Paleocurrents. Why measure paleocurrent directions? Features that give paleocurrent directions. Correction to paleocurrent measurements

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1 Why measure paleocurrent directions? Paleocurrent measurements can provide valuable information on ancient flow conditions, which can often shed light on paleogeography. For example, paleocurrent data can reveal how sediment was dispersed through a basin or the direction of depositional dip. In marine settings, paleocurrent indicators are often the start for reconstructing paleoceanography. Features that give paleocurrent directions Linear structures are easy features on which to measure paleocurrent azimuths. Such features include groove marks, gutter casts, parting lineation, and channels. In some cases, they cannot give the direction of the current, only the trend. For example, a groove mark running from northwest to southeast typically cannot be used to tell whether the current originated from the northwest or the southeast. On contrast, flute marks offer unambiguous paleocurrent directions because their blunt noses point upstream. Cross-stratification is one of the most abundant paleocurrent indicators. However, threedimensional exposures are necessary to obtain true dip directions, and incomplete exposure will give erroneous current directions. Foreset laminae dip in the downstream direction, but these dip directions vary more widely for trough cross-sets. In trough crosssets, the azimuth of trough axes are reliable paleocurrent indicators because they are aligned parallel to flow. Currents commonly orient fossils, although in many cases experimental work is needed to understand how a current will orient a particular shape of bioclast. Elongated fossils that are not noticeably wider at one end (such as a straight log or orthoconic nautiloid) commonly align parallel to unidirectional currents and perpendicular to waves. Conical shells and elongate fossils that are larger or heavier at one end (such as a log with attached roots) are typically anchored at the larger end, while the lighter end rotates to point downstream. In waves, conical forms align with their axes perpendicular to the flow, that is, parallel to wave ripple crests. Where fossils are aligned, a quick sketch should be made showing their alignment and the azimuth of the preferred orientation should be recorded. The imbrication or stacking of flat pebbles and intraclasts can indicate current direction, but care must be taken to determine whether the clasts are part of a foreset lamina. Where clasts are contained within foresets, they dip downstream but at a shallower angle than the foreset laminae. Where clasts are not contained within foreset laminae, they dip preferentially upstream. Correction to paleocurrent measurements In flat-lying strata and in strata inclined less than approximately 30, the azimuth of the paleocurrent indicator gives the true paleocurrent direction. The azimuth will not give a Sedimentary Geology

2 true paleocurrent direction if the bed is tilted more than 30, if the bed is part of a fold with an inclined axis, or if the bed has been axially compressed. In these situations, a separate correction must be made to obtain true paleocurrent directions. Both stereographic and trigonometric corrections are available; these corrections are described in most structural geology texts. Presentation of data When paleocurrent data are scarce, individual measurements may be plotted as vectors next to a measured stratigraphic section. These are typically plotted as arrows, with up indicating north and right indicating east. In general, larger structures such as large foresets and channel directions better reflect large-scale fluid flow, whereas smaller structures record more localized flow directions that may deviate substantially from overall flow direction. When paleocurrent data are abundant, they are best shown on a rose diagram, a type of 360 histogram or frequency distribution. Rose diagrams can plot vectors for individual observations, but more commonly they group observations into bins ranging from 10 to 30. Smaller bins allow more finer details to be recognized, but may also mask larger trends. Here is an example of a rose diagram shown below, with data grouped into 20 increments. The concentric rings can either be in absolute numbers of observations or in percentage of total observations. Using percentages is better because it makes it easier to compare diagrams from data sets that have greatly different sample sizes. Note that the total number of observations is indicated adjacent to the rose diagram with standard notation, such as n=83, as shown here. Rose diagrams typically plot the direction of the current, which in some cases may lie at some angle to the structure measured. For instance, the direction of a reversing current lies at 90 to the trend of symmetrical ripple crests, so it is more informative to plot the current direction rather than the trend of the ripple crests. Interpretation Once paleocurrent indicators are measured, plotted, and statistically treated, the real work begins in trying to understand what the currents say about the original flow conditions. Natural fluid flows are variable at many length and time scales. This variation is crudely preserved in the rock record in the form of paleocurrent indicators. Thus, paleocurrent indicators are variable because flow conditions change through space and time. Sedimentary Geology 2

3 Ideally, we would like to map the changes in both space and time, but this requires very detailed correlations that are difficult at best. More commonly, we are forced to pool data formed at different times in different places. Interpretations should consider how the data have been pooled. For example, one might pool all data from a particular sedimentary structure from a particular facies, which will describe flow at a particular spatial scale in one environment. Lumping different types of sedimentary structures lumps different flow conditions, and lumping different sedimentary environments may lump different types of flow. Several workers have described a hierarchy of paleocurrent variability. In general, ripples and structures tend to have more variable paleocurrent azimuths than dunes and large structures. For example, the channel direction in a river commonly has much less variability than the dip directions of ripple foresets. Smaller structures are more variable because they are a response to not only the overall flow of the system, but also the localized flow in response to bedforms and other obstacles in the flow. When interpreting paleocurrent data, the relationship between the scale of the feature and what it indicates must be continually kept in mind. Optional reading Cooper, M. A., and Marshall, J. D., 1981, Orient: a computer program for the resolution and rotation of paleocurrent data: Computers and Geosciences, v. 7, p Curray, J. R., 1956, The analysis of two-dimensional orientation data: Journal of Geology, v. 64, p Potter, P. E., and Pettijohn, F. J., 1977, Paleocurrents and basin analysis: Berlin, Springer- Verlag, 425 p. What to do In this lab, you are given four sets of paleocurrent azimuths, each from a different facies that you will see in the next lab. For each set of paleocurrent data, prepare a rose diagram. You will produce four rose diagrams in all. Each rose diagram should be scaled in terms of percentage. The rings on the rose diagrams should be integer numbers, not fractions. For example, having each ring represent 2%, 5%, or 10% would make good sense, but rings of 5.7% or 2.18% would not. Ideally, the longest spoke on each diagram should come close to the outer ring; given that, the scales may differ among the rose diagrams. Add the sample size (e.g., n=23 ) next to each rose diagram. For each set of paleocurrent data, write a 3-5 sentence paragraph describing the flow pattern in terms of overall direction and spread or scatter. Given what you know about the sedimentary structure on which the rose diagram is based, interpret what the results say about the type, direction, and consistency of flow. Speculate on what environments Sedimentary Geology 3

4 might have this type of flow, but bear in mind that you may change your interpretations when you see the next lab exercise, which adds substantially more data. Data Set 1 These data are derived from Facies A and are measurements of foresets of large-scale trough cross stratification Data Set 2 These data come from the lower portions of Facies B and are measurements of foresets of large-scale trough cross stratification Data Set 3 These data are derived from the lower portions of Facies D, and they are measurements of prod marks Sedimentary Geology 4

5 Blank rose diagram This is a blank rose diagram that you can use as a template for your plotting. Sedimentary Geology 5