Sedimentological and biofabric patterns for hybrid coquina deposits : Insights from wave tank experiments

Abstract

Hybrid bioclastic/siliciclastic coquinas are sedimentary accumulations typical of coastal- and shallow-water settings, where wave breaking, longshore drift and wave shoaling drive sediment transport and control the final accumulations. These processes resulted in thick packages of sediments, as identified in the Lower Cretaceous depositional record of Brazilian marginal basins. In this context, 3D physical modelling experiments were performed in a wave tank using a mixed sediment composition to reproduce the coastal processes at a small scale to verify their control on the sedimentological and taphonomic (biofabric) pattern formation in hybrid coquinas. Several parameters, including composition distribution, grain size, sorting, packing, shell orientation, concavity position and porosity, and the morphological evolution of a simulated wave-built deposit, were evaluated. Five morphodepositional units showing morphologies similar to those of typical beach systems were identified in the physical model: (i) washover and (ii) double shore ridges, forming an elongated spit at sub-aerial zones; (iii) an inner bar; (iv) an outer bar; and (v) a lower shoreface in the sub-aqueous domain. The shell-rich shore ridges presented larger grain size and higher sorting values than the siliciclastic sand-rich outer bar. The inner bar and shore ridges in the surf and swash zones, respectively, showed biofabric patterns with well-developed imbrication of concordant/oblique and concave-down shells. The inner bar presented clusters of imbricated oblique shells that were related to the migration of transversal bedforms in the longshore direction. Conversely, the washover deposited by the overwash processes and the outer bar accumulation positioned below the wave-breaking point exhibited random biofabric patterns. Experimental results reveal a direct correlation between (i) the sedimentological and biofabric patterns and (ii) the wave-driven (cross-shore) and longshore drift processes. Furthermore, a comparison was made between the morpho-depositional units and the ancient coquina facies to refine the interpretation of the beach palaeoenvironment.