Stratigraphy, sedimentary facies and diagenesis of the Ecca Group, Karoo Supergroup in the Eastern Cape, South Africa.

Abstract

This is a MSc research project, and is aimed at the new insight on the stratigraphy, sedimentary facies, diagenesis and depositional environments of the Ecca Group, Karoo Supergroup in the Eastern Cape Province. Methodologies used in this research include field investigation, stratigraphic logging, thin-section microscope study, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses. The stratigraphy of the Ecca Group is divided into five formations, namely the Prince Albert Formation, Whitehill Formation, Collingham Formation, Rippon Formation and the Fort Brown Formation from bottom upward. Based on the field investigation and laboratory correlation, the Prince Albert, Whitehill, Collingham, and Fort Brown Formations can each be subdivided into two new members, i.e. lower member and upper member; whereas three new members have been proposed for the Rippon Formation, i.e. lower, middle and upper members. The Ecca Group sediments were accumulated in various depositional environments, from bottom of deep marine environment, passed through the middle of deltaic environment, and ended in a lacustrine environment. The Prince Albert Formation, Whitehill Formation and the Collingham Formation were all deposited in a deep marine basin, whilst the Rippon Formation was laid down in a deltaic environment. As the climate gradually became warmer and drier, the top Fort Brown Formation was lastly deposited in a lacustrine environment. The stratigraphic succession of the Ecca Group constitutes a perfect regression sequence, indicating that the marine water gradually retreated and the sea-level gradually dropped. The rocks in the Ecca Group are mainly terrigenous sandstone and mudstone with some coarse grain-sized siliciclastic rock of conglomerate. The sandstones are dominated by feldspathic graywackes with minor quartz-wackes, and there are no arenites in the Ecca Group. Whereas the mudstones are dominated by grayish mudrocks and black shales, purer claystone was found in the turbidite facies of the Collingham Formation, which probably has economic significance for the future since the reserve is quite large. Optical microscope, XRD and SEM analyses demonstrated that the minerals in the Ecca Group include detrital minerals of quartz, orthoclase, microcline, plagioclase, biotite, muscovite; and clay minerals (smectite, kaolinite, illite and sericite). These minerals constitute the rock framework grains and cements whereas; the authigenic minerals of calcite and hematite were formed during diagenesis. Accessory minerals such as rutile and zircon are the heavy minerals present in the strata, and occur only in a small amount. Based on the lithologies, sedimentary structures and sequence stacking patterns, ten sedimentary facies have been recognised, namely 1) Grayish laminated and thin bedded shale facies, 2) Grayish laminated shale and intercalated chert facies, 3) Grayish rhythmite facies (all the three facies above were deposited in deep marine water); 4) Flat and lenticular bedded graywacke facies, 5) Grayish alternating mudstone and sandstone facies, 6) Dark organic rich mudstone facies, 7) Fossil bearing mudstone facies, 8) Laminated and thin bedded black mudstone with lenticular siltstone facies, 9) Interbedded grayish sandstone and mudstone facies (above Facies 4-9 were deposited in deltaic environment and appeared in the Rippon Formation); and 10) Varved rhythmic mudstone facies, which occurs only in the Fort Brown Formation and represents lacustrine sediments. Four types of cements have been identified in the rocks of the Ecca Group, including quartz, smectite, calcite and feldspar cements. The first three cement types are the major cement types, whilst the feldspar cement is minor and occurs only locally. Recrystallisation in Ecca sediments includes quartz, feldspar, clay mineral recrystallisation and conversion from smectite and kaolinite to illite and then to sericite. Replacement involves calcite replacing quartz, feldspar and clay matrix; accompanied by albitization, i.e. albite replacing other feldspar minerals in a deep burial environment. Dissolution in the Ecca Group involved calcite and kaolinite dissolving and leaching, which created more pore-space and increased porosity. The sediments of the Ecca Group went through three stages of digenesis, namely the early stage, the late stage and the up lift stage which led the rocks being exposed on the Earth’s surface and being weathered. In each stage, some minerals became unstable, then replaced by a more stable mineral suitable for the new diagenetic environment. Precipitation of cements and formation of authigenic minerals mostly occurred in the early diagenetic stage, which led the soft sediments becoming a hard rock; whilst recrystallisation, replacement, and dissolution took place mostly in the later diagenetic stage due to increase of temperature and pressure resulting from increase of burial depth.