High-frequency relative sea level variation cycles recorded in sedimentary discontinuities

In the Lodève region (“Caussenard High” southwestern margin of the South-East Basin), upper Hettangian to Lotharingian carbonate series shows more than twenty sedimentary discontinuities of various types. Some of these discontinuities are polyphase surfaces, which record high-frequency sea-level variation cycles. One of these discontinuities has been studied in detail, in order to understand its origin (processes, chronology of formation). Several stages of formation have been recognized. A transgressive erosion occurred first. Next, development of Skolithos and Diplocraterion burrows, associated with iron mineralizations, could be indicative of a condensation phase corresponding to the subsequent maximum-flooding period. The burrows sparitic fills may characterize the highstand period following the maximum-flooding period. Next, alteration and corrosion observed on the surface and in the burrows may indicate an emersion related to a regressive phase and the subsequent lowstand period. Finally, micro mud-mounds (formed by pelecypods and corals trapping fine lime sediment) may indicate a new condensation phase, associated with the transgression and the following maximum-flooding period. Thus, this discontinuity records one and a half relative sea level variation cycle. Counting sequences observed in the deposits and in the discontinuities leads to an estimate of 100,000 years duration for these cycles. That could correspond to the eccentricity variation cycles. Moreover, the local synsedimentary tectonics may have influenced the formation of this type of discontinuity, by forming shallow restricted environments favourable to their development. Recognition and understanding of these particular discontinuities are essential because they may be laterally correlated with other surfaces in continental settings (paleokarst) or with classical sequences in marine environments.