Distribution of Diagenetic Alterations in Fluvial, Deltaic, and Shallow Marine Sandstones Within a Sequence Stratigraphic Framework: Evidence from the Mullaghmore Formation (Carboniferous), NW Ireland

Abstract

ABSTRACT The distribution of diagenetic alterations in the fluvial, deltaic and shallow marine, arkosic to subarkosic sandstones (average Q72F26L02) of the Mullaghmore Formation (Carboniferous, NW Ireland) can be predicted within a sequence stratigraphic framework. Eogenetic calcite (18OPDB = -13.3‰ to -6.5‰, 13CPDB = -3.0 to +3.4‰, and 87Sr/86Sr = 0.706721 to 0.709227) and ferron dolomite (FeCO3 = 8-12 mol%; 18OPDB = -14.2‰ to -7.8‰, 13CPDB = -1.4‰ to -1.0‰, and 87Sr/86Sr = 0.709051 to 0.709167) occur in bioclast-rich, transgressive lag deposits at parasequence boundaries and transgressive surfaces, and in wave-influenced, deltaic, highstand systems tract (HST) deposits. Mesogenetic illite, chlorite, baroque dolomite (FeCO3 = 16 mol%; 18OPDB = -14.2‰ to -12.7‰, 13CPDB = -3.8‰ to -1.0‰), quartz, and calcite (18OPDB = -15.7‰ to -12.5‰, 13CPDB = -5.8‰ to -3.7‰, and 87Sr/86Sr = 0.709016 to 0.709122) were formed mainly in the bioclast-poor deposits, which were not pervasively cemented by carbonates during near-surface eodiagenesis. These deposits include fluvial, incised-valley sandstones of lowstand systems tract (LST), and fluvial-dominated, deltaic sandstones of transgressive systems tract (TST) and HST. Illite is the dominant diagenetic clay mineral in the fluvial, incised-valley sandstones of LST, possibly because of simultaneous albitization of K-feldspars. Conversely, chlorite, dominates in the fluvial-dominated, deltaic sandstones of TST and HST, because of the presence of suitable precursor clays. The integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications of sandstone deposits.