Belize Shelf Research Articles

Mixed carbonates and siliciclastics in the Quaternary of southern Belize: Pleistocene turning points in reef development controlled by sea-level change

The Belize barrier and atoll reefs represent one of the largest reef structures in the Atlantic Ocean. The southern shelf of Belize is a classic location of a modern mixed carbonate–siliciclastic system. Whereas knowledge of the Holocene deposits in the area is extensive, data on the Pleistocene system are fragmentary. Open questions include: (i) the nature of the reef foundations (carbonate versus siliciclastics); (ii) the ages of the deposits including the initiation of the barrier reef; and (iii) the response of the mixed system to sealevel fluctuations. The results of a study of borings on the southern Belize shelf are presented here. Six, up to 105 m long borings were made to better understand the history of this important mixed system. Uranium-series dating in the Pleistocene was not possible because of diagenetic alteration; however, lithostratigraphy, strontium isotopes and calcareous nannofossil biostratigraphy were used to constrain stratigraphic ages. Results support the contention that the Quaternary development in Belize was quite similar to that of other major barrier reefs such as the Florida Reef Tract and, further afield, the Great Barrier and the New Caledonian Barrier Reefs. All of these barrier reefs are mixed carbonate–siliciclastic systems and significant reef growth only began after the onset of high-amplitude, eccentricity-controlled sea-level changes and as late as during the exceptionally long and warm marine isotope stage 11, some 400 ka. In Belize, Early Pleistocene sections at bases of borings include mollusc-rich wackestones, rare coral packstones and marls, which were deposited under low to moderate energy conditions in a ramp setting before ca 900 ka, during the high sea-levels of marine isotope stage 25 and possibly earlier (marine isotope stage 31 or 37). The Belize shelf was subaerially exposed for most of the mid-Pleistocene and was dominated by siliciclastic sedimentation, possibly during marine isotope stages 24 to 12 when highstands were comparatively low. Continuous reefs at the shelf margin were developing during highstands. In the Late Pleistocene, beginning with the long and high highstand of marine isotope stage 11 (some 400 ka), the southern shelf was flooded entirely and carbonates started to dominate once more. Reefs developed on top of siliciclastic deposits on the shelf. A continuous barrier reef came into existence and largely developed on top of carbonates at the shelf margin. During Late Pleistocene lowstands, siliciclastics presumably no longer reached the shelf margin because of the topographic high of the barrier reef platform. The Quaternary Belize example may serve as a model for reconstructing ancient mixed systems in icehouse worlds, however, any extrapolations are limited by the fact that fast-growing Scleractinian reef-builders had not yet evolved in the Palaeozoic.

Numerical study of circulation, dispersion, and hydrodynamic connectivity of surface waters on the Belize shelf

A nested grid ocean circulation modeling system is used to examine the circulation, dispersion, and hydrodynamic connectivity of surface waters on the Belizean shelf. The nested grid system consists of a coarse‐resolution (∼19 km) outer model of the western Caribbean Sea, an intermediate‐resolution (∼6 km) middle model of the southern Meso‐American Barrier Reef System (MBRS), and a fine‐resolution (∼2 km) inner model of the Belizean shelf. The nested system is forced by climatological monthly mean surface forcing and integrated over 5 years. The near‐surface circulation on the Belize shelf produced by the inner model is characterized by a strong and persistent northwestward flow as a direct influence of the Caribbean Current on the northwestern shelf and a weak and spatially variable flow on the inner and southern shelf. The monthly mean model currents are used to calculate retention and dispersion of conservative, near‐surface particles carried by the ocean currents. The near‐surface dispersion is relatively higher in areas seaward (east) of Lighthouse and Glovers Reef atolls and lower on the inner shelf, particularly within the Inner Channel and in the vicinity of South Water Cay. To examine hydrodynamic connectivity of reefs in the surface waters of the Belize shelf, we calculate upstream and downstream retention areas for coral reefs at Turneffe Islands and Glovers Reef atolls. The potential sources of passive, near‐surface particle supply reaching these two reef atolls within 30 days include both the shallow waters surrounding the two sites, the deep waters between them, and the coastal waters of the Bay Islands (Honduras). The 30‐day downstream retention areas of the Turneffe and Glovers Reef atolls cover the central and southern Belize shelf, respectively.

