TECTONIC AND SEDIMENTARY HISTORY OF THE MID NATAL VALLEY (S.W. INDIAN OCEAN)

ABSTRACT The Natal Valley is a sediment-filled marine basin situated between the east coast of southern Africa (Natal) and the Mozambique Ridge. Geophysical and sedimentological t~chniques are used in a broad geological study of the mid Natal Valley. Major emphasis is directed to: (a) basin history and tectonic evolution; (b) seismic stratigraphy of the basin fill; (c) recent sedimentary processes and responses. General basin morphology is defined by five major physiographic provinces: continental shelf and slope, Tugela Cone, Central Terrace, Mozambique Ridge and deep basin plain. Thinned (20-25 km) continental crust, attenuated and subsided in response to Gondwana rifting and drifting, underlies the Central Terrace, Tugela Cone and Mozambique Ridge. Southern margins of the Central Terrace and Tugela Cone are cored by a series of subsea floor ridge and pinnacle complexes (Naude, East Tugela and South Tugela Ridges). Geochemical analyses of East Tugela Ridge basalts suggest a transitional origin but with continental affinities. These volcanic marginal ridges may approximately delineate the continental-oceanic crust boundary (COB) in the Natal Valley. To the south, the deep basin plain is underlain by oceanic crust. Recognition of Mesozoic magnetic anomalies MO-M12 in the southern Natal Valley constrains the early relative motion between the Falkland Plateau (South America) and Africa. Optimum continuity of inter-continental structural lineaments is achieved by overlapping anomaly M10N of the Natal Valley and Georgia Basin. Simultaneously, the leading apex of the Falkland Plateau ' I l I i . ! ii is rotated against the South Tugela Ridge, a marginal ridge suspected to have been emplaced within continental crust imm~diately north of the COB. It is implied that, in this zone of SW Gondwana, sea floor spreading commenced at ~125 Myr. With improvement in definition of the COB of the Mozambique Ridge and Natal Valley, morphological fits suggest that the southern Mozambique Ridge may presently be situated 160 km east of its pre-drift palaeo-position. Palaeo-fracture zone trends (S72°W) accommodating the translation are recognised by basement morphology, magnetic lineaments, non-magnetic fracture ridges and bathymetric offsets on the Mozambique Ridge flanks. Crustal thickness and structural style, East Tugela Ridge basalt geochemistry and the absence of demonstrable magnetic and fracture zone lineaments suggest that the northern Natal valley is underlain by subsided attenuated continental crust. crustal status invalidates Gondwana refits in which East Antarctica is abutted against the Lebombo Line. This Seismic stratigraphic analysis of the basin fill is established relative to four regional depositional sequence boundaries: acoustic basement, horizon McDuff (CenomanianTuronian), Angus (early Oligocene) and Jimmy (early Pliocene). These sequence boundaries define the four major depositional sequences (sediment units A-D). Time-depth and time-thickness maps,for the sequence boundaries and depositional sequences respectively help reconstruct the basin sedimentary history. Over basement-high zones (Central Terrace and Mozambique Ridge), sediment unit A (pre-McDuff) growth-faulted onlap fill has aggraded to >1,0 sees time-thickness in irregular graben. In the deep basin, unit A onlap fill is locally >1,2 sees thick. iii Major progradational growth of the Tugela Cone during deposition of unit B (McDuff-Angus) permitted thick (max >2,0 sees) delta and·deep-sea fan aggradation. Unit B deep-sea fans over the Central Terrace flanks are locally thicker than 1,0 sees. Mixed offlap and onlap sequences within unit C (Angus-Jimmy) and unit D (post-Jimmy) indicate periodic Tugela Cone growth and inactive phases. Both units are locally thicker than 0,6 sees. Within units C and D, local deep-sea fan growth continued along the flanks of the Central Terrace although low-energy onlap fill and sheet drape facies characterises the Central Terrace crest and deep basin plain. Contourite mounds within the uppermost sequences of units B and C and throughout unit D are important indicators of extant current-control on sedimentation. Depositional style evolution from onlap fill (unit A) through mixed offlap, onlap and onlap fill (units B-C) to offlap and sheet drape (unit D) is typical of early rift basin fill and post-rift passive offlap fill. Total sediment thickness reaches. a maximum of 7,25 km under the upper Tugela Cone declining to 2,8-3,5 km under the eastern Tugela Cone/deep basin. Structural configurations of the major provinces have changed little since the early Oligocene. Sedimentation rates for unit A are fast (41-119 m/my) declining through units Band C (14-53 m/my) before dramatically increasing in unit D (18-132 m/my). Major controls over this trend include: the balance between sediment supply and basin subsidence, climatic change and relative sea-level. Modern sediment supply to the basin is 5-8 times higher than average rates in the geologic past, probably as a result of poor modern land management. The Agulhas Current has strongly influenced sedimentation iv processes through time by initiating: (1) slow post-Angus net sedimentation rates in scoured zones; (2) local exposure of units B artd C strata in scoured zones; (3) generation of currentcontrolled microtopography and large-scale b~dforms; (4) asymmetric sedimentation and scour moating; (5) sediment reworking and winnowing. Regions subjected to strong Agulhas Current flow inciude the Central Terrace, Mozambique Ridge and ·eastern and upper slopes of the Tugela