Influence of shallow habitats on the development of a mixed siliciclastic-carbonate depositional system in a semi-arid coast

Tropical semi‑arid shelves commonly sustain shallow‑water carbonate factories that export bioclasts to adjacent coasts; however, their interactions with relict and modern siliciclastics remain under‑constrained. We investigate a mixed siliciclastic–carbonate system along the Ceará coast (Brazil) by integrating sedimentology, XRF geochemistry, and remote‑sensing mapping across 48 sediment samples spanning inner shelf to coastal environments (barriers, beaches, dunes, lake/lagoonal settings). Inner‑shelf substrates are bioclastic‑rich, sourced primarily from calcareous algae, benthic foraminifera, and molluscs, with mud‑rich seagrass meadows acting as sediment traps. Siliciclastic grains tend to dominate most coastal settings; however, the medium‑sand fractions in some beaches and barriers are bioclastic‑rich, especially where the shelf slope decreases (< 10 m water depth) near the Itapagé Inflexion. A gentle slope, high shallow-water carbonate production, alongshore drift, cross-shore exchange, aeolian sediment transport, and low siliciclastic input from fluvial sources is interpreted to favor accumulation of mixed siliciclastic-carbonates in this semi-arid coastal study area. These results highlight the importance of understanding local controlling factors when developing models of mixed siliciclastic–carbonate sedimentation on semi-arid tropical coasts, rather than relying on a universal model. Future work should quantify bioclastic production and coastal storage, to constrain sediment interaction between highstand coastal zones and the inner shelf.