Organic-walled microphytoplankton from the Middle Devonian (Givetian) Gravel Point Formation, Michigan, USA

Abstract

ABSTRACTAn abundant, diverse, and moderately well-preserved assemblage of organic-walled microphytoplankton is reported from two exposures of the Middle Devonian (Givetian) Gravel Point Formation at Little Traverse Bay, Michigan. The palynoflora consists of 13 prasinophyte and 19 acritarch species, together with some scolecodonts and chitinozoans. This is the first microphytoplankton assemblage to be described from the Middle Devonian of Michigan, and is clearly Givetian in age. It is closely comparable with, albeit less diverse than, Middle Devonian acritarch/prasinophyte assemblages reported from elsewhere in North America. Based on the diversity and abundance of the acritarch and prasinophyte taxa, along with evidence from marine faunas, the Gravel Point sediments accumulated in a normal, low-energy, offshore environment, with periodic regressions indicated by stromatoporoid–coral bioherms. Four of the acritarch species are restricted to the Middle Devonian of North America; the other specific components are typically, but not exclusively, Middle Devonian in age. Although some species are restricted to North America, others are cosmopolitan in distribution.