The Holocene shallowing-upward parasequence of north-west Andros Island, Bahamas

Abstract

Many pre-Mesozoic records of Earth history are derived from shallow water carbonates deposited on continental shelves. While these carbonates contain geochemical proxy records of climate change, it is the stratal architecture of layered carbonate units that often is used to build age models based on the idea that periodic astronomical forcing of sea-level controls the layering. Reliable age models are crucial to any interpretation of rates and durations of environmental change, but the physical processes that actually control this stratal architecture in shallow water carbonates are controversial. In particular, are upward-shallowing stacks of carbonate beds bounded by flooding surfaces ('parasequences') truly a record of relative sea-level change? The purpose of this study is to examine a tidal flat that is actively accumulating carbonate stratigraphy, and to determine the relative importance of tidal channel migration (poorly known, but investigated here) and Holocene sea-level rise (well-known) in controlling post-glacial parasequence architecture. This work represents a field study of peritidal carbonate accumulation at Triple Goose Creek, north-west Andros Island. By integrating surface facies maps with differential global positioning system topographic surveys, a quantitative relationship between facies and elevation is derived. Sedimentary facies are sensitive to elevation changes as small as 5 cm, and are responding to both internal (distance to nearest tidal channel) and external (sea-level rise) controls. The surface maps also are integrated with 187 sediment cores that each span the entire Holocene succession. While flooding of the Triple Goose Creek area should have occurred by ca 4500 years ago, preservation of Holocene sediment did not begin until 1200 years ago. The tidal channels are shown to be stationary, or to migrate sluggishly at up to 6 cm per year. Therefore, while the location of tidal channels is responsible for the modern mosaic of surface facies, these facies and the channels that control them have not migrated substantially during the ca 1200 years of sediment accumulation at Triple Goose Creek. Once the region was channellized, vertical and lateral shifts in facies, such as the landward retreating shoreline, expanding mangrove ponds and seaward advancing inland algal marsh, are driven by changes in relative sea-level and sediment supply, not migrating channels. While stratigraphic columns look different depending on the distance to the nearest tidal channel, the overall parasequence architecture everywhere at Triple Goose Creek records an upward-shallowing trend controlled by the infilling of accommodation space generated by post-glacial sea-level rise.

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