PETROGRAPHY AND GEOCHEMISTRY OF RHOMBIC, CALCITE PSEUDOMORPHS FROM MID‐TERTIARY MUDSTONES OF THE PACIFIC NORTHWEST, U.S.A.

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Abstract
Marine, mid‐Tertiary mudstones and siltstones of northwestern Oregon and western Washington contain unusual calcite pseudomorphs that commonly form the nuclei of carbonate concretions. These pseudomorphs typically have the shape of rhombic prisms, with dipyramidal terminations, that are much elongated due to multiple twinning (?). They appear similar to the glendonite pseudomorphs of New South Wales, however, their rhombic (orthorhombic?), prismatic form is at variance with the reported monoclinic form of the glendonite pseudomorphs. Internally, the pseudomorphs consist of granular calcite that takes the form of large ovoid crystals, radial or spherulitic crystals, fibrous but non‐radiating crystals, and mosaic crystals of clear spar.The original mineral, believed to be aragonite or one of the hydrated carbonates, probably formed in soft, organic‐rich mud, possibly in the burrows of marine organisms. Changes in physico‐chemical conditions caused early solution of the original mineral, leaving hollows or molds that quickly filled by precipitation of granular calcite. Judging from the low δC13 values of the pseudomorphs, the carbonate carbon of this granular calcite was supplied from CO2 produced through decay of local masses of organic matter. Concretionary calcite, supplied from a different source as shown by much higher δC13 values, formed around the pseudomorphs. The entire pseudomorphic and concretionary process took place only a short distance below the depositional interface, prior to compaction and lithification of the host mudstone.

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