NEW, HIGH-RESOLUTION LITHOFACIES DATA FROM FINE-GRAINED SEDIMENTS: A REAPPRAISAL OF FUNDAMENTAL PROCESSES OPERATING TO CONTROL FACIES VARIABILITY IN EPICONTINENTAL SEAS

Ancient epicontinental seas were major sites of mud deposition and carbon sequestration. On the basis of field observations that fine-grained sediments collected from these settings are commonly fissile, apparently homogeneous, unbioturbated, and organic matter-rich. Sedimentologists have argued that deposition occurred in low-energy conditions, beneath long-term stratified water columns; where bottom water layers were prone to anoxia, specially during periods when surface productivities were high.

Recent high-resolution lithofacies work, utilizing optical and electron optical techniques combined with whole rock geochemical analyses (total organic carbon contents), provide evidence that contradicts this widely accepted paradigm.

Using, examples collected from a variety of fine-grained, epicontinental Mesozoic successions; this paper illustrates, that contrary to expectations, successive depositional units (beds) in these strata contain a diverse range of sedimentological textures (e.g. thin, upward-fining beds; intraclasts; shell pavements; starved ripple laminae; diminutive burrows and in-situ benthic foraminifer). In addition, to being composed of varying proportions of constituent materials derived from detrital, productivity and diagenetic processes.

These observations suggest that in epicontinental shelf seas: the bottom water layers were rarely anoxic, conditions at the seabed were commonly energetic, and that physical, chemical and biological processes contributing individual sedimentary components to the site of burial were highly variable, even on short time-scales. Moreover, they also imply that sedimentary detritus was delivered to the sediment water interface in pulses, rather than as “continuous rain”.

These observations seriously challenge prevailing views of the sedimentological processes operating to control lithofacies variability in epicontinental seas.