The Belize margin revisited. 2. Origin of Holocene antecedent topography

The importance of antecedent topography in dictating Holocene facies patterns has been generally recognized. There is, however, disagreement as to origin or lithology of the antecedent topography, particularly with respect to the siliciclastic or carbonate nature of the underlying topography and structural patterns. To help resolve these problems, published and unpublished information have been compiled to produce a structural fabric map of onshore and offshore Belize that includes a new geologic map of the country. The map, along with illustrated seismic lines, demonstrates the occurrence of a number of NNE-trending transpressional faults in which landward directed thrusting is consistently displayed along with tectonic inversion. Offshore wells in conjunction with the seismic lines document the inversion as post-Eocene, suggesting a similar age for transpressional fault movement. Presumably, the landward-directed thrusting reflects the opposing force of eastward-directed subduction along the western margin of Central America relative to the westward seafloor spreading of the Caribbean Cayman Ridge. The Belize faults show little current seismicity, but, nevertheless, the resulting structures have affected Quaternary carbonate deposition as evidenced on an illustrated seismic line by both seaward and landward (bi-directional) progradation of the reef margin from an underlying structural high. The structural influence on the development of Holocene antecedent topography is further suggested by the occurrence of a Bouguer gravity plateau in the same shelf area that marks the occurrence of the Belize lagoon rhomboid shelf atolls. The youngest documented lithology of 12 illustrated offshore exploration wells is Miocene carbonate. In some wells, the carbonate is interpreted as extending into the Pliocene and Holocene although no age diagnostic criteria are in evidence. In other wells, siliciclastics of unknown age and thickness are identified as overlying Neogene carbonates. The regional distribution and age of onshore limestones suggests that unroofing of the Cretaceous carbonate cap of the Maya Mountains siliciclastic sediment source did not occur until late in Neogene time, perhaps no earlier than late Pliocene. Consequently, the Maya Mountains could not have been a major offshore source of siliciclastics until the Quaternary. Information on the lithology of the immediately underlying pre-Holocene is provided by limited penetration core data and shallow resolution seismic lines. These show that antecedent Pleistocene limestones beneath the Holocene reefs were deposited around 130,000 B.P. (isotope stage 5e). The thickness of the overlying Holocene, shelf margin, reef-capped carbonates increases along depositional strike from a few meters in the north to more than 25 m in the south. In contrast, piston cores and seismic data from the southern shelf lagoon collectively document the occurrence of antecedent siliciclastic topography. The southward dip of both carbonate and siliciclastic antecedent surfaces is presumably a reflection of increasing southerly subsidence. Increasing antecedent erosional relief in the same direction reflects the pronounced southerly increase in paleo-rainfall that presumably paralleled that of present-day rainfall.