REEF TO OOID SEQUENCES AND LEEWARD
PLATFORM MARGIN EVOLUTION: THE HOLOCENE
AND PLEISTOCENE OF WEST CAICOS
Overview
Paraphrasing the guidebook assembled by Wanless and Dravis (1989), the island of West Caicos exposes the architecture of a Pleistocene platform-margin barrier
reef system and overlying oolitic sand complex thus enabling the examination of the larger-scale evolution of leeward platform-margin sedimentation during the late Pleistocene and Holocene. Outstanding outcrops and core-boring transects provide data on reef and
oolitic grainstone development, their interaction and facies evolution. This data base permits critical evaluation of how environmental processes operated during the
Pleistocene to control 'leeward margin carbonate deposition.
Physiographic Setting
The island of West Caicos is approximately 6 km in length and 1.5 km in width and is located less than 1 km from the steep western leeward margin of Caicos
Platform. The island contains an older central spine of two north-south trending
oolitic/peloidal grainstone ridges. The internal structure of these ridges indicate that they were formed by transport from east to west. Younger Pleistocene ridges have extended the island to the north, south, east and west (Wanless and Rossinsky, 1986; Waltz, 1988). Holocene beach accretion ridges further extended the northern and eastern island margins.
The western margin of the island is a continuous cliff sequence, 2-6 m in height, which exposes spectacular outcrops of Pleistocene reefal deposits and oolitic grainstones.
From the western reefal cliff margin, the limestone surface slopes gently eastward to Lake Catherine, which fills the low area between the western reefal/oolitic ridge and the central ridge spine. Lake Catherine has fairly normal salinity because of exchange with ocean waters through karst pipes. Other low inter-ridge areas are occcupied by hypersaline
ponds and salinas. Great Salina is a large, elongate salina between the central spine and the Holocene beach ridges to the east. Company Point Salina, at Northwest Point, is impounded by Holocene beach ridges and contains a thick Holocene sediment sequence.
A broad, low area in the interior of southern West Caicos has an intensely karstic surface with solution holes ranging from several centimeters to tens of meters in diameter. The surface calcrete over this low area appears to be a composite or coalesced calcrete,
representing two Pleistocene exposure periods without intervening marine sedimentation. Eastward, these two soil crusts separate in the artifically cut outcrops at
an abandoned railway cut and towns and drilling transects R-R' and S-S'.
Exposed Section of Reefal Associations
Excellent exposures of a Late Pleistocene leeward margin barrier reef sequence occur along the extreme western margin of West Caicos.Also reefal exposures occur along the southern twothirds of the western coast and occur as a linear belt as much as 80 meters in width. Up to 25 meters of emergent and submergent vertical reefal section are exposed. The seaward edge of this Pleistocene reef sequence is only a few hundred meters from the present-day platform margin. The upper Pleistocene reefal sequence contains four reefal lithologies.
(1) Coralline algal framestone and bindstone comprised either of coralline algal branches in
an oolitic and skeletal sand matrix or of surficial and cavity-filling encrustations. This lithology forms a broad reef flat, especially along the southern portion
of the reef.
(2) sorted rubble Acropora palmata framestone and rudstone consisting of either in situ
and fronds of Acropora palmata in a poorly skeletal grainstone matrix or oriented frond
This lithology forms the seaward reef crest deposits.
(3) Head coral framestone dominated by the coral Montastrea annularis, followed by Diploria sp. and Porites astreoides. Corals are mostly in growth position and occur in a skeletal
grainstone matrix. This lithology is the most abundant of the reefal facies and is located behind and beneath the Acropora palmata framestone.
(4) Acropora cervicomis bafflestone and rudstone consist of Acropora cervicornis branches in a skeletal grainstone matrix. Burrowing is common in the Acropora cervicornis bafflestone, where branches are widely spaced and more vertically oriented. The
Acropora cervicornis rudstone is dominated by horizontally oriented branches in contact with each other.
These lithologies are encountered only in core and in the deeper portions of the underwater cliff and represent deeper forereef deposition.
Physical features and strong lateral and vertical gradients in facies illustrate the maturity of the Pleistocene West Caicos reef. The more platform marginal reef crest, as in the Holocene, is dominated by thick, massive branching Acropora palmata, present both as in situ growth and rubble. Some of this rubble was subsequently colonized by head corals.
Extending seaward from the southernmost exposures are preserved outcrops of fossil spur and groove structures which are visible both from the air and underwater.
The classic shallowing-upwards reefal sequence from deeper Acropora cervicornis deposits overlain by head coral framestones is well exposed. Head coral framestones formed a
low energy buildup both in shallow water platformward of the Acropora palmata / coralline algae reef crest and in deeper water between the A. palmata zone and the deeper A. cervicomis zone. The Acropora palmata framestones represent the shallow-water wave break
environment (Storr, 1964). In southern exposures, coralline algae cap the reef crest platform ward of the A. palmata framestone. The coralline algal lithology extends to about 3.5 m above present mean water level.
