Thin-skinned extensional tectonics on a salt detachment, northern Kwanza Basin, Angola

Thin-skinned extensional tectonics on a salt detachment, northern Kwanza Basin, Angola Erik R. Lundin* Conoco Norway Inc., PO Box 488, 4001 Stavanger, Norway

Received 11 October 1991; revised 1 December 1991; accepted 13 December 1991 Thin-skinned extension on a salt detachment, of a style known as raft tectonics, characterizes deformation of the late Aptian to Recent section in the northern Kwanza Basin, Angola. Large rafts and intervening grabens are interpreted to have developed as a result of gravity gliding (increased dip) and gravity spreading (asymmetrical loading of a sedimentary wedge). Major raft movement occurred primarily from the Middle Tertiary, but some deformation appears to have started as early as late Cenomanian or early Turonian times. Individual grabens measure up to 70 km in length, 15 km in width and nearly 3 km in depth, and show switching polarities in the sense of thickening of the Tertiary sedimentary infill. Graben geometries were controlled by syndepositional extension and the local availability of salt. From the data examined, which includes both seismic and well data, the grabens appear to become younger from east to west, reflecting basinward progression of the deformation. As the age relationships could be related to insufficient sampling of strata deposited in complex sedimentation patterns, they may have no relevance to the timing of the deformation; the diachronous deformation is therefore speculative. Extension in the northern Kwanza Basin is estimated at 60%; the outermost raft is translated 55 km westwards. In addition to the extensional structures, compressional folds, thrust faults and salt structures mark the toe of the gravity gliding and spreading system. All of the rafts lie within the African portion of the Aptian Salt Basin, on rifted continental crust, and they are generally unrelated to structures below the salt. Biostratigraphy and lithostratigraphy indicate that the grabens formed on the shelf and upper slope from the Middle Eocene. Associated compressional features developed near the slope rise. The unusual expression of raft tectonics is attributed to a combination of: (1) the presence of a salt layer; (2) a stable basement configuration that allowed quiescent accumulation of the raft sequence; and (3) an increase in basin dip and sediment supply. Keywords: Kwanza Basin, Angola; extension tectonics; salt tectonics; raft tectonics

Introduction

The Kwanza (or Cuanza) Basin of Angola is located on the continental margin of West Africa between latitudes 8°S and 11°30'S [Figure I (upper panel)]. The area lies within the West African Aptian Salt Basin that extends between southern Cameroon and Angola (Schlumberger, 1983). The paper of Brognon and Verrier (1966) remains the most comprehensive published treatment of Kwanza Basin geology to date. The concept of raft tectonics in the Kwanza Basin was introduced by BuroUet (1975) and was more recently described by Jackson and Cramez (1989) and Duval and co-workers (1990; 1992). Raft tectonics refers here to large lateral movements of rigid blocks [the pre-kinematic layer of Vendeville and Jackson (1992a)] separated by grabens that are filled with syndeformational strata [the synkinematic layer of Vendeville and Jackson (1992a)]. The lateral dimensions of individual rafts and grabens are as much *Present address: Statoil Research Center, Postuttak, N-7004 Trondheim, Norway

0264-8172/92/040405-07 ©1992 Butterworth-Heinemann Ltd

as 70 km in length and 15 km in width (Figure 1). The graben infill, referred to as the gravity deformation sequence, displays a switching polarity in the sense of sedimentary infill thickening. Based on geometry, the gravity deformation sequence is subdivided into: (1) a core; (2) a landward expanding and rotated growth sequence; and (3) a basinward expanding and rotated growth sequence (Figure 2). Previous work on the Kwanza Basin has addressed the origin of the grabens. Verrier and Branco (1972) proposed that the graben geometries resulted from syndepositional vertical salt evacuation from large salt walls, without any horizontal extension. Their model is rejected here because: (1) it cannot be restored; (2) major extension is now recognized in other parts of the basin; and (3) it is difficult to dissolve a large salt pillow concurrent with significant sedimentation. Although Burollet (1975) recognized extension, he proposed gravity gliding to be the sole cause of rafting. This paper presents an alternative, extensional and restorable model that relies on gravity spreading in addition to gravity gliding as the driving mechanism. The study included mapping of ~4500 km of offshore seismic data and a study of the available well logs, published data and internal company reports. Although extensional grabens in brittle rocks overlying salt detachments are well known (e.g. McGill

