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Título

SEDIMENTOLOGICAL AND BIOFABRIC PATTERNS FOR HYBRID COQUINA DEPOSITS: INSIGHTS FROM 3D FLUME EXPERIMENTS

Texto do resumo

Hybrid bioclastic/siliciclastic coquinas are sedimentary accumulations typical from coastal/shallow water settings where wave breaking, wave-induced currents (alongshore drift) and wave shoaling (non-breaking waves zone) drive the sediment transport and control the final accumulation. These processes have generated thick packages of sediments identified in the records from the Early Cretaceous depositional record of Brazilian marginal basins, such as in Morro do Chaves formation (Sergipe/Alagoas Basin), Coqueiros formation (Campos Basin) and Itapema formation (Santos basin). In this context, 3D physical modelling experiments were performed in a wave tank using a mixed sediment composition to better understand the interaction of coastal processes with sedimentological and taphonomic (biofabric) patterns of hybrid coquinas. Several parameters, including composition distribution, grain size, sorting, packing, shell orientation (in sections), concavity position and porosity, and the morphological evolution of the simulated wave-built deposit were evaluated. Five morpho-depositional units similar to typical beach system morphologies were identified on the physical model: washover and double shore ridges, forming an elongated spit at sub-aerial zones; an inner bar, an outer bar and a lower shoreface at the sub-aqueous domain. The shell-rich shore ridges presented higher grain size and sorting values, whereas the siliciclastic sand-rich outer bar had lower ones. The inner bar and shore ridges that are placed at the surf and swash zones, respectively, showed a biofabric 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 to the alongshore direction. Conversely, the washover deposited by overwash processes and the outer bar placed under the line of wave collapse had random biofabric patterns. The experimental results suggest a straight relation between the sedimentological and biofabric patterns of the simulated hybrid coquina deposit and the wave (cross-shore) and alongshore drift processes. Furthermore, a comparison was made between the morpho-depositional units and the ancient coquina facies to refine the interpretation of the beach palaeo-eoenvironment.

Palavras Chave

hybrid coquinas; bio-sedimentology; biofabric; shallow water physical modelling; beach system