The palynology of the Lower-Middle Jurassic transition of the North Caucasus (southwest Russian Federation), calibrated with index ammonites

Abstract

Palynomorphs from the Toarcian and Aalenian Djigiat Formation of Karachay-Cherkessia in the Kuban River Basin (North Caucasus, southwest Russian Federation) are reported for the first time. Five samples carefully selected for their biostratigraphical content were studied to document the palynological assemblages. Four samples (1, 2, 3, 4) were collected from pieces of rock with key index ammonite specimens, and sample 5 comprises fragments of ammonite shells from a sideritic concretion. Three of these samples (4, 2 and 1) yielded relatively abundant and diverse aquatic microplankton (acritarchs, dinoflagellate cysts, prasinophytes and zygnematalean algae) and terrestrial palynomorphs (pollen and spores); the remaining two samples (3 and 5) were less productive and contained fewer than 50 palynomorph specimens. The study of the dinoflagellate assemblages provided characteristic and key information. These are the uppermost Toarcian Nannoceratopsis Assemblage and the uppermost Toarcian to lowermost Aalenian Parvocysta Suite Assemblage. The former is overwhelmingly dominated by Nannoceratopsis with sparse numbers of the Parvocysta suite, and the latter yielded more diverse and common specimens of Parvocysta and its relatives. This is consistent with the hypothesis that the Parvocysta suite migrated from the Boreal Realm further south into Laurasia during the Toarcian. Certain dinoflagellate cysts such as Mancodinium semitabulatum and Scriniocassis spp., which are typical of Europe and much of Greater Laurasia are absent. Composition of the pollen and spores proved relatively out to be monotonous, and cannot be subdivided. However, in general, the composition of the assemblages of spores and pollen is consistent with the Toarcian–Aalenian transition.