Rhodri M. Jerrett

The significance of hiatal surfaces in coal seams

Abstract: A widespread misconception is that coals correspond to single palaeo-peat bodies, which represent continuous and time-invariant records of peat accumulation. Evidence for the occurrence of intra-seam hiatal surfaces within datasets from bituminous coals, lignites and modern peats suggests that existing depositional models for peat and coal require modification. Recognition that coals may represent a succession of stacked mires separated by hiatal surfaces has implications for palaeoenvironmental and sequence stratigraphic studies that assume a continuous record of peat accumulation, as well as for the prediction of whole-seam composition and thickness trends.

Extrinsic and intrinsic controls on mouth bar and mouth bar complex architecture: Examples from the Pennsylvanian (Upper Carboniferous) of the central Appalachian Basin, Kentucky, USA

A fundamental architectural element of deltas is the mouth bar. Although process-based facies models have been developed to reconstruct the influence of different external controls on mouth bar geomorphology, depositional architecture, and grain-size distribution, few studies have documented the internal architecture of ancient mouth bars and mouth bar complexes, in order to analyze extrinsic and intrinsic controls on these parameters. Two exceptionally well-exposed ancient examples show that the increasing influence of inertial forces in friction-dominated mouth bars results in increasing deposition from gravity flows (hyperpycnites and turbidites), with increasing bypass of the mouth bar foreset and deposition in a detached frontal lobe on the basin floor ahead of the mouth bar. The increasing influence of inertial forces also results in increased bed length, and the better development of clinothem bottomset beds. Within these friction-dominated mouth bars, following initiation and aggradational-progradational growth, choking results in lateral accretion on the mouth bar flanks, but discharge may not be maintained symmetrically on both flanks. Additionally, “choking” of the feeding distributary can result in its upstream avulsion and abandonment of the mouth bar. This process generates laterally accreted fining-up successions that downlap onto the floor of the receiving basin, contrasting with standard coarsening-up facies successions predicted for mouth bars. Within mouth bar complexes, superposition of individual mouth bars causes gradual shallowing of the water column, reducing gradients in, and increasing confinement of successive mouth bars. Hence, early mouth bars are more strongly influenced by inertia, flows have long runout distances, and they are more likely to develop a succession of detached prodelta turbidite lobes. Later mouth bars are more strongly dominated by friction, and flows have short runout distances, since they are less able to achieve autosuspension. Earlier mouth bars display more “normal” aggradation-progradation, lateral accretion, and retrogradation in an unconfined setting, whereas later mouth bars are more strongly confined and progradational. The two case studies presented here illustrate that upward changes in mouth bar architecture and facies distributions within a mouth bar complex are a predictable product of shallowing and increasing confinement during delta progradation.

Sedimentology and microfacies of a mud-rich slope succession: in the Carboniferous Bowland Basin, NW England (UK)

A paucity of studies on mud-rich basin slope successions has resulted in a significant gap in our sedimentological understanding in these settings. Here, macro- and micro-scale analysis of mudstone composition, texture and organic matter was undertaken on a continuous core through a mud-dominated slope succession from the Marl Hill area in the Carboniferous Bowland Basin. Six lithofacies, all dominated by turbidites and debrites, combine into three basin slope facies associations: sediment-starved slope, slope dominated by low-density turbidites and slope dominated by debrites. Variation in slope sedimentation was a function of relative sea-level change, with the sediment-starved slope occurring during maximum flooding of the contemporaneous shelf, and the transition towards a slope dominated by turbidites and then debrites occurring during normal or forced shoreline progradation towards the shelf margin. The sediment-starved slope succession is dominated by Type II kerogen, whereas the slope dominated by low-density turbidites is dominated by Type III kerogen. This study suggests that mud-dominated lower slope settings are largely active depositional sites, with consistent evidence for sediment traction. Additionally, the composition and texture of basin slope mudstones, as well as organic content, vary predictably as a function of shelf processes linked to relative sea-level change.