Temporal patterns among Phanerozoic oolitic ironstones and oceanic anoxia

The stratigraphic distribution of Phanerozoic oolitic ironstones corresponds to temporal patterns recorded in common detrital deposits. These patterns, in which ironstones share significant associations with black shales, comprise successively smaller-scale sequences, each defined mainly by major transgressions and regressions, changing rates of erosion and sedimentation, and inter-regional unconformities. Two 300 Ma episodes of continental dispersal and Pangaeic assemblage of cratons encompassed the maximum rise and fall of sea level. Deposition of ironstones and black shales was essentially limited to 150-170 Ma phases (Ordovician-Devonian; Jurassic-Palaeogene) marked by extensive epicontinental seas and ineffective oxygenation of deep-water masses. Smaller-scale tectonically distinct phases of cratonic submergence, several to many tens of Ma long, were separated by briefer erosional intervals, or by longer episodes of oscillating conditions during which several interregional unconformities developed. Widespread submergence generally favoured major production of ironstones and black shales. Within this framework both were especially common during the Ordovician and Jurassic periods. A hypothetical quasiperiodic (c. 32 Ma) pattern of global climate change was accompanied by recurring relatively high sea level and expansion of oxygen-depleted water masses. Many of these phases led to widespread development of both ironstones and black shales. Episodes of fluctuating coastal onlap one to several Ma long reflect variations in sea level, sediment supply, and/or subsidence. Major successions of ironstones repeated on a similar time scale were limited to ten tectonic provinces comprising foreland basins, cratonic margins and unstable cratons, and intracratonic basins. During the favourable phases black shales and ironstones were deposited alternately in some successions. In many of the major successions an ironstone developed between each of several repeated small-scale sequences in a cadence of several hundred thousand years. Ironstones in these asymmetric units record a long lapse in normal sedimentation during the initial stage of renewed transgressions that commonly spread organic-rich mud across the shelf. The repeated association of ironstones and black shales with sequences of common sedimentary rocks helps constrain speculation about conditions controlling their origin, and suggests that ironstones reflect the more local development of productive conditions.