SEDIMENTARY ENVIRONMENTS/Deltas 537 Figure 15 A delta front gully (lower left) filled by mudflow deposits in an Upper Carboniferous deltaic succession in the Clare Basin, western Ireland One margin of the gully is exposed on the right side, truncating earlier strata; the gully is m deep and extends over at least 20 25 m laterally to the edge of the cliff Figure 16 A mud diapir in an Upper Carboniferous deltaic succession in the Clare Basin, western Ireland Note that the bedded sandstones that overlie the diapir are thin above the crest and thicker on the flank of the diapir, indicating that the diapir grew during deposition of the sandstones Cliff height is 25 m reveal that the mudlumps are growth anticlines related to reverse faults The growth nature of the structures is depicted by thickness variations in the overlying mouth bar deposits that are thin above the diapir crest and thick in areas between diapers Synsedimentary deformational features can also be observed in exposed, ancient deltaic successions (Figures 15 and 16) Shelf-Edge Deltas in Quaternary Lowstand Periods Presently, because of the Holocene transgression and the current highstand of sea-level, most major, ocean-facing deltas are located at the inner margin of a broad continental shelf In contrast, during periods of sea-level lowstand associated with peak glacial in the Quaternary, these deltas were located at the outer edge of the continental shelf where they border on the continental slope Falling sea-level forced the deltas into this position and they remained there until they retreated back across the shelf during sea-level rise associated with the onset of interglacial conditions These shelf-edge deltas have been examined extensively in recent years using shallow, high-resolution seismic data that have demonstrated several distinctive features The deltas are fed by widespread fluvial systems that are often incised into the former shelf (incised valleys) These systems supply large volumes of sediment to the shelf-edge deltas at a time when the accommodation space available for sediment accumulation is limited As a result, the delta front slopes of lowstand deltas, seen as dipping clinoforms in seismic data, are typically steep ($2–4 ) and frequently display evidence of large gravitational failures Slump scars can cut 200–300 m of strata and extend laterally for several kilometres Instantaneous bathymetric relief associated with the scars can be as great as 30 m and the scars may grow further by subsequent faulting Slump scars therefore have the potential to release significant volumes of sediment to the deep basin as slides, slumps, debris flow, or turbidity currents, and to define minibasins that are repaired by subsequent deltaic sedimentation Seismic data from these slump scar minibasins indicates that the repair of the scars is rarely simple Healing by sediment accumulation alternates with renewed sediment failure, and the evolution of the scar fill can be critical in understanding the delivery of sediment to the deeper water setting during lowstand periods Economic Aspects Deltaic deposits are commonly preserved in the geological record as thick, expanded sections of predominantly clastic sedimentary facies As already mentioned, these deposits are of great economical importance They contain vast reserves of hydrocarbons, both solid, in the form of coal, and fluid, as gas and oil (see Petroleum Geology: Overview) Because deltaic sediments are commonly rapidly deposited, with little reworking, organic matter is preserved in them When buried and heated, this organic matter may generate oil and gas In humid climates, the waterlogged surfaces of deltas are colonized by lush vegetation that is preserved in anaerobic marshes and swamps These environments are sites of formations of peat that can accumulate to significant