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and Stromquist, 1979), analogues of the Kwanza Basin grabens have not been described to the author's knowledge. However, it has been possible to find many similarities with scaled deformation experiments (Jackson et al., 1990; VendeviUe and Jackson, 1992a; 1992b). General stratigraphie

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The stratigraphy of the Kwanza Basin can be simplistically subdivided into pre-salt and post-salt sequences that were originally separated by an Aptian salt layer (Figure 2). The pre-salt sequence consists of Neocomian alluvial and fluvial lake margin sandstones 406

that grade upwards into carbonates and basinal lacustrine shales that infilled sub-basins in the block faulted rift topography. These are, in turn, overlain by a regional Barremian siliciclastic wedge deposited in fluvial and lacustrine settings during thermal subsidence. Early Aptian rebound of the rift shoulders resulted in regional erosion and deposition of a thin blanket sand (McHargue, 1990). Subsidence related to the drift between Africa and South America allowed the first marine transgression and consequent deposition of the Aptian salt in a restricted marine setting (Evans, 1978; Brice et al., 1982). Within the study area, the post-salt sequence

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referred to as the raft sequence consists of Albian through Cenomanian sediments that were deposited before the onset of gravity deformation, as well as Turonian to Maastrichtian sediments that appear to have experienced some extension. The Tertiary section that was deposited during major raft movement is called the gravity deformation sequence (Figure 2). Regionally, the gravity deformation sequence appears to transgress time, becoming younger to the west (Figure 3G). These age relationships are supported by published information from the Quenguela Graben (Verrier and Branco, 1972), proprietary well data in the Gaivota Graben, scout information from the Calombaloca Graben and seismic mapping of the Western Graben. However, it remains possible that the observed age relationships are related to insufficient sampling of strata that were deposited in

complicated sedimentation patterns caused by sediment supply, eustacy and instability of the salt. The apparent age relationships may therefore have no explicit relationship to the timing of structural events. Structural

model

The structural model presented here is derived from analyses of the graben infill geometries (Figure 2), age relationships between grabens (Figure 3G), restoration of the extended part of the basin (Figure 3A-F) and cross-section restoration of the Western Graben (Figure 4). The Western Graben was selected for restoration because of good seismic coverage of the graben and adjoining rafts. This graben is also better imaged seismically because it has not experienced late stage crestal faulting (Figure 4H).

