New insights on sedimentology, sequence stratigraphy and palaeogeographic reconstruction of the Tortonian-Early Messinian Kechabta series in Kechabta Foreland Basin (Northern Tunisia)

Abstract

The Kechabta Basin is part of the Tunisian Foreland Basin in front of the Numidian and Tellian nappes. During the Tortonian, this basin records a strong subsidence rate due to flexural process, in front of the nappes. Contrary to previous interpretations, a sedimentological investigation of the thick Tortonian-Early Messinian Kechabta Formation (over 1300 m) reveals that this siliciclastic series corresponds to a complex deltaic shorelines deposit dominated by waves and influenced by tides. This formation is mainly made of a large number of high frequency, mudstone to sandy prograding parasequences representing repeated progradational phases of the coastal wedge. Facies successions within many of the parasequences show normal regressive trends. These parasequences present offshore grey-brown mudstone, followed by transition offshore sandy storm beds alternating with mudstone, then by shoreface sandstones bearing hummocky-cross stratification and wave ripples, then by foreshore facies or tidal flat facies. However, many other parasequences reveal forced regressions during the progradational phases. This evidences that the progradational phases were driven by small, stepped, relative sea-level falls due to a tectonic activity related to the compressive Atlassic phase.This new paleoenvironmental interpretation suggests new elements for the palaeogeographic reconstruction of Northern Tunisia foredeep depozone during the Tortonian-Early Messinian. The base of the Kechabta Formation record the sedimentary response to regional marine transgression of the Mediterranean Sea from the East. This basin was an elongated shallow marine gulf parallel to the thrust front. The fluvio-deltaic systems of the paleo-Medjerda Rivers provided the bulk of the Kechabta siliciclastic sedimentary flux and extensive prograding delta are formed.