Stratigraphic Patterns Among Phanerozoic Oolitic Ironstone

F. B. Van Houten

Most of the Phanerozoic oolitic ironstones accumulated during two principal (150-170 Ma) episodes of dispersal of major cratonic blocks and relatively highstand of sea level. They were especially numerous and widespread in the Ordovician and Jurassic.

Under persisting favorable regional conditions, such as relative tectonic stability and deep weathering, accumulation of ironstone recurred on average every few to several million years in 10 major sedimentary basins. This pattern persisted for about 14 m.y. in foredeeps and unstable cratons, to more than 100 m.y. in intracratonic basins.

When especially congenial conditions prevailed, an ironstone developed during a hiatus between each of several successive marine coarsening-upward detrital sequences that recurred at estimated average intervals of 250,000-600,000 years. During an unusually long hiatus between sequences, rare stacked ironstones accumulated on the same short time scale.

Long and short episodes of ironstone repetition suggest two different modes of control; they also introduce the common problem of distinguishing between allogenic and autogenic factors. In the well-constrained, long time-scale Jurassic record of Europe and Arctic Canada, ironstones are consistently associated with the initial stage of repeated widespread transgression, suggesting eustatic control of sea level may have been effective. Speculation about the cause of the short time-scale repetitions is poorly constrained, but several other patterned successions with a cadence of several hundred thousand years have been tentatively attributed to eustatic control.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.