GIANT COLUMNAR CARBONATE CHIMNEYS IN THE UPPER CRETACEOUS WILLIAMS FORK FORMATION, NORTHEASTERN UTAH: AN EXCEPTIONALLY PRESERVED ANCIENT SEEP SYSTEM?

The Williams Fork Formation consists of a succession of fluvial channel sandstones, crevasse splays, floodplain mudstones, paludal coals, and shallow marine deposits that were deposited by meandering- and braided-river systems within coastal- and alluvial-plain settings on the margin of the Western Interior Seaway in northwestern Colorado and northeastern Utah. Within this stratigraphic setting, nine massive carbonate chimneys have been exposed. They lie on the surface of the steep bedding planes, trending east-west, with lengths reaching up to 5.3 meters and circumferences up to 2.3 meters. Due to erosion of the surrounding soft mudstones, these irregular columnar structures are collapsing and the once perfectly stacked symmetrical, cylindrical, clearly laminated sections, each ranging from 25-40 cm in thickness, are now toppling. Secondary infilling is indicated by a contrasting red course-grained sediment going through the cylindrical center of the tubes. Concentric rings radiating from the center of each tube appear to indicate precipitation of methane-derived cements, occurring in a succession of multiple sequences. These specimens closely resemble the carbonate chimneys reported from the northeastern slope of the South China Sea, and ancient carbonate chimneys from Bulgaria. However, because the bases of these structures are still buried it is unclear if they formed parallel to the bedding surface or standing up vertically as in other examples. There are numerous oil shales and natural gas fields in the region, suggesting a source of hydrocarbons for the methane seeps on the shallow seafloor.