SEARCH FOR MILANKOVITCH PATTERNS AMONG OOLITIC IRONSTONES

The stratigraphic record of about 175 oolitic ironstones is sparse, yet it portrays several meaningful patterns. Repeated ironstone episodes occurred in only about 10 major sedimentary basins which were marked by relative tectonic stability or subdued orogeny. Intervals between episodes varied but were mostly a few to several million years long. Repetitions on this scale are commonly construed as global rise and fall of sea level. The pattern persisted for a few tens of million years in fore-deeps, several tens of million years on unstable cratons and microcontinents, and more than 100 m.y. in intracratonic basins. Within the congenial habitats an ironstone developed locally between each of several successive shallow marine upward shoaling detrital sequences. These asymmetric units, normally a few meters to a few tens of meters thick, recurred on average every 250 to 500 kyr. Commonly, fewer than 10 ironstone-bearing sequences are involved. During a very long lapse in normal sedimentation between such sequences stacked ironstones accumulated on the same short-time scale. Speculation about short-time scale patterns is poorly constrained, but several other repetitive stratigraphic successions with a several hundred thousand year cadence have been attributed tentatively to the 400,000-year Milankovitch climate cycle. It is unclear how this would exert a eustatic control without the waxing and waning of continental glaciers.