An outline of the inshore submarine geology of southern South West Africa and Namaqualand

Abstract

Abstract provided by author:

Offshore, the unconsolidated sediment thins and terminates between 15 km and 20 km from the shoreline to expose the surface of the Intermediate Sediment. Lenses of the Acoustical Blanking Layer (A. B. L.), a sediment apparently impervious to the acoustic impulses of the Sparker-profiling system, screen details of the underlying sediment and bedrock between the Buffels River mouth and Cliff Point and also between Wreck Point and Mittag in the M. D. C. A wide and thick sediment body reflects the presence of the Orange River, at whose mouth a buried channel system is also present. The inshore coastal fringe between Mittag and Chameis is also largely sediment-free. The seaward edge of the unconsolidated sediment trends shorewards, being at least 30 km off the Orange River and only lO km off Chameis. Inshore, between Chameis and Elizabeth Bay, is a persistent and relatively thin wedge of sediment. At Elizabeth Bay itself there is another well developed buried river channel. The Intermediate Sediment suboutcrop is relatively shallow (-50 m) off Mittag but increases in depth to -95 m off Plumpudding Island, at which depth it remains at least as far as Luderitz

The sediment distribution north of Hottentot Bay in the T. D. C. is more irregular than that further south. Areas of sediment-free bedrock are breached at several places by prominent tongues of sediment striking inshore and occupying depressions in the bedrock. A striking northeasterly trending rocky ridge, the summit of which comprises Hollams Bird Island, has trapped the sediment in a thick, curved belt on its southern side. The thickness of the sediment in general and the widespread presence of the ABL have combined to obscure the details of the sediment distribution on the TDC

The Intermediate Sediment's appearance on the Sparker records is illustrated. Work carried out elsewhere supports the contention that it is pre-Quaternary in age. The ABL is thought to be a gas-rich, fine-grained sediment

Work on the grab samples collected at irregular intervals from the unconsolitdated sediment surface is described. Determinations of the grain size distributions, supplemented by a section on the theory behind such determinations, show that in general the sand is fine to very fine grained, well sorted and only slightly skewed. The size distribution curves tend to be leptokurtic. The only statistically meaningful relationship is that between mean size and skewness: as the mean grain size increases, the sediment has a wider range of fine grained constituents asoiated with it. The sediment is characteristic of those associated with unidirectional currents

There is a highly significant statistical difference in the mean percentage (by weight) of heavy minerals in the surface sediment of the three sub-areas: MDC has a mean value of 25 percent, TDC has 15 percent, and SDC about 8 percent. The epidote-pyroxene-amphibole group of minerals is the most abundant, but garnet and ilmenite are also common. The local rivers appear to have been important contributors of heavy minerals

There is also a highly significant statistical difference in the mean percentage (by weight) of total carbonate content between grab samples of sediment located south of the Orange River mouth and those north of the mouth. The mean carbonate content of the S. D. C. is 16 percent but it is only some 4 percent for both M. D. C. and T. D. C. The Orange River mouth thus marks the boundary between sediments with relatively high heavy mineral and low carbonate contents, and vice versa

The general stratigraphic sequence of sand, shell, gravel and compacted silt on basement bedrock or shelly sandstone/conglomerate is described and amplified by detailed descriptions of the Plumpudding Island and Hottentot Bay deposits

The Elizabeth Bay and Orange River buried river channels are described in detail. The Elizabeth Bay channel is 12 km long, 1 km wide and has been incised some 40 m to 50 m into the local bedrock. The offshore end of the Orange River channel is obscured but its dimensions appear to be comparable with th eElizabeth Bay channel. Each slopes some 40 m per 8 km seaward. A submarine cliff about 3m high has been traced for 15 km in the MDC area. Its base lies between -18m and -20 m below mean sea level, which agrees closely with prominent bedrock flattening between -18 m and -24 m traced intermittently through most of the areas for which reliable information is available. Twenty eight bedrock profiles located between Pomona and Dreimaster Bay in the MDC were examined. They show tha the bedrock at depths of -32 to -34; -40 to -42 and -52 to -54 is anomalously flat; in more general terms the bedrock between -44 and -58 m appears to be planed off. The closing chapter deals with events of the Pleistocene Epoch and their local effects. A postulated reconstruction of Quaternary events is presented, the most significant being as follows: Illinoian glacial stage: marine regression to about -90 m; Sangamon or last interglacial: marine transgression. Erosion of the major cliff and deposition of the upper CDM beaches 9major emergence?). Coastal warping; Wurm I stadial: minor marine regression, coastal warping concluded; Wurm I interstadial (Epimonastirian) : marine transgression, erosion of the minor cliff and depostion of the lower CDM beaches (minor emergence?); Wurm II stadial and interstadial: minor fluctuations of sea level. Deposition of the younger CDM beaches at about 36 000 to 39 000 years BP; Wurm III glacial (?) stage: marine regression to -50 or -60 m ±12 000 yrs BP. Post glacial Flandrian transgression with halting [?] stages; Fianl recovery of sea level: progradation of coastline

There is also a highly significant statistical difference in the mean percentage (by weight) of total carbonate content between grab samples of sediment located south of the Orange River mouth and those north of the moauth. The mean carbonate content of the S. D. C. is 16 percent but it is only some 4 percent for both M. D. C. and T. D. C. The Orange River mouth thus marks the boundary between sediments with relatively high heavy mineral and low carbonate contents, and vice versa

The Elizabeth Bay and Orange River buried river channels are described in detail. The Elizabeth Bay channel is 12 km long, 1 km wide and has been incised some 40 m to 50 m into the local bedrock. The offshore end of the Orange River channel is obscured but its dimensions appear to be comparable with the Elizabeth Bay channel. Each slopes some 40 m per 8 km seaward

A submarine cliff about 3m high has been traced for 15 km in the MDC area. Its base lies between -18 m and -20 m below mean sea level, which agrees closely with prominent bedrock flattening between -18 m and -24 m traced intermittently through most of the areas for which reliable information is available

Twenty eight bedrock profiles located between Pomona and Dreimaster Bay in the MDC were examined. They show that the bedrock at depths of -32 to -34; -40 to -42 and -52 to -54 is anomalously flat; in more general terms the bedrock between -44 and -58 m appears to be planed off

The closing chapter deals with events of the Pleistocene Epoch and their local effects. A postulated reconstruction of Quaternary events is presented, the most significant being as follows: Illinoian glacial stage: marine regression to about -90 m; Sangamon or last interglacial: marine transgression. Erosion of the major cliff and deposition of the upper CDM beaches (major emergence?). Coastal warping; Wurm I stadial: minor marine regression, coastal warping concluded; Wurm I interstadial (Epimonastirian) : marine transgression, erosion of the minor cliff and depostion of the lower CDM beaches (minor emergence?); Wurm II stadial and interstadial: minor fluctuations of sea level. Deposition of the younger CDM beaches at about 36 000 to 39 000 years BP; Wurm III glacial (?) stage: marine regression to -50 or -60 m ±12 000 yrs BP. Post glacial Flandrian transgression with halting [?] stages; Final recovery of sea level: progradation of coastline