Ripple lithofacies influenced by tides

Lenticular and flaser bedding in Late Pleistocene intertidal – estuarine deposits. The sand ripples are dark grey hues, the mudstones pale grey. Manukau Harbour, west Auckland. Coin diameter at lower center is 25 mm.

Lenticular bedding, flaser bedding, and interference ripples

Most crossbeds and their associated bedforms cannot be linked directly to specific environments of deposition. For example, planar and trough crossbeds form in marine, fluvial, and aeolian settings and pin-pointing any one of these usually requires additional information. Likewise, current ripples are ubiquitous in marine and terrestrial environments. However, a small group of ripple bedforms can, with some confidence, be associated directly with tidal currents, where bedforms react to tidal current asymmetry (the fancy name for ebb-flood reversal of tidal currents). The value of this association is enhanced when other sedimentary structures of shallow marine origin are present. Tidal currents interact with the sea floor on all continental shelves and platforms, but their most profound influence on sedimentation is in intertidal environments. Three of these bedforms are described here: lenticular and flaser ripple bedding, and interference ripples.

Use this link to read the introduction to the lithofacies series.

Flaser and lenticular bedded lithofacies

General occurrence:

Lenticular and flaser bedding are commonly found together. There is a complete gradation between the two types of bedding. They are best developed on mixed sand-mud tidal flats.

External form:

Reineck and Wunderlich (1968) provide a detailed description and classification of lenticular and flaser bedding – this is still one of the best sources of information. The term ‘lenticular’ derives from the ripple profile, where the lenticularity refers to lee face – stoss face symmetry. Lenticular bedding is characterized by sand ripples overlain by or encased in mudstone. Thus, ripples appear isolated, disconnected, or as locally connected bedforms. The proportion of mud in beds containing lenticular ripples is usually equal to or greater than that of sand.

Flaser bedding forms where the proportion of sand exceeds that of mud and is characterized by mud veneers or streaks that line or fill the troughs between successive ripples. The mud layers may overlap ripple crests.

Lenticular and flaser bedding form with both current and wave ripples. Thus, bedforms may have asymmetric or symmetric profiles, and crests that are straight, linguoid, or lunate.

Mud flasers (pale brown) have accumulated in the troughs of these linguoid (and a few lunate) ripples on an intertidal flat – the ripples formed during a flood tide. In this example, clay and silt were stirred into the shallow tidal waters during a storm. The mud was deposited from suspension to the intervening troughs during slack tide when bottom currents were at their lowest velocity. Depending on the relative strength of the subsequent ebb currents, the flasers may be preserved or modified. The bed in the lower half of this view is partly covered by water. Field of view is 80 cm wide.

Straight crested sand ripples (deep red hues) have migrated over a muddy substrate; trails in the mud layer have been over-ridden by the ripples. This is a recent example of lenticular bedding, where successive ripples are disconnected. There is a superficial resemblance to flaser bedding, but in this example the mud layer was deposited before the ripples. Here, the ripples formed during the flood tide cycle. Coin diameter (top left) is 24 mm.

Internal structure:

Diagrammatic representation of lenticular (left) and flaser bedding (modified from Reineck and Wunderlich, 1968). Lenticular bedding is represented by isolated and connected wave and current ripples. The flaser bedding panel shows straight crested (2D) and linguoid – lunate (3D) bedforms and (grey) mud flasers. Note, the 3D bedforms have scoured, spoon-shaped bases and concave crossbed foresets). Each field of view is 50 cm wide.

Current ripple bedforms in lenticular and flaser bedding contain typical lee-stoss face asymmetry, and foresets that dip downflow. Wave-generated bedforms are symmetrical and may contain opposing foresets, or foresets having a dominant dip direction depending on wave orbital geometry. Each ripple may be isolated or overlap (and partly erode) other ripple sets. Mud that drapes or encases lenticular ripples may be laminated and commonly contain discontinuous sand laminae. Bioturbation is common; there may also be macro- and microfossils.

