upper flow regime Archives

Conditions for bedform formation presented in different hydrodynamic contexts

An addition to the Lithofacies Series

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

General occurrence

One of the subordinate but no less significant sandy lithofacies, low-angle crossbedded sandstone is commonly is interbedded with plane-bed laminated sandstone. Recognition of both lithofacies in the rock record is important because they preserve one of the few lines of evidence for upper flow-regime hydrodynamic conditions. Low-angle crossbedded sandstone is generally found in fluvial and shallow marine depositional environments. Several authors include low-angle antidune bedforms in this category, but in this series antidune bedding will be treated separately.

External structure

Low-angle in this context refers to the inclination of crossbed foresets with respect to their substrate and the discordant contact between crossbed sets. Their low angle confers a low relief surface expression such that recognition of distinct bedform surfaces such as stoss and lee faces is difficult. In many situations the lithofacies appears more wedge-like. The boundaries between crossbed sets tend to be planar.

Internal structure

The lithofacies general occurs in relatively clean, fine- to medium-grained sandstones. Crossbed laminae dips are generally less than 15^o. Likewise, the contacts between crossbed sets also have very shallow dips; in 2D outcrops these contacts are flat. Foresets are usually tangential at their base. If tangential tops are present the foresets have an asymptotic geometry.

Parting lineation is common.

Outcrop view of numerous low-angle crossbed sets in medium-grained sandstone. The discordant contacts between successive crossbed sets are <10^o to 12^o (a few contacts indicated by arrows). Foreset dips are <15^o. The lithofacies is part of a prograding shoreface succession. Pen (left centre) is 15 cm long. Jurassic, Bowser Basin, northern British Columbia.

Miall (2006) has noted that the bedforms can accumulate on surfaces that are initially flat or inclined. Distinguishing between the two conditions is possible by comparing the orientation of the lithofacies with underlying lithofacies, such as laterally extensive ripple bedded units that probably originated on relatively flat substrates.

Formation – hydrodynamic conditions

Deposition of low-angle crossbed lithofacies probably takes place under conditions that span the hydrodynamic interval between upper plane bed flow and formation of antidunes; the presence of parting lineation confirms this interpretation. This corresponds to the transition from subcritical (tranquil) to critical flow (standing waves and antidunes) and supercritical flow.

Summaries of the hydraulic conditions for deposition of plane bed lamination, compared with other bedforms, in terms of: 1- Flow Regime (left); after J.C. Harms & R.K. Fahnestock, 1965, 2- Median grain size and mean current velocities (centre); modified from Ashley, G.M. 1990, 3- Reynolds and Froude numbers as a function of flow depth and flow velocity (right); modified slightly from J.R.L. Allen,1992, Fig. 1.21. Upper flow plane bed and low-angle crossbed fields are indicated in bold red type.

Summaries of the hydraulic conditions for deposition of upper flow regime bedforms, compared with other bedforms. Data for the graphical plots is mostly from flume experiments:
1- Flow Regime (left); after J.C. Harms & R.K. Fahnestock, 1965,
2- Median grain size and mean current velocities (centre); modified from Ashley, G.M. 1990,
3- Reynolds and Froude numbers as a function of flow depth and flow velocity (right); modified slightly from J.R.L. Allen,1992, Fig. 1.21.
Upper flow plane bed and low-angle crossbed fields are indicated in bold red type.

Common environments

The low-angle crossbedded sandstone lithofacies commonly is associated stratigraphically with upper plane bed laminated sandstone lithofacies. Both lithofacies are important because they preserve upper flow regime hydrodynamic conditions.

In fluvial systems the lithofacies most commonly develops in the proximal regions of crevasse splays that spill from river channels and delta distributary channels. In this setting the lithofacies will be associated stratigraphically and laterally with ripple, interbedded mudstone, and possible organic paleosols that develop across adjacent flood plains. The lithofacies can also fill shallow channel scours.

A section of beach deposits exposed during storm surge erosion and subsequent redistribution of sand to the shoreface. The section contains low-angle crossbeds in well sorted fine- to medium-grained sand – discordant contacts indicated by arrows. The section is parallel to the shoreline (i.e., parallel to depositional strike). Raglan, west coast New Zealand.

Low-angle crossbedded sand and sandstone is common on sandy beaches, particularly in the wave swash and backwash zone. In this case the depositional surface is initially inclined seaward. Like the laminated upper plane bed lithofacies, it will generally occur near the top of coarsening-upward, prograding shoreface successions, as shown in the example below.

Low-angle crossbedded sandstone at the top of a shoaling upward shoreface parasequence (a couple of crossbed set discordances indicated by arrows). The top of the lithofacies is capped by a transgressive, pebbly, fossiliferous limestone – the base of the limestone is the maximum regressive surface (MRS, and the top a flooding surface (FS). Lens cap (bottom centre) is 50 mm wide. The complete parasequence is shown opposite – the location of the crossbed lithofacies is shown by the red outline (top right). Low-angle crossbed and upper plane bed laminated lithofacies are common in the upper parts of all five parasequences (cycles) seen here. The vertical bar scale (lower right) is 5 m long. Mid Jurassic Bowser Basin, northern British Columbia.

