The 100‐ka and rapid sea level changes recorded by prograding shelf sand bodies in the Gulf of Lions (western Mediterranean Sea)

Abstract

Thick forced regressive units on the wide continental shelf of the Gulf of Lions (western Mediterranean) recorded the composite effect of sea level changes during the Quaternary. They are mostly composed of coastal siliciclastic and bioclastic wedges showing clinoform geometry. These deposits have been intensively explored through high‐resolution seismic investigations, but only recently it was possible to ground truth seismic interpretations, based on a long (100 m) borehole that crossed the succession and recovered a large part of the mainly sandy deposits (∼84% recovery). A multiproxy analysis of the sedimentary succession shows that (1) the stratal architecture of the shelf margin is defined by major bounding surfaces that are polygenic erosion surfaces associated with coarse‐grained material incorporating abundant and diverse shells, including cold‐water fauna (presently absent from the Mediterranean Sea). Between each surface, coarsening upward units with steep (up to 5°) foresets are made of massive (more than 20 m thick) sands with possible swaley and hummocky cross‐stratification, passing seaward to sands with muddy intervals and, further offshore, alternating highly boiturbated sands and silts. Each prograding wedge corresponds to a forced‐regressive shoreface (or delta front/prodelta), deposited during the overall sea level falls occurring at (relatively slow) interglacial/glacial transition and therefore represents the record of 100 ka cyclicity. Higher‐frequency Milankovitch cyclicities are also probably represented by distinct shoreface/delta front wedges; (2) detailed examination of the architecture and chronostratigraphy of the most recent sequence shows that minor bounding surfaces, corresponding to abrupt shallowing of sedimentary facies, separate downward stepping parasequences within the last 100 ka sequence. These events are in phase with millennial‐scale glacial climatic and sea level variability, the downward shift surfaces corresponding to the falls during the coldest stadials. These deposits provide a comprehensive and well‐constrained Pleistocene analog to the numerous shoreface deposits attributed to falling‐stage systems tracts recognized in ancient stratigraphic records, studied at the outcrop scale.