Supercritical-flow structures on a Late Carboniferous delta front: Sedimentologic and paleoclimatic significance

Abstract

Deposits of fluvial systems in highly seasonal tropical climates possess unique architectural and facies characters owing to a flood-prone regime alternating with lengthy periods of ineffective discharge. Distally linked deltaic successions should also feature distinctive attributes, with great potential to preserve the stratigraphic evidence of exceptional discharge events. We describe Late Carboniferous deltafront, valley-confined sandstones from the Pennine Basin (UK), originally deposited at paleoequatorial latitudes during final assembly of the Pangean megacontinent and characterized by giant sedimentary structures with repetitively sigmoidal geometry. Facies traits indicate geologically instantaneous deposition of a large sediment volume from a density current at sustained supercritical-flow conditions, leading to aggradation of cyclic steps, recently identified bedforms developing in high-energy flows and of which this is the first complete outcrop example. The lack of unconformable erosional surfaces and absence of different associated facies point to a single aggradational event during which the structures attained dimensions comparable to those indicated by seismic data sets from which they are remotely detected on modern seafloors. Cyclic-step formation in a deltaic setting suggests that Pangean megamonsoons could have triggered hydrologic events capable of imprinting sedimentologic signatures on shallow-marine deposits.

Ice‐marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large‐scale depositional architecture

“…Related bedforms on the delta slope include deposits of antidunes, chutes-and-pools and cyclic steps, which might be partly misinterpreted as scour fills or waveinduced hummocky cross-stratification (cf. Deposits of supercritical density flows may record high-magnitude (glacial) floods (Ghienne et al 2010;Winsemann et al 2011;Girard et al 2012Girard et al , 2015Carling 2013;Ventra et al 2015) or represent delta slope failure events producing slides, slumps, debrisflows and/or turbidity currents (Talling 2014;Dietrich et al 2016;Hughes Clarke 2016). Deposits of supercritical density flows may record high-magnitude (glacial) floods (Ghienne et al 2010;Winsemann et al 2011;Girard et al 2012Girard et al , 2015Carling 2013;Ventra et al 2015) or represent delta slope failure events producing slides, slumps, debrisflows and/or turbidity currents (Talling 2014;Dietrich et al 2016;Hughes Clarke 2016).…”

“…The grain size of the foreset-bed packages commonly decreased during progradation and a lateral facies transition from FA1.2.4 to FA1.2.5 can be observed. The finer-grained sandy foreset beds, deposited from migrating (humpback) dunes and ripples (Figs 7F-G, 8E) also require sustained turbidity currents that may reflect plunging hyperpycnal flows (Plink-Bj€ orklund & Steel 2004;Winsemann et al 2007;Ghienne et al 2010;Ventra et al 2015;Carvalho & Vesely 2017) during low rates of delta-front aggradation. Alternatively, the fining during progradation may be related to a decreasing water discharge and sediment supply and deposition from lower-energy density flows.…”

“…Facies association FA1.2.5 (Figs 7G, 8D, E) is dominated by climbing-ripple cross-laminated sand (facies Sr), deposited from sustained subcritical turbidity flows. These coarser-grained deposits might represent major slope failure events(Talling 2014;Dietrich et al 2016; Hughes Clarke 2016) or major meltwater discharge events(Ghienne et al 2010;Ventra et al 2015;Carvalho & Vesely 2017). This zone is characterized by flow expansion and the formation of upslope directed large vortices (cf Jopling 1965;Clemmensen & Houmark-Nielsen 1981;Winsemann et al 2007)…”

Morphodynamics and depositional signature of low‐aggradation cyclic steps: New insights from a depth‐resolved numerical model

“…Developments in physical and numerical modelling of supercritical‐flow bedforms (Kennedy, ; Jorritsma, ; Foley, ; Winterwerp et al ., ; Parker & Izumi, ; Alexander et al ., ; Fagherazzi & Sun, ; Sun & Parker, ; Taki & Parker, ; Fildani et al ., ; Kostic & Parker, ; Alexander, ; Sequeiros et al ., ; Spinewine et al ., Kostic et al ., ; Paull et al ., ; Cartigny et al ., , ; Kostic, ; Balmforth & Vakil, ) have sparked a large number of observations of supercritical‐flow bedforms in modern systems (Fildani et al ., ; Lamb et al ., ; Duarte et al ., ; Jobe et al ., ; Babonneau et al ., ; Maier et al ., ; Covault et al ., ; Hughes Clarke et al ., ; Fricke et al ., ; Tubau et al ., ; Zhong et al ., ; Normandeau et al ., ; Symons et al ., ). Despite this common and well‐documented occurrence of supercritical‐flow bedforms, outcrop examples of deposits indicating these flow conditions in a fluvial setting (Fielding, ; Duller et al ., ; Fielding et al ., ; Ghienne et al ., ; Lang & Winsemann, ) or in a (deltaic‐) marine setting (Postma et al ., , ; Ventra et al ., ; Dietrich et al ., ) are sparse. The recent flurry of recognition of supercritical bedforms in modern environments makes it implausible that sedimentary structures indicative of these bedforms should be rare in deposits formed in comparable ancient environments.…”

“…The sparsity of supercritical sedimentary structures is often attributed to poor preservation potential of supercritical‐flow regime deposits, due to reworking by subcritical flows in the waning stages of high‐discharge events. Froude‐supercritical flows also tend to form in parts of the sedimentary system that are net‐erosive on a geological timescale, such as mountainous terrains (Middleton, ; Foley, ; Yagishita & Taira, ; Wynn & Stow, ; Fielding, ; Duller et al ., ; Ponce & Carmona, ; Lang & Winsemann, ; Macdonald et al ., ; Cartigny et al ., ; Postma et al ., ; Ventra et al ., ). An alternative explanation for the sparse recognition of supercritical regime facies is that their depositional signature is poorly understood.…”