sign in sign up
<<
Home , Atlantic Ocean

THE OPENING OF THE SOUTH ATLANTIC

How one has opened Seen from an African perspective

Thick piles of accumulate on the”passive” or “Atlantic-type” margins of 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 , where there is very little post-rift sediment, showed listric normal faults and flow of the at depth. Both accommodated extension. The transition between (ca.35 km continental crust) and ocean-floor (ca.10 km basaltic crust) showed a ca.150 km wide transitional . 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-”. 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 ( ) 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 HAVE BEEN DOMINATED BY EVENTS IN THE . (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

Continent to Ocean transition north of 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 and South America began to separate ca 126 Ma (end Barremian) about 7 My after Tristan Plume erupted and 14 My after rift initiation on the FLOOR: ocean site.

•M11 (131Ma ?) Strike-slip motion Off Durban began at ca 126 Ma (Neves data) *M4 (126 Ma ? ) SOUTH Potiguar Off AMERICA 3 anomalies strike AFRICA obliquely into the shore Tristan between the Plume ca 133 Ma and the Walvis Ridge. Erupted (Hawkesworth) *Oldest Marine faunas S. Atlantic magnetics ca 126 Ma Ca.126 Ma. *Subaerial ocean floor

formed north of the Durban Basin magnetics Walvis ridge: No ca 126 Ma (Karen Marks) 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 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. (on crest of ) and SW Africa (S.of Walvis) are different Mod. from Gallagher et al., 1998, AREPS • (1) AFT reset near coast while shoulders eroded (? 5km)(180- 140 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 specialModified ( b of fromscarp Gallagherretreat. FT resetet al .neaIIr (1998), coast AREPS when shoulders 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) draining inward have farther to travel and are less steep than rivers going directly to coast.

Fission track predictions Now retreat (1) and erosion of a to For 3 models. Model 2 (3).applies Distinguishing to swell forming the scarp FT retreatProcess event(active fromonly since the 30later Ma). A age swellModel erosion 1 and a late eventr model is critical.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. Sirt fed byl ong rivers into a broad estuary

Cret.desert Arabia mainly carbonates and sabkha

s Bulge from A to B r e d a) ocean floor from 180 Ma tlas l low lying near sea level north of this A A u o b) carbonate shel f 180-125 Ma h line, shoreline fluctuated rapidly 150-30 Ma s c) deltas 135-65 Ma Tan Tan Abu Gharadig Casamance thin non-marine “Nubian” and ”Hamadi an” sandstones B high rainfall I M later i tes and bauxi tes formed forest between desert latitudes, silcretes and calcretes 10 m di nosaur-catching crocodiles Anza further south 80 Ma NEC Amazon until 125 Ma pulse Lamu Delta 150-34 Ma Interi or mar i ne transgressions Ogooué deltas in Benue ri ft mari ne transgressi on up r ift? before 85 Ma, onto desert over a) I (Iullemmeden Basi n, Gao rift) short ? ocean floor much of the delta five between 100 and 50 Ma rivers from 80 Ma Rufiji b) M (Benue and Maidugari deltas on Zambezi Delta Basins) five between 100 and carbonate ? shelf • major delta 150-65 Ma 50 Ma earl y thin Kalahari • no 65-30 Ma delta c) NEC ( North East Congo) Zambezi only late Cretaceous. Kalahari or Orange Delta mainly 122-95 Ma, MM Limpopo MM (Mahura Muthla) 80 Ma pulse, little si nce 70 Ma ) land plants in gravels Tugela, 1st pulse ca. 140 Ma Karroo like Orange, but (no 65-30 Ma delta) 140 to more deposition in Caledon erosi on from rift shoul ders R. Cenozoic Agulhas, deposition mai nly in rifts 30 Ma deltas No African rifts, rivers and deltas ca. 150 Ma (LateJurassic) to 3083 Ma Ma (early ). 84 Ma (Santonian) tectonics not emphasized.Events EFFECTS OF A COLLISIONAL / TRANSPRESSIONAL EVENT ON PETROLEUM SYSTEMS SANTONIAN Santonian 84-80 Ma Reversal of PERTURBATIONKermanshah, Neyriz and Oman ophiolites Traps formed, movement on emplaced by arc collision over 2000 km suture Structures -Gibraltar later filled transform places Formedwith hydrocarbons N Africai nto Sirt rifting compression and Syri an arc folds ProducingTraps formed, Intra Africa E-W faults Exploration potential plate boundary Dark Abu Gharadig folds green Casamance Benue intense Cretaceous Equator prospects folding 80-85 Ma Reservoirs deposited Doba folding Yellow in response to event ; folding Sediment sediment ity form pulse in un- con pulse. Rio E/Guinea thrusting on Anza Muni Charcot FZ)

indications Reservoirs of erosion eroded or uplifted in AFT record eruption of Mari on Plume at 85 Ma Structuresand exposed at carbonatite Mari on Plume di es at 60 Ma Surface. 86 Ma Breached.Probable sediment pul se Folding K K destruction of any rivers in Orange Ri ver K K toopreviously filled deltas intensetraps

widespread k i mberlites in S. Africa 86 Ma

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