G432 ó Lecture 12

Paleocurrents and paleocurrent indicator analysis:

Paleocurrents are ancient currents. Paleocurrent indicators are oriented sedimentary structures interpreted to have been deposited by ancient flows. Cross-beds slip faces, pebble imbrication, parting lineation, tool marks and groove casts, and ripple crest orientation are all examples of paleocurrent indicators. Some paleocurrent indicators are unidirectional ó that is, their shape provides unique information about the direction of the ancient paleoflow. Unidirectional paleocurrent indicators include foresets (ripple or dune), flute casts, and clast imbrication. Some paleocurrent indicators are bidirectional ó that is, their shape eliminates all but two possible directions (either upcurrent or downcurrent). Bidirectional paleocurrent indicators include oriented wood fragments or other elongates clasts, tool marks or elongate sole marks, and parting lineation.

Paleocurrent indicator analysis is an important part of sedimentology, because it provides direct information about the orientation of the sedimentary systems. Paleocurrent analysis can yield information about the flow directions of rivers, longshore currents, sediment gravity flows, and paleowinds. This in turn helps establish a model of paleogeography, which can lead to better predictability of economically-important facies (e.g., oil-saturated porous, permeable sandstone facies).

Below are some photos of common unidirectional and bidirectional paleocurrent indicators.  Iíve included both planar and linear indicators.

Unidirectional Paleocurrent Indicators:

Here are some flute casts on a sandstone bedding sole from the Gaspe Peninsula, Quebec. Photo by Cathy Summa, Winona State University. Flow in this photograph was lower right to upper left. Flute casts such as these are unidirectional, linear paleocurrent indicators.
 

Another very nice shot of some flute casts (unidirectional paleocurrent indicators) from the Gaspe Peninsula, Quebec from Cathy Summaís collection. Flow was right to left.
 

Hereís a shot illustrating cobble imbrication along the west coast of Baja, California, Mexico near San Carlos. Notice how the surf has stacked most of the cobbles so their long axes are dipping seaward. This is the most hydrodynamically stable position and provides a nice unidirectional planar paleocurrent indicator.
 
 

Hereís a shot of some Sinian sandstone from northwestern China, showing well developed tabular cross-bedding. This is an excellent unidirectional planar paleocurrent indicator.
 

Hereís an unusual unidirectional linear paleocurrent indicator from some Cretaceous marine strata in central Utah. Black lens cap for scale. The prominent object below and right of the lens cap is a large oyster valve. Turbulence downflow from the valve created an elongate scour pit that subsequently filled in with coarse sand, resulting in an unusual but very useful unidirectional paleocurrent indicator.
 
 

Bidirectional Paleocurrent Indicators:

Here are some nicely developed groove casts on a bedding sole. Photo by Duncan Heron. Flow was either from upper left to lower right or lower right to upper left (probably lower right to upper left as suggested by the large flute cast in the middle of the photo). Groove casts such as these are good bidirectional linear paleocurrent indicators.
 

Hereís a shot of parting lineation in sandstone. Flow was either from the top to bottom of the photo OR from the bottom to top of the photo. This is a good example of a bidirectional, linear paleocurrent.
 
 

Procedure for measuring the orientation of paleocurrent indicators and extracting paleoflow directional information.
The orientation of paleocurrent indicators can be easily measured in the field with a brunton compass or other simple measuring device. For planar paleocurrent indicators (cross-beds, pebble-cobble imbrication planes), the strike and dip of the planar feature is measured. The dip direction in cross beds is the paleoflow direction. In pebble and cobble imbrication, the dip direction of the long-intermediate axis of the pebbles is upstream, because flat clasts are in their hydrodynamically most stable position if they are leaning downstream (see sketch above), For sinuous-crested ripples and dunes, a complication arises in that the trough limbs donít dip downstream but rather dip at a high angle to paleoflow direction. However, the trough axis dip direction is parallel to the paleoflow direction, and trough limb orientation information can be statistically treated to obtain a vector average of the paleoflow direction. For a good discussion of these statistical techniques, check out the following paper, available on the G432 ERES site.

DeCelles, P.G., Langford, R. P., and Schwartz, R. K., 1983, Two methods of paleocurrent determination from trough-stratification: Journal of Sedimentary Petrology, v. 53, p. 629-642.

