SEDIMENTOLOGICAL ANALYSIS OF DEEP WATER, UPSLOPEMIGRATINGCROSS-BEDDED DEPOSITS IN A DISTALLY STEEPENEDCARBONATE RAMP (MENORCA, BALEARIC ISLANDS, SPAIN)
2009
The upper Miocene units cropping out along the southern coast of the Island of
Menorca (Balearic Islands, Spain), are mainly represented by two carbonate
depositional systems: an early Tortonian distally steepened ramp (Lower Bar Unit)
and an upper Tortonian – lower Messinian reef-rimmed platform prograding complex
(Reef Complex). Within the distally steepened ramp, Pomar et al. (2002)
distinguished four facies belts: fan-delta conglomerates passing upwards to
bioturbated packstones (inner ramp), cross-bedded grainstones (middle- ramp),
clinostratified rhodolithic rudstone (ramp slope) and fine-grained wackestonepackstone
with planktonic foraminifera (outer ramp).
The backset-bedded units analysed in this work are placed at the transition
between toe-of-slope and outer ramp sediments, below the wave-base-level. They
infill the axial depression of large slide/slump scars. These scars truncate the gently,
10°- 12° basinward dipping, slope-to-outer ramp clinoforms.
Backset beds are cross-bedded forms that dip against the direction of flow of the
depositing currents, therefore they present foresets migrating upcurrent (Gary et al.,
1972).
These sedimentary structures are well known and largely described on the foreset
and toeset of Gilbert-type fan delta (Postma, 1984; Massari, 1984, 1996; Nemec,
1990). In carbonate depositional systems these type of bedforms are rarely found
and only little described.
The backset-bedded units, here analysed, are channel-like, wedge-shaped, 10-12
m thick, pinching out landward and extend laterally for tens of meters. Each unit is
formed by several amalgamated set of backset beds, 40 cm to 2 m thick. These units
are mainly conglomerates composed by bioclastic coarse-grained grainstone to
rudstone. Large components are rhodoliths, bivalves, skeletal and ooid-rich pebbles
to boulders, gastropods and corals. Matrix is of a bioclastic coarse-grained sand to
fine gravel, made of fragments of bivalves, gastropods, rhodoliths, bryozoans, algae,
echinoids, loose ooids and planktonic and benthic foraminifera. Ooids are locally very
abundant both in matrix and as main components of pebbles. Pebbles are mainly
flattened, elongated, of average size 6-8 cm (a-axis) and sometimes have mollusc
borings on their surface: large (20-30 cm) rounded and spherical boulders are locally
present. Intergranular and intergranular porosity is very high, cementation low and
dolomitization patchy.
Foreset laminae dip upslope with varying angles ranging from almost horizontal to
30°; higher angles are mostly found in the basinward part of the unit. Lamination is
underline by the orientation along laminae of coarser components especially of
bivalves, pebbles and rhodoliths. Grain-size distribution has a particular trend that shows a progressive decrease in size landwards and upwards. Sorting may noticeably
vary being high or absent in different bodies.
The lower boundary of the backset-bedded units is represented by scour surfaces
which, on a parallel-to-flow section are almost concordant with the stratification
below, while on a perpendicular-to-flow section are concave-up shaped, presenting
the very steep walls.
The study of different outcrops along the coast evidenced some important
variation in components: moving northward composition changed from almost
completely rhodolithic-dominated to rhodolith-bivalve-ooid-pebble-dominated to
bivalve-ooid-pebble-dominated with first findings of corals.
Upslope bedform migration has been explained as forming when a supercritical
flow encounters a local obstruction or a local break on the slope, and a hydraulic
jump may occur within the flow, upcurrent from the obstruction. Sediment will be
therefore deposited at the obstruction forming an up-flow-dipping slipface that will
tend to accrete and migrate in the upflow direction (Nemec, 1990 and reference
therein).
The backset deposits of Menorca are found in deep-water settings but they are
composed of shallow-water sediment. The formation of these backset beds is
interpreted to be related to high energy storm-events able to remove sediment from
shallow water and to transport it into deeper position. The sediment-rich outgoing
flows channalized and accelerated along slide-scar axis, eroding and rapidly infilling
up-slope the scours. In this portion of the ramp preservation potential is higher
thanks to sediment deposition which buries and preserves these structures.
The repetitive occurrence of backset bedded units within the outer-ramp
sediments and the progressive variation in composition suggest that those processes
where probably active at the transition between the ramp and the reef systems.
Therefore the formation of these sedimentary structures is interpreted to be strictly
link to concurrence of peculiar morphological features, hydrodynamic energy and
grain-size availability.
Computational fluid dynamic (CFD) numerical simulation have been performed as
an integrated part of this work to improve the understanding of the development of
hydraulic jumps within concentrated density flows. The simulated parameters do not
refer to the example of Menorca but to turbidity currents for which finer-grain size
(sand-size) have been used in a smaller-scale topography compared to the one
studied in outcrop. The work presented proposes some new stating points for further
simulations to constrain more precisely the main parameters controlling and
determining the occurrence of a hydraulic jump and the consequent deposition of
sediment with backset bedding.
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