Diagenetic Study of the Knysna and Robberg Formations in the Plettenberg Area, Western Cape Province of South Africa

Abstract

Knysna and Robberg Formations are both Cretaceous deposits composed of conglomerates and sandstones. These represent a transgressive sequence which shifted from fluvial deposition to marine deposition in the Plattenberg area of South Africa. This research investigated the mineralogical properties, provenance and diagenetic properties of the Knysna and Robberg Formation sandstones. Thin sections of some selected rock samples were prepared and studied under an optical microscope. Modal mineral composition analysis (point-counting) was carried out on the thin sections. The results of point counting were later normalised to a 100% and then plotted on QFL diagrams. Rock textures, mineralogy, morphology and diagenetic changes were determined using the petrographic microscope as well as SEM and XRD analyses. Modal composition analysis indicated that the major frame work grains of the sandstones are monocrystalline quartz, feldspar and lithic fragments, with the most abundant mineral being quartz. The sandstones of Robberg Formation have been classified as quartz-arenites, which are more mature than the Knysna Formation which are mostly arkosic and lithic arkosic sandstones. Provenance analysis revealed that the sediments were derived from craton interior recycled orogeny, most likely recycled from weathering and erosion of underlying Cape Supergroup rocks. Diagenetic studies showed there were six types of cements present in these formations, namely: smectite, kaolinite, hematite, chlorite, quartz and calcite cements. Three diagenetic stages were identified, i.e. early, burial and uplift diagenesis. Each diagenetic stage showed specific diagenetic textures and changes in the diagenetic processes, such as cementation, recrystallization, replacement and grain compaction. Deposition of clay matrix, and formation of smectite, kaolinite, quartz and hematite cements and point and long grain contacts occurred during the early diagenetic stage. Quartz overgrowths, calcite replacement, muscovitization, concavo-convex grain contacts and suture grain contact were formed during later diagenetic stage. Uplift related diagenesis was indicated by calcitization (calcrete), saussuritization and weathering. This study has provided the mineral, textural and diagenetic framework for the Cretaceous Knysna and Robberg Formations and will therefore serve as a good platform for future studies on these Cretaceous deposits.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018