Spatial and temporal distribution of calcareous nannofossils along a proximal–distal transect in the Lower Jurassic of the Umbria–Marche Basin (central Italy)

摘要

Three Upper Pliensbachian/Lower Toarcian successions of the Umbria–Marche Basin were analysed for sedimentology and calcareous nannofossil content across a 120-km-long transect from proximal to distal areas with respect to the shallow Latium–Abruzzi Platform. The uppermost part of the Corniola Unit (Uppermost Pliensbachian) and the Marne di Monte Serrone Formation (p.p., Lower Toarcian; Polymorphum to Levisoni Ammonite Zones) that contains organic matter-rich levels were studied in the three localities. The analysis of the sedimentary structures in the studied successions indicates that the basin was a shallow-water system, above storm-wave base, during the Late Pliensbachian/Early Toarcian. The stratigraphic evolution of the sedimentary structures recorded in the studied localities shows that the Upper Pliensbachian corresponded to a regressive sedimentary interval, whereas a complete transgressive–regressive relative sea-level cycle occurred during the Lower Toarcian Polymorphum Zone. A further transgression occurred in the lowermost Levisoni Zone (Lower Toarcian). Changes from carbonate-dominated to marl-dominated sediments form decimetre- to metre-scale lithological units, that can be correlated from succession to succession, and may have resulted from short-term sea-level and/or palaeoenvironmental variations. The mean duration of these lithological units suggests that they formed in tune with the first orbital eccentricity cycle (100 ka). The analysis of calcareous nannofossil quantities (abundance per mm~2 of the smear slide, percentage, mean abundance) shows both spatial and temporal changes in the community structure. Schizosphaerella spp. and Crepidolithus crassus were more abundant in proximity to the shallow Latium–Abruzzi Platform. These two taxa were likely affiliated to oligotrophic conditions in marine surface waters, explaining why they were generally more abundant in carbonate-rich intervals than in marls. Mitrolithus jansae is interpreted as being a deep-dwelling species, which was more abundant during rising sea level in the earliest Toarcian. Species of the genera Lotharingius, Biscutum and Calyculus were more abundant when higher trophic conditions occurred, and were thus more common during periods of marl deposition. The stratigraphic distribution of calcareous nannoplankton and its interpretation in terms of palaeoecological affinities allow three main phases in the environmental evolution during the Early Toarcian anoxic event to be recognised. The first phase corresponds to a long-term transgression, and high-nannoplankton abundance in distal areas (mainly Lotharingius, Biscutum and Calyculus species) suggests high productivity that may have resulted from upwelling. The beginning of accumulation and preservation of organic matter in the sediments occurred during this phase. The scarcity of nannoplankton during the second phase may indicate oligotrophic conditions in surface waters, possibly due to water stratification during the highest sea-level stand. A high abundance of Schizosphaerella spp. and C. crassus is recorded in proximal areas, probably resulting from sporadic nutrient input from continental or shallow-platform areas. The maximum of organic matter preservation is recorded during this phase. During the third phase, relative sea-level fall and efficient water mixing by frequent storm events led to enhanced nannoplankton productivity in proximal and distal areas and to a decrease in organic matter accumulation in the Umbria–Marche Basin.