Diagenetic formation of bedded chert: Implications from a rock magnetic study of siliceous precursor sediments

Abstract

Bedded chert is a distinctive rock type characterized by a rhythmic alternation of quartz-rich chert beds and clay-rich interbeds. The origin of the lithologic variations is not well understood. A rock magnetic study of a sequence of alternating bands of radiolarian-rich and clay-rich bands recovered by ODP leg 129, which is considered to be the best-known analog for the bedded chert precursor sediments, shows that the relative abundances and characteristics of magnetic phases are similar in both sediment types, with only a minor (1.5 times on average) increase in bulk concentration in clay-rich bands. Comparison with magnetic properties of the diagenetically mature chert-shale couplets suggests that siliceous precursor sediments undergo strong differential diagenetic modification. A simple silica transfer model, with dissolution of silica in proto-shale donor beds and deposition in proto-chert receiver beds, indicate that the known features of the bedded chert sequences, such as the difference in thickness, cosmic dust content between chert and shale, can be reproduced in case of donor beds having initially lower clay (higher biogenic silica) content compared to receiver beds. While contrary to the current depositional model explaining chert beds formation by episodic radiolarian bloom on a background of slow accumulation of clays, an association of proto-shale beds with high productivity episodes is supported by an independent paleoproductivity proxy – their higher Al/Ti values, and is in full agreement with the known silica diagenesis trends, particularly, with the retarding influence of detrital minerals on silica transformations. The impetus for diagenetic silica segregation is provided by initial variability in clay content within a siliceous sedimentary succession. Silica dissolution starts in clay-poor intervals. When diffusing silica encounters clay-rich beds, adsorption to clay lowers dissolved silica content below the saturation level of the higher solubility phases and leads to deposition of low solubility quartz. Early deposition of quartz cement prevents significant compaction of chert layers, while the loss of silica in donor beds leads to their compaction and concomitant increase in non-reactive clay and cosmic dust particles. Severe distortion of primary environmental signals during differential diagenesis questions the use of bedded chert sequences for cyclostratigraphic studies.