Crossbedded gravel lithofacies

Braided rivers are a common sight in mountainous, eastern Canadian Arctic, this one draining into Strand Fiord, Axel Heiberg Island. Flow is seasonal, and at a maximum during spring-early summer thaw. Late summer low water levels expose large mid-channel gravel bars, dissected by chutes during peak flow runoff. Partially submerged active bars are developed around more sand prone tabular bedforms during late summer low-flow conditions (this view). The floodplain has only sparse vegetation – bank collapse supplies slugs of gravel to the active channels.

Featuring tabular, trough, and horizontally crossbedded gravels

General occurrence

Tabular and trough crossbedding in gravel and mixed sand-gravel deposits are comparable to their sandy counterparts in some respects:

The basis for defining them as tabular and trough bedforms uses the same criteria, namely the 2D and 3D geometry of their lower crossbed set boundaries – planar and spoon-shaped respectively.
Trough, tabular (and horizontal bedded) gravels are closely associated in fluvial-alluvial depositional systems.
They are a response to bedload transport of sediment, under conditions that vary from subcritical to supercritical flow.

The main point of departure from these commonalities is the significantly greater shear stresses required to entrain clasts >2 mm diameter, particularly for pebble and coarser clast sizes. In many cases, such flows will be Froude supercritical. Furthermore, in natural channels and sheet flow there is much variation in flow velocity (and bed shear) from one part of a channel to another, where flow fluctuates between supercritical to subcritical. Whereas the bedload movement of sand may be relatively continuous under subcritical (lower flow regime) conditions, the transport of gravel will likely be discontinuous.

Tabular and trough crossbedded gravel lithofacies

Tabular and trough crossbeds are assigned to separate lithofacies, but here are treated together.

External structure

Tabular and trough crossbedded gravel bedforms tend to be larger than their sandy counterparts by virtue of their grain size. The best place to observe them in their entirety is in river and alluvial fan channels during low flow conditions. In coarse-grained braided systems, tabular crossbeds are one of the foundational structures of within-channel bars. Note however that most channel bars are complex amalgams of overlapping, discordant, and truncated tabular and trough crossbed sets rather than a single bedform, because of changing flow directions and flow competences as channel discharges wax and wane. Unlike their sandy counterparts, identification of 2D and 3D bedform geometries is more difficult. In reality, confident identification of either tabular or trough gravel bedforms requires cross-section exposure of foresets and crossbed set boundaries.

Internal structure

Tabular foresets are generally planar or slightly concave upward, dipping 20^o-30^o. Trough crossbed foresets are more spoon-shaped, mimicking to some extent the trough-like lower boundary. Crossbed set thickness generally ranges from a few decimetres to 3 metres and more. Grain-size grading is common where the largest clasts occur at the base of foresets, becoming finer-grained up dip. Clast-supported frameworks predominate. Successive crossbed sets of both types truncate earlier formed sets.

Unfortunately, in real world exposures crossbedding can be difficult to discern because the grain-size range along a single gravel foreset and among groups of foresets commonly ranges over two and even three orders of magnitude (keep in mind the Wentworth grain-size scale is logarithmic). If you want to decipher sedimentary structures in gravel deposits, you really do need to stand back from the outcrop.

Two sedimentary structures provide some relief from this dilemma.

Thin sandy deposits on foresets provide good contrast in grain size within the gravel accumulations, and
Imbrication or alignment of platy and oblate clasts along foreset planes.

A massive conglomerate of probable fluvial origin, but lacking obvious crossbedding. Framework is clast-supported but sorting is extremely poor, with clasts ranging from small pebbles to 150+ mm diameter. The conglomerate is interbedded with other coarse-grained, crossbedded units. Buchanan Lake Fm, Ellesmere Island (?Eocene).

Large tabular crossbeds in an Eocene (?) conglomerate, with sets up to a 120 cm thick. Crossbed set contacts are mostly planar. Foresets are more easily identified on weathered exposure.

