Late Cretaceous to Cenozoic geodynamic evolution of the Atlantic margin offshore Essaouira (Morocco)

Abstract

After Mesozoic rifting, the Atlantic margin of Morocco has recorded the consequences of the continental collision between Africa and Europe and the relative northward motion of the African plate over the Canary Island hotspot during Cenozoic times. Interpretation of recently acquired 2D seismic reflection data (MIRROR 2011 experiment) presents new insights into the Late Cretaceous to recent geodynamic evolution of this margin. Crustal uplift presumably started during the Late Cretaceous and triggered regional tilting in the deep-water margin west of Essaouira and the formation of the Base Tertiary Unconformity (BTU). An associated hiatus in sedimentation is interpreted to have started earlier in the north (presumably in the Cenomanian at well location DSDP 416) and propagated to the south (presumably in the Coniacian at well location DSDP 415). The difference in the total duration of this hiatus is postulated to have controlled the extrusion of Late Triassic to Early Jurassic salt during the Late Cretaceous to Early Palaeocene non-depositional period, resulting in regional differences in the preservation of salt structures: the Agadir Basin in the south of the study area is dominated by salt diapirs, whereas massive canopies characterised the Ras Tafelnay Plateau farther north and salt-poor canopies and weld structures the northernmost offshore Essaouira and Safi Basins. Accompanied by volcanic intrusions, a presumably Early Palaeogene reactivation of previously existing basement faults is interpreted to have formed a series of deep-water anticlines with associated gravity deformation of shallow-seated sediments. The orientation of the fold axes is roughly perpendicular to the present day coast and the extensional fault direction; therefore, not a coast-line parallel pattern of extensional faults, related to the rifting and break-up of the margin, but rather a coast-line perpendicular oceanic fracture zone might have caused the basement faults associated with the deep-water folds. Both the volcanic intrusions and the formation of the deep-water anticlines show a comparable age trend which gets progressively younger towards the south. A potential tempo-spatial relationship of the BTU and the reactivation of basement faults can be explained by the relative northward motion of the African plate over the Canary Island hotspot. Regional uplift producing the BTU could have been the precursor of the approaching hotspot during the Late Cretaceous, followed during the Early Palaeogene by a locally more pronounced uplift above the hotspot centre.

Anticline growth by shortening during crustal exhumation of the Moroccan Atlantic margin

“…Halokinesis and salt tectonics are well expressed in the area (Hafid et al, 2006;Hafid, 2000) and proposed to happen during this period in the Central High-Atlas (Saura et al, 2013) and in the offshore Essaouira-Agadir Basin (Pichel et al, 2019a). Although extensive diapirism exists offshore Morocco, no clear interpretations of pre-Cretaceous timing and mechanism(s) of salt mobilisation are available, and it thus may occur in relation to different mechanisms than in the onshore (Neumaier et al, 2016). Moreover, salt mobilisation may potentially lead to syn-sedimentary deformation along the sides of the diapir and sedimentary dykes (Morley et al, 1998;Giles and Lawton, 2002 respectively;see examples in Poprawski et al, 2014).…”

Crustal structure variations along the NW-African continental margin: A comparison of new and existing models from wide-angle and reflection seismic data

“…The region was later on affected by volcanism due to the Canary Islands and Cape Verde Islands hotspots. Basement has formed bulges possibly due to the hotspot influence (Holik et al, 1991;Duggen et al, 2009;Patriat and Labails, 2006;Neumaier et al, 2015).…”

Syn-depositional Mesozoic siliciclastic pathways on the Moroccan Atlantic margin linked to evaporite mobilisation

“…Early and Middle Jurassic evaporite mobilisation in the Central High Atlas has been inferred from sedimentary onlaps described by Saura et al (2014), Martin-Martin et al (2017, and later reviewed in Moragas et al (2018). While the evaporite mobilisation mechanisms in the offshore EAB may differ from those onshore, timing of early evaporite mobilisation is similar (Neumaier et al, 2016;Pichel et al, 2019, Fig. 11b).…”

“…Alternatively, and although we favour the above-presented mechanism, evaporite mobilisation may be due to substantial temperature gradients in the evaporite bodies (i.e., thermal loading; Hudec and Jackson, 2007). Processes that could lead to such gradient include: i) extensive magmatic activity in the Late Triassic (Central Atlantic Magmatic Province; e.g., Davies et al, 2017) following the end of rifting, ii) by reactivation of ~NE/SW basement normal faults (e.g., Hafid, 2000;Piqué and Carpenter, 2001), as described for the offshore areas further north in Morocco (Neumaier et al, 2016), iii) significant changes in the supra-evaporite thickness, and/or vi) different lithosphere structure within the basin.…”