Abstract

The Pliocene deposits from Ventimiglia to Bordighera belong to a gravelly fan-delta complex genetically related to disjunctive Neogene tectonic phases which involved the continental margin (fig. 1). The following seven lithofacies occur: (1) angular-subangular breccias with silty-sandy matrix and dispersed rounded gigantic boulders (up to 3-4 m <sup>3</sup> ); 2) heterometric (2), coarse-grained, unstratified, clast-supported, massive and disorganized conglomerates, with prevailingly spherical cobbles and blocks and dispersed angular clasts; their lower bounding surfaces are strongly erosional; (3) roughly organized, massive, sporadically matrix-supported conglomerates, with a crude stratification marked by bands of different grain size and discontinuous sandy beds; their bases are erosional; outsized blocks are frequent; (4) stratified, well-sorted, sandy matrix-supported conglomerates with beds ranging from a few decimeters to 2-3 m; they vary to sandy conglomerates; their bases are gently erosional or nonerosional; (5) gravelly, coarse-to medium grained, massive sandstones, lacking in matrix, with 40-50% of gravel and floating blocks; they contain vertical tubular burrows; (6) medium to fine-very fine laminated turbidite sandstones with graded beds up to 40 cm thick and arenite/pelite ratio <1; these units gradually pass upward or downward to the pelitic deposits; (7) pelites, mainly formed by gray-blue, massive silty-marly clay; sandy and microgravelly laminae are sometimes present; this lithofacies occurs either as conspicuous deposits up to 150 m thick, or as discontinuous horizons up to 1 m thick dispersed in the conglomeratic units (in the latter case, they become silty-sandy and laminated). These lithofacies form a continuum in both progradation and aggradation directions and they are interpreted in terms of sedimentary processes as follows. Breccias are classed as deposits of debris avalanching (sensu Prior & Bornhold, 1990); they might result from the reworking of alluvial and colluvial deposits during the initial stages of formation and discharge of the feeder basin; they occur only in contact with the substratum, in internal and proximal areas. Conglomerates are grain-flow deposits resulting from mass-transport mechanisms; these deposits may be genetically related to catastrophic floods which allowed large amounts of sediment-water mixtures to enter the basin with enough inertia to cut the mouth bar, bypass the delta front and finally set off down the slope as gravity flows (see Prior & Bornhold, 1990; Mutti et alii, 1996). Such flows were transformed from high-energy, channelized, cohesionless debris flows to unconfined, laminar flows of decreasing energy. Gravelly sandstones may have formed through interaction between hyperpycnal flows and the encompassing fluid. Turbidite lithofacies might result from the transfer to distal areas of the fine fraction of hyperpycnal flows as turbidity currents. Pelites result from the settling of suspended plumes during long retraction periods of the system (very thick horizons) or during temporary diversions of the system (thin horizons). Three different conglomeratic units, named as "lower unit" (outcropping only in the axial zone of the fan delta), "intermediate unit" and "upper unit", form the stratigraphic pattern of the fan-delta complex (figs. 3 and 5) and their overall thickness amounts to about 500 m. They are separated by two so-called "main pelite levels". A first main pelite unit, named "lower pelite level", up to 150 m thick, generally lies directly on the substratum, except at Mt. Baraccone and Santa Croce, where it overlies the conglomeratic "lower unit". Other main pelite levels, ranging in thickness from 15 m to 35-45 m, occur at three different heights within a conglomerate band 100-200 m thick, and together form the "upper pelite level". The grain size of the gravelly fan-delta complex generally decreases downflow (following the direction of progradation) and upward (following the direction of aggradation). The same fining-upward trend occurs both within the individual, above-mentioned conglomerate units, and within different subunits of lower rank. Their crosswise shape is strongly lenticular. They lie on surfaces dipping at 6-8 degrees on which downlapping foreset beds are unconformable by 5-15 degrees . Upcurrent, their lower boundaries are strongly erosional, but downcurrent lightly erosional or sometimes transitional. Other unconformities also occur into their interior, caused perhaps by significant lateral digressions of the system. Foreset beds form almost the whole of the three coarse units. They are SE- and SW-dipping, towards the basin axis, with maximum inclination ranging from 15 degrees (western area) to 22 degrees (eastern area), so delineating a vessel-type feature (fig. 4). Such a feature could be produced by two different supply directions or an effective confinement guided by the opposite basin sides. Flow directions inferred from a-axes of imbricated discoidal pebbles are congruent with the foreset-bed attitudes. However, a NW-SE direction is prevalent. The alternation of main pelite levels (abandonment periods of the fan delta) and conglomeratic units (activity periods) suggests a cyclic succession of transgressive and regressive phases. As already observed with regard to the nearby Pliocene deposits from Taggia-Castellaro (Marini, 2000), such cyclicity could result from relative sea-level variations caused by local tectonic activity, prevailingly consisting of collapse and basculing movements of the margin. The individual coarse units are compared to "depositional sequences" sensu Vail et alii (1977). Although they have a regressive character, their marked fining-upward trend denotes their nature as filling sequences. Several spectacular unconformities observed within individual conglomerate units, along with the eastward displacement of the different stacked bodies, suggest the lateral shifting of the whole system in the same direction, probably owing to tectonic causes.

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