Recognition of cyclic steps in sandy and gravelly turbidite sequences, and consequences for the Bouma facies model

Abstract

Preservation of cyclic steps contrasts markedly with that of subcritical‐flow bedforms, because cyclic steps migrate upslope eroding their lee face and preserving their stoss side. Such bedforms have not been described from turbidite outcrops and cores as yet. A conceptual block diagram for recognition of cyclic steps in outcrop has been constructed and is tested by outcrop studies of deep water submarine fan deposits of the Tabernas Basin in south‐eastern Spain. Experimental data indicate that depositional processes on the stoss side of a cyclic step are controlled by a hydraulic jump, which decelerates the flow and by subsequent waxing of the flow up to supercritical conditions once more. The hydraulic jump produces a large scour with soft‐sediment deformation (flames) preserved in coarse‐tail normal‐graded structureless deposits (Bouma Ta), while near‐horizontal, massive to stratified top‐cut‐out turbidite beds are found further down the stoss side of the bedform. The architecture of cyclic steps can best be described as large, up to hundreds of metres, lens‐shaped bodies that are truncated by erosive surfaces representing the set boundaries and that consist of nearly horizontal lying stacks of top‐cut‐out turbidite beds. The facies that characterize these bedforms have traditionally been described as turbidite units in idealized vertical sequences of high‐density turbidity currents, but have not yet been interpreted to represent bedforms produced by supercritical flow. Their large size, which is in the order of 20 m for gravelly and up to hundreds of metres for sandy steps, is likely to have hindered their recognition in outcrop so far.

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…They also resemble the transitional antidunes of Fielding (2006) or the long-wavelength bedforms of Cartigny et al (2013) described as unstable antidunes, chutes-and pools or cyclic steps, formed under supercritical flow conditions associated with hydraulic jumps in flow expansion areas. More precisely, thick (N10 cm) and graded laminae individually similar to top-cut-out sandy turbidites point to an origin related to flow events generating cyclic steps (Postma et al, 2014). Similar deposits are illustrated in the Quaternary glacial record by Lang and Winsemann (2013).…”

Supercritical and subcritical turbidity currents and their deposits--A synthesis

“…Unit Te represents the hemipelagic interval, the final suspension rain that covers the entire turbidite deposit. The Tb facies were expanded for high-concentration flows (e.g., Lowe, 1982, Mutti, 1992, and recently by Talling et al (2012) and Postma et al (2014) to include facies (see Fig. 2) that have experimentally been related to the sediment concentration of the basal flow layer (see Cartigny et al, 2013;Postma et al, 2014).…”

“…Antidune bedforms were previously inferred from turbidite sequences, and their wavelength and height ratios have been related to flow depth, flow density, and flow velocity (Hand, 1969(Hand, , 1974. Cyclic steps have only recently been recognized in modern turbidite systems (e.g., Fildani et al, 2006;Kostic, 2011;Hughes Clarke et al, 2012 and in outcrops (Cartigny, 2012;Lang and Winsemann, 2013;Postma et al, 2014). Cyclic steps are dune-shaped bedforms that migrate upflow by lee-face erosion and stoss-side deposition, and contrast markedly with subcritical dunes (Winterwerp et al, 1992;Fig.…”

“…Nevertheless, supercritical bedforms such as antidunes, chutes and pools, and cyclic steps (e.g., Hand, 1969;Winterwerp et al, 1992;Alexander et al, 2001) have not been merged into the classical turbidite facies models (e.g., Bouma, 1962;Lowe, 1982;Mutti, 1992), most likely because they have not been recognized, their architecture being far beyond bed scale (Postma et al, 2014).…”

“…Consideration of density flow criticality by the densimetric Froude number is of prime importance for understanding large-scale flow hydraulics and their developing turbidite facies (e.g., Migeon et al, 2001;Kostic, 2011;Cartigny et al, 2011). Not only is the flow's velocity profile very different for super and subcritical flows (Sequeiros, 2012), which has important consequences for the flow's carrying capacity (Hiscott, 1994;Winterwerp, 2006), but a supercritical flow passing through a hydraulic jump leaves distinct sedimentary facies (Russell and Arnott, 2003;Postma et al, 2009Postma et al, , 2014.…”