Simulating the upper ocean circulation on the belize shelf: An application of a triply nested-grid ocean circulation model

We present a three-level nested-grid ocean circulation modeling system for the Belize shelf of the western Caribbean Sea. The nested-grid system has three subcomponents: a coarse-resolution outer model of the western Caribbean Sea; an intermediate-resolution middle model of the southern Meso-American Barrier Reef System; and a fine-resolution inner model of the Belize shelf. The two-way nesting technique based on the semi-prognostic method is used to exchange information between the three subcomponents. We discuss two applications of the nested-grid system in this study. In the first application we simulate the seasonal mean circulation in the region, with the nested system forced by monthly mean surface fluxes and boundary forcing. The model results reproduce the general circulation features on the western Caribbean Sea and meso-scale circulation features on the Belize shelf. In the second application, we simulate the storm-induced circulation during Hurricane Mitch in 1998, with the nested-grid system forced by the combination of monthly mean forcing and idealized wind stress associated with the storm. The model results demonstrate that the storm-induced currents transport a large amount of estuarine waters from coastal regions of Honduras and Guatemala to offshore reef atolls.

Historical Climate Variation in Belize (Central America) as Recorded in Scleractinian Coral Skeletons

Abstract Three cores from the scleractinian coral Montastraea faveolata and one core from the scleractinian coral Siderastrea siderea from the Belize barrier and atoll reef complex, Central America, were analyzed with regard to sclerochronology (skeletal extension rates) and stable isotope geochemistry (δ18O and δ13C). The core material covers the time span from 1815 to 2000. The four coral time series were compared with available instrumental climate data, such as sea surface temperatures (GISST), cloud cover (GHCN), and precipitation (COADS). Skeletal extension rates measured in the cores average 8.5–14 mm/year in M. faveolata and 8 mm/year in S. siderea. No systematic correlations between extension rates and instrumental climate data were detected. Annual variation in oxygen isotopes is 0.6– 0.8‰, which accounts for monthly averages of sea-surface water temperature fluctuations of 3–4°C. In three cores from the Belize shelf and barrier reef, negative correlations of δ18O with the GISST 2.3b data were o...

Population recovery and differential heat shock protein expression for the corals Agaricia agaricites and A. tenuifolia in Belize

Over recent decades, coral reefs worldwide have experienced severe sea-surface temperature (SST) anomalies. Associated with an El Nino-Southern Oscillation (ENSO) event of 1997-1998, nearly 100% mortality of the space-dominant coral Agaricia tenuifolia was reported at several shelf lagoonal sites of the Belize barrier reef system; a less abundant congener, A. agaricites, had lower mortality rates. We assessed A. agaricites and A. tenuifolia populations at coral reef ridges in the south-central sector of the Belize shelf lagoon and forereef sites to document recovery following the 1998 ENSO event and subsequent passage of Hurricane Mitch. To investigate the difference in heat stress tolerance between the 2 species, heat shock protein (HSP) expression was examined in the laboratory under ambient (28°C) and elevated (+6°C) temperatures. Populations of A. agaricites and A. tenuifolia surveyed at forereef sites in 1999 showed after effects from the 2 disturbances (partial colony mortality was ∼23 and 30% for A. agaricites and A. tenuifolia, respectively), but partial mortality declined by 2001. At reef ridge sites, A. tenuifolia exhibited 75 to 95 % partial colony mortality in 1999 compared to 18 % in the less abundant A. agaricites. We measured a significant increase in percentage live cover at ridge sites for both Agaricia species from 1999 to 2001, except at Tunicate Ridge; at this site, which has restricted water flow, live A. tenuifolia cover remained low (∼10 %) 3.5 yr after the 1998 warming event, due in part to high sponge cover (> 75 %). Immunoblotting results indicated that A. agaricites had twice as much HSC 70 (16.9 μg cm -2 ) as A. tenuifolia (8.7 pg cm -2 ) at ambient temperatures and 6x as much under the +6°C treatment. In addition to the inducible response by A. agaricites, this species expressed HSP 90, whereas A. tenuifolia did not. The distinctive patterns of population recovery and HSP expression suggest that A. tenuifolia has a lesser ability to produce HSPs for protection against environmental stress than A. agaricites. Such differences in resilience to large-scale environmental disturbances such as intermittent ENSO episodes may drive a dramatic change in coral species abundance patterns.

