--> --> Defining Chronostratigraphic Framework and Clinothem Evolution Through Seismic Analyses of the Po River Lowstand Wedge (Adriatic Sea): Changes in Sediment Supply and Compartmentalized Basin-Floor Deposits

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Defining Chronostratigraphic Framework and Clinothem Evolution Through Seismic Analyses of the Po River Lowstand Wedge (Adriatic Sea): Changes in Sediment Supply and Compartmentalized Basin-Floor Deposits

Abstract

Although studies of ancient shelf-edge deltas based both on seismic and outcrop data have proliferated during the past three decades, most examples lack sufficiently high-spatial and temporal resolution for detailed analysis of their basic architectural elements and facies distribution. Moreover, stratigraphic models strive to interpret stratal architecture and sediment distribution as the result of the interaction between accommodation and sediment supply rates, but infer those rates from stratigraphic patterns, without independent evidence. Integration of a newly acquired set of high-resolution seismic profiles (1500 line-km) with sediment attributes, microfossil content, and detailed age control from a continuous-recovery borehole allowed definition of the chronostratigraphic framework and clinothem evolution of the Po River Lowstand Wedge (PRLW), deposited during the Last Glacial Maximum (LGM), in the Adriatic Basin. Using PetrelĀ®, we reconstructed a 3D model of the elemental clinothems composing the PRLW, identifying key stratigraphic surfaces and seismic facies with a loop-tie method. Within the PRLW, clinothem types are defined by topset geometry, shelf-edge and onlap-point trajectory, and associated slope and basin-floor deposits. The main variations in clinothem types record short-term supply fluctuations during an interval of overall enhanced sediment accumulation into the basin (during the LGM Chronozone). In the basin-floor, seismic facies typical of channel-lobe complexes (DLCs) alternates with facies ascribed to mass-transport complexes (MTCs). MTCs developed following short phases of rapid relative sea level fall and formation of erosional surfaces on the outer shelf. Micropaleontological analysis and sediment accumulation rates suggest that DLCs were associated with phases of increased fresh-water and sediment supply in the basin, whereas MTCs developed when sediment supply from the shelf was decreasing. Our findings suggest that, even within a single lowstand sediment wedge, sediment supply to the basin is far from constant and that shelf-edge clinothems prograde under pulses associated with different mechanisms of sediment delivery. These concepts may be useful when interpreting low-resolution or non-continuous datasets, where sequence stratigraphic models consider sediment supply constant and tend to ascribe compartmentalization of basin-floor deposits to major cycles of sea level change and shutdown of the sediment flux to the basin.