SEDIMENTARY INFILLING OF AN OPEN SEAWAY - BAWIHKA CHANNEL, NICARAGUANRISE

Recent oceanographic cruises to Bawihka Channel have revealed the domi nant sedimentary products along and within this open seaway separating two carbonate platforms and the mechanisms and style of infilling. Th is feature is a seaway 125 km long and 8 km wide through which pass st rong currents associated with the regional western-boundary-current sy stem. This seaway is in a tropical setting near a tectonically active plate boundary making it a suitable, comparative counterpart to the mu ch better known and well-studied seaways to the north associated with the Bahama Banks. Multichannel seismic data indicate that the long and narrow Bawihka Channel evolved from a broad, open basin 50 km wide in the Eocene by basin-margin aggradation and progradation. Basin infill ing was asymmetric, with the western margin becoming stabilized and ag grading by the middle Miocene while progradation to the west along the eastern margin was accelerated. Strengthening of the western boundary current during this time via tectonic foundering of the Nicaraguan Ri se megabank, as well as by stronger easterly trade winds due to climat ic deterioration, stimulated this style of basin-margin development an d led to narrowing of the seaway. High-resolution seismic data reveal that more recent infilling has been dominated by debris-flow deposits, turbidites, large displaced blocks, and widespread algal-bioherm grow th. These sediment-gravity-flow deposits, particularly the debris flow s and displaced blocks, may be a result of the active tectonic setting of the northern Nicaraguan Rise. The distribution of sediment texture s and constituents as well as the widespread presence of well-cemented bioherms and hardbottoms results from strong currents flowing through the seaway. Shallow-water surface sediments are dominated by coarse H alimeda fragments, while base-of-slope and channel-axis sediments are finer grained and contain higher concentrations of planktonic forams a nd pteropods. Piston-core data indicate a late Pleistocene or early Ho locene phase of turbidite infill followed by a phase of pelagic sedime ntation. The pelagic-infill phase resulted from decreased turbidite fr equency and lack of shallow-water sediment production on the bank tops during early sea-level flooding. Midslope mounds, interpreted to be a lgal bioherms, and associated midslope basins also retard downslope mo vement of shallow-water sediments. The infilling of Bawihka Channel is significant in that it reveals how rapid and under what conditions tw o shallow-water carbonate platforms might coalesce into a larger bank. It also reveals the potential complexity of sedimentary facies and st ratigraphy that may lie beneath the interiors of large carbonate banks .