STRANGE BREW: PERVASIVE PATTERNS OF UNUSUAL CHEMICAL SEDIMENTATION AND TRANSIENT OCEAN CHEMISTRY AT THE DAWN OF THE PHANEROZOIC

Continents worldwide experienced protracted denudation during the Late Neoproterozoic and Early Phanerozoic, resulting in the removal of most preexisting sedimentary rock and the exposure of crystalline basement rocks across an area that is unprecedented in the past 1 billion years. It has recently been suggested that chemical weathering of this surface, known as the Great Unconformity, led to an enhanced flux of continental weathering products to the Cambrian oceans. Here, we use the Macrostrat database in concert with petrologic and geochemical data to explore patterns of unusual chemical sedimentation that provide evidence of ocean chemistry during the advent of biomineralization. These patterns extend across all three facies belts that onlapped paleocontinents during the Cambrian: the inner detrital belt, the carbonate platform belt and the outer detrital belt. The inner detrital belt is marked by a Phanerozoic peak in the abundance of glauconite and non-analogue conditions of glauconite authigenesis, in which precipitation occurred more rapidly and across a broader range of environments than it has subsequently. In addition, expressions of anomalous chemical sedimentation include widespread Fe-ooids and early hematite and carbonate cements in sandstones, as well as abundant sandstone intraclasts that were cemented by carbonate at or near the seafloor prior to wave disturbance. The carbonate platform belt is marked by Phanerozoic highs in both area and burial flux of carbonate, the majority of which was photosynthetically-derived and grain-poor. Peaks in the abundance of flat-pebble conglomerates and oncolites also reflect the influence of early carbonate precipitation. The mudstone-dominated outer detrital belt is characterized by pervasive carbonate cementation at the seafloor, a factor that was critical in promoting widespread preservation of Burgess Shale-type fossils. Carbonate nodules and concretions bearing seawater ∂¹³C values are also common. Together, these observations are indicative of an enhanced flux of HCO₃^-, Ca²⁺, Fe³⁺, H₃SiO₄^-, Mg²⁺, and K⁺ to the terminal Neoproterozoic-Cambrian oceans, coincident with the initial acquisition of skeletons by metazoans and the Cambrian explosion.