ARE CONTINENTAL MARGIN CHERTS A MOVING TARGET?

The Phanerozoic temporal and spatial distribution of siliceous sedimentary rocks suggests that marine chert has repeatedly retreated from and advanced toward shallow-water settings since the Early Paleozoic. Several pulses of shallow nearshore and shelf chert accumulation mark the mid-late Paleozoic, and also the Eocene. Evaporitic shallow-water cherts should not be time restricted.

Onshore-offshore evolutionary patterns may have driven the Ordovician retreat of chert from shallow-water facies as radiolarians and perhaps siliceous sponges evolved from relatively nearshore to more offshore dwellers. Subsequent advances and retreats might be subject to the same control. Alternatively, bursts in supply of dissolved silica may have triggered shoreward advances of chert. Potential sources of silica pulses include: 1) chemical weathering of fresh silicates generated during continental collisions, 2) land plant innovations such as expanded root systems and evolutionary expansions of biosiliceous plants, 3) vigorous ocean circulation during glaciation, and 4) major volcanic events. Rapid transgressions may have shut down siliciclastic dilution sufficiently to explain some Paleozoic shallow-water cherts.

Only tentative linkages can as yet be drawn between shallow-water chert intervals and potential controlling factors described above. A mid-late Devonian interval is consistent with onshore-offshore evolution and with increased silicate weathering via orogeny and/or expanded land plant root systems. Orogeny and and/or glaciation may explain a Mississippian interval. Coal-swamp expansion of Equisitaceae might account for Pennsylvanian nearshore cherts, if Paleozoic horsetails were siliceous. Shallow-water Eocene cherts may have been derived from opalline phytoliths in nascent grasses. If so, shallow-water cherts are expected during sod-forming grassland expansion in the Early Miocene and during C4 grassland expansion later in the Miocene.