DEPOSITIONAL ENVIRONMENTS OF THE WILKES FORMATION AT SALMON CREEK, WASHINGTON: SOURCE OF SIDERITE 'COPROLITE' CONCRETIONS

The Wilkes Formation exposures near Toledo, Washington are famous for the occurrence of coprolite-shaped siderite concretions in exposures along Salmon Creek, where flat-lying strata are exposed in bluffs up to 30 m high. Continued documentation (after Mustoe, 2001, GSA Bull.v.113, p.673) of these strata in SE1/4 Sec. 34, R.1W, T.1N reveals a volcaniclastic character for the sediments. Sediments are characterized by poor sorting and high clay content, sparse sand layers and contain common carbonized wood throughout and a rich pollen/spore assemblage. There is a lower unit of volcanic mudflows, a middle unit of interbedded woody lignite and mudstones with thin mudflows and airfall ash layers and a top unit of pond/lake sediments containing siderite concretions. These are overlain by 2 m of terrace gravels with some clasts eroded from Wilkes strata. The lowest Wilkes unit (5 m) consists of multiple fine grained volcanic mudflows, with some coarser, texturally mature sediment, deposited on forested ground, preserving tree stumps in growth position. The mid unit (6 m) has three lignite couplets of lower and upper lignites with large pieces of wood sandwiching a mudstone; couplets are separated by 1-2 m volcanic mudflows. The mid mudstone of a couplet is interpreted as standing water deposit and along pond margins lignites accumulated with wood winnowed from mudflows. A 15 cm airfall ash bed with abundant phenocrysts and coarse vermiform kaolinite prisms provides a poorly constrained age of 11.6±0.4 Ma, indicating an mid-late Miocene age. The top unit (6 m) is mudstones and minor sand that probably accumulated in standing water. Siderite concretions of diverse sizes and shapes are common in the upper 2 m. Deposition is related to nearby volcanic activity generating mudflows that blocked existing drainages. Botryoidal shapes of concretions are common, both as primary forms and as secondary accretions on sinuous masses. Siderite concretions consist of unzoned micritic siderite (crystallite size ~ 10mm) partially altered to hydrous iron oxides. Carbon isotope ratios (d13C = -13.0 to -11.4 o/oo PDB) of siderite indicate that dissolved carbonate was probably generated near decomposing plant matter along with microbial methane production.