Siliciclastic Foundations of Quaternary Reefs in the Southernmost Belize Lagoon, British Honduras

Siliciclastic foundations of Quaternary reefs in the southernmost Belize Lagoon, British Honduras

DONG RYONG CHOI \ Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, ROBERT N. GINSBURG j University of Mu '

ABSTRACT

Analyses of a closely spaced lattice of seismic profiles from the southernmost Belize Lagoon, combined with logs of nearby drill holes, reveal that the Quater- nary (late Pleistocene plus Holocene) reefs and carbonate sediments are growing on siliciclastic sediments which have an alluvial and delta-like morphology. The residual relief of these coastal-plain deposits localized initial coral growth or an accumulation of calcareous skeletal material; favored sites for these were elevations such as levees and bars of remnant channels, and deltaic lobes.

INTRODUCTION

The extent of substrate control on the initiation of Quaternary coral reefs is of considerable interest to reef specialists, to those interested in Quaternary sea level, arid to carbonate sedimentologists. The Belize (British Honduras) Reef Tract (Fig. 1) is a particularly attractive area for studying the foundations of reefs for two reasons: (1) Belize has the largest, most varied, and most luxuriant array of Holocene reefs in the western Atlantic; (2) it is an area where reefs and carbonate sediments mix and interfinger with siliciclastic sediments, an unusual association of Holocene sediments. The three works on the foundations of Holocene reefs of Belize have established that both the numerous lagoon reefs and the semicontinuous barrier reef are founded on elevations of Pleistocene limestone. Purdy (1974a, 1974b) used seismic profiles to determine the configurations of the foundations of Holocene barrier and lagoon reefs; complementing this study with core bor-

I ' ] IS-ÎS ftttomt

• - « 0 falhoffti Figure 1. Index map of Southern Belize Shelf, showing study areas. Bathymetry r ; Í' • • p « r Thon 40 folhonti compiled from bathymétrie charts by U.S. Naval Océanographie Office, No. 1496 (1970) and No. 1497 (1965). Locations of drill holes from Purdy (1974b).

Geological Society of America Bulletin, v. 93, p. 116-126, 13 figs., February 1982.

116

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/116/3434352/i0016-7606-93-2-116.pdf by guest on 30 September 2021 M6°00' 16°00J Figure 2. Bathymetry of the southernmost Belize Reef Tract, showing the three grids of seismic profiles obtained from ESSO Interna- tional. See also Figure 1 for locations of Areas 1 to 3.

ings, he established that the foundations are Tentati - Acoustic Assumed Thick- Estimated mostly Pleistocene limestone. Halley and ve age veloci - ness Unit 1i thology others (1977) made five borings through a ty* (Meters) single, small, Holocene lagoon reef (Boo Bee Patch Reef). They found an elevation of coral-bearing Pleistocene limestone beneath CM o o the reef and, topographically lower, ce- CM CNJ C\J mented Pleistocene muddy limestone in n-45 Channel fills three off-reef borings. 0-32 2.0 Si 1iciclasti c Purdy (1974a, 1974b) proposed that the Fl uvial 15-90 elevations of Pleistocene limestone on Limestone or which the Holocene reefs are positioned are +32 Shalv Ls. karst pinnacles and remnants formed during a period of subaerial erosion in the late Pleistocene. An alternative explanation was T2 +100 suggested by Halley and others (1977), who Shale or interpreted their five borings to indicate 2.5 Shaly that one Holocene lagoon reef was positioned on a Pleistocene reef knoll. Limestone*

n +160

* Km/sec Figure 3. Summary of acoustic stratigraphy of southernmost Bel- ** Purdy (1974a , 1974b) recovered these lithologies „. .. in drill holes, ize Shelf.