The Belize margin revisited: 1. Holocene marine facies

Belize is best known for its 260-km-long barrier reef and associated lagoon reef shoals, but also exhibits a complete transition from near-shore siliciclastics to pure carbonate deposits across a narrow shelf lagoon and, in addition, has three of the very few occurrences of Caribbean atolls. Published Holocene facies relationships on the Belize shelf have been semi-quantitative in that they are based on quantitative point count thin section data in the northern shelf lagoon and on qualitative estimates of thin section composition in the southern shelf lagoon. This contrast has been rectified by point counting southern shelf lagoon thin sections and incorporating the result with published point count data from the northern shelf, supplemented by additional information, to produce a modified factor analysis facies distribution for the entire shelf. The resulting nine facies are extended seaward to include previously published analysis of sediment on the three Belize off-shelf atolls, resulting in a total of 11 facies. In addition, the carbonate mud distribution has been mapped over both the shelf and atolls with the not unexpected result that, in places, the distribution pattern clearly indicates a barrier platform source of lagoon-deposited carbonate mud. Dunham's limestone classification terminology has been used as a basis for description to make these relationships more useful to those reconstructing ancient limestone depositional environments.

Coral diversity and mode of growth of lateral expansion patch reefs at Mexico rocks, northern Belize shelf, central America

Mexico Rocks is a large patch reef complex on the outer shelf of northern Belize, to the lee of the barrier reef. The complex consists of approximately 100 patch reefs, clustered on a topographic ridge of Pleistocene limestone, and is composed predominantly (83%) of head corals (Montastrea annularis). Biotic zonation is not apparent on any of the patch reefs. Concomitant with increasing area of individual reefs is an increase in surficial areas of dead coral, areas degraded by physical and bio-erosion, and cavities. Such areas enhance habitat complexity and provide zooid-free substrates that are colonized rapidly by more cryptic coral species. The result of such processes is an increase in spatial heterogeneity and species richness on larger reefs relative to smaller patch reefs in the complex.

Setting of and Mixed Carbonate-Siliciclastic Facies Associated with Holocene Nearshore Sabellid Worm Reefs, Northern Belize: ABSTRACT

Communities of sabellid worms (Polychaeta) in northern Belize (mouth of Northern River Lagoon) occur as areally discontinuous, unlithified patch reefs cresting positive features on an irregular depositional topography of Holocene and older sediments. They are found in nearshore marine, moderate energy, tidally influenced environments (intertidal to 60-cm depths) of normal salinity (36%) adjoining subtidal deposits. These colonies, as much as 30 cm thick, are composed of dense thickets of agglutinating worm tubes (1.0 mm diameter, 3.0 cm long) that trap and bind sand to silt-size bioclastic debris and micrite. Worm population density in these communities averages 30 tubes/cm{sup 2}. The antecedent depositional topography beneath the worm reefs and subjacent to adjoining Holocene deposits is characterized by ridges oriented normal to the present shoreline. These ridges are held up by pre-Holocene (latest Pleistocene ) terrigenous sands of probably fluvial distributary origin, occurring at approximately 2.0 m in the subsurface. The overlying Holocene deposits on these ridges are 2.0 m thick and consist of a general upward-coarsening section of terrigenous sandy, carbonate sandy muds to muddy sands punctuated by mangrove peats and capped by the worm reefs. Low areas on the antecedent topography are presently the sites of deposition of deeper subtidalmore » (1.0 m), muddy carbonate sands with some admixed, reworked terrigenous sand. The Holocene section records drowning due to recent (approximately 6 Ka) transgression of the older, lowstand distributary sands on the northern Belize shelf. This mixed carbonate-siliciclastic section and the worm reefs presently are being reshaped and modified by littoral processes.« less

