Microspores, megaspores, palynofacies, and depositional history of the upper Givetian Maywood Formation, Northern Wyoming, USA

Abstract

Incised valley-fill deposits of the Middle Devonian Maywood Formation at Cottonwood Canyon, Wyoming, USA, contains a significant palynological record. Twenty-six palynotaxa were recovered, of which 21 species have wide paleogeographical distribution. The presence of the microspores Samarisporites triangulatus and Contagisporites optivus, as well as other Givetian index taxa, points to a late Givetian age (C. optivus/S. triangulatus palynozone) of the Maywood Formation. The palynoflora investigated may be correlative with the hermanni conodont Zone or a younger zone, and thus they record the latest Taghanic crisis or the earliest post-Taghanic interval. The playnoflora includes progymnosperms that had a tree-like habit (Aneurophytales and Archaeopteridales) characteristic of proximal fluvio-lacustrine settings of floodplains and paralic environments of internal basins. Spores of the herbaceous lycopsid Selaginellales, shrubby lycopods Protolepidodendrales, and primitive ferns, indicate a proximity of terrestrial plant habitats near the Maywood paleovalley. Additionally, the palynofacies (1) have amorphous organic matter and few algal remains of marine origin (Quadrisporites, Dictyotidium), (2) exist in association with assemblages of monospecific microconchid tubeworms that may have colonized non-biomineralized algae, and (3) are in strata that lack other benthic fossils, all of which indicate that the Maywood paleovalley record fluctuations of salinity. These were generated by mixing of marine and freshwater inputs within the estuarine channel, which was developed along the paleoshorelines of the Maywood Sea.