Ripple-drift Cross-lamination Research Articles

Sequence of structures in fine-grained turbidites: Comparison of recent deep-sea and ancient flysch sediments

A comparative study of the sequence of sedimentary structures in ancient and modern fine-grained turbidites is made in three contrasting areas. They are (1) Holocene and Pleistocene deep-sea muds of the Nova Scotian Slope and Rise, (2) Middle Ordovician Sevier Shale of the Valley and Ridge Province of the Southern Appalachians, and (3) Cambro-Ordovician Halifax Slate of the Meguma Group in Nova Scotia. A standard sequence of structures is proposed for fine-grained turbidites. The complete sequence has nine sub-divisions that are here termed T 0 to T 8. “The lower subdivision (T 0) comprises a silt lamina which has a sharp, scoured and load-cast base, internal parallel-lamination and cross-lamination, and a sharp current-lineated or wavy surface with ‘fading-ripples’ (= Type C etc. …).” (= Type C ripple-drift cross-lamination, Jopling and Walker, 1968). The overlying sequence shows textural and compositional grading through alternating silt and mud laminae. A convolute-laminated sub-division (T 1) is overlain by low-amplitude climbing ripples (T 2), thin regular laminae (T 3), thin indistinct laminae (T 4), and thin wipsy or convolute laminae (T 5). The topmost three divisions, graded mud (T 6), ungraded mud (T 7) and bioturbated mud (T 8), do not have silt laminae but rare patchy silt lenses and silt pseudonodules and a thin zone of micro-burrowing near the upper surface. The proposed sequence is analogous to the Bouma (1962) structural scheme for sandy turbidites and is approximately equivalent to Bouma's (C)DE divisions. The repetition of partial sequences characterizes different parts of the slope/base-of-slope/basin plain environment, and represents deposition from different stages of evolution of a large, muddy, turbidity flow. Microstructural detail and sequence are well preserved in ancient and even slightly metamorphosed sediments. Their recognition is important for determining depositional processes and for palaeoenvironmental interpretation.

Backset Bedding in Turbidites: A Further Example from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec

ABSTRACT Backset bedding was found in a thick (144 cm) turbidite bed in the Cloridorme Formation, Gaspe Peninsula, Quebec. The backsets occur in coarse-medium sand below a train of three trochoidal profiles (wavelength 30 cm, amplitude 4 cm) and above a basal division (9-13 cm thick) of parallel lamination. Flute casts on the sole of the bed, and ripple-drift cross-lamination--forming the preservation blanket covering the trochoidal profiles and backsets--allow paleocurrent direction measurements. The backset bedding is interpreted to result from the upstream migration of antidunes. A suitable analysis for the formation of antidunes by density currents uses an equation appropriate for internal waves at a density interface between two superimposed fluids which are moving parallel and in the same direction.

Sedimentology of a lacustrine barrier system at Wasaga Beach, Ontario, Canada

In an area of 4 by 15 miles (6.5 × 24 km) at Wasaga Beach, Ontario, a Holocene system is exposed that can be considered sedimentologically to be a lacustrine “barrier island complex”, although the initial features were formed as a spit-baymouth bar combination. The sedimentary facies of the system are comprised of lagoonal or bay deposits: calcareous, fossiliferous clays and silts interstratified with sandy layers, some showing regular alternation and which originated as storm layers; beaches of the barrier itself that range from older coarse shingle-beaches to more recent sandy and locally fossiliferous ones. The beach sediments show several cycles of deposition. A type of macrocyclicity is related to coarsening-upward cycles in the pebble beaches, the coarser and higher layers indicating strong storm conditions. Smaller cycles within the sections have characteristic fining-upward sequences indicating different storm of fair-weather conditions. Swash zones migrating during falling stages of storms are well defined by typical open work granule layers. Cycles of the sandy beaches are characterized by erosional surfaces and alternation of plane beds and ripple-drift cross-lamination. The whole barrier is capped by foredunes, transverse and high parabolic dunes made up of very well sorted sands showing regular to slightly modified large-scale cross-bedding. Unlike marine systems, no intense bioturbation has occurred in these lacustrine units, the strandlines are well defined because of lack of significant influences from tides, the various subenvironments can be readily recognized, the approximate energy conditions existent at the time of growth of the barrier can be reconstructed, and sedimentological models can be built. A descriptive analogue model of the lacustrine barrier system is presented.

Ripple-Drift Cross-Lamination in Tidal Deposits: Examples from the Precambrian Bhander Formation of Maihar, Satna District, Madhya Pradesh, India

Ripple-drift cross-laminations are described from tidal deposits of the Precambrian Bhander Formation, which belongs to the Vindhyan Supergroup of central India. Considering the transport processes operative in modern midflats, ripple-drift cross-laminations should be common in ancient counterparts. The generation of ripple-drift cross-laminations is, however, usually hindered by the absence of persistent, unidirectional current flow. These structures can only develop in areas of the midflat where there is persistent, unidirectional current. Because of periodic velocity changes of tidal currents, rhythmically size-differentiated foreset laminae appear to be a diagnostic feature of tidal ripple-drift cross-laminations. Size differentiation is normally absent in the ripple-drift cross-laminations of turbidites and fluvial sequences, for in the environments responsible, there exists no cyclic pattern of water movement (that is, currents that alternate with periods of slack water) as is found in the tidal-flat environment.