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Figure 4. At lop, seismic profile (uniboom) WB-19, Area 2. Below, interpretation showing stratigraphic relationships of acoustic units

and internal reflectors. Note the clear erosional channels, their fills, and their thickly layered wavy reflectors (Pt], Pt2), which are considered terrigenous clastics. These sediments are overlain by sediments forming mounds (Pmj, Hi) or burying depressed

areas (Pm2, H2).

To provide additional information on the sediments, probably a coastal-plain succes- 530 km of recorded data (Fig. 2). Both foundations of Holocene reefs, we analyzed sion that accumulated during a low stand of sparker (1,000 to 4,000 joules) and boomer a series of high-resolution seismic profiles in sea level in the Pleistocene. (800 joules) systems were employed in two the lagoon. Three grids of closely spaced areas (areas 2 and 3), but the boomer alone seismic profiles in the southernmost lagoon AREA AND METHOD OF STUDY was used in Area 1 because of its superior (Fig. 2) were used to establish a seismic stra- resolution. Navigation was by ONI Shoran, tigraphy for the Quaternary deposits to a Seismic profiles were taken in three PDI Mark 3 System. calculated depth of about 250 m. Sample separate areas of the Belize Shelf Lagoon in Acoustic-stratigraphic units were deline- logs from nearby exploratory oil tests pro- October 1976 for ESSO International. Each ated on the basis of their boundaries and vided support for the interpretations of area is about 4 by 6 km in size; one is internal reflectors. Often, some of the lithology from the profiles. This approach located just behind the barrier reef of Zapo- boundaries were indistinct on individual has led to recognition that the Quaternary tilla Cays, and the other two are landward profiles, but distinct reflectors in crossing carbonates (Holocene and late Pleistocene), (Fig. 1). and adjacent profiles allowed us to infer in at least this southernmost part of the Bel- The grid line spacing was 200 m by 500 m, their locations. The thicknesses of the three ize Shelf Lagoon, are founded on elevations giving a total of 100 lines plus two tie lines major units recognized were derived by in an underlying sequence of siliciclastic connecting the three areas, totaling about assuming a sound velocity for each unit

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/116/3434352/i0016-7606-93-2-116.pdf by guest on 30 September 2021 Figure 5. At top, seismic profile WB-33 in Area 1. Below, interpretation. Reefal(?) sediments (Pni|) are localized on both sides of a

channel which has compound history of formation. The older channel, filled with Ptb was re-eroded (during Pt2 time) before deposition of Pm unit. The younger channel preserved its original topography throughout Pm and H times.

(Fig. 3) based on lithologie data from bor- SEISMIC STRATIGRAPHY Lowermost Unit (T) ings in Belize and the authors' experiences Three major acoustic units are differen- in other areas. The rough description of the tiated based on their boundaries and in- The T unit is a group of thick, wavy, yet borings at Palmetto and East Snake Cays ternal reflectors: (1) a lowermost unit traceable reflectors whose upper boundary (Purdy, 1974a, 1974b) and at Boo Bee Cay designated T in Figures 4 through 8, (2) a in Areas 2 and 3 is an acoustic unconfor- (Halley and others, 1977) provided direct middle unit designated Pt, and (3) an mity (Figs. 4, 5). This unit can be sub-

information on lithology to support the uppermost unit with subdivisions Pm and divided into three sub-units (T|, T2, and

interpretations of the seismic profiles. H. T3), two of which are shown in Figures 4, 5,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/116/3434352/i0016-7606-93-2-116.pdf by guest on 30 September 2021 Figure 6. Seismic profile WB-7u (uniboom) in barrier-reef tract, Area 3. Note the difference in characters of reflectors between reefs

(Pm| and H|) and interreef sediments (Pm2 and H2). The Holocene reefs (H]) are growing on underlying topographic highs (Pm]) which also show reefal features. The apparent topographic highs below Pirij are caused by velocity "pull-up" and are not real highs. In Area 3, it is difficult to determine the influence of basement topography on initiation of Pmj sediments from only seismic records.

Figure 7. Contour map on the base of Ptj, in Area 1, showing an erosional channel system that deep- ens southward.

