Holocene sedimentation and coastal response to rising sea level along a subtropical low energy coast, Ten Thousand Islands, southwest Florida

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Geology and Geophysics

First Committee Member

H. R. Wanless - Committee Chair


The Ten Thousand Islands (TTI) are a myriad of low relief mangrove islands which lie along the low energy, subtropical southwest Florida coast. The region has been subjected to a relative rise in Holocene sea level, which has continuously decelerated up to its present position.Data derived from surface and subsurface sampling indicate that the Holocene sediment package of the TTI area consists of two sediment sequences. The lower sediment sequence is transgressive and was generated as a mangrove fringed shoreline passively retreated landward under the influence of rising sea level and was replaced by a shallow subtidal coastal marine setting. The upper sediment sequence consists of (1) biogenic shallowing upwards sequences or (2) thickened mangrove peat layers, reflecting island emergence and shoreline stabilization, respectively. Island emergence compartmentalized the area, reducing wave and current energy and promoting the deposition of organic rich shelly mudstone and wackestone in protected inter-island bays.Based on coastal stratigraphy and Carbon-14 dates, these changes in coastal response are directly related to changing rates of Holocene sealevel rise. The initial rapid rise, which averaged about 26 cm/100 years (Neuman, 1976, unpublished; Wanless, 1982) prompted shoreline retreat and the development of a transgressive facies sequence. All deposits associated with the deepening phase dated at $>$3,500 ybp. As the rate of rise slowed to an average of $<$10cm/100 years (ibid), biological sediment production began to out pace the transgressive effects of sealevel rise, as evidenced by the numerous shallowing upwards sediment sequences which initiated upon the subtidal substrate and underlie most of the mangrove islands. Carbon-14 dates indicate that these biologic buildups initiated at $<$3,500 ybp. As pointed out by Scholl (1964a) and Wanless (1982) for south Florida, and Neuman (1976, unpublished) for the Bermuda area, 3,500 ypb marks the transition from a rapid to a slow rate of Holocene sealevel rise.The transgressive/regressive sediment couplet generated during the Holocene sealevel rise in south Florida is remarkably similar to the thin, short-term cycles which repeat 10's to 100's of times throughout the geologic record. The results of this study support the allocyclic mechanism as a visible hypothesis for the generation of these rock cycles.



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