Rutile, although not a major component of detrital heavy mineral deposits, is a valuable source of titanium oxide. Theoretically rutile is pure titanium dioxide (TiO2) and should form white or colourless tetragonal crystals with a density of 4.25gm/ml. However, natural rutile although tetragonal, displays a variety of colours ranging from red through brown to black, yellow or blue, variable density between 4.23 to 5.50g/ml as well as a range in the magnetic susceptibility and electrical conductivity. In addition to these variations exhibited by natural rutile, samples from detrital heavy mineral deposits normally contain, in addition to homogenous grains, composite grains, in which rutile is intergrown with one or more mineral species, commonly quartz, feldspar and ilmenite. The Sibaya Formation, like most detrital heavy mineral deposits, has a polymictic source, and as such contains rutile grains formed in many different chemical environments. Homogenous rutile grains display a chemical variation with a preference for the select few elements, which are compatible with the rutile cyrstallographic structure. The ions that substitute for titanium (Ti4+) in the crystal lattice are a reflection of chemical environment in which the crystal formed. The size and charge of the Ti4 + ion greatly restricts the species that may enter the rutile crystal lattice, with Sb3 +, V3 +, Fe3 +, Cr3 +, Sn4 +, M04+, W4+, Mn4+, 8i5+, Nb5+, Ta5 +, Sb5 +, V5 +, being theoretically compatible with the size and charge of the Ti4+ ion. Electron microprobe analysis of detrital rutile grains from the Sibaya Formation, KwaZulu-Natal show that elements, Nb5 +, Ta5+, A13+, Zr4+, Si4+, Fe3+, Cr3 +, and V5 +, commonly substitute for the Ti4 + ion. However, Sb3+, Sn4+, M04+, W4 + and 8i5 + were not present at detectable levels implying that the provenance area is not enriched in these elements. Although the high Fe3+ values were expected in the rutile grains, as Fe3 + is common in many rocks, the high Si4+ values encountered were not expected, as Si4 + is not normally compatible with Ti4 + ion, as noted by their distinct separation in rutilated quartz. The anomalous Si4 + content of certain grains suggests that within the provenance area rutile bearing rocks formed under unusual conditions, such as high pressure, temperature and silicon activity where the high charge density of the Si4 + ion would favour the inclusion of Si4 + into the rutile lattice. The chemical variation of the rutile grains causes significant variation in the magnetic susceptibility and electrical conductivity, and thus has marked effects on mineral processing, which relies heavily on magnetic and electrostatic separation techniques. The data presented indicates that individual homogenous rutile grains displays significant range of chemical composition, commonly containing other oxides from a fraction of a weight percent to well over 10wt%. Data plots of TiO2, FeO and 'other' oxides (Nb2O5, Ta2O5, A12O3, ZrO2, SiO2, Cr2O3 and V2O3), showed that many of the more magnetic rutile grains appeared to be FeO enriched and contained a higher proportion of 'other' oxides. However, some grains that just had higher proportions of 'other' oxides and a lower FeO content were also magnetic. Thus magnetic susceptibility although strongly influenced by the presence of FeO, can also be enhanced by the substitutions of other oxides. The vast majority of rutile grains from the electrostatic fractions were relatively TiO2 pure, and contained low concentrations of 'other' oxides. However, some grains did have slightly enhanced SiO2 and V2O3 concentrations, which appear to enhance the conductivity of the grains. Four main colour groups were differentiated from the population of rutile grains from the Sibaya Formation, these being, reddish brown, black, blue and yellow. No single oxide seemed solely responsible for the colour of rutile grains. However, the red rutile grains had a slightly but significantly higher Cr2O3 and Nb2O5 content, whereas black rutile grains appeared to be V2O3 and Nb2O5 enriched. The blue colour of rutile grains appears to be influenced by a combination of SiO2, Al2O3 and Nb2O5 substitutions. The yellow rutile grains had slightly enhanced FeO and Nb2O5 concentrations. Although these differences are very small, trace quantities of certain elements and different combinations of elements can have a strong effect on colour. Apart from Fe3+, no single element; appears to be solely responsible for variations noted in the physical characteristics (magnetic susceptibility, electrostatic conductivity and colour) of homogenous rutile grains from the Sibaya Formation. However a combination of substituting elements appears to influence magnetic susceptibility and electrical conductivity. An enhanced Fe3+ content normally increases the magnetic susceptibility although combinations of other elements may have the same effect on Fe3+ poor grains. In general terms, the purer the rutile grain, the more likely it is, to be non-magnetic and conductive. Substitutions of 'other' oxides appear to decrease the conductivity of rutile grains. The relationship between grain colour and chemistry is also not very clear, verifying the widely held view that grain colour is often the result of more than just mineral chemistry. / Thesis (M.Sc.)-University of Durban-Westville, 2002.