H "Basement" (T unit)

I 0J Lower Pleistocene channel t1—I systems (contour, bottom depth in milliseconds) I 1 Area where Pt, unit was eroded

Ends of track lines. See figure 2 AREA I Seismic profile

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/116/3434352/i0016-7606-93-2-116.pdf by guest on 30 September 2021 Figure 8. Suberop map of Area 1 made on the base of Pm unit with distribution of Pmj (reefs?) superimposed. The distribution of reefal(?) sediments shows close relationships between underlying fluvial morphology and distribution of reefal(?) sediments. Pm] is located around younger channel margins and bars. See also Figures 5 and 11 for profiles.

AREA 2

Figure 9. Contour map on the base of sub-unit Ptj in Area 2, showing development of channel systems. These channels are filled with sediments (Ptj) and escaped later channeling. For legend, see Figure 7.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/2/116/3434352/i0016-7606-93-2-116.pdf by guest on 30 September 2021 Figure 10. At top, seismic profile WB-16 (uniboom) Area 1. Interpretation below,

showing acoustic features of Pt3. Note well- layered internal reflectors and erosicinal upper and lower boundaries.

o o

100

V ER • HOR =8 3: I

ASSUMED VELOCITIES: T, - Ts, 2.5 km/sec ; Pt,-P t3, Pm26 Pm3, 2.0 km/sec; Pm, a H( ,2.2 km/««c ; Figure 12. Interpreted geologic profile along composite seismic line A-H. For location, see Figures 1 and 2. This profile is corrected by assumed velocity (see Fig. 3).

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Channel systems on the base of Pt2

yr^ Coast line of Pm age (upper Pleistocene) Figure 11. Subcrop map in Area 2 made on the base of the Pm unit, with distribution of reefs on Pm, superimposed. Reefs(?) (Pm,) or

carbonate-sand shoals are positioned along channels and edges of deltaic lobes (Pt2). See also Figures 4 and 12 for profiles.

Figure 12. (Continued).

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and 7. Each of the sub-units seems to be bounded by disconformities. The lithology of the T unit is inferred MIDDLE PLEISTOCENE from a combination of the log of a nearby boring and its acoustic characteristics shown in Figures 4 and 5. The boring at Palmetto Cay (Purdy, 1974a, 1974b) penetrated a section of shaly limestone from 90 m downward; therefore, the lower two

sub-units, T] and T2, are interpreted as

bedded shaly limestone. Sub-unit T3 may be the remnants of a continuous layer of limestone or shaly limestone that was dissected

by erosion. The age of the T] and T2 sub- units is believed to be Miocene to Pliocene.

The T3 sub-unit could well be younger, pos- sibly late Pliocene or early Pleistocene.

Middle Unit (Pt)

The middle unit is most distinctive in Areas 2 and 3 where both upper and lower I ^ 88°20' ' boundaries are unconformable and the unit 1 has distinct and generally thick layering (Figs. 4, 5). Three sub-units are recogniza- EARLY PLEISTOCENE ble, Ptb Pt2, and Pt3, each having distinctive boundaries and internal acoustic layering. The base of Pt] in Areas 1 and 2 is a high-relief unconformity that cuts well down into the underlying T unit (Figs. 4, 5). The resulting asymmetrical erosional depressions are filled with well-stratified sediments that also show internal truncations (Fig. 4). These depressions range from 300 to 1,000 m wide and from 15 to 90 m deep, and they can be contoured and connected in adjacent profiles (Figs. 7, 9). Extending the trends between Areas 1 and 2 gives a pattern of sinuous, branching channels. The erosional depressions are absent in Area 3; there, the Pt unit is thin (less than several metres) and far less distinctive than in Areas 1 and 2 (Fig. 6).

