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Storm deposits: evidence of event sedimentation in the Bajocian of the Central High Atlas, Morocco

Alt Addi A.

Volumina Jurassica

2008

Vol. 6, no. 1

23-32

Storm beds interbedded with marls, claystones and beds of nodular to undulose bioclastic limestones have been recognized in the Central High Atlas of Morocco. These provide the first evidence of event sedimentation in the Bajocian of the area. The outcrops studied are included in the Agoudim and Tazigzaout formations in the center of the basin, and are parts of the Bin El Ouidane Group at the margins of the basin. The storm deposits comprise calcarenites and bioclastic packstones-grainstones. These beds have symmetrical wave-ripples at their tops and an internal structure of hummocky cross-stratification. They are considered to have been formed by tropical storms and hurricanes. Multiple reworking and winnowing of siliciclastic or bioclastic material by oscillatory currents related to storm and to currents resulting from storm/tide interactions is inferred. Bed features are the most important evidence in favour of a palaeobathymetric interpretation of related pelagic sediments, with calcareous tempestites representing episodic resedimentation, mainly coincident with relative sea-level falls during which major storm waves affected the sea bottom. This interpretation is in agreement with the regional palaeogeography and is further supported by evidence of Jurassic storm-controlled sedimentation in adjacent Mediterranean basins.

Storm deposits: evidence of event sedimentation in the Bajocian of the Central High Atlas, Morocco

Ait Addi A.

Volumina Jurassica

2006

Vol. 4, no. 1

75-75

During Bajocian time, the Central High Atlas of Morocco received several hundred meters of marls, shaly marls and carbonate deposits. These are spread out in two main domains: southern and northern platforms, situated N of Errachidia and S of Midelt cities, respectively, where shallow marine carbonates corresponding to the group of Bin El Ouidane formations dominate (Monbaron 1981); and central basin, presently forming the centre of High Atlas, where hemipelagic facies to shallow marine carbonates represent Agoudim (members II, III and IV) and Tazigzaout formations (Ait Addi 2000, Ait Addi 2002). The first evidence of event sedimentation in the Middle Jurassic of central High Atlas of Morocco has been provided by Ait Addi (2002). These deposits are Middle Jurassic (Bajocian) in age and are included in Agoudim and Tazigzaout formations in the central basin; and parts of Bin El Ouidane Group in the borders. The storm deposits are calcarenite beds of bioclastic packstone-grainstone character. These beds present symmetrical wave-ripples at their tops and show an internal structure with hummocky cross-stratification. The deposits are considered to be formed by tropical storms and hurricanes. The above characteristics are the effects of multiple reworking and winnowing of siliciclastic or bioclastic and/or carbonate material by oscillatory currents related to storm and to storm/tide currents interaction. Palaeobathymetric interpretation of related pelagic sediments indicates that the sedimentation occurred fairly deep, and that calcareous tempestites resulted from episodic resedimentation, mainly coincident with relative sea-level falls, in which major storm waves and tsunamis affected the sea bottom. This interpretation is in agreement with the regional palaeogeography and is further evidence of Jurassic storm-controlled sedimentation in adjacent Mediterranean basins.