Syn-rift carbonate platform [Dorobek,2008 ]

1. GEOL 534
Assignment 2
Tectonic and depositional controls on syn-rift carbonate platform sedimentation
Dorobek, 2008
Omar A. Radwan
PhD Student – Geosciences Dept. - KFUPM

2. 2

3. Outline
Introduction
• Rift
• Syn-Rift
• Syn-rift carbonate platform
Extensional Settings
• Extensional basins
• Stretching Gradients & Transform Segments
• Subsidence
• Structures
Carbonate Platforms in Extensional Settings
• Controls
• Timing
• Models
Summary
3

4. Introduction
Rift
• occurs where tensile stresses stretch the lithosphere, causing;
• extensional faulting of the brittle upper crust
• either brittle faulting or ductile flow in the lower crust
• probable ductile thinning of the mantle lithosphere
Syn-Rift
• The onset of the syn-rift phase of basin evolution is usually
interpreted to begin when the first brittle faults cut the upper
crust.
• The onset of seafloor spreading typically marks the “rift-to drift
transition” in the development of passive continental margins
because stretching of continental lithosphere generally stops
once seafloor spreading begins.
Syn-rift carbonate platform
• form important petroleum reservoirs within rift-basin systems.
• provide critical records for understanding the tectonic evolution
and depositional history of rift systems.

5. Extensional Settings: Extensional basins
• Intracontinental sag basins
• Limited stretching of the continental lithosphere before
rifting ends
• Typically less than 10% extension
• Occur within continental interiors
• Deformation is best described as broad-wavelength
subsidence or “sagging” ≈ semicircular basins
• Intracontinental rift basins
• Greater amounts of stretching within continental
lithosphere
• Typically 10–80% stretching
• Located in interior parts of continental plates, or they may
project inboard from passive or convergent continental
margins
• Deformation is best described as multiple rift depocenters
with intervening basement highs

6. It is difficult to estimate when shallow-water flooding of synrift
topographic features might begin during the rift history of an
intracontinental rift system.
• Gulf of Suez and Red Sea region
• Formed on pre-rift Precambrian basement substrates very
early in the rift history.
• This is largely due to the very low pre-rift relief across the
Middle East and northeastern Africa.
• South China Sea region
• Initiated at a variety of times in the synrift history.
• This depends on local time of rifting, regional pre-rift
topography, the location within the complexly evolving rift
network, and proximity to major siliciclastic input points.
In general, however, shallow-marine carbonate sedimentation
typically commences during later syn-rift stages, when enough
stretching and syn-rift subsidence has occurred and the evolving rift
system becomes linked to an adjacent ocean basin.
Extensional Settings: Extensional basins

7. Extensional Settings: Stretching Gradients & Transform Segments
Stretching Gradients
• Narrow, with steep, often step-like structural and depositional
profiles toward the rift axis or continent–ocean boundary
• Wide, with more gentle, regional depositional gradients
• Complex, with many localized depocenters that are separated by
intervening, fault-bounded, and elevated crustal blocks
Transform Segments
• Form where fault systems trend nearly parallel to regional (i.e.,
basin-scale) stretching directions.
• Strike-slip and transtensional styles of deformation dominate
transform segments.

8. Extensional Settings: Subsidence
Fault-Controlled Subsidence
• Fault displacement can occur either;
• aseismically or
• during earthquakes (i.e., incremental cosesimic
displacements)
• semicontinuously (i.e., creeping faults),
• quasi-periodically (i.e., coseismic displacements), or
• in a pulsed fashion (i.e., where regional tensile stresses wax
and wane over geologic time scales).
• Tectonically created surface deformations range
from development of;
• terraced profiles,
• curved to ramp-like surfaces bounded by fault scarps, or
• fault-bounded blocks with characteristic horst–graben or
half-graben crosssectional profiles.
Long-Wavelength Subsidence

9. Extensional Settings: Structures
Faults
• Normal faults & Oblique slip
• Strike-slip deformation or shortening may develop along accommodation zones between major normal faults
• Full grabens, half-grabens, and horsts, are the basic tectono-geomorphologic building blocks of rift systems.
Syn-Rift Fault Growth, Linkage and Interactions
• Hard linkage develops where fault segments propagate toward each other and directly link together to
become a longer, through-going fault zone.
• Soft linkage is characterized by more highly distributed extensional strain in deformation zones between
neighboring faults, without the development of through-going faults.

