Silica‐rich seawater in the early Cambrian: Sedimentological evidence from bedded cherts

The effects and timing of the evolution of siliceous sponge and radiolarians on the mode of silica burial and oceanic silica cycling in the early Cambrian are still controversial. Bedded cherts of the lower Cambrian Yurtus Formation deposited in a subtidal shelf environment. They display diagnostic sedimentary features, including intraclastic chert grains, edgewise chert breccias, weakly compacted grains and rip-up clasts, and rapid silica cementation, collectively indicating primary and very early silicification at and near the sediment-water interface. This depositional mode contrasts with Phanerozoic biogenic cherts but is comparable with that of Precambrian cherts precipitated from silica-rich seawater. The Yurtus cherts therefore provide direct and compelling sedimentological evidence of the continuation of silica-rich seawater into the early Cambrian, consistent with the extensive coeval chert depositions elsewhere. These results contrast with the long-held view of rapid decline in seawater silica concentrations in the earliest Phanerozoic, and elucidate coeval Si-coupled geochemical cycling, silica-biomineralization and fossil preservation.