What is the topset of a shelf‐margin prism?

Abstract

AbstractThe siliciclastic topset of a continental margin, or a shelf‐margin prism in subsiding nonplate‐margin deepwater basins, is the flat‐lying upper part of the margin succession; it is coeval basinwards with deepwater slope clinoforms. Topsets develop by the aggradation of repeated, cross‐shelf, shoreline regressions and transgressions, thereby hosting the shelf portion of stacked, fourth‐order stratigraphic sequences. Sediment spreading downdip and along strike during the cross‐shelf transit of the sediment delivery system, as well as process regime changes of deltas and shorefaces (regressive) and of estuaries, barrier–lagoon systems and shelf ridges (transgressive) are highly variable over short distances, so that correlation within a single stratigraphic sequence is far more difficult than correlation of the cross‐shelf maximum flooding surface boundaries. Thickness of individual regressive–transgressive, fourth‐order sequences is given by shelf accommodation, typically <10 m in embayment or on the inner shelf and up to 200 m on outer shelf. Tectonic subsidence and compaction will enhance this thickness only if rates are very high compared to shelf‐transit time. In very high subsidence rate settings, the transgressive tracts are well preserved and often thickly developed. Topset sequences in an Icehouse climate setting tend to have a high proportion and greater landward development of marine vs nonmarine deposits, compared to Greenhouse sequences, because of the importance of eustatic rise of sea level in the former. Previous numerical experiments show that even for very wide shelves and irrespective of Icehouse or Greenhouse conditions, deltas rarely take more than 10 s of ky to reach their shelf edge, suggesting that it is fourth‐order (or higher) sequences that are the fundamental ones in sequence stratigraphy.