Sédimentation détritique continentale synchrone d'un volcanisme explosif dans le Trias terminal à infra-Lias du domaine atlasique (Haute Moulouya, Maroc) (Late Triassic to infra-Liassic continental detrital sedimentation synchronous with an explosive volcanic event in the Atlas area [Hgh Moulouya, Morocco

Abstract

A volcano-detrital formation (the Tizi-n-Ghachou Formation), essentially composed of reworked basaltic material and volcanic fallout deposits, has been discovered in a southwest-northeast-trending intracontinental decakilometric sedimentary basin. This basin is located on the South Middle Atlas Fault, which separates the Middle Atlas in the northwest from the high Moulouya area to the southeast. The study of the space and the time distribution of these epi- and pyroclastic deposits enables the palæomorphology and the dynamics of this basin to be deciphered. In the subsiding axis of the southern part of the basin, sedimentation was roughly organised in a positive mesosequence with, from bottom to top: coarse conglomerates with basement pebbles corresponding to a piedmont fan; relatively finer fluvial deposits found in conglomeratic channels and drainage deposits from floodplains; clayish siltstones and carbonate sediments (limestones and dolomites) includeing pyroclastic basaltic fragments. This formation was mainly derived from the erosion of Late Triassic tholeiitic continental flood basalts contemporaneous with aerial explosive volcanic events. Erosion, as well as limited distribution of the deposits, suggests regional uplift of the basaltic traps after their emplacement, synchronous with reactivation of magmatic activity on the uplifted margins of the intracontinental basin. Laterally, toward the northwest, an important asymmetry suggests two discrete blocks, whose mobility was controlled by southwest-northeast syn-sedimentary active faults. Lower Liassic carbonate platform deposits overlay these structures. A wide area nalysis of these deposits shows structural trends superimposed on a major crustal suture zone, and suggests a tectono-sedimentary evolution in an uplift-related, extentional, or transtensional, geodynamic setting during the Late Triassic-infra-Liassic period.