The origin and regional distribution of dolomite cement in sandstones from a Triassic dry river system, Corrib Field, offshore west of Ireland

Abstract

Dolomite cement in the Triassic Sherwood Sandstone Group of the Corrib Field predominantly has a very early diageneticorigin (dolocrete) having formed in evaporated ground and riverwater as it flowed from southwest to northeast in a braided river environment. The mainly non-ferroan dolomite is the dominant carbonate cement in the Corrib Field, whereas calcite is dominant in well data 30 km southwest of Corrib. Petrographic and cathodoluminescence (CL) data show characteristic spherical to prolate nodules of primary dolomite with intercalating clay that is overgrown by non-ferroan. The carbon isotope composition of dolomite fluctuates from −3.8 ‰ to +2.1 ‰ PDB and can be correlated in six wells to a certain stratigraphic datum. Oxygen isotopes vary from −14‰ to −4 ‰ PDB depending on the type of dolomite cement. Burial diagenesis resulted in minor overgrowth of ferroan dolomite but did not involve the introduction of any new carbonate. The distribution of calcrete, dolocrete and gypcrete in the Corrib Field and the Slyne Basin can be broadly predicted on the basis of knowing the palaeocurrent direction and the relative position in the river basin.