LITHOSTRATIGRAPHY, DIAGENESIS AND CARBONATE CEMENT ISOTOPE CHARACTERIZATION OF UPPER DEVONIAN OLD RED SANDSTONE; IMPLICATIONS FOR FLUID FLOW DURING DIAGENESIS

The Upper Devonian Old Red Sandstone (ORS) in the Irish Midlands comprises three major lithofacies. Stratigraphically lowest ORS consists of reddened, alluvial to fluvial lithic pebble conglomerates, arenites, greywackes, and minor siltstones and is developed across the Midlands. This facies grades upwards into lithologically similar cream colored fluvial facies. Uppermost ORS consists of fluvial to marginal marine, grey colored, fine-grained arkose and subarkose interlayered with red, green, and grey mudstones, calcareous litharenites and grainstones.

Earliest diagenetic cements in all lithofacies include calcite, ferroan dolomite and Mn-ankerite. Pore lining iron oxide cements are best developed during early diagenesis and responsible for reddening of lowest lithofacies; cream and grey lithofacies were not reddened but may preserve reddened hematitic rock fragments. Early diagenetic calcite has wide ranging carbon and oxygen isotopic signatures interpreted as mixing between meteoric fluids, soil-derived CO2 and fluids locally interacted with siliciclastic material. Early ferroan dolomite and Mn-ankerite isotopic signatures reflect influx of Devonian to Lower Carboniferous seawater that interacted with bacteriogenic, reduced sediments and vadose soil horizons. Quartz overgrowths with minor illite are products of intermediate diagenesis throughout the ORS. Late diagenesis produced partial to pervasive ferroan dolomite-ankerite alteration of rock components and sericitization of aluminosilicates. Chlorite is developed in hematitic units. Carbon-oxygen isotope ratios of late ferroan dolomite-ankerite are atypical of modified lower Carboniferous sea water and interpreted to have sourced from ORS formation waters equilibrated with siliciclastic material.

Previous hypotheses for fluid flow have commonly treated ferroan dolomite as forming from a single event. The new data demonstrate early carbonate cementation in the meteoric and marginal marine subsurface from downwards flow of fresh and sea waters. Later ankerite-ferroan dolomite precipitated from formation waters sourced from deeper portions of the basin driven laterally along units probably by topographic gravity-induced flow.