Enigmatic origin of ferruginous "coprolites": Evidence from the miocene wilkes formation, southwestern Washington

Excrement-shaped masses of siderite and limonite have been reported from cl ay-rich sedimentary rocks that range in age from Late Permian to Holocene, These objects have been widely accepted as being coprolites, but the ferrug inous composition, absence of internal inclusions, and scarcity of associat ed vertebrate remains suggest that they may instead be pseudofossils create d by mechanical deformation of plastic sediment. Possible abiotic mechanism s include coseismic Liquefaction, intrusion of sediment into hollow logs, o r expulsion of sediment in response to gravity. Alternatively, carbon stabl e isotope ratios indicate that sediment extrusion may have been related to emission of biogenic methane during early stages of diagenesis.