The interplay between tectonics and eustasy in a modern mixed carbonate-siliciclastic system: Southern Belize lagoon.

摘要

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 ({dollar}sim{dollar}25{dollar}spcirc{dollar} east of north). The folds are oriented between 38{dollar}spcirc{dollar} and 52{dollar}spcirc{dollar} 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 {dollar}sp{lcub}14{rcub}{dollar}C dates indicate all the sediment cored in this study (0-4 m) is Holocene in age (between 4,740 +/{dollar}-{dollar} 60 and 11,230 +/{dollar}-{dollar} 90 {dollar}sp{lcub}14{rcub}{dollar}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.