The Distribution of Diamonds on a Late Cainozoic Gravel Beach, Southwestern Namibia

Apollus, Leonard (1995) The Distribution of Diamonds on a Late Cainozoic Gravel Beach, Southwestern Namibia. MSc(R) thesis, University of Glasgow.

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Abstract

The Late Cainozoic diamondiferous pocket beach deposits studied here are exposed in the Chameis area along 8.8km of the storm-dominated, Atlantic coastline of the Sperrgebiet of southwestern Namibia, 100km north of the mouth of the Orange River. Several pocket beaches are present in the area, but only the deposits of the most extensive pocket beach (number 4) were studied. The main aim is to derive a model for the concentration of diamond placers in pocket beaches. In persuit of this aim the characteristics of the sediments and the shore platform on which they rest were recorded across and along the beach, and within that sedimentary framework, variations in both concentration and size of diamonds were recorded. In addition, the general characteristics, as well as the abrasion textures of the diamonds recovered from these sediments were described in detail for possible evidence of their original source and subsequent transport history. The exposures available for study were largely dependent on the progress of the mining operations in the study area which strip open the diamond bearing marine sediment otherwise buried under thick modern aeolian and beach sand. The sediment in the most southern part of the pocket beach was mined-out before this study commenced, nonetheless information about this part of the beach could be obtained from in-house reports. Two sharply bounded lithostratigraphic units are recognized and are subdivided into an eastward (landward) red unit, Eemian in age, and a westward (seaward) Holocene grey unit. Both units comprise of a basal gravelly unit interpreted to represent the transgressive phase of beach accretion and an upper finer regressive phase. The downward progradational offlapping gravels of the red unit are interpreted as beachface (intertidal) facies with a more sandy and thicker seaward shoreface facies. The westward grey unit is interpreted to be wholly of the shoreface (subtidal) facies. Both the mean and maximum grain size of the gravel fraction of both units increase towards the northern end of the pocket beach as does the sorting of the red unit gravel fraction. The high specific density banded ironstone formation, Orange River derived, exotic clasts also increase in abundance in the same direction as well as the relative concentration of diamonds. This trend indicates an increasing energy response towards the northern end of the pocket beach. The Chameis deposits largely record the regressive phases of a sequence of transgressions and regressions. On the basis of the stratigraphic data from these deposits, the general sequence of the Namibian (Sperrgebiet) beaches and the global Pleistocene sea-level records, the following sequence of events is deduced: 6) Deposition of dune sand (TOP) 5) Regression and deposition of grey unit 4) Transgression and partial erosion of red unit 3) Sheetflows and mass-wasting from land: subaerial exposure 2) Regression and deposition of red unit, c. 130 000 BP 1) A major transgression: recutting and/or modification of shore platform (BASE) During these phases of erosion and deposition normal to the coastline, diamonds were thought to be moving northwards along the shore and probably from the offshore, to be trapped in these raised marine terraces. The shore platform morphology suggests it to be an inherited feature formed at least partly by marine transgressions during Middle to Late Pleistocene. As a result of a rapid sea-level rise during the last interglacial highstand, the shore platform has probably remained relict. The most important factors that probably influenced the present morphology of the shore platform are the rate of sea-level rise and bedrock geology. Because the morphology of the shore platform is considered to be highly sensitive to local conditions the commonly accepted alongshore correlation of shore platforms on the basis of similar heights is rejected until the morphology and genesis of the shore platform in each area has been studied in some detail. There is no significant spatial variation in the general characteristics and abrasion features of the diamonds in the study area but the diamond characteristics are consistent with patterns reported from other studies of southern African alluvial diamond populations. It is therefore concluded that the diamonds were presorted before being introduced into the study area and therefore the distribution of their characteristics was independent of the sedimentary processes operating in the pocket beach. However, the maximum diamond concentration is in the beachface (intertidal) facies of the red unit beach. In the grey unit diamonds concentrate best in areas with the highest intensity of gullies and potholes. Therefore sedimentary processes are more important mechanisms for the entrapment of diamonds in the red unit gravel whereas gullies and potholes take precedence in the subtidal grey unit. This study has established a database which may be useful in comparing, the distribution of lithofacies and textural characteristics of the sediments, as well as the characteristics of the diamonds, with placer deposits from different parts of the Sperrgebiet with a view to determine their similarities and thus probable primary source. A comparison of the general characteristics of the Chameis diamond population with their offshore counterparts may also be useful.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Additional Information: Adviser: John Ward
Keywords: Geology, Sedimentary geology
Date of Award: 1995
Depositing User: Enlighten Team
Unique ID: glathesis:1995-75434
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 20:08
Last Modified: 19 Nov 2019 20:08
URI: http://theses.gla.ac.uk/id/eprint/75434

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