Stratigraphy and sedimentology of the Mzamba formation in the Eastern Cape, South Africa

摘要

This research project is aimed at providing new information to the stratigraphy, sedimentology, palaeontology and diagenesis of the Mzamba Formation. The study area is located at the south of Port Edward, Eastern Cape. The methodologies employed in this study include field geological investigation and sampling, stratigraphic measurement and logging, thin-section microscope study, powder samples of XRD analysis, and SEM-EDX analysis of rock textures and mineral compositions. The stratigraphy of the Mzamba Formation can be divided into three newly established members, i.e. the Lower Conglomerate Member, Middle Silt/Mudstone-Shell Bed Member and Upper Mudstone-Shell Bed Member with a total thickness of 31.26m in an inland borehole and 30.05m in the field measurement. The Lower Conglomerate Member is 2.65m thick and consists of pebbly conglomerate with coarse sandstone, shell fragments and silicified wood trunks, representing shallow marine nearshore deposits. The Middle Silt/Mudstone and Shell Bed Member is 9.5 m thick and consists of black mudstone and fine-grained siltstone alternated with medium grained pecten beds, which was deposited in a storm influenced deeper marine environment. The Upper Mudstone-Shell Bed Member is 17.9m thick and is made up of fine-mudstones with articulated pecten layers which were deposited in a deep and quiet marine environment. Petrology studies showed that the Mzamba Formation consists of mixed sediments of carbonate and siliciclastic rocks. Siliciclastic rocks include pebbly conglomerates, medium to coarse sandstones and fine-grained mudstones, whereas carbonate rocks include packstone, wackstone and grainstone (pecten beds). The formation shows cyclical pattern of a series fining-upward cyclicities, changing from bottom conglomerate to sandstone, then upward repeated series of cyclotherms from pecten bed to mudstone. Mineralogy of the Mzamba Formation consists of terrigenous minerals of quartz, orthoclase, plagioclase, muscovite and various igneous and metamorphic rock-lithics, clay minerals of smectite, illite and sericite, and carbonate minerals of calcite and dolomite; with minor diagenetic minerals of pyrite, glauconite, hematite, gypsum, albite and organic maceral of vitrinite. Heavy minerals of garnet, zircon and rutile are minor minerals in the strata, which were detrital in origin. Mzamba Formation is a fossiliferous sequence, and contains both fauna and flora fossils in the strata. The pecten beds host well-preserved bivalve, gastropod, brachiopoda, ammonite, and echinoderm; whereas trace fossils of coprolites, burrows and tracks, as well as plant fossils of silicified wood trunks were also found in the formation. Some new fossil species were collected and studied, which include Bivalve: Pteriaceae, Pinnacea and Ostreacea; Gastropod: Cerithiacea and Mesogastopoda; Echinoderm: Echinocystoidea and Crinoidea. The benthonic species predominate in the lower part in the succession, whilst the planktonic species are abundant in the upper part of the sequence, which points to increase in water depths of the depositional environment. Based on lithology, sedimentary structures, and stratum architecture, seven different facies have been distinguished. Facies A (Flat bedded pebbly conglomerate), Facie B (Cross-bedded coarse calcareous sandstone facies), Facies C (Burrowed sandstone facies), Facies D (Shell-fragmental fine-grained calcareous sandstone facies), Facies E (Horizontal bedded calcareous mudstone facies), Facies F (Calcareous patch reef), Facies G (Wash out reef facies). Wash out reef facies is rich in algae, bivalve shells, broken oysters, coral fragments and small pebbles. Four types of cements were found in the Mzamba Formation, including calcite, smectite, illite and quartz. Calcite cement can be further classified into two types, micrite calcite cement and sparite calcite cement. The clay cement consists of smectite and illite and mainly occurs as matrix. The isopachous rim calcite and bright isopachous rim of silica cements indicate diagenesis in a marine phreatic zone. Authigenic minerals which formed in early diagenetic stage include quartz, plagioclase, glauconite and organic maceral of vitrinite. Three stages of diagenesis have been recognised in the sequence, i.e. syndiagenesis, early and late diagenesis. Glauconite pellets and worm faecal pellets were formed in syndiagenetic stage; cementation and authigenic minerals were formed in early diagenetic stage; whereas clay mineral conversion of smectite to illite, quartz overgrowth, bioclast recrystallization and calcite replacement took place during late diagenetic stage. The pebbly conglomerate at the bottom of the Mzamba Formation represents high energy deposits in a shallow marine environment; the grain-size gradually becomes finer in the middle succession and finest mudstone facies at the top of the succession, which represents deep marine deposits. Meanwhile, benthonic fossils are dominant in the bottom succession while plankton fossils are more abundant in the top succession. These features indicate that the Mzamba Formation constitutes a perfect transgression sequence, and the depositional environments started from shallow marine near shore environment, and gradually shifted to a deep marine quiet water environment.