Middle to Outer Shelf
The sediments of the Moyvoughly Beds within the Moyvoughly-Moate area of the Irish Midlands comprise ooid grainstones, skeletal ooid grainstones, and marine sandstones, punctuated by rarer calcareous mudstones. These sediments were deposited in agitated environments on a gently sloping south- or southeast-facing ramp. Core investigations and construction of isopach maps have shown changes of thicknesses within individual sedimentary units across known faults. These are interpreted as the result of synsedimentary faulting during sediment deposition. Changes in sediment lithologies can be related both to relative changes in sea level plus faulting activity, with the sandstones being sourced from erosion of local, active, fault-bounded highs. Relative sea-level highs are characterized by calcareous mudstones, with periods of lower sea level being characterized by grainstones or sandstones dependent on local siliciclastic sourcing. No major sequence breaks have been recognized with the Moyvoughly Beds. However, a regional change in sedimentation occurs at the base of the Moyvoughly Beds, where corals colonized the lithified surface of the underlying Micrite Unit. These lower sediments were deposited in a shallow lagoon, with periodic marsh progradation and subsequent exposure with paleosol formation. The Micrite Unit can be traced across the Irish Midlands and, although no regional detailed studies have yet been carried out, appears to be of similar lithologies throughout. In contrast, units within the Moyvoughly Beds, although correlatable within the area studied, cannot be traced to other areas, and their characteristics are more controlled by local tectonic movements.
Figures & Tables
Mixed Carbonate-Siliciclastic Sequences
The study of carbonate-siliciclastic mixed sequences has seen an increase in the number of investigations that focus on mixed settings as part of the continuum between the carbonate and clastic end members. Cyclic deposition in mixed basins by reciprocal sedimentation has become one of the foundation blocks for sequence stratigraphy. In addition, these mixed sequences have a variety of distinctive petroleum reservoir characteristics, important for both exploration and development programs. The emphasis now is on reevaluating ancient sequences in the light of a more dynamic understanding of spatial and temporal variations and controls on these sequences. Examples in this volume are subdivided under the following headings: Shelf Wide, Coastal and Inner Shelf, Middle to Outer Shelf, Slope to Basin and Paleokarst. Many mixed sequences have been described in the literature, but understanding the controls of these sequences from a process approach in now in an adolescent stage.