WAS CORDAITES A LATE CARBONIFEROUS MANGROVE?

Late Carboniferous coals from the Kalo Formation of Iowa with abundant and diverse cordaitalean gymnosperms have been reconstructed as mangrove (salt water) swamps. Two lines of evidence previously suggested that these paleoequatorial coals formed in salt water: 1, similarities in the root morphology of Cordaites and the modern mangrove, Rhizophora mangle, and 2, the distribution of pyrite surrounding, but not within Cordaites rootlets, which indicates that these plants remained alive when submerged in salt water. New stratigraphic data suggest that Cordaites-dominated coals from Iowa probably formed in fresh water environments. Mangrove peats are rare, forming only on carbonate platforms or coasts with low siliciclastic sediment influx. The depositional setting and characteristics of Cordaites-dominated coals from Iowa are similar to other paralic freshwater coals of the Late Carboniferous. Most Late Carboniferous paralic freshwater coals experienced marine influx prior to compaction. Pyrite associated with Cordaites roots suggests that Cordaites may have tolerated these marine incursions better than other Late Carboniferous swamp plants. Despite this, coals with abundant Cordaites probably formed in a dominantly fresh water environment in which occasional saline flushing occurred. Since Late Carboniferous equatorial coals with abundant Cordaites appear confined to the Atokan – early Desmoinesian, the presence of Cordaites in these coals may reflect relatively drier climates rather than growth in more saline conditions. No Cordaites-dominated swamps have been found in the mid-to-late Desmoinesian, which has been reconstructed as an interval of wet climate. Instead, hydrophylic lycopsids dominated mid-to-late Desmoinesian swamps, implying another change in paleoclimate.