Carbonaceous cherts of the Swaziland Supergroup, Barberton Mountain Land, southern Africa

摘要

The well-preserved carbonaceous cherts of the early Archean (3,200 to 3,500 Ma) Swaziland Supergroup of the Barberton Greenstone Belt, South Africa contain fossils that are among the Earth's oldest. Fossils are rare in the carbonaceous cherts, however. Petrographic and geochemical analyses of the carbonaceous cherts yield information useful to the study of early life and to the general question of the nature of silicification of early Archean rocks.;The filamentous microfossils whose morphologies, as well as the texture of the encompassing laminations, suggest an affinity to modern microbial mat-dwelling cyanobacteria or bacteria, are the most probable biological structures identified during this study. Spheroidal and ellipsoidal structures of various types in cherts of the Hooggenoeg and Kromberg Formations are considered only possible microfossils because of their simple shape. Thick-walled ellipsoids are interpreted to be akinetes, or resting spores of cyanobacteria. Because akinetes are presently found only in the more advanced families of modern cyanobacteria, their occurrence supports assertions that cyanobacteria were well-established by the early Archean.;Black-and-white banded cherts are the most promising targets in the search for Archean microfossils. They are well-preserved, early-silicified accumulations of fine carbonaceous laminations, interpreted as remains of microbial mats, interbedded with layers of simple or composite carbonaceous grains. Massive black cherts contain a large proportion of lithic grains as well as carbonaceous detritus, but lack mat-like laminations. Laminated black cherts are also detrital accumulations of both lithic and carbonaceous matter, but are commonly finer-grained and more compacted than massive black cherts with higher TOC values than the other chert types.;Shallow-water platform sediments of the Hooggenoeg, Kromberg and Mendon formations contain an abundance of mat-like laminations and composite grains. Units dominated by volcaniclastic sediments generally contain a large proportion of carbonaceous detritus. Deeper-water sediments and basin sediments of the upper Kromberg and Mendon formations are dominated by well-compacted carbonaceous detritus and lack direct evidence of microbial activity.;The relationship between grain shape and TOC indicates that TOC in carbonaceous cherts is a function both of primary carbon content and the amount of pre-lithification sediment compaction. H/C ratios suggest that considerable alteration of the organic matter has occurred. The carbon isotope values indicate that all of the carbonaceous matter may have had a biological origin.