Diagenesis of Platform Carbonates, Wodejebato Guyot, Marshall Islands

Abstract

Cores from five sites on Wodejebato Guyot show that diagenesis played a major role in forming the platform limestones. Five morphologically distinct cement types representing several diagenetic environments, extensive dissolution porosity, and widespread neomorphism of metastable components to calcite with mimic or coarsely crystalline textures are observed. Radiaxial cement (RC) and a fibrous variant (RC-FIC) occur almost exclusively in primary pores in the upper 70 m of the inner perimeter ridge at the platform edge. These cements precipitated from shallow, warm marine waters. Prismatic limpid cement with uniform extinction (PLUC) occurs throughout the platform limestones in both primary and secondary porosity. Geologic, petrographic, and isotopic data indicate formation in deeper, cool marine waters, after drowning of the platform. Columnar, bladed, magnesian-calcite (7 mole % MgCO 3) cement with sweeping extinction (CC) is confined to a single core of a reef interval on the inner perimeter ridge. It occupies primary porosity as the first cement or overlies RC. This is an early, shallow-marine cement. Many caprinid rudist shells in this interval maintain their primary aragonite composition. Syntaxial overgrowth cement (SOC) is widely distributed, in small quantities, mainly in grainstones. It appears to have precipitated concurrently with or somewhat later than PLUC, presumably in the same deeper, cool, marine water. Dissolution formed extensive molds, some vugs, and some small caverns, particularly in grainstones. Dissolution was most intense in the outer perimeter ridge at the platform edge. Molds also occur in muddier rocks and in reworked cobbles of wackestone. Geologic and petrographic evidence suggests that dissolution occurred both in meteoric waters during early subaerial exposure of Wodejebato Atoll and in cold marine waters after drowning, perhaps continuing to the present. Neomorphism is most apparent in skeletons that were originally aragonitic, rudists and other mollusks in particular. Where the neomorphic calcite closely mimics the original wall structure, the original isotopic composition is only slightly altered. Coarsely crystalline neomorphic calcite has lower δ 18 θ and δ 13 C values than the original shells, indicating meteoric alteration. Burial diagenesis is not an alternative on this guyot. Matrix and magnesian calcite shells, such as benthic foraminifers and red algae, rarely show petrographic evidence of neomorphism, but some samples show dramatic isotopic shifts to lower 18 O and C, suggesting meteoric alteration. Isotopic shifts to lower values suggest two incursions of meteoric water at the lagoon site and on the inner perimeter ridge. Traces of root structures indicate a third exposure episode within the lagoon. The clearest signal of meteoric water is in a shift to lower isotopic values, primarily in 18 O, at the top of platform carbonates. Isotopic data are available only from the lagoon and inner perimeter ridge sites, but a probable microkarst surface truncates platform limesto...