The Barberton Drilling Project’s Buck Reef Chert core BARB3 – Sedimentary facies and depositional environment of a 3.4 Ga marine platform succession

Abstract

The ∼ 3.4-billion-years old Buck Reef Chert (BRC) is a ∼ 400 m thick succession of banded chert and iron-formation that has proved to be an important site for the study of Archaean surface processes. It is exceptional in its degree of preservation of primary textures and microbial matter in chert, but full comprehension of its depositional history requires subsurface investigation. As part of the International Continental Drilling Program-funded Barberton Drilling Project a single drill core (BARB3) with a total length of 899 m was obtained from the steeply dipping succession. The BRC overlies an eroded volcanic edifice of shallow intrusive to extrusive felsic volcanic rocks and is separated from overlying ultramafic lapillistone by an ultramafic sill. Drilling commenced in the sill at an angle of ∼ 45° and ∼ 200 m of serpentinised peridotite were intersected. The remaining ∼ 700 m of the core include a variety of sedimentary and minor intrusive mafic to felsic igneous rocks. The chemical sedimentary rocks are dominated by chert and siderite forming four thinly interbedded lithofacies: (1) white chert, (2) granular chert, (3) grey chert/carbonate, and (4) carbonate, largely reflecting marine precipitation under varying physico-chemical conditions of the depositional environment. These facies form three distinct facies associations of shallow-water banded granular chert, banded ferruginous chert and deep-water banded iron-formation, reflecting gradual deepening of the environment in agreement with previous studies. Subordinate facies include carbonaceous matter-rich siliceous shale distributed randomly throughout the succession and thin beds of jaspilite in banded iron-formation. The former reflect blooms of planktonic microbes linked to enhanced nutrient supply and the latter reflect episodes of variation in marine environmental parameters. Both planktonic and benthic microbes were thriving in an acidic, anoxic, shallow-marine platform environment dominated by chert precipitation. Siderite is common in both shallow- and deep-water deposits and is regarded as an early diagenetic precipitate from alkaline pore fluids. Stratiform and cross-cutting veins of botryoidal chert and quartz ± siderite are common and formed from ascending formation waters derived from compaction of chert precursor sediment. Deposition took place in a volcanically active setting with high geothermal heat flow, resulting in early diagenetic thermal decomposition of organic matter and some hydrocarbon generation, now preserved as pyro-bitumen. Exceptional preservation of primary textures provides a remarkable record of processes that operated on a 3.4 Ga marine sedimentary platform.