Sediments, facies tracts, and variations in sedimentation rates of Holocene platform carbonate sediments and associated deposits, northern Belize - Implications for "representative" sedimentation rates

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 km(2) 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 C-14 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 242460 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 (R-2 = 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 sealevel 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.