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A. Slootman et al. / Earth and Planetary Science Letters 444 (2016) 192–204                                      197

Fig. 4. Analysed section of the examined backset-bedded clinoform unit. Close-up of Fig. 2. (A) Photo panel. Dotted lines indicate upper and lower boundaries of the studied
unit. (B) Line drawing of internal stratification. Part of the outcrop is not visible in the photo panel due to recent debris in front of the exposure. However, the exposure is
for a large part accessible behind the scree. Such additional data were used in the palaeohydraulic reconstruction of Fig. 5.

Table 1
Flow parameters associated with the deposition of backset-bedded and dune cross-bedded clinoform units.

  Deposition of studied backset-bedded clinoform unit

Description                                                                Symbol                 Unit         Flow 1           Flow 2  Flow 3

Sediment concentration flow                                                 C flow                  %            4.5              10.0    15.0
Sediment concentration ambient/low-density layer                           C amb                  %            0.0               4.5     4.5
Mean flow velocity over antidunes (Hand, 1974)                              U                      m/s          1.2               1.3     1.8
Average flow thickness                                                                             m            1.9               1.9     2.0
Flow duration for examined backset-bedded clinoform unit                   d                      h            6.2               2.6     1.2


Deposition of equivalent dune cross-bedded clinoform unit                          Symbol                Unit          Case 1   Case 2  Case 3

Description                                                                        θ                     (–)              0.15     0.2   0.5
                                                                                   U 50                  m/s              1.4      1.6   2.6
Mobility parameter (Van Rijn, 1984)                                                U 100                 m/s              1.5      1.7   2.7
Flow velocity in 50 m water depth                                                  tcum.                 days          838      463     91
Flow velocity in 100 m water depth                                                 tsingle,n=50          days           16.8       9.3   1.8
Cumulative flow duration for dune cross-bedded clinoform unit                       tsingle,n=500         days             1.7      0.9   0.2
Average flow duration single dune-activating event (in case of 50 events)
Average flow duration single dune-activating event (in case of 500 events)

U = g L ρflow − ρamb                                                        (1)     concentration (4.5%, 10%, 15%) were estimated between 1.2 and
       2π ρflow + ρamb                                                              1.8 m/s (Table 1).

where g is acceleration of gravity, and ρflow and ρamb are the den-                     Laboratory experiments have shown that specific bedforms are
                                                                                   linked to distinct ranges of the densiometric Froude number Fr
sities of the flow and the ambient water. The density of the flow is                 (Hand, 1974; Cartigny et al., 2014);
a direct function of its sediment concentration, which is reflected
in the stratification type of individual backset-beds (Cartigny et                            U                                          (2)
al., 2013). The preserved spaced stratification (sensu Hiscott, 1994)               Fr =
consists of cm-thick strata separated by erosion surfaces. This
suggests a reduced turbulence at the base of the flow that pre-                                gd
vented the deposition of mm-thick upper-stage plane-bedding,
while maintaining enough turbulence to allow shear-stress pulses,                  where g is submerged gravity (g = g[ρflow − ρamb]/ρflow) and d
generated higher in the flow, to cause intervals of bed erosion.
This points to basal sediment concentrations between 5 and 15%                     is flow thickness. Antidunes associated with breaking waves form
by volume (Cartigny et al., 2013). Above a 9% threshold of sedi-
ment concentration (Bagnold, 1954) a dense basal layer develops.                   at densiometric Froude numbers of about Fr = 1.1 (Cartigny et al.,
Such flow stratification should be accounted for in the calculations,                2014), corresponding to a flow thickness of ca. 2 m (Table 1).
by replacing the ambient fluid density by the density of a dilute,
overriding layer. Following Bagnold (1954) such dilute layer has                       Sediment flux qs (m2/s) is defined as the amount of sediment
a sediment concentration of 9% at its base (interface with dense                   passing a vertical line on the slope per unit time. It equals the
basal layer) and 0% at its top (ambient water). As an approximation
a linear sediment concentration profile is assumed; hence average                   product of flow velocity, flow thickness and sediment concentra-
sediment concentration of a dilute layer is 4.5%. Using the mea-
sured antidune wavelength of 28 m (Fig. 5) and the relation of                     tion Cflow. Flow duration can now be estimated by dividing the
Hand (1974), the velocities of three flows of different sediment                    dip-parallel, cross-sectional area of the examined backset-bedded
                                                                                   clinoform unit ( A ≈ 2400 m2) by sediment flux;

                                                                                         AA                                             (3)
                                                                                   t= =

                                                                                        qs U dCflow

                                                                                       These calculations show that the sediment gravity flows associ-
                                                                                   ated with the generation of backset-bedded clinoform units lasted
                                                                                   a few hours only (1–6 hrs, Table 1).
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