Unconsolidated sediment distribution within the KwaZulu-Natal Bight
Ron Uken and Nokwanda Mkize
School of Geological Sciences, University of KwaZulu-Natal, Durban
[email protected]
ABSTRACT
Sea floor sediment grain size characteristics, total organic content and carbonate content were established in the
KwaZulu-Natal Bight delineating three subzones within the bight, a southern, central and northern zone. Sediment
across the bight is dominated by medium grained sands (0.5 mm to 0.25 mm) except in the central mud-dominated
central zone. Very coarse and coarse grain sizes (2 mm – 0.5 mm) are due to a high bioclastic component, mimicking
the carbonate content distribution derived from reef systems. These form two localized concentrations on the midshelf in the southern zone, and at the shelf edge in the northern zone. Two mud banks situated off the Thukela River
Mouth define the central zone and are mimicked by areas of high organic content. The magnetic component of the
sediments, a function of the magnetite content, revealed a large northerly directed plume extending along the mid shelf
from the Thukela River Mouth northwards, dividing the inner and outer mud banks if the central zone. The magnetite
plume is interpreted as a submerged palimpsest strand line that existed during lower sea level. Sediment distribution
patterns are thus explained partly by inherited features as well present day circulation patterns.
INTRODUCTION
It has long been known that sediment dispersal in the KwaZulu-Natal Bight is controlled by the complex interaction of
shelf morphology, the Agulhas Current, wave regime, wind-driven circulation, sediment supply – mainly from the
Thukela River and the presence of the semi-permanent gyre within the KwaZulu-Natal Bight, developed in the lee of
the Mtunzini coastal offset (Flemming, 1981; Lutjeharms, 2006; Bosman et al., 2007). Sediment in the outer shelf is
influenced solely by the strong Agulhas Current forming large-scale subaqueous dunes with a southwesterly transport
direction (Flemming, 1978, 1980, 1981). Subaqueous dunes in the inner and mid shelf are prone to current reversals.
This produces zones of bedload parting and is ascribed to the semi-permanent gyre which moves sediment
northwesterly, counter to the prevailing southwesterly flowing Agulhas Current. The bounding bedload parting zones
shift along the coast, depending on where the Agulhas Current meets the shelf at any particular time (Flemming,
1980).
In this paper we present the results of bottom sediment sampling undertaken within the KwaZulu-Natal Bight during
February 2010 aboard the marine research vessel FRS Algoa. One hundred and seventy five sea floor surface samples
were collected from selected sites from localities ranging between -40 m and -75 m depth (Figure 1). This was done
by ShipekTM grab sampling with location determined by the onboard Geographic Positioning System (GPS) system.
Grain size distribution, total carbonate content and total organic content and magnetic content of each sample was
determined. Samples were rinsed to remove the mud fraction, dried and sieved using SABS test sieves of decreasing
aperture. The mass of each sand fraction was determined and from this the sorting, skewness, median and mean were
calculated. Carbonate content was determined using the carbonate bomb method, following the procedures by Schink
et al, (1979). In this method, the amount of CO2 liberated from CaCO3 by dilute HCL is related to the carbonate
percentage by the pressure increase that occurs in a closed system during the reaction. The organic content was
determined treating the samples with Hydrogen Peroxide (H2O2) oxidation. The organic content is calculated as the
difference between the initial and final sample mass. Magnetite was extracted from each sample using a hand held
magnet. Spatial data analysis was performed using imaging software Surfer 8.
1 northern zone central zone southern zone Figure 1: Map showing sample collection sites within the KwaZulu-Natal Bight. 1 Durban,
2 Thukela River Mouth, 3 Richards Bay, 4 Cape St Lucia and sediment subzones within
the bight.
SEDIMENT DISTRIBUTION
Grain size distribution maps (Figure 2) indicate that the KZN bight can be subdivided into three zones. A southern,
central and northern zone (Figure 1). The southern and northern zones are dominated by medium grained sand with a
central zone defined by two mud belts. The central zone extends from the Thukela River Mouth to Mtunzini, the
inflection point in the bight.
Very coarse and coarse grain sizes (2 mm – 0.5 mm) are due to a high bioclastic component, mimicking the carbonate
content distribution and derived from reef systems and form two localized concentrations on the mid-shelf in the
southern zone, and at the shelf edge in the northern zone (Figure 2).
The two mud banks situated off the Thukela River Mouth define the central zone and are mimicked by areas of high
organic content (Figure 2).
The magnetic component of the sediments, a function of the magnetite content, revealed a large northerly directed
plume extending along the mid shelf from the Thukela River Mouth northwards (Figure 3), dividing the inner and
outer mud banks of the central zone. The magnetite plume is interpreted as a submerged palimpsest strand line that
existed during the last glaciations when the Thukela Mouth was situated at the present-day mid shelf.
2 A B
C
D
E
F
G
H
Figure 2 Grain size distribution characteristics for the KZN Bight. A) gravel, B) very coarse sand, C) coarse sand, D)
medium sand, E) fine sand, F) very fine sand, G) mud, H) Mean grain size.
3 A
B
C
Figure 3 A) Total carbonate content distribution, B) Total organic content distribution, C) Magnetic mineral
concentration distribution.
DISCUSSION
This work confirms previous studies that the mud banks of the central zone are relict and that these are separated by a
palimpsest dune beach complex high, marked by a higher magnetite content. Magnetite is considered to mark the
ancestral Thukela River Mouth matching the high magnetite contents on the present day beaches extending
northwards from the present day Thukela River Mouth (Hugo, 1993). The inner mud belt may represent a back barrier
lagoonal system. Even in the absence of a recognisable preserved dune barrier complex, it is also likely that outer mud
belt represents a similar lagoonal complex that existed closer to the last glacial maximum.
The concentration of very coarse sediment in the southern zone may mark the postion of the central “swirl” that
produces high current activity of the shelf resulting in winnowing of finer sediment and builup of a coarser sediment
lag. This would also explain the coarser sediment concentration in the northern zone due to winnowing by the Agulhas
Current.
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