Polystrate fossil
A polystrate fossil is a fossil of a single organism that extends through more than one geological stratum. The word polystrate is not a standard geological term. This term is typically found in creationist publications.
This term is typically applied to "fossil forests" of upright fossil tree trunks and stumps that have been found worldwide, i.e. in the Eastern United States, Eastern Canada, England, France, Germany, and Australia, typically associated with coal-bearing strata. Within Carboniferous coal-bearing strata, it is also very common to find what are called Stigmaria within the same stratum. Stigmaria are completely absent in post-Carboniferous strata, which contain either coal, polystrate trees, or both.
Geological explanation
In geology, such fossils are referred to as either upright fossil trunks, upright fossil trees, or T0 assemblages. 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.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. Typically, this period of rapid sedimentation was followed by a period of time - decades to thousands of years long - characterized by very slow or no accumulation of sediments. 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. For example, geologists such as John W. F. Waldron and Michael C. Rygel have argued that the rapid burial and preservation of polystrate fossil trees found at Joggins, Nova Scotia directly result from rapid subsidence, caused by salt tectonics within an already subsiding pull-apart basin, and from the resulting rapid accumulation of sediments. The specific layers containing polystrate fossils occupy only a very limited fraction of the total area of any of these basins.
Yellowstone
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. This type of volcanism generates and deposits large quantities of loose volcanic material as a blanket over the slope of a volcano, as happened during the 1991 eruption of Mount Pinatubo. Both during and for years after a period of volcanism, lahars and normal stream activity wash this loose volcanic material downslope. 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. As with modern lahar deposits, the sedimentary layers containing upright trees of the Yellowstone petrified forest are discontinuous and very limited in areal extent. Individual layers containing upright trees and individual buried forests occupy only a very small fraction of the total area of Yellowstone National Park.Fossil soils
Geologists have recognized innumerable fossil soils throughout the strata containing upright fossils at Joggins in Nova Scotia, in the Yellowstone petrified forests, in the coal mines of the Black Warrior Basin of Alabama, and at many other locations. The layer immediately underlying coal seams, often called either "seatearth" or "underclay", typically either consists of or contains a paleosol. Paleosols are soils which were formed by subaerial weathering during periods of very slow or no accumulation of sediments. Later, renewed sedimentation buried these soils to create paleosols. These paleosols are identified on the basis of the presence of structures and microstructures unique to soils; animal burrows and molds of plant roots of various sizes and types; recognizable soil-profile development; and alteration of minerals by soil processes. In many cases, these paleosols are virtually identical to modern soils.Geologists, who have studied upright fossils found in sedimentary rocks exposed in various outcrops for decades, have described the upright fossil trees as being deeply rooted in place and typically rooted in recognizable paleosols. Researchers such as Falcon and Rygel et al., have published detailed field-sketches and pictures of upright tree-fossils with intact root systems, which are rooted within recognizable paleosols. In the case of the upright fossil trees of the Yellowstone petrified forests, it has been found that the upright fossil trees, except for relatively short stumps, are rooted in place within the underlying sediments. Typically, the sediments within which trees are rooted have paleosols developed within them. Retallack has published pictures and diagrams of the Yellowstone upright fossil trees having intact root systems developed within paleosols found within these strata.
Formation by regeneration
Geologists have also found that some of the larger upright fossil trees found within Carboniferous coal-bearing strata show evidence of regeneration after being partially buried by sediments. In these cases, the trees were clearly alive when they were partially buried by sediments. The accumulated sediment was insufficient to kill the trees immediately because of their size. As a result, some of them developed a new set of roots from their trunks just below the new ground surface. Until they either died or were overwhelmed by the accumulating sediments, these trees would likely continue to regenerate by adding height and new roots with each increment of sediment, eventually leaving several meters of former "trunk" buried underground as sediments accumulated.Formation by Carboniferous deglacial meltwater-pulses
In addition, part of the Carboniferous Period was a period of extensive and thick continental ice sheets. During the Carboniferous ice age, the repeated glacial – interglacial cycles caused major changes in the thickness and extent of continental ice sheets. When these ice sheets expanded in extent and thickness, eustatic sea level typically fell by over. When these ice sheets shrank in extent and thickness, eustatic sea level typically rose again by typically over. As occurred during the Holocene Epoch for Meltwater pulse 1A and Meltwater pulse 1B, brief episodes of rapid melting of Carboniferous, Gondwanan continental ice sheets likely caused very rapid rises in sea level that would have abruptly inundated low-lying coastal swamps and drowned the forests growing on them. Based on the sedimentology of roof strata of surface and underground coal mines and cyclothems containing the fossils of upright and in situ tree trunks, geologists proposed that the flooding of coastal swamp by deglacial meltwater pulses resulted in the rapid flooding of coastal forests, particularly along preexisting coastal rivers and streams, over large areas of coastal swamp. During and after their submergence, upright trunks of drowned coastal forests were buried by tidally influenced sedimentation.Association with marine fossils
Geologists find nothing anomalous about upright fossil trees found in Carboniferous coal-bearing strata being associated with marine or brackish-water fossils. Because they lived on subsiding coastal plains or pull-apart basins open to the coast, it was quite frequent for subsidence to periodically outpace the accumulation of sediments such that adjacent shallow marine waters would periodically inundate coastal plains in which the trees were buried. As a result, sediments containing marine fossils would periodically accumulate within these areas before being replaced by coastal swamps - either as sediments filled in the shallow sea or as the sea level fell. Also, according to ecological reconstructions by geologists, specific assemblages of the types of trees found as upright fossils occupied brackish water, even saline coastal swamps much like modern mangrove swamps. Thus, finding marine and brackish water fossils associated with these trees is no different than finding brackish water or marine animals living in modern mangrove swamps.A detailed study by Taylor and Vinn 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. Taylor and Vinn discovered that the "Spirorbis" fossils found in sedimentary strata, including the Joggins and other Carboniferous coal measures deposited from the Ordovician to Triassic periods are the remains of an extinct order of lophophorates unrelated to modern marine tube-worms to which the genus Spirorbis belongs. This contradicts arguments made by Harold Coffin and other creationists that "Spirorbis" fossils within strata containing polystrate fossils indicate their deposition in a marine environment, because these fossils are classified as the remains of extinct fresh and brackish water microconchids instead of the remains of the marine genera Spirorbis as they have been misidentified in the geologic literature.