Holocene Carbonate-Siliciclastic Shoreline and Ravinement Stratigraphy of the Belize Lagoon: a Mixed-System Model

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Abel Hugh Murphy
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1 Holocene CarbonateSiliciclastic Shoreline and Ravinement Stratigraphy of the Belize Lagoon: a MixedSystem Model DONALD F. MCNEILL AND CLINTON A. COWAN Key findings o Mixedsystem facies heterogeneity during slow sealevel rise. o Mixing mechanism: infaunal reworking significant. o Estuarine trapping of fines = siliciclasticfree skeletal cap. o Change in reef style between slow and rapid sealevel rise. Initial results of mapping and coring program The lowenergy southern Belize coast has estuarine, deltaic, shoreface and palimpsest siliciclastic sediments (clays, quartzose sands and rare gravels) that are being intricately mixed with modern carbonates. Cores and surface samples are used to map facies and characterize the recent history of 25 km of coast from Placencia to the Monkey River, from shoreline to 10 m depth (a 25 km wide area; Fig. 1). Waves and longshore currents are the principle agents of marine sediment transport; tides are insignificant. Persistent longshore currents usually restrict deltaic plumes of turbid water to areas close to shore. This allows photozoan coralalgal carbonates to exploit areas immediately adjacent to suspended and deposited siliciclastics. There is a strong feedback, in that partitioning of coastal turbidity is enhanced by shoaling (aggrading) mudbanks, coral cays and especially mangroves that create shelters (local catchments for fines), headlands (reef sites) and conduits (intercay and mangrove channels). Additionally, mixing of siliciclastic and carbonate occur as a result of marine bioturbation. We have found that infauna locally rework older, submerged coastal plain sediments into modern environments across the ravined Holoceneflooding surface. The Belize coast sediments were deposited during a relatively slow rise in sea level (Fig. 2) as compared to the open Belize lagoon sequences recently studied with seismic and core data (Esker et al., 1998). Cores from the coastal sequence show backstepped estuarine facies, and aggraded mudbank, mangrove and coral facies during 1

2 Holocene flooding (Fig. 3ac). The slowrise conditions have allowed proliferation of dense mangrove cover over marine facies after the initial transgression. These laterstage mangroves have acted to baffling and retain suspended sediment giving rise to widespread estuarine facies and a heterogeneous suite of coastal facies. This contrasts with offshore shelf deposits that generally lack widespread estuarine and appreciable heterogeneous facies because these areas experienced more rapid sealevel rise in the earlier Holocene. In addition, the mangrove colonization and the formation of islands appear to have provided a stable foundation for widespread fringing reef development along seawardfacing headlands. The coastal reef morphology is significantly different from open lagoon reef deposition during the earlier period of rapid sealevel rise. The coastal reefs tend to form shoreline parallel, elongate deposits that expand at both ends through the shedding and mixing of coral debris with the encroaching mangrove deposits. This elongate geometry is in distinct contrast to the open lagoon where the reefs appear to reoccupy preexisting highs (elevated paleoreefs, channel bars, channel levees) and usually form steepsided pinnacles. Application and significance of findings o Surface facies mapping has developed a 2D example of facies heterogeneity in a coastal mixed carbonatesiliciclastic system in a slowrise sea level condition. The surface distribution and scale of potentially porous facies are quantified as a modern example of a mixedsystem. o The mechanism for mixing of coarse quartz sand in modern marine facies has now been identified. The introduction of quartzose material occurred through the infaunal reworking of a concentrated lag of quartzsand and gravel that caps the exposed coastal plain. This residual lag is first reworked at the ravinement and later under openmarine conditions that favor extensive bioturbation. o Shallow cores have documented the nature of facies development and evolution during a relatively slow rise in sea level. Consistent with the surface mapping, the subsurface contains appreciable heterogeneity in lithofacies. This heterogeneity is in apparent contrast with cores from offshore, where open lagoon deposits generally lack estuarine and other shallowwater facies. 2

3 o The late stage stabilization of the coastal deposits by extensive mangrove development appears to have resulted in the estuarine trapping of siliciclastic fines. This restricted input of terrestrial suspendedload is manifest in core by a change in lithofacies from a mixed siliciclasticmud and skeletal carbonate to a predominantly siliciclasticfree "clean" skeletal carbonate cap. o The style of reef deposition during a slow sealevel rise appears to be distinct from the fastrise reefs in the open lagoon. The coastal reefs are accommodationspace limited and result in shoreparallel, elongate deposits. In contrast, the deeper parts of the open lagoon have a reef morphology characterized by steepsided, pinnacle features that were initiated during a period of rapid sealevel rise. 3

4 N Northern Area Benthic Facies Map CARBONATE SHORELINE SUBTIDAL QUARTZMOLLUSC SANDS ESTUARINE MUDDY BOTTOM SHELTERED LAGOON MUDDY BOTTOM MUDBANKS THALASSIASKELETAL COASTAL PLAIN SPIT AND CAY QUARTZ MOLLUSC SANDS MODERN SANDS MANGROVE PORITESAMPHIROA CARPET AND MOUNDS PATCH REEF AREAS HEADLAND REEF OPEN LAGOON MUDDY BARE BOTTOM Figure 1a. Benthic facies map of the northern study area, southern Belize lagoon.

5 N Mudbanks Reef Mounds Southern Area Benthic Facies Map CARBONATE SHORELINE SUBTIDAL QUARTZMOLLUSC SANDS ESTUARINE MUDDY BOTTOM SHELTERED LAGOON MUDDY BOTTOM COASTAL PLAIN SPIT AND CAY QUARTZ MOLLUSC SANDS MODERN SANDS MANGROVE MUDBANKS THALASSIASKELETAL PORITESAMPHIROA CARPET AND MOUNDS PATCH REEF AREAS HEADLAND REEF OPEN LAGOON MUDDY BARE BOTTOM Figure 1b. Benthic facies map of the southern study area, southern Belize lagoon.

6 Facies Initiation Window Belize coastal deposits coastal mixed system 5 Depth (mbsl) multipinnacle reefs individual pinnacle reefs southern Belize lagoon reefs and incised valleys Depth (feet bsl) southern Belize Holocene SeaLevel Curve Western Atlantic (Fairbanks, 1989) Age (C Figure 2. Holocene sealevel curve for the western Atlantic based on corals from Barbados and Florida (Fairbanks, 1989). The Belize coastal system was deposited during a period of relatively slow sealevel rise compared to pinnacle reefs and incised valley systems in the deeper lagoon.

1 Shoreline Landforms 2. 2 Emergent v. Submergent 2. 3 Wavecutting 3. 4 Planview 4. 5 Marine Terraces 5. 6 California 7. 7 Tombolos, Sea Stacks 8

1 Shoreline Landforms 2. 2 Emergent v. Submergent 2. 3 Wavecutting 3. 4 Planview 4. 5 Marine Terraces 5. 6 California 7. 7 Tombolos, Sea Stacks 8 Shorelines November 9, 2008 Contents 1 Shoreline Landforms 2 2 Emergent v. Submergent 2 3 Wavecutting 3 4 Planview 4 5 Marine Terraces 5 6 California 7 7 Tombolos, Sea Stacks 8 8 Active Processes 9 9 Emergence