flute casts Archives

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

Sedimentary structures that indicate paleoflow. Measuring and plotting paleocurrent indicators are treated in a separate post.

This post is part of the How To… series

Sediment that is moved along a substrate (e.g. the sea floor, river bed, submarine channel, wind-blown surface) will commonly generate structures that record its passing. Sedimentary structures that preserve directionality (paleoflow) are indispensable for deciphering whence the sediment came and where it went; for interpreting sedimentary facies (local scale) and sedimentary basins (regional scale). Paleocurrents are a measure of these ancient flows.

A single structure, such as a ripple will give a unique measure of paleoflow at a certain point in space and time. An important question for this single piece of data is – how relevant is it to the bigger picture of sediment dispersal? To get a sense of regional flow and sediment transport patterns, we need many measurements so that we can tease the overall pattern of flow from whatever local variations might exist.

We can illustrate this central problem by looking at flow in a fluvial meander belt with depositional settings like the main channel (arrows), point-bars and adjacent flood plain. This snapshot in time shows clearly the huge variation in local flow directions. We also need to account for other ‘snapshots’ in time, because even at a local scale (e.g. one meander bend and point-bar), the directions of flow and sediment transport will vary from flood to low water stage. We can try to circumvent this problem if we measure a large number of flow directions over an equally large area of the river and floodplain. In modern drainage basins this is straight forward but for the rock record, exposure is likely to be discontinuous and even structurally disjointed.

Structures indicating unique flow directions

Subaqueous dunes and ripples: These bedforms are built by 2-dimensional (straight-crested) dunes and ripples. Hence, the boundaries between adjacent crossbed sets tend to be planar (cf. trough crossbeds). Flow direction is approximately at right angles to dune or ripple crests.

Trough crossbed, or 3D subaqueous dunes Spoon-shaped troughs filled by migrating, sinuous dunes produce trough crossbedding. This kind of crossbed is common in confined, channelized flow (e.g. fluvial and tidal channels). The mean flow direction is along the axis of the trough.

Left: Festooned trough crossbeds exposed approximately parallel to bedding. Paleoflow is the direction of the hammer handle Proterozoic Loaf Fm.). Right: Cross-section view of multiple trough crossbeds – only apparent flow directions can be surmised from outcrop (Eocene Buchanan Lake Fm.).

A caution about wave-formed ripples; This bedform does not arise from bedload transport in flowing currents, but from wave orbitals. Wave ripples are not paleocurrent indicators. However, wave ripple crests will be oriented approximately parallel to the strike of the ancient shoreline.

Imbrication Flat and platy clasts are commonly oriented by strong currents, such that the ‘plates’ dip upstream. These fabrics are common in gravelly fluvial deposits.

Imbricated platy cobbles and pebbles in a modern stream. Flow is to the right.

Flute casts Flutes originate from erosion of a soft, commonly muddy substrate and are filled with sand – they are part of the overlying bed and are usually seen as casts on the sole of the overlying bed. Flow direction is towards the open, shallow end of the flute.

Large flute casts on a turbidite bed sole (Omarolluk Fm, Belcher Islands). Flow was from top left to bottom right

Structures indicating ambiguous flow directions:

Groove casts Objects dragged across a soft substrate by strong currents (e.g. bottom currents, turbidity currents) will scour linear grooves that become filled by the overlying sedimentary layer. Like flutes, they are usually seen as casts on the soles of beds. In the absence of other indicators, the two possible paleoflow directions are 180^o apart.

Groove casts on a bed sole, indicate flow in either direction. other criteria, like flute casts, are need to specify unambiguous flow directions.

Parting lineation These are subtle structures 2 or 3 grains thick, that are visible only on exposed laminated bedding. The word ‘Parting’ refers to rock breakage along planar laminations. Parting lineation is attributed to high flow velocities where the long axes of sand grains become aligned (in Flow Regime terminology this corresponds to Upper Plane Bed conditions). Paleocurrents are measured parallel to the long direction of parting, but like groove casts, are ambiguous.

Paleoflow indicated in this parting lineation was either to the left or right.

Current alignment of elongate fossils, rod-shaped clasts, or bits of wood can generally be treated like groove casts in terms of their paleocurrent value. There are exceptions; for example turreted gastropods may be aligned with their apices pointing downstream. The example shown here shows fairly consistent alignment of Permian Fusulinid foraminifera parallel to the prevailing flow (but the actual flow direction is ambiguous).

The classic text that deals with paleocurrent analysis is – Potter, P.E. and Pettijohn, F.J. (1977) Paleocurrents and Basin Analysis. 2nd Edition, Springer-Verlag, New York, 425 p.

Some more useful posts in this series:

Measuring a stratigraphic section

Measuring and representing paleocurrents

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime