Holocene Sediments of the Belize Shelf

Dr. Clif Jordan, 2002

Summary of the geography and Quaternary geology of Belize

Belize, formerly the colony of British Honduras, became independent in 1981, assuming a position in the Commonwealth and enjoying the protection of the United Kingdom. Its mainland is approximately 180 miles long north to south and 20-60 miles wide east to west. Belize is bordered on the north by Mexico and on the west and south by Guatemala, who, to this day, considers Belize a territory of their country.

The long eastern coast of Belize faces the Caribbean Sea and is protected by the second longest barrier reef in the world; in perspective, it is about 10% the length of Australia's Great Barrier Reef. The northern half, of Belize is low-lying, swampy terrain and represents the southern limit of the Yucatan Platform with its characteristic internal drainage and karst topography. The southern half of the country includes the heavily forested Maya Mountains with over 3,600 feet of relief and has a well developed river system. The climate of the country is tropical with an average annual rainfall of 49 inches in the north, 70 inches in Belize City, and over 150 inches near the southern border.

Off the northern coast of Belize, from San Pedro northward to the Mexican border, lies a narrow, shallow-water shelf behind a well-developed curvilinear barrier reef. This shelf is generally a half-mile wide or less and contains numerous small patch reefs, biogenic banks, and a variety of generally high- to moderate-energy sands. It is bounded on the west by a long, low-lying peninsula about 60 miles long which varies from less than one mile to about 20 miles in width. Due to high marine energies, there is little lime mud on this shelf. The peninsula is called "Ambergris Caye" because it is broken by a channel at its northern end, which was excavated by the Mayans and is now maintained by tidal exchange flowing through it. This is where the Belize-Mexican border is situated. The peninsula is underlain by Pleistocene limestone with a thin, discontinuous cover of Holocene sediment, some of which is dolomitic. Behind the peninsula, to the west, is a large, shallow bay, Chetumal Bay, that is about 50 miles long ~ 15 miles wide, connecting to the sea at its southern end. This broad bay is a protected, low-energy depositional environment, floored with lime mud and foraminiferal mixed-skeletal sands, and is a restricted marine environment that does not support normal-marine-salinity organisms, such as corals or echinoids.

South of Chetumal Bay, the shelf off Belize is about 15 miles wide, extending south to the latitude of Placentia; then it widens to around 20 to 25 miles from there southward to Amatique Bay. On the southern shelf of Belize, the barrier reef is a nearly continuous line of reefs near the shelf edge. Shelf waters behind the barrier reef become increasingly deeper from the latitude of Belize City southward, i.e., from about 15 feet off Belize City to over 125 feet off Placentia. Furthermore, the southern shelf is receiving an influx of terrigenous siliciclastics from the eroding crystalline and siliciclastic rocks of the Maya Mountains. These sediments are being swept southward by longshore currents, and they dominate shoreline sedimentation and influence sediment composition for several miles offshore. In contrast, the northern shelf is of low relief and receives little terrigenous siliciclastic input.

Offshore from the Belize Barrier Reef are three large atolls rising out of very deep waters. Northward, along the coast of the Yucatan1 are three more atolls of similar size, about 12 to 25 miles long, by 4 to 12 miles wide. From north to south, these are Arrowsmith Bank, Cozumel, Banco Chinchorro, Turneffe Islands, Lighthouse Reef, and Glovers Reef. The Belize atolls are rimmed by an almost continuous coral reef, but the enclosed lagoons are in various stages of maturity. The atolls vary in form and occur as large single islands (e.g., Cozumel), as a complex of mangrove islands (e.g., Turneffe Islands), or as many small, submerged patch reefs (e.g., Lighthouse Reef). These atolls grew on tilted fault blocks that form subparallel, south-southwest trending ridges. The faults are downthrown to the basin (i.e., to the southeast). The shelf off Belize is also influenced by this structural grain, as well as by east-west structural alignments.

Our field trip consists of 19 scheduled STOPS and an overflight. Before presenting a description of each field STOP, a general overview is in order that treats the geology of Belize, hydrographic conditions on its shelf, and regional mapping of marine sediments. The table below summarizes the main differences between the Northern and the Southern Shelf of Belize.

Northern Shelf Southern Shelf
bathymetry generally shallow shallow to deep
shelf width moderate wide
tectonic activity mild strong
facies patterns transitional facies patterns distinct facies belts
reef development only at outer shelf middle•and outer shelf

Regional geology of Belize
Belize is situated near the southern edge of the North American Plate; the plate boundary between that and the Caribbean Plate to the south is shown in Figure 1 and Figure 2 as an extension of the Cayman Trough which is a spreading center. The southern half of Belize is thus structurally complex compared to the flat-lying sediments of the northern half of the county. Offshore of Belize there are five fault-controlled submarine ridges trending subparallel in a northeast-southwest direction. Ridges 1 through 3 are in shallow water and affect atoll distribution; 4 and 5 are in deep water and are partially buried. Number 5 ridge connects to the northeast with the Cayman Trough.

A generalized geological map of Belize (Figure 2) shows the Maya Mountains to be composed of Triassic granites intruded into Late Paleozoic (Pennsylvanian and Early Permian) siliciclastic sediments, which were subjected to varying degrees of metamorphism. This formed a high that was rimmed on all sides by Cretaceous limestones. Following this, Tertiary clastics were deposited to the south of the high and Tertiary carbonates to the north. Finally, there is the onlapping by Quaternary clastics in southern Belize and Quaternary carbonates in northern Belize. A NW-SE cross-section from the Maya Mountains to the Cayman Trough (Figure 5) shows the effects of structural control to form antecedent topography as foundations for modern reef growth. A regional bathymetric map of Belize shows that the shelf deepens southward. Seismic profiles show that Pleistocene surfaces generally mimic those of Holocene sediments, with the notable exception of paleo-lows being filled in with a relatively greater thickness of modern sediment, all of which creates a smoothing effect on the seafloor surface of today.

The marine setting
Since this field trip deals largely with reefs, a review of reef terminology is timely. Figure 12 shows the main features of the Belize Barrier Reef . The most distinctive features are: 1) that the reef rises right up to sea level, 2) that the active zone of reef growth is on the open seaward, side, 3) that a large apron of reef-derived sediment is developed in back of the reef, and 4) that there is a vertical face to the reef (this is called "the wall" by divers) that bypasses sediment to the forereef slope. Second-order features include reef pavement at the crest of the reef, islands being built from coral rubble and skeletal sands, and spur-and-groove structure. We will see later that patch reefs have their own distinctive profile.

As seen in the bathymetric-perspective diagram of Figure 13, the north-to-south trend of the Belize Barrier Reef coincides with the eastern edge of the Belize Shelf. Note the strange "J" pattern of the barrier reef at its southern end and how barrier reefs surround each of the three offshore atolls.

A map of the main surface currents off Belize (Figure 14) shows that the Belize Shelf seems to be in a cul-de-sac with respect to the main flow of currents northward up along the Mexican coast, flowing past Cuba and out into the Gulf of Mexico. As a result, modern carbonates in Cancun and the nearby islands are mainly dominated by ooids and coated-grains, not by reefs and skeletal grains. In contrast, there are no oolitic sediments in Belize at all.

Wave energies that impinge upon the barrier reef generally originate from the east¬northeast (about 74.5°); however, they change from northeast sources to east sources, from north to south. These waves hit four offshore atolls before coming to the barrier however. Their energies are reduced, and an "energy shadow" is established in the lee of each atoll (Figure 15). It is not certain if this aids or limits barrier reef growth.

