towards south east of Fars province, unconformity has same former characteristics (fig 3 as profiles a-c, b-c), with this difference that local erosion has ruined all of limestone in Pabdeh Fm. from late cretaceous and Asmari carbonate. So vassal deposits of Asmari Fm. are perched directly glauconitic and weathered shales of Cazhdumi Fm. (of Albian age), where Pabdeh Fm. is substituted gradually by Jahrum dolomite. Unconformity is as an weather surface of hematite with a thin layer of shale and sandstone with glauconite or argillic limestone. To northwards, in all of belts, unconformity separates fine detrital shapes and carbonates from Sachun Fm. (late Maastrichtian to Pliocene) and or Kashkan conglomerate from Tarbur limestone. Locally, location of Sachun Fm. is ruined and instead Sachun separates carbonates from Tarbur with an unconformity. Towards north eastern of Zagros folded-thrust belt, in thrust sheets as fels shape (Sanandaj- Sirjan zone); ophiolites have been deformed on Sarvak Fm. or carbonates equivalents that in all of parts of Zagros thrust- folded belt, conglomerate and thrust folds are on Amiran Fm. which have been transported from Kashkan Fm. as a siliciclastic wedge from southwestern side with an alternation of grey- green shale sequences and dark grey shales (dark) are interfingering in this formation. Towards south west of Amiran Fm. and mudstones rich from calcareous fossils of globotruncana fossil in Gurpi Fm., is more with fossils in fauna characteristics of deep marine limestone in Amiran Fm.. In external parts, Zagros folded- thrust belt is near to change frontier of Zagros (fig 1). Gurpi Fm. covers Ilam Fm. limestone with an unconformity boundary, by shallow marine limestone in Ilam Fm. that this formation is as erosional unconformity as interformational on Sarvak mudstone and wackestones. Towards, south west of Sourgah, it is turned gradually, to equivalent of Lafean Fm. (lowest part of aroma) in UAE and Qatar, Bahrein and it converts to Khasib and Kuma in Kuwait and south west Iraq (fig 3). These units loaded older beds, its resulted load results in movement of them and locating of them against to a minor unconformity results in well recognition in Persian gulf and Saudi Arabia. In upper stratigraphy surfaces (Maastrichtian, Gurpi Fm. is covered by Amiran carbonates as isoclines and isotope (normal). Tarbur Fm. has extended in Zagros zone as lateral with diversion of Facieses, in zone of Sandandaj- Sirjan. In external belt, Tarbur fm. includes some formations like thin layer pelagic limestone and hemi-pelagic limestone and argillic-limestone mudstones rich from fossils of planktonic organisms. In this part of area, a gradually boundary to lower formation, have Gurpi. These formations are almost without any characterized belt. Toward north east, Zagros folded-thrust belt is situated to Amiran Fm. as unconformity or ophiolitic complex (with a bed of polymictic conglomerate with a thickness of 2m) and or Amiran Fm. and composed of shallow limestone in marine regions with biologic riffs in shallow and energetic regions, inside of this belt, Tarbur Fm. carbonates are as interfingering to Sachun Fm. shales and with dolomite in lower part of this formation. In external part of this belt, anyway, erosion has ruined upper part of tarbur Fm. from lower part late Maastrishtian to lower Pliocene. Relation between this unit to upper units is uncharacterized, from Ilam and Surgah fm., likely this unit is towards two units of Tarbur and its trial to north eastern parts (fig 5). Development of sedimentary sequence is towards late cretaceous (XI) as emplacement of NeoTethys ophiolites on Arabian- African separated margin plate as a trail of primary collide Iron plate with western plates to south western parts (fig 5, a and b). Emplacement of ophiolites and thrust plates on African- Arabian plate, has caused a considerable load on that results pro-foreland with development of sedimentary basin in fore- blogue and back blogue in their frontiers. Stages of late emplacement have begun from Santonian (late Toronian) and it extends forward into Maastrichtian (a period about 15 m.y). Erosional products with thrust Zagros plates results in accompanied with overgrowth plates in the fault and formation of Tarbur and Facieses of sandstone conglomerate and a sequence from shale and sand (flisch during this period. Hemi - pelagic sediments related to Asmari carbonate Fm. in marine shallow regions related to Ilam Fm. in fore blougue argillic limestone in Surgah (or Lafean) in back- bulge are aggregated. Pebels and Amiran Fm. fragments indicate location and establishment of faults in Amiran and Surgah Fm. indicative of transportation of south western parts (fig 4). During middle Maastrichtian, carbonates are with Facieses in back- bulge environment are related to Tarbur Fm. towards fore bulge side (towards northwest sides) and they are equivalent to carbonates in shallow and deep depths and progressive of outcrop deposits and Amiran siliciclastic sediments in this formation (fig 5,b). Towards north east, regressive transportation to fore deep and accompanied sedimentary basin in pro-foreland basin are as serious of plates operations of thrusts and loading in length of African- Arabian continental Edge with beginning of collide to Iran plate is interpreted. So this event is resulted from first orogeny collide to Zagros orogeny and it seems related to middle Maastrichtian (about 68 m.y). Late Maastrichtian sequence to late Eocene (35 to 65 m.y): Second sequence in pro-foreland (sequence of x) is uncontinous on late cretaceous sequence (megasequence of IX) is covered and it includes Sachun, Kashkan, Shahbazan, Pabdeh and Jahrum (fig 3 and 4). From north eastern to south western side, upper cretaceous beds are from middle Eocene with progressive Facieses while middle to upper Eocene has several regressive Facieses. Lowest unit (Sachun Fm.) is limited to several Facieses. But it can be as a source for lowest unit (Sachun Fm.) is limited to internal parts in north east of Zagros folded belt and it includes evaporites and dolomitic interlayers with shale and sandstone which are in contact to Kashkan Fm. toward north east and they are mainly sandstone and argillic as interfingering. Kashkan Fw. Is with conglomerate and coarse- grained sandstones and it includes shale interlayers with fine- grained sandstone in south west which is covered by Shahbazan Fm. dolomites as interfingering to limestone and dolomites of Shahbazan Fm. Shahbazan Fm. is with gradual formation, between fluvial siliciclastic deposits from Kashkan Fm. and Facieses of deep waters in calcareous mudstone – calcareous shales and marls in Pabdeh Fm. is extensively with Shahbazan Fm. carbonate towards north eastern part and it covers Kashkan conglomerate from Shahbazan. It is near change frontier in current structure of Zagros, upper parts of sedimentary sequence, from carbonates and argillics in deep marine regions from Pabdeh Fm. which is covered by Jahrum dolomites which is indicative of immigration towards north east in the basin. Apex of this sedimentary sequence (X) cut by minor regional discontinuity (fig 3 and 4). Discontinuity is seen well in border of Jahrum – Asmari in external parts of the belt. Which is as heterogenous beds from hematitized dolomite and they remind conglomerate bodies. Uppermost part is Eocene deposits in Jahrum fm. due to erosion before deposition of Asmari Fm. and lower parts (lower Oligocene) is not present currently. Towards western parts, near to change frontiers of Zagros, where Jahrum dolomites are covered by sandstone. Stratigraphical information are underground of early Miocene age for lowest parts of Ahwaz Fm. parts and middle Eocene age is suggested for u