Modes of accumulation of carbonaceous matter in the early Archean: A petrographic and geochemical study of the carbonaceous cherts of the Swaziland Supergroup (1999)

Three main types of carbonaceous chert occur in the Swaziland Supergroup, Barberton Greenstone Belt, South Africa: black-and-white banded chert, massive black chert, and laminated black chert. These cherts are composed of six main morphological types of carbonaceous matter: carbonaceous laminations, simple grains, composite grains, wisps, diffuse carbonaceous matter, and crystalline carbonaceous matter. The black bands in black-and-white banded cherts are generally composed of well-preserved fine carbonaceous laminations, representing the remains of microbial mats, interbedded with layers of simple and composite carbonaceous grains. Massive black cherts contain a large proportion of lithic grains as well as carbonaceous detritus, but lack matlike laminations. Laminated black cherts are also accumulations of detrital lithic and carbonaceous matter, but are commonly finer grained than massive black cherts and contain a high proportion of carbonaceous wisps. Comparison of the aspect ratios of carbonaceous grains among the various chert types suggests that the original sediments were silicified at different times relative to compaction. Black-and-white banded chert and to a lesser extent massive black chert contain round or lobate carbonaceous grains and were silicified before sediment compaction. Laminated cherts are dominated by wispy grains, indicating that compaction largely preceded silicification. The relationship between grain shape and total organic carbon (TOC) indicates that TOC in carbonaceous cherts is a function of both primary carbon content and the amount of prelithification sediment compaction. In general, laminated cherts show the greatest presilicification sediment compaction and the highest TOC contents. Carbon isotope values indicate that all of the carbonaceous matter probably had a biological origin. Most cherts in the Hooggenoeg, Kromberg, and lower cycles of the Mendon Formations contain carbonaceous matter deposited in shallow water as both loose detritus and microbial mats. During periods of explosive volcanism, volcaniclastic debris was locally mixed with accumulating carbonaceous matter. These shallowwater sediments were generally lithified soon after deposition, probably through the interaction of the uppermost sea-floor sediment layers and sea water. During deposition of basaltic sequences, detrital carbonaceous matter accumulated in deeper water along with fine volcanic ash. Cherts in the upper part of the Mendon Formation represent deep-water sediments that rarely contain mat accumulations. Carbonaceous matter was preserved mainly as fine detritus mixed with lithic grains deposited under low-energy conditions. Lithification and silicification occurred after sediment compaction, probably well below the sea-floor. The abundance of in situ bacterial mats and composite grains in shallow-water deposits and their paucity in deep-water carbonaceous cherts is consistent with the interpretation that some early Archean organisms were photosynthetic, with much primary production occurring in the photic zone.