GEOLOGIC SETTING AND PRESERVATION OF A LATE PLEISTOCENE BALD 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 of water. The forest is preserved as stumps in life position with little evidence of decay and large pieces of trunks. Analysis shows the forest is older than can be dated with AMS C-14 methods. Comparison of Pleistocene sea level curves with the study area depth suggests that the forest developed and was likely buried during marine isotope stage 3 or 4, or perhaps older stages. Condition of sampled wood suggests that the forest was buried and preserved in anoxic sediments for millennia, prior to recent exhumation. To better understand the puzzling geological conditions that could allow forest preservation during sea level fall and shelf exposure spanning >30,000 years, submersible vibracores (to 6 m length) and geophysical data (swath bathymetry, sidescan, and chirp subbottom) were collected. Cores are being analyzed using a Geotek Multi Sensor Core Logger, granulometric and sediment composition analyses. This presentation focuses on the geological setting and mode of forest preservation.

Preliminary analysis of sediment types and stratigraphy in cores shows that the local stratigraphy is broadly consistent with previous regional shelf-stratigraphic studies, consisting of (top to bottom) a surface facies of Holocene transgressive sands (high and uniform bulk densities, to ~3 m thick) overlying (as yet undated) Pleistocene terrestrial and coastal facies that have been penetrated by core to depths of <2 m. Pleistocene facies include sediments of interbedded sand and mud, overlying a swamp or delta plain facies containing woody debris and mud, of low and variable bulk density. This deeper facies is thought to include the soil horizons of the ancient cypress forest. However, the Pleistocene lithofacies (fluvial, backswamp, or possibly delta plain muds) appears to differ considerably in both depositional environment and degree of environmental preservation from previous studies (mostly described as highly oxidized paleosols and bay-floor muds). Ongoing analysis will focus on more detailed facies description and interpretation, and elucidating the succession of events that allows preservation of this unique Pleistocene sedimentary record.