CHANGING ENVIRONMENTS OF CHERT ACCUMULATION THROUGH THE ORDOVICIAN

Preliminary analysis of siliceous sedimentary facies suggests that shallow-water chert was relatively common in the Early Ordovician, but had diminished in abundance by the Late Ordovician. The timing and rate of this retreat of chert to deeper water environments is as yet poorly constrained. Ongoing study of the spatial and temporal distribution of nodular cherts has the potential to better constrain the timing and nature of this change so as to better understand its cause(s).

Potential mechanisms for instigating the change include 1) onshore-offshore evolutionary patterns of silica-secreting organisms and 2) changes in availability of dissolved silica for use by silica-secreting biotas. Movement of the Cambrian Fauna and the Paleozoic Fauna from nearshore to offshore facies during the Ordovician may have included the silica-secreting biotas. Radiolarians appear to have originated in nearshore facies in the Cambrian, but then moved offshore in the Ordovician. Where siliceous sponges originated is less clear, but by late Paleozoic they were often confined to offshore settings. Whether these silica-secretors moved offshore in concert with the Cambrian or Paleozoic Fauna is unclear.

Late Ordovician orogeny and associated volcanism is expected to have increased dissolved silica input to the ocean by generating increased volumes of rock that are easily susceptible to chemical weathering. Late Ordovician glaciation probably stimulated thermohaline ocean circulation that promoted increased upwelling of dissolved silica and nutrients to surface waters. Improved temporal distribution of Late Ordovician cherts can potentially contribute to resolution of the ongoing controversy over whether the glaciation was short-lived (~1 m.y. long) or much longer lasting.