HOLOCENE SEDIMENT FILL IN RHOMBOID REEF LAGOONS AND ENGLISH CAYE CHANNEL, MIXED CARBONATE SILICICLASTIC CENTRAL CONTINENTAL MARGIN IN BELIZE

The main purpose of this research program focuses on analyzing and understanding the Holocene sediment fill of the Rhomboid Reef lagoons, English Caye Channel, and some incised valleys in the Belize Central Shelf Lagoon. Contrarily to the incised valleys within the Belize Central Shelf lagoon which are fully buried by Holocene sediment, the Rhomboid Reef lagoons and English Caye Channel have remained underfilled. Being less than 2 km² in area and because of their close proximity of siliciclastic sources, one would expect the lagoons of Channel and Elbow Rhomboid Cayes to be completely filled, however they are not. A question that begs to be addressed is simply why one observes these underfilled lagoons and channel.

9 vibrocores and 2 gravity cores were retrieved from the Belize Central Shelf Lagoon. Since early flooding, the Holocene sediment fills of the Rhomboid Reef lagoons and English Caye Channel, in particular the variations through time of neritic and pelagic carbonates, in addition to siliciclastics, archive a transgressive record of the surrounding coralgal rim evolution and influx of terrigenous sediment. A time frame was established based upon radiocarbon analyses of different core horizons containing intact non-transported macro fossils, assemblages of micro fossils (benthic and planktic foraminifera), peat, and organic rich material. Faunal assemblage analyses provide important information on depositional environments such as salinity (open marine, versus brackish, or even freshwater).

The underfilled lagoons of the Rhomboids Reefs and English Caye Channel can be explained by limited export of the neritic carbonates produced along the rims and back barrier edges toward the lagoons and channel. Although the siliciclastic sources appear to be in close proximity of the Rhomboid Reefs and English Channel, longshore currents apparently have kept the siliciclastics close to the coastline, therefore limiting their fluxes towards the atoll lagoons and English Caye Channel. Moreover, the neritic carbonate exports towards the lagoons and channel might have been significant, however the exported unstable carbonates were perhaps dissolved in the deep lagoons of the Rhomboid Reefs because of the lack of ventilation and underlying peat oxidation and transported away by tidal current in English Caye Channel.