Gravel overpassing on humpback bars supplied with mixed sediment: examples from the Lower Old Red Sandstone, southern Britain     

J. R. L. ALLEN 《Sedimentology》1983,30(2):285-294

ABSTRACT The Lower Old Red Sandstone (Downton to ?Emsian) in southern Britain is a largely fluviatile sequence of increasing upward sand-dominance. The highest beds at two groups of localities include many sedimentation units composed of gravelly foresets in depositional continuity with overlying topsets of parallel-laminated sandstone. These units are thought to have been fashioned by humpback bars having a crest a considerable way upstream from the brink at the top of a gravelly slipface. The overall textural composition but internally segregated character of the bar units suggests that a bimodal sediment load of mixed sand and gravel was supplied at the upstream end of the bar, but that this load became texturally differentiated as it moved downstream. Differentiation is suggested to have occurred because the comparatively large and well-rounded gravel particles behaved on the sandy topset as though on a smooth surface, and were transported under similar flow conditions to the sand, much of which eventually lodged on the topset instead of being passed on, like the gravel, to the slipface beyond. A quantitative model is outlined which justifies the proposed gravel overpassing. In terms of the control of sedimentary structures exerted by grain size under laboratory conditions, the association of cross-bedding (gravel) with simultaneously formed parallel lamination (sand) seems to be a natural consequence of the efficient textural differentiation of the supplied load by the overpassing of the gravel component under a single flow condition.  相似文献   

Wave-generated structures in the Devonian lacustrine sediments of south-east Shetland and ancient wave conditions     

PHILIP A. ALLEN 《Sedimentology》1981,28(3):369-379

The most common wave-generated structures in the nearshore lacustrine sediments of the south-east Shetland basin are cosets of undulatory and unidirectional ripple cross-lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post-vortex) ripples, and the unidirectional cross-sets were formed by the migration of vortex (orbital) ripples at lower strengths. Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand-starved substrate. Some lenses were reworked during periods of more prolonged wave activity. The Inman-Komar plot of near-bottom orbital diameter versus ripple spacing (λ= 0.80d₀ for small d₀, or λ= 0.65d₀ as modified by Miller & Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimum d₀ conditions) is utilized in this study since wave periods in lakes are small. The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the order of 20 km wide.  相似文献   

DISTINCTION OF ELEMENTARY SAND POPULATIONS IN THE GIRONDE ESTUARY (FRANCE) BY R-MODE FACTOR ANALYSIS OF GRAIN-SIZE DATA     

GEORGE P. ALLEN  PATRICE CASTAING  RE KLINGEBIEL 《Sedimentology》1972,19(1-2):21-35

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Changes in intertidal dunes during two spring-neap cycles, Lifeboat Station Bank, Wells-next-the-Sea, Norfolk (England)   总被引：1，自引：0，他引：1  

J. R. L. ALLEN  P. F. FRIEND 《Sedimentology》1976,23(3):329-346

Twice-daily observations over a fixed range were made covering three consecutive sets of spring tides. Some dune population dimensions (e.g. mean wavelength) show a long-lasting (at least weeks) alteration during the most vigorous springs, but a short-lived (tides or days only) change over the weaker ones. Other properties (e.g. mean height) experience merely short-lived alterations. The changes seem to be linked to the creation during spring tides of new dunes of an initially small size (minor dunes), which introduces new matured dunes into the populations if there is sufficient subsequent sediment transport, and to the limited ability of individual dunes to respond to changing hydraulic conditions during their life-spans. The dunes are active during only a very small proportion of each spring-neap cycle, but the bank is permanently covered by them, and cross-stratification caused by dune movement is the only internal structure. Without a knowledge of the history and dynamics of the dune populations, the quality of the environment at Lifeboat Station Bank cannot be satisfactorily assessed from the sedimentary products.  相似文献   

THE NATURE AND ORIGIN OF BED-FORM HIERARCHIES   总被引：3，自引：0，他引：3  

J. R. L. ALLEN 《Sedimentology》1968,10(3):161-182

The bed forms generated by liquid flows over loose-grain boundaries assume only two orientations relative to flow, being either transverse or longitudinal, but range enormously in physical scale. Two types of bed form, current ripples and parting lineations, take a characteristic length (2) which is independent of boundary conditions as expressed by flow depth (d). We have that: λ^L≤ 0.025 m (parting lineations) λ^R≤ 0.60 m (ripples). The characteristic length of other kinds of bed form is correlated with flow depth, according to the equations: λ^D= 1.16D1.^1.55 (dunes) λ^A 6.3d (antidunes, Fr = 1.0) λ^SR= 1.35d^1.31 (sand ribbons) λ^M - 672d^1.11 (meander bars) λ^T= 116d² (tidal current ridges) From these relationships can be deduced the compositions of theoretical hierarchies of bed forms, where a hierarchy is defined as a series of bed forms graded as to size which in combination are adapted to a single overall flow condition. The compositions of these hierarchies, which are closely matched by naturally occurring hierarchies, are broadly related to available flow depth, general flow conditions (lower or upper regime), and availability of sediment. Bed forms are considered to be due to unstable interactions between the bed material and the over-bed flow, of liquid or grains or both. Hierarchies of bed forms arise because the quantities determining the flow are suficiently numerous that several mutually unstable combinations can exist, each combination being expressed in terms of a bed form of a characteristic physical scale and orientation relative to flow.  相似文献