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Stratigraphic architecture and depositional processes across lower Paleozoic siliciclastic shallow-marine platforms: insights from the Late Ordovician of the Anti-Atlas (Southern Morocco)

Abstract : During Cambrian-Ordovician times (542-444 Ma), the Anti-Atlas (Southern Morocco) was part of the Gondwana supercontinent and drifted from mid- to subpolar latitudes. The area was part of a shallow marine platform on the so-called northern passive margin of the continent. The stratigraphic succession records a long-term flooding of the platform from the Cambrian to the Silurian. Above Lower Cambrian fluvial to estuarine deposits, an essentially shallow-marine succession (from offshore shales to tidal sandstones) is punctuated by major flooding events in the Middle Cambrian, the lowermost Ordovician, the middle Ordovician and the lower Upper Ordovician. A superimposed eustatic sea-level drop due to the Late Ordovician glaciation marks the last transgressive-regressive cycle that includes the Ktaoua group (mid-Sandbian to Upper Katian) and the glaciation-related Second Bani Group (Hirnantian). In spite of well-known stratigraphy, details of the depositional processes, the overall geometry of the clinoforms (shelf vs. ramp) and resulting stacking patterns of the high-frequency units remain poorly understood in the platform domain. Here, we present a stratigraphic correlation made through a field-based logging of seven sections along a 50 km long, well-exposed cliff located in the Central Anti-Atlas. The Ktaoua to Second Bani groups, part of the Jbel Bani Mountain, are investigated between the villages of Tissint and Foum Zguid. The Ktaoua succession is dominated by high-order regressive parasequences grading from shales into fine to coarse-grained bioturbated sandstones, presenting paraconformable contacts. Incursions of sandstones showing hummocky-cross-stratifications (HCS) are common and considered to represent offshore-transition storm deposits. In the Second Bani Group, which includes glacially-related depositional facies (diamictites), fine to coarse-grained sandstones are dominant as well, though HCS are virtually absent. In the study area, the upper Second Bani Group forms an unconformable unit, severly truncating the Ktaoua succession and emphasizing a regressive sequence initiated within the Ktaoua deposition. These archives enable to constrain the basin geometry in the Upper Ordovician. Indeed, whereas offshore shales in the region of Zagora (Central Anti-Atlas) characterize the depocenter of the basin (eastward from the study area), the studied succession is surrounded by a belt of coarser-grained deposits to the north-east (Alnif, Eastern Anti-Atlas) and west (Tissint/Foum Zguid, Western Central Anti-Atlas). The stratigraphic correlation of the Ktaoua parasequences will be used to reconstruct the orientation, direction, and dip of platform clinoforms, which will allow to dissociate a scheme based on chronostratigraphic units from the usual lithostratigraphy. Besides, we aim to further characterize the geometry, shape, and stratigraphic position of HCS bodies in order to advance our understanding of the setting (e.g., paleobathymetry, position relative to regressive vs. transgressive trends) and hydrodynamic processes generating sandstones bodied with prevailing HCS beds.
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Contributor : Isabelle Dubigeon <>
Submitted on : Tuesday, November 10, 2020 - 10:39:48 AM
Last modification on : Thursday, November 12, 2020 - 3:07:27 AM


  • HAL Id : insu-02997610, version 1



Déborah Harlet, Guilhem Douillet, Jean Francois Ghienne, Philippe Razin, Pierre Dietrich, et al.. Stratigraphic architecture and depositional processes across lower Paleozoic siliciclastic shallow-marine platforms: insights from the Late Ordovician of the Anti-Atlas (Southern Morocco). Swiss Geoscience Meeting, Nov 2020, online, Switzerland. ⟨insu-02997610⟩



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