This study represents the first detailed geological, geophysical and geochronological investigation of the continental shelf surrounding the Aliwal Shoal, ~5 km offshore of Scottburgh, in southern KwaZulu-Natal. Mapping of the seafloor geology using geophysics and direct observations from SCUBA diving transects were integrated with the seismic stratigraphy and constrained by new geochronological data. Four seismic stratigraphic units (A to D) were identified and interpreted with the subsequent sequence stratigraphic model consisting of four incompletely preserved stratigraphic sequences separated by three sequence boundaries (SB1 - SB3) comprising complex reworked subaerial unconformity surfaces. Sequence 1 is the deepest, subdivided by a basin-wide marine flooding surface (MFS1) into a lower Campanian (and possible Santonian) TST and an upper Maastrichtian combined regressive systems tract comprising HST/FRST deposits. SB1 follows Sequence 1 and spans most of the Tertiary representing multiple erosional events. Shelf sedimentation resumed during the Late Pliocene to early Pleistocene with deposition of Sequence 2, the shelf-edge wedge, which again was followed by erosion and non-deposition during the high frequency and amplitude Early to Middle Pleistocene sea-level fluctuations resulting in the formation of SB2. Sequence 3 consists of coast-parallel, carbonate cemented aeolianite palaeo-shoreline ridges of various ages overlying Sequence 1 and 2. Sequence 4 unconformably overlies all the earlier sequences and comprises a lower TST component displaying characteristic retrogradational stacking patterns and an upper local HST clinoform component showing progradation and downlapping. Inner and middle shelf TST units constrained between Sequence 3 ridges form thick sediment deposits showing a progression from lagoonal and lower fluvial-estuarine deposits, overlain by foreshore and shoreface sands, documenting the changing depositional environments in response to a sea-level transgression. Laterally, in the absence if Sequence 3 ridges, TST sediments comprise only a thin transgressive sand sheet. The upper HST component comprises a prograding shore-attached subaqueous-delta clinoform sediment deposit, the Mkomazi Subaqueous-Delta Clinoform (MSDC) which evolved in four stages. An initialization and progradation stage (Stage 1) (9.5 to 8.4 ka cal. B.P.) was interrupted by retrogradation (Stage 2) and backstepping of the system due to rapid sea-level rise between 8.4 to 8.2 ka cal B.P. Stage 2 backstepping of the clinoform controlled the subsequent overlying topset morphologies resulting in later stages inheriting a stepped appearance upon which shoreface-connected ridges (SCR’s) are developed. Stages 3 (8.2 to 7.5 ka cal. B.P.) and 4 (7.5 to 0 ka cal. B.P) show a change from ‘proximal’ topset aggradation to ‘distal’ foreset progradational downlap, linked to a change in the dominant sedimentary transport mechanism from aggradational alongshore to progradational cross-shore related to variations in accommodation space and the rate of sediment supply. Morphologically the MSDC is characteristic of sediment input onto a high energy storm-dominated continental shelf where oceanographic processes are responsible for its northward directed asymmetry in plan-view, for the lack of a well defined bottomset and for the re-organisation of its topset into very large SCR’s. The SCR’s are 1 - 6 m in height, spaced 500 to >1350 m apart and vary from 3 km to >8 km in length, attached on their shoreward portions to the shoreface between depths of -10 m to -15 m (average at -13 m) and traceable to depths exceeding -50 m, although the majority occur on the inner shelf between -20 m to -30 m. Several individual crests can be identified forming a giant shoreface-connected sand ridge field with a sigmoidal pattern in plan-view postulated to be a surficial expression of the subjacent retrogradational phase (MSDC Stage 2). SCR’s development occurred in two stages. Stage 1 involved deposition of sediment on the shoreface and ridge initiation during the MSDC Stage 2 retrogradational event. Sediment was reworked during sea-level rise generating clinoforms with proximal along-shore aggradation and distal across-shore progradation. This occurred during the last post-glacial sea-level rise from ca. 8.4 ka cal. B.P. SCR Stage 2 represents modern maintenance of the SCR system which is continually modified and maintained by shelf processes and consists of two physical states. State 1 considers SCR maintenance during fair-weather conditions when transverse ridge migration is dominant and driven by the north-easterly flowing counter current shelf circulation. State 2 considers SCR development during storm conditions when longitudinal ridge growth is suggested to occur as a result of storm return flows. Following the storm, the regional coast-parallel current system is restored and the fair-weather state then moulds the SCRs into a transverse bedform. Deposition on the MSDC is ongoing on a continental shelf that is still in a transgressive regime. The exposed seafloor geology comprises late Pleistocene to Holocene aeolianite and beachrock lithologies, deposited as coastal barrier and transgressive shoreface depositional systems. Extensive seafloor sampling was combined with a multi-method geochronological programme, involving the U-series, C14 and optically stimulated luminescence (OSL) to constrain the evolution of the aeolianite and beachrock complex. The Aliwal Shoal Sequence 3 ridge comprises three distinct aeolianite units (A1 to A3) which represent different types of dune morphologies deposited during the climatic and associated sea-level fluctuations