LATERAL AND VERTICAL VARIATIONS IN SEDIMENTATION RATES OF HOLOCENE SUBTIDAL PLATFORM CARBONATE SEDIMENTS, BELIZE

Sedimentation rates are critical in stratigraphic studies for converting stratal thickness into depositional duration, and typically are assumed to be constant for meter-scale beds of similar lithology. Rates of sedimentation of Holocene, shallow-subtidal carbonate sediments from within a 700 km² area of Chetumal Bay, Belize, were evaluated to test this assumption based on thickness data from 348 locations. Rates were determined on the basis of C¹⁴ age dates of basal deposits overlying Pleistocene bedrock limestone and reference to a published sea-level curve for the area. Sedimentation rate of the total Holocene section varies spatially in the bay from 0-118 cm/ky and averages 32±27 cm/ky. Rates are high in depocenters of thick mud-mound deposits (42±31 cm/ky at Cangrejo Shoals and 41±17 cm/ky at Bulkhead Shoals), and are considerably lower elsewhere (15±14 cm/ky). Sedimentation rates correlate well with bedrock depth (R²=0.75) and poorly with water depth (R²=0.06). Vertical variations in sedimentation rate within the Holocene section also are indicated. The highest rates are in early-transgressive mangrove peats (214-938 cm/ky) and the lowest rates in overlying or laterally correlative early-transgressive carbonate sediments (20-48 cm/ky). Rates of combined late-transgressive and early highstand carbonates are 112-166 cm/ky, and 242-460 cm/ky for highstand deposits alone. These trends correspond to transgressive catch-up and highstand keep-up sedimentation during Holocene sea-level rise. Sedimentation rate correlates poorly with sediment thickness (R²=0.003) but exponentially with duration (R²=0.63, 28 data points). Results suggest that considerable variations in sedimentation rate occur both vertically within the section and spatially across this shallow-water shelf during a single cycle of sea-level rise. Variations are controlled mainly by interaction among bedrock topography, mechanisms of sediment redistribution and accumulation, and sea-level rise history. Hence, the assumption of constant sedimentation rate should be used cautiously in interpreting depositional duration from stratal thickness for ancient meter-scale platform carbonate cycles.