A depositional model for the Liassic Minette ironstones (Luxemburg and France), in comparison with other Phanerozoic oolitic ironstones

Abstract

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.

Sedimentological and geochemical controls on ooidal ironstone and ‘bone-bed’ formation and some comments on their sequence-stratigraphical significance

“…It has also been recognized that ooidal ironstones commonly cap coarsening upward cycles (e.g. Hemingway 1951;Van Houten & Karasek 1981;Van Houten & Bhattacharyya 1982;Rawson et al 1983;Bayer 1989;Teyssen 1989;Young et al 1991) and that they are overlain by marine mudstones. The stacking patterns of the ironstones and their associated sediments have been variously interpreted as indicating ironstone formation either at sea level lowstands (e.g.…”

“…The presence of these components is usually interpreted to indicate that the ironstones were deposited in shallow high-energy conditions (e.g. Sellwood 1972;Hallam & Bradshaw 1979;Bayer et al 1985;Bayer 1989;Guerrak 1989;Teyssen 1989;Young et al 1991). In addition, it has been proposed (e.g.…”

Ooidal ironstones and ferruginous microbialites: origin and relation to sequence stratigraphy (Aalenian and Bajocian, Swiss Jura mountains)

“…bathymetry, hydrodynamic regime, sedimentation rate, salinity, and their temporal changes due to sea-level fluctuations are also topics represented by different opinions: according to Knox (1970), Gygi (1981), and others, ooidal ironstones formed in a low-energy environment, whereas Hallam (1975), for example, envisages turbulent conditions. Ooidal ironstones occurring at the top of shallowing-upward parasequences are considered to have accumulated either during a sea-level fall (Hallam & Bradshaw, 1979; Bayer eta]., Teyssen, 1989) or a sea-level rise (Van Houten & Purucker, 1984;McGhee & Bayer, 1985;Young, 1989Young, , 1993, or independently of sealevel fluctuations, through local environmental changes (James & Van Houten, 1979).…”

Evidence of dilatant and non-dilatant damage processes in oolitic iron ore:P-wave velocity and acoustic emission analyses

“…These iron ore beds, known by miners as the ‘Green’, ‘Black’, ‘Brown’, ‘Grey’, ‘Main Yellow’, ‘Savage Yellow’ and ‘Red’ beds from the base (deepest bed) to the top of the deposit, are separated by intercalated marl beds. An iron ore bed is composed of a sequence of three lithological formations materialized in a littoral environment (Bubenicek 1961; Teyssen 1989). There are, from the base to the top of the bed, an argillaceous and carbonated siltstone, an oolitic iron ore and a coarse shelled limestone.…”