Morphology and Hydraulic Significance of Climbing Ripples with Superimposed Micro-Ripple-Drift Cross-lamination in Lower Quaternary Lake Silts, Nebraska

ABSTRACT Climbing in Quaternary lake silts in the glaciated part of Nebraska are defined by continuous laminae over some of the ripple systems, but more commonly, by laminae that are discontinuous and, in part, cross-laminated. Laminae on stoss sides of the climbing are formed of trough sets of micro-ripple-drift, which except for their smaller size, are similar to erosional Type A and gradational Type B and S sets of ripple-drift described by Jopling and Walker (1968). Most of the sets are erosional Type A micro-ripple-drift cross-lamination with heights between two and 10 millimeters. Gradational sets of micro-ripple-drift usually occur on the upper three-quarters to two-thirds of the stoss side where parallel laminations pass laterally downcurrent into Type S, then Type B, a d finally, into Type A micro-ripple-drift. Patterns of micro-ripple-drift on the backside of the are indicative of ripple fans, which migrated up the stoss side of the larger straight-crested and cascaded clown the slipface that is made up of alternating coarse-fine laminae. The size of the larger ranges up into the dimensions of and down into the dimensions of with no apparent hiatus in scale. These dimensions for have not been reproduced in flume experiments using silt size sediment (Southard and Harms, 1972). The scale of the larger bedforms, therefore, is believed to be the result of a difference in scale for ripples and dunes because of the smaller grain size and/or the result of rapid disequilibrium deposition. Patterns of micro-rip le-drift indicate a movement of fluid over the larger like that predicted by Allen (1968) for flow patterns over straight-crested with grains transported and deposited froth bedload and suspension. The angle of climbing displayed by the larger is defined by the thickness of sets of micro-ripple-drift preserved on the stoss side and the thickness of sediment added to the slipface from avalanching of grains, cascading of micro-ripples over the brinkpoint, and deposition from suspension. Thickness of sediment added to the slipface contemporaneous with migration of a single set of micro-ripple-drift cross-lamination over the stoss side to the brinkpoint, is the major factor that determines the angle of climbing of the larger ripples. This relationship implies that the downcurrent climbing of these is significantly influenced by the rate of migration of micro-ripples over the stoss side onto the slipface and the amount of sediment supplied to the stoss side and slipfa e by bedload sediment transport and from suspension. The ripple-drift cross-laminated silts were deposited by density currents (under flow) in a small shallow lake, perhaps no more than 7 m deep, that existed for only a few years. Topography on the lake floor was irregular and mounds of gravel may have reached the lake surface in a few areas. Silt layers of varves deposited over these mounds show changes in thickness, type of ripple-drift cross-lamination, and changes in ripple shape which probably were caused by increased flow or the shallow water depths associated with the larger mounds. Cyclical repetitions in the steepness of climbing of in the silt summer layer of the basal varve couplet indicate a simple-harmonic variation of local flow velocity with time. Such simple fluctuations characterize the diurnal runoff from gla iers today. In the early Pleistocene, similar diurnal variations in meltwater added to the lake from nearby glaciers could have significantly influenced underflow on the lake floor.

Ripple-drift cross-lamination in the Hawkesbury sandstone, New South Wales

Ripple-drift cross-lamination in the Hawkesbury sandstone, New South Wales

Antidune Cross-Stratification in Turbidite Sequence, Cloridorme Formation, Gaspe, Quebec: ABSTRACT

Large-scale, single set cross-stratification, considered to have been produced by the upstream migration of antidunes, is present in the basal division of turbidite beds 2-3 m thick in the s1 member of the Cloridorme Formation (Ordovician), Gaspe, Quebec. The local westward paleocurrent direction is approximately parallel with the east-west strike of the vertical beds, which crop out on the wave-cut platform. The beds generally show 3 broad divisions. The basal division (2-15 cm) consists of very coarse-grained sand with a smooth or fluted base and sinusoid top (wave length from 45 to 100 cm and amplitude from 3 to 7 cm) and internal laminae inclined at a shallow angle (less than 15°) toward the east. Where flutes occur they give a paleocurrent direction toward the west. A second division consists of spindle or globular-shaped nodules (pseudonodules) in an argillaceous matrix and commonly occurs above the basal division. In 1 bed, which occurs in a sequence with beds exhibiting ripple-drift cross-lamination, a ripple-drifted horizon is developed between the basal and pseudonodule divisions, the ripple-drift produced by a paleocurrent toward the west. The upper division consists of f ne-grained siltstone or shale. Because the basal very coarse sandstone is a small percentage of the total thickness of any bed, it appears that the settling velocity of the coarser grains probably was reduced by a high concentration of fine suspended material which increased the viscosity and reduced the density difference between the coarse grains and the suspension; the result was a high transport rate and the formation and preservation of antidunes, because of a delicate balance between traction and suspension in the current. Calculations suggest that the current moved at velocities of approximately 1 m/sec. End_of_Article - Last_Page 871------------

