paleocurrent Archives

This post is part of the How To… series

Eocene fluvial tabular crossbeds exposed along MacKenzie River, northern Canada

Crossbeds are ubiquitous in sedimentary rocks. They can be found on the deep ocean floor, the driest desert, and pretty well any depositional environment in between. They are most common in sandy deposits. They are less common, but no less important in gravels (e.g. low sinuosity – braided rivers). Crossbeds form where air and water flow across a bed of loose sediment, so long as the individual sediment grains are cohesionless (non-sticky). Mud crossbeds are rare because individual clay particles tend to bind to one another (a result of residual electric charges).

Crossbeds in the rock record are visible in bed cross-sections, or as exhumed 3D ripples and dunes on exposed bedding planes. The term crossbed refers to their internal structure; i.e. laminations that are usually inclined in the down-flow, or down-stream direction.

The laminae are called foresets. In a 2D cross-section view, a single crossbed consists of any number of foresets bound above and below by flat or curved boundaries. The geometrical arrangement of foresets, their bounding surfaces and their size or amplitude gives us the information needed to decipher:

the kind of crossbed,
the hydraulic conditions under which the crossbed formed, and to some extent
the paleoenvironment in which they formed; keep in mind that most crossbeds can be found in a range of paleoenvironments but used in conjunction with other criteria such as body and trace fossils, sediment composition and stratigraphic trends (e.g. fining upward) will help pin-point specific depositional settings.

Our interpretations can be advanced further if we are lucky enough to see exhumed structures on bedding, such that we can define:

the shape of the ripple or dune crest line (is it straight or sinuous?)
the wavelength between successive ripple or dunes, and

a relatively unambiguous measure of ripple-dune migration across the bed (i.e. paleocurrents).

Most of our knowledge about ripples and dunes (collectively referred to as bedforms) and how they form has been garnered from studies of modern environments. Afterall, if on your walks across a tidal flat or subaerial dune field you see ripples that look identical to those preserved in rocks, it is quite reasonable to predict that the ancient bedforms developed in ways similar to the modern analogues (this is the Uniformitarian Principle at work).

In fact, they have also been videoed forming in real time on Mars.

This terminology has evolved from an original 1953 description by McKee and Weir (see references at the end of the post). An SEPM workshop in 1987 (Ashley,1990) sought to incorporate in a revised terminology, the 3-dimensional aspects of bedforms larger than common ripples and their inherent hydraulic properties. They recommended that the term dune be used, with the basic distinction between subaerial and subaqueous dunes, of all sizes. Subaqueous dunes can be further separated into:

2 dimensional subaqueous dunes having relatively straight crest lines and planar foreset contacts; they correspond to tabular crossbeds (in the above diagram), and
3 dimensional subaqueous dunes having sinuous crest lines and spoon- or scour-shaped foreset contacts. These correspond to the classic trough crossbeds.

Trough crossbeds are most common in channelized, or confined flow (rivers, tidal inlets and channels, rip currents). Three dimensional subaqueous dunes tend to form at higher current velocities than their 2D counterparts.

The SEPM nomenclature is widely used, but deeply entrenched terms like trough and tabular crossbed are still popular.

Here are some classic older texts on the topic (and just because they are older than 10 years doesn’t mean they are irrelevant!)

Allen, JRL. 1963. The classification of cross-stratified units. With notes on their origin. Sedimentology, v. 2, p.93-114

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.

McKee, E.D. and Weir, G.W. 1953. Terminology for stratification and cross-stratification. Geological Society of America Bulletin v. 64, p. 381-390.

Some more useful posts in this series:

Sedimentary structures: Fine-grained fluvial

Determining stratigraphic tops

Identifying paleocurrent indicators

Measuring and representing paleocurrents

The hydraulics of sedimentation: Flow Regime

Sediment transport: Bedload and suspension load

This post is part of the How To… series

Identifying sedimentary structures that indicate paleocurrent directions is an important task in any study of sedimentary rocks. Knowing the direction of sediment transport will help you decipher paleoenvironments and sedimentary facies, paleoslope dip directions, possible sources of sediment, and the location of sediment sinks.

It all begins with a humble crossbed, flute cast, or current aligned object. Identifying any of these is reasonably straight forward; knowing what to measure can be a bit tricky.

