Abstract The distribution and spatial patterns of surficial nearshore sediments off the north coast of Ireland were mapped in detail using three repeated (over one year total duration) dual-frequency (100–500 kHz) side-scan sonar surveys which were ground-truthed using a bucket-grab sediment sampler. In the study area (40 km 2 , < 30 m water depth), three acoustic facies were identified (interpreted as sand, gravel, and bedrock). Planar sand with small sand waves dominates most of the study area; bedforms developed on sand and gravel substrates are located around the margins of a bedrock-floored bathymetric trench. Unusual sea-going gravel ‘ribbons’, which morphologically resemble rip current chutes, feed into the trench. Temporal changes in substrate and bedform patterns, mapped from the side-scan sonar data, suggest bedform development and facies boundary migration (by > 100 m distance) over the winter season followed by sediment migration back again during the summer. Generally, substrate and bedform mobility is likely to be related to changes in the relative strengths of waves and tides along this exposed, mesotidal coast. More specifically, tidal asymmetry and the formation of a headland leeside eddy can account for scouring of the trench, trench-flank bedform patterns developed in sand, and the formation of the gravel ribbons.

Rim cements of high magnesium calcite, with exceptionally high magnesium carbonate content, are presently lithifying quartz sands at the surface along the South Louisiana Gulf Coast. Cements accumulate at the margin of a marsh environment on the leeside of a coastal beach complex. Based on X-ray diffraction analyses, surface cements range from 35 to 50 mole % magnesium carbonate. Cements show a well defined calcite lattice structure with no indication of dolomite ordering. Surface cements are associated with evaporites and blue-green algae but are not restricted to areas of algal activity. Surface crusts are periodically buried by storm action and preserved in the subsurface. X-ray diffraction analyses of buried samples indicate a magnesium carbonate content of 17 to 33 mole %. Scanning electron microscopy indicates that cements are “fiber-bundles” with individual fibers constructed of crystallites with an average diameter of 0.3μm Sandstone clasts, found along beaches of South Louisiana, are interpreted to be ripped-up fragments of older surface crusts. Clasts are cemented by high magnesium calcite with 8 to 17 mole % magnesium carbonate. Cements appear as blades with well defined trigonal terminations. These blades are also constructed of crystallites, averaging 0.3 μm in diameter. The surface crusts, buried crusts and rip-up clasts can be categorized into three cement types based on composition, crystal morphology and environment of occurrence. The three cement types reported here are considered to be representative of diagenetic stages involving dissolution and precipitation. After burial, magnesium carbonate content decreases while the δC 13 values become more negative. Isotope analyses yield average δC 13 values of −1.0%c, −4.0%c, and −37.4%c PDB for surface, buried, and rip-up-clast cements, respectively. Oxidation of organics is probably the controlling mechanism of the dissolution/precipitation process. Extreme negative δC 13 values of the rip-up-clast cements and a progressive decrease in values with time and depth of burial support this interpretation.

Abstract The period since the 1960s witnessed significant progress in our ability to decipher the clastic rock record from a wide range of sedimentary environments, and spanning many spatio-temporal scales, from millimetric to that of the sedimentary basin, and involving processes acting on timescales of seconds to millions of years. This review assesses advances in four areas of fluvial sedimentology: the nature of alluvial dunes, the role of fine-grained suspended sediment, the linking of facies models and channel planform, and the reconstruction of drainage networks within ancient sedimentary successions. The synthesis reveals that we require new thinking and research to: (1) address the range of stratification produced by dunes and their palaeohydraulic implications; (2) evolve new bedform phase diagrams capable of incorporating the reality that many fluids transport fine-grained sediment, both in flow and within the bed, which may significantly modify the bedform morphology and phase space when compared with existing bedform stability diagrams; (3) develop new alluvial facies models in which planform channel pattern is not the fundamental discriminant; and (4) re-establish consideration of process mechanics as the heart of developing ideas and debates concerning fluvial deposit preservation and alluvial architecture.