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Lithologic signals of uplift and relaxation phases of Zagros Belt during Maastrichtian and Paleocene: Examples from Kurdistan Region, NE-Iraq
BAKHTYAR M. AMEEN
Published in GRMENA 3
In this study, the lithologic representation of the uplift and relaxation phases of the Zagros Fold-Thrust belt is studied during Maastrichtian and Paleocene. The study concerned with northeastern Iraq which includes important part of Western Zagros mountain belt. The sedimentology of this part show two main tectonic uplift phases during Lower Maastrichtian and Lower Paleocene. Both uplifts have followed by relaxation phases during Middle-Late Maastrichtian and Upper Paleocene. The uplift signal consists of very thick successions of the sandstone-shales (flysch facies) and conglomerates-red claystone (molasse facies) at basin plain and marginal areas (proximal area) of the foreland basin respectively. Representing lithologies of the relaxation phases consist of thick succession of marl (hemipelagite) and shale at the proximal area and distal part of the basin respectively. The thickness of some of these above successions is more than four hundreds meters thick. The boundary between above high and low energy lithologies are sharp and showing rapid relaxations and uplifts. Now the area of the study is located in front (directly to the southwest) of the present Zagros Main Thrust and Reverse Fault which covered during the above ages by foreland basin. All the above ltihilogies now organized, in literature, into three stratigraphic units under the name of Tanjero, Kolosh Formations in addition to Red Bed Series. During Maastrichtian and Paleocene, the contrasts of the tectonic signals (sediments) are clear and high but those of the post Paleocene are weak and not clear. This attributed to concentration and release of stress along narrow strip near the Iraq-Iran Border while during post Paleocene the stress distributed and releases through wide are extended from Sanandij Serjan to the south of Sulaimani City. This is attributed to migration of the stress along low depth decollement thrusts toward southwest.
This paper is conducted in the northeastern Iraq near the Iran-Iraq border which includes both a well known Zagros Suture and Folded Zones of Northeastern Iraq (Fig.1).The area was part of the New-Tethys where the oceanic parts of Iranian and Arabian Plates were collided during lower Cretaceous while colliding of the continental part till now controversial. But some geologist agree that the colliding of the continental parts was occurred in the Eocene such as Buday (1980), Buday and Jassim (1987) and Numan (1997, 2001a, 2002) and Al-Qayim (2000). They cited that the Iranian plate thrusted over Arabian plate and a foreland basin is formed at Eocene. The idea of this colliding at Eocene is based on disappearance of the flysch deposits and beginning of deposition of the continental red beds molasses. The same age is also assigned for collision of Indian and Asian plates from Himalaya Mountain by De Sigoyer et al. (2000), Klootwijk et al. (1992), Searle et al. (1997) and Najman et al (2005).
The ideas of the colliding of the continental parts in Eocene are changed recently to Early Maastrichtian by Karim (2004) Karim and Surdashy (2005 and 2006). Therefore, in the present study the latter ideas are more applicable on the studied area as concerned to the interpretation of the uplift and relaxation phases during Late Cretaceous and Early Tertiary (Fig.2). During Early Maastrichtian (or late Campanian), the southern Neotethys are closed and the area of the Sanandij Sergan area (inside Iran and near to Iraqi border) has uplifted above sea level and a positive lands are generated in between the two plate after colliding (Fig.3).
Fig.(1) Simplified geological map of the studied area (modified from Sissakian, 2000) .
Early Maastrichtian: First phase of the Uplift
The generation of the positive land by stresses of the colliding of the two continental parts is the first prominent uplift phase which transformed the uplifted area into positive land (source areas). According to Allen and Allen (1990) this period is called active thrusting by which a clastic wedge is deposited. This uplift phase is reflected by deposition of 500m of boulder and pebbles conglomerate at proximal area (near the source area) (Fig.4). About 400m of sandstone–shale is derived from this conglomerate and deposited in deeper part of the foreland basin by turbidity currents and deposited as flysch facies (Fig.5). These sediments are now know as Lower part of the Tanjero Formation which described, in literature, in detail by Bellen (1959), Buday (1980) and Jassim and Goff (2006). This author indicated the formation as sediments of deep trench (or forearc basin) while Karim et al (2007) and Karim and Surdashy (2005) inferred that it deposited in a foreland basin with both deep and shallow environments.
Fig.(2) A) Carton show of the four tectonic phases of uplift and relaxation of Zagros Fold –Thrust belt during Maastrichrian and Paleocene with lithological representation as shown by stratigraphic column the two ages of each phase (B).
Middle-Late Maastrichtian: First phase of the Relaxation
After deposition of the signals (conglomerate and sandstone-shale) of the Early Maastrichtian Uplift phase of the Tanjero Formation, a sudden change of the lithologies can be seen at the top of the both later lithologies. At the top of the conglomerate the lithology changes to 50meters of red claystone and 80meters of reefal limestone in Marginal are of the basin (proximal area) while the sandstone-shale (at the basin plain) suddenly changes to marl (hemipelagite) (Fig.2 and 3). These sudden changes from coarse grain rocks to fine ones is relatively denoting sudden subsidence of the source area and part of the basin by relaxation of the uplifted and deformed sediments in between the two plates. In this phase the ratio of the coarse clastics is decreased by more than 80% and the deep water sediments transgressively overlain the shallow and coastal ones. According to Allen and Allen (1990) this period of quiescence is due to visoelastic relaxation. The sediments of this phase are now known as Middle and Upper part of Tanjero Formation. They are studied by Karim (2004) and Karim and Surdashy(2006) whom divided it into transgressive and high stand systems tracts.
