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Abstract: Holocene Reef Development on the Belize Shelf

William F. Precht

Reconstructing the history of Holocene reef development requires an accurate measure of the local relative sea level rise. In Belize, a C14 based sea level curve has been developed using mangrove peats as the sea level datum. By combining this curve with the southward-sloping configuration of the pre-Holocene surface, it is possible to model the progressive flooding of the shelf in time slices and to superimpose the development history of the reefs.

In the southernmost portions of the lagoon, the antecedent underpinnings of the reefs were flooded by 10 ka and aggradational reef growth started to develop about 8 ka. Since 8 ka, the average measured accretion rate for these reefs is less than 1 m/1000 yr. This is more than two times slower than the calibrated sea level during that interval. These reefs fail to grow to sea level and are incipiently drowned. Accordingly, they are classified as "give-up" reefs.

The reef templates of the central shelf lagoon were flooded between 9 and 5 ka and reef growth initiated between 8 and 3 ka. Measured accretion rates for these reefs range between 1.5 and 8 m/1000 yr and reveal an extremely rapid accumulation profile as compared to the coeval sea level history. The internal anatomy and shallowing-upward profile of these reefs indicate a "catch-up" style of growth.

The shallow, northernmost portions of the shelf lagoon were inundated by sea level rise between 4 and 5 ka. The reefs of this area are the youngest in Belize and allow for the investigation of reef inception and development that is concomitant with the final, decelerating stages of Holocene sea level rise. Measured vertical accretion rates for these reefs range between 1 and 3 m/1000 yr. The internal facies of these shallow structures reveal a reefal history that tracks sea level characteristic of "keep-up" type reefs.

In conclusion, the southward sloping ramp-like profile of the Belize lagoon provides examples of backstepping reef deposits that initiated and developed at various stages of the Holocene transgression. This study demonstrates that the rate of sea level rise coupled with shelf position and depth controls the growth history of reefs. The most significant implication of this research details a direct causal relationship between sea level history and reef drowning events in Belize.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994