Benthic communities and sedimentary facies recording late Quaternary environmental fluctuations in a Po delta subsurface succession (Northern Italy)

The upper Quaternary of the Po Delta region consists of thick marine and continental deposits, that accumulated in a strongly subsiding portion of the Apennines Foredeep. These successions record large eustatic and climatic fluctuations, dramatically affecting the depositional dynamics of the area. The paper is focused on the palaeoenvironmental interpretation of a 188 m deep borehole, recently cored through the upper Quaternary succession, in the northeastern portion of the Ferrara Province. This is the deepest continuously cored stratigraphic borehole ever drilled in the Po Delta area. The work integrates sedimentological and palaeontological analysis to produce an interdisciplinary reconstruction of the palaeoenvironmental evolution. Palaeoeontological study was focused on dif¬ferent taxonomic groups, ranging from ostracods to foraminifers and mollusks. Several discrete fos¬sil associations were described for each taxonomic group, typifying various depositional environ¬ments, such as freshwater (F), brackish (B), marine (M), and reworked-transported (R) associations. In the marine sediments, four foraminiferal groups (Ma, b, d, e) and varying ostracod and mollusk associations record important salinity and water depth variations. The depositional history of the succession ranges from more than 200,000 yr ago to the present, as suggested by the regional strati-graphic correlation and the 14C dating of the younger portion of the succession. The stratigraphic column is dominated by two thick continental bodies, intercalated by three paralic and marine inter¬vals, corresponding to the last two interglacial highstand phases and to the Holocene post-glacial interval. The two continental units consists of lower alluvial plain deposits, showing scattered fresh¬water ostracod and mollusk faunas, capped by cold-climate coarse sands, deposited in middle allu¬vial plain environments, under lowstand conditions. The lower portion of the borehole records two marine transgressive pulses, developed during interglacial highstand times. In the middle and upper portion of the succession, two transgressive-regressive cycles show a similar asymmetric architectu¬re, with a thin, deeping upward basal unit, followed by a thick regressive portion consisting of pro-delta muds, in turn overlain by delta front sands, with reworked microfossils and storm concentra¬ted mollusk coquinas. The evolutionary trend of the entire succession can be well correlated with the late Quaternary sea level variations, suggesting that the eustatic forcing played a major role in the shaping of the analysed depositional history, even in a tectonically active foredeep area. keywords. Benthic communities, late Quaternary, Po Delta.