A Plio-Pleistocene eustatic and storm-controlled mixed carbonate–siliciclastic marine ramp deposit in south-west Florida: An example of sediment homogenisation with maintenance of carbonate-producing organisms

Mixed siliciclastic and carbonate sediments are common in the stratigraphic record, but fully homogenised mixes are not. Many occurrences of mixed sediment sequences are dominated by end-members with stacking of ‘nearly pure’ lithfacies (e.g. cyclothems containing alternating sandstone, limestone and coal units). The Plio-Pleistocene sediments within south-west Florida provide insights into the occurrence of fully homogenised siliciclastic/carbonate deposits. In all defined environments from lagoon to supratidal to inner tidal to beach to offshore to coral reef, quartz sand coexists with carbonates. Perhaps the key feature that allowed full homogenisation of the sediments within all facies and subfacies was the relatively shallow water (<10 m), which facilitated mixing during low-order eustatic sea-level events and storms. However, four factors contributed to the full homogenisation of the sediment types without termination or inhibition of carbonate organism growth. These factors are (1) the shallow water allowing wave-driven sediment transport (all environments within the wave orbital depth), (2) close proximity and perhaps irregular nature of the depositional environment boundaries, (3) low influx rate of quartz sand via longshore transport, and (4) the lack of significant terrigenous mud transport into the system. Mixing processes at the large-scale included movement of sediments from one depositional environment to another during storms, mixing along facies boundaries, and in situ mixing within autochthonous and parautochthonous mollusc death assemblages. At the smaller scale, mixing occurred by bioturbation and diagenetic dissolution of carbonate skeletal grains during minor high sea-level stands.