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Thin-skinned extension in the Kwanza Basin: E. R. Lundin sequence continues to develop and rotate until the salt is depleted beneath the western raft. At this final stage. Thickness variations in the Turonian and Maastrichtian the graben and both adjoining rafts remain stationary section (Figure 4B) indicate early salt movement. A and are thereafter bypassed by sediments. The change in the sense of thickening of the overlying Quenguela and Gaivota Grabens have reached this Middle Eocene section is also evident. Both of these configuration, whereas the Western Graben is still thickness trends may be related to early extension and extending. associated salt diapirism. Figure 18 of Vendeville and Polarity switches in the sense of thickening, from Jackson (1992a) supports this mechanism. Depositional early landward to later basinward thickening, as a thickening of the Turonian and Maastrichtian section result of salt withdrawal have also been described from towards the graben axis may be attributed to reactive the Corsair fault system in the northern Gulf of Mexico diapirism. Tectonic thinning of the overburden related (Worall and Snelson, 1989). to extension would have resulted in differential loading, allowing the salt to rise between the rafts while Regional development of grabens a graben developed above the diapir (Figure 4C). Well and seismic data suggest that the grabens become Subsequent shouldering of the rafts during active progressively younger to the west (Figure 3G). As diapirism could account for the observed depositional discussed above, these age relationships could be thinning of the overlying Middle Eocene section artifacts of complicated sediment distribution patterns. towards the graben axis. A measure of scepticism is therefore appropriate with In the restored sections, the once triangular shaped respect to the diachronous nature of the graben piece of Turonian and Maastrichtian section (Figure development and the concept should be tested bv 4B) cannot be accounted for. Slivers of the 'missing' future drilling. section are schematically shown to be downfaulted into It is proposed that in addition to gravity gliding, the graben. The section could also reside on the updip differential loading related to the westward side of the eastern remnant salt pillow (Figure 4G). progradation of shelf sedime.nts drove the deformation Alternatively, the section may have been pushed out by (gravity spreading) and propelled the salt basinward. the rising salt and eroded. Thus the basinward side of the system would have remained free to move while the landward side Major raft separation successively became welded, resulting in sediment Structurally, the time of core development is bypass and basinward progression of the deformation. considered to mark the onset of significant raft In support of the proposed mechanism, salt movement. Cross-section restoration shows that from accumulations are observed in the outer part of the the time of core development, the salt level in the basin (Figure 1). Salt flow in a basinward direction due graben was falling due to its limited availability in to gravity spreading has been suggested for the Sigsbee relation to the increasing gap between the extending salt nappe in the Gulf of Mexico (e.g. Worall and rafts (Figure 4C-H). This behaviour is also observed in Snelson, 1989). In other aspects, however, this salt experimental deformation [see figure 6 of Vendeville deformation differs from the described raft tectonics. and Jackson (1992b)]. The age of the oldest dated Scaled deformation experiments also support the sediments in the grabens is Middle Eocene, indicating concept of basinward flow of salt during gravity that major raft movement started no later than this spreading (Jackson et al., 1990). (Figures 3G and 4D). It is difficult to determine whether gravity gliding The cores of the grabens are striking features, the dominated over gravity spreading or vice versa; geometries of which are established by drilling in the extensional deformation can be induced by both Quenguela Graben, by seismic and well data in the mechanisms (Jackson et al., 1990; Vendeville and Gaivota Graben and by forward cross-section Jackson, 1992a; 1992b). A combination of the two is modelling (Figure 4H). The reason for the symmetry of probably responsible for the observed raft the cores, in contrast to the asymmetric overlying deformation. sequences, is not established. As the genetic Although the age of the oldest graben sediments significance of the cores is unclear, they should perhaps indicate that significant raft movement started no later be included in the landward expanding growth than the Middle Eocene, the rate of deformation seems sequence. to have increased during the Oligocene and Miocene. In all the grabens studied, the landward expanding Accelerated raft movement was probably related to growth sequence developed before the basinward shelf progradation initiated by the major Oligocene sea expanding growth sequence. It is proposed that level fall at 29 My (Haq et al., 1978) and particularly to prograding sediments loaded the landward side of the uplift of the African craton starting in the Late grabens and initiated the asymmetry, in a manner Oligocene (Bond, 1976; Walgenwitz et al., 1990). similar to the development of other growth faults (e.g. Restoration of the extensional regime yields an Daftly, 1976). The landward expanding growth estimated total extension of 55 km (60%). Comparison sequence continued to grow and rotate until it between stretching in the extensional regime and grounded against the pre-salt sequence and became shortening in the compressional regime is presently not welded to it (Figure 4G). possible because of the western limit of seismic data. Growth of the basinward expanding sequence The apparent discrepancy between extension and represents the final phase of graben development. The compression seen in Figure 1 (lower panel) is related to forward modelled cross-section (Figure 4H) illustrates the limits of the seismic data. A true test of the how the sequence rolls over as the western raft proposed model would be to balance extension and continues gliding basinward; the eastern raft and the shortening; this will be possible when more seismic data graben infill remain welded to the pre-salt section. The are acquired in the future.

First salt movement and possible extension

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Thin-skinned extension in the Kwanza Basin: E. R. Lundin /

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Figure 5 Schematic drawing from seismic data showing an Upper(?) Miocene distributary channel downfaulted into the Western Graben. The channel acts as a structural marker, showing extensional deformation and providing an estimate of age by seismic tie to the drilled Gaivota Graben. The channel is downfaulted in a direction approximately N80W and the throw and heave are about 5 and 2 km, respectively

Discussion The extensional origin of the deformation is supported by several independent lines of evidence. The most direct support is provided by an Upper(?) Miocene distributary channel that is downfaulted into the Western Graben (Figure 5). Other indications of extension are transfer zones between slightly offset grabens. Transfer zones typically occur between offset extensional features at all scales (e.g. Sempere and Macdonald, 1986; Morley et al., 1990), but are not commonly associated with the vertical evacuation of salt. The compressional folds, thrust faults and salt structures at the toe of the raft system provide a coherent structural couple with the extensional grabens. Examples of such coupled structural regimes are well documented (e.g. Buffler et al., 1978; Winker and Edwards, 1983). Further support for extension is furnished by the similar seismic character of rafts on opposite sides of the Western Graben, which would be unlikely if the grabens were caused by salt withdrawal from a large pillow. It would also be difficult to explain the loss of salt from a large pillow, in conjunction with major sediment input, by a mechanism other than extension (Vendeville and Jackson, 1992b). From the cut-off points of the downfaulted channel (Figure 5) it can be determined that the grabens generally opened perpendicular to their axial trends. An exception is the anomalous trending Sangano Graben (Figure 1, upper panel). It is interpreted to have been initiated as a tear fault, which developed into a graben by left lateral motion. Its location was influenced by the Cabo Ledo Uplift that acted as a buttress, limiting extension to the east. The easternmost, arcuate, Calombaloca Graben is 410

