Author Name: Medi Alavai
Publisher Date : October 2005
Title : Regional stratigraphy of folded belt- Zagros Thrust and evolution of fore-ecosystem h is depended on it
Description :
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.
Full Description:

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.

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