THE OPENING OF THE SOUTH ATLANTIC
How one ocean has opened
Seen from an African perspective
Thick piles of sedimentary rock accumulate on the”passive”
or “Atlantic-type” margins of oceans of this kind. Data are generally too incomplete for establishing what has happened.
Speculations and models abound. Many are helpful but
recognizing the assumptions involved and assessing the
utility of the methods applied in constructing the models is a
personal responsibility.
Caveat Emptor ( Let the buyer beware)
HOW TIGHTLY CAN OCEANS BE CLOSED ?
Classic studies from the north side of the Bay of Biscay, where
there is very little post-rift sediment, showed listric normal
faults and flow of the continental crust at depth. Both accommodated
extension.
The transition between continent (ca.35 km continental crust)
and ocean-floor (ca.10 km basaltic crust) showed a ca.150 km wide
transitional region. The conjugate shore transition zone is likely
to have been narrower.
Too many terms are used e.g.:” rifted and thinned continental
Crust”, “ stretched continental crust”
“Transitional crust” and “proto-oceanic crust”. Some see two
or more varieties of crust.
(1)MOST SOUTH ATLANTIC INTRA-CONTINENTAL RIFTS
WERE INITIATED 140 Ma +/- 5 (mid-Berriasian). (IN K-pippe)
(2) THEY EVOLVED AS INTRA-CONTINENTAL RIFTS TILL
ca. 125 Ma (Late Barremian) WHEN OCEAN FLOOR BEGAN TO
FORM.
(3)WHAT IS NOW THE TRISTAN PLUME ERUPTED AT ca 133 Ma
(Hauterivian) TO FORM PARANA (S. AMERICA) AND ETENDEKA
( Africa) LARGE IGNEOUS PROVINCES.
(4) OCEAN FLOOR BEGAN TO FORM ALL THE WAY FROM
DURBAN TO THE DEMERARA RISE (at 125 +/- 3 Ma
within BARREMIAN times.) M 9 130 Ma? M8: 129, M2:124 Ma).
(5) EVAPORITES WERE DEPOSITED ON OCEAN FLOOR
(UNDER AIR) NORTH OF THE TRISTAN PLUME TRACK
AND IN NEARBY CONTINENTAL MARGIN RIFTS AT 112 Ma
(Late Aptian).
(6) RIFT SHOULDERS WERE ERODED BY ca. 90 Ma.
(7) SUBSEQUENT TECTONIC PERTURBATIONS AND THE ELEVATION OF ERODABLE AREAS IN SOUTH AMERICA
HAVE BEEN DOMINATED BY EVENTS IN THE ANDES.
(8) FOR AFRICA THE SANTONIAN (C.84 Ma) AND AFRICA
ARREST EVENTS (30 Ma) HAVE BEEN TECTONIC DRIVERS
SINCE THE TRISTAN HOT SPOT TRACK WAS BREACHED
Rifted and thinned continent with listric faults. 150-200 km wide
Offshore ANGOLA
Continent to Ocean transition north of Walvis ridge
where ocean floor formed under air from 126-112 Ma.
Sea water then spilled in to form a 1M sq.km area of
evaporites.
BEST :From Durban to Guinea Nose the start of Ocean formation
was between 131 and 124 Ma (128 +/- 4 Ma) Burke et al. 2003
Macmillan 2003 Fig.5,Tikku & Marks, Neves 1989
Africa and South America began to separate
OLDEST SOUTH
ca 126 Ma (end Barremian) about 7 My after Tristan
ATLANTIC OCEAN
Plume erupted and 14 My after rift initiation on the
ocean site.
FLOOR:
•M11 (131Ma ?)
Strike-slip motion
began at ca 126 Ma
Off Durban
(Neves data)
*M4 (126 Ma ? )
SOUTH
Potiguar
Off Namibia
AMERICA
AFRICA
3 anomalies strike
obliquely into the shore
Tristan
Plume
between the Cape
ca 133 Ma
Erupted
(Hawkesworth)
and the Walvis Ridge.
S. Atlantic magnetics
*Oldest Marine faunas
ca 126 Ma
Ca.126 Ma.
*Subaerial ocean floor
formed north of the
Durban Basin magnetics
ca 126 Ma (Karen Marks)
Walvis ridge: No
Magnetic anomalies and
No marine faunas
How long do escarpments at the continental margin last?
Coward et al.( Ed Purdy’s strat columns1998)
showed:
South Atlantic rifts formed 145 ± 5 Ma.
Rift deposition (pink) persisted in rifts and on
ocean floor, under air, till sea water spilled
in to form evaporites at 112 Ma (Late
Aptian)
After evaporites were deposited, carbonate
deposition (blue) followed ending at 20-40
My after ocean floor began to form. This
was the”Rift shoulder duration time”.
Rift shoulders had been eroded by the time silici-clastic deposition (green) began at 105-90 Ma. No Long-lived escarpments
Conclusion: Escarpments lasted a few tens of
millions of years.
