Marginal marine depositional processes from the Archaean Moodies Group, Barberton Mountain Land, South Africa: Evidence and significance

Abstract

Abstract The Moodies Group, which is approximately 3300 Ma in age, contains the oldest recognizable sediments of marginal marine origin. These accumulated in estuarine-deltaic, barrier beach and back-barrier depositional environments. A paleogeographic model recognizes the interplay of different shallow marine processes and the spatial relationships of the depositional environments. Delta plain sediments consist of tidal channel and estuarine tidal flat deposits. Elongate sand shoals, oriented perpendicular to the palaeoshoreline, fronted the delta plain and indicate that sedimentation occurred under macrotidal conditions in a non-barred estuary. Rapid gravity flow sedimentation on the delta front resulted in ubiquitous soft-sediment deformation and water escape. Longshore reworking of riverborne sediments led to the development of barrier island and back-barrier deposits lateral to the delta. Chemical and suspension sedimentation predominated in the offshore, and low-energy bedload and storm-surge sedimentation in the nearshore depositional environments. Shoreface sediments display widespread evidence of rip current activity and were succeeded shorewards by tidal inlet and barrier-spit deposition. Tidal flat as well as flood tidal delta and washover fan sediments accumulated behind the barrier island. Spit, washover fan and abundant rip current deposits, coupled with the absence of extensive ebb tidal delta accumulations, are indicative of a microtidal coastline. The coeval existence of micro- and macrotidal conditions is attributed to irregularities along the coastline and/or variations in width of the continental shelf. While providing no new clues as to the original composition of the earth's crust, the nature of the Moodies sediments indicates the existence of widespread exposed granitic terrains by 3300 Ma. In addition, the abundance of marginal marine orthoquartzitic sandstones in the Moodies Group suggests a relatively stable crust on which extensive reworking of sediments occurred. The proposed micro- and only local macrotidal ranges, however, imply a fairly narrow shelf during deposition of the Moodies Group. The conformably underlying Fig Tree sediments, which are considered to be a deep-water facies equivalent of the Moodies Group, are further indicative of a narrow shelf. The above criteria have been used to suggest that the Moodies and Fig Tree Groups accumulated along an ancient continental margin. Marginal marine facies are apparently lacking in other Archaean terrains, most notably in Canada; rather alluvial and turbidite sediments occur and were deposited in localized graben-like basins. These two styles of Archaean sedimentation may reflect responses to different stages of continental rifting and spreading.