For linear paleocurrent indicators, the long-axis of the indicator is usually assumed to be parallel to flow direction. This is the case with elongate clasts that are entrained in a flow. A complication arises with linear indicators that "roll" and are oriented with their long axis perpendicular to flow. Itís important to recognize this difference in the field.
 

This illustration shows how elongate clasts with different masses can behave differently in the same flow, depending on whether the clasts are rolled by the flow or whether they are completely entrained in the flow. This photo was taken at the strand line of the Great Salt lake in Utah and shows a set of small wood fragments that are oriented perpendicular to flow direction. Incoming, small (cm-scale) waves from the lake traveled from the top of the photo toward the bottom of the photo. The wood fragments were apparently rolled by those waves and oriented with their long axes perpendicular to the primary flow direction. In contrast to that orientation isthe orientation of the smaller elongate clasts which are either insect larvae shed exoskeletons dead insect larvae (I wasnít sure which). The insect larvae are oriented mainly parallel to the incoming waves because they were small enough to be completely entrained in the flow.
 
 

Most paleocurrent measurements are acquired from deformed (i.e. tilted) beds, so the beds and paleocurrent measurements must be restored to paleohorizontal before the directions of paleoflow can be meaningfully interpreted. This may be accomplished manually through stereonet rotation, or it may be done using a computer. Iíve found that the free Stereonet software written by Rick Allmendinger (Cornell University) works very well for treatment of paleocurrent data.

Below is a document on the field and laboratory treatment of paleocurrent data, compliments of William R. Dickinson at the University of Arizona. This document describes how to treat paleocurrent data in the field and how to restore it to horizontal if the paleocurrent indicators were in tilted strata.

PALEOCURRENT-PALEOSLOPE INDICATORS
A. General

Orientations of key sedimentary structures can be measured as indicators of directions of paleocurrents or paleoslopes. Geometrically, the indicators are either planars or linears; the geometric manipulation required to obtain paleocurrent or paleoslope directions differs for the two. For planars, the strike and dip is measured directly, and stereographic techniques are used, in conjunction with the strike and dip of the bedding of the strata in which the planar indicators occur, to restore the observed structures by stereographic rotation to their inferred original attitudes. For the two main types of planars, the inferred direction is then either down-dip (cross-lamination) or up-dip (imbrication). For linears, stereographic techniques can also be used, starting with bearing and plunge as observations, but direct measurement of pitch, rather than plunge, allows automatic restoration to inferred original position. For the two main categories of linears, the inferred direction is then either parallel or perpendicular to the lineation. For different types of indicators in each category, the indicated azimuth may be uniquely unidirectional, or bi-directional and hence ambiguous. Both planar and linear indicators may be on a scale large enough to measure in the field, or on a scale so small that oriented specimens must be collected in the field so that measurements can be made in the laboratory. Techniques for summarizing accumulated data include the calculation of vector means, the construction of rose diagrams and concentric azimuth circles, and the sequential plotting of maps of outcrop means, maps showing moving averages of subarea means, and interpreting maps of smoothed data.

B. Type of Indicators

Types of paleocurrent and paleoslope indicators can be catalogued in six groupings by the following conventions: (a) root term plan or lin for planar of linear, (b) prefix uni or bi for unidirectional or bi-directional, (c) suffix down or up for direction with respect to dip of planars, and (d) suffix parl or perp for direction with respect to strike of linears. Most indicators define only local paleocurrents directly, but a few which involve deformation of previously deposited but unconsolidated and still-exposed strata define only local paleoslopes directly; inferences about gross regional paleoslopes and dispersal paths require interpretation in conjunction with data on provenance and depositional environment. A partial list of major paleocurrent-paleoslope indicators:

1. Uniplandown
(a) cross-bedding: multiple measurements vital, as only vector mean is significant
(b) ripple-drift cross-lamination: should measure at right angles to strike of ripple trend; is good directional indicator but insensitive to azimuth, hence best used in conjunction with other indicators of bi-directional linear type sensitive to azimuth but ambiguous as to direction.

