THE STRUCTURAL AND SEDIMENTOLOGICAL CONTROLS ON THE REOCCUPATION OF QUATERNARY INCISED VALLEYS, BELIZE SOUTHERN LAGOON

A grid of high-resolution single-channel seismic data from the Belize southern lagoon has documented the development of several generations of stacked late Pleistocene (400 ka-Holocene) incised-valley fills. A feedback mechanism between tectonics, reefal buildups, and incised-val ley location is apparent in the data. Incised valleys are recognized a s medium- to high-amplitude reflections that truncate reflections prod uced by older strata on the seismic data. The Holocene valley fills in the Belize southern lagoon contain three seismic facies units (from b ottom to top): (1) a basal unit characterized by moderately high-ampli tude, chaotic reflections, or progradational patterns, that downlap on to the valley floor; (2) a middle unit characterized by continuous, ho rizontal, near-transparent reflections that drape the basal unit and o nlap the valley walls; and (3) an upper unit that consists predominant ly of transparent to low-amplitude, low-angle downlapping reflections. The middle and upper units are separated by an erosional surface. Bas ed on core evidence and seismic facies analysis, the basal, middle, an d upper seismic and sedimentary facies units are interpreted as fluvia l and carbonate sands, estuarine muds, and marine sands and muds, resp ectively. The erosional surface separating the upper and middle units is interpreted to be a ravinement surface that marks the transition fr om brackish estuarine conditions to open-marine conditions. Faulting d ecreases and reefal buildups increase upsection. The oldest valleys mi mic structural trends and flow through areas currently occupied by ext ensive carbonate reefs. Rapid, high-amplitude late Pleistocene sea lev el rises may have led to widespread reef development, creating a templ ate that subsequent,incisions (reoccupation valleys) followed, regardl ess of structural trends. The intimate relationship among tectonics, s ea level change, carbonate reef buildups, and incised-valley reoccupat ion serves as a model for understanding ancient, tropical mixed-carbon ate-siliciclastic systems. Accelerator mass spectrometer C-14 dates of sediment core samples (marine gastropods, marine mollusks, barnacles, worm tubes, peat, and oysters) indicate all the sediment cored (1-4 m ) in this study is Holocene in age (between 4740 +/-60 and 11,230 +/-3 0 C-14 age). This radiocarbon dating consistently links the erosional surface between the upper and middle units with the intersection of th e regional sea level curve (nonmarine/marine boundary).