How to Make a 350-m Thick Lowstand Systems Tract in 17,000 Years: The Late Pleistocene Po River Lowstand Wedge
This study investigated clinothem characteristics, 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. The 350-m thick succession of the Po River Lowstand Wedge (PRLW) deposited during the Last Glacial Maximum (~17 kyr long) contains stratal architecture at a physical scale that is commonly attributed to much longer time intervals, with complex, systematically varying internal clinothem characteristics. Within the PRLW, three clinothem types are characterized by distinctive topset geometry, shelf-edge and onlap-point trajectory, internal seismic facies, and interpreted bottomset deposits: Type A) moderate topset aggradation, moderately ascending shelf-edge trajectories, and thin mass-transport bottomset deposits, Type B) eroded topset, descending shelf-edge trajectories, and bottomset distributary channel-lobe complexes, and Type C) maximal topset aggradation, ascending shelf-edge trajectories, and draped concordant bottomsets. Measured sediment accumulation rates suggest that Type A and C clinothems had reduced sediment bypass and delivery to the basin, whereas Type B clinothems were associated with short intervals of increased sediment export from the shelf to deep water and formation of distributary channel-lobe complexes. This interpretation is supported by micropaleontological analysis that highlight increased delivery of sediment and fresh water to the basin during the progradation of Type B clinothems, as suggested by the reduced frequency of Cassidulina leavigata carinata and and the peak frequency in the abundance of Nonioum species. All clinothems formed in very short interval, from 0.4 to 4.7 kyr, contemporaneous with significant eustatic and climate changes, but their stacking patterns resemble those of ancient successions with significantly longer duration indicating that: 1) the response time of ancient continental-margin-scale systems to high-frequency variations in eustasy 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) sediment export to the basin is episodic, even in a single and short-duration lowstand systems tract. These notions can be used in ancient records to broaden the interpretation range where chronological resolution is inherently limited.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017