ABSTRACT

In stratigraphic analysis and simulation, sedimentation rates are typically assumed to be constant for meter-scale sedimentation units of similar lithology. The rates of Holocene, shallow-marine carbonate and associated sediments within an 820 km² area of Chetumal Bay in northern Belize were evaluated to test this assumption. Rates were determined from thickness data from 363 locations, durations derived from ¹⁴C age dates of mangrove peat on Pleistocene bedrock limestone and of overlying cerithid gravels, and reference to a sea-level curve for this area. The rate of entire Holocene sections (basal transgressive mangrove peat, shelly gravel, and overlying carbonate) varies from 0 to 118 cm/ky and averages 32 ± 26 cm/ky. Rates are the highest at two thick mud-mound depocenters (41 ± 27 cm/ky) and considerably lower elsewhere (16 ± 16 cm/ky). In general, sedimentation rate correlates positively with depth of bedrock below sea level. Basal mangrove peats beneath the mud mounds have the highest rates (214-938 cm/ky), whereas overlying to laterally correlative transgressive shelly gravels have the lowest rates (20-48 cm/ky). Rates of combined transgressive and earliest-highstand carbonates, the latter deposited in a catch-up mode, are 112-166 cm/ky, and rates of overlying youngest highstand carbonates deposited in a keep-up mode are 242-460 cm/ky. Sediment thickness may correlate positively with duration but does not correlate with sedimentation rate. A power-law relationship between sedimentation rate and duration (R² = 0.63, 30 data points) is related to the completeness of the Holocene record.

The large vertical and spatial variations in sedimentation rate across this shallow inner shelf during a single phase of sea-level rise are controlled by interactions among bedrock topography, mechanisms of sediment redistribution and accumulation, and rate and magnitude of sea-level rise. The assumption of a constant "representative" sedimentation rate may not be viable in qualitative and quantitative studies of ancient, meter-scale, platform subtidal carbonate units. The thickness of a time-stratigraphic unit is not a faithful proxy for duration of deposition, just the best-available.