Asymmetric current ripple and dune bedforms exposed on bedding planes can be measured by noting the facing direction of lee slopes (face down current – see the image above). In such cases, measured bearings for individual bedforms provides a unique sense of flow.

However, it is more common to find crossbed cross-sections in 2-dimensional exposures like cliffs and road-cuts. In situations like this, the crossbed foresets are more likely to present an apparent dip direction, rather than true direction of flow. The trick here is to look for nooks, crannies, joint or fracture faces that present a degree of three-dimensionality to the outcrop.

In the example below, crossbed foresets are exposed in two rock faces of a joint block, presenting us with two apparent dips. Measure both plunge and bearings, and find the true dip using stereographic projection. In most cases, the direction of maximum foreset dip will be close to the paleoflow direction. BUT! You must be certain that the foresets belong to the SAME CROSSBED SET. This stipulation is important in situations where multiple crossbed sets cut one another – a feature of sandy fluvial and shallow marine deposits.

In exposures where crossbeds have been eroded parallel or slightly oblique to bedding, crossbed laminae are outlined in sinuous and festoon patterns. Trough crossbeds, and various 3D ripples exposed in this way (e.g. lunate ripples) provide an opportunity to take multiple paleocurrent measurements. In the example of festooned crossbeds shown here the concave aspect of each trough set faces downstream.

Of all the sole structures, flute casts are the most useful, providing (relatively) unambiguous paleoflow; flow is parallel to the length of the flute, from the deeper spoon-shape scour to the thin feather edge. However, paleoflow determined from groove casts is ambiguous, the two possible directions 180^o apart. Data from sole structures is improved if flutes and grooves occur together.

Graphical representation of paleoflow

How you treat the data graphically and statistically depends on the number of measurements at any one locality, and the geographic – stratigraphic distribution of data. A few questions you need to ask are:

Does the number of data points at each locality warrant separate treatment for each locality, or should the data be lumped into a single point of analysis?
Is the data distributed over a narrow stratigraphic interval (e.g. 1 or 2 beds, or a single coarsening upward sequence of beds), or a more extensive stratigraphic interval?
If data from multiple localities or stratigraphic intervals is aggregated, will important variations in paleocurrent trend be represented. For example, if there are local bimodal trends representing tidal ebb and flood currents, will these be ‘lost’ if all coastal data is analysed as a single block of data?
If mean flow direction is calculated, how useful is this measure of central tendency in the context of the overall spread of paleoflow directions?
Are corrections needed to account for structural dip?

Rose diagrams provide the simplest way of representing data in diagrammatic form. Data is plotted as a circular histogram through 360^o. Several software programs are available to do these plots, but it is also a simple task to do it by hand. The inset shows you how to do this.

With the data in hand, choose a bearing interval (the example here is 20^o intervals)
Organize the data in the intervals and calculate the percentage of measurements for each interval.
Plot each interval so that the length of each sector of the rose is proportional to the number of measurements for that interval. The example here uses intervals of 20%.

The distribution is clearly unimodal. We could have chosen a 10^o or 15^o bearing interval for the plot which would probably show some finer detail about the paleocurrents.

Paleocurrent distributions in sedimentary basins generally fall into 3 or 4 categories: Unimodal (one primary direction), bimodal bipolar (2 directions 180^o apart), bimodal oblique 2 directions at different angles), and polymodal (widely distributed). Vector means for unimodal distributions are useful for comparing paleoflow among locations and assessing regional patterns of flow. However, the mean directions for strongly bimodal or polymodal distributions may have little real-world value in this context.

The Mean paleoflow vector can also be calculated, but the usual arithmetic methods DO NOT APPLY to azimuthal data. Calculation of the mean for our unimodal distribution is shown below.

Note: All the bearings are in the SW quadrant and therefore Sine and Cosine values are all negative, and Tangent values are positive. Other distributions may have a mix of positive and negative values – make certain you use the correct sign.

Here are a couple of free Rose plot programs (there are lots of commercial programs available):

GeoRose (free) for Windows and Mac

GEOrient (free for academic users)

Additional posts in this series:

Measuring a stratigraphic section

Identifying paleocurrent indicators

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime