Sedimentary architecture and genesis of Holocene shallow-water mud-mounds, northern Belize



Abstract Cangrejo and Bulkhead Shoals are areally extensive, Holocene biodetrital mud-mounds in northern Belize. They encompass areas of 20 km2 and 35 km2 in distal and proximal positions, respectively, on a wide and shallow-water, microtidal carbonate shelf where storms are the major process affecting sediment dynamics. Sediments at each mound are primarily biodetrital and comprise part of a eustatically forced, dominantly subtidal cycle with a recognizable deepening-upward transgressive systems tract, condensed section and shallowing-upward highstand systems tract. Antecedent topographic relief on Pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7·6 m thick at Cangrejo and 4·5 m thick at Bulkhead. Despite differences in energy levels and location, facies and internal sedimentological architectures of the mud-mounds are similar. On top of Pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels. Deposition of these basal transgressive, premound facies tracked the rapid rate of sea-level rise from about 6400–6500 years BP to 4500 years BP, and the thin basal sedimentation unit of the overlying mound-core appears to be a condensed section. Following this, the thick and complex facies mosaic comprising mound-cores represents highstand systems tract sediments deposited in the last

≈ 4500 years during slow and decelerating sea-level rise. Within these sections, there is an early phase of progradationally offlapping catch-up deposition and a later (and current) phase of aggradational keep-up deposition. The mound-cores comprise stacked storm-deposited autogenic sedimentation units, the upper bounding surfaces of which are mostly eroded former sediment–water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with Thalassia-colonized surfaces. Vertical stacking of these units imparts a quasi-cyclic architecture to the section that superficially mimics metre-scale parasequences in ancient rocks. The locations of the mud-mounds and the tidal channels transecting them have apparently been stable over the last 50 years. Characteristics that might distinguish these mud-mounds and those mudbanks deposited in more restricted settings such as Florida Bay are their broad areal extent, high proportion of sand-size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas.