Reef demise and back-stepping during the last interglacial, northeast Yucatan |
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Authors: | Paul Blanchon |
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Institution: | (1) Reef Systems Unit, Institute of Marine Sciences and Limnology, National Autonomous University of Mexico (UNAM), Ap 1152, 77500 Cancun, Q. Roo, Mexico |
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Abstract: | The elevation of reefs and coastal deposits during the last Interglaciation (MIS-5e) indicates that sea level reached a highstand
of as much as 6 m above the present, but it is uncertain how rapidly this level was attained and how it impacted reef development.
To investigate this problem, I made a detailed sedimentological analysis of a well-dated reef from the northeast coast of
the stable Yucatan Peninsula. Two linear reef tracts were delineated which are offset and at different elevations. The lower
reef tract crops out along northern shore for 575 m and extends from below present mean sea level to +3 m. The reef crest
facies consists of large Acropora palmata colonies dispersed within a coral boulder-gravel and is flanked by an A. cervicornis-dominated reef-front and a large area of lagoonal framework formed by coalesced patches of A. cervicornis and Montastraea spp. Constituents in the upper centimetre of the lower tract are heavily encrusted by a cap of crustose corallines and, in
places, are levelled by a discontinuous marine-erosion surface. The upper reef tract crops out ~150 m inland up to an elevation
of +5.8 m and parallels the southern section of shore for ~400 m. It also consist of an A. palmata-dominated crest facies flanked by reef-front, back-reef and lagoonal frameworks. In this case, however, lagoonal frameworks
are dominated by a sediment-tolerant assemblage of branching coralline algae. Also different is the lack of encrustation by
corallines, and the infiltration of upper tract facies by beach-derived shell-gravels from regressive shoreface deposits above.
These results indicate that the lower reef tract and lagoonal patch-reefs formed at a sea level of +3 m. Final capping by
crustose corallines and discontinuous marine erosion indicates that the lower tract was terminated by the complete demise
of corals on the crest but only patchy demise in the lagoon. Areas of continuous framework accretion between the lagoonal
patch reefs and the upper reef-tract, however, require that the demise of this reef was ecologically synchronous with initiation
of the upper reef-tract, which had back-stepped 100 m into the lagoon. In this new position, the upper tract developed a reef
crest that corresponded to a final sea-level position of +6 m. Reef flat development at +5 m and large in-place colonies of
A. palmata at the base of the crest unit indicate, however, that sea level must have risen rapidly from +3 to more than +5 m to accommodate
back-stepping. This sea-level jump created a higher energy wave field that mobilized back-reef and lagoonal sediments, and
the resulting high sediment flux eroded lagoonal framework and prevented the recovery of the submerged lower reef crest. So
this single jump in sea level was responsible not only for reef demise and back-stepping but also for marine erosion and suppression
of subsequent reef development—features that elsewhere have been used to support multiple sea-level excursions during the
last interglacial. |
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