Outsized Clasts Research Articles

A proglacial origin for the upper Proterozoic Rockfish Conglomerate, central Virginia, U.S.A.

The upper Proterozoic Rockfish Conglomerate is a lenticular unit of stratified pebbly sandstone and conglomerate as much as 500 m thick at the base of the Lynchburg Group in central Virginia. It has been interpreted as a deep-water resedimented deposit, but glacial influence is suggested by the presence of dropstones and coarse-grained rhythmites. The coarsest deposits in the Rockfish Conglomerate are confined to the lower 30 m and consist of stratified pebbly sandstones containing outsized clasts and lenses of cobble conglomerate. They are interpreted as subaqueous glacial outwash deposited on a delta front. Coarse-grained, rhythmically-bedded sandstones containing thin, laterally persistent pebbly horizons make up the bulk of the unit and probably accumulated by a combination of mass flow and ice-rafting processes seaward of the delta front. Overlying this rhythmite facies is a thin horizon of dropstone-bearing turbidites. The Rockfish Conglomerate may reflect local progradation of highland glaciers across the margin of the Lynchburg basin during the initial stages of late Proterozoic rifting. Recognition of glaciogenic rocks in the Lynchburg Group provides further evidence for regional glaciation in the southern Appalachians during late Proterozoic time. Glaciation in the southern Appalachian Blue Ridge may correlate with the worldwide Vendian glacial event, compatible with geochronologic data that indicate rifting between 690-570 Ma in the southern Appalachians.

Possible Late Palaeozoic Glaciation in the Central Parts of the Yemen Arab Republic

Abstract Occasional outsize clasts, some of which are striated, and laminae bent against the enclosed clasts have been recorded throughout the shales and siltstones forming the lower part of the Kohlan Group in the central parts of the Yemen Arab Republic. Such features are interpreted as an indication of continental glaciation. On stratigraphical grounds, these shales and siltstones are assigned to the Permian.

Possible Late Palaeozoic Glaciation in the Central Parts of the Yemen Arab Republic

AbstractOccasional outsize clasts, some of which are striated, and laminae bent against the enclosed clasts have been recorded throughout the shales and siltstones forming the lower part of the Kohlan Group in the central parts of the Yemen Arab Republic. Such features are interpreted as an indication of continental glaciation. On stratigraphical grounds, these shales and siltstones are assigned to the Permian.

Sedimentological evidence on the origin of the Precambrian Great Conglomerate (Kundelungu Tillite), Zambia

The Great Conglomerate (Kundelungu Tillite) of the Zambian Copperbelt is largely a pebbly mudstone. Detailed study of drill cores from the Ndola area shows erosional basal contacts, slump structures, fabric and correlatable pebble-size variations, indicative of a mode of deposition by mudflows. The pebbly mudstone is interbedded with andpasses laterally to finely-bedded argillites. The resemblance of the finely-bedded argillites with glacial varves and the presence of outsized clasts that cut the underlying laminae of the argillite are considered evidence of glacial origin. The Great Conglomerate of Zambia is interpreted as the glacio-marine facies of the Precambrian continental glaciation of Zaire, formerly Congo (Kinshasa). The pebbly mudstone could have originated from the resedimentation of unsorted glacial material in a nearshore area.

Grain-Flow Deposits and their Implications, Santa Ynez Mountains, California

ABSTRACT A conformable 4000 meter thick marine Lower Tertiary section, well exposed in the Santa Barbara to Ojai segment of the Santa Ynez Mountains, reveals an upward progression from deep-water turbidites with flysch features to shallow-water deposits, and is succeeded by nonmarine beds. Sandstones and conglomerates which occur generally between the flysch and the shallow-water deposits are characterized by: thick, ungraded beds with sharp borders; outsize clasts; diffuse, flat lamination; dish structure (defined here); peculiar sole markings; and direct evidence of mass flow. These primary features indicate deposition by pseudo-laminar flow of granular material, here called grain flow. Such flow may be explained on the basis of the 'Bagnold effect' of grain interaction. Depositional directions within these grain-flow deposits and associated slide deposits are generally transverse to the basin axis, in contrast to the longitudinal directions in both the older turbidites and the younger shelf deposits. These different trends are interpreted as typical of a shelf-slope-bottom configuration in a linear, fault-controlled basin, and in this instance, their succession records progressive filling of the basin. Deposition by grain flow is probably important in many steep-sided basins, particularly linear troughs along continental margins, and many massive sandstones and conglomerates probably represent grain-flow deposits. Some examples for California are cited.