Other posts in this series

Sedimentary lithofacies – An introduction

Ripple lithofacies: Ubiquitous bedforms

Climbing ripple lithofacies

Ripple lithofacies influenced by tides

Tabular and trough crossbed lithofacies

Laminated sandstone lithofacies

Hummocky and swaley cross-stratification

Antidune lithofacies

Lithofacies beyond supercritical antidunes

Subaqueous dunes influenced by tides

Introducing coarse-grained lithofacies

This post is part of the How To… series

We introduce some basic hydraulics of sediment movement, bedforms and the concept of Flow Regime.

Ripples and dunes form when a fluid (usually water or air) flows across a sediment surface. Structures formed by air flow are called subaerial ripples or dunes; those in water have the qualifier subaqueous. These structures are given the general name bedform. The construction of bedforms requires certain conditions:
–    The sand must be cohesionless (i.e. grains do not stick together).
–    Flow across the sediment surface must overcome the forces of gravity and friction, and
–    There is a critical flow velocity at which grain movement will begin; this also depends on the mass of individual grains, and to some extent their shape.

Ripples and dunes form under a relatively limited range of flow conditions. We can illustrate this in a graph of flow velocity against grain size, plotting areas on the graph that correspond to bedform growth. Most of the data for plots like this are derived from flumes where experimental flow conditions can be monitored closely. The plotted distribution shows that bedforms can be categorized according to flow and sediment conditions. This partitioning of bedforms was used to construct the Flow Regime hydraulic model, first published in the now classic 1965 paper by J.C. Harms & R.K. Fahnestock and used widely ever since.

The Flow Regime model considers three fundamentally different states of flow:

No bed movement where there is too little energy in the system to initiate and maintain sand grain movement,
A Lower Flow regime in which all common bedforms develop. Here, plane bed (basically parallel, planar laminae with no ripples) represents the lowest velocity, or energy conditions where sediment movement is initiated. It has been observed in flumes and in natural channels that the size of bedforms increases from ripples to large subaqueous dunes in concert with flow velocity. Dune type also changes from two dimensional structures (straight crests and planar crossbed bounding surfaces), to three dimensional structures that have sinuous, arcuate and lunate outlines and spoon or scour-shaped bounding surfaces (commonly seen as trough crossbeds).
An Upper Flow Regime where the power of stream flow washes out ripples and dunes, replacing them with plane bed (this kind of plane bed commonly has parting lineations), plus antidunes, and erosional chutes and pools.
As stream flow increases the transition from Lower to Upper flow regime produces one of the more interesting bedforms – antidunes. They are mostly found in shallow channels (e.g. fluvial and tidal channels). You can recognize that this transition has taken place when you see standing surface waves – watch closely and you will see the waves migrate upstream. Antidunes are the bedforms that develop immediately below standing waves (the two are in-phase). If high flow is maintained, the antidunes will also migrate upstream. However, once flow slackens they tend to wash out; the preservation potential of antidunes is low.

The example above shows standing waves in a tidal channel on an out-going tide. Tidal flow is to the left; standing wave migration is to the right (Mangawhai Heads, North Auckland).

Hydraulic jumps: The transition from Lower to Upper flow regime passes with a change in bedform, in particular washing out of subaqueous dunes, but there is no sudden break in surface flow – the transition is reasonably smooth. This is not the case for an Upper to Lower flow regime transition that is marked by an abrupt increase in water level and turbulence – a hydraulic jump. Hydraulic jumps can be thought of as standing waves. They are caused by a reduction in Upper Flow Regime velocity, a change in stream-bed gradient or water depth, or combinations of all three.

You can generate a hydraulic jump in your kitchen or laundry sink, so long as the sink floor is reasonably flat. Turn on the tap until you see a flow pattern like the one in the image above. Flat laminar flow is generated by the downward force of tap water – this is the plane bed. The hydraulic jump is located where the flow changes abruptly, with an increase in surface amplitude. Beyond the jump is lower flow regime flow.

We can use the Flow Regime concept in the field as a quantitative indicator of changes in paleoflow in time (i.e stratigraphically) and space (laterally). For example, a stratigraphic sequence that shows a layer of ripples overlain by a layer of trough crossbeds indicates that flow velocities, and hence stream power increased abruptly. What kind of paleoenvironment might this have occurred in? This is one of the central questions for any sedimentological analysis.

Here are a couple of important references:

Ashley, G.M. 1990. Classification of large-scale subaqueous bedforms: A new look at an old problem. Journal of Sedimentary Petrology, v.60, p. 160-172.

Harms, J.C. and Fahnestock, R.K. 1965. Stratification, bed forms, and flow phenomena (with an example from the Rio Grande). S.E.P.M. Special Publication 12, p. 84-115.

Some more useful posts in this series:

Identifying paleocurrent indicators

Measuring and representing paleocurrents

Crossbedding – some common terminology

Sediment transport: Bedload and suspension load

Fluid flow: Froude and Reynolds numbers

Fluid flow: Shields and Hjulström diagrams