Identifying the Sequence Boundary in Over- and Under-Supplied Contexts: The Case of the Late Pleistocene Adriatic Continental Margin
Claudio Pellegrini, Kevin M. Bohacs, Tina M. Drexler, Fabiano Gamberi, Marzia Rovere, Fabio Trincardi, "Identifying the Sequence Boundary in Over- and Under-Supplied Contexts: The Case of the Late Pleistocene Adriatic Continental Margin", Sequence Stratigraphy: The Future Defined, Bruce Hart, Norman C. Rosen, Dorene West, Anthony D’Agostino, Carlo Messina, Michael Hoffman, Richard Wild
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Even in a system whose stratal record is well expressed, it can be challenging to confidently differentiate sequence boundaries from other erosional surfaces because of lateral changes in stratal patterns due to variations in accommodation and sediment-supply rates and routes. Identifying a sequence boundary, as originally defined by Mitchum et al. (1977), is based on objective geometric relations. The original and standard criteria for defining a sequence boundary include not only the recognition and interpretation of stratal terminations but also an assessment of the spatial distribution of such terminations. Key geometric relations, however, are not always apparent on every single seismic line and are commonly inferred solely from vertical sections from boreholes. Hence it is essential to correlate and map the three-dimensional distribution and character of potential sequence boundaries (and any other sequence-stratigraphic surfaces) for a more confident interpretation. Variations in observed geometric relations are a function of profile location and orientation with respect to sediment-entry points and shelf-edge, as well as to spatial changes in rates of change in accommodation relative to the rates of sediment supply (e.g., Madof et al., 2016).
We illustrate this interpretation process using our work in the Adriatic continental margin taking advantage of the preserved and well-expressed strata and surfaces of the late Pleistocene succession. In addition, in the study area, independent evidence of accommodation changes (eustasy and subsidence), sediment-supply (rates and routes), and robust geochronological control are available (Pellegrini et al., 2017). On the Adriatic margin, the late Pleistocene stratigraphy consists of a succession of regressive depositional sequences bounded by shelf-wide erosional surfaces, each recording approximately 100-ky glacio-eustatic cycles (Trincardi and Correggiari, 2000; Ridente and Trincardi, 2005). The most recent depositional sequence shows at its top different erosional surfaces developed during higher frequency changes in accommodation and sediment supply. Following the classic definition of sequence boundary by Mitchum et al. (1977), we are able to differentiate the sequence boundary from other erosional surfaces by their different types and extents of onlap on the slope and different basinal deposits. We compare and contrast the character of these surfaces in three dimensions, taking into account the importance of along-strike variations in supply regime along a continental margin, and show how this aids interpretation of causal mechanisms and the consequences for predictions of rock properties.