Drowning of a Lower Jurassic carbonate platform: Jbel Bou Dahar, High Atlas, Morocco |
| |
Authors: | Dr Dierk P G Blomeier Dr John J G Reijmer |
| |
Institution: | 1. Dept. of Paleoceanography, Geomar Research Center for Marine Geosciences, Wischhofstr. 1-3, D-24148, Kiel
|
| |
Abstract: | Summary The high-plateau of the Jbel Bou Dahar, situated in the Central and Eastern High Atlas of Morocco, represents a Lower Jurassic
carbonate platform that drowned at the beginning of the Toarcian. Three phases of platform evolution can be distinguished:
During thepre-drowning phase (upper Sinemurian— upper Pliensbachian) the platform interior facies reflects a restricted-marine lagoonal environment, protected
by scattered buildups and cemented debris at the platform margin. Upper and mid-slope are dominated by coarse-grained, poorly
sorted limestones, deposited through debris flows during sea-level lowstands. Sea-level highstand deposits occur at the toe
of slope and are formed by an alternation of fine-grained litho- and bioclastic pack- to grainstones (turbidites), marls and
mud- to wackestones (hemipelagic oozes).
A condensed section, reflecting an abrupt and fundamental environmental change along the entire platform, characterises thedrowning phase (upper Pliensbachian— lower Toarcian). Within the platform interior densely packed biosparites represent the switch to high-energy
environments, causing erosion of the former pre-drowning lagoonal sediments. These erosional products were redeposited on
the platform slope, leading to the formation of coarse-grained non-skeletal sparites and micrites. Both platform interior
and slope successions show a series of cyclic variations in sediment composition that could have been triggered by small-scale
sea-level fluctuations.
In contrast to the abrupt facies change at the pre-drowning —drowning boundary, the transition to thepost-drowning phase (lower Toarcian—Aalenian) is gradual. During this phase, biopelmicrites and pure micrites were deposited in all platform
sections, followed by the deposition of calcistiltites. The facies point to quiet-water conditions below storm-wave base and
display a uniform deep-marine sedimentation.
This analysis shows that the drowning of the Jbel Bou Dahar carbonate platform was caused by abrupt and fundamental changes
in the shallow-water realm. After exposure of the platform, these changes prevented the carbonate factory from re-establishing
itself and made it impossible for the platform to keep up with the subsequent rise in sea level. These local changes were
probably triggered by high-frequency sealevel variations in combination with regional or even worldwide changes in ocean circulation
patterns. |
| |
Keywords: | Carbonate Sedimentology Platform Drowning Microfacies Analysis Depositional Geometries Morocco (High Atlas) Liassic |
本文献已被 SpringerLink 等数据库收录! |
|