Carbonate Facies and Landsat Imagery of Shelf Off Belize, Central America: ABSTRACT

A reevaluation of Holocene sediments on the Belize shelf is based on (1) a newly constructed composite of 7 Landsat images, enhanced and registered to form a regional base map, and (2) a Holocene facies map based on a rigorous treatment of compositional and textural parameters for approximately 600 bottom samples. The sediments are mapped in terms usually applied to lithified carbonate rocks, allowing direct comparisons with carbonate facies in the subsurface. By combining Landsat imagery with this facies map, it is possible to point out the following geologic features: (1) major tectonic elements, such as the Maya Mountains, the Yucatan Plateau, several offshore ridges, and 3 large atolls, (2) major physiographic features such as the Belize barrier reef with its reef platform and crest, middle-shelf shoal deposits, middle-shelf patch reefs (including lagoon reefs or rhomboid reefs), (3) Holocene facies patterns with potential reservoir facies of foraminifera-grainstone bars, Halimeda grainstones, and branching-coral, encrusting red-algae boundstones, and (4) nearshore clastics and a sharp transition eastward to carbonate sediments. An understanding of Holocene facies patterns on the Belize shelf is important to the explorationist, because these facies patterns are living examples of exploration fairways and invite comparisons with several petroleum provinces: (1) Cretaceous reefs of Texas, (2) upper Paleozoic skeletal-grainstone bars in west Texas, and (3) Devonian reefs of the Alberta basin. End_of_Article - Last_Page 271------------

Foundations of Quaternary Reefs in South-Central Belize Lagoon, Central America: GEOLOGIC NOTES

Seismic profiles and drilling show that the Quaternary (late Pleistocene and Holocene) lagoon reefs in the south-central Belize Lagoon have grown on topographic highs constructed by fluvial, deltaic, and/or marine processes. The basic pattern of the Quaternary reefs was determined in the early Pleistocene when the entire Belize Shelf was exposed. The levees of the rivers that flowed across the exposed shelf produced topographic highs which became major sites for late Pleistocene coral colonization. The Holocene reefs have grown on these carbonate mounds, atop deltaic sediments deposited during latest Pleistocene to earliest Holocene and on marine sand bars in the near coastal lagoon area. The unique (rhomboidal and long-sinuous) reefs are derived from the underlying early Pleistocene river morphology (Victoria River) rather than from a fault-dominated karst morphology. The swarm of reefs in the coastal lagoon area reflects the differences in their foundations and geologic history. This study supports a model of lagoonal reef formation and/or modification of reef foundation controlled by alternating sea level fluctuation and coastal plain fluvial processes.

POROSITY MODELS AND OIL EXPLORATION OF AMAPÁ CARBONATES, PALEOGENE, FOZ DO AMAZONAS BASIN, OFFSHORE NW BRAZIL

The extensive, thick Paleogene carbonate platform (Amapá Formation) of the Foz do Amazonas Basin, probably the largest coralgal‐foraminiferal platform of the geological record, developed through four depositional cycles expressing global eustatic changes of sea level. It shows five environmental belts: slope, apron, coralgal platform, large foraminifer shoal, finger coral bank and restricted lagoon, all of which associate bioaccumulated and bioclastic microfacies. At all times, a terrigenous environment consisting of fan deltas and lagoonal fine‐grained clastics existed immediately behind the carbonate platform and was connected with the open ocean by transverse troughs filled with shales and carbonate olistoliths. This combination of clastic and carbonate environments is unique in the geological record, and today only the Belize Shelf displays comparable conditions.The patterns of distribution of porous carbonates are different for each depositional cycle and the percentage of porous carbonates increases from cycle II to cycle IV. This indicates distinct and successive reservoir conditions generated during episodes of subaerial exposure of an expanding platform by large‐scale circulation systems at the end of each depositional cycle when high‐stand sea level changed to low‐stand. Porosity is mainly enlarged interparticle and moldic with a minor contribution of intercrystalline type related to dolomitization by mixing freshwater‐marine waters. It is not known to what extent the terrigenous environment contributed to the meteorically supplied groundwater system and related dolomitization, but the scale of the circulation is comparable to that presently displayed by Florida and Yucatan.The possibility of hydrocarbon flushing from such carbonate reservoirs is an important factor in the evaluation of the petroleum potential of the Amapá carbonates.