Annual rainfall in Belize is about 60 inches (150 em) in the north and increases to over 160 inches (400 em) in the south (Figure 16). One result is that surface waters on the Belize Shelf are brackish nearshore, becoming more saline offshore (Figure 17). On the Southern Shelf, surface waters are fresher than anywhere else on the Belize Shelf. Freshwater runoff, flowing out from Amatique Bay, produces a relatively less dense wedge of water that floats on top of normal-marine sea water (comparing Figure 17 and 18). After heavy rains, this wedge of fresh water has been observed as far out as Laughing Bird Caye.

Sedimentary facies maps
Seafloor sediments have been collected at several hundred locations and analyzed for their texture and composition. These data form the basis for these Holocene sediment maps of Belize (Figure 19 and Figure 20). The maps present data from years of study students at Rice University with a total of some 600 samples. The distribution of carbonate material in sediments is shown in Figure 19 to increase eastward toward the outer-shelf carbonate platform. The sediment facies are differentiated into fourteen types by composition (terrigenous, carbonate, or transitional) and by texture (sands, sandy muds, and muds). Differences between bottom types on the northern and southern shelves apparent, as is the distribution of reefs which is mainly on the open-ocean side of the shelf and the offshore atolls. The final product is a map of bottom types, categorized sedimentologically.

Over 100 samples were collected by the author from areas sampled by previous workers, so that their data could be normalized and mapped uniformly. The map is made in the context of ancient rocks; in other words, nearly 1150 sediment samples were classified as their lithified equivalents. This meant wet-sieving all samples to determine the percent mud, then dissolving that mud in hydrochloric acid to determine percent carbonate versus non-carbonate mud, and making thin-sections of the sand-size fraction for positive grain identification. A special scheme of facies nomenclature was employed to assure regularity in this mapping. The net result is a Holocene map that can be laid down next to Paleozoic or Mesozoic facies maps, with the possibility of meaningful comparisons.

The distribution of molluscan biofacies across the Belize Shelf is shown in Figure 22. One significant observation is that thick-shelled robust forms are usually associated with high-energy regimes, whereas thin-walled delicate "razor clam" shells are from pelecypods that burrow into muddy sediment and are indicative of low-energy environments. Many of us will probably end up collecting specimens of various types of molluscs as we go from field stop to stop on our trip. (see Figure 23)

Our field stops on the Southern Shelf are shown in the LANDSAT image in Figure 24. Today we are essentially making a shelf-to-basin transect across the shelf in the dip direction. We start at the coast north of Placentia and work our way eastward to the reef platform that makes up the outer shelf. The charts of Figure 26 show inner-shelf environments of the Placentia Spit, Placentia Lagoon, and the coastline up to the mouth of South Stann Creek. Sedimentological details of the Southern Shelf Traverse appear in Figure 26, our field stops are indicated in the upper panel.

Stop 1 South Stann Creek Delta
South Stann Creek originates about 11 miles west of the coast in the Cockscomb Basin (a Holocene drainage basin) along the southeastern edge of the Maya Mountains, which rise to an elevation of over 3,000 feet above sea level. As the stream meanders its way to the sea, its banks are stabilized by dense jungle vegetation, and moderately small point bars are formed. At the coast, a small wave-dominated cuspate delta is developed with an arcuate river-mouth bar in front of it; the creek flows around this bar on the south side. Offshore, there are two to three sand bars, the most seaward being about a quarter mile offshore.

Delta sands are moderately sorted, angular, coarse-grained deposits comprised mainly of quartz with minor amounts of granitic rock fragments and potassium feldspar. Offshore from the delta, sands become finer-grained and better sorted, and the amount of rock fragments decreases. These sediments include minor amounts of molluscs, echinoids (sand dollars), and benthonic forams as the only carbonate components.

If preserved in the rock record, coarse-grained, well-sorted sands of this high-energy delta would have good reservoir potential. Assuming progradation of the delta, the sands would be underlain by argillaceous, fossiliferous wackestones of the lagoon and overlain by beach and marsh deposits of the inner shelf.

Stop 2 on the Southern Shelf Traverse
Along this transect we will see the transition from terrigenous clastics to pure carbonates and from moderate-energy of the inner-shelf to high-energy associated with shallow-water shoals and reefs of the outer shelf. We will also note a sharp bathymetric change as we leave the siliciclastics being deposited in the Inner Channel.

Cruising offshore about half a mile from the mouth of South Stann Creek, we encounter nearshore terrigenous facies of sandstone and calcareous siltstone. Coarse-grained, angular quartz grains make up the bulk of the sediment here. Quartzite rock fragments (locally up to 20%) are common; potassium feldspar grains (2%) occur, but are rare. Skeletal grains make up 20% of the sediment, and they are dominated by molluscs. At this stop, note the relative turbidity of the water, the benthic flora and fauna, depositional structures, and the sediment.

We now head progressively eastward toward the shelf-edge, taking bottom samples along the way. We will cross a relatively deep-water channel called by various names, the Inner Channel or the Main Channel. This channel is aligned with the fault block along the eastern side of Ambergris Caye and under Banco Chinchorro off Yucatan. We will take our next bottom sample about 6 to 7 miles from shore in about 50 feet of water. The sediment is argillaceous wackestone (marl) with molluscs and benthonic foraminifera. This is a low-energy deposit, well below wave base and is the home of the protected "black coral".

"Marl" is an old term loosely applied to a variety of materials, mostly unconsolidated, earthy deposits consisting of clay and calcium carbonate. Pettijohn (1957) defined the sediment as ranging from 65% calcium carbonate and 35% percent clay, to 65% clay and 35% calcium carbonate. This latter definition was the one used by Purdy in describing these lagoonal sediments. The sediments are more accurately classified in a modified Dunham classification as an argillaceous molluscan wackestone.

Continuing eastward, we will take a grab sample about 13 to 14 miles from shore and about 5 miles behind the shelf-edge reef. This is about the deepest portion of the shelf waters at this latitude - about 75 feet deep. This is also the thickest accumulation of Holocene lagoonal sediments on this traverse at about 35 feet thick. The bottom sample is a gray-green mixture of terrigenous and lime mud. Grains are sparse and consist of molluscs and foraminifera.

About a mile farther to the east, we abruptly cross a steep escarpment 1finging the bottom up from 85 to about 20 feet. This escarpment forms a trap for the muds of the lagoon with the thickest accumulation just behind the platform. The sediment also changes facies very sharply, perhaps in less than 100 feet, to a Halimeda mixed-skeletal packstone, with a large reduction in terrigenous clay content. Bottom samples collected just off this escarpment indicate very little dispersion of the Halimeda-dominated platform sediments westward into the lagoon. We will take a grab sample a short distance east of the edge of the outer shelf platform. This sample is representative of the deeper parts of the carbonate platform, ranging from 50 to 75 feet of water. The sediment is a Halimeda mixed-skeletal packstone with many fragments of reef organisms. The bottom is covered with Thalassia which is encrusted with the red algae Meiobesia, and other epibionts, such as bryozoans and worm tubes. Turtle grass normally leaves no fossil record, but the presence of such a bottom floral cover may be inferred by the following: 1) the presence of encrusting epibionts that sometimes bear the impression of the Thaiassia leaves on their calcified skeletons, 2) by occasional carbonized leaves in euxinic environments such as a mangrove swamp, and 3) rarely as imprints of sea grass in skeletal wackestones, as in Lower Cretaceous carbonates in central Texas.

Seafloor topography is very irregular on the outer shelf; reefs here have underpinnings of Pleistocene reefs (Figure 27 and Figure 28). Our first reef is an amoeba-shaped patch reef called Boo Bee patch reef.