Sub-unit Pt2 is present in Areas 1 and 2, but its thickness and geometry vary. In Area 1, it is generally less than 10 m thick, but there are local thicknesses of as much as Figure 13. Successive stages in the paleogeography of southernmost Belize Shelf. 20 m. In Area 2, the maximum thickness is In the early Pleistocene, river systems incised into the T unit. In the middle Pleisto- 32 m, but there are large variations. The cene, fluvial (Pt] and Pt3) and deltaic sediments (Pt2) have buried the earlier T unit, and the major channel has shifted to northeast. In the late Pleistocene, the sea first base of Pt2 has concave and v-shaped seg- ments, some of which truncate the layering penetrated along the pre-existing channels and the first carbonates, reefs or mounds of sediments, were localized along river banks, channel bars, and on deltaic lobes (see of the underlying Ptj (Fig. 4). The top of Pt2 is a prominent reflector that forms low- also Figs. 8 and 10). Contemporaneously with these early lagoon reefs, broad barrier relief mounds of varying size (Fig. 4). Inter- reef developed on the shelf margin. Present day reefs (HI) show broader development than those of the late Pleistocene. nally, Pt2 is mostly layered and, although the reflectors are not as distinct as those of

Pt], there are areas of inclined layers ar.d Sub-unit Pt3 is present in all three areas We interpret sub-units Ptj and Pt2 as sili- rapid changes of slope (see area between as narrow, sinuous, and branched bands. In ciclastic sediments on the bases of their crosslines 16 and 18, Fig. 4). Within some of the profiles, it has the form of a channel that erosional bases and their distinct layering of the mounds at the top of Pt2, the reflec- is filled with distinctly layered sediments. with internal truncations. Support for this

tors are noticeably weaker than in adjacent The top of Pt3 clearly shows erosion before interpretation comes from the logs of two parts of the profile. deposition of Pm3 (Fig. 10). borings. Siliciclastic sediments were found

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shoals of carbonate sediments and the second as bedded interreef deposits. Core PRESENT borings in the southern lagoon have established that the Holocene reefs overlie Plei- stocene coral-bearing limestone (Purdy, 1974a, 1974b; Halley and others 1977). Accordingly, the H sub-unit is Holocene, and the Pm sub-unit is considered late Pleis- tocene. Maximum thickness of the total upper unit (Pm and H) is 60 to 65 m in the marginal shelf area (Area 3), but it thins markedly to less than 15 m toward the coast. The thickness of Holocene lagoonal sediments varies from 5 to 15 m, and the boundary between Pm and H occurs con- sistently between 30 and 35 m below sea level.

DISCUSSION

Siliciclastic Foundations of Quaternary Lagoon Reefs

The profiles, combined with the sample logs of drill holes, make it clear that a LATE PLEISTOCENE sequence of siliciclastic sediments underlies what is interpreted as Quaternary carbonates (late Pleistocene and Holocene). Ear- lier, Purdy (1974a, 1974b) also recognized these deposits as siliciclastics, but he interpreted them to be of karst-fluvial origin, because the channels were not continuous from profile to profile along the shelf lagoon. Furthermore, in his profiles made in the 1960's, Purdy was unable to see that the siliciclastic sequence extended beneath the Quaternary carbonates. The channels and their fills of units Pt|

and Pt2 are the key to interpreting the environments of deposition. Contour maps of the bases of the channels of Pt i (Figs. 7, 9) show a south-southeast-oriented pattern which we believe reflects drainage from the Maya Mountains to the Gulf of Honduras. We consider that this drainage pattern and the relatively thick, well-layered nature of the channel fills indicate that the sediments Figure 13. (Continued). are fluvial and were probably deposited on a coastal plain. The discontinuity between

Pt| and Pt2 truncates reflectors of Pt| (Fig. from 36 to 90 m in the Palmetto Cay boring, relief erosional disconformity (Figs. 4 4) and is therefore interpreted as resulting 28 km north of Area 1, and from 26 to 39 m, through 6). The base of Pm is also a low- from a period of erosion caused by a lower- the maximum depth penetrated in the bor- relief disconformity marked by a prominent ing of sea level.

