The Cenomanian–Turonian boundary event on the Moroccan Atlantic margin (Agadir basin): Stable isotope and sequence stratigraphy

Abstract

Correlations of four sections, constrained by stable isotope stratigraphy and micropalaeontology, along a 80 km-long, E/W-oriented transect from the western High-Atlas to the present-day coast (Taghazoute), show that the Cenomanian–Turonian boundary event (CTBE) occurred within shallow-water, oyster-bearing limestones and marls, and not in the overlying black shale. The age of the laminated shale is early Turonian as the planktonic marker Helvetoglobotruncana helvetica is found from its first centimetres upward. The underlying deposits hosting the CTBE event are bounded by two unconformities interpreted as emersion surfaces, and represent the strongest seawardshifts of facies in the local Cenomanian sedimentary wedge. The δ 13C anomaly is only recorded in the coastal Taghazoute section, not in other sections landward, meaning that the CTBE occurred here during a short transgressive phase within an overall regressive trend. The lower Turonian black shale of the Taghazoute section is transgressive onto the second unconformity and pass landward to white, evenly laminated, chert-bearing mudstones, then to bioturbated mudstones in the easternmost sections of the transect (western High-Atlas).