The study was done in the Mpenjati estuary. It was found more briefly if sediment grain size and mineralogy distribution change across a beach profile changes and how it change. 4 zoneS of a beach profile were sampled. Quartz is the most stable sediment composition in the surface of the earth, compared to heavy minerals and feldspar (Marshak, 2008).
The aim of the study was to find out how does the sediment grain size and mineralogy distribution change across a beach profile. Mineral is a homogenous, naturally occurring solid inorganic substance with a definable chemical composition (Marshak, 2005). The impacts of the depositional environment, transport and climate on the morphology of minerals oceans are the focus of discussion during the present study of grain morphology (Shaffer,2006). It goes without saying that morphology and texture as well as the intergrowths among ocean minerals are also of utmost importance for the and industrial use of placer minerals (Pipkin, 2007) and by this oceans profiles must not be protected from pollution and disasters. The Mpenjati Estuary has been subjected to extensive sand and stone mining for approximately 40 years and the mining operations affected sediment patterns (matthias et al,2012).
Although geologists tend to classify estuaries according to their geomorphology, physiography, sedimentation or tidal patterns but an estuary is an enclosed coastal body of water which is periodically or permanently open to the sea (Whitfield, 2010). The study was done by the University Of KwaZulu-Natal students of BIOL 231 (Marine environments) together with their demonstrators and Lecturer. The study was done in the Mpenjati estuary on the third of August 2013 which is the winter season while the estuary was closed at the mouth. The Mpenjati estuary is an open/ closed estuary located at 30 ͦ 58’15” and approximately 165km southwest of Durban Kwazulu-Natal south coast
FIGURE 1: Locality map showing Mpenjati estuary (Whitfied, 2003)
FIGURE 2: beach profile showing different zones (Schwarts, 2008)
MATERIALS AND METHODS
Sampling was done when mouth of the estuary was closed. Four different zones of the beach profile were sampled which is Swash zone 1 which was closer to the sea followed by swash zone 2, and back beach 1 which is closer to the estuary followed by back beach 2. In each zone 2 clear plastic bags were filled with half way with sand using our own hands and were sealed with sell tape for later processing at the Lab. The plastic bags which had sand were named according to their zones collected from. The collected sediments were then taken to the soil laboratory at the Geological science Department (UKZN) for drying for 3 days. In the laboratory the sieve trays ware weighed using balanced scales after weighing. Sand which came out from drying was crushed and grinded and stirred on to the bowls using a thick pin as if cooking.
Sieve trays were stacked together and then placed in a sieve machine where the dry grinded sand was stirred more on the machine for 2 minutes. After the 2 minutes the sand or sediments were poured on to the sieve trays which have different sizes of open holes measured in µM, the top sieve tray had the biggest porosity and the bottom one had the smallest and was consisting of finest sand. When sieving was done the sieve trays were weighed again in order to obtain the mass of the sediment grains collected on the estuary. From the masses of the dishes the value of Phi (Ø) was calculated:Phi (Ø) = -log2 (d) d = Mass of a sieve /1000
The calculated phi was then used as a dependent variable in plotting graphs for distribution and the cumulative frequency that are displayed in the results. Phi ranged from -1 – 3.47 then the base.This was done for each zone I the four zones which were sampled. A small portion of sediments was collected In each zone of sampling was then placed into a petri dish and was examined by a light microscope for analyzing different types of minerals which were in each set of sediment zone by looking at their grain size, texture, and the mineral composition of the sand. This was done for each zone.
The field observations are also confirmed by the graphs that in figure 4 the mass content, sediment mean size goes from high to low from swash zone 1 to back beach 1 showing that as one goes closer to the sea the sediment grain size of the sand and mass increases. Table 1 tells us that the back beach has more quarts e.g The back beach has the highest quarts. DISCUSSION
It has been found out from figure 2 that the beach profile is divided into different zones, 4 zones were sampled at the mpenjati estuary namely swash zone 1, swash zone 2, back beach 1 and back beach 2. It was also found in figure 3, figure 4 and table 1 that the 4 zones have different mineralogical distributions and grain size. It was found that the soil colour and texture of the sand changed from zone to zone. The sand was damp with and had shells, little fragments, oysters afflicted by waves and washed to the sand by wave deposit. The ripples indicated wind direction and the wavelength of ripple was 6.5 cm. The deeper side of the ripples is gentle and shorter on the side of the ripples; this is due to north east blow of the wind (Marshak, 2005)) The swash zone is a zone of constant deposition from the beach characterised by moderately to poorly sorted sediments as sediments of first deposition are sub angular with a grain shape of low sphericity compared to other zones in the beach (table 1). The long shore drift which passes across the beach water body creating wave action energy is insufficient to carry these types of sediments (Shaffer, 2006).
The reason why small sediments in size are mostly found at the back beach (figure 3 and 4) is because the wave action slows down its energy in the swash zone so only small sediments in size can be carried towards the back beach leading to well sorted sediments with mature texture as high transportation of sediments leads to re-deposition and re-transportation (Pipkin, 2007)). Closer to the sea were larger stones and burrows (figure 4) swash zone 1 had the highest mass content followed by swash zone 2 and the reason for this is because closer to the sea there is more wind action (Shaffer, 2006). Crabs adapted to the environment with burrows. Quartz is the most stable sediment composition in the surface of the earth, compared to heavy minerals and feldspar (Marshak, 2008). Quartz and feldspar are dominant in all beach zones (Table 1) this is because they are primary rock minerals, they are found in parent rock e.g. granite which contains these two minerals (Schwarts, 2005). Since soil is the deposited by the chemical weathering of rocks, the minerals also are as a result that deposition. Feldspar makes about 60% of the earth’s crust (Marshak, 2005). Mineral sands contain suites of minerals with high specific gravity known as ‘heavy minerals’.
They were found mostly on the back beach 2 (Table 1) and the swash zone 2 because the wind current was not strong enough to transport them away. Those that are found on the back beach 2 are probably deposited by the river inflow and that on swash zone 2 is deposited by the ocean current and cannot be transported further because they are resistant to wave action (Pipkin, 2001). The swash zone was found to have more shells (Table 1) and some broken up and there were smooth, dishaped stones (cables) because of wave action on tide and also pedals. Lithic fragments were found mostly the back beach (Table 1) and only one on swash zone 1 because they are light and easily transported by wave and wind action. In conclusion as the profile is divided into zones it was found that mineralogical distribution and grain size change across the zones of a beach profile. As one go closer to the sea the gran size becomes larger and there was found to be more mineralogical diversity to zones closer to the sea compare to zones closer to the estuary. Heavy minerals were found in zones closer to the sea and light minerals were found to zones far from the sea, this is all due to processes like wind and wave action.
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