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Fining-upwards Sedimentary Sequences Generated in Seagrass Beds
Harold R. Wanless
Shallow-marine clastic and carbonate environments of southeast Florida contain fining-upwards sedimentary sequences being generated by beds of the seagrass, Thalassia testudinum. Lateral seaward migration of flute-shaped sand pockets (storm blowouts) in the grass platform produces a lenticular fining-upwards package 10-50 m in width, 20-100 m in length, and 0.3-2 m in thickness. Migration and vegetative restabilization of recurring "blowouts" on a grass platform can produce a composite sequence containing numerous complete or stacked incomplete fining-upwards packages.
A complete fining-upwards sequence consists of 1) an erosional base, 2) a coarse basal shell layer representing the winnowed lag of the eroded, preexisting grass-stabilized platform, 3) a ripple-laminated, well-sorted, medium-grained sand zone representing the mobile sand on the leeward slope of the blowout, and 4) a shelly, bioturbated sand or silty sand having a strongly bimodal texture (modes at 100 and 500 µm) in which the fine mode dominates upwards. This upper portion records increased trapping of storm suspensions and decreased bedload transport as seagrasses are reestablished. The mollusks Chione cancellata and Codakia orbicularis characterize the grass-stabilized zone. The distinctive textural sequence and molluscan assemblage serve to differentiate these iogenically induced fining-upwards sequences from others in the geological record.
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