Abstract
Seismic sequence and seismic facies analysis are conducted on 1031 km of high-resolution single-channel seismic reflection data collected in 1992, and calibrated by 4 long (100's m) core borings, and 34 sediment cores (1-4 m) to delineate the post-Miocene-Pliocene tectonic evolution of the Belize southern lagoon, to document the development of several generations of incised-valley fills, and to determine the late Quaternary evolution of depositional systems and the depositional record of the Holocene transgression.Mapping of the fault and fold patterns indicate that by the end of the Pliocene, the lagoon consisted of a normal-faulted and folded western margin and a less faulted eastern margin. Separating the folded and unfolded strata is a near-linear zone of faults. This zone of faults is termed the transition zone ($\sim$25$\sp\circ$ east of north). The folds are oriented between 38$\sp\circ$ and 52$\sp\circ$ east of north, at an acute angle to the transition zone. A majority of the faults are either aligned with or normal to the fold axes. Many of these structures are truncated by a regional unconformity here termed the Great Belizean Unconformity. This fault and fold pattern is interpreted as a left-lateral wrench fault system, probably active throughout the Pliocene, and possibly linked to the Motagua-Polochic fault zone. Quaternary accommodation space was controlled by topography on the structures related to the wrench fault.Incised valleys on the seismic data are first recognized on the stratigraphic level that might represent deep-sea oxygen isotope stage 10 (400 ka) deposition. These valleys often stack laterally and/or vertically. Faulting frequency decreases towards the top of the strata studied and the amount of seismic facies attributed to carbonate build-ups increases. The oldest valleys mimic structural trends and flow through areas currently occupied by extensive carbonate reefs. Rapid, high-amplitude late Pleistocene sea-level rises may have led to widespread reef development. The reefs created a template which subsequent incised valleys (reoccupation valleys) followed, regardless of structural trends. By about 200 ka, faulting waned and valleys became entrenched between the shore-parallel carbonate barriers. The intimate relationship between sea-level change, incised valley reoccupation, and carbonate reef build-up serves as a model for understanding ancient, tropical mixed carbonate-siliciclastic systems.The Holocene incised-valley fills can be divided into three seismic facies units. These units are, from bottom to top: (a) a basal unit characterized by moderately high-amplitude, chaotic reflections and/or progradational patterns which downlap onto the valley floor; (b) a middle unit characterized by continuous, horizontal, near-transparent reflections which drape the basal unit and onlap the valley walls; and (c) an upper unit which consists predominantly of transparent to low-amplitude, low-angled downlapping reflections. The middle and upper units are separated by an erosional surface. Sediment cores contain carbonate-rich (primarily marine skeletal sands) sediments that often unconformably overlie predominately organic-rich siliciclastic mud. The basal, middle and upper seismic and sedimentary facies units are interpreted as fluvial and carbonate sands, estuarine muds and marine sands and muds, respectively.Accelerator mass spectrometer $\sp $C dates indicate all the sediment cored in this study (0-4 m) is Holocene in age (between 4,740 +/$-$ 60 and 11,230 +/$-$ 90 $\sp $C age), and links the erosional surface between the upper and middle units with the intersection of the regional sea-level curve. It is therefore interpreted as a marine ravinement surface that records the passing of the shoreface.