Fining-upwards sedimentary sequences generated in seagrass beds.

Shallow-marine clastic and carbonate environments of SE 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. 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 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 biogenically induced fining-upwards sequences from others in the geological record. -from Author