THE ROLE OF ACCOMMODATION SPACE ON THE PRESERVATION OF IN SITU TREES IN THE COAL FIELDS OF THE APPALACHIAN BASIN

Fossilized erect tree trunks in coal fields of the world have fascinated geologists since the early 1800s. In Pennsylvanian-age coal fields of the Appalachian Basin, preserved in situ trees are mostly lycopsids and calamites, which vary in height from a few centimeters to more than 4 m. Shorter trunks are more commonly preserved than taller trunks. Relatively rapid burial rates are likely, as these plants contained very little wood for structural strength. The standing vegetation represents disproportionately high local sedimentation rates of meters in tens of years, rather than meters in tens of thousands of years or more, for the discrete intervals in which the trees are preserved. As such, facies containing erect trees preserve temporal snapshots of Carboniferous wetlands. Aside from indicating relatively rapid burial and allowing more accurate reconstructions of paleowetlands, the fossilized trees provide data on the available accommodation space at the time of burial. Accommodation space for rapid burial of Carboniferous vegetation in the Appalachian Basin was provided by the 1) height of levees marginal to vegetated floodplains, which controlled the thickness of crevasse-splays and height to which floodplains could aggrade during any short interval of time; 2) the depth of abandoned channels, or paleotopographic depressions; 3) compaction of underlying peats (by dewatering, drying, etc.); 4) changes in local water level (tidal, fluvial, or other); and possibly 5) syn-depositional faulting. During deposition of the Fire Clay coal there was also a volcanic ash fall, but no large in situ trees have been documented encased in this tonstein.