Mud flasers are preserved as isolated, wispy veneers or lenses overlying or filling ripple troughs. Ripple cosets may contain several flasers.

Paleocene estuarine or tidal flat deposits containing lenticular ripple bedding (light coloured sandstone in upper half of image) encased in and draped by dark grey, carbonaceous mudstone laminae. Ripple migration was to the left. Ellesmere Island. Coin diameter is 24 mm.

Formation – hydraulic conditions:

Lenticular bedding and the ripples overlapped by flaser bedding form in the same way as standard current and wave ripples. Their distinctiveness derives from an interaction with ebb and flood tidal currents. Both types of structure form when either the ebb or flood tidal currents dominate, with the mud layers accumulating during the opposite, much weaker flow.

Common environments

Both structures form on mixed sand-mud tidal flats in marine embayments, lagoons and estuaries that are subjected to tidal exchange. Note that neither bedform indicates which tidal flow was responsible for their development, but instead points to the relative strength or competence of either ebb or flood flows. The real value of the lithofacies lies in the stratigraphic repetition of the bedforms that indicate a degree of regularity or periodicity of opposing current strengths, a periodicity that is difficult to explain in depositional settings other than a tidal environment (current reversals can develop locally in fluvial and aeolian depositional settings, but there is rarely any periodicity to such events).

Interference ripple lithofacies

General occurrence:

Interference ripples occur when one bedform coset, formed during either flood or ebb tidal flow, is modified by ripples formed during the opposing tide. For ripple interference to occur the directions of current flow must be less than 180^o such that the two sets of ripple crests are oblique.

Interference ripples in fine-grained sand, Minas Basin, Bay of Fundy. The directions of tidal flow are indicated. The flood tide ripple sets have larger wavelengths than the ebb tide sets, indicating weaker currents in the latter. Both sets have relatively straight crests. Coin diameter is 24 mm.

External form:

Each ripple coset is formed by currents and hence will usually show lee – stoss face asymmetry. However, the first-formed ripples will also be modified – typically crests will be rounded or flattened by the opposing tidal flow. In addition, the crest lines of earlier-formed ripples will be dissected in a fairly regular fashion by the latest ripple coset.

Interference ripple patterns are more easily identified in straight crested bedforms than in 3D bedforms such as linguoid ripples. Exposure of both ripple sets on bedding is necessary for confident identification.

Two billion year-old interference ripples in fine-grained, mixed carbonate-siliciclastic sandstone. Both ripple sets have similar amplitudes but the wavelength of set 1 is about half that of set 2, indicating that the opposing tidal currents had slightly different velocities. Both ripple sets have straight to sinuous crests. Set 1 ripples (flow to the right) formed after and modified Set 2 ripples. Thus, the flow direction of Set 2 is ambiguous in this bedding exposure. Lens cap diameter is 50 mm. McLeary Formation, Belcher Islands.

Internal structure:

Ripple foresets will show lee-face preservation typical of most current ripples. In profile views there is little to distinguish interference ripples from other ripple types; bedding exposure is usually a prerequisite for positive identification.

Formation – hydraulic conditions:

The hydraulic conditions necessary for current ripples to form apply to both ripple cosets (see the companion post on current ripple formation). The change in direction of ripple migration is a function of the degree of asymmetry of opposing tidal flows.

Common environments:

Interference ripples are valuable indicators of tidal current asymmetry over tidal flats associated with lagoons, estuaries, and coastal embayments, particularly if they occur with lenticular and flaser bedding. Interference ripples will not form if the flood-ebb flow directions are directly opposite (i.e., 180^o). They are more likely to form where ebb flow is deflected, for example by the formation of larger bedforms over the tidal flat, or by wind shear that pushes the shallow water masses in different directions (for example, during storm surges).

Neither ripple set can be equated with an ebb or flood tide. However, if the strike of a paleoshoreline can be determined from independent data, then it may be possible to assign ebb or flood status to a particular ripple set.