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Sedimentologic Framework of Late Devonian Oolitic Iron Formation, Shatti Valley, West-Central Libya
F. B. Van Houten, R. M. Karasek
Late Devonian paralic to shallow-marine Shatti Formation exposed along the northern margin of Murzuk Basin in west-central Libya is in the upper part of an essentially continuous Paleozoic detrital succession. Iron-bearing Shatti deposits, a maximum of 90 m thick, comprise five coarsening-upward regressive sequences that record intermittent detrital influx along a low-energy coast. Quartz arenite sandbodies include long breaker bars on a chenier coast, thin storm-generated littoral sheet sands, wave-dominated delta-front sands, thin crevasse splays, and minor deeply scoured channel fill. Processes and products along the Shatti shoreline ranged from those characteristic of the low-energy shelf of southwestern Louisiana to those of fluvial-wave interaction on the Rhone Delta. Shatti seq ences are marked by the absence of large well-winnowed shoreface sandbodies and other evidences of major transgressive nearshore sedimentation and also of any associated platform carbonate facies.
As many as 10 beds of ferric oxide-chamosite ooids are present in the iron-bearing section. Most of the thicker oolitic ironstones overlie coarsening-upward deltaic sequences where they accumulated during episodes of delta abandonment. Some interfinger laterally with sheeted shoreline sandbodies. The ooids were formed on low-energy mudflats and then concentrated in thick nearshore bars by local currents. Interruptions in accretion produced burrowed, ferruginous horizons. When building ceased, the ooid bars were deeply penetrated by large vertical burrowers. Commonly an ooid bar was succeeded by or graded laterally into a thin, ferruginous phosphatic intraclast lag deposit that developed when relatively higher coastal energy of storms or renewed transgression winnowed the sediment-star ed mudflats. Then the oolite and intraformation conglomerate were buried under marine mud that accumulated during the succeeding transgression.
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