Evidence and Timing of a Forced-Regressive System in Passive Continental Margin Setting. The Late Barremian to Aptian of the Essaouira-Agadir Basin, Morocco.

The Essaouira-Agadir Basin (EAB) of Morocco contains the only continuous Mesozoic succession exposed in the Central Atlantic passive margin. These outcrops provide a record of the basin margin dynamics and analysis of the... [ view full abstract ]

The Essaouira-Agadir Basin (EAB) of Morocco contains the only continuous Mesozoic succession exposed in the Central Atlantic passive margin. These outcrops provide a record of the basin margin dynamics and analysis of the temporal and spatial evolution is critical to understanding the controls and processes of sediment delivery in the underexplored basins offshore Morocco.

Extensive sedimentary logging and high-resolution biostratigraphy of the Early Cretaceous interval allows characterisation of the depositional environments and generation of high-resolution palaeogeographic reconstructions. Two main regressive intervals have been identified as primary candidate periods to deliver coarse clastic sediment into the otherwise mudstone-dominated succession.

In the early Hauterivian Tamanar Fm. widespread carbonate production with patch-reef build-ups and oolitic shoals developed, often overlain by prograding mixed carbonate and clastic-rich shoreface deposits. This system is interpreted to have developed as a late highstand.

The late Barremian to Aptian of the Bouzergoun Fm. interval crops out along the modern coast of Morocco and can be traced inland for 100 km. It comprises an upward-coarsening succession of thick marls with minor sandstones, interpreted to record typical highstand progradation. This interval is overlain by sharp-based, fossil-poor marine sandstones. Discrete intervals of widespread soft sediment deformation can be recognised, suggesting rapid sedimentation rates on over-steepened slopes. Depositional environments range from lower shoreface to delta mouth bar and strand plain. Exceptional exposure allows correlation inland, with evidence for earlier shoreface deposits capped by erosional fluvial succession. This is interpreted to record a classic zone of separation following a forced regression. The succession is often truncated by a regionally recognized erosion surface, in some areas developing incised valleys, recording a sequence boundary developed during maximum base-level fall. Infill of the incised valleys ranges from fluvial and floodplain to tidal deposits. Regionally this termination of this regressive succession can be well constrained, as it is overlain by an abrupt transgression recorded by fossil-rich sandstones and marls of the early Aptian (D. Forbesi ammonite zone).

Our study has defined the late Barremian-Aptian as the main period when coarse clastics are likely to be delivered in the deepwater EAB. Improved biostratigraphic resolution has enabled sequential palaeogeographic maps to be produced that constrain lateral variability in the system and illustrate the potential provenance of the coarse clastics from a hinterland suffering inversion. The forced regression does not correlate with a global eustatic event, suggesting a complex interplay of local tectonics, local eustasy and climate as the driving mechanism.