The Great WestAfrican Tertiary Coastal Uplift Fact or
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The Great West-African Tertiary Coastal Uplift: Fact or Fiction? A Perspective from the Angolan Divergent Margin Bureau of Economic Geology Martin Jackson Michael Hudec Kerry Hegarty AGL - 2005
The Great West-African Tertiary Coastal Uplift: Fact or Fiction? A Perspective from the Angolan Divergent Margin Bureau of Economic Geology We explore the pattern of exhumation in the coastal Kwanza Basin by combining analyses of Tertiary hiatuses and apatite fission tracks. Planktonic biozones show five major hiatuses in the Oligo-Miocene and Plio-Pleistocene. Between gaps, Oligo-Miocene strata accumulated under marine conditions. A marine setting refutes the idea of a massively raised coastal plateau in the mid-Tertiary. Marine conditions continued until ~5 Ma. Fission-track data suggest three thermal events: ~150 Ma, during rifting and volcanism; ~100– 70 Ma, during shortening and volcanism; and ~20– 10 Ma, during exhumation. Tertiary uplift was spatially highly variable. For the Kwanza Basin, we infer that Tertiary uplift on the West African margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas. Massive uplift (2000– 4000 m) of the Precambrian craton had little structural effect in the outer basin. Instead, minor uplifts on the shelf drove late-Tertiary deformation on the slope. AGL - 2005
Acknowledgments Bureau of Economic Geology Total Proprietary data Sonangol Cooperation Geotrack International Apatite fission-track data and analysis Pennsylvania State University Nick Harris: discussions AGL - 2005
Significance of Uplift Bureau of Economic Geology Drives Deformation Tilt triggered much Tertiary gravity-driven deformation. Supplies Sediments Erosional unroofing fed offshore Tertiary sediments; created a massive sediment dump without major rivers. Sediment differentially loaded the salt. Sediment dump matures hydrocarbons Depressed source rocks (including post-salt) into oil window. AGL - 2005
What’s New? Talks on this topic were given at AGL meetings in 1999, 2000, and 2003. The following paper written for Tectonics produced new ideas. Jackson, M. P. A. , Hudec, M. R. , and Hegarty, K. A. 2005, The great West African Tertiary coastal uplift: fact or fiction? A perspective from the Angolan divergent margin: Tectonics, v. 24, TC 6014, doi: 10. 1029/2005 TC 001836. This talk summarizes the main conclusions. The Tectonics paper (2005) has details. New data sources: Distribution of cratonic cover Apatite fission track analysis (AFTA) Biostratigraphy. Main new product is a map of inferred Neogene uplift. AGL - 2005 Bureau of Economic Geology
Evidence Cited For Epeirogenic Uplift Bureau of Economic Geology Topography (southern Africa) Present hypsometry of Africa (especially lower elevations) is far higher than that of other continents since at least the Miocene (Bond 1978). Cretaceous shoreline elevations (Africa) Presently elevated by as much as 2– 3 km, especially in southern Angola (Sahagian 1988). Erosional truncation (offshore basins) Suggests 1– 2 km uplift in the Oligocene (Lavier et al 2000, Steckler et al. 1997; Karner et al. 1997). Basement ramps (offshore basins) The Atlantic Hinge Zone is vertically offset by 1– 2 km. Hydrocarbon maturity (onshore Kwanza) Inner Kwanza Basin was differentially uplifted 1– 2 km (Lunde et al. 1992). (This chapter challenges this conclusion) Fission-track dating (Namibia) Indicates 1– 2 km of Tertiary uplift (Gallagher & Brown 1999). AGL - 2005
Two Phases of Rafting in Kwanza Basin Bureau of Economic Geology Phase 1: Late Cretaceous. Cooling margin tilted seaward. Produced many small tilted rafts. Phase 2 : Tertiary Wider rafts separated by troughs. Why? Sudden uplift of coast. AGL - 2005
Sediments load salt and enhance maturity Tilt drives deformation Unroofing supplies sediments Phase 2 rafts This schematic cross section illustrates the idea of Lunde et al. (1992). Further details in the next slide. AGL - 2005 8
Funda-3 2000 m 1000 m 0 m ela u ng ce e Qu ovin Pr Lunde et al. (1992) inferred a 2 -km-high coastal plateau. Uplift from Miocene to present; coast is now eroded to near sea level. Idea based on maturity profiles in the 3 wells shown here. They assumed that exhumation caused the cooling. a im ce x Mu ovin Pr ma e a ss c Ki ovin Pr iso L rro nce o M ovi Pr Abacaxi-1 AGL - 2005 Morro Liso-3 im bo m A to ce r Po ovin Pr Geologic map simplified from Total-Sonangol, 1987
