Early Precambrian Tidal Sedimentation in the Pongola Supergroup of South Africa

Abstract

ABSTRACT Sedimentary sequences which resemble the regressive intertidalite model of Klein (1971, 1972) occur associated with volcanic rocks in the 3,000 m.y. Pongola Supergroup. Two inliers in northern Natal which were selected for the present study include prominent orthoquartzitic sandstones with thinner siltstones and mudstones. The arenite member at the base of the Wit-Mfolozi section is attributed mainly to shallow marine deposition. The upper 5 m display features which are suggestive of a low tidal sandflat environment. These include reactivation structures, herringbone cross-stratification, and ripple forms indicative of decreasing water depth and possible subaerial exposure. Above this is the alternating arenaceous-argillaceous member with a distinctive association of shrinkage cracks, mudclast breccia, and double-crested, flat-topped and interference ripples. Similar features occur in modern mid-tidal flat deposits. A high tidal mudflat environment is envisaged for the uppermost unit, the argillite member. A hematitic (jaspilite) bed is locally present at the top of the sequence and suggests the possibility of biological liberation of oxygen in periodically flooded depressions in the inner mudflat. Three similar upward-fining sequences are present in the Mpongoza inlier. Paleotidal ranges of between 12 and 25 m are inferred. The maximum figure is considerably in excess of any present-day tidal amplitudes, and lends support to the hypothesis of an early Precambrian origin for the earth moon system.