Early Paleocene: Second phase of the uplift and thrusting
The hemipelagite of the Late Maastrichtian is overlain by coarser clastic sediment at the boundary between Maastrichtian and Paleocene. These sediments represented by sandstone-shale, in the basin, and conglomerate with red claystone in the coastal area. The thickness of these sediments reaches more than 1500meters. These rocks are signifying the second uplift phase of the Zagros Orogenic Belt. These rocks are classified stratigraphically as Lower part of Kolosh Formation and Red Bed Series respectively. The representative lithologies of this phase are nearly similar to that of the first phase except for dividing them into two separate units, i.e Kolosh Formation and Red Bed Series. Although these two units are time equivalents, yet their depositional basins are separated, in literature, in to two different basin by Buday (1980), Lawa (2004) and Jassim and Goff (2006). According to these authors the two basins are separated during Paleocene by positive land that was coincide with the present position of High and Imbricated Zone of Iraq. But Barzinjy (2005) and Karim et al (2008) showed by lithological correlation and field study that the two units are deposited in one foreland basin as coastal facies (Red Bed Series) and basinal facies (Kolosh Formation). Sharbazery (2008) showed by planktonic forams biozonation that the Cretaceous/Tertiary boundary, in the distal of this basin is continuous in the sedimentation which contain several biostratigraphicaly important species as shown in fig (7).
Fig.(3) A) Closure of southern Neotethys during Lower Maastrichtian which is associated with uplift of sedimentary rocks. B) Subsidence of the source area and foreland basin (relaxation phases) during Middle Maastrichtian. The above phases are applicable also for uplift and relaxation phases of Paleocene (Only the location Tectonic front moved more southwestward).
Fig.(4) Sirwan Valley outcrop section (2km south of Halabja town) shows the stratigraphic units at distal part (basin) of Zagros foreland basin showing the relaxation and uplift phases.
Middle-Late Paleocene: Second phase of the Relaxation
This phase is represented by the lithology of the Middle part of Kolosh Formation and the upper part of the unit two of the Red Bed Series. These parts consist of hemipelagite and shale respectively which signifying the occurrence of the second phase of the relaxation. But the amount of subsidence and deepening are lesser than the first phase. The thickness of the hemipelagite within the Kolosh is about 200m while in the Red Bed Series it has less than 30m which can be seen at the southeast of the Qalachualan village.
Fig.(5) 1 and 2)Thin section photos of sandstone of Kolosh Formation which consist of chart (grey), bitumineous limestone ( grey or black) and quartz (white) grains at distal part of basin. 3 and 4) rip up clasts and graded bedding of conglomeratic sandstone of Tanjero Formation at distal part of the Maastrichtian Foreland basin showing high energy during uplift phases.
Concentration and dispersal of tectonic stress
It is evidence from the above phases that the tectonic forces that associated with southwest advance of the tectonic front of the Iranian plate (as apart of Zagros Orogen) are concentrated in the narrow belt. This belt coincides with present position of the Iraq-Iran border and Sanandij Serjan areas. Due to this concentration the uplift and relaxation signal is showing high contrast as represented by the shallow and deep facies. The contrasts in the post Paleocene rocks ( signals) are weak and show no clear evidence of uplift and subsidence although some week signals present. These week signals are most probably attributed to dispersal of the tectonic force and its distribution on large area by partial migration of stress toward southwest along shallow decollement thrusts surfaces. During post-Paleocene ages, the stresses were distributed on an area which was twice larger than the earlier ages. This area was extended from Sandij Serjan to the boundary between high and Low Folded Zone to south of Sulaimani City. This migration causes many irregularities and splitting of the main foreland basin into two basins which is shown schematically by Karim et al (2008) (Fig.9). One of these weak signals of post Paleocene ages is mentioned by Ameen and Karim (2007) during Middle Miocene in Lower Fars Formation. They showed occurrence of possible turbidite (with tempestite) in the middle part of the formation.
Fig.(6) Planktonic existence as indicators of the continuous sedimentation at the Cretaceous/Tertiary boundary(Sharbazery.2008).
Fig.(7) Concentration and dispersal of the tectonic stress of the Zagros thrust front during Campanian-Middle Eocene. A) Dispersal of stress by shallow decollement thrust. C) Concentration of tectonic stress on the narrow belt has generated one large foreland basin with strong signal of Uplift and relaxation (Modified from Karim et al (2007).
1-Two Phase of uplift and relaxation are described from Western Zagros of northeastern Iraq during Maastrichtian and Paleocene.
2- Strong signals of these processes are shown by sudden change of the of high and low energy sediments.
3- Strong signals can be seen during Late Cretaceous and Paleocene when the tectonic stress was concentrated on narrow belt along Iraq-Iran border.
4- The weak signals can be seen during post Paleocene ages when the tectonic stresses were dispersed on large area of the Iraq interior.
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