Grain-size distribution and cementation of the Cretaceous Mzamba Formation of Eastern Cape, South Africa: a case study of a storm-influenced offshore sequence

Abstract

The Late Cretaceous Mzamba Formation of the Eastern Cape, South Africa, is a storm-influenced succession deposited in an offshore marine environment. The succession is composed of two fundamental lithofacies, very fine sandstone and packstone (shell-bed) lithofacies; the former represents background sedimentation in a relatively deep-water, low-energy offshore environment, whereas the latter represents storm deposits occurring with variable periodicity within the quiet offshore environment. The storm-generated packstone lithofacies shows a bimodal grain-size distribution with two saltation fractions on the cumulative frequency diagrams, together with negative skewness parameters and a relatively wide kurtosis of grain-size distribution. These characteristics are indicative of offshore storm-generated sediments. Initial cementation of the storm-influenced sediments occurred in a marine phreatic environment when isopachous rims of phosphate, isopachous fibrous rims of smectite, and micrite calcite cements were formed; these cementation textures also support the offshore tempestite-origin of the Mzamba sediments. The original marine phreatic diagenetic environment of the Mzamba sediments changed to a meteoric phreatic environment probably at the end of Mesozoic. Following deposition of the overlying Cainozoic sediments, the Mzamba Formation entered a burial diagenetic environment where the sediments were affected by elevated temperature and pressure. Finally, the sediments were entered into a meteoric vadose zone due to tectonic uplift or sea level fall, new dissolution pores and pendant calcite cement were created.