Sediment distribution and Cenozoic sedimentation patterns on the Agulhas Plateau BRIAN E. TUCHOLKE Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 GEORGE B. CARPENTER ABSTRACT Hayes, 1971; Francheteau and Le Pichon, 1972; Ladd, 1974) show that prior to about 110 m.y. B.P. the Falkland Plateau must have Unconsolidated sediments on the Agulhas Plateau range in age occupied the present position of the Agulhas Plateau. Because the from Maestrichian to Quaternary and consist of calcareous ooze. A Falkland Plateau appears to be underlain by continental crust (Ew- composite sedimentary record from piston cores suggests three ing and others, 1971; Barker, Dalziel, and others, 1974), it there- hiatuses in sedimentation which can be correlated with angular un- fore would appear that the Agulhas Plateau cannot be older than conformities inferred from seismic profiles. These hiatuses encom- about 110 m.y. (Barrett, 1977). However, we cannot rule out the pass late Miocene—Pliocene, middle Oligocene, and late possibility that the Agulhas Plateau predates 110 m.y. B.P. and that Paleocene-middle Eocene time and correlate with similar strati- it moved to its present position during or after the westward dis- graphic gaps widely distributed in Deep Sea Drilling Project placement of the Falkland Plateau. boreholes. The deeper basins surrounding the plateau contain ter- This report summarizes available geological and geophysical evi- rigenous and calcareous or siliceous hemipelagic sediments; sedi- dence pertaining to the depositional history of the sedimentary sec- ment composition is controlled by depth and locality of deposition. tion on the Agulhas Plateau. Relatively little attention has been de- Strong influence of bottom currents on sediment distribution dur- voted to the plateau in the literature, but the position of the plateau ing Cenozoic time is demonstrated by a well-developed erosional in the Cretaceous breakup of Gondwanaland (Larson and Ladd, zone near 4,500-m depth around the plateau as well as by local 1973) suggests that its sedimentary column contains an important erosion and sediment drifts on the plateau itself. The plateau can be record of late Mesozoic and Cenozoic oceanographic events. The divided into a northern and a southern province on the basis of dif- effects of abyssal currents should be clearly recorded in these sedi- ferences in the nature of acoustic basement. Basement in the south- ments because of the high southern latitude of the plateau and its ern province is smooth with some faulting and contains weak proximity to the source of Antarctic Bottom Water. We report here internal reflectors unconformable with the basement surface; the the results of examination of 12,500 km of seismic profiler data reflectors may represent layered basalt flows or consolidated and 33 piston cores as a preliminary step in investigating sedimen- Mesozoic sediments deposited shortly after the formation of the tation dynamics on the Agulhas Plateau. Profiler records were ob- plateau. Acoustic basement in the northern section is intensely tained using a 20-in.3 airgun sound source, and they were recorded faulted and shows no evidence of internal stratification. at a variety of band passes between 10 and 200 Hz. INTRODUCTION SEDIMENTARY RECORD IN PISTON CORES The Agulhas Plateau is a large, aseismic structural high in the Thirty-three piston cores ranging in age from Quaternary to southwesternmost Indian Ocean, lying just southeast of the Cape of Cenomanian have been recovered from the study area; 23 of them Good Hope. Its dimensions are about 400 by 700 km, with the long are from the Agulhas Plateau (Figs. 2, 3). Three general lithofacies axis extending roughly north-south, and its minimum depth is are recognized, all of which are dependent on depth and locus of slightly less than 2,500 m or about 2.5 km above the surrounding deposition. ocean floor (Fig. 1). Heezen and Tharp (1964) described the general Foraminiferal and nannoplankton oozes are found on the morphology of the plateau and made a distinction between the Agulhas Plateau in water depths less than about 4,000 m. A clear rough topography of the northern third of the plateau and the sub- distinction exists between the composition of the calcareous com- dued topography of the southern part. Barrett (1977) has shown ponent of Pliocene-Quaternary sediments and of pre-Pliocene sed- that this difference is caused primarily by variations in basement iments. Whereas foraminiferal oozes dominate the younger sedi- morphology under variable sediment cover; the dividing line be- ments, nannoplankton oozes or nannoplankton-foraminiferal tween the provinces is near lat 38.5CS. oozes are prevalent in sediments of Miocene to Maestrichtian age. The origin, structure, and age of the Agulhas Plateau are as yet This compositional change may reflect either changes in the com- poorly defined. Graham and Hales (1965) used a free-air gravity position of the surface-water biocoenose or preferential