Stop 3 Boo Bee Patch Reef
This patch reef rises up from about 65 feet of water and grows upward in a haystack shape. The dominant corals are head corals with lesser amounts of platy and branching corals. The surface is encrusted by red algae and makes sediment sampling difficult. Try to scoop up a handful of sediment on the reef top!

Sediment constituents and a profile across the reef are shown in Figure 29. The pattern of sediment dispersion off the reef is radial, producing a typical "halo" of reef-derived sands around the patch reef, a pattern that will be seen repeatedly on our reef overflight coming up.

This patch reef was drilled into by Halley et al. (1976) who found that a Pleistocene reef underlies the Holocene one here. So, this reef, as well as others we will visit, brings up the matter of reef foundations. What caused the reef to grow where it is? Three explanations have been proposed: 1) structural highs (Lara, 1993; Ferro et. ai., 2000), 2) antecedent karst topography (Purdy, 1974), and 3) depositional highs - levees and other relatively high-elevation features of fluvial-deltaic sedimentation (Choi and Ginsburg, 1982).

Stop 4 The Outer-Shelf Carbonate Platform
Our next bottom sample is on the skeletal sand flat behind the barrier reef , where water depths range from 1-15 feet. This facies is a narrow, linear belt of winnowed skeletal sand (a Halimeda mixed-skeletal grainstone). It is entirely sand-sized carbonate, derived from the physical and biological breakdown of the reef. Halimeda plates dominate the grain types, but fragments of coral, red algae, thick -shelled molluscs, and encrusting foraminifera are common. The most common and most easily recognized encrusting foram here is Homotrema rubrurn, a wine-colored coarse sand-size grain that gives a red tint to reef-derived sediment. The sand flat varies from a narrow belt a few hundreds of meters wide to broad flats several kilometers wide behind the reef. These sediments display very high primary intergranular and intragranular porosity and commonly are porous and permeable in the subsurface, forming a major exploration fairway along strike.

The outer shelf at the latitude of the Southern Traverse consists of a well developed carbonate platform. This platform is about five miles wide and is about six miles offshore from the mainland. It has a steep slope on both its seaward and shelfward sides. Water depths are generally less than 20 feet over the platform and about 75 feet in the lagoon behind it. The barrier reef here consists of two outer ridges of coralgal boundstone developed at the deep end of the reef front.

On a depositional profile like this, the reef and near backreef are topographically higher than the broad lagoon behind the outer shelf platform and, with a lowering of sealevel, are much more likely to be exposed later to vadose diagenesis, leaching, and/or cementation than lagoonal sediments.

The last sample of our traverse is immediately behind the barrier reef, on a broad apron of coarse-grained, gravel-rich, Halimeda mixed-skeletal grainstones. These are reef-derived sands, pushed shoreward by pounding surf and high waves hitting the reefs, waves with a wind-fetch as far as Africa. It is interesting in this high-energy backreel setting to overturn large blocks of coral to see the beautiful color of encrusters on the bottom side of coral blocks (mainly sponges, foraminifera, small bivalves, and red algae) and to observe the numerous brittle stars, surprised and fleeing on all five legs. Also note the high degree of boring that riddles many of these same blocks.

Weather and time permitting we will swim/wade to the back side of the barrier reef and see the thriving coral reef environment... the heart of the sediment factory. Here, Acropora palmata is the dominant reef-building element; a limited variety of other corals also live here, including Agaricia (the lettuce leaf coral), Acropora cervicornis, Montastrea cavernosa, and two species of Porites.

The Belize Barrier Reef is believed to have formed in the last half million years during brief intervals of transgression and highstand during the late Quaternary (Ferro et al., 2000). This interpretation is based on high-resolution single channel seismic, shot in front and in back of the reef. The Plio-Pleistocene section below the reef consists of shelf-edge siliciclastics.

Stop 5 Bakers Rendezvous
This elongate patch reef (Figures 31-32), referred to as a "ribbon reef', was dominated by Acropora cervicornis, almost to the exclusion of the other corals (Figure 24 and Figure 25). A dense thicket of branching corals flourished on the reef top, and is barely visible. Crinoids are sometimes seen on reefs such as this, crawling around in the mesh of branching corals. The dominance of Acropora cervicornis as the primary frame-builder here was described from field studies in the 50's and 60's. In the 90's, however, there was a noticeable decline in the abundance of this once prolific branching coral due to a "bleaching event."

Cary Caye is located on the emergent crest of this ribbon reef. On the eastern (windward) side of the island is a storm ridge that consists of coral debris, in the form of large blocks of Acropora palmata and broken branches of Acropora cervicornis.

Lineament analysis of the Belize Shelf shows a direct correspondence between the distribution of modern reefs and underlying structures (Precht, 1994). The linearity of the ribbon reefs is most dramatic. This reef type probably represents reef growth along one edge of a structural block that dips down on one side. Using the same logic, rhomboid reefs that we'll see tomorrow most likely grew evenly on all sides of structural blocks.

Stop 6 Placentia Spit
Today will be spent making a long, second transect across the Southern Shelf, this time where the shelf is almost at its widest. This part of the shelf hosts the most spectacular reefs off southern Belize. If the weather cooperates, we will try to see the shelf-edge barrier reef on both its forereef and backreef sides. In the water today, we will examine Placentia Spit, nearby patch reefs, a National Park on Laughing Bird Cay, Victoria Channel, and the Queen Cays. From the air tomorrow, we will see three varieties of patch reefs (linear, amoeboid, and rhomboid forms) found on the southern shelf.

A walk along the beach here shows granule-sized quartz sand, forming a spit some 13 miles long that is migrating southward. Actually, these sands have migrated over and buried several small nearshore patch reefs. Coarse quartz sands shed from the nearby Maya Mountains make up white sand point bars seen along the rivers. They meet the sea and are transported southward by longshore drift. These sands would make an excellent reservoir facies if this spit deposit were preserved intact.

A swim across the narrow channel to Placentia Caye shows quartz sands on the beach, but muddy carbonates on the island. In the channel between the two, Halimeda mixed-skeletal wackestones occur, demonstrating a very rapid change from clastic to carbonate deposition. In the near geological future, one would expect Placentia Caye to be buried by the migrating spit.

Stop 7 Bugle Cayes
A small cluster of patch reefs occurs here in this middle-shelf setting and makes up the Bugle Cayes. Reef top faunas of coral and red algae are similar to those of Boo Bee patch reef. Both are amoeba-shaped reefs (Figure 36), as opposed to being ribbon or rhomboid reefs (Figure 37 and Figure 38).

A comment: as we travel about the southern shelf we will rarely, if ever, be out of sight of islands, covered by mangroves and palm trees. Usually, there will be several islands in sight. The effect of these "points of exposure" on the rocks that these sediments will produce depends on several variables, but certainly marine sediments are exposed to meteoric diagenesis on these islands now. If sea level drops a few feet or tens of feet, the exposure and weathering profile will be even deeper. Dolomitization could occur locally on these islands, or the formation of cements might occlude porosity. Unconformities that might be recognized on islands would certainly not occur between islands in subtidal sediments. Thus, correlations between "local highs" would be difficult, since off-reef, low-energy mudstones and wackestones are deposited in settings over 100 feet below time-equivalent sediments on highs.

The reefs of the Bugle Cayes have been probed and cored, and their foundations appear to be similar reefs of Pleistocene age (Precht, 1993).

