--> Sedimentation in Structurally-Confined Narrow Basins: Toward the Identification of Uncommon Depositional Systems

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Sedimentation in Structurally-Confined Narrow Basins: Toward the Identification of Uncommon Depositional Systems

Abstract

Structurally controlled mini-basins, block-faulted compartments or narrow marine passageways can generate unpredictable sediment routings in association with variable accommodation as they fill. In such settings both oceanographic forcing (e.g., tides) and morphological constraints can result in sedimentation that departs from classical models. Consequently, analysis of such settings in the subsurface is often problematic. The resultant uncertainties can, however, be reduced by comparison with appropriate outcrop analogues. Here, we review three Neogene-to-Quaternary successions of southern Italy, to assess how marginal- to deep-water sedimentary systems change their depositional features within structurally-confined, narrow basins. The first example covers a number of structurally confined tidal straits, developed after Plio-Quaternary marine inundation of a fragmented orogenic arc. These narrow passageways linked two adjacent wider basins, generating convergent, axially flowing tidal currents, and the associated accumulation of thick cross-stratified tidal sandbodies. Tidal hydrodynamics also controlled the progradation of marginal fan deltas, whose delta-front deposits accumulated with elongate morphologies following flow-deflection, to produce uncommon deltaic facies stackings and depositional architectures. The second case study is of a middle Miocene sand-prone turbiditic complex filling a narrow elongate (thrust-top) deep-marine basin. Superbly exposed, 100s-m-thick channelized sandbodies transit laterally to thinly bedded heterolithic deposits, and show how the vertical-lateral facies stacking in a deep-marine fan system may show stratal patterns and depositional architectures that relate to lateral confinement. The final (Pliocene) analogue is located at an orogenic front, where structural culminations produced a series of elongate shallow-marine basins and the progradation of mixed siliciclastic-carbonate coastal wedges some 30-40-m-thick. A number of depositional units, accumulated in shoreface to offshore marine environments, where the waves and currents combined to produce mud-free deposits. Undulations of the structural front produced intermittent low accommodation depocenters, in which coastal wedges rapidly pinched out laterally onto the adjacent structural high. The resulting shallow-marine systems thus lacked both clinoformal depositional geometries and fine-grained distal deposits.