Main Points to be Made in This Paper Bureau of Economic Geology Three thermal episodes. 1 – 4 km Neogene uplift in the Precambrian hinterland. No coastal high plateau existed in the Neogene. Minor Neogene uplift on the shelf had a major effect on deep-water salt tectonics. Neogene uplift varied greatly. Mid-Miocene thermal event has implications for hydrocarbon maturation. AGL - 2005
Three Thermal Events Apatite Fission-Track Analysis Fission tracks form by decay at uniform rate. Destroyed by heating. Sample location Thermal Events Edge of Kwanza Basin Neogene, 20– 10 Ma Cretaceous, 100– 70 Ma Jurassic, ~150 Ma National geologic map of Angola AGL - 2005
Building a Thermal History Bureau of Economic Geology Boxes show 95% confidence limits for time when cooling began and peak temperature. Precambrian samples record all 3 thermal events. Solutions distinguish between Required vs. Allowed Boxes overlapping in time confirm thermal events. Assumed: Heating rate Cooling rate Paleogeothermal gradients. AGL - 2005
Thermal History Bureau of Economic Geology Solid lines = required thermal event Dashed lines = allowed thermal event Overlapping ages define at least 3 thermal events (more complex scenarios are also possible) AGL - 2005
What’s Known……What Isn’t Bureau of Economic Geology Red box shows a wellconstrained range of age and temperature for each event. Depth of paleoburial (D) is constrained by temperature and by assumed geotherm. Exhumation (E) is unconstrained. Two different paths shown are equally possible, as are other paths passing through the red boxes. AGL - 2005
Massive Neogene Uplift of Precambrian Hinterland Bureau of Economic Geology Data sources for this region: Present topography Map pattern of Precambrian-Cretaceous cover Apatite fission-track analysis Slides 24. 18. 16 to 24. 18 cover this region AGL - 2005
Precambrian fold belt Escarpment absent; relief declines Kwanza Topography Mountains are too rugged to have formed entirely in the Mesozoic, but interpreting them further is difficult? If the hinterland was flat in the Paleogene, the present relief records minimum uplift for the Neogene. 2081 m 220 m Great Escarpment AGL - 2005 2620 m If the hinterland was mountainous in the Paleogene, Neogene uplift was less than the present relief. If the mountains eroded faster than the coastal plain, then uplift was greater than the present relief.
What the Geologic Map Reveals about Differential Uplift Bureau of Economic Geology Lower Congo West Congolian FB Base Cretaceous at 1200 m Coastal arch: formed between Neoproterozoic orogeny and Tertiary arching. Exhumation increases to south. Neoproterozoic cover shifts 400 km inland in S AGL - 2005 Base Cretaceous at 1500 m
Tertiary Uplift of Hinterland Bureau of Economic Geology 30– 45°C cooling, ~1200 m exhumation Kwanza edge Maximum uplift of >4000 m is decreased if: Mountains were above sea level before the Neogene. Some cooling was caused by a falling geothermal gradient. Now at +300 m, ~2000 m exhumation Now at +2600 m, >4000 m exhumation AGL - 2005
No Coastal High Plateau in Neogene Bureau of Economic Geology Data sources: Kwanza geologic map Biostratigraphy from wells Structural restoration AFTA Slides 24. 18. 20 to 24. 18. 26 cover this region in the northern Kwanza Basin (Quenguela province). AGL - 2005
Basin rim uniformly uplifted Tertiary Uplift of Kwanza Rim Restoration suggests rim unroofed by 600– 2000 m, depending on assumed geometry Little exhumed in N; mostly Miocene/Pliocene outcrop More exhumed in S; mostly Eocene outcrop AGL - 2005 Geologic map simplified from Total-Sonangol, 1987 20
Tertiary Uplift of Kwanza Rim AFTA allows 80 -90°C cooling, equivalent to ~2000 m unroofing. We conservatively assign ~1500 m to allow for possible higher paleogeothermal gradient and for restoration results. AGL - 2005
Kwanza Wells Basement 0 m 20 AGL - 2005 22
Blocks were more eroded than troughs were Hiatuses merge on uplifted rim 5 hiatuses VE x 5 AGL - 2005 23
Wells AGL - 2005 24
Mid-Miocene Hiatus in Inner Basin Bureau of Economic Geology This is the largest of all 4 hiatuses in time-space. Age is 12– 11 Ma. Coincided with eustatic fall of 115 m at 11. 7 Ma. Slumping began on the shelf at 15 -10 Ma (Lunde et al. 1992). Erosion was deep enough to cut all 3 main depotroughs in the inner basin. This hiatus ended marine conditions in the E of the basin. Marine conditions returned in the W of the basin. AGL - 2005
