High Resolution Climate Eco-Stratigraphy in Better Solving Pleistocene Successions

摘要

Conventional biostratigraphy may lack in resolution within the Quaternary, especially in the very expanded (as far as almost 2000 m thick) Pleistocene series of the Adriatic offshore, Italy. In this context, an eco-biostratigraphic approach, based on the analysis of the oscillating abundance trends of the microfossil assemblages, must be preferred. These variations in the composition of the assemblages are mostly linked to regional/global paleoclimatic and/or paleoceanographic events. Such a climate eco-biostratigraphy, well consolidated in the academic world, has been poorly applied in the oil industry, because of the high degree of disturbance caused by reworking and caving in drilled successions. The individuation of Glacial-Interglacial excursions, as well as the distinction of the sapropel events (dark and laminated sediments, cyclically recurring, linked to periods of enhanced rainfall), allows the tuning of the successions with the Milankovian periodicity of 100 kyr, 41 kyr and 23 kyr. Milankovitch cycles are linked to the orbital parameters of the planet Earth (Eccentricity, Obliquity and Precession, respectively), regulating the annual insolation and the heat distribution at the Earth surface. Climate fluctuations have a direct role in modulating the oceanographic regime and in regulating both the sedimentation and the life of the microfauna and the nannoflora. Therefore, unravelling the climate oscillations by studying variations in microfossil assemblages, grants the definition of a high resolution stratigraphic framework. The Adriatic Sea is probably the most suitable area for testing a new highly-resolutive methodology, because of the availability of a large number of cored wells and the possibility of tuning (on a local scale) the stratigraphic framework with both the seismic and the log dataset. The analysis of a 300-m thick section, cored by Agip in the Adriatic offshore within a prodelta setting affected during the Pleistocene by several turbiditic episodes, acts as case study to calibrate the method. The paleoclimatic cyclicity inferred by the variations in the microfauna and nannoflora has been compared with the astronomically- tuned stable oxygen isotope curve, allowing the distinction between interglacial (warm) and glacial (cold) stages. Few constraints derived from stratigraphic bioevents and from the identification of sapropels helped in better delineating the chronological framework of the studied climatic succession. The cored sediment interval straddled the Mid-Pleistocene Revolution, a period during which the orbital forcing switched from an obliquity-dominated climate to an eccentricity-regulated one. The impact of climate on the depositional processes is particularly evident by watching at sedimentation-related parameters, as the spontaneous potential log and the grain-size curve, and the stacking pattern of sedimentary cycles, marked by the turbiditic events. The application of this sort of climate eco-biostratigraphy, therefore, allows the achievement of a high resolution chronostratigraphy and helps in better understanding the mechanisms regulating the sediment deposition.