11. Carbonate Platforms in Extensional Settings
Tectonic Versus
Eustatic
High frequency
sea-level changes
Interfere significantly with the
short-term changes in
accommodation space created by
fault displacements
Longer-term
eustatic changes
Interact more significantly with
long wavelength syn-rift tectonic
subsidence across rift systems

12. Carbonate Platforms in Extensional Settings
Sedimentation in
extensional
settings
Early to middle
syn-rift stages
Terrestrial
depositional
systems
later syn-rift to
early post-rift
stages
Transition into
marine conditions

13. Carbonate Platforms in Extensional Settings
Actual time and
location of initial
marine incursion
Rift
Active
Not flooded by
seawater until late
syn-rift to early
post-rift stages
Passive
Earlier marine
flooding
Shear
Pure
Both sides of the
rift system should
be flooded at
approximately the
same time
Simple
The lower plate
should be flooded
first.
Pre-rift topography
Low-elevation
(peneplain surfaces
)
Flooded over wide
areas during the
earliest stages of
rifting
High-elevation
(orogenic belts)
Flooding may not
occur across rift
zones until the
lithosphere is
highly stretched
The amount of
stretching (β)
Marine incursion is
more likely to
occur earlier in the
tectonic history of
extensional basins
and over larger
areas if β values
are higher.
Proximity of the
rift to an existing
ocean basin.
Marine incursion is
likely to occur
earlier where the
rift system
develops close to
an existing ocean
basin.
History of eustatic
sea level during
syn-rift to early
post-rift stages
Marine flooding is
obviously more
likely to occur
sooner in the
history of an
extensional basin
during times in
Earth history when
eustatic sea level
was rising or at a
long-term
highstand.

14. Carbonate Platforms in Extensional Settings
Syn-rift
deformation
At regional scales
Influences platform morphology and
location within the rift system
At local scales
Influences facies development, the location
of facies tracts, the internal growth stratal
patterns of individual platforms, and the
degree of mixing between coeval siliciclastic
and carbonate facies

15. Carbonate Platforms in Extensional Settings

16. Carbonate Platforms in Extensional Settings
stretching gradients
low
Syn-rift isolated carbonate
platforms may form on
numerous fault bounded
highs across the rift
system, and these
platforms may coalesce
into larger composite
platforms during the late
syn-rift to early post-rift
phase.
high
Syn-rift carbonate
platforms will likely be
limited to a narrow zone,
just downdip from the
tectonic hinge.

17. Carbonate Platforms in Extensional Settings
Fault growth
Soft linkage
Surface deformations related to soft linkage
are commonly expressed as relay ramps
Hard linkage
Surface deformations related to hard
linkage are commonly expressed as
terraced or step-like topography between
the main extensional faults.

18. Carbonate Platforms in Extensional Settings
Carbonate platforms
location
Closer to the rift flank
Thinner and shorter-lived because
they are overwhelmed sooner by
prograding siliciclastic strata
Farther offshore
Thicker and much longer lived
because it may take many millions
of years to fill up-dip depocenters
and regional accommodation space

19. Carbonate Platforms in Extensional Settings

20. Carbonate Platforms in Extensional Settings
• Incremental synrift deformation results in
rotation of the hanging wall
• Deposition across the rotated hanging wall
results in the divergent stratal patterns

21. Carbonate Platforms in Extensional Settings
• Block rotation is not likely to occur
• Subtle growth stratal patterns may be
recognizable within syn-rift isolated platforms
on horsts only if the horst is at least 30 km
wide.

22. With regard to sequence stratigraphic relationships within syn-rift
platforms, unconformity-bounded transgressive to early highstand
systems tracts should dominate footwall highs, whereas late
highstand to lowstand strata should be more common down
hanging-wall dip slopes of half-graben elements (Fig. 8).
22
Carbonate Platforms in Extensional Settings
Similar stratal patterns should characterize horst platforms, although
lowstand systems tracts may not be well developed within platform
interiors and are preferentially developed on leeward slopes (Fig. 10).

23. Summary
• This review showed the complexity and significance of syn-rift carbonate platforms and the stratigraphic
successions that are characteristic of these systems.
• When? Where? Typically form during later synrift stages, after significant fault linkage has already
occurred, the amount of extension has progressed enough so that the evolving rift system can link to an
adjacent ocean basin and becomes flooded with seawater
• What? Typically form isolated platforms that are constructed on fault-bounded topography
• With regards to sequence stratigraphic relationships,
• Unconformity-bounded transgressive to early highstand systems tracts should dominate footwall highs
• Late highstand to lowstand strata should be more common down hanging-wall dip slopes