Stop 8 Laughing Bird Caye
This stop is on one of the larger rhomboid reefs (also referred to as "shelf atolls") on the Southern Shelf. A volcano-like shape is created on the seafloor by a rim of living reefs that in map view make a closed loop (Figure 41 and Figure 42). (How do those coral do that? How do they know where to meet up?) The lithofacies map shows a rim of reef facies and a central area with steep-sided amoeboid patch reefs. The internal structure, as best we understand it, is fault-related (Figure 43). We shall make a traverse across this feature in our boat and land on the leeward side of Laughing Bird Caye. Watch out for pelicans and an extremely large school of small baitfish. We plan to swim around the island, examining the reef on the eastern side first.

Holocene sediments of rhomboid reefs reach a maximum thickness of 13 meters. These deposits rise up some 30 meters from the lagoon floor. This is accounted for, of course, by a thick section of underlying Pleistocene, reef that also rises up from the lagoon floor, and on this the modern reef is founded. The rhomboid reefs are of the same scale and display comparable reef core facies as reef-reservoired fields of the Devonian Alberta Basin. Purdy (1974) cited "conical karst" as seen on the mainland to account for reef foundations. Despite their steepness of 50-80°, the Recent rhomboid reefs are uncemented structures, held together by dense networks of branching corals that physically baffle and trap sediment. Radiocarbon dates indicate these reefs grew up in the last 9000 years and had a growth rate of 1.4 meters/millennium (Westphall, 1985), which in general agrees with growth rates cited by Wilson (1975) for Holocene carbonate sediments of one meter per millennium. This rate holds fairly constant, even when considering a wide variety of modern depositional environments.

Two areas of well developed rhomboid reefs were detected by specially enhancing the data to bring out deeply submerged reef structures. The features are rhomboid reefs concentrated in the upper and lower parts of Victoria Channel and also at the southern terminus of the Belize Barrier Reef. Many of these features do not show up well or at all on hydrographic charts of this area.

The origin of rhomboid reefs has not easily demonstrated, despite coring efforts in the last few years to resolve this problem. Re-interpretation of these data suggest that there is an underpinning of Pleistocene reefs below the rhomboid pattern of the modern reef. This, however, only pushes the question back further into the Pleistocene regarding the origin of these reefs. Underlying structural features apparently are seen 'on seismic lines as bonafide reef foundations.

Stop 9 Victoria Channel
Since we are in about 100 feet of water at this stop (Figure 28), this major channel will be sampled by our bottom sampler. The bottom here is a green mud, with abundant skeletal fragments. Be careful and run your fingers gently through the mud; notice lots of sharply broken, thin-walled pelecypod shells, as well as needle-like spines of echinoids, as well as various types of spiny worms working their way through the sediment; you'll only want to do this once. The argillaceous content of this mud is over 35%, and the green color is due to the prest1nce of montmorillonite, derived from erosion of the Maya Mountains. The net result is a deposit of argillaceous Halimeda mixed skeletal wackestones with a microfauna of coccoliths, diatoms, pteropods, and planktonic forams, which all attest to the deep-water nature of this deposit. In the rock record, the recognition of a relatively deep-water fauna like this might complicate an environmental interpretation. Instead of a deep-water shelf interpretation, open-marine faunas would suggest open oceanic waters of abyssal depths, which this is not.

Stop 10 Barrier Reef Near Queen Cay
We plan to snorkel the reef flat at this point, and, if the sea is calm, we may try to swim to the reef crest and forereef. The living barrier reef proper is a narrow zone ranging from one-half kilometer to less than one kilometer wide. It appears as a brown zone from the air, and its seaward side is marked by white surf. The crest of the reef consists almost exclusively of
(moosehorn corals). On the forereef side of this zone Acropora palmata grows with its branches straight and pointed and directed like spears into the waves. Behind this zone and onto the reef flats, the branches flatten out in the more common moosehorn shape. These corals grow right up to sea level and are often slightly exposed during low tide. This Acropora palmata crestal zone is a very high energy zone, but notice that the actual bottom of the sediment-water interface is far down inside the coral thicket and is in a somewhat lower-energy zone because of the protection afforded by branching and platy corals. Thus, high-energy reefs locally may contain mudrich pockets, representing relatively quiet-water micro-environments.

Behind the reef crest is the broad reef flat. Here the high wave-energy has been absorbed by the crestal zone, and the biota changes to one of the most diverse varieties found anywhere. The corals change to large head corals, such as Montastrea annularis, Diploria labyrinthiformis, and Porites asteroides. The fire coral, Millepora alcicornis (actually a hydrozoan), is abundant, as are many gorgonians (the sea fans, sea whips, and sea feathers). Coralline red algae, Homotrema, and green codiacean algae, especially Halimeda, are abundant. Echinoderms are common, as are several kinds of brittle stars and mobile crinoids. In addition, several varieties of sponges are found. Smaller life forms such as worms, clionid sponges, and many others are common, as are a great variety of crustaceans, fishes, and other vagrant benthic and nektonic creatures. Note that reefbuilders are abundant, but most of the volume of the reef flat is sand- to boulder-size sediment, not in growth position. A facies map showing this pattern on the Belize Reef about five miles north of South Cut appears in Figure 48; al similar pattern occurs at South Cut as well.

Behind the reef flat is a broad zone of winnowed sand, a grain flat. This sand is kept mobile by wave action and is a clean, white, bare sand with few organisms living"in it. It consists predominantly of Halimeda plates. The living Halimeda does not catch the eye as much as large framework corals, but these algae produce several crops of plants per year, and each plant furnishes scores of heavily calcified leaves, whereas the corals are long-lived, slow-growing, and produce relatively little sand-size sediment. Note also that calcified algal leaves disaggregate upon the death of a Halimeda plant and that reconstruction of the plant's growth form would be very difficult from individual fossilized grains. The same problem occurs with many Paleozoic algae, such as Epimastopora, Ivanovia, and Eugonophyllum from the Permian of West Texas.

The barrier reef certainly represents reservoir-quality facies, unless diagenesis later plugs all the various forms of porosity involved with this eco-system. This includes all elements of the reef as shown in a schematic block diagram by James and Ginsburg, (1979); note the spur-and-grove structures on the front side of the reef at Queen Cay (Figure 50). The grain flat behind the reef actually represents one the best potential reservoir facies of the entire shelf. Its variable width and digitate back side would offer potential for stratigraphic trapping.

We may see more Agaricia (lettuce coral) here especially in the forereef, and perhaps more Millepora (the fire coral). Also, we may see crinoids here or on the rhomboid reefs. They are green, orange, or pink in color and climb slowly around on the corals. These are not sessile attached forms today as they were in the past.

A drill hole was made on Queen Cay, and unconsolidated Holocene carbonate sediments were drilled to a depth of about 17 meters. Once again, Pleistocene carbonates, not necessarily reef facies however, underlie modern ones.

Of the facies we have seen on the Southern Shelf Traverse, potential reservoir facies exist in the quartz beach and delta sands on the landward end, and the reef and skeletal sands on the carbonate platform at the other end. The barrier reef, of course, has very high primary porosity. Packstones shelfward of this have good primary porosity, but may be tight in the subsurface. On the other hand, diagenesis can make packstones nearly as porous as grains tones. Argillaceous lagoonal rocks are a potential source rock, as are basinal rocks in deep water in front of the reef. Either of these two might bury the platform, forming a seal.

A successful play concept, where opposite a tidal pass in the barrier reef, there is a cluster of patch reefs, backset from the pass, but in its direct path. Possibly, the concentration of energy in this "bottleneck" supplies more food or nutrients to reef-growing organisms, so that patch reefs might be expected to flourish in such localized settings.

Two geological problems that are evident from the traverse are: 1) the stratigraphic correlation of time-equivalent but very dissimilar facies (deposited at very different water depths), and 2) the prediction of major facies belts (such as the extensive carbonate platform of the outer shelf), when there is virtually no evidence of the platform in lagoonal marls, even as close as only 100 feet or so from its leeward edge.

