Due to the fact that the Strait of Hormuz area that separates the Persian
Gulf from Oman Sea is surrounded by four major structural-stratigraphic
provinces: (1) the Zagros fold belt to the North-West, (2) the Arabian
platform to the Southwest, (3) the Makarn basin to the east and (4) the
Oman Mountains and Musandam Peninsula to the South (Fig.
1), the Strait of Hormuz area is considered the most complicated region.
Furthermore, Oman and Zagros Orogeny had a significant role in complexion
of area structure and triggering Cambrian Hormuz Salt. This, in turn,
led to deformation of Gurpi and Pabdeh Formations dated as upper Cretaceous
and lower Paleocene.
Understanding how the complex deeper upper cretaceous formations structures
created are still somewhat ambiguous. Therefore, the study try to describe
tectono-depositional structures resultant of Cretaceous, Tertiary and
Miocene tectonic activities for deciphering the geological history of
the Strait of Hormuz area.
High quality 3D-seismic data covered the offshore
area of Strait of Hormuz in a regular grid.
||Structural elements map of Eastern North part of Arabian
plate included Strait of Hormuz area
Interpretation of 3D-seismic
data and restoration technique could clarify structural imaging and retort
some structural ambiguities in the study area.
Restoration is a suitable technique for validation seismic interpretation
and better understanding of complex structures through analysis tectonic
events that occurred in Cretaceous until recent time in the study area.
Balanced cross-sections can be restored so that the beds are placed back
into their depositional, pre-deformational position. Balanced cross-sections
link the deformed and undeformed states; therefore, finite strain analysis
can be performed, which can be used as a predictive tool for fracture
distribution and orientation. Furthermore, a balanced structural model
validates the seismic interpretation through restoration to a depositional,
pre-deformational state achieved by applying certain geometric rules.
And it promotes a better understanding of the geological history of the
REGIONAL TECTONO-DEPOSITIONAL SETTING
The study area locates in the Eastern part of Persian Gulf and is a part
of the Zagros Fold-Belt (ZFTB). The belt is bounded to the NE by the Main
Zagros Thrust Fault (MZT) and to the SW by the Persian Gulf, which represents
its present-day active foreland basin. Topography along the Northern shoreline
of the Persian Gulf represents the present day Zagros deformation front
The complex geodynamic history of ZFTB is summarized in five stages:
(i) platform phase during the Paleozoic, (ii) rifting during the Permian
and Triassic, (iii) passive continental margin of the Neo-Tethys ocean
in the Jurassic-early Cretaceous, (iv) ophiolite emplacement dated as
late Cretaceous and (v) collision and crustal shortening since the Neogene
(Sherkati et al., 2006; Abdollahiefard et al., 2006).
During Permian, a regional shallow marine transgression with basal coastal
clastics covered the entire region (Jahani et al., 2007). Vertical
movement of salt bodies affected this Permian basin. As, there are evidences
for thinning of Permian Dalan Formation at crest of salt related structures
in central part of the Persian Gulf based on seismic profiles). Movement
of Hormuz Salt continued up to present time. The climax of salt movement
in the Persian Gulf area is Turonian time as the thickness of the Upper
Cretaceous Sarvak Formation varies tremendously based on seismic profiles
and well data. In addition, compressional events of Oman Orogeny in Late
Cretaceous-Early Tertiary time (ophiolites emplacement in Oman; Glennie,
2000) affected the study area (folding and thrusting of Upper Cretaceous
Gurpi and Lower Tertiary Pabdeh Formations beneath the pre Mid-Miocene
base Guri Unconformity surface). Continental collision probably began
in late Paleocene-Oligocene at the Northern promontory of the Arabian
plate (Yilmaz, 1993) and propagated Southeastwards into the Lower Miocene
(Sherkati et al., 2006), creating a considerable unconformity surface
which middle Miocene sediments overlain eroded Paleocene-early Miocene
sediments. Growth strata of Mid Miocene Mishan and Upper Miocene-Pliocene
Aghajari Formation are attributed to the main phase of Zagros folding
(Sherkati et al., 2005; Abdollahiefard et al., 2006).
