Polystrate fossil
According to mainstream models of sedimentary environments, they are formed by rare to infrequent brief episodes of rapid sedimentation separated by long periods of either slow deposition, nondeposition, or a combination of both.
[3][4][5] Upright fossils typically occur in layers associated with an actively subsiding coastal plain or rift basin, or with the accumulation of volcanic material around a periodically erupting stratovolcano.
In river deltas and other coastal-plain settings, rapid sedimentation is often the end result of a brief period of accelerated subsidence of an area of coastal plain relative to sea level caused by salt tectonics, global sea-level rise, growth faulting, continental margin collapse, or some combination of these factors.
[6][8] The upright fossil trees of the Gallatin Petrified Forest in the Gallatin Range and the Yellowstone Petrified Forest at Amethyst Mountain and Specimen Ridge in Yellowstone National Park, occur buried within the lahars and other volcanic deposits comprising the Eocene Lamar River Formation as the result of periods of rapid sedimentation associated with explosive volcanism.
These processes result in the rapid burial of large areas of the surrounding countryside beneath several meters of sediment, as directly observed during the 1991 eruption of Mount Pinatubo.
[9] As with modern lahar deposits, the sedimentary layers containing upright trees of the Yellowstone petrified forest are discontinuous and very limited in areal extent.
[10][17][18] Retallack (1981, 1997) has published pictures and diagrams of the Yellowstone upright fossil trees having intact root systems developed within paleosols found within these strata.
[13][14][15] A detailed study by Taylor and Vinn (2006) of the microstructure of fossils which have been traditionally identified as "Spirorbis" in the geological literature revealed that they consist of the remains of at least two completely different animals.
This position of geologists is supported by numerous documented examples, a few of which are discussed in the paragraphs below, of buried upright tree-trunks that have been observed buried in the Holocene volcanic deposits of Mount St. Helens, Skamania County, Washington, and Mount Pinatubo, Philippines; in the deltaic and fluvial sediments of the Mississippi River Delta; and in glacial deposits within the midwestern United States.
[10][17] Within excavations for Interstate Highway 10 in the United States of America, and in borrow pits, in landfills, and archaeological surveys, unfossilized upright trees have been found buried within late Holocene, even historic, fluvial and deltaic sediments underlying the surface of the Mississippi River Delta and the Atchafalaya Basin of Louisiana.
In one case, borrow pits dug in the natural levees of Bayou Teche near Patterson, Louisiana, have exposed completely buried, 4 to 6 feet (1.2 to 1.8 meters) high, upright trunks of cypress trees.
[31] Unfossilized, late Pleistocene upright trees have been found buried beneath glacial deposits within North America along the southern edge of the Laurentide Ice Sheet.
Sediments released by the melting of the adjacent ice sheet rapidly filled these lakes, which quickly buried and preserved the submerged forests lying within them.
[32] Excavations for a tailings pond about Marquette, Michigan, exposed an in situ forest of unfossilized trees, which are about 10,000 years old, buried in glacial lake and stream sediments.