Pelotas (on crest of Rio Grande Rise) and
SW Africa (S.of Walvis) are different
Mod. from Gallagher et al., 1998, AREPS
• (1) AFT reset near
coast while shoulders
eroded (? 5km)(180140 and 130-80 Ma)
• Too little erosion
(<2km) in interior to
reset AFT
• Interior eroded and
Africa low by 90 Ma
• (2) Swell initiated at 30
• Ma.Erosion mostly
outboard of newly formed
Great Escarpment
• Too little erosion (<2km)
to reset AFT
AI special ( b of scarp retreat. FT reset neaIIr coast when shoulders
Modified
from
Gallagher
et
al.
(1998),
AREPS
eroded
• Too little erosion <2km in interior to reset FT
• Africa flat and low by 90 Ma.
In this model (1)Scarp stays fixed for
> 100 My because virtually no rain
falls on the crest. (2)Rivers draining
inward have farther to travel and are
less steep than rivers going directly to
coast.
Fission track predictions
retreat
(1) and erosion of a to
For 3 models. Model 2
(3).
Distinguishing
applies
to swell formingthe scarp
Process event
(active from
only since
Ma). A
retreat
the 30
later
Modelerosion
1 and a later
model
2 event
swell
event
is critical.
Now
FT
age
.
work
for Africa and for all
other rifted margins with a
near coast escarpment.
Great
Scarps
are
products
of
erosion
during
the past
30 My
Image courtesy of NOAA and USGS.
Sirt fed by long rivers
into a broad estuary
Cret.desert
Arabia mainly carbonates and sabkha
A
Atlas
low lying near sea level north of this
line, shoreline fluctuated rapidly 150-30 Ma
sho
Bulge from A to B
a) ocean floor from 180 Ma
b) carbonate shelf 180-125 Ma
c) deltas 135-65 Ma
ulde
rs
Europe
Tan Tan
Abu Gharadig
thin non-marine “Nubian” and ”Hamadian”
sandstones
Casamance
B
I
M
Cretaceous Equator
high rainfall
laterites and bauxites formed
between desert latitudes,
silcretes and calcretes
further south
forest
10 m dinosaur-catching crocodiles
Interior marine transgressions
a) I (Iullemmeden Basin, Gao rift)
five between 100 and 50 Ma
b) M (Benue and Maidugari
Basins) five between 100 and
50 Ma
c) NEC (North East Congo)
only late Cretaceous.
MM (Mahura Muthla)
land plants in river gravels
deltas in Benue rift
before 85 Ma, onto
ocean floor
from 80 Ma
Anza
NEC
80 Ma
pulse
Amazon until 125 Ma
Lamu Delta 150-34 Ma
Ogooué
short
rivers
deltas on
carbonate
shelf
Kalahari or Orange Delta
mainly 122-95 Ma,
80 Ma pulse, little since 70 Ma)
desert over
much of the
Congo Basin
?
delta
Rufiji
?
early thin Kalahari
sands
MM
marine transgression up rift?
Zambezi Delta
• major delta 150-65 Ma
• no 65-30 Ma delta
Zambezi
Limpopo
Tugela, 1st pulse ca. 140 Ma
(no 65-30 Ma delta)
140 to
R.
erosion from rift shoulders
Agulhas, deposition mainly in rifts
30 Ma
deltas
No
African rifts, rivers and deltas ca. 150 Ma (LateJurassic) to 3083
MaMa
(early Oligocene). 84 Ma (Santonian) tectonics not emphasized.
Events
Karroo like Orange, but
more deposition in
Cenozoic
Caledon
EFFECTS OF A COLLISIONAL
/ TRANSPRESSIONAL
EVENT ON PETROLEUM SYSTEMS
SANTONIAN
Santonian
84-80 Ma
Reversal of
Kermanshah, Neyriz and Oman ophiolites
PERTURBATION
Traps formed,
movement on
emplaced by arc collision over 2000 km suture
Structures
Azores-Gibraltar
later filled
Formed
transform places
with hydrocarbons
Sirt rifting
N Africa into
Atlas
and
compression
Syrian arc
Producing
Traps formed,
folds
E-W faults
Intra Africa
Exploration potential
Dark
plate boundary
Abu Gharadig
folds
green
Casamance
prospects
Benue intense
Cretaceous Equator
folding 80-85 Ma
Reservoirs deposited
Doba folding
Yellow
in response to event ;
folding
Sediment
sediment
y
rmit
pulse in
o
f
n
n
pulse. Rio
o
u c
E/Guinea
Anza
thrusting on
Muni
Charcot FZ)
indications
of erosion
in AFT record eruption of Marion Plume at 85 Ma
Marion Plume dies at 60 Ma
carbonatite
86 Ma
rivers
sediment pulse
in Orange River
K K
K K
deltas
Reservoirs
eroded or uplifted
and exposed at
Structures
Surface.
Breached.
Probable
Folding
destruction of any
too
previously filled
traps
intense
widespread kimberlites
in S. Africa 86 Ma
Tectonics, erosional, depositional and igneous events related to the Santonian
event (ca. 84 Ma).