SEISMIC DATA ACQUISITION
The seismic survey in the study area consisted of 192 prime 3D lines
shot with a full fold of 62. The acquisition bin geometry was 6.25x25
m. The sail lines were shot using a two gun and six streamer configurations.
Streamer cable length was 4600 m and each streamer recorded 368 channels.
Recorded lengths recorded during these surveys were generally 6 sec. Sample
rate was 2 msec.
THE SEISMIC INTERPRETATION OF STUDY AREA FROM MIOCENE TO RECENT TIME
The seismic data of the study area shown on seismic profiles (Fig.
2, 3), illustrate presence of two main horizons represent
two regional unconformities occurred in Miocene and Touronian epoch are
respectively Guri Unconformity and Bangestan Unconformity (equivalent
Wasia Group in Oman area).
Map of the study area showing Hormuz and Larak salt-related
islands and a hidden salt diapir and also showing parallel NE-SW trending
graben and horsts and fault zones discovered by seismic data
||(A) Uninterpreted and (B) interpreted seismic profile
showing different formations of the study area. Location of the seismic
profile is shown on Fig. 2
The latter event has simultaneously formed with Oman Orogeny, whereas the first event happened
at onset of Zagros Orogeny. Both of the unconformity surfaces have been
tied to HD-1 well located approximately 21 km north of the seismic profile
(Fig. 2, 3).
The shallowest Guri base Unconformity surface consists of limestone rock.
Therefore, it is a strong and characteristic reflector. The event is created
by a drop in impedance at base of Guri base Unconformity surface covering
the Pabdeh Formation (marls and shales) underlying, creating a strong
trough; this is made stronger by a thin high impedance interval just above
the Padbeh. Guri base Unconformity surface constitutes base of Mid-Miocene
The Guri base Unconformity surface is the most conspicuous seismic reflector
mapped in the study area. It clearly shows the deep truncation of older
strata. Nearly all the deeper reflectors and associated sediments have
been truncated, forming an evident angular unconformity. In general, the
unconformity surface dips toward the East and the hiatus increases in
the same direction. In the Eastern most part of the area, at the boundary
to the Oman Sea, the unconformity has been tied to the Musendam Unconformity
(Fig. 1, 5).
Wells data in Strait of Hormuz area denote to the amount of truncation
increases eastward. For example, on the HA-1 well located approximately
14 km Southwest of the seismic profile (Fig. 2, 3);
the upper part of the Eocene Pabdeh Formation is eroded. Eastward, on
the HD-1 well to the Northeastern part of the study area, the erosion
becomes more severe and nearly all of the Pabdeh Formation is eroded.
Furthermore, overlying Gachsaran Formation disappears completely (Fig.
In the South Eastern most part of the study area, the erosion is less
severe. Oligocene-lower Miocene Fars Salt has been seen beneath Guri base
Unconformity surface which seems completely displaced and depleted and
led to creation of salt welds due to vertical movements of Zagros events
(Fig. 4, 5).
(A) Uninterpreted and (B) interpreted seismic profile
showing channels in Mishan Formation. And also displaying Fars salt
diapir dated as Oligocene below Guri unconformity surface. Location
of the seismic profile is shown on Fig. 2
Showing the important tectonic events affecting on Hormuz
area that occurred in all Mesozoic and Cenozoic and also displaying
stratigraphic correlation for the Iran- Oman (Michaelis and Pauken,
These welds appear that
had an important role in forming minibasins within Mishan Formation the
container Mass Transport Complexes (MTC) resultant of submarine turbidity
The sediments thickness the covering the Guri base Unconformity surface
composite of limestone, marls and minor shales of the Mishan Formation
and shales with minor limestones and sandstones of the overlying Aghajari
Formation varies from less than 350 m (300 msec of two way time) in the
Western part to more than 2000 m (1.8 sec of TWT) in the East. These formations
contain plenty of channels with turbidites which is believed that were
created by different factors such as salt tectonic and sea level variations.
The sea level variations have major effects on rate of sediment supply
and type of sediment and subsequently affect on creation turbidity currents
and sedimentation rates. A relative rise in sea level creates increased
accommodation space within the shelf and slope region, whereas a relative
drop in sea level decreases accommodation space within the basin. Locally
confined accommodation can also be generated by syn-depositional tectonic
activity. The salt tectonic also plays a major role in creation of accommodation
of deposits in this area and subsequently, formation of turbidites.
