Volcaniclastic turbidites are high-volume, in-situ clay mineral factories

Bulk clay mineralogy is one of the most powerful proxies for tracing sediment provenance and climate history across a sedimentary basin's source area. However, there are numerous instances where high volumes of pore-bridging clay minerals form by burial diagenetic processes, and where clay mineral formation has no direct connection to climate-driven processes in the hinterland. One of the best exposures to study in-situ clay mineral formation is the early Ordovician ( 385 Ma) Power Steps Formation at Ochre Cove, Bell Island, Newfoundland. This formation developed when high volumes of highly-reactive, detrital minerals (feldspar and volcanoclastics) were delivered via mechanical weathering from a non-vegetated hinterland. Systematic sedimentological and compositional analyses of mud and sandstone along the clinothem topset, foreset and bottomset reveal that sediment was delivered by uni-directional currents and experienced episodic reworking by storm-waves. Petrographic examination and quantitative X-ray diffraction reveal significant variability in the distribution of illite versus chlorite. In the present-day clay mineral fraction illite is detrital whereas chlorite originates preferentially through alteration of highly unstable mafic (volcanoclastic) grains that were, initially, delivered to the clinothem as silt-sized framework components. These lithic fragments were diagenetically alerted, via in-situ weathering, to chlorite before significant compaction and yield significant volumes of diagenetic silica cement that eliminates primary porosity while its presence influences the modern-day rock strength of mudstone beds. Compositional, diagenetic and textural attributes across the Ochre Cove mud clinothem vary as a function of starting composition (provenance), hydrodynamic sorting and grain stability. Future studies must consider the formation of in-situ weathering-derived chlorite separately from detrital chlorite when aiming to employ clay mineralogy as a proxy for provenance, which can also impact the loss of porosity and modify rock strength via formation of large volumes of pore-bridging silica cement.