Association of Stylolitic Carbonates and Organic Matter: Implications for Temperature Control on Stylolite Formation: ABSTRACT

Abstract

Petrographic and geochemical examination of carbonate-rock samples collected from three cores penetrating the Slave Point Formation (Middle Devonian, northwestern Alberta, T96-R4W6) have demonstrated a local association between high concentrations of organic matter, the presence of ferroan calcite and ferroan dolomite, and significant stylolite development. The stratigraphic succession of stylolitic and nonstylolitic rocks indicates that overburden pressure alone was not the main control on stylolite formation. Light-colored, pelletal/skeletal grainstones contain fewest stylolites and lowest concentrations of organic content (TOC 0.2%). In contrast, dark-brown lime wackestones and mudstones contain abundant stylolites and contain organic carbon contents as high as 1.0% an extractable organic matter (EOM) contents as high as 1,600 ppm. Organic matter is concentrated as an insoluble residue along stylolites; concentration developed during diagenesis as a result of selective solution of soluble carbonate-rock matrix. However, the greater abundance of stylolites in mudstones relative to grainstones suggests that factors inherited from original depositional environments have affected the tendency for later stylolite formation. We suggest that acidic species, principally CO2, released by catagenetic alteration of autochthonous organic matter, can dissolve sufficient carbonate to initiate stylolite formation prior to significant pressure solution. Solution of carbonate in the reducing environment of organic diagenesis also had led to the precipitation of ferroan calcite and ferroan dolomite along stylolites. These minerals are notably absent from non-stylolitic intervals. Stylolite formation in carbonate rocks may be related to type and distribution of autochthonous organic matter, which then is related to depositional environment. In addition, thermal history (rank level) of the section will also effect the depth at which stylolites form. Thus, in some carbonate rocks, subsurface temperature rather than pressure may be a more significant factor in determining the depth of stylolite formation. It is proposed that carbonate solution by organic-derived acidic species may act as an important mechanism by which carbonate rocks may locally concentrate organic matter, and produce conduits along which generated hydrocarbons may be expelled. End_of_Article - Last_Page 922------------