On the forereef side we will see the deep, dark waters of the slope, shallowing sharply toward the lighter blue waters of the reef. The grooves or channels widening downward are floored with bare, mobile sand which is funneled down onto the forereef slope. The spurs bounding the grooves are covered with corals, a highly digitate seaward extension of the reef. This geometry apparently produces the most effective resistance to break the force of large storm waves coming in off the Caribbean. A good analogy would be to try to record sounds waves bounced of the long edge of a hair comb; they would be mostly absorbed by the structure.

Corals of the forereef include Agaricia, Montastrea, Diploria, and some other head corals. Acropora cervicornis, commonly is seen up to and near wave base, and Acropora palmata occurs almost exclusively at the crest of the reef within the surf zone.

We won't be able to observe forereef slope deposits directly (unless someone has SCUBA tanks), so we will rely on computer perspectives (Figure 54) and Nekton dives that explored "the wall" in front of the barrier reef. This is the well cemented, oversteepened edge of the continental shelf (reaching down to about 160 meters off Queen Cay) that gives way below to relatively coarse-grained detrital carbonate sediments mixed with indigenous deep-water deposits that comprise proximal slope deposits. These become finer seaward and form distal slope deposits. Seismic profiles were shot over slope deposits in front of the Belize Barrier Reef. They show early infilling of lows on the shelf-to-basin profile and localized slump deposits.

Everything seen on the outer shelf testifies to the enormous wave energy to which the reef is subjected and how the reef protects the shelf and the coastline behind it. In comparison, however, the maximum wave energy in the Caribbean is only a fraction as strong as the wave energy impinging on South Pacific reefs where different types of corals and massive walls of coralline red algae (with very little coral) are present.

Slope deposits consist of the remains of indigenous deep-water fauna mixed with shallow-water skeletal fragments and large blocks of shallow-water origin that have calved off the reef and come down the slope. The steepness of these slopes is maintained, at least in part, by a unique type of seafloor cement that forms in slope settings as large botryoids several centimeters across. These have been found by Grammer et al. (1993) to grow at very rapid rates, cementing steep sediments well beyond the angle of repose within tens of years. It is likely that this process of cementation is widespread across slope environments and accounts for their steepness.

Our flight path will take us over the Southern Shelf, the Belize Barrier Reef, and finally the Northern Shelf and Ambergris Caye. Note the low, dense, mangrove-covered islands and shallow, clear water that looks turquoise in color; white areas on the seafloor are bare sand or mud bottom. Reefs are brown in color; dark, subtidal areas are covered with vegetation, usually Thalassia (turtle grass) or algae. The deeper water is dark blue ... and we call it "abyssal blue".

As we fly over the Southern Shelf, we observe the three types of patch reefs that we dove on the last two days. We see the sharp color change at the shelf edge, where the reef slope is steep, vertical in places.

As we approach the atolls off Belize, we notice major differences between them, despite that each is a reef-rimmed structurally high block. Glovers Reef and Lighthouse Reef are both current swept, but Glovers has more patch reefs across it. Lighthouse Reef has numerous patch reefs, widespread reef-derived sands, and more islands. It also hosts the famous Great Blue Hole. In contrast, Turneffe is a collection of mangrove islands on a relatively low-energy platform. Banco Chinchorro to the north is included for comparison. All these were mapped in detail by Gischler and Lomando (1999) who collected over 450 bottom samples.

Further northward, where the shelf is shallow and clear, we start seeing tidal channels cut through the sediment. A careful scrutiny of these reveals that some of them form a dendritic pattern like a drainage system or distributaries in a delta. We may also see sinkholes in Pleistocene bedrock nearest to Ambergris Caye. Both the drowned drainage system and the sinkholes are evidence of a much lower sea level in the not too distant past. There are several large Mayan ruins on Ambergris Caye that are thought to have been centers of maritime commerce and perhaps seaside resort cities for the Indians, but are now in the center of an uninhabited mangrove swamp.

As we approach Ambergris Caye from the air, we may be able to see the north-northwest pattern of fractures parallel to the island. Note the low, swampy mangrove jungles, the savannas floored mainly by Pleistocene rock, and the inlets, bays, ponds, algal mats, and karst features on the bay side of the island Figure 72. The white wall of surf a short distance offshore is the barrier reef, which grows to sea level and breaks large Caribbean waves.

San Pedro is a small picturesque fishing village. We will be staying in hotels along the beach a short distance north of the airport. The only way to reach San Pedro is by air or boat. The village has small grocery stores, drug stores, restaurants, bars, and several gift shops where coral jewelry and woodcarvings are probably the best buys. Ambergris Caye has enjoyed a spurt of growth the last few years with a real estate boom along the beach for miles north and south of San Pedro.

We will have some time to be a tourist today, but do not ignore the geology of Ambergris Caye as we look around; an east-west cross-section appears in Figure 73.

Today we will travel south of San Pedro and begin a traverse that will take us from inner and middle-shelf settings to the shelf-edge barrier reef. We will see a variety of depositional environments in a short distance, which are very important to the petroleum geologist since these facies include reservoirs, source beds, seals, and traps.

STOP 11 Tidal deltas near Cangrejo Caye
The Cangrejo tidal deltas (Figure 74) occur as a series of mud shoals between Cangrejo Caye and the southern end of Ambergris Caye, measuring some 18 square kilometers in size. Although the tidal range is microtidal in northern Belize, the exchange of diurnal tides requires a transfer of large volumes of water through a limited number of tidal channels, enough to service a large bay like Chetumal. This produces a Bernoulli effect in channels twice a day, moving lime mud alternatively east and west.

Depositional processes at the mouth of the bay are strongly influenced by these currents (Figure 75). One might expect to find predominantly high-energy, winnowed sands in this area, and there are some locally developed as bars at the mouths of tidal channels, but most of the sediment at the mouth of the bay is lime mud with lesser amounts of sandy mud. Local sand production is dominated by foraminifera and pelecypods; muds appear to have a net westward direction of transport (i.e., shelfward as opposed to basinward). Hence, sand particles of these deposits are provided by local faunas of an inner to middle-shelf setting, whereas muds appear to be derived locally and from the breakdown of particles from the outer-shelf reef tract to the east.

Lime mud at Congrejo is mainly (70%) high-Mg calcite and formed these deposits during the last 6500 years; authigenic rhombs of dolomite, ten microns in size, constitute an average of 5% of the mudbank, from a depth of 0.6 meters down (Mazzullo et al., 1994).

The formation of low islands in this area progressed in the following way. First, mud accumulated over the Pleistocene limestone (deposited first in the lows of the Pleistocene surface). This was then colonized by Thalassia grass, which then trapped more mud by the baffling action of its leaves and the binding action of the holdfasts. As the mud shoal approached sea level, the red mangroves (Rhizophora mangle) became established and still more mud was trapped between their roots. As the mangrove area expanded and the interior was filled in by more sediment, blue green algal mats, black mangroves (Avicennia nitida), and palmettos became established there. All these phases of island growth can be observed in this area. Eventually these islands will probably coalesce and accrete to the southern end of Ambergris Caye, and a skeletal sand beach may be developed over the sequence. Coring on the southern tip of Ambergris Caye indicates the Pleistocene limestone there is overlain by mangrove peats and lime muds and is capped by beach sand. The importance of biologic activity in the carbonate ,depositional process cannot be overemphasized, even though many of the important organisms have no hard parts and leave no fossil record.

