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A. Slootman et al. / Earth and Planetary Science Letters 444 (2016) 192–204                 199

Fig. 6. Close-up of high-angle cross-stratification formed by the migration of subaqueous dunes. For location see Fig. 2. (A) Photo panel. Dotted lines indicate unit boundaries.
(B) Line drawing of internal stratification. Encircled numbers have similar meaning as those in Fig. 2. (C) Conceptual model for the formation of dune cross-bedded clinoform
units by the progradational stacking of large-scale composite dunes.

Ashley, 1990) parent bedform, forming compound cross-beds on          4.3.2. Palaeohydraulic reconstruction
the lee-side of such composite dunes (Fig. 6). Composite dunes            Previous studies have shown that dune migration on the
had a high preservation potential once they migrated below storm
wave base and down the ramp slope, where they would ultimately        Mediterranean shelf is locally related to strong, sustained winds
be preserved as compound cross-bedded sets. Advancement of the        capable of moving the entire water column (e.g. Bassetti et al.,
carbonate ramp occurred by the progradational stacking of such        2006). The height of the water column must have exceeded the
sets (Figs. 1C, 6C). The formation of a single dune cross-bedded      preserved clinoform height by at least the depth of the storm
clinoform unit finished with the blanketing of the ramp slope dur-     wave base (Pomar and Tropeano, 2001), hence water depth is
ing the rapid deposition of a backset-bedded clinoform unit from      estimated at 50 to 100 m. Flow velocities required to form sub-
a high-energy gravity flow.                                            aqueous dunes in sands with the measured grain size distribution
                                                                      (median grainsize D50 = 1.0–2.0 mm) were calculated, using the
    Based on the variation in bed thickness, the presence of nu-      scale-independent bedform stability diagram of Van den Berg and
merous reactivation surfaces and different degrees of Thalassi-       Van Gelder (1993), to range from 1.4 to 2.7 m/s for such water
noides bioturbation (Fig. 6), dune cross-bedded clinoform units       depths (Table 1).
clearly reflect a polycyclic evolution. The number of distinct ‘dune-
activating events’ within a single clinoform unit is estimated            Subaqueous dunes form only under bed-load-dominated flows,
from outcrop between 50 and 500. The average recurrence pe-           as high sediment concentrations suppress their formation (Wan
riod of backset-generating events also constrains the average time    and Wang, 1994). For bed-load-dominated flows, the dimension-
contained within single dune cross-bedded clinoform units at          less sediment flux qs∗ can be estimated using bed-load predictors,
14–35 kyr. This estimated overall duration and the number of          e.g. Meyer-Peter and Mueller (1948);
dune-activating events, imply that on average subaqueous dunes
were active only once every 46 to 458 years (Table 2).                qs∗ = 8 θ − τcr∗ 3/2  (4)
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