ABSTRACT: Multiple Causality of Drowned and Incipiently Drowned Holocene Reef Complexes from Belize

PRECHT, WILLIAM F., Reef Resources, Miami, FL

Previously undocumented pinnacle reef complexes from southernmost Belize are unique and enigmatic, as they are examples of reefs "caught in the act" of being drowned. It has been documented that growth rates of Holocene reefs should easily keep pace with sea level, yet drowned reefs are common in the geologic record. These reefs, therefore, offer an unprecedented opportunity to evaluate the causality of reef drowning.

All reefs identified grow no higher than 8 m below mean sea level. Branching coral communities are restricted to the rim of the pinnacles with their tops consisting primarily of coralgal skeletal debris. The flanks have living coral communities (platy varieties) to depths of 28 m.

Analyses of these reefs in a sequence stratigraphic framework reveal a complex history, directly related to the progressive flooding of the ramp-style platform. Lowstand siliciclastics were replaced by thin backstepping, transgressive carbonate units. As these carbonates backstepped, bathymetric highs provided the sites for reef initiation. These antecedent templates were flooded no earlier than 10 Ka and aggradational reef growth did not develop until 7-8 Ka. Average accretion rates for these reefs are approximately 1 m/1000 years, which is some two times slower than the calibrated sea level rise, and up to eight times slower than previously documented reefs from Belize.

The ultimate height to which these reefs grow is controlled by the size of the initiating highs, their depth at the time of reef inception, and the relative rate of sea level rise. Other factors such as unfavorable juxtaposition with siliciclastics and the effect of "inimical" waters related to runoff have also led to the demise of these reefs.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)