A palynological investigation of the Lower and lowermost Middle Jurassic strata (Sinemurian to Aalenian) from North Yorkshire, UK

A palynological investigation of the Lower and lowermost Middle Jurassic (S inemurian to Aalenian) strata of North Yorkshire has been undertaken which incorporates material from the BGS Brown Moor Borehole, near Acklam and the coastal outcrops between Staithes and Robin Hood's Bay. The resulting data set is an additional contribution to the Jurassic dinoflagellate cyst biost ratigraphy of northern England. Two distinctive variants of Liasidium varia bile appear to have biostratigraphical significance in the Upper Sinemurian of North Yorkshire. Specimens of L. variabile from the Caenisites turneri Ammonite Zone, the Eparietites denotatus Ammonite Subzone and the Oxynotice ras simpsoni Ammonite Subzone have short apical and antapical horns. Forms from the overlying uppermost Oxynoticeras oxynotum Ammonite Zone, however, are characterized by elongate apical horns and rounded hypocysts. The regio nal dinoflagellate cyst diversity increase in the late Pliensbachian and th e disappearance of many dinoflagellate cysts in the Lower Toarcian are both interpreted as being related to eustatic fluctuations. The paucity of dino flagellate cysts close to the Dactylioceras tenuicostatum-Harpoceras falcif erum Ammonite zonal transition in the Lower Toarcian was caused by the spre ad of widespread marine anoxia during the Lower Toarcian maximum flooding e vent. A new species of dinoflagellate cyst, Nannoceratopsis symmetrica, is described.