Geodynamic Significance of Stacking Lower Cambrian Sequences Units in the Western Anti-Atlas

Mohammed Benssaou

doi:10.11648/j.ijmsa.20170603.15

Abstract

In the Anti-Atlas the lower Cambrian succession was traditionally considered as a post-rift series. Recently the Western Anti-Atlas appears to be affected by an early Cambrian rifting process. In the Igherm area (Western Anti-Atlas), this succession is represented by an alternating carbonate and siliciclastic facies reflecting several sedimentary environments. The vertical stacking of these facies define four Composite-Sequences constituted, each one, by two or more transgressive-regressive sedimentary cycles. The differential and progressive extensional tectonic events seem to be the main control on the genesis and on the vertical stacking transgressive-regressive sedimentary cycles. The accommodation space is generated by a strong tilting related to syn-rift tectonics. The lack of lowstand system tracts and other erosive features, suggests the occurrence of platform drowning just below the falling sea level stage. So stratigraphic stacking of the Lower-Cambrian strata exposed in the Igherm area, constitutes the response to the rifting process in the Anti-Atlas and consequently a probably fragmentation indicator of a neoproterozoic Gondwana.

Highlights

In the Moroccan Anti-Atlas, the succession that was traditionally assigned to the Adoudounian-Lower Cambrian (Choubert, 1963), is recently attributed to a Lower Cambrian age [1]
We address the potential impact of the Gondwana Lower Cambrian rifting on the genesis of vertical stacking sequence units that filled wide graben and half graben of the Souss basin
The alternating sedimentary carbonates platform and reworked deltaic sediment within the Lower Cambrian succession in the Irherm area allows the distinction of four Composite Sequences similar to those defined by Mitchum et al [3]

Summary

Introduction

In the Moroccan Anti-Atlas (fig. 1), the succession that was traditionally assigned to the Adoudounian-Lower Cambrian (Choubert, 1963), is recently attributed to a Lower Cambrian age [1]. In the Irherm area, the succession including, in ascending order, the Adoudou Formation, the “série lie de vin” Formation, the Igoudine Formation”, the “Amouslek Formation and the “Issafene” Formation This alternating carbonate and siliciclastic facies reflects two major carbonate transgressive deposits separated by a major glacio-eustatic regressive event represented by the siliciclastic “série lie de vin” Formation [2]. Through sedimentologic analysis and the recognition of sequence units by using sequence-stratigraphy concepts, we characterise the Lower Cambrian relative sea level change in the anti-atlasic margin of the Western African Craton. It is important to note that the principles of sequence stratigraphy have been, successfully, applied recently to analysis of Precambrian and Cambrian successions in spite of the stratigraphically fossils scarcity. In the light of this it is interesting to note that the sedimentologic and stratigraphic patterns constitute the most sensitive indicators of syn-sedimentary tectonism in the absence of significant relic tectonic related structures

Stratigraphic Stacking of the Irherm Lower-Cambrian Succession

Interpretation of Stacking Sequence Units

Conclusion