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Stratigraphic Patterns |
Department of Geology, UNCC, PO Box 914, Cardiff, CF1 3YE, UK
Ooidal ironstones are a characteristic feature of the Ordovician successions of south-west Europe and adjacent areas. They occur in shallow marine clastic sequences deposited on a broad low-topography shelf. Three stratigraphic intervals have a particularly widespread development of ironstones: the early Llanvirn, the early Caradoc and the early Ashgill. These intervals, corresponding to periods of major global eustatic sea-level rise, are marked by the development of thin, but persistent, ooidal ironstones even in relatively offshore areas. The ironstones produced by these three events have been studied in the previously contiguous areas of central Portugal and Armorica.
The development of the ironstones, and the nature of the successions within which they occur, show remarkable similarities among the three horizons; this recurrence of similar sedimentary facies defines a cyclicity. In each of these ‘first-order’ cycles the upper part is characterised by the occurrence of a second order of cyclicity, represented by asymmetric coarsening— and thickening—upwards sequences of siltstone and sandstone tempestites, commonly capped by phosphatic conglomerates. The latest ‘second-order’ cycle in each case is characterized by a more distal sequence of dark mudstones with graptolites. The chamositic ooidal ironstones lie disconformably above these upper mudstone-dominated cycles, and are interpreted as forming during ‘second-order’ transgressions late in major global transgressive events. Both ‘first-’ and ‘second-order’ sedimentary cyclicity is attributed to eustatic variation. The ‘first-order’ eustatic cycles have a period of 10–15 Ma, and largely correspond to the international stratigraphic series, while the period of the ‘second-order’ cycles is estimated as 0.2–0.5 Ma.
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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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