considered to be the breakaway fault zone for the extensional system. This graben probably follows the original updip limit of the salt analogous to the location of the Mexia-Talco fault zone at the updip limit of the Louann Salt in eastern Texas (Cloos, 1968). The unique style of extensional deformation described in this paper is attributed to: (1) the presence of a salt layer, which (2) was overlain by the raft sequence that accumulated over a stable basement, followed by (3) an increase in westward basin dip and sediment supply. In contrast, in other parts of the basin the extensional deformation is characterized by closely spaced listric faults that were active by the Albian and that generally became inactive before the Tertiary [see figures 12 and 13 of Nunns (1991)]. In these areas the pre-salt surface appears to have developed basinward dip relatively early. This resulted in early gravity gliding, the breakup of the post-salt sequence into smaller rotated blocks, associated redistribution of the salt and grounding of the blocks. Consequently, rafts and grabens of the type described in this paper did not develop. Certain parts of the Kwanza Basin appear to have experienced both the Late Cretaceous extensional phase that produced closely spaced listric faulting and the Tertiary extensional phase that created large rafts (Duval et al., 1990; 1992). This study indicates that extensional deformation has resulted in significant lateral translations of the Aptian to Recent section in the northern Kwanza Basin. If correct, recognition of this deformation has direct implications to hydrocarbon exploration of the post-salt sequence. In particular, the relationship between the spacial location of reservoirs, the time of hydrocarbon migration and distribution of the salt is critical. The complex interplay between hydrocarbon generation,

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Thin-skinned extension in the Kwanza Basin: E. R. Lundin Duval, B., Cramez, C. and Jackson, M. P. A. (1990) Raft tectonics migration and trapping is in many ways analogous to in the Kwanza Basin, Angola GeoL Soc. Am. Abs. Prg. 22, A48 the situation in the Campos Basin, Brazil (Cobbold and Duval, B., Cramez, C. and Jackson, M. P. A. (1992) Raft tectonics Szatmari, 1991), the South American rifted counterpart in the Kwanza Basin, Angola Mar. Petrol GeoL 9, 389-404 of the Kwanza Basin. Evans, R. (1978) Origin and significance of evaporites in basins around Atlantic margin Am. Assoc. Petrol GeoL Bull. 62, The model put forth is based largely on field 223-234 observations, geological methods such as restoration Haq, B. U., Hardenbol, J., Vail, P. R., Wright, R. C., Stover, L. E., and the integration of recent concepts of salt tectonics. Baum, G., Loutit, T., Gombos, A., Davies, T., Pflum, C., The model has neither been tested by numerical Romine, K., Posamentier, H. and Jan du Chene, R. (1988) Mesozoic-Cenozoic chronostratigraphic- and eustatic-cycle modelling, nor has it been matched in detail by chart. In: Sea-level Changes: An Integrated Approach (Eds experimental deformation, and as such should be C. K. Wilgus, C. G. Hastings, C. Kendall, H. W. Posamentier, regarded a hypothesis to be tested by further drilling, C. A. Ross and J. C. van Wagoner), Soc. Econ. PaleontoL seismic interpretation and other geophysical methods, Mineral Spec. PubL No. 42 as well as theoretical and experimental modelling. Jackson, M. P. A. and Cramez, C. (1989) Seismic recognition of

