Evolution of an isolated carbonate bank during Oligocene,Miocene and Pliocene times,Cayman Brac,British west Indies |
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Authors: | Prof Dr Brian Jones Mr Ian G Hunter |
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Institution: | (1) Department of Geology, University of Alberta, T6G 2E3 Edmonton, Alberta, Canada |
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Abstract: | Summary Modern carbonate sedimentation in the Caribbean Sea commonly occurs on banks that are surrounded and isolated by deep oceanic
water. This depositional regime also occurred during the Tertiary, and many islands, such as Cayman Brac, have sequences that
evolved in such settings.
Cayman Brac is a small (about 39 km2) island, located on the Cayman Ridge, that has an exposed Oligocene to Pliocene succession which encompasses three unconformity-bounded
formations. The upper Lower Oligocene Brac Formation is formed ofLepidocyclina limestones and sucrosis dolostones that locally contain numerous bivalves and gastropods. The overlying Lower to Middle Miocene
Cayman Formation is formed of pervasively dolomitized mudstones to grainstones that contain an abundant, diverse biota of
corals, gastropods, bivalves, foraminifera, and algae. Rhodolites are locally common. The Pliocene Pedro Castle Formation
is formed of limestones, dolostones, and dolomitic limestones that contain a biota which is similar to that in the Cayman
Formation. The unconformities between the formations represent substantial periods of time during which the previously deposited
carbonates were lithified and eroded to produce karst terrains.
All facies in the Brac, Cayman, and Pedro Castle formations on Cayman Brac developed on a bank that was no more than 20 km
long and 3 km wide. There is no evidence of reef development other than isolated thickets ofStylophora and/orPorites and no systematic stratigraphic or geographic changes in the facies patterns of the formations. Comparison with modern Caribbean
banks shows that the depositional regime was primarily controlled by water depth and energy levels. Limestones of the Brac
Formation probably accumulated in low-energy conditions in water less than 10 m deep. The overlying Cayman Formation contains
facies that formed in water 15 to 30 m deep with good cross-bank circulation. The Pedro Castle Formation formed in slightly
shallower water (5–25 m) and lower energy conditions. The disconformities between the packages correlate with world wide eustatic
drops in sea level. |
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Keywords: | Carbonate Banks Unconformities Cayman Brac (British West Indies) Tertiary |
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