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Facies Reconstruction of a Late Pleistocene Cypress Forest Discovered on the Northern Gulf of Mexico Continental Shelf
A previously buried bald cypress forest (Taxodium distichum) was discovered on the continental shelf seafloor, offshore of Orange Beach, Alabama, USA, in ~20 m water depth. The forest was likely buried in the late Pleistocene, possibly exhumed by Hurricane Ivan in 2004, and is now exposed as stumps in life position. In August 2015 and July 2016, submersible vibracores and geophysical data were collected to investigate local stratigraphy and mode of forest preservation. This study focuses on analysis of the longest and most stratigraphically complete vibracore, DF–1 (4.78 m). This core revealed, from top to bottom, a surface of Holocene transgressive sands, underlain by interbedded sand and mud (potentially Holocene or Pleistocene), overlying a swamp or delta plain facies (likely Pleistocene) containing woody debris and mud that has been provisionally dated using radiocarbon to ca 41–45 ka. One core collected in 2016 revealed a Pleistocene paleosol beneath Holocene sands in a nearby trough.
We hypothesize that floodplain aggradation in the area was a key factor that might have allowed forest preservation. A sea-level rise pulse of 10–15 m occurred ca. 40 ka that could have produced widespread floodplain aggradation, likely burying the swamp and forest sediments. During the subsequent glacial lowstand, sediments that comprise the floodplain were eroded and paleosols were formed in other nearby areas. It is hypothesized that some swamp sediments located in paleo-topographic lows might have been preserved and buried due to the deep coverage of the eastern-trending channel infill sediments. Coastal wave erosion during transgression likely eroded high ground but enough sediment remained to keep the cypress forest blanketed and therefore allowed preservation of stumps and woody debris.
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