Imbrication of platy and elongate clasts accentuates crossbed foresets and set boundaries. In this modern example, there is a strong imbricated fabric atop an inactive gravel bar. River flow was to the right.

Formation – hydrodynamic conditions

Tabular crossbedded pebble conglomerate in scoured contact with underlying tabular crossbedded lithic sandstone. Flow in both units was to the left. The apparent dip of gravel foresets is 20-25o (dashed lines outline general trends). The lowermost conglomeratic tabular bed can be traced laterally for about 8 m. It is overlain and locally scoured by tabular and trough crossbedded conglomerate. Lower Cretaceous Elk Fm, southern Alberta. — Tabular crossbedded pebble conglomerate in scoured contact with underlying tabular crossbedded lithic sandstone. Flow in both units was to the left. The apparent dip of gravel foresets is 20-25 deg (dashed lines outline general trends). The lowermost conglomeratic tabular bed can be traced laterally for about 8 m. It is overlain and locally scoured by tabular and trough crossbedded conglomerate. Lower Cretaceous Elk Fm, southern Alberta.

Tabular and trough bedforms in gravels develop from bedload transport – a combination of traction carpet and saltation load. For this to take place, flow velocities must be high enough to initiate (entrain) and maintain grain movement. For entrainment to begin, the shear stresses generated by flow of water over a sediment bed must be greater than the combined forces of gravity and frictional drag; these are referred to as critical stresses. Some of the variables that influence critical stress include:

Flow characteristics at the water-bed interface. In most natural flows this will involve turbulence.
Flow velocities.
Clast diameter; large clasts will have a greater surface area exposed to flow than their smaller neighbours.
Clast density.
Clast shape; spherical clasts will roll more easily than platy clasts of the same diameter and density (Cassel et al., 2021, Open access).
Overall bed roughness, or the degree to which some grains stand proud against their nearest neighbours, is determined by grain size, sorting, and clast packing.

Bed roughness can have a significant impact on clast entrainment. In a gravel bed where size sorting is relatively poor (a common feature of gravels), smaller clasts trapped by larger neighbours will not move until the larger clast is dislodged. This means that some sections of a gravel bed may be static, while other sections are on the move.

Assuming entrainment is in full swing, clasts will roll, bounce, and jostle their way along the top of the submerged bar or bedform. Clasts will cascade down the lee face. Tabular crossbedding is the most common type formed in this way. Any entrained sand will probably be swept beyond the lee face but may fall out of suspension in the backflow eddies because of reduced flow competence.

The rate at which bars and bedforms migrate downstream is also dependent on sediment supply (e.g., Nelson and Morgan, 2018). There are three main sources of coarse- (and fine) grained material:

Newly introduced sediment from upstream sources such as alluvial fans during floods or peak flow during spring thaw.
Erosion and collapse of channel margins (e.g., older channel deposits). Supply of this kind will be discontinuous.
Reworking of material from within-channel bars during peak flow as channels migrate laterally or on bars dissected by chutes.

Tabular crossbedded pebble conglomerate in sets up to 1.5 m thick. You can gain a sense of the truncation and erosion by successive bedforms by tracing crossbed set boundaries through the exposure. Overall, the channel fill thickness exceeds 8 m. This does NOT necessarily mean the channel depth exceeded 8 m deep at the time of deposition but could imply a continuity of accommodation space generation in a channel that maintained its location. Late Jurassic, Bowser Basin, northern British Columbia.

Common environments

We generally associate these lithofacies with gravel-bed fluvial channels in low sinuosity river systems. Alluvial fans may also contain ephemeral, braided channels. In both cases, within-channel transverse, point, and longitudinal bars consist mostly of amalgamated, tabular crossbedded lithofacies, with the trough crossbedded lithofacies created by scouring in pools that form at the junction of active channels (see N. Smith, 1974 for one of the first descriptions of these bar types – PDF available). These conditions also apply to glaciofluvial environments, and to fluvial channels on continental shelves or platforms that are exposed during sea level lowstand.

