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ABSTRACT: Holocene Facies Relationships in a Mixed Siliciclastic-Carbonate Barrier Complex, Placentia, Belize, Central America

T. H. Wiedemeier, W. E. Full, S. J. Mazzullo

Holocene and Recent sediments in the Placentia region of southern Belize compose a complex of siliciclastic, carbonate, and mixed siliciclastic-carbonate depositional environments. These facies developed in a microtidal, wave-dominated system that is the result of Flandrian transgression.

Eight major facies are recognized in the study area: (1) fluviodeltaic siliciclastic muds and sands deposited along the mainland; (2) a narrow, areally restricted quartz-sand beach tract on the mainland; (3) mangrove swamp peats; (4) restricted lagoonal and (5) lagoonal muds of Placentia Lagoon, both of mixed siliciclastic-carbonate composition; (6) barrier beach and backbeach mixed quartzo-feldspathic and carbonate sands of Placentia spit; (7) patch reef carbonates seaward of Placentia spit; and (8) interior southern Belize shelf back-reef mixed siliciclastic-carbonate muds.

Textural and compositional analyses suggest that the siliciclastic fractions represent three distinct depositional events: (1) initial transportation and deposition of the coarsest fractions in a fluviodeltaic setting during Pleistocene lowstand; (2) a reduction in sediment supply to the system during the ensuing Flandrian transgression, which drowned the deltaic complex, subsequently reworked the sediment, and established local carbonate deposition; and (3) modern siliciclastic sediment being supplied to the area from the north by longshore drift. This three-stage development resulted in the facies geometries observed in the Holocene siliciclastic sediments in the area.

The Holocene carbonate facies were deposited in response to the changing influx of the terrestrial sediment such that areas on the older, drowned deltaic complex contain more carbonate than areas with still active siliciclastic influx and carbonate sands mixed with siliciclastic sediments are presently being deposited in proximity to patch reefs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990