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The R/V Lone Star in Belize: A Strong Showcase of Siliciclastic-Carbonate Co-habitation

Abstract

Since the sixties, the modern Belize central continental margin has been recognized has a clear case of juxtaposed siliciclastic coastline with, offshore, a well developed barrier and back barrier reef. Later studies considered that siliciclastics only played a secondary role in the Plio-Quaternary evolution of the Belize margin. Results of the seismic surveys on the R/V Lone Star in the nineties initiated the model that the Belize central margin was essentially siliciclastic with only a relatively thin carbonate cover. English Caye Channel, an impressive unfilled incised valley crossing the modern Belize barrier reef, triggered enough curiosity for John Anderson to agree to sail the R/V Lone Star to Belize. High resolution seismic acquisition was conducted in the Belize Shelf Lagoon and offshore Turneffe and Gladden basins. Two well-developed and organized lowstand incised valley systems were imaged behind the barrier reef, split by a divide at the latitude of Dangriga; one system was draining towards the south to merge into the channels separating the Rhomboid Reefs, whereas the second one was draining north feeding into English Caye Channel with, at its mouth, a well developed lowstand shelf edge delta. In merging the R/V Lone Start seismics with industry seismic grids, and integrating a R/V Marion Dufresne 800ky-long record from Gladden Basin, it became obvious that the Belize Barrier Reef was sitting on top of an early/middle Quaternary essentially siliciclastic margin. Starting at about 2.7 my ago, a significant, long-term sea level fall produced a downward shift of the eastward siliciclastic progradation along the eastern flank of Camels Hump. The early/middle Quaternary coastal and fluvio deltaic deposits were redistributed by alongshore currents into beach ridges. Further west, a fluvial plain formed throughout the area of the present shelf lagoon. During several relatively brief sea level rise intervals in the late Quaternary, in particular an initial and very marked mid-Brunhes (~0.45 my) reflooding event, the former lowstand shoreline shifted a considerable distance to the west, whereby siliciclastic sediments remained confined along the newly coast by alongshore currents, promoting offshore the establishment of the Central Belize Barrier Reef and the Rhomboid Reefs, through the initial growth of coralgal reef accumulation over preserved lowstand topographic positive reliefs such as paleo fluvio deltaic highs, channel levees, and beach ridges.