Tidal channels here are floored with mud, not eroded down to Pleistocene bedrock, despite relatively strong, regular currents. Ten such channels cut across the Congrejo shoals from northwest to southeast. They are as deep as 10 feet, are grass-free, and have numerous burrow mounds of the ghost shrimp, Callianassa.

The facies map for this area (Figure 75) shows mud shoals consisting of skeletal wackestone, flanked by foraminiferal molluscan packstones to the north and west and by Halimeda skeletal packstones to the east. Sediment-probing with a metal rod indicates the thickness of unconsolidated sediment ranges from 5 to 15 feet thick on Cangrejo tidal deltas (Figure 76).

The muds on Cangrejo Shoal are extremely rich in organic material and they are saturated with water. Beneath a thin layer of white, oxidized sediment, we will find the mud is nearly black and very fetid due to the reducing environment and high organic content. We will also notice that the mud offers very little support or resistance if we try to stand on it or walk through it. Beware of stinging and biting decapods that live in lime mud; wading through the mud can result in numerous bites that may give the victim a neurotoxic reaction.

The only obvious signs of life in the mud are Thalassia and a scum of brown algae. If we imagine a similar mud mound with phylloid algae instead of Thalassia as the baffling agent, we have an analog to the phylloid algal bioherms of the Upper Pennsylvanian and Wolfcampian of the Permian Basin.

Development of this mud shoal complex of tidal deltas at the southern end of Ambergris Caye represents a sharp protrusion of inner-shelf facies out into middle-shelf facies. Recognizing this pattern in the rock record depends on the realization that tidally influenced sedimentation does not have to occur in front of major landmasses or in narrow passes between islands. In other words, these inner-shelf sediments represent an accretion along the strike of Ambergris Caye by mudbank and tidal delta facies. The strong tidal influxes are caused by the shape of the basin on the inner shelf (i.e., Chetumal Bay) which produces a "bottleneck" effect as waters are exchanged by tidal cycles. Thus, the Cangrejo tidal deltas are important from the standpoint of the complexities involved in facies reconstruction. This facies is not a typical reservoir facies unless it becomes diagenetically altered by dissolution of foraminifera and molluscs to produce moldic porosity or by dolomitization to produce intercrystalline porosity.

Stop 12 Bulkhead Shoals
Bulkhead Shoals (Figure 75) is another large mud shoal similar to Cangrejo Shoal. Some maps refer to it as "Bulkhead Reef', the original definition of reef being any hazard to navigation. Bulkhead Shoals certainly fits that definition, but not the ecologic one.

Bulkhead Shoals is a large three-pronged mudmound complex (Figure 76) about seven miles long E-W and about two miles wide N-S. It nearly blocks the mouth of Chetumal Bay, limiting the exchange of marine waters of the open shelf with seasonally hyposaline and hypersaline waters of the bay. Water depths surrounding the mudmounds are 7-8 feet deep, with the crest of the mounds rising to within 1-2 feet of sea level. On the western side of the Bulkhead a large single mudmound is developed with several tidal channels cut through it. On the eastern side, it bifurcates, producing a southern and a northern mudmound, the northern one having two mangrove-covered cays developed on it. Bulkhead Shoals is a mud mount 2 to 8 feet thick consisting of black, fetid carbonaceous mud with thin beds of foraminiferal sand containing miliolids, with lesser amounts of molluscs and peneroplid forams. Blades of Thalassia grass are seen in cores taken through this deposit. The shoals are forming in an area subject to periodic, moderate to strong winds and tidal currents, and several tidal channels cut the southwestern portion of the shoals.

The composition of the muds and sands at Bulkhead Shoals is high-Mg calcite with approximately 10 mole% MgC03, which is similar to the other facies seen in Chetumal Bay. In contrast, the sediments of the outer shelf on the seaward side of the island are predominately aragonite. This basic mineralogic difference between the sediments of Chetumal Bay and those of the outer shelf is well defined and would be an important factor in the diagenesis of the sediments.

The origin of these muds, according to Reid et al. (1992) is from the abrasion of silt-sized cryptocrystalline grains (i.e., micritized silt-sized skeletal grains). This differs from the explanation of the origin of the muds at Cangrejo which are reported to result from physical and biological breakdown of reefal outer-shelf deposits which are transported shoreward as mud-sized particles (Mazzullo et at., 1995). The production of lime mud at The Bulkhead and also in Chetumal Bay in general has been attributed to the physical and biological breakdown of strongly micritized skeletal grains (Reid et at., 1992), although there is also the possibility of direct precipitation of lime mud from sea water. This latter hypothesis is supported by the observance of "whitings' in Chetumal Bay; however, some attribute "whitings" to the movements of schools of fish. Nothing definitive here yet; there is good evidence on both sides.

From the standpoint of depositional facies developed here, The Bulkhead has no reservoir potential because the sediments are too muddy. However, considering possible diagenetic alteration later, reservoir potential could be produced by dolomitization of the wackestones (thus producing intercrystalline porosity) or by the dissolution of bioclastic material (producing moldic porosity).

With regard to mapping geologically ancient facies patterns, The Bulkhead (an extensive mudmound with a few feet of vertical relief) can serve as an example of a muddy "barrier" to water-circulation patterns in middle- and inner-shelf environments. This sizable physiographic feature essentially causes the lack of exchange of marine waters deep into Chetumal Bay. In the rock record, it is common in cores and cuttings to observe "restricted faunas" or "restricted facies". This type of interpretation is usually based on the lack of normal-marine faunas or the presence of sedimentary structures that suggest the nearness of the shoreline. However, the existence of an ancient mudmound complex with limited vertical relief, such as The Bulkhead, might be completely obscured, if muddy sediments of the inner- or middle-shelf bury and mask the mound in more lime mud.

Stop 13 Miliolid-Pellet facies of Chetumal Bay
About a mile or two north of Mosquito Cay we are in the cryptocrystalline grains facies of Pusey (1964) with miliolid forams and pellets as common constituents. This low energy facies covers nearly all of the northern and southwestern parts of Chetumal Bay and extends to the mainland. The Holocene sediment here is a thin veneer over Pleistocene limestone. It is dark colored and organic-rich, slightly argillaceous, with pelletal skeletal packstone and wackestone textures. The only visible flora are occasional Thalassia plants and algal scum. The silt-size pellets of cryptocrystalline grains that comprise two-thirds of the sand fraction are of uncertain origin. Some may be fecal pellets, but many are formed as micritized grains by microboring algae or fungi boring into the margins of skeletal grains. A complete spectrum of bioclast alteration is observed, from thin surface effects (comparable to the micrite rim of Bathurst, 1966) to complete obliteration of skeletal microstructure. The precise mechanism of recrystallization is not understood, but there is not a significant change in mineralogy upon alteration to cryptocrystalline material. The mineralogy of this facies is predominantly high- Mg calcite, as in the other facies in the bay. These particles should not be confused with ooids, since even slightly altered grains do not show typical ooid microstructure. Other grain types present include molluscs (averaging 16%) and foraminifera (averaging 10%). Argillaceous material is present throughout Chetumal Bay and averages 10%. Many grain-producing organisms dependent on normal-marine salinity (36  0/00) are not present or abundant in Chetumal Bay because inflow from rivers dilute the salinity of bay waters to as low as 26  0/00.

Questions one might ask include the following. Would one expect to find a reservoir in this facies in the subsurface? Could it become a source bed? A seal? If these sediments were encountered in the subsurface, would we call it deep water or shallow, protected? What clues would we look for?

Chetumal Bay represents a broad, shallow depression in the inner shelf. Argillaceous pelloidal packstones with highly altered grain types are being deposited here, and skeletal grain types indicate a restricted marine condition. Unless diagenesis later intercedes (e.g., forming moldic porosity by dissolving framework grains), the facies will have little reservoir potential.

