Palaeoenvironmental significance of a monospecific assemblage of dinoflagellate cysts from the Miocene Clarkia Beds, Idaho, USA

A single dinoflagellate cyst species occurs abundantly in nonglacial varved lacustrine deposits of the Miocene Clarkia Lake succession in northern Idaho. Previously of uncertain affinity, it is now identified as Pseudokomewuia (Pseudokomewuia) aff. granulata He 1980. The variety of other biological groups that are represented by fossils in the succession provides a basis for making observations on ecological associations and climatic conditions during the period of deposition of the sediments. The cysts do not occur in the alluvial (floodplain) and poorly laminated, early lacustrine facies that underlie the varved unit, nor in the overlying late lacustrine, oxidised, silty clay facies. They are associated only with sediment-couplets considered to reflect accumulation in temperature-stratified water approximately 8–12 m deep that overturned periodically. It is likely that the dinoflagellates inhabited the warm, epilimnic, oxygenated layer above the cool deep water in which oxygen levels were low, rendering this environment inhospitable to both animals and plants, but favouring the preservation of organic matter. Their absence from the unlaminated deposits of the early and late lacustrine facies may not mean that the motile dinoflagellates were not present in the shallower waters they represent but simply that the cysts were not preserved. These beds accumulated in oxidising environments and are palynologically barren. The land plant mega- and microfossil records suggest that the vegetation of the surrounding hills consisted of mixed deciduous–coniferous forest of a character that is typical of a moist (summer-wet) warm-temperate climate. This is also implied by some of the fauna (e.g., the salmonid fish and the gastropod Bellamya). These observations add significantly to palaeoecological conclusions drawn from the few records of nonmarine dinoflagellate cysts from Cretaceous and younger successions elsewhere, most of which have been associated with quiet lacustrine or lagoonal habitats.