Stacking pattern transition caused by submarine channel inception and internal levée development

Abstract

ABSTRACTThe shift from highly amalgamated and laterally offset to aggradational and vertically stacked submarine channels is a pattern that has been widely recognized in seismic reflection datasets as well as in outcropping successions worldwide. However, the sedimentary and stratigraphic details of such an important part of channel evolution and its implications have not been discussed extensively. That gap is addressed here by characterizing a previously undocumented, seismic‐scale outcrop of the Tres Pasos Formation (Magallanes Basin, Chile) that records the transition from a laterally offset/low‐aggradational channel complex to a vertically stacked aggradational complex associated with the development of an internal levée that enhanced channel aggradation. The boundary between the two complexes is defined by a composite erosional surface (up to ca 35 m relief) with two adjacent element‐scale channelized incisions with contrasting sedimentary infills that provide unprecedented insight into this key phase of submarine channel evolution. Observations indicate that, in between these two depositional architecture styles, there is a significant phase of deep incision and bypass. The relief achieved via this deep incision of one or multiple simultaneously active conduits is interpreted to be necessary to set up the conditions for flow stripping and, subsequently, overbank deposition. Finally, the siltstone‐rich intra‐channel lithofacies association observed directly overlying part of the incision is interpreted to represent the stratigraphic expression of the earliest deposits of an active submarine channel preserved due to the abrupt abandonment of this pathway and the development of an adjacent conduit, which resulted in the former being covered with fine‐grained overbank deposits.