dissolution model to infer a Moho depth of about 21 km beneath the plateau, of foraminifera in the older samples. In general, foraminifera in the leaving open the question of whether it indicated thinned continen- pre-Pliocene sediments are moderately to poorly preserved, and tal or thickened oceanic crust. Scrutton (1973), on the basis of nannoplankton exhibit moderate preservation. Because foraminif- morphology, suggested that the plateau might be a relict spreading era tend to be less solution-resistant than nannoplankton, foram- center. More recently, Barrett (1977) reported the results of seismic iniferal dissolution may have resulted in relative enrichment of refraction and magnetic studies that suggest that at least the rugged nannofossils. northern one-third of the plateau may be of oceanic origin. Sedi- Even if this mechanism is valid, the dissolution cannot be attrib- ment as old as Cenomanian has been reported from the western uted to deposition below the carbonate compensation depth flank of the plateau (Saito and others, 1974). Reconstructions of (CCD); sediments in all but two cores were deposited several South America and Africa in Cretaceous time (Le Pichon and hundred metres to more than 1,000 m above the probable contem- Geological Society of America Bulletin, v. 88, p. 1337-1346, 7 figs., September 1977, Doc. no. 70912. 1337 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/9/1337/3418608/i0016-7606-88-9-1337.pdf by guest on 29 September 2021 1338 TUCHOLKE AND CARPENTER porary CCD, as deduced from JOIDES borehole data for both the Transkei Basin contain sharp-based, graded turbidites that have Atlantic and Indian Oceans (van Andel, 1975). Nannoplankton in been derived from southeast Africa. Turbidites are rare in cores two cores containing sediments deposited near or slightly below the from the western Agulhas Basin, where the Agulhas Fracture Zone contemporary CCD are poorly preserved (V24-213, Maestrichtian) and Agulhas Ridge block southward transport of terrigenous de- and well preserved (V24-215, Paleocene). The best explanation for tritus (Fig. 3). the dissolution may be that the calcareous sediments were exten- A third sediment type was cored in the southern Agulhas Basin. sively reworked by slightly to moderately corrosive bottom cur- This is a hemipelagic diatom-radiolarian clay found at depths rents. Very good evidence for such sediment reworking and redis- below 5 km. All these cores are Pliocene to Quaternary in age, and tribution is seen in both the cores and in the seismic profiler rec- they reflect the surface-water production of siliceous organisms ords, which show erosional unconformities and internal reflectors near the northern limit of the Antarctic polar front zone (Lisitsin, conformable with neither the sea floor nor with deeper acoustic 1972, p. 156). horizons. Many cores, notably V22-124, V22-125, and V22-133, Manganese nodules and crusts several centimetres to tens of cen- contain abundant reworked nannoplankton and foraminifera that timetres in diameter or thickness are also a common component in bottom currents have apparently eroded from older outcrops and cores recovered from the Agulhas Plateau. They usually occur at redeposited at the coring sites. levels in the cores where unconformities exist, notably at the The second most common sediment is detrital clastic (terrige- boundary where Pliocene-Pleistocene sediments unconformably nous) and hemipelagic material. This kind of sediment is found on overlie older deposits. Manganese stains and micronodules are the Agulhas Plateau below about 4,000 m, in the basins west and common at other levels in most cores. east of the plateau, and in the Transkei Basin. Between about 4-and Two cores from the west-central plateau contain significant 5-km water depth, calcareous organisms are a common component quantities of clastic sand. Core V22-123 contains medium sand (< 40%) of these mostly clayey sediments, but barren clays, silty that is mostly iron-stained and represents a suite of mafic-igneous clays, and sand are predominant below 5 km. Cores from the minerals (V. Kolla, 1976, personal commun.); core V22-133 con- Figure 1. Bathymetry of Agulhas Plateau, modified from Simpson (1974). Lamont track control is shown by light lines. Heavy lines show position of profiles in Figure 6. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/9/1337/3418608/i0016-7606-88-9-1337.pdf by guest on 29 September 2021 SEDIMENTATION PATTERNS ON THE AGULHAS PLATEAU 1339 tains as much as 20% of similar minerals in the Pleistocene crest of the plateau. Bottom-water potential temperatures range foraminiferal ooze (Fig. 2, 3). Both of these cores were taken near from about 0.2 °C in the western Agulhas Basin (<0.0 °C in the localized, rugged basement peaks that protrude through the sedi- southern Agulhas Basin) to greater than 1.8 °C over the Agulhas ment cover of the plateau (Fig. 5); the sands are probably derived Plateau (Wyrtki, 1971; Kolla and others, 1976).