A MODEL FOR THE HOLOGENE EVOLUTION OF REEF ENVIRONMENTS IN SOUTHWESTERN TOBAGO

DEREK HUDSON
Exploration Department
Trinidad and Tobago Petroleum Company Limited
Santa Flora, Trinidad


Drowned reefs at sheif edges and relatively young fringing reefs (.<8,000 yr B.P.) along coastlines are characteristic of the eastern Caribbean. It is now clearly established through analyses of cores and age dating that these fringing reefs kept pace with rising sea level during the Holocene transgression. This information, along with the data available on Holocene eustatic sea level changes In the Caribbean and available field evidence, provide an excellent framework for postulating the evolution of reef environments in southwestern Tobago, including the Buccoo Reef Complex, over the last 18,000 yr.
The proposed model suggests that: 80,000 - 120,000 yr B.P. Similar accretionary reef limestones which were deposited during preceding marine transgressions may also underlie the present reef systems.
Studies such as shallow sparker profiling, age dating and compositional analyses of cores should be carried out in order to clarify the ideas proposed in this model for Holocene reef evolution in southwestern Tobago.
(I) Southwest Tobago Reefs are situated on a Pleistocene coral and lagoonal limestone, which unconformable overlies sands, gravels and clays of the Rockly Bay Formation and voicanics of the Bacolet Formation.
(ii) The exposed Pleistocene coral has been dated at approximately 125,000 yr B.P. and represents reef growth during the last major interglacial high of
(iii) At the initiation of the Holocene transgression, approximately 18,000 to 10,000 yrB.P., ahermatypic corals, algae and sponges would be establIshed on sand and rock substrates in the deeper areas of the reefs, 120 to 30 m below mean sea level (m.s.l.). Constructive reef growth may have occurred at this time on the margins of the Drew Bank, which Is SSE of Buccoo Reef Complex.

(iv) From 10,000 to 8,000 yr. B.P., flooding of lower sheif areas with associated turbid conditions may have resulted in the drowning of the constructive reef at Drew bank. This relict reef which Is probably covered by sediment, occurs at present as a shoal, 6-12 m below m.s.l.

(v) From 7,000 to 2,000 yr. B.P., rapid vertical accretion of reef growth, accompanying rising sea level, occurred along the shell margin, to approximately 2 m below m.s.l. probably coincident with the return of warm, clear, saline waters. Present day reef flats, especially at Buccoo Reef would be Initiated during the latter haif of this period on antecedent topographic highs. back-reef, lagoonal and patch reef environments would also be established during this period, as a result of inundation of the upper, shallow shell by waters being exchanged through antecedent topographic lows or channels.

(vi) Slower rates of sea level rise from 1500 yr. B.P. to present probably resulted In some vertical build-up and extensive lateral accretion. More restrictivewater circulation patterns created by the lowered rates of sea level rise and vertical build-up at the reef flats to m.s.l., resulted in rapid sediment deposition in back-reef and lagoonal areas; and species diversification and coral rubble development in patch reef areas.

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