PRIMARY MICROFOSSILIFEROUS CHERT IN THE APTIAN BARRA VELHA FORMATION

The Aptian Barra Velha Formation records the rifting of Gondwana and the formation of the proto-Atlantic basin. Previous studies have characterized the carbonate spherulite and shrub facies in the Barra Velha Formation and other pre-salt deposits and suggest that the proto-Atlantic basin was a continental-scale alkaline lake. However, the exact water chemistry, the water sources, and the silica cycle in the lake are not fully understood. Additionally, the microbial communities that inhabited the lake, their role in mineral formation, and the taphonomic windows in the lake are poorly documented. We describe rare authigenic chert in the Bara Velha Formation which provides new insight into the water chemistry, evolution, and microbial communities of the pre-salt basin.

Microcrystalline chert is present in five samples across three wells and is distinct from late diagenetic silica phases. Petrographic textures reveal that the microcrystalline chert pre-dates compaction of carbonates but post-dates carbonate precipitation and subsequent dissolution, suggesting that chert and carbonate precipitation were temporally decoupled. Secondary ion mass spectrometry (SIMS) δ³⁰Si and δ¹⁸O results reveal that this chert likely formed from a fluid that was compositionally distinct from the lake water, pointing to a fluid source that was introduced sporadically into the basin.

Light microscopy and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS) reveal that the authigenic chert preserves primary organic material that is compositionally and texturally different from bitumen in the carbonate facies. This primary organic material is present in the form of clotted and wispy organic textures as well as morphologically diverse microfossils. The chert therefore represents a rare taphonomic window and provides an in situ record of the microbes and simple eukaryotes that inhabited this ancient basin. The decoupling of chert and carbonate precipitation points to chemical complexity of the lake with specific microenvironments that saw the precipitation of chert and preservation of organic material in the absence of carbonate precipitation. These results provide a path toward understanding the lake water chemistry and complexity, mineral formation and organic preservation, and microbial communities that inhabited this complex ancient basin.