ing at East Snake Cay, 10 km west of Area 1 reflector, and the upper boundary of the H Sub-unit Pt2 also has what we interpret as (Purdy, 1974a, 1974b). From its fine and sub-unit is the present sea floor. Both the channels (Figs. 4, 11), but they are much continuous layering, sub-unit Pt3 is inter- Pm and H sub-units have two different smaller and narrower than those of Ptj. The

preted as marine sediment that filled acoustic signatures: (1) areas with discon- convex upper surface of Pt2 suggests to us channels. tinuous reflectors, inverted v's, or a zig-zag the possibility that this sub-unit is a series of pattern that has steep sides (Figs. 4, 6); and small distributary lobes that are part of a Upper Unit (Pm and H) (2) areas with closely spaced horizontal lay- low-relief deltaic system.

ering (Fig. 6). The layered Pt3 sediments that fill ero- These two sub-units, Pm below and H We interpret the first type of signature sional depressions suggest that this sub-unit above, are separated by a distinct but low- (discontinuous reflectors) as reefs and/or is marine or estuarine and record the filling

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of an earlier, clanneled landscape during a the lagoon floor (Fig. 1) and from the REFERENCES CITED period of rising sea level (Figs. 12, 13). north-to-south increase in the thickness of Holocene reef deposits on the barriers (see Gvirtzman, G., Buchbinder, B., Sneh, A., Nir, Y., Relationship between Quaternary Purdy, 1974b, Fig. 10). and Friedman, G. M., 1977, Morphology of the Red Sea fringing reefs: A result of the The shapes of the late Pleistocene lagoo- Carbonates and Underlying Siliciclastics last glacial low-stand sea level and the fol- nal reefs in Figures 8 and 11 resemble the lowing Holocene recolonization, in Second To examine the relationships between sili- shapes of Holocene reefs in the central part Symposium Intern. Sur les Coraux et récifs ciclastic foundations and overlying Qua- of southern Belize Lagoon, the rhomboidal coralliens fossiles, Paris Sep. 1975, Mém. 89, Bureau de Recherches Géologiques et ternary carbonates, the distribution of the and long sinuous reefs illustrated by Purdy Minières, p. 480-491. (1974a, 1974b). It is tempting to speculate initial carbonates (sub-unit Pni|) was super- Gvirtzman, G., and Buchbinder, B., 1978, Recent imposed on subcrop maps drawn on the that these reefs are also founded on silici- and Pleistocene coral reefs and coastal sedi- base of Pm] (Area 1, Fig. 8; Area 2, Fig. clastics and that their shapes are controlled ments of the Gulf of Elat: Sedimentology in 11). The comparison shows that initial car- by underlying fluvial morphology as seen in Israel, Cyprus and Turkey, Guidebook, pt. II, Post-congress Excursion Y4, Tenth In- bonates, probably reefal, are positioned the southernmost Belize Lagoon, rather ternational Congress on Sedimentology, over residual elevations of the underlying than by a fault-dominated karst morphol- Jerusalem, p. 163-191. siliciclastics. This relationship is most con- ogy as proposed by Purdy (1974a, 1974b). Halley, R. B„ Shinn, E. A., Hudson, J. H., and vincing in Area 1 (Fig. 8) where the initial Reefs growing on siliciclastic sediments Lidz, B., 1977, Recent and relic topography Pm] carbonates are localized along the have been reported from Spain, the Red of Boo Bee Patch Reef, Belize: Third Inter- margins of channels and over what were national Coral Reef Symposium Miami, Sea, and the Great Barrier Reef. Gvirtzman Proceedings, v. 2, Geology, p. 29-35. probably channel and point bars in the and others (1977) and Gvirtzman and Maxwell, W.G.H., 1968, Atlas of the Great Bar- underlying unit (see also Fig. 5). In Area 2 Buchbinder (1978) described Holocene reefs rier Reef: Amsterdam, Elsevier Publishing (Fig. 11), the comparison shows two princi- growing on the margins of alluvial fans. Co., 