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Abstract
Abstract The 350-m-thick succession of the Po River lowstand wedge (Italy) associated with the Last Glacial Maximum (deposited over ∼17 k.y) contains stratal architecture at a physical scale commonly attributed to much longer time scales, with complex, systematically varying internal clinothem characteristics. This study investigated clinothem stacking patterns and controls through the integration of seismic reflection data with sediment attributes, micropaleontology, regional climate, eustacy, and high-resolution age control possible only in Quaternary sequences. Three clinothem types are differentiated based on topset geometry, shelf-edge and onlap-point trajectory, internal seismic facies, and interpreted bottomset deposits: type A has moderate topset aggradation, ascending shelf-edge trajectory, and mass-transport bottomset deposits; type B has eroded topset, descending shelf-edge trajectory, and bottomset distributary channel-lobe complexes; and type C has maximal topset aggradation, ascending shelf-edge trajectory, and concordant bottomsets. Type A and C clinothems exhibit reduced sediment bypass and delivery to the basin, whereas type B clinothems are associated with short intervals of increased sediment export from the shelf to deeper water. Clinothems individually span a range of 0.4–4.7 k.y., contemporaneous with significant eustatic and climate changes, but their stacking patterns resemble those found in ancient successions and ascribed to significantly longer durations, indicating that (1) the response time of ancient continental margin–scale systems to high-frequency variations in accommodation and sediment supply could be as short as centuries, (2) even millennial- to centennial-scale stratal units can record substantial influence of allogenic controls, and (3) sandy deposits can be compartmentalized even in a short-duration lowstand systems tract.
Highlights
Sequence-stratigraphic models predicted that third-order lowstand systems tracts would be characterized by the deposition of sand-prone channel-lobe complexes topped by the progradation of a mudprone slope wedge (Vail et al, 1991)
Clinothems individually span a range of 0.4–4.7 k.y., contemporaneous with significant eustatic and climate changes, but their stacking patterns resemble those found in ancient successions and ascribed to significantly longer durations, indicating that (1) the response time of ancient continental margin–scale systems to high-frequency variations in accommodation and sediment supply could be as short as centuries, (2) even millennial- to centennial-scale stratal units can record substantial influence of allogenic controls, and (3) sandy deposits can be compartmentalized even in a short-duration lowstand systems tract
Clinothem Character: Types A, B, and C The Po River lowstand wedge comprises a set of clinothems with hundreds of meters of relief characterized by cyclic changes in topset geometry, shelf-edge and onlap-point trajectory, seismic facies, and slope and bottomset deposits
Summary
Sequence-stratigraphic models predicted that third-order lowstand systems tracts (durations of 1–2 m.y.) would be characterized by the deposition of sand-prone channel-lobe complexes topped by the progradation of a mudprone slope wedge (Vail et al, 1991). We used a grid of high-resolution seismic reflection profiles through this expanded stratigraphic succession to characterize the internal architecture of each component clinothem and tied these observations to a high-resolution micropaleontologic and chronostratigraphic framework constrained by borehole data (Piva et al, 2008) This enabled us to relate clinothem stratal geometries and stacking to independently constrained late Quaternary eustatic and climate changes (e.g., Lea et al, 2002; Monegato et al, 2007) to better understand the relation of clinothem character to high-frequency variations in accommodation, sediment supply, and coarse-sediment bypass to the basin
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