Stop 14 Peneroplid Foram Sand Shoals and Algal Stromatolite Beds at Blackadore Caye
Just when we were about convinced that we could write off any reservoir potential in Chetumal Bay sediments, here is a clean, winnowed, foraminiferal sand-a foram grainstone at Blackadore Caye. Why are these washed sands here in what normally would be expected to be a low-energy environment? Apparently, bay waters here have fairly strong currents produced periodically by winds. The hydraulic diameter of platy forams like peneroplids is small, and they are easily moved and winnowed on local highs. Linear highs in Chetumal Bay are aligned with the regional structural grain observed in LANDSAT imagery. The obvious interpretation (with no seismic data in hand, however) is that local faulting has produced linear highs that produce shoaling environments at right angles to the predominate wind which blows west to east. These shoals allow the accumulation of platy foraminifera (Figure 78d) in a mud-free environment... an excellent example on a local scale of the interaction between tectonics and sedimentation.

These foram-rich sediments are about 3 to 5, feet thick and rest on a bed of mangrove peat or directly on Pleistocene limestone. A few genera of forams are present in the sands, but Archais angulatus predominates. The sands thus are predominantly high-Mg calcite. The sand has been worked into large ripples. Look at the ripples and see if we can determine if they are wave or current ripples, and, if, current ripples,' in which direction the current is flowing. Blackadore Cay is a narrow, mangrove-covered island about two miles long and one-eighth mile wide, consisting of foraminiferal grainstones and packstones. An extensive bar is formed off the southern end of Blackadore Cay and trends NNW-SSE along the strike of the cay, as do other island bar complexes between Blackadore Cay and Ambergris Island. This is compatible with the regional pattern of faulting in the Chetumal Bay area, suggesting that the island and the bar formed on the edge of an upthrown fault block.

The pattern of carbonate facies developed here lends support to the possibility of tectonic control. Foraminiferal packstones blanket broad middle-, and inner-shelf areas west of Ambergris Cay and south around Bulkhead Shoals. High structural blocks would raise these muddy sediments into a higher energy regime where mud would be winnowed to form a grainstone lag of foraminiferal tests.

The peneroplid foraminifera, Archais angulatus (Wantland, 1967), dominates the foraminifera population of this facies. Other grain types occurring here include miliolid foraminifera, molluscs, and cryptocrystalline grains. Referring back to the geologic record, we see that sands of orbitolinid foraminifera are common in the Lower Cretaceous of the Texas Gulf Coast, and fusulinid sands are common in the Late Paleozoic of West Texas and southeastern New Mexico. Both are common reservoir-related facies.

As we approach Blackadore Cay, note the red mangroves growing out into the water. There are three main kinds of mangroves in the Caribbean: the red mangroves (Rhizophora mangle) that mainly grow farthest out into the sea, the black mangroves (Avicennia nitida) that look very much like the reds but grow more on the beach, slightly further back from the sea, and the white mangroves that live on high, well drained land. Another shrub or tree that looks very much like the mangroves has crinkly leaves and secretes a poison similar to poison ivy. It can cause considerable discomfort if touched or even approached closely. The mangroves have no hard parts to be preserved as fossils, but they serve a very important sedimentologic function. They trap lime mud with their many roots, which form a very effective wave baffle; therefore, the islands can accrete seaward with the aid of the mangroves. Elsewhere we will see many mangrove-covered islands across the shelf and especially in the bay. They have a very clever way of propagation: they produce seeds in a slender pod about a foot long that is weighted to float upright like a fishing float. When ripe, these pods fall off the tree into the water and float until the bottom of the pod touches the bottom of the bay. There they quickly take root and begin to grow. The roots of the tree trap mud and the accretion begins anew. The black mangroves propagate differently-mainly by sending up new shoots from the horizontally spreading roots.

Mangroves are also one important bioturbation process. The roots destroy laminations or bedding features that might otherwise be formed in the sediment. Sometimes casts of the roots are preserved and are recognizable in ancient rocks, but this is fairly rare. We often think of bioturbation as caused by burrowing animals, but plants can also produce a thoroughly bioturbated sediment.

On Blackadore Cay, there are localized algal flats, fed by storm washover, as opposed by tidal channels. Note the abundance of small, high-spired, cerithid gastropods (Batillaria minima) grazing on the algal surfaces in great concentrations. In this stress environment, the only signs of life are the algae and gastropods that live on it. Large population numbers and low diversity characterize this stress environment. As it turns out, in the Late Precambrian, before gastropods had evolved, blue-green algal deposits went unchallenged and formed build-ups a few meters thick, relatively thick for an accumulation of millimeter-thin algal layers. ~

Our final effort near Blackadore Cay will be to take a core of Holocene sediments off the foram bar. Stop 15 Hol Chan Pass
Our last field stop of the day is at a break in the barrier reef on the northern shelf of Belize. The Hol Chan Cut is protected as a national park, and numerous fish take advantage of the "no fishing" regulations here. It's a good place to wash off all our lime mud from Chetumal Bay. Several medium- to large-sized fish will come up to us in the water, looking for a handout. This is an extremely photogenic area, but pay some attention to the marine geology as all the beautiful fish swim by.

Note the red-algal-encrusted reef rubble in the axis of the channel, accumulating as a current-swept lag deposit. As the tides change, there is an increase in current velocity as a tidal bulge of water piles up on one side or other of the reef and is forced through the pass. Through this break in the linear reef pattern along the shelf margin tremendous volumes of water are exchanged from the open ocean to middle- and inner-shelf areas behind the reef. Extensive lime mud accumulations at Bulkhead Shoal and in the Cangrejo area form obstacles to this flow of water onto the shelf and direct the flow back into Chetumal Bay.

Stop 16 Reef Point
On our way to Reef Point, Robles Point may be as far north as the boat can go safely without going outside the barrier reef. If sea conditions do not allow us to take the boat closer to Reef Point, we will walk nearly two miles northeast from Robles Point along the rough beach ridge which is covered with cobble-size coral rubble. If weather permits, we will land the boat further north, closer to Reef Point and avoid the walk.

In contrast to Florida and the Bahamas, where Pleistocene carbonate rocks form numerous keys and cayes, rocks of Pleistocene age are relatively rare in Belize. Only on Ambergris Cay and on nearby smaller islands do they occur, and they are usually low-lying outcrops with thick vegetal cover. The result is a two-dimensional understanding of Pleistocene facies in this area.

Pleistocene facies of Ambergris Caye, as described by Tebbutt (1967), include the following rock types, which are relabeled in our maps and cross-sections for a more straightforward comparison with the Holocene facies map of Belize presented herein.

1. Coralgal boundstones - deposited in outer-shelf reef environments. These consist of branching corals and encrusting red algae (commonly Lithothamnion) with an interstitial fill of skeletal wackestone forming the matrix between reef framebuilders.

Bioclasts include coral fragments, molluscs, echinoids, green algae, and the encrusting foraminifera, Homotrema rubrum. Two main types of coralgal boundstone occur as linear facies belts at the shelf edge: one is dominated by Acropora palmata and the other, occurring immediately shelfward of this, is dominated by Acropora cervicornis. (In 1967, Tebbutt referred to these respectively as Facies I-Reef Crest, and Facies II-Back Reef.)

