BIOGENIC AND ORTHOCHEMICAL MICROFACIES AND SEDIMENTARY ENVIRONMENTS OF THE EARLY ARCHEAN BUCK REEF CHERT, BARBERTON GREENSTONE BELT, SOUTH AFRICA

The Buck Reef Chert is a 250–400-m-thick unit of banded carbonaceous and ferruginous chert deposited 3,416–3,334 Ma. Water depths increased upward. The lowest 5–50 m include evaporitic units deposited in coastal brine ponds. Overlying black and black-and-white banded cherts were deposited under shallow, wave- and/or storm-dominated conditions. These grade upward into finely layered ferruginous units deposited in quiet, deeper waters. Locally, shallower-water units reappear at the top of the unit. Clastic input was low to absent.

Five principal microfacies have been identified in thin section. Microfacies I shows current-deposited fine to coarse detrital kerogen grains interlayered with crinkly carbonaceous laminations. Intraformational detrital silica grains, ripped up carbonaceous chunks, and quartz-filled water escape cavities are common. Microfacies II is characterized by weakly carbonaceous chert containing ferruginous materials, originally iron-bearing carbonates but now mainly iron oxides formed by carbonate oxidation. Carbonate grains are disseminated, form discrete layers, and coat fine detrital silica and carbonaceous grains. Microfacies III is characterized by chert containing flattened mostly fine kerogen grains and crinkly to anatomosing carbonaceous laminations. Flattened or elongate fine silica grains may be present. Microfacies IV is similar to microfacies III but also contains ferruginous material. Microfacies V is a mixture of chert and orthochemical ferruginous material lacking detrital components.

Microfacies I makes up much of the shallow-water part of the section. Thick units commonly show soft-sediment deformation and brecciation, possibly due to storms. Microfacies II and III overlap with microfacies I, but are most common in sections transitional to deeper-water units. Microfacies IV and V are most common in the upper part of the section deposited below storm wave base and show thin, evenly banded layers.