of MIS 5. Units A1 and A2 deposited during the MIS 6/5e (~134 to ~127 ka cal. B.P.) transgression represent contemporaneous evolution of a coastal barrier system which consisted of two different dune forms associated with a back-barrier estuarine or lagoonal system. Unit A1 most likely originated as a longitudinal coastal dune whilst Unit A2 comprised a compound parabolic dune system that migrated into the back-barrier area across an estuary mouth/tidal inlet of the back-barrier system. The coastal barrier-dune configuration established by Unit A1 and A2 was most likely re-established during similar subsequent MIS 5 sea-level stands which during MIS 5c/b resulting in the formation of the back-barrier dune system of Unit A3. Palaeoclimatic inferences from Units A1 and A2 aeolianite wind vectors indicate a change from cooler post-glacial climates (lower Unit A1) to warmer interglacial-like conditions more similar to the present (upper Unit A1 and Unit A2). Unit A3 palaeowind vector data show variability interpreted to be related to global MIS 5c climatic instability and fluctuations. For Units A1, A2 and A3 pervasive early meteoric low-magnesium calcite (LMC) cementation followed shortly after deposition protecting the dune cores from erosion during subsequent sea-level fluctuations. Sea-spray induced vadose cementation in Units A1 and A2 may have been a key factor in stabilising dune sediment before later phreatic meteoric cementation. The final preserved Late Pleistocene depositional event in the study area was that of the storm deposit of beachrock Unit B5. Induration followed shortly after deposition by marine vadose high-magnesium calcite (HMC) cementation. Following deposition and lithification, Units A1, A2, A3 and B5 underwent a period of cement erosion associated with decementation and increased porosity due to either 1) groundwater table fluctuations related to the high frequency MIS 5 sea-level fluctuations and/or 2) carbonate solution due to complete subaerial exposure related to the overall MIS 4 - 2 sea-level depression towards the LGM lowstand. In addition to the decementation and porosity development Unit B5 also experienced inversion of the original unstable HMC cement to LMC. During MIS 4 to 2 the Aliwal shelf comprised an interfluve area which was characterised by subaerial exposure, fluvial incision of coast-parallel tributary river systems and general sediment starvation. Beachrock Units B1 to B4 were deposited in the intertidal to back-beach environments and subsequently rapidly cemented by marine phreatic carbonate cements comprising either aragonite or HMC. Unit B1 was most likely deposited at 10.8 ka cal. B.P., B2 at 10.2 ka cal. B.P, B3 at 9.8 ka cal. B.P and B4 <9.8 ka cal. B.P. thereby indicating sequential formation during the meltwater pulse 1b (MWP-1b) interval of the last deglacial sea-level rise. Unit B3 marks the change from a log-spiral bay coastal configuration established by Units B1 and B2 to a linear coastline orientation controlled by the trend of the pre-existing aeolianite units. This change in the morphology of the coastline is also documented by the shape of the underlying transgressive ravinement surface (reflector TRS, Sequence 4) which again was controlled by the subjacent sedimentary basin fill architecture and subsequent transgressive shoreline trajectory (Sequence 4). Sea-level rose at an average rate of 67 cm/100 years from B1 to B2 and 86 cm/100 years from B2 to B2 indicating an acceleration in the rate of sea-level rise supporting enhanced rates of sea-level rise during the MWP-1b interval which also seemed to have altered the coastal configuration and resulted in the closure of the southern outlet of the back-barrier estuarine system. Two cycles of initial aragonite followed by later HMC cement are tentatively linked to two marine flooding events related to different pulses of enhanced rates of sea-level rise during MWP-1b which are considered responsible for significant changes in the marine carbon reservoir ages. Comparisons of the U-series, C14 and optically stimulated luminescence (OSL) methods have shown OSL to be the most reliable method applied to dating submerged aeolianites and beachrocks. OSL not only provides the depositional age of the sediment but also does not suffer from open system behaviour, such as marine reservoir changes and contamination. Acoustic classification of the unconsolidated sediment samples resulted in the demarcation of 3 major acoustic facies, C to E, interpreted with sample analyses as quartzose shelf sand (C), reef-associated bioclastic-rich sand (D) and an unconsolidated lag and debris deposit (E). Grain size distribution patterns of the unconsolidated seafloor sediments indicate that the SCR system delivers fine and medium sand to the inner and middle shelf and imparts a general N-S trending pattern to the gravel and sand fractions. In addition grain size distributions support selective erosion of the seaward flank of the Sandridge with the remobilised sediment deposited in the Basin as low amplitude bedforms over the Facies E lag and debris pavement. The mud fraction is interpreted to be deposited by gravity settling from buoyant mud-rich plumes generated by river discharge. Integration of acoustic mapping, field observations and sample analyses indicate that the present distribution of the unconsolidated sediment is the result of a highly variable distribution of modern and palimpsest sediments which are continually redistributed and reworked by a complex pattern of bottom currents generated by the interaction of opposing oceanographic and swell driven circulation patterns. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.