2. A mixture of three middle-shelf patksione facies: a) burrowed skeletal pelletal packstones with associated coralgal bounds tones of patch reef facies, b) pelletal coated-grain packstones, and c) skeletal pelletal packstones . Massive head corals such as Montastrea and Diploria are the dominant corals in patch reef facies, but branching corals such as Acropora cervicornis and Porites format also occur; Millepora may also be common locally. Clasts consist mainly of molluscs, Halimeda, or pellets and form fine-grained packstones with some thinly coated grains (with only 1-3 layers of aragonite). The mud content of this facies belt indicates a moderate-energy regime and differs from the classic high-energy environments in which well-formed oolites are deposited. These three facies were referred to by Tebbutt, respectively, as Facies IIIa (outer, middle, and inner shelf) and correspond to outer, middle, and inner parts of the middle-shelf environment.

3. Burrowed mudstones - deposited in middle-shelf lagoonal environments, possibly in the form of mud mounds. At Reef Point, the Holocene barrier reef becomes a fringing reef as it comes ashore. Here, we have an excellent horizontal exposure of a Pleistocene coral reef. The rock consists of broken Acropora cervicornis rubble, in situ thickets of Acropora palmata, Diploria, Montastrea annularis, and Agaricia, enclosed in a matrix of muddy skeletal sands, rich in pelecypods and gastropods. Petrographically, the rocks are poorly sorted, fine- to coarsegrained skeletal sands composed of fragments of corals, pelecypods, forams, worms tubes, and the large gastropods Busycon and Strombus. The sediments are lightly-cemented and have excellent intergranular porosity. Core Lab measured 25.3% porosity and 17 md permeability in samples of these calcarenites. Samples of the Pleistocene reef facies exposed to surf spray were measured at 15.6% porosity and 0.36 permeability. Also, the reef shows extensive micritization of all grains by endolithic algae.

Another exposure of a Pleistocene reef is present at the Dredge Cut in the Holocene barrier reef about four miles south of San Pedro. A ship ran aground on the reef at this point cutting a notch through it. This notch was later dredged out and opened up for a navigation pass through the reef. The rocks dredged out were piled into a small island and indicate that the substrate is a Pleistocene barrier reef at that locality also.

Reef Point is an important field stop from the standpoint of the development of facies belts. The modern barrier reef here becomes a fringing reef at the headlands created by an outcropping Pleistocene reef. The Holocene reef grows on top of and lateral to the Pleistocene reef. The Pleistocene reef occurs below the modern barrier reef and provides a foundation for it. Figure 84 shows a map of the top of this Pleistocene surface as it dips southward. Considering the- brief span of geologic time involved, the "minor" unconformity between these two reef systems here might not be well developed or easily recognized in core samples.

Lateral facies relations in the Pleistocene show a zone of Acropora cervicomis on the shelfward (western) side of the main barrier reef. Like the modern barrier reef, the Pleistocene barrier reef was composed mostly of Acropora palmata. In modern reef systems, the Acropora cervicornis zone may occur on either side of the main barrier reef, where wave energies are moderate. In Holocene reefs off Belize, this zonation is like the Pleistocene, and this rock type is found on the shelfward side of the barrier reef. It also has been observed in abundance on numerous patch reefs of the middle shelf as one of the main frame-building elements of the reef. All the old reports from Rice University in the 1960's mention dense stands of the "elkhorn coral".

However, in recent years (since about 1990 on), there is a noticed and obvious decline in the population of Acropora cervicornis on middle-shelf patch reefs of Belize. Thirty years ago there were living colonies of this branching finger coral everywhere on these reefs, forming dense thickets, much like a hedges or "piles of brush" onshore. Such well developed colonies are hard to find today and occur only rarely and locally. A coral blight, some sort of coral disease, is believed to be responsible for the recent demise of this species. Since this same phenomenon is seen on the far reaches of the southern shelf of Belize, as far south as the Seal Cayes, which are still fairly remote, it seems that humans are not responsible for the decline in this species. Something natural is changing on the shelf today.

Stop 17 Mexico Rocks and/or Punta Azul
Mexico Rocks has some of the largest patch reefs in the Ambergris Caye area; Punta Azul is another similar area here. They host mainly growths of the coral Montastrea annularis (the star coral), and comprise 83% of the living coral population here (McHenry, 1996). However, several other corals are also present, including the brain corals Diploria labyrinthiformis, Diploria clivosa, and Porites asteroides (the porous coral); the branching corals, Acropora palmata ("moosehorn coral"), Acropora cervicornis ("staghom coral"). The hydrozoan Millepora (fire coral), the encrusting foraminifera Homotrema, and the encrusting red algae Lithothamnion are also common on the reefs here. Note also the small clumps of the green codiacean algae Halimeda, growing like small shrubs on the corals. Fish, lobsters, echinoids, octopi, and other life forms are to be found, especially if we look carefully under rocks and in crevices. Note the burrowed carbonate sands around the reefs and the turtle grass meadows where the large bright pink and brown gastropod or "conch", Strombus gigas, is common. We will also see numerous Halimeda plants growing on the bottom here and also on corals. Halimeda plates are a prominent constituent of the lime sands, making up more than two-thirds of the sediment. The sands also include miliolid and soritid forams, as well as coral detritus, pelecypods, worm tubes, and gastropod fragments. Wave energies in this area are high compared to other areas on the northern shelf, because there is a break in the barrier at this point. This contributes to the prolific growth of these relatively large reefs. Note the large, colorful parrot fish which bite off mouthfuls of living coral, chew them up, and spit out (or excrete) sand-size skeletal sediment, representing an important process in reducing coral to sand-size sediment.

Here, in addition to small patch reefs, we see meadows of turtle grass, Thalassia. We will also be able to see the cone-shaped burrow mounds of the ghost shrimp, Callianassa. We may also see the small green algae Penicillus (which resembles a green shaving brush), Udotea, and Rhipocephalus. These algae secrete aragonite in the form of very small needles (less than four microns long). When the plant dies and the tissue decays, aragonite needles are released into the water and are a major source of lime mud. Other sources of lime mud include physical and biologic breakdown of larger grains and weakly calcified skeletons of many small forams, worms, epibionts, and other biota that live in great abundance in these warm, clear waters. It appears that there is always enough lime mud produced to be a matrix for carbonate sediments; therefore, if a mud matrix is not present in a limestone, we may assume it is because the energy level if the environment of deposition was too high to permit the mud to be retained in the deposit due to winnowing by wave or current activity.

In 1995, a coral bleaching event, caused by excessively warm waters (32-34°C) in backreef settings, killed numerous corals at Mexico Rocks, decreasing coral cover by 13 % (Burke, 1996). This was accompanied by an increase in soft brown and blue-green algae, inhabiting once live coral surfaces and by an increase in the amount of boring and cavity formation on the reef. Such disturbances in the balance of reefs must represent normal stresses they face through geologic time.

Considering that the Holocene transgression inundated the northern part of Belize long after flooding the deeper southern part of the shelf, the reefs at Mexico Rocks must be the youngest on the shelf. They are age-dated at 4500 years before present, as compared with ages of 7000-9000 years before present for patch reefs on the Southern Shelf (McHenry, 1996).

Stop 18 Shark-ray Alley
This is our final look at the Belize Barrier Reef, and the setting is a special one. For years, fishermen have cleaned their daily catch here. The word traveled fast in two animal populations: nurse sharks and manta rays. Both come to feed on daily scraps, given freely, and the fish will literally "eat out of your hands". It's like a "Pavlov response"-boats arrive, fish scraps are provided, and sharks and rays start swarming. This is your chance to feel the texture of shark or ray skin or to see the feeding habits of these animals close up. Be careful; don't give them your fingers!

Click here for information on Marco Gonzales Maya site.
Click here for information on the Holocene Sediments of the Belize Shelf.
Click here for information on the Geologic origins of Ambergris Caye.

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