The oolitic ironstones of the Minette were deposited during Toarcian/Aalenian times in a nearshore environment of the Paris Basin. The sedimentary sequence comprises up to 13 coarsening upward depositional cycles. The development of the cycles and different facies types are described. The iron ooids accumulated in a subtidal environment under the effects of tidal currents. Sand waves, which form an important part of the Minette sediments, were deposited under both time‐velocity symmetrical and time‐velocity asymmetrical tides. From the study of their internal structures the time‐velocity patterns of the currents, current speeds (up to 0±9m s−1) and net transport rates have been estimated (29 g m−1 s−1 in the case of large‐scale sand waves controlled by time‐velocity symmetrical tides). Sand wave heights and calculated near‐bed current speeds suggest a mesotidal regime. Sedimentary facies include large‐scale and waves, subtidal shoals with channels and an offshore muddy shelf. Conclusions as to bathymetry are drawn from ichnofaunal associations and from the study of wave ripple marks. Finally a depositional model of the Minette iron formation is proposed which may be applicable to other oolitic ironstones.
Summary
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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