Cone. ·Below 2500 m, North Atlantic Deep Wa~er flow around the deep basin margins inhibits fast sedimentation by inducing winnowing and redistribution processes. Invigorated deep current action is invoked to explain generation of the major early Oligocene (Angus) and early Pliocene (Jimmy) erosive hiatuses. Sea-level low stands may have initiated the equivalent coastal/shelf hiatuses. A low sea-level stand allied to regional tectonism may have triggered the nondepositional Cenoma~ian-Turonian hiatus (McDuff). The Tugela Canyon is an erosional feature that was formed during a post-early Pliocene (Jimmy) low sea-level stand. In contrast, the 29° 25'S Canyon has acted as the major disiributary fan valley on the Tugela Cone since at least the early Oligocene fostering signific~nt levee/fan complex aggradation over the eastern Tugela Cone. Post-Jimmy (unit D) sediment instability over the northernmost Tugela Cone has resulted in generation of four sediment slides, each 200-1100 km 2 in areal extent. Instability in this region is a response to fast sedimentation rates and slope oversteepening. the final trigger. Seismic shocks may have acted as Analysis of core and grab samples recovered at 54 stations v forms the basis for a regional discussion of sedimentology, lithofacies and sediment dispersal. Modern sediment composition and carbonate content is strongly controlled by the influx and dispersal of terrigenous detritus. Major terrigenous depocentres and dispersal routes are characterised by low carbonate contents and abundant detrital minerals. Pelagic environments are dominated by foram and coccolith oozes. Hinterland-sourced illite is the dominant marine clay mineral group and is concentrated in the terrigenous depocentres. Smectites and kaolinites are locally hinterland-derived but there is significant import from the Mozambique/Madagascar area by Agulhas Current flow. Textural analyses reveal five major provinces related to energy regime and detrital type. Equilibrium mud deposition is active over the south Tugela Cone terrigenous depocentre. Pelagic provinces are coarser-grained because of the large sandsize foram population. Winnowing and sand-enrichment occurs beneath regions of vigorous current flow. Sand fraction sizefrequency distributions verify the presence of two overlapping and mixing hydraulic populations within the basin: sand (biogenic) and very fine sand (terrigenous). medium-fine Progressive mixing is best developed over the south Tugela Cone and adjacent deep ·basin. Five modern lithofacies are discriminated: turbidite, hemipelagite, pelagite, muddy contourite and sandy contourite. Facies assemblage distributions are primarily controlled by proximity to terrigenous sources, dispersal routes and the strength of current influences. v forms the basis for a regional discussion of sedimentology, lithofacies and sediment dispersal. Modern sediment composition and carbonate content is strongly controlled by the influx and dispersal of terrigenous detritus. Major terrigenous depocentres and dispersal routes are characterised by low carbonate contents and abundant detrital minerals. Pelagic environments are dominated by foram and coccolith oozes. Hinterland-sourced illite is the dominant marine clay mineral group and is concentrated in the terrigenous depocentres. Smectites and kaolinites are locally hinterland-derived but there is significant import from the Mozambique/Madagascar area by Agulhas Current flow. Textural analyses reveal five major provinces related to energy regime and detrital type. Equilibrium mud deposition is active over the south Tugela Cone terrigenous depocentre. Pelagic provinces are coarser-grained because of the large sandsize foram population. Winnowing and sand-enrichment occurs beneath regions of vigorous current flow. Sand fraction sizefrequency distributions verify the presence of two overlapping and mixing hydraulic populations within the basin: sand (biogenic) and very fine sand (terrigenous). medium-fine Progressive mixing is best developed over the south Tugela Cone and adjacent deep basin. Five modern lithofacies are discriminated: turbidite, hemipelagite, pelagite, muddy contourite and sandy contourite. Facies assemblage distributions are primarily controlled by proximity to terrigenous sources, dispersal routes and the strength of current influences. vi Modern sediment input to the basin has four main sources: (1) hinterland-sourced bedload detritus; (2) hinterland-derived suspended muds; (3) bedload and suspensates introduced from the north by current flow; (4) pelagic rain. Terrigenous bedload is funnelled to modern basin depocentres under Agulhas Current traction while suspensates are more widely distributed. remote from the hinterland are dominated by pelagic and Areas hemipelagic sedimentation. Winnowing and redispersal of muds and very fine sands is common in current-scoured areas. Only suspended muds have the potential for significant export from the Natal Valley basin. Development of the Tugela Fan (Tugela Cone and deep basin) has followed standard models of deep-sea fan evolution with the 29° 25•5 Canyon acting as the major mid fan distributary channel/levee system. Major fan growth has probably taken place during low sea-level stands and/or phases of deltaic progradation to the outer shelf. Large regions of the fan are temporarily inactive because of the modern high sea-level stand and changes in canyon status. The large size (upper-mid fan is 230 x 170 km in dimension) but atypical morphology of the Tugela Fan favours its classification as an hybrid fan type but with close affinities to the elongate fan type end-member.