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Abstract

Recent sediments of the Belize continental shelf are characterized by a transition from detri- tal sediments near the shore to carbonate sediments near the barrier reef at the shelf edge. Studies of the less-than-2µ fraction of these sediments show that the major clay minerals are montmorillonite, kaolinite, and illite; chloritic mixed-layer clay and minor chlorite are also present.

The clay minerals of the shelf sediments are principally detrital in origin. Major differences in parent material and environment of soil formation are reflected in the clay minerals transported by the rivers and deposited on the adjacent shelf. Thus the montmorillonite-rich sediment of the northern shelf is derived from lime-enriched soils of northern Belize and the kaolinite-illite-montmorillonite sediments of the southern shelf reflect the more thoroughly leached soils of southern Belize.

Distribution of montmorillonite, illite, and kaolinite on the shelf is governed by the differential settling tendencies of these minerals, effects of longshore currents, and geographic distribution of shoals on the shelf. The relatively slow-settling montmorillonite is preferentially deposited in deep-water areas offshore. Relatively large concentrations of illite and kaolinite are found in the nearshore area and on the shallow barrier platform near the reef. On the barrier platform, agitation by currents and waves resuspends the sediment and permits montmorillonite to be transported away from the shoal into deeper water. Longshore currents transport montmorillonite-rich sediment southward from the northern shelf, creating a tongue of montmorillonite-rich sediment opposite rivers carrying sediment from an environment of kaolinite-rich, acid-leached soils.

Kaolinite is preferentially deposited in the nearshore area, although kaolinite and illite have approximately the same settling velocities. The formation of slower settling illite-montmorillonite settling entities may cause the preferential transport of illite farther from shore. Chlorite and chloritic mixed-layer clay are present in the sediments on and near the barrier platform, but are not found in sediments on other parts of the shelf or in the river sediments analyzed. The chlorite may be forming in situ by diagenesis of montmor-illonite, or it may be brought to the area by ocean currents.

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