Time-transgressive Confinement on the Slope and the Progradation of Basin-floor Fans: Implications for the Sequence Boundary of Deep-water Deposits

Observations from outcrop and subsurface datasets indicate that key stratigraphic surfaces in ancient submarine slope successions are diachronous and form during periods of seascape degradation and sediment bypass. Evidence for time transgressive confinement of submarine channel–levee systems includes composite basal erosion surfaces, cut-off bends and hanging valleys, and external levees overlying lobe deposits. After the onset of a sediment supply cycle, progressive confinement will develop on the submarine slope, through a combination of incision and external levee construction, such that sequential sediment gravity flows will maintain their downslope energy farther into the basin. This way, frontal lobe deposits are incised by channel systems and overlain by external levee deposits as the channel–levee system becomes more entrenched and propagates farther into the basin. The stratigraphic response on the related basin floor is fan growth and net progradation until a maximum basinward extent is reached, which corresponds to the time of maximum through-channel sediment transfer (bypass). At this time a maximum regressive surface forms, although in reality this is challenging to identify in the rock record. Conceptually, this basinfloor process response to progressive slope confinement at a point could be autocyclic, but would be amplified with an allogenically driven waxing-then-waning sediment supply cycle. The coupled progressive confinement of the submarine channel–levee system and basin-floor fan growth will result in a diachronous lithological basal surface to the system. This challenges the idea of the deep-water sequence boundary being isochronous and passing into a single correlative conformity at the base of the basin floor fan, or that there is temporal distinction between deposition by high- and low-density turbidity currents.