2. Uniplanup
(a) clast imbrication: measure in field for gravel but in lab for sand (also some apparent imbrication may reflect foreset bedding, hence be uniplan down)
(b) flame-structure vergence: also insensitive to azimuth
(c) axial surface of contorted bedding: also insensitive to azimuth

3. Unilinparl
(a) flute casts, and some furrow casts
(b) prod marks, and some bounce or skip marks
(c) trough sets of cross-strata

4. Unilinperp
(a) current-ripple marks
(b) axes of flow rolls

5. Bilinparl
(a) groove casts, and some furrow casts
(b) clast lineation: measure in field for gravel but in lab for sand (also some may be bilinperp)
(c) axes of convolute bedding (also some may be bilinperp)
(d) parting lineation
(e) channel fills

6. Bilinperp
(a) wave-ripple marks
(b) axes of contorted bedding

C. Field Planars

Measurement and manipulation of planar indicators visible in the field is handled as follows (see first three attached sheets for stereonets and stereographic procedures):

(1) Measure "bad," the strike and dip of the bedding of the strata in which the planar indicators occur.

(2) Measure "isd," the strike and dip of the planar feature serving as indicator. For cross-strata, this measurement may be on an individual lamination or a small packet of laminations within a set. For ripple-drift cross-lamination, use of a clipboard as a device to enlarge the planar surface to be measured is helpful. For imbricate clasts, this measurement may be made on individual clasts or on a visual estimate of the general orientation of a group of nearby clasts.

(3) Plot the poles of bsd and isd as points on a stereonet.

(4) Rotate the pole for bsd to horizontal (the stereonet origin) about the strike of bsd as axis of rotation, and rotate the pole for isd the same amount about the same axis; the rotated pole for isd is "oisd," or the inferred original attitude of the planar indicator.

(5) Read off the stereonet the dip or antidip of oisd; this is the indicated paleocurrent or paleoslope.

D. Field Linears

Measurement and manipulation of linear indicators visible in the field is handled as follows:

1) If the dip is low (see top left of fourth sheet attached), the bearing and plunge of the linear may be measured directly (for unilinparls, record as plunge of antiplunge; for unilinperps, record indno or indso to indicate northerly or southerly paleocurrent or paleoslope much as northerly or southerly dips are denoted for given strike lines).

(2) If the dip is high, measured bearings and plunges can be rotated stereographically to inferred original position, much as for planars, if the lineation is plotted as a point on the stereonet (where it should lie initially the cyclographic projection of the contained bedding plane, and its restored original position should lie on the primitive circle)

(3) A more efficient general technique is to ignore the problem of measuring accurate bearings and plunges of linears, and instead to measure the pitch of the linear in the bedding plane. This can be done with simple homemade devices (see fifth sheet attached) which include two arms hinged together at one end. One arm has a level mounted on it, and thus can be laid along the strike-line of a bedding surface. The other arm is then rotated at the hinge to lie simultaneously parallel to the linear whose orientation is being measured. A protractor mounted suitably in the hinge area is then used to measure the pitch angle between the trend of the linear and the trend of the bedding strike. If the bed is then restored mentally to its original position by rotation about the strike, the pitch angle gives the trend of the linear directly by simple addition or subtraction. When this method of measuring linears is used, recording conventions must be systematic or confusion between whether adding or subtracting is appropriate will cause errors. If strikes are recorded in clockwise azimuths, clockwise (c) pitches are positive on bed tops but counterclockwise (c-c) pitches are positive on bed soles. If strikes are recorded in quadrants, pitches should be recorded as c or c-c, and as top or sole. For unilinparl and unilinperp indicators, if not for all linears, the required addition or subtraction should be performed on the outcrop where results can be checked briefly by comparison with visual relations (thus, the paleocurrent or paleoslope direction is derived immediately, much as strike-and-dip symbols can be plotted at once on a map).

E. Lab Measurements

Laboratory determinations of sand-grain fabrics (imbrications and lineations) as illustrated by sixth sheet attached) are handled as follows:

(1) An oriented specimen is collected in the field by (a) measuring the strike-and-dip of the natural outcrop surface, (b) marking and labeling, with a felt-tip pen, the strike-line and dip tick on the surface measured, and (c) collecting the portion of the outcrop so marked (thus marked, the hand specimen can be re-oriented correctly in space at any time).

(2) The specimen is sawed parallel to bedding, and the surface examined with a hand lens or binocular microscope to detect any grain orientation present. If not visible initially, etching with hydrochloric or hydrofluoric acid may display it. If still not visible on the etched rock surface, an acetate peel made by pressing acetate film against the surface while it is flooded with acetone may show it.

(3) The specimen is re-sawed twice perpendicular to bedding: once parallel and once perpendicular to the grain orientation detected (to distinguish between imbrication and lineation). Etching and peeling may be helpful in this step also.