Geometrical analysis of ripple-drift cross-lamination

The geometrical parameters of ripple-drift cross-lamination (climbing ripples) have been measured in coastal exposures of the Cloridorme Formation (Middle Ordovician turbidites of northern Gaspé Peninsula, Québec), in order to determine the factors that control the angle at which the ripples climb, and to investigate the context of the ripple-drift relative to the other sedimentary features in the Cloridorme Formation. Purely geometrical analysis shows that the angle of the stoss side is more sensitive in controlling the angle of climb than the angle of the lee side, but the single most important factor is the ratio of thickness of the stoss side and lee side laminae. When the lee laminae are about twice as thick as the stoss laminae, the angle of climb is about 30–40 degrees. If the ratio is greater than about 10, the angle of climb is reduced to about 5 degrees.The lamina thicknesses are controlled by the rate of deposition from suspension relative to the rate of bed load movement. At high flow regimes, sediment is swept onto the lee side and the angle of climb is low. In the Cloridorme Formation, the turbidites associated with the ripple-drift are proximal, and hence a rapid flattening of the basin floor gradient is suggested to cause the rapid deposition leading to ripple-drift formation.

Ripple-Drift Cross-Lamination in Turbidites: ABSTRACT

Ripple-drift cross-lamination is the name given to a series of ripples which upon the backs of each other as a result of the addition of sediment from suspension during ripple migration. The type found in turbidites is characterized by continuity of lamination across the ripple system, changing composition of laminae from lee side (mud) to stoss side (silt and sand), and gradual upward decrease of ripple amplitude. Geometric analysis has shown that the angle at which the ripples climb is a function of lamina thicknesses on the lee and stoss sides, the angle of the lee and stoss slopes, and the symmetry of the ripples. Computation of many angles of climb and different ripple geometries has shown that all three factors (thickness, angle, and symmetry) are equally important in determining the angle of climb. Field measurements of ripple-drift morphology in Ordovician turbidites of the Gaspe Peninsula, Quebec, indicate angles of climb from about 3 to 40°, with cosets of ripple-drift cross-lamination ranging in thickness from 4 to 37 cm. There is a direct correlation between coset thickness and angle of climb, and lee-side lamina thicknesses tend to be two to four times greater than stoss-side t icknesses. The factor controlling the angle of climb is the ratio of volume of sediment deposited from suspension to the volume of sediment moved on or very close to the bed and deposited on the lee sides. The velocity at which the ripples move downstream is also a function of the same ratio, plus a function of the hydraulic parameters controlling the formation of the ripples. Because of the complex interactions of these variables, it is not yet possible to estimate accurately the time taken for formation of the ripple-drift cosets. Crude estimates suggest a period of less than 1 or 2 hours for a coset 40 cm thick. End_of_Article - Last_Page 748------------

Morphology and Origin of Ripple-Drift Cross-Lamination, with Examples from the Pleistocene of Massachusetts

ABSTRACT Many different types of ripple-drift cross-lamination occur in juxtaposition on the gently sloping foresets of a late Wisconsin kame delta near Concord, Massachusetts. A morphological classification is proposed, based on the two end members of a complete sequence of forms. Type A ripple-drift cross-lamination is characterized by non-preservation of stoss-side laminae, and hence consists only of climbing sets of lee side laminae. Sinusoidal ripple lamination (new term) consists of a series of ripples with symmetrical, sine-wave profiles and continuous laminae across the ripple system. One intermediate type (B) is defined between these two end members; the type is characterized by climbing sets of lee side laminae and by preservation of sandy stoss side laminae. The morphological classi ication is extended into a genetic classification, based upon the ratio of sediment fallout from suspension to sediment moved as bed load. Type A represents dominant bed-load movement, and sinusoidal ripple lamination represents dominant fallout from suspension. The cross-lamination described in the paper originated from density underflows of sediment-laden meltwater flowing into a glacial lake. The different types of ripple-drift cross-lamination are attributed here to small fluctuations in the current velocity, and to variations in the composition and concentration of suspended sediment. It is believed that at times of high fallout from suspension, the presence of fines stabilized the lake floor, and sinusoidal ripple lamination was formed. Flume experiments on the formation of sinusoidal ripple lamination tend to confirm this conclusion.

Deduction of flow regime from bedding character in conglomerates and sandstones

Deduction of flow regime from bedding character in conglomerates and sandstones