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.

中文翻译：

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寒武纪早期富含二氧化硅的海水：来自层状燧石的沉积学证据

硅质海绵和放射虫的演化对早寒武世硅质埋藏和海洋硅质循环模式的影响和时间仍存在争议。沉积在潮下陆架环境中的下寒武统 Yurtus 组的层状燧石。它们显示出具有诊断意义的沉积特征，包括碎屑内燧石颗粒、边缘燧石角砾岩、弱压实颗粒和破裂碎屑以及快速硅石胶结，共同表明沉积物 - 水界面处和附近的原始和非常早期的硅化作用。这种沉积模式与显生宙生物成因燧石形成对比，但与从富含二氧化硅的海水中沉淀的前寒武纪燧石相媲美。因此，Yurtus 燧石提供了直接和令人信服的沉积学证据，证明富含二氧化硅的海水延续到早寒武世，与其他地方广泛的同时期燧石沉积相一致。这些结果与最早显生宙海水二氧化硅浓度迅速下降的长期观点形成对比，并阐明了同期的硅耦合地球化学循环、二氧化硅生物矿化和化石保存。