Important Observation Bureau of Economic Geology Between hiatuses, all Oligo-Miocene strata accumulated under marine conditions (indicated by planktonic forams) Conclusion: this part of Inner Kwanza Basin remained at or below sea level between hiatuses. Implication: contradicts the idea of a high (1– 2 km) coastal plateau across the basin in the Neogene. AGL - 2005
Neogene Uplift Was Highly Variable Bureau of Economic Geology Map shows the variability, which is exemplified by the next two slides. Pre-Cretaceous crustal structures probably controlled the site and size of irregular uplifts. Conclusions about uplift should be extrapolated for short distances only. Fact: uplift affected Kwanza Basin. Fiction: estimates of uplift timing and size that are extrapolated to other areas and basins. AGL - 2005
150– 250 m exhumation Cacuaco uplift da Fun la n gue Q ue Praia Increasing exhumation of shelf to N; amount uncertain Cabo Ledo uplift COASTAL FOLDBELT 500 to ~2000 m exhumation n it o c Se EAST KWANZA FOLDBELT Next slide shows cross section Morro Liso uplift Exhumation of shelf increases to S AGL - 2005 Gonga uplift 200 m exhumation 1000– 1500 m exhumation locally Basement uplift Anticline Thrust Sedimentary trough GONGA FOLDBELT 28
Previous slide shows line of section Updip fold-and-thrust belt Neogene contraction Cabo Ledo Uplift Crestal graben Congo Craton Uplift Synclinal trough--Eocene or younger 2 km shortening AGL - 2005 29
Evidence for Uplift: Truncated Shelf Bureau of Economic Geology Toplap TWT 0 Sea level Slumped mid-Miocene dates uplift Maboque-1 Older units unroofed landward 1 2 Aptian salt 5 km AGL - 2005 Truncation of shelf Pleistocene Pliocene Upper Miocene Lower Miocene Lunde et al. 1992 30
Small Uplift…. Big Effects Bureau of Economic Geology Largest Neogene uplift was in the Precambrian hinterland: 2– 4 km uplift Uplift shed a mass of Neogene sediments onto the continental margin without major rivers. Large Neogene uplift of Cabo Ledo: ~1000– 1500 m uplift of the coast and shelf Probably had a major tectonic effect (not quantified. ) Small Neogene uplift on the shelf: ~150– 250 m uplift tilted shelf and upper slope seaward. This uplift had a major effect on deep-water salt tectonics. Hudec & Jackson (2004) estimated ten-fold increase in seaward translation rate in the Miocene. AGL - 2005
Neogene Uplift and Translation Bureau of Economic Geology Northern part of Kwanza Basin W E 24 Ma, End Oligocene, Pre-uplift 0 km 5 Translation 16 km 18 km 20 km Depotroughs 3 km Updip Extension P Downdip overthrusting Present day 0 km 5 AHZ V: H = 5 AGL - 2005 150– 250 m exhumation of shelf 50 km m 500– 2000 exhumation of rim
Implications for Hydrocarbon Maturation Bureau of Economic Geology Biostratigraphy in N. Kwanza precludes significant paleoburial (except the E rim). Thus, Neogene heating (20 – 60°C) in the coastal plain is likely due to local advection of hot fluids or to increase in the paleogeothermal gradient. Both processes introduce heat, which could enhance maturation of post-salt source rocks. Walgenwitz et al. (1990) reported Miocene heating of 30 – 40°C on Angolan margin farther N. Neogene rise of the Precambrian hinterland supplied a massive sediment dump, which enhanced hydrocarbon maturity in the deep-water region. AGL - 2005
Conclusions: Fact & Fiction Bureau of Economic Geology Massive Neogene uplift of hinterland. The Kwanza rim and Precambrian hinterland were exhumed 1– 4 km, but tilting did not appreciably affect the northern coastal plain of the Kwanza Basin. No coastal plateau. Coastal plain of N. Kwanza remained near sea level in the Tertiary until 5 Ma ago; then the plain was uplifted by ~150– 250 m and thinly blanketed by continental deposits. An extended stay near sea level refutes the idea of 1– 2 km Miocene uplift of the coastal plain. Minor distal uplift was important. Minor uplift of the continental shelf and upper slope—not the inner basin—triggered Neogene gravity spreading of the Kwanza margin and Phase 2 extensional rafting. Highly variable uplift. Tertiary uplift of coast varied greatly over small distances. Be careful in extrapolating uplift observations. Mid-Miocene thermal event affected hydrocarbon maturation. Thermal effect in Kwanza coastal plain was not due to massive exhumation. Instead, probably caused by advection of hot fluids or increased geothermal gradient. This conclusion fits with previously reported Miocene heating of 30– 40°C on the north Angolan margin. AGL - 2005
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