Fluvial channels that incised shelf deposits during a sea level lowstand (now exposed as a coarsening upward parasequence), produced spoon-shaped scours filled by trough crossbedded conglomerate and pebbly sandstone, and a few tabular crossbeds that represent migration of 2D bedforms over the channel floor. Callovian Bowser Basin, northern British Columbia. — Fluvial channels that incised shelf deposits during a sea level lowstand (now exposed as a coarsening upward parasequence), produced spoon-shaped scours filled by trough crossbedded conglomerate and pebbly sandstone (TrXB), and a few tabular crossbeds (TabXB) that represent migration of 2D bedforms over the channel floor. Callovian Bowser Basin, northern British Columbia.

Fluvial channels that supply sand to the topset beds of coarse-grained fan deltas contain both lithofacies. Larger versions of the tabular crossbedded gravels also resemble the foreset layers of Gilbert deltas.

Beach storm ridge gravels tend to be structureless internally but may develop crude tabular crossbeds if the gravels are pushed landward. In this case the foresets will dip landward. Associated lithofacies and biofacies will provide a reasonable basis for making the distinction between beach settings and fluvial, glaciofluvial, and fan delta environments.

Horizontally bedded gravel lithofacies

The lithofacies is included in this post because in coarse-grained fluvial deposits it is associated with the crossbed lithofacies, as a bedform itself and as a transitional or precursor bedform to tabular crossbedded gravels (Hein and Walker, 1977 PDF available; Miall, 2006).

External structure

The lithofacies consists of flat bedded, low relief gravels deposited as diffuse sheets. They are generally a few decimetres thick.

Erosional cross-section through a subhorizontal gravel sheet, deposited on an alluvial fan that periodically discharges into Peel River, Yukon. This view shows the crude low angle layering in very poorly sorted gravel. Framework clast sizes range from fine pebble to 150-200 mm diameter cobbles. Matrix is a mix of fine and coarse sand. Flow was left to right. Abandonment of the bar was accompanied by erosion and cannibalization of gravel that was deposited on younger bars farther down the fan slope.

Internal structure

Stratification mimics the external bedform geometry and thus tends to be flat with very low-angle, downstream dip. Stratification tends to be crude and in coarser gravels may not be easily identified, although it may be enhanced by clast imbrication. Clast-supported frameworks predominate. In most deposits the sand matrix has infiltrated clast interstices during low flow conditions.

Formation – hydrodynamic conditions

Based on their observations of bar development in Kicking Horse River, Hein and Walker (1977) inferred that these bedforms developed during peak flows as a gravel lag one to two clasts thick. If the supply of clasts is high then the sheets aggrade, but their overall geometry does not change. The overall effect is the development of flat, horizontal or low angle stratification. If clast supply rates remain high, then the gravel sheet will also expand downstream.

In the Hein and Walker model, lower rates of sediment supply also result in vertical aggradation of the bedform but in this case a lee or slip face develops at the downstream end of the gravel sheet. Clasts that tumble down the slip face will contribute to the development of tabular foresets. Thus, if the Hein and Walker hypothesis is correct, horizontal bedded sheets and bars can act as a precursor to tabular crossbedded bars.

A cartoon version of the Hein and Walker (1977) model for development of gravel sheets and tabular crossbeds. Modified from their Fig 6. The image of a modern braid channel (right) helps put these two bedforms into context. The example shows low-flow conditions (mid-summer on Axel Heiberg Island, Canadian Arctic) where bar tops are exposed. Low-flow exposure accentuates the changing locus of channels, and bar-top chutes that formed during a lowering of water levels. Observations from the channel bank identified tabular gravel and sand bedforms in the active channels.

Common environments

Like the tabular and trough crossbed lithofacies, there is a general expectation that these gravel sheets occur in coarse-grained river and ephemeral alluvial fan channels, including channels that transport sediment across fan delta topsets.

Low-angle and horizontally bedded gravels also occur in marine shoreface and beach deposits. However, the association with other lithofacies and biofacies in these environments should serve to distinguish them from fluvial-dominated settings.