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 butrecognizing the assumptions involved and assessing theutility 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 wideOffshore 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

OLDEST SOUTH ATLANTIC OCEAN FLOOR: •M11 (131Ma ?)Off Durban*M4 (126 Ma ? )Off Namibia3 anomalies strike obliquely into the shore between the Cape and the Walvis Ridge. *Oldest Marine faunas Ca.126 Ma. *Subaerial ocean floor formed north of the Walvis ridge: No Magnetic anomalies and No marine faunas

Potiguar SOUTH

AMERICA AFRICA

Africa and South America began to separateca 126 Ma (end Barremian) about 7 My after TristanPlume erupted and 14 My after rift initiation on theocean site.

Strike-slip motionbegan at ca 126 Ma(Neves data)

Tristan Plume

Erupted ca 133 Ma (Hawkesworth)

S. Atlantic magneticsca 126 Ma

Durban Basin magneticsca 126 Ma (Karen Marks)

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 (180-140 and 130-80 Ma)

• (<2km) in interior to reset AFT

• Africa low by 90 Ma

• (2) Swell initiated at 30 •

outboard of newly formed Great Escarpment

• to reset AFT

(? 5km)

Too little erosion

Interior eroded and

Ma.Erosion mostly

Too little erosion (<2km)

et al. (1998), AREPS

retreat (1) and erosion of a to (3). Distinguishing the scarp retreat event from the later swell erosion event is critical.

Modified from GallagherAI special ( b of scarp retreat. eroded

• •

.

FT age

Now

FT reset neaIIr coast when shoulder

Too little erosion <2km in interior to reset FT Africa flat and low by 90 Ma.

s

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 For 3 models. Model 2 applies to swell forming Process (active only since 30 Ma). A Model 1 and a later model 2 event 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.

African rifts, rivers and deltas ca. 150 Ma (LateJurassic) to 30 Ma (early Oligocene). 84 Ma (Santonian) tectonics not emphasized.

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140 to 30 Ma No

Cretaceous Equator thin non-marine “Nubian” and ”Hamad an” Casamance

low lying near sea leve north of this line, shoreline fluctuated rapidly 150-30 Ma

Anza

Zambez

Zambezi Delta • major delta 150-65 Ma • no 65-30 Ma delta

Lamu Delta 150-34 Ma

delta mar ne transgress on up r

Tugela pulse ca. 140 Ma no 65-30 Ma delta)

Limpopo Kalahar or Orange Delta

mainly 122-95 Ma, 80 Ma pulse, little s nce 70 Ma

Karroo like Orange, but more deposition in

Cenozoic Caledon

, deposition ma nly in rifts

ear y thin Kalaharsands

desert over much of the Congo Basin

Inter or mar ne transgress I (Iullemmeden Bas n, Gao rift)

five between 100 and 50 Ma M (Benue and Maidugari

five between 100 and

NEC North East Congoonly late Cretaceous.

eros on from rift shou ders

deltas

ater tes and baux tes formed desert latitudes,

silcretes and calcretes further south

Bulge from A to B ocean floor from 180 Ma carbonate she f 180-125 Ma deltas 135-65 Ma

Amazon until 125 Ma 80 Ma pulse

short rivers

deltas on carbonate

Abu Gharadig

Sirt fed by ong rivers into a broad estuary

sandstones

Arabia mainly carbonates and sabkha

10 m d nosaur-catching crocodiles

forest

Cret.desert

Tan Tan

MM (Mahura Muthand plants in river gravels

deltas in Benue r ft before 85 Ma, onto

ocean floor from 80 Ma

gh rainfall

83 MaEvents

Tectonics, erosional, depositional and igneous events related to the Santonianevent (ca. 84 Ma).

EFFECTS OF A COLLISIONAL / TRANSPRESSIONAL EVENT ON PETROLEUM SYSTEMS

Traps formed, later filled with hydrocarbons

Reservoirs eroded or uplifted and exposed at Surface.Probable destruction of any previously filled traps

Reservoirs deposited in response to event ;

Traps formed, Exploration potential

SANTONIAN

Atlas

sediment pulse in

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KK

86 Ma

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conformityun-

pl

laces i

PERTURBATION

deltas

Cretaceous Equator

Casamance

Anza Guinea

Abu Gharadig folds

Syr an arc folds

Doba folding

Sirt rifting

indications of erosion

in AFT record

carbonatite

sediment pu se in Orange R ver

despread k mberlites in S. Africa 86 Ma

eruption of Mar on Plume at 85 Ma

Mar on Plume d es at 60 Ma

thrusting on Charcot FZ)

E-W faults

Kermanshah, Neyriz and Oman ophiolites emplaced by arc collision over 2000 km suture

rivers

folding

Intra Africa ate boundary

Benue intense folding 80-85 Ma

Reversal of movement on Azores-Gibraltar transform pN Africa nto compression

Santonian 84-80 Ma Structures Formed and Producing Dark green prospects Yellow Sediment pulse. Rio Muni

Structures Breached. Folding too intense