Outcrops expose numerous small, circular to elongate patch reefs dominated by head corals and
fossil meadows of Porites porites behind the reef crest. The distribution, composition and geometry of these reefs is similar to the Holocene fringing and patch reefs north of Providenciales.
Pleistocene oolitic grainstones
Exquisite exposures of Pleistocene oolitic grainstones on West Caicos form a shallowing-upwards cap to the reefal sequences. North and east (platformward) of the reefal
occurrences, oolitic facies also form complete and repetitive shallowing-upwards sequences. Oolitic grainstone lithologies and interpretations were determined from outcrops and core borings (Waltz, 1988). Lithologies are presented from bottom to top as they occur in outcrop.
(1) Burrowed oolitic grainstone is moderately sorted and dominated by burrow structures or burrow-mottled fabric. Lined Ophiomorpha burrows dominate the upper, more sparsely burrowed portions of this lithology. Smaller sub horizontal to randomly oriented sand-filled burrow tubes are prevalent in the lower, more intensely burrowed zone, and can yield to intensely bioturbated, mottled fabrics. Burrowed oolitic grainstones represent a stabilized subtidal sand environment.
The burrow-mottled and cross-bedded oolitic grainstones (stabilized and mobile subtidal) cover both the backreef patch reefs and portions of the reef crest facies.
(2) Cross-bedded oolitic grainstone is planar and trough cross stratified and consists of medium- to coarse-grained, moderately well-sorted oolite. Physical structures, together with scattered Ophiomorpha burrow structures and burrowing sea anemone (Phylactis) structures (Shinn, 1968), indicate a mobile, high-energy, shallow subtidal depositional environment. Trough cross-bedded units are commonly exposed in plan view and record a southerly longshore sediment transport direction in the northern outcrops and a northerly transport direction in the southern outcrops. Cross stratification in larger planar cross stratification
generally has an onshore dip direction. Cross-bedded oolitic grainstones occur 10 channels
and other areas of low reefal paleotopography.
(3) Oolitic/coral block conglomerate forms wedges and layers at the base of and within the oolitic sequences. conglomerates are comprised of subrounded to rounded oolitic blocks, some containing keystone fenestrae (beachrock-derived conglomerate), and/or coral blocks in an oolitic to oolitic-skeletal grainstone matrix. Oolitic conglomerates predominate in the northern outcrop exposures while coral conglomerates dominate the
southern exposures. These conglomerates may be incorporated in an oolitic grainstone sequence, commonly occurring just below the beach deposits, or may directly cap an erosional surface on top of the reefal sequence and at the base of the oolitic sequence. By far the
most spectacular conglomeratic outcrops are adjacent to the small cove at the south tip of West Caicos where coral-dominated conglomerates forms several seaward-dipping wedges 3-5 meters thick.
Layers of storm-derived conglomerates are observed to occur in both the stabilized and mobile oolitic grainstones. In some cases, several stacked conglomerate layers occur in an outcropping sequence.
(4) Layered oolitic grainstone is dominated by low angle, seaward-dipping planar beds of well-sorted, medium-grained ooids. Keystone fenestrae, parting lineations, coarse basal skeletal lags at the base of the sequence, swash markings and lines, and rill markings
indicate deposition in a beach environment.
(5) Fine-grained oolitic grainstone is composed of fine-grained ooids that are well-sorted and layered to root-mottled. It is interpreted as a wind-deposited back beach and dune deposit.
The transition from beach to back-beach and dune is best seen in the higber ridges capping the reefal sequences towards the southern end of the island.
The photographers of the images in the galleries ranged from Ned Frost, Ryan Phelps, Jerome Bellian, Steve Steve Bachtel, Enzo Aconcha, and Christopher Kendall, all of whom participated on a field trip by Jackson School of Geosciences from Austin Texas.
The image galleries the West Caicos Pleistocene rocks, linked above, are listed as four groups:
(1) Pleistocene Part 1
(2) Pleistocene Part 2
(3) Pleistocene Part 3
(4) Pleistocene Part 4
References
Waltz, M.D., 1988, The evolution of shallowing-upwards reef to oolite sequences at the leeward margin of Caicos Platform, BWI, M.S. Thesis, Univ. Miami, Coral Gables; FL, 95p.
Wanless,H.R. and Dravis, J.J., 1989, Carbonate Environments and Sequences of Caicos Platform, Field Trip Guidebook T3 74. American Geophysical Union, Washington, DC. 75 pp.
Wanless, H.R., and V. Rossinsky, Jr., , 1986, Coastal accretion on leeward margins of carbonate platforms, Turks and Caicos Islands, BWI, Amer. Assoc. Petroleum Geologists Bull., 70, 660.