What is the topset of a shelf‐margin prism?

Abstract

The 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.

Facies and architectural variability of sub‐seismic slope‐channel fills in prograding clinoforms, Mid‐Jurassic Neuquén Basin, Argentina

“…The presence of pebbles in slope channels limits the river mouth‐to‐shelf‐edge distance to less than 10 km, as this is the suggested maximum distance pebble‐sized grains should be transported across the shelf into the deep water slope channels (Sweet & Blum, ). This is also consistent with steep hinterland slopes and limited distances of shelf transgression during a greenhouse climate (Steel, Olariu, Zhang, & Chen, ). An intermittent connection to the Pacific Ocean introduced a mesotidal range to the Neuquén back‐arc basin (Dean, ) that has left extensive tidal deposits on the shelf.…”

River‐dominated and tide‐influenced shelf‐edge delta systems: Coarse‐grained deltas straddling the Early–Middle Jurassic shelf–slope break and transforming downslope, Lajas–Los Molles formations, Neuquén Basin, Argentina

“…C). Single topset sequences on the shelf consist of an upward coarsening regressive unit (often with several internal parasequences) followed by an upward fining, transgressive unit (Olariu et al ., ; Steel et al ., ). These phases represent the regressive transit of deltas across the shelf followed by transgressive backstepping of the same deltas, leading to clinoform buildout (Ross et al ., ; Carvajal & Steel, ).…”

Clinoforms and clinothems: Fundamental elements of basin infill

“…Steel et al, 2019 present a comprehensive, up‐to‐date review of topset strata in shelf‐margin depositional systems. The authors focus on regressive and transgressive deposits formed during repeated cross‐shelf shoreline transits, with topset thicknesses varying from 10’s to several 100’s of metres from inner to outer shelf due to water‐depth increase.…”

“…Amorosi et al, 2019;Plink-Bjorklund, 2019) to shelf-edge scale (e.g. Bryn et al, 2019;Gamberi et al, 2019;Gan, Steel, Olariu, & Almeida Jr, 2019;Olariu et al, 2019;Patruno et al, 2019;Steel et al, 2019) and continental-margin scale (Chen, Steel, Wang, Zhao, & Olariu, 2019;Midtkandal et al, 2019). Continuous and repeated erosion is commonplace in the topset areas, F I G U R E 3 Conceptual section of nested clinoforms as the result of incremental margin growth: as in Matryoshka dolls, progressively smaller-scale clinoforms and clinothems 'appear' as we gradually zoom into a continental margin succession.…”