258 p. pal relationships: (1) over most of the area, Maxwell (1968, 1970) mentioned the possi- 1970, Deltaic pattern in reefs: Deep-Sea Research, v. 17, p. 1005-1018. the carbonates [reefal or bank(?)] are posi- ble influence of ancient drainage-system Purdy, E. G., 1974a, Reef configurations: Cause tioned along the margins of the subcrop of morphology on the distribution of Holo- and effect, in Laporte, L. F., ed., Reefs in sub-unit Pt2 that are just seaward of the cene reefs. Reefs growing on a variety of time and space: Society of Economic Pale- inferred shoreline of an island of Pm age; siliciclastics in the Teritary and Mesozoic of ontologists and Mineralogists Special Publi- and (2) in the upper left, a few bodies of Spain were described by Santisteban (1980) cation no. 18, p. 9-76. carbonate occur along the edge of a channel. 1974b, Karst-determined facies patterns in and Santisteban and Taberner (1980), the British Honduras: Holocene carbonate sed- We interpret the relationships between foundations including deltas, fan-deltas, imentation model: American Association of initial buildups of carbonate (Pm sub-unit) beaches, and tidal bars. Petroleum Geologists Bulletin, v. 58, p. 825-855. and their foundations to mean that differen- These examples, as well as the Belizean Santisteban, C., 1980, Composition, morphol- tial relief determined their locations. Thus, one described here, make it clear that reefs ogy and structure of the fore-reef fan. The residual elevations of siliciclastic sedi- may originate on substrates produced in Tortonian "Desastre Reef', Murcia (Spain) ments—channel banks, bars, and slopes environments quite different from that [abs.]: International Association of Sedi- of islands—were favored sites for initial required by the reef-building community. mentologists First Europ Mtg., Bochum, p. 212-214. accumulations of carbonates. These first The example from Belize indicates that rela- Santisteban, C., and Taberner, C., 1980, The sili- accumulations of carbonates, which per- tively small differences in elevation may ciclastic environments as a dynamic control petuated the residual relief of the siliciclas- serve to localize reefs and determine their in the establishment and evolution of reefs. tics, became, in turn, the foundations for initial shapes. Sedimentary models [abs.]: International some of the Holocene (Hj) reefs and shoals Association of Sedimentologists First Europ Mtg., Bochum, p. 208-211. of carbonate sediment. However, not all of ACKNOWLEDGMENTS U.S. Naval Oceanographic office, 1965, Bathy- the Holocene reefs are positioned on pre- métrie chart, Ranguana Cay to Columbus existing carbonates; some occur on the This paper is a contribution from the Cay, No. H. O. 1497. mound-like elevations of Pt2 believed to be Rosenstiel School of Marine and Atmos- 1970, Bathymétrie chart, Gulf of Honduras, No. H. O. 1496. deltaic sediments,, and others lack indica- pheric Science, University of Miami. Re- tions of underlying elevation. search was supported by the Cooperative Present-day lagoon reefs cover much Research Group in Sedimentology. broader areas than those of the late Pleisto- We thank ESSO International and cene (Fig. 13). We believe this difference is EXXON Production Research for the seis- the result of increased flooding of the mic profiles. We are grateful to James Mul- Southern Shelf produced by differential holland and Bonnier McGregor for advice MANUSCRIPT RECEIVED BY THE SOCIETY subsidence. Additional evidence for post- and encouragement, and to Christopher JANUARY 15, 1981 Pleistocene differential subsidence comes Harrison, John Vedder, and Donald Zenger REVISED MANUSCRIPT RECEIVED MARCH 16, 1981 from the north-to-south increase in depth of for thoughtful editorial suggestions. MANUSCRIPT ACCEPTED MARCH 23, 1981

Primed in U.S.A.

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