Mangrove swamp stratigraphy and Holocene transgression, Grand Cayman Island, West Indies

The island of Grand Cayman, in the western Caribbean, has an extensive mangrove swamp vegetation. Numerous cores have been taken in and adjacent to these swamplands, and these reveal that the swamps are generally underlain by a transgressive sequence of sediments. The basal unit of this sequence consists of a laminated or non-laminated crust, formed in a subaerial environment. Locally, these crusts are overlain by plastic mud, deposited in seasonally-flooded environments. Mangrove peat forms the upper unit of the transgression within most of the swamplands, overlying the other units where these are found, and reaching thicknesses of more than 4 m. The final stage of marine incursion is recorded to the east of North Sound, where, in water depths of 10–200 cm, shelly mud occurs overlying mangrove peat. In contrast to this, to the west of North Sound, there is a regressive sequence in which shelly marl underlies a localised sea grass peat and mangrove peat. This regressive sequence records local progradation of mangroves into marine environments. The transgressive sedimentary sequence on Grand Cayman is similar to transgressive sedimentary sequences described from the Florida Everglades-mangrove complex, Florida Bay and the Belize Shelf. On Grand Cayman, however, the stratigraphy is less complex because there are not extensive freshwater peat-forming environments, or intertidal and supratidal carbonate environments. Samples of mangrove peat were collected from the peat/bedrock interface along a surveyed profile across Barkers Peninsula on Grand Cayman Radiometric dating of these implies that sea level was approximately 185 cm below the present level at least 2100 years BP. Comparison of these dates with dates on mangrove peat from elsewhere suggests that submergence on Grand Cayman has taken place at a similar rate to that in Florida.

Microfacies and Depositional-Diagenetic Model of Amapa Carbonate Rocks (Paleogene) of Foz do Amazonas Basin, Offshore NE Brazil: ABSTRACT

The extensive, thick Paleogene carbonate platform (Amapa Formation) of the Foz do Amazonas Basin developed next to the ancient shelf edge through four depositional cycles. This important stratigraphic-structural unit of the basin became the natural target of the search for potential hydrocarbon reservoirs. The Amapa carbonate platform shows six environmental belts: slope, apron, coralgal platform, large foraminifer shoal, finger coral bank and restricted lagoon. These belts consist of zones of intense bioaccumulation by red algae and large foraminifers separated by transverse channels where the products of their mechanical reworking accumulated as calcarenites. At all times, a terrigenous environment consisting of fan deltas and lagoonal sediments existed immediately behind the carbonate platform. It was connected with the open ocean by transverse canyons cutting across the carbonate platform and filled with shales containing carbonate olistoliths. Distinct reservoir conditions were generated by underground circulation systems during episodes of subaerial exposure at the end of each depositional cycle when high-stand sea level changed to low-stand conditions. Excellent porosity exists in all microfacies except for the facies of the apron and slope belts, and consists of mainly enlarged interparticle and moldic porosity with a minor contribution of intercrystalline porosity related to dolomitization by mixing of freshwater and marine waters. The Amapa platform is unique in the geologic record and the only modern analog is the Belize shelf. However, similar conditions of underground circulation and dolomitization by mixing waters exist today at a comparable scale in Florida and Yucatan. End_of_Article - Last_Page 687------------