SEDIMENTOLOGICAL ANALYSIS OF DEEP WATER, UPSLOPEMIGRATING CROSS-BEDDED DEPOSITS IN A DISTALLY STEEPENED CARBONATE RAMP (MENORCA, BALEARIC ISLANDS, SPAIN)

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.