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Enigmatic origin of ferruginous "coprolites": Evidence from the Miocene Wilkes Formation, southwestern Washington

¹ Geology Department, Western Washington University, Bellingham, Washington 98225, USA

Excrement-shaped masses of siderite and limonite have been reported from clay-rich sedimentary rocks that range in age from Late Permian to Holocene. These objects have been widely accepted as being coprolites, but the ferruginous composition, absence of internal inclusions, and scarcity of associated vertebrate remains suggest that they may instead be pseudofossils created by mechanical deformation of plastic sediment. Possible abiotic mechanisms include coseismic liquefaction, intrusion of sediment into hollow logs, or expulsion of sediment in response to gravity. Alternatively, carbon stable isotope ratios indicate that sediment extrusion may have been related to emission of biogenic methane during early stages of diagenesis.

Key Words: coprolite • methane • Salmon Creek • siderite • Wilkes Formation