ORDOVICIAN AND SILURIAN CHERTS AND THEIR RELATION TO PALEOCLIMATE, PALEOCEANOGRAPHY, AND ONSHORE-OFFSHORE EVOLUTION

Temporal and spatial distribution of bedded chert, nodular chert, silicified oolites, and other siliceous facies may help resolve several questions in the Ordovician and Silurian. These include: 1) the timing of Late Ordovician glaciation, 2) the veracity and nature of an Early Silurian chert gap, and the relationship between siliceous facies and onshore-offshore patterns of evolution.

Brenchley et al (1994) suggested that the Late Ordovician glaciation may have lasted less than 1 m.y. during an otherwise long greenhouse interval. Conversely, Pope and Steffen (2001) suggest that extensive chert in the Montoya Group represents an glaciation-related upwelling system that persisted for up to 15 m.y. A conspicuous and short-lived chert pulse that is coeval with the glaciation would not only support the short glaciation hypothesis, but would argue that Ordovician cherts responded to abrupt changes in ocean circulation much as diatom cherts have done in the Cenozoic. A longer interval of chert deposition would be consistent with a glaciation of longer duration and/or might mean that siliciceous sponge and radiolarian cherts were not as responsive to oceanographic changes as are diatom cherts. Preliminary global data are consistent with the longer-lived glaciation supported by the Montoya cherts.

A gap in bedded chert in the Early Silurian may reflect sluggish ocean circulation similar to the Middle Cretaceous and Early Triassic, both of which are marked by lows in chert abundance. Presently available data are insufficient to determine whether the apparent chert gap is real or merely a function of undersampling, but a gap in Early Silurian phosphorite deposition supports the possiblity of sluggish ocean circulation.

Temporal distribution of silicified oolites and shallow-water bedded cherts suggest that shallow-water siliceous facies were relatively common in the Cambro-Ordovician, and then largely absent until returing in moderate abundance in the mid-late Devonian and late Carboniferous. This pattern roughly matches patterns of onshore-offshore evolution, particularly with respect to the Paleozoic Fauna.