Rock Magnetic Clues to Diagenetic Formation of Bedded Chert Sequences

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 has long been a matter of contention. The best-known analog for the bedded chert precursor sediments is a sequence of siliceous sediments recovered by ODP leg 129. A rock magnetic study of the sequence found no significant differences in properties of the adjacent clay-rich and clay-poor bands. Apart from a minor difference in bulk concentration of magnetic phases, clay-rich and clay-poor intervals are similar in the relative abundances of magnetic fractions, including biogenic magnetite. This contrasts with diagenetically mature chert-shale interlayers that show up to 2 orders of magnitude difference in bulk magnetic concentrations and a conspicuous absence of biogenic magnetite in shale. The difference suggests that precursor sediments undergo differential diagenetic modification.

All known features of bedded chert sequences, such as differences in thickness, detrital elements, Al/Ti ratios, and cosmic dust content between chert and shale, can be accounted for by the transfer of silica from clay-poor (silica-rich) donor beds to clay-rich receiver beds. The silica transfer is driven by variability in the clay content of sediments. Silica dissolution starts in clay-poor intervals. When diffusing silica encounters clay-rich beds, adsorption to clay lowers dissolved silica content and leads to deposition of low solubility quartz. The early nucleation of quartz provides seed for crystal growth, maintains a diffusion gradient, and facilitates the continuing transfer of silica to proto-chert layers. 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 detrital particle concentration.

The distinct bedding in chert sequences thus arises from the diagenetic redistribution of silica within a sequence with variable clay content. While the precursor sediments may have recorded climatic variability, differential diagenesis severely distorts primary environmental signals. Mature bedded chert sequences are poorly suited for cyclostratigraphic studies.