Channels with evident erosional bases occasionally occur internally within
the interval. Some of the scoured features display internal large scale
sigmoidal cross bedding. These beds, which form in the direction from
western north to eastern south, may have originated from submarine progradation
or, more likely, as channel fill by lateral accretion.
In most the study area, the internal reflections of Mishan and Aghajari
Formations the deposited simultaneously with Zagros events are parallel
or semi parallel, (Fig. 3). Besides, the faults intra
the aforementioned formations range from normal faults with small throws
originated by salt solution in crests of salt diapirs to very small faults
and fractures originated by turbidity currents intensity. This indicates
to the sediments are gently folded by Zagros Orogeny. In other words,
Zagros events have had very little effect on these formations.
THE SEISMIC INTERPRETATION OF STUDY AREA FROM LOWER CRETACEOUS TO
LOWER MIOCENE TIME
Turonian Unconformity interpreted as top Bangestan group shown on seismic
profile (Fig. 3) is picked on a black peak it represents
the transition between marl and shale in the Gurpi Formation to the limestones
of Sarvak Formation (Bangestan group). This transition is characterized
by a large increase in acoustic impedance. The horizon is also a characteristic
and strong seismic reflector in most of the study area, but presence of
a lot of multiples, makes the interpretation difficult, especially below
The nature of the sediments between the two regional unconformities (Guri
and Turonian Unconformity surfaces) is speculative and is probable to
be Eocene Pabdeh, Upper Cretaceous Gurpi marls and shales or a combination
Ophiolite obduction (Oman Orogeny) that took place during much of late
Cretaceous time affected the on sedimentological pattern of Gurpi Formation.
In addition, another related factor that caused local variations in depositional
environment and thickness were halokinesis related to Cambrian Hormuz
Salt (Fig. 3, 5). Hence, The significant
compressional and extensional tectonic movements of Oman Orogeny played
an effective role in creation of complex structure of the Hormuz area
particularly, the interval intercepted between two unconformity surfaces
through making of reverse, thrust and normal faults and these features
had a vital role in salt flow activations from Cambrian Hormuz Salt Formation
and also acted as conduits for active diapirism.
The faulting seen in some zones over top of Bangestan of the study area
shows clear indications of intense compression and lateral movements related
to the Late Cretaceous-Early Tertiary event of the Oman Orogeny (Fig.
In the study area, where this thrusting appears, there is a chaotic zone
in the Pabdeh/Gurpi interval, where some characteristic and strong intra
Gurpi reflections seen elsewhere suddenly disappear. It seems that the
iterated lateral compressional movements of Oman Orogeny occurred along
some of the faults in soft Pabdeh/Gurpi Formations the eastern part of
the area, led to squashing rocks and creation chaotic zones (Fig.
3). Besides, the late compressional events with rejuvenation of thrusting
related to the Oman Orogeny had a major influence on the structuring of
the study area. Moreover, presence harpoon type structures across the
faults reveal inversion tectonics in this area. The original movement
in these faults is normal, but compressional movements forced them to
act as reverse faults at later stages.
PALINSPASTIC ANALYSIS OF SEISMIC PROFILE
The seismic data and the depth-converted geoseismic section used as input for
the restoration are presented in (Fig. 3). Due to intense deformation
of the overburden overlying Turonian unconformity, the geoseismic section was
restored to its paleostructure in four stages, from upper Cretaceous (Turonian
Unconformity) to lower Miocene (Guri base Unconformity), using 2D move software
(Fig. 6). The geoseismic section was balanced by fault parallel
flow and flexural slip unfolding methods.
Restoration of seismic profile in the Fig.
