Abstract

Early diagenetic dissolution removed aragonitic molluscan debris which served as nuclei for ooids in the Jurassic Twin Creek Formation in northwestern Wyoming. The resulting hollow elliptical shells were subsequently crushed during burial and compaction to produce the crushed ooids typical of this Jurassic unit. Alternatively, ooids with nuclei composed of calcite in the form of crinoidal, foraminifera, and molluscan debris remained intact and were unaffected during this early burial event. Precompactional dissolution of aragonitic ooid nuclei is indicated by flattened ooids and confirmed by the orientation of fractures developed in the cortical coatings. Hollow colds were flattened by tensional microfracturing of the cortical exterior in planes oriented parallel and perpendicular to bedding. Boring by microendoliths preceding and accompanying ooid formation provided additional permeability for carbonate diffusion from ooid interiors during their dissolution. Cortices of Twin Creek ooids are now composed of radial calcite. Textural evidence suggests that both the fabric and mineralogy of the radial calcite are primary. This evidence supports the postulate of Sorby (1879) and Sandberg (1975) that radial fabric in many ancient calcite oolites is primary and not related to postdepositional diagenetic modification.

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