Characterization of upslope migrating cross-bedded deposits in a carbonate ramp (Upper Miocene, Menorca, Balearic Islands, Spain)

Menorca is subdivided in main regions with different stratigraphic and tectonic characteristics. To the north, folded, thrusted and faulted Paleozoic, Mesozoic and Paleogene rocks compose the Tramuntana region. To the south, upper-Miocene carbonate and, locally, siliciclastic rocks compose the Migjorn region. Upper Miocene rocks unconformably overlie Paleozoic to middle Miocene basement. In the Migjorn, three Miocene depositional units were recognized (Pomar et al., 2002 and references herein): the Basal Conglomeratic Unit (Burdigalian p.p.), interpreted as near-shore to fan-delta deposits; the Lower Bar Unit (early Tortonian), interpreted as a distally steepened carbonate ramp; the Reef Complex (upper Tortonian - lower Messinian), a reef-rimmed platform prograding complex. Within the ramp, Pomar et al. (2002) distinguished four facies belts: fan-delta conglomerates and bioturbated packstones (inner ramp), crossbedded grainstones (middle- ramp), rhodolithic rudstone clinobeds (ramp slope) and fine-grained wackestone-packstone with planktonic foraminifers (outer ramp). The cross-bedded units here analysed occur at the transition lower slope-outer ramp. They infill the axial depression of large slide/slump scars. These scars truncate the gently, 10°-12° basinward dipping, slope-to-outer ramp succession. The wedge-shaped cross-bedded units, 10-12m thick, pinch out basinward and extend laterally for tens of meters. Several amalgamated cross-laminated bodies, 40cm- to 2m wide each with erosive base compose each unit. They are composed by rhodolithic-mollusk rudstone with coarse grain/packstone matrix. Porosity is very high, cementation low and dolomitization patchy. Matrix consists of red-algal-, bivalve-, echinoderm, coral fragments, and benthic foraminifers. Ooids are, locally abundant along with limestone pebbles. Pebbles are flat, 5- to 10-cm wide, of oolitic grainstone, sometimes with mollusk borings. Cross bedding is underlined by the abundance and orientation of coarser components. Cross-laminae within units dip upslope (8° to 35°) forming backstepping backsets. 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. In Menorca, these sedimentary structures have been interpreted by Pomar et al. (2002), as backset beds related to up-stream migrating hydraulic jump where slide scars acted as slope channels funneling platform debris down the slope. The occurrence of aragonitic components (oolites, corals) within these units, absent in the ramp system, and the scars development at the end of the ramp progradation, suggest the involvement of Reef Complex sediment. Therefore, those events, involved both sediment of the upper Tortonian distally steepened ramp and part of the Reef Complex. These structures have been mostly described in different environments of terrigenous systems. In carbonate systems are scarcely described and none from carbonate ramps. The architecture characterizing these upslope cross-bedded beds seams to call for a scour-and-fill process, therefore initially the action of a strong scouring event immediately followed by depositional events. The presence of these bodies in various intervals suggests the repetitive occurrence of this process. In the studied system, deep-scouring events on the platform could be represented by slope failures that induced tsunami events or by eustatic variations, able to create strong downslope flows directed offshore. Scours have been rapidly infilled by shallow-water sediment.