(using 2D move) carried out in four stages from Turonian to
present Time. The restoration panel is a schematic representation
of deformation in the Pabdeh and Gurpi Formations above Turonian Unconformity
The restored section was decompacted
at each stage to investigate scenario for the structural evolution of the eastern
part of Persian Gulf. The model describes tectonic activities which occurred
after Turonian time and also extrapolates scenario of Cambrian Hormuz Salt movement
and creation of salt diapirs in this area
The interpretation shows that through
time, Pabdeh-Gurpi horizons lengths increasing rate relative to the original
section lengths gradually decreases upward. Where amount of extension of Pabdeh-Gurpi
Formations did not exceed 1% of the original length, this generally indicates
to tectonic events during the initial stages of deposition of Gurpi Formation
was active, lower layers thickness variation of Gurpi Formation along the seismic
section is clear evidence (Fig. 6).
Presence of the normal and thrust faults intra Pabdeh-Gurpi Formations
refers to the area intensely exposed to extensional and compressional
tectonic movements. The area, in the initial stages, was stroked by intense
extensional events in Maastrichtian- Paleocene time led to rotational
movement of the overburden upward about 3° (Fig. 6C)
and this corresponds to Oman Orogeny. Besides, the interpretation indicates
to that Pabdeh-Gurpi Formations also exposed to compressional activities
in Paleocene time led to form thrust faults (the left side of Fig.
3, 6), it is likely that the latter activities may
be represent the latest phase of Oman Orogeny or onset of Zagros events
(onset of Arabian and Eurasia continents collision). These events caused
to trigger Cambrian Hormuz Salt and creation of normal fault with big
throw due to salt flow Strength upward (the right side ofFig.
3, 6. The uniform thicknesses of Pabdeh-Gurpi Formations
on two sides of normal and thrust faults denote to that formation of the
normal fault and salt flow occurred post-deposition of Gurpi Formation
(Fig. 3, 6C, B). Seismic
interpretation reveals that the normal fault is not regional; it does
not extend to the basement. It is likely that in upper Cretaceous-Lower
Miocene time, the majority of Hormuz Salt moved up and settled beneath
Guri base unconformity. After that, during deposition of Mishan and Aghajari
Formations dated as mid Miocene-present time diapirs have mainly grown
passively by downbuilding. They originated by maintaining their crest
at or close to the sea floor while sediments accumulated in depotroughs
between salt bodies. In other words, the diapirs crest remained at or
near the surface of the sedimentation while its base sank together with
the surrounding, subsiding Aghajari strata. The weight of the Aghajari
sediments at synclines accelerates displacement of salt bodies and somehow
uplifting of diapirs. By downbuilding, a diapir could pierce many kilometers
of strong overburden, because the latter was being deposited only along
the flanks of the diapir (Fig. 3). The interpretations
show that in Mid-Miocene time, a significant part of Hormuz salt was evacuated
upward and creation salt welds.
||Based on a kinematic model from restoration, which describes
evolution of the study area history, Hormuz Salt has intensely flowed
post-deposition Pabdeh-Gurpi Formations in Paleocene-Oligocene time
that caused to create normal fault. The extensional and compressional
movements (Oman Orogeny) were the main factor in triggering Cambrian
||The Fault Parallel Flow algorithm and flexural-slip unfolding technique
are determined suitable approaches in balancing faulted anticlines
of the area.
||The seismic imaging indicates that the compressional events with
rejuvenation of thrusting related to the Oman Orogeny had a major
influence on the structuring of the study area.
||The Zagros Orogeny has had very little effect in the area as Guri
base Unconformity surface is gently folded by this Orogeny. Furthermore,
the Zagros Orogeny triggered a new salt tectonic event in the study
area as both the Hormuz Salt and Fars Salt are affected.
||Evolution of salt diapirs in the study area has passed two active
and passive diapirism stages. Cambrian Hormuz Salt rose up mainly
in active form by tectonic events until Lower Miocene time. After
that, it continued to flow mostly in passive form by downbuilding.
||Hormuz Salt tectonic, Zagros events and sea level variations had
an effective role in creation of turbidites and mass transport complexes
(MTC) within Mishan Formation.
This research was prepared under supervision and permission of NIOC-Exploration
Directorate and by cooperation with Amir Kabir University of Technology.
The authors would like to thank Mr. M. Mohadess, Mr. M.M. Khorasani and
Mr. M.A. Naini, for their supports and permission to publish this paper
to include the seismic profiles. And we are also grateful to staff of
Amir Kabir University for help and close cooperation.