Palynostratigraphy and palaeoclimatic analysis of Lower-Middle Jurassic (Pliensbachian - Bathonian) of the Inner Hebrides, NW Scotland

Abstract: A palynostratigraphic investigation of partly ammonite dated, Pliensbachian to Bathonian (Early – Middle Jurassic) strata on the Isles of Skye and Raasay, Inner Hebrides in Northwest Scotland, has recovered a diverse but not always well preserved miospore flora and a less diverse dinoflagellate cysts flora. The studied interval has been subdivided into five biozones, A - E, based on the first occurrence, common occurrence, first common occurrence, and relative abundance of pollen and spore key taxa. Biozone A (Pliensbachian) is defined by the co-occurrence of several long ranging taxa, e.g. Corollina spp. and Cerebropollenites spp., as well as the presence of rare Spheripollenites subgranulatus. Biozone B (late Toarcian-early Aalenian) is defined by abundant presence of Spheripollenites subgranulatus. Biozones A and B are separated by a large hiatus. The lower boundary of Biozone C (late Aalenian - early Bajocian) is marked by the first co-occurrence of Callialasporites dampieri, C. turbatus and C. minus. The lower boundary of Biozone D (middle Bajocian - early Bathonian) is defined by the first occurrence of Neoraistrickia gristhorpensis, while the uppermost zone, Biozone E (middle/late Bathonian), is marked by a significant increase in the abundance of Perinopollenites elatoides and Araucariacites australis. The erected biozones correlate well with other biostratigraphic zonations and palynological results from Northwest Europe. Existing spore-pollen zonations from NW Europe often lack independent stratigraphic dating. Hence, the results from this study provide, for the first time, biostratigraphical constraints on some important spore-pollen events during the Early - Middle Jurassic. The depositional environments are interpreted as marine during the Pliensbachian – Toarcian (Early Jurassic), marine near shore in Aalenian – Bajocian (Middle Jurassic) and brackish/lagoonal to terrestrial in Bathonian (Middle Jurassic). This is consistent with previous interpretations of the depositional environment in the Inner Hebrides Basin during this time interval. An environmental interpretation based on ecological and climatological preferences of the probable parent plants has also been performed for the Toarcian – Bathonian interval. The results indicate a warm and humid climate in Late Toarcian, followed by a cooling period in Early Aalenian. In the late Aalenian/Early Bajocian the conditions become warm and dry, after which the climate becomes gradually cooler and more humid during the mid Bajocian – middle/late Bathonian. This interpretation correlates well with other climatic interpretations over the interval.