Controls on Devonian hemi-pelagic limestone deposition analyzed on cephalopod ridge to slope sections,Eastern Anti-Atlas,Morocco |
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Authors: | Stefan Lubeseder Jochen Rath Martin Rücklin Ralf Messbacher |
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Institution: | 1.Institut Und Museum Für Geologie Und Pal?ontologie,Universit?t Tübingen,Tübingen,Germany;2.Wintershall Holding AG,Kassel,Germany |
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Abstract: | The hemi-pelagic Tafilalt Ridge separating the Maider Basin from the Tafilalt Basin developed progressively from an Early
Devonian homoclinal ramp, through a Middle Devonian ramp-slope stage of moderate topography, to a mature cephalopod ridge
during the Late Devonian and formed a spur-like element extending from the southern shallow-water Maider Platform to the broader
northern hemi-pelagic Tafilalt Platform. During the Middle Devonian, thick lowstand aprons were shed onto the ridge-slope
from an active mid-ramp carbonate factory in the south (Maider Platform). The shallow-water derived sediments first by-passed
the central paleohigh, but then onlapped the ridge during start of sea-level rise. The allodapic limestones (storm-induced
turbidites, tempestites, and debrites) of the aprons consist of large parts or entirely of reworked lithic peloids. The lithic
peloids are interpreted as the main foundation of all micritic lithofacies types in the distal ramp locations, including those
on the hemi-pelagic cephalopod ridge. Lateral facies transition of allodapic limestones into nodular cephalopod limestones
on the ridge suggests the latter originate from allodapic beds and were subsequently transformed into nodular limestones by
bioturbation, early diagenesis, and the faunal input from a hemi-pelagic community. Iron-rich hardgrounds formed on the ridge
during major regressive phases as a result of increased winnowing by the wave-base and correlate with siliciclastic turbidite
deposition on the slope. The hardgrounds can be correlated with hiatuses on the Tafilalt Platform, which formed a broader
hemi-pelagic swell to the north. During the Late Devonian, the southern Maider Basin carbonate factory became unproductive
or disappeared, and a ridge facies of truly hemi-pelagic autochthonous limestones developed. Allodapic deposits on the slope
were shed from the ridge itself at this time but did not form well-defined aprons. |
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