Acknowledgements I thank Conoco Inc, Conoco Cegohna (Angola) Ltd, Sonangol, Total, Unocal and Citizen Energy for allowing publication of this paper. S. Scott and M. Spangler in Conoco laid much of the groundwork and many other colleagues provided help and guidance during the project - - I thank you all. Discussions with the Applied Geodynamics Laboratory in Austin, Texas under the direction of M. P. A. Jackson strengthened the structural model. I am grateful for the opportunity to meet with this group and acknowledge their significant contributions to salt tectonics. The manuscript benefited from constructive reviews by J. Hossak and M. P. A. Jackson. References Bond, G. (1976) Evidence for Late Tertiary uplift of Africa relative to North America, South America, Australia, and Europe J. GeoL 86, 47-65 Brice, S. E., Cochran, M. D., Pardo, G. and Edwards, A. D. (1982) Tectonics and sedimentation of the South Atlantic rift sequence: Cabinda, Angola. In: Studies in Continental Margin Geology (Eds J. S. Watkins and C. L. Drake), Am. Assoc. Petrol GeoL Mem. No. 34, pp. 5-18 Brognon, G. P. and Verrier, G. R. (1966) Oil and geology in Cuanza Basin of Angola Am. Assoc. Petrol GeoL Bull. 50, 108-158 Buffler, R. T., Shaub, F. J., Watkins, J. S. and Worzel, J. L. (1978) Anatomy of the Mexican Ridges, southwestern Gulf of Mexico. In: Geological and Geophysical Investigations of Continental Margins (Eds J. W. Watkins, L. Montadert and P. W. Dickerson), Am. Assoc. Petrol GeoL Mere. No. 29, 319-327 Burollet, P. F. (1975) Tectonique en radeaux en Angola Bull. Soc. G#oL Fr. XVll, 503-504 Cloos, E. (1968) Experimental analysis of Gulf Coast fracture patterns Am. Assoc. Petrol Geol. Bull. 52, 420-444 Cobbold, P. R. and Szatmari, P. (1991) Radial gravitational gliding on passive margins Tectonophysics 188, 249-289 Dailly, G. C. (1976) A possible mechanism relating progradation, growth faulting, clay diapirism and overthrusting in a regressive sequence of sediments Can. Petrol. Geol. Bull. 37, 92-116

salt welds in salt tectonic regimes Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists Tenth Annual Research Conference, Houston, TX, USA Program and Abstracts, pp. 66-71 Jackson, M. P. A., Vendeville, B. C. and Schulz-Ela, D. D. (1990) Applied Geodynamics Laboratory 1989 Annual Report to Industrial Associates, Bureau of Economic Geology, University of Texas, Austin, TX, pp. 1-21 McGill, G. E. and Stromquist, A. W. (1979) The grabens of Canyonlands National Park, Utah: geometry, mechanics, and kinematics J. Geophys. Res. 84, 4547-4563 McHargue, T. R. (1990) Stratigraphic development of protoSouth Atlantic rifting in Cabinda, Angola: a petroliferous lake basin. In: Lacustrine Basin Exploration: Case Studies and Modern Analogs (Ed. B. J. Katz), Am. Assoc. Petrol GeoL Mere. No. 50, pp. 307-326 Morley, C. K., Nelson, R. A., Patton, T. L. and Munn, S. G. (1990) Transfer zones in the East Africa rift system and their relevance to hydrocarbon exploration in rifts Am. Assoc. Petrol GeoL Bull. 74, 1234-1253 Nunns, A. G. (1991) Structural restoration of seismic in extensional regimes Am. Assoc. Petrol GeoL Bull. 75, 278-297 Sempere, J. C. and Macdonald, K. C. (1986) Overlapping spreading centers: implications from crack growth simulation by displacement discontinuity method Tectonics 5, 151-163 Schlumberger (1983) Well Evaluation Conference, Afrique de I'Oest, Schlumber, Paris, Ch. 1, pp. 1-28 Vendeville, B. C. and Jackson, M. P. A. (1992a) The rise of diapirs during thin-skinned extension Mar. Petrol. Geol. 9, 331-354 Vendeville, B. C. and Jackson, M. P. A. (1992b) The fall of diapirs during thin-skinned extension Mar. Petrol GeoL 9, 354-371 Verrier, B. and Branco, C. (1972) The Tertiary trough and the oilfield of Quenguela North (Cuanza Basin) Rev. Inst. Fr. Petr. XXVll, 51-72 Walgenwitz, F., Pagel, M., Meyer, A., Maluski, H. and Monie, P. (1990) Thermochronological approach to reservoir diagenesis in the offshore Angola Basin: a fluid inclusion, Ar4°-Ar 39and K-Ar investigation Am. Assoc. Petrol Geol. Bull. 74, 547- 563 Winker, C. D. and Edwards, M. B. (1983) Unstable progradational clastic shelf margins. In: The Shelf Break: A Critical Interface on Continental Margins (Eds D. J. Stonley and G. T. Moore), Soc. Econ. PaleontoL Mineral Spec. PubL No. 33, pp. 139-157 Worall, D. M. and Snelson, S. (1989) Evolution of the northern Gulf of Mexico, with emphasis on Cenozoic growth faulting and the role of salt. In: The Geology of North America ~ an Overview (Eds A. W. Bally and A. R. Palmer), Geological Society of America, Boulder, CO, pp. 97-138

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