RETURN OF MARINE CHERT TO SHALLOW-WATER SETTINGS IN THE PENNSYLVANIAN

Marine chert deposition appears to have returned to shallow-water settings in the Pennsylvanian, temporarily reversing a trend of offshore accumulation that began in the early Paleozoic. Cherts were tabulated and analyzed to test this hypothesis. They show that 47 of 73 (64%) of cherts accumulated in shallow-water environments. Cherts that accumulated seaward of the shelf break were much less abundant than anticipated. Only 9 shelf cherts and 5 basinal cherts were identified. These new data confirm earlier suggestions of shallow-water Pennsylvanian chert accumulation as recorded by bedded shelf chert and silica-replaced oolites. The somewhat common occurrence of nearshore spicular flints near the eastern edge of the North American epicontinental sea supports the return of chert to nearshore environments.

Several factors were considered as potential influences on the siliceous sponges, which likely buried the silica in many of these cherts. We rule out an apparent return of the Paleozoic Fauna to nearshore settings because it is younger than most of these chert deposits, and it is not clear if the siliceous sponges were even members of that fauna. River input of dissolved silica may have increased as the result of chemical weathering of silicate rocks exhumed during Carboniferous orogenesis. Silicon-accumulating plants such as horsetails and ferns may have taken up dissolved silica from soil pore waters and delivered soluble opaline phytoliths to coastal waters where they dissolved and nourished siliceous sponges. The proximity of some spicular flints to deltaic facies is consistent with this mechanism. We speculate that ferns may have mined soil pore-water silica over large upland drainage areas. If silicon accumulated in swamp-dwelling horsetails, much of that silica may have been trapped in coals.