Trough crossbeds and tabular crossbeds – two common lithofacies

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

 

General occurrence

Next to ripples, tabular and trough crossbeds are the two most common sedimentary structures you will encounter in sandstones. Unlike the smaller-scale ripples that form in fine-grained sand, tabular and trough crossbeds occur in all grain size grades of sand and gravel, and at scales measured in centimetres to 10s of metres. These two crossbed types occur across the spectrum of aeolian, fluvial, alluvial, and marine depositional environments, from the giant aeolian foresets in the Navajo Sandstone that record blustering winds across the Early Jurassic North American continent, to sand-gravel dune-like bedforms in deep water submarine canyons (e.g., Kaikoura Canyon, east New Zealand; Mountjoy et al., 2018). Tabular and trough crossbeds are important paleocurrent indicators.

Tabular and trough crossbeds are ubiquitous, so much so that they deserve separate lithofacies designations.

 

Internal structure – tabular crossbeds

The distinction between trough and tabular crossbedding follows McKee and Weir’s (1953) definitions and is based on the geometry of the crossbed set basal contacts and the three-dimensional shape of the foresets.

Tabular crossbed sets have planar basal contacts; in 2D profiles they commonly appear flat. Foresets are also planar to slightly concave (downstream). The contact between foresets and the base varies from abrupt to tangential depending on the relationship between grain size and flow velocity (explained in the paragraph on hydraulics). Foreset contacts at the upper set boundary may also be tangential although erosion by the overlying bedform commonly removes these.

Foreset inclinations vary from 34^o in dry sand (aeolian settings), and 15^o to 30^o in saturated aqueous conditions. In unsaturated but wet sand foreset dips can be as high as 45^o because of grain surface cohesion. Foreset thickness also varies according to grain size and the overall size of crossbed sets – from one or two grains thick, to several centimetres in large bedforms.

Grain size grading parallel to foreset dip can also occur where larger clasts accumulate at the base of the foreset, becoming finer grained up-dip. Grain size grading along foresets is more easily identified in large bedforms (decimetres to metres thick), and in sediments having a range of clast sizes. Stacked tabular crossbeds commonly show partial erosion of underlying sets, but the contacts remain relatively flat or planar.

 

Internal structure – trough crossbeds

The basal contacts of trough crossbeds are distinctly curved upward in profile views, and concave or spoon-shaped in 3D views. Foresets are also curved. Other properties such as foreset thickness and grain-size grading follow trends similar to tabular crossbeds. Trough crossbeds frequently occur as cosets (multiple sets) where successive bedforms partly erode previously deposited sets. The basal contacts in all of these are concave. Trough crossbed cosets exposed in two-dimensions on bedding are typically festooned – a term used to describe the arcuate intersections of multiple troughs

 

 

 

 

External form

Tabular and trough crossbedding are generated by two types of bedform: two-dimensional and three-dimensional bedforms respectively.

[Note, such bedforms are also called dunes, subaqueous dunes, sandwaves, ripples, megaripples, bars]

Two-dimensional (tabular) bedforms have straight to slightly sinuous crests. They migrate downflow along relatively planar, flattish beds (sand, gravel) as sediment moves down the slip face (lee face). They are designated 2D because they are adequately described by a single profile parallel to the direction of flow (and therefore normal to the crest line). Migration of 2D bedforms produces tabular crossbedding.

Three-dimensional bedforms have arcuate or highly sinuous crest lines, and slip (lee) faces that migrate across a scoured, spoon-shaped depression that develops immediately downflow of the lee face; scouring takes place as the bedform migrates. In this case, the bedform geometry requires at least two profiles to capture its overall shape – one parallel to flow and normal to the crest line, and the other at approximately 90^o and parallel to the crest line.

 

Formation – subaqueous hydraulic conditions

Data from flume experiments and monitoring of natural flow in river and tidal channels, and shallow marine settings indicate that the 2D – 3D morphological separation of bedforms reflects 2D and 3D conditions in flow hydraulics (e.g., Allen, 1985; Ashley, 1990). In general, 2D (tabular) bedforms are generated at lower flow velocities than 3D (trough) bedforms with respect to grain size – as grain size increases, so too does the necessity for higher velocities. Thus, depending on (median or mean) grain size, higher velocities result in higher energy vortices and eddies in front of the lee face, resulting in scour.

