Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Sedimentary rocks of the 3.2-3.2 Ga old Fig Tree Group in the southwest part of the Barberton Greenstone Belt in South Africa can be divided into three informal formations. The lowest unit, Formation A, is composed mostly of silicified pyroclastic debris and carbonaceous cherts. Formation B has four lithofacies--siltstone; sandstone; jaspilite; and bedded barite. Two coarse-grained lithofacies make up Formation C. In the western portion of the study area there is chert-clast conglomerate and in the eastern part of the study area is volcanic-clast conglomerate, breccia, and sandstone. In the western portion of the study area Formations B and C form a coarsening-upward sequence of siltstone, sandstone, and conglomerate that is interpreted to represent a fan-delta and associated low-energy basin. Three facies define the fan-delta: (1) a subaerial, delta-plain facies consisting of alluvial conglomerate and sandstone in a coarsening-upward sequence; (2) a delta-front facies consisting of subaqueous, sediment-gravity-flow-deposited conglomerate and sandstone derived from an alluvial/fluvial system that prograded into a low-energy body of water, and; (3) a subaqueous-basin facies consisting of laminated siltstone and thin beds of sediment-gravity-flow-deposited sandstone, conglomerate, and breccia. The pyroclastic and volcaniclastic rocks found in the eastern portion of the study area formed in response to dacitic volcanism which occurred during Fig Tree time. Rocks of the Fig Tree Group have been subjected to diagenetic alteration and low-grade metamorphism which has resulted mainly in silicification, along with the production of phyllosilicate. This had produced rocks that are now composed mainly of chert and sericite. Provenance studies indicate that the source for the Fig Tree fan-delta included uplifted portions of the underlying greenstone belt sequence and penecontemporaneous Fig Tree volcanic rocks. The tectonic evolution of the Barberton Greenstone Belt from latest Onverwacht through Fig Tree times can be summarized in three stages: (1) a predominantly volcanic, anorogenic stage; (2) a stage of evolving tectonic instability, and; (3) a final orogenic stage in which there was thrust faulting and erosion of both intra- and extra-belt rocks.

Pages

196

Share

COinS