Ripple Cross-lamination Research Articles

Depositional Structures and Processes in the Non-barred High-energy Nearshore

ABSTRACT The marginal marine environment consists of an offshore where the wave form is approximately sinusoidal and a nearshore where the wave form is either solitary or that of a bore. Within the nearshore, shoaling waves become progressively higher and steeper until they break. After breaking, the waves progress as bores through a surf zone; these bores ultimately terminate within a swash zone on the beach itself. In a high-energy coastal environment where long-period swell enters a nearshore uncomplicated by offshore bars, sedimentary structures develop on the seafloor in facies that trend parallel to the zones of different wave activity. In the offshore, small sand ripples are the most common depositional structure, but in the nearshore larger bed forms predominate. Seaward from the line of breakers, in the zone of wave build-up, are landward-oriented lunate megaripples. Near the outer portion of the surf zone the bed form is planar (outer planar facies), but, in the inner portion of the zone, a area of large-scale bed roughness (inner rough facies) commonly is present. Within the swash zone, the bed form is again planar (inner planar facies). The boundaries of the facies shift in response to changes in waves or tide, and certain of the zones are sometimes missing. The relative position of the zones, however, is invariable. The major features of the bed forms can be interpreted in terms of flow regime. The stronger of the two opposing transient currents caused by passing waves produces structures analogous to those produced by continuous, unidirectional currents. Landward wave surge is dominant in the outer three structural facies, whereas seaward surge predominates in the innermost (inner planar) facies. Wave surge over the intermediate inner rough facies is more complex, and the direction of strongest surge may be variable. In the outer three facies, the landward sequence from small asymmetric ripples to lunate megaripples to plane bed suggests an increase in flow regime from the lower part of the lower regime to the upper regime; this shoreward increase in flow regime is associated with a shoreward increase in orbital velocity at the bottom. The inner planar facies is produced by flow in the upper regime. The inner rough facies, situated between two zones of flow in the upper regime, is apparently a product of flow in the upper part of the lower regime. Within each of the structural facies a distinctive set of internal structures is produced. Internal structure of the asymmetric ripple facies consists of shoreward-inclined ripple cross-lamination and gently inclined cross-stratification. The lunate megaripples produce medium-scale landward-dipping foresets. Within the outer planar facies bedding is nearly horizontal. Structures in the inner rough facies produce medium-scale foresets that mostly dip directly or obliquely seaward, although landward-dipping foresets also occur. Within the inner planar facies bedding is gently inclined seaward. Migration of the facies in response to changes in waves or tide produces distinctive assemblages of structures where the facies overlap. These assemblages provide criteria for paleoenvironmental i terpretation, particularly where interrelated assemblages occur in a meaningful spatial distribution.

Glacial Sedimentology of the Precambrian Gowganda Formation, Ontario, Canada

The Gowganda Formation is part of the thick Huronian sequence of Precambrian sedimentary rocks that crop out in central Ontario from Lake Superior to Quebec. Although it has long been considered to be glacial, recent work on submarine slump and turbidite deposits has reopened the question of its origin. This study was made to determine its origin and paleogeography. Till-like conglomerates, varved argillites, and abundant dropstones characterize the Gowganda and provide strong evidence for a glacial origin. Pebble fabric parallel to regional paleocurrents, rare striated and grooved pavements, and abundant unweathered detritus are also in harmony with a glacial origin. Local thin-bedded sandstones contain flame structure, graded bedding, contorted bedding, and rippled tops, suggesting deposition by turbidity flows. Association of these sandstones with varved argillites and rafted stones indicates that Gowganda turbidites are glaciolacustrine. Along the southern margin of the Gowganda, at Whitefish Falls, thick, laterally continuous till-like conglomerates contain internal stratification indicative of subaqueous deposition. Argillites that lack varved structure and associated silty limestones suggest a glacial marine environment. In the Cobalt region the apparent long axes of pebbles in conglomerates show a predominant north-south alignment. In the Bruce Mines-Elliot Lake region, the orientation of the long axes of pebbles is more variable. Differences in orientation are related to stratigraphic position. Ripple cross-lamination in sandstones that are interbedded with argillite indicates south-trending paleocurrents. Distribution of varved argillite, silty limestone, and probable marine argillites suggests a northern fresh-water facies and a southern marine facies. Abundant plutonic pebbles and feldspar indicate that the Gowganda ice sheets eroded a northern plutonic terrane and deposited much of their sediment load on the underlying Huronian sediments to the south.

LARGE SCALE RIPPLES IN SILURIAN LIMESTONES

Large scale ripple marks occur in Silurian (Landovery-Wenlock) limestones of the Oslo fjord area, Southern Norway. The limestones are biosparites with a drifted assemblage of fossils and sandstone pebbles. Ripple cross-lamination is developed. It is concluded that the limestones accumulated under the action of strong currents. Features of the ripple cross-lamination suggest that these currents may have been tidal in origin.

Cross-stratification as a criterion of palaeocurrent direction in flysch deposits

Synopsis Measurement of the line of intersection between foreset laminae and normal bedding gives the most reliable estimate of palaeocurrent direction in ripple cross-lamination. Accepting a possible error of ± 20°, it is necessary to make only one or two measurements per locality.

Microfacies and Sedimentary Structures in Deeper-Water Lime Mudstones: ABSTRACT

Limestone deposited in downslope or in basinal positions possesses several distinctive criteria based on microfacies, bedding, and fauna. The microfacies include (1) homogenous lime mudstone, (2) millimeter-laminated End_Page 485------------------------------ lime mudstone, and (3) common micropelletoid lime grainstone (calcisiltite). The latter may show fine ripple cross-lamination. These rock types may occur in graded sequences but non-graded alternations are also common. Several kinds of minor sedimentary rhythms are recognized. The limestone types generally are dark, but light colored and even pink or red varieties are known. Distinctive monotonous bedding, consisting of planar relatively thin (½- to 2-foot) limestone beds interlayered regularly with thinner beds of dark shale, is common. Chert beds may be intercalated in such sequences. Spectacularly large cut-and-fill structures (stretching more than 100 yards) and presumably caused by submarine penecontemporaneous sliding are seen in places. Convolute bedding and flame structur s indicative of soft sediment slumping are scarce. Distinctive faunal assemblages exist in these beds. Siliceous and calcareous microplankton (radiolarians, diatoms, calcispheres, and tintinnids), sponge spicules, graptolites, pelagic or nektonic pelecypods, pelagic Foraminifera (globigerinids), pteropods, and certain ammonoids are especially characteristic. Field observations of this limestone type show that it commonly forms a sort of apron down very gentle slopes from typical carbonate shelves and in many places regionally is peripheral to a depositional basin, the center of which contains a thin section of dark shale. Recognition of this facies permits one to predict proximity to a carbonate-shelf margin. In several locations lime mudstone mounds are seen slightly upslope from basinal lime mudstone. Both the even and regularly bedded lime mudstone and calcisiltite and the mudstone mounds contrast with characteristic limestone turbidite (allodapic limestone) which probably represents accumulations down from relatively steeper slopes, in more tectonically active environments, and perhaps downslope from shelves with higher water agitatio . Numerous examples of deeper-water but non-turbiditic limestone are discussed from basins and geosynclinal troughs in the late Paleozoic of the western United States, the Cretaceous and Jurassic of Mexico, the Middle East, and Europe. End_of_Article - Last_Page 486------------