Migration of both bedforms takes place in much the same way as smaller ripples with most of the action taking place in the bedload; grains are rolled, jostled or bounced along the (upstream) stoss face, and beyond the crest they cascade down the lee face. Flume experiments have demonstrated that the geometry of foreset basal contacts in 2D bedforms will change from abrupt to increasingly tangential as flow velocities increase. Foresets in 3D bedforms are usually curved and tangential to the sediment bed as they fill the scoured depression.

Both bedforms are included in the upper part of the lower flow regime. Using the terminology of Froude, both bedforms form in subcritical flow conditions; the Froude number for 3D bedforms is usually greater than that for 2D tabular bedforms.

 

Formation – subaerial hydraulic conditions

The fundamental difference between subaqueous and subaerial depositional conditions lies in the significantly lower fluid density and viscosity. Aeolian bedforms tend to form within a restricted sediment grain-size range compared with subaqueous structures because flow in air exerts much lower magnitude shear stress across the sediment-air interface. Thus, subaerial ripples and larger dunes generally form only in fine- to medium-grained sand.

Notwithstanding these restrictions, the mechanics of bedform initiation and migration are very similar to those developed in subaqueous conditions. Most sand is moved as a traction carpet and a saltation load up the bedform stoss face, subsequently cascading down the lee face. Under dry conditions, the foreset dip is close to the angle of repose – about 34^o.

An additional mechanism responsible for moving sand down the subaerial dune lee face involves dry grain flows. Grain flows are usually initiated when the upper part of a lee face is oversteepened during rapid sedimentation, gravitational collapse generating small grain flows. Underlying foresets may be disrupted during this process. Each grain flow event is small and may be difficult to recognise in the rock record.

 

 

Common environments

Trough and tabular lithofacies, found together or separately, characterise many terrestrial and shallow marine depositional environments. They are usually included in the various architectural elements and facies models that encapsulate these settings.

Both lithofacies develop where current flow is confined as in fluvial and tidal channels, and unconfined where flow is more sheet-like. There is a propensity for trough crossbeds (3D bedforms) to form in channels because flow velocities there tend to be higher than in unconfined conditions. However, tabular bedforms may also form in channels during lower-flow conditions such as waning flood stages in fluvial environments or waning tidal flows during ebb-flood transitions.

Unconfined tidal flow is typical of continental shelves and platforms. Trough and tabular lithofacies can be observed frequently on sandy tidal flats, and platforms adjacent to tidal channels, although my own observations indicate that the 3D varieties accumulate during tidal flow maxima, and 2D types during the waning stages of ebb or flood tide.

Extensive subaerial and submarine dune fields are well documented in the modern and rock record.  For example, subaqueous dunes, or sandwaves commonly develop on continental shelves where there is a sufficient supply of sand. Many of these structures have 2D bedform profiles with relatively straight or slightly sinuous crests. Amplitudes in some are measured in metres and wavelengths in 10s to 100s of metres. In most cases, each sandwave or subaqueous dune is a composite structure, made up of many overlapping, smaller scale bedforms. Many of these subaqueous dune complexes are dynamically linked to sedimentation on the shoreface. They are frequently modified by storms.

Likewise, deserts, ergs, and sand seas are replete with large and small bedforms, most of which are composite structures. And as we have discovered over the last couple of decades, the Martian planet surface has a fascinating variety of subaerial dunes for which our home-grown varieties provide good analogues for Martian bedform geometry and hydraulics.

Other posts in the series

Sedimentary lithofacies – An introduction

Ripple lithofacies: Ubiquitous bedforms

Climbing ripple lithofacies

Ripple lithofacies influenced by tides

Laminated sandstone lithofacies

Low-angle crossbedded sandstone

Hummocky and swaley cross-stratification

Antidune lithofacies

Lithofacies beyond supercritical antidunes

Subaqueous dunes influenced by tides

Introducing coarse-grained lithofacies