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Willwood Formation
The Willwood Formation is a sedimentary sequence deposited during the late Paleocene to early Eocene. It consists of fine grained clastic rocks (mudstone and shale) interbeded with medium grained clastic rocks (sandstone) and sporadic conglomerates. The formation underlies portions of the Bighorn Basin of Big Horn, Hot Springs, Park and Washakie counties of Wyoming.〔(【引用サイトリンク】url=http://tin.er.usgs.gov/geology/state/sgmc-unit.php?unit=WYTwl%3B0 )〕
Radiometric dating of volcanic tuffs, combined with comparisons with other formations using magnetostratigraphy, using numerous samples from various levels of the formation suggest an age range of 55 - 52 million years ago, placing the Paleocene-Eocene boundary near the base of the formation.〔Tauxe, L., Gee, J., Gallet, Y., Pick, T., & Bown, T. (1994). Magnetostratigraphy of the Willwood Formation, Bighorn Basin, Wyoming: new constraints on the location of Paleocene/Eocene boundary. Earth and planetary science letters, 125(1), 159-172.〕
Trace fossils have been found in the Willwood Formation. Fossil bird include ''Gastornis'', ''Neocathartes'' and ''Paracathartes''. A fossil alligatorid, namely ''Orthogenysuchus'', was also found in this formation.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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