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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Chronostratigraphy, Sedimentary Facies, and
Architecture of Tectono-Stratigraphic
Sequences
within a
Miocene Rift, Gulf of Suez, Egypt
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By
1Geological Consultant, Houston, Texas
2Consulting Biostratigrapher, Katy, Texas
The Miocene synrift stratigraphy of the Suez Rift records the complex interplay between the structural development of the rift and the sedimentary response to tectonics. Along the Sinai margin of the Gulf of Suez (GOS), the Miocene sediments were deposited during the three main phases of rifting: rift initiation, clysmic rifting, and early post-rift. In general, this stratigraphy records the progressive subsidence and drowning of the basin during rift initiation and the clysmic stage, followed by isostatic adjustment and shallowing during the early post-rift stage.
A regional synthesis of the Miocene stratigraphic sections
exposed along the Sinai margin of the Gulf of Suez has resulted
in a depositional
and sequence stratigraphic model for these
strata that integrates tectonic history and sedimentary response
during the early, clysmic, and post-rift phases of basin evolution.
The development of this model was made possible by establishing
rigorous chronostratigraphic control based upon micropaleontology
and magneto-stratigraphy. Application of this model,
along with 3-D seismic, has had a major impact on the ability to
recognize stratigraphic and subtle combination traps within this
mature basin, resulting in a 75% increase in exploration drilling
success (1993-1997) and an increase in production of 164,000
BOPD (IP).
Graphic correlation of paleontological data
from
wells and outcrops reveals
that the Neogene section consists of at least eight biostratigraphic
sequences
(S10-S80) separated by graphic terraces (T00-T70) or geologic hiatuses (gaps in time). Outcrop
analysis of terraces T00 to T30 and their associated fossil assemblages
indicates that they represent either regional regressive or
transgressive events. The number of
depositional
sequences
(those bounded by regressive erosional surfaces) is therefore less
than the number of paleontologically defined
sequences
.
Terraces T00 and T20 are sequence boundaries, and subsurface
evidence suggests that T40 is a condensed interval and that T50
is an erosional unconformity (sequence boundary). Field observations
of T10 at Wadi Thal in the Sinai indicate that it consists
of at least two ravinements and a condensed section within a
narrow stratigraphic interval. Similarly, the T30 hiatus associated
with the Markha Anhydrite at Wadi Feiran is composed of
several stacked
flooding and regressive surfaces. These surfaces
at both Wadi Thal and Wadi Feiran represent minor time breaks.
These small lacunae cannot be individually resolved by graphic
correlation, but their sum total within a thin rock (hiatal) interval
is detectable as a graphic correlation terrace. Despite limitations
in resolution, graphic correlation of paleontological
data
was crucial for
recognizing
key surfaces and intervals that
allowed us to decipher the sequence stratigraphy of the Miocene
synrift section of the Gulf of Suez and to make more precise correlations
within the basin.
The key stratal surfaces and intercalated sequences
can then be
related to the tectonic evolution of the GOS. T00, for example,
represents the pre-rift unconformity. The initiation phase of the
Suez Rift is recorded by deposition of the Aquitanian Nukhul
Formation (S10). Rift initiation was characterized by northerly
flowing fluvial systems that occupied the downthrown segments
of asymmetric half-grabens. Increased subsidence resulted in a
relative sea-level rise that flooded the half-grabens and created
End_Page 16---------------
elongate estuaries and, ultimately, shallow marine environments.
Nukhul depositional
facies include: continental alluvial valley fill,
estuarine, tidal flat, tidal channel complexes, and shallow offshore
marine. The clysmic phase began with uplift of the rift
shoulders and concomitant basin subsidence. This resulted in a
period of progressive sediment starvation within the basin (T10)
as sediment sources adapted to the new topography. The clysmic
stage of rifting is recorded by deposition of relatively deep
marine mudstones, basin-floor fan sandstones, and footwall-margin
conglomeratic-talus cone and fan delta deposits of the
Mheiherrat Formation (S20). Continued extension and crustal
thinning resulted in isostatic uplift, shallowing of detachment
depths, and increased rotation of fault blocks (T20). The later
stages of the clysmic rift were documented in the channelized
submarine fan, offshore marine, deltaic, lacustrine, and hypersaline
lagoon/sabkha deposits of the Hawara, Asl, and Ayn Musa
formations (lower S30). The early post-rift phase is recorded by
open marine mudstones and delta front deposits of the Lagia
and Ras Budran members of the Ayun Musa Formation (upper
S30 and S40).
Within the subsurface of the Gulf of Suez, seismic data
are generally
poor because of energy attenuation by shallow evaporites,
multiple reflections, and complex structure. These conditions
make traditional seismic sequence stratigraphy techniques difficult
to apply. However, the tectono-sequence stratigraphic
model developed
from
outcrops and the paleontologically-defined
chronostratigraphic framework provides tools that allow
for better subsurface correlations by systematically mapping
stratal geometries using sequence boundaries and flooding surfaces
defined by high-resolution biostratigraphic
data
. The
stratigraphic picture that emerges
from
application of these concepts
creates a profound change in quantification of fault throws
and recognition of stratigraphic and combination traps. In
addition to revealing new plays, application of the tectono-stratigraphic
model also results in a better understanding of reservoir
geometry and distribution.
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