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Stratigraphic Patterns |
Shell Research BV, Volmerlaan 6, 2288GD Rijswijk, The Netherlands
Petroleum Development Oman, Exploration Department, PO Box 81, Muscat, Sultanate of Oman
Oolitic ironstones generally mark the top of a regressive, coarsening- and shallowing-upward depositional megasequence. The iron ooids themselves are preferentially formed during sea-level lowstands or in condensed sections. This general model is valid for the Minette iron formation which was deposited in Toarcian and Aalenian times at the north-eastern margin of the Liassic Paris Basin in a near-shore, shallow marine, mostly high-energy environment. The top of the regressive sequence shows intraformational reworking. The Minette, however, is a remarkably thick unit (up to 60 metres), also internally composed of coarsening-up sequences. In many places these sequences represent large-scale subtidal sand waves, elsewhere large subtidal shoals advancing over their distal finer grained facies. Strong tidal currents building up large sand waves are preferentially active during transgressions. For the Minette base level rise events (transgressions) are therefore required at the top of a base level fall sequence (regression). This apparent discrepancy can be resolved when considering the tectonic setting. Reactivation of structures of the underlying Variscian basement led to subsidence events in syndepositionally formed troughs. This model can explain a high aggradational potential and deposition of thick ironstone deposits at the top of a regressive sequence while erosion took place laterally along the same palaeocoastline.
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  J. H. S. Macquaker, K. G. Taylor, T. P. Young, and C. D. Curtis Sedimentological and geochemical controls on ooidal ironstone and 'bone-bed' formation and some comments on their sequence-stratigraphical significance Geological Society, London, Special Publications, 1996; 103: 97 - 107. [Abstract] [PDF]
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