Erosional Discontinuities Research Articles

Upper Quaternary sedimentation and hillslope evolution; southeastern Brazilian Plateau

Morphostratigraphic evidence indicates that sediment sources and relative rates of erosion over slopes have changes with time. The pattern of change seems to be more compatible with the variable runoff source models than with Horton9s infiltration theory of surface runoff. Older deposits were reworked by successive denudational episodes, resulting in the reshaping of the landforms; erosional discontinuities dominate within the stratigraphic sequences, and stonelines occur at more than one level.--Modified journal abstract.

Sand waves: A model of origin and internal structure

Sand waves are large flow-transverse bedforms coupled to oscillatory boundary-layer currents of tidal origin. Like the much smaller ripple-marks generated by short-period wind waves, sand waves are observed to grow more asymmetrical with increase in the time—velocity asymmetry of the governing currents, that is, with increase in the steady component of flow (mass-transport strictly related to wave-motion) relative to the periodic component. Wind-wave ripple-marks owe their origin directly to a mass-transport component dependent on bed-curvature, which arises naturally wherever a sufficiently powerful oscillatory flow is imposed on a deformable grain boundary. This curvature-related current, flowing from troughts to crests on the bed, drifts grains mobilized by the periodic component into transverse bands. Sand waves may also owe their origin and growth to an unstable interaction within tide-generated oscillatory boundary layers between the mobile bed and a curvature-related mass-transport. Sand-wave internal structure apparently depends on the strength and degree of asymmetry of the governing currents. Relatively symmetrical forms associated with currents of low asymmetry are expected to contain comparatively small, intricately related herring-bone or climbing cross-bedding sets. Relatively asymmetrical currents shape sand waves with one side so steep that large-scale flow separation is inevitable. The predominant structure is then expected to be long avalanche bedding broken into sets by mud drapes and bioturbated zones representing periods of gentle flow, and/or by erosional discontinuities recording tidal reversals. The expected structures have parallels in the stratigraphic record and amongst modern sand waves.

Coral encrusted concretions: a key to recognition of a ‘shale on shale’ erosion surface

Coral encrusted septarian concretions have been found along an obscure diastem within the Penn Yan Shale Member (Genesee Group, Upper Devonian) near Canandaigua Lake, New York. These hiatus-concretions (sensu Voigt) are found associated with an erosional discontinuity surface in the shale. The restriction of coral growth to exposed concretion and wood found on the erosion surface, indicates a differential substrate preference exhibited by these animals. Diagenesis, both prior to and following this erosion event, produced various physical structures whose time of formation can be estimated from observed cross-cutting relationships with one another. An erosion-related feature establishing an early-diagenetic age for septarial fractures within concretions is described, and the origin and significance of the diastem is discussed.