Paleocurrent Analysis

Introduction

Paleocurrent analysis is the systematic recovery and statistical treatment of directional information preserved in sedimentary structures. Many structures - cross-bedding, current ripples, flute casts, groove casts, parting lineation - formed in response to the flow that deposited or deformed the sediment. The orientations of these structures are therefore fossil records of that flow. By measuring many structures across an outcrop or formation, geologists can reconstruct not just local flow directions but, when data are compiled regionally, the entire drainage or ocean circulation pattern of an ancient sedimentary basin.

Directional Data Sources

Many sedimentary structures yield directional data showing the direction ancient currents flowed at the time of deposition. The dip direction of cross-bed foresets, the asymmetry and orientation of current-ripple crests, and the orientation of flute casts, groove casts, and current lineation are all examples of directional data obtainable from sedimentary structures. ^[1]

Cross-bedding is one of the most useful sedimentary structures for determining paleocurrent direction. Because the foreset laminae in cross-beds are generated by avalanching on the downcurrent (lee) side of bedforms, the foresets dip in the downcurrent direction. Measuring paleocurrent direction from cross-beds requires that they be exposed in a three-dimensional outcrop. The strike of the foreset laminae is determined first; the dip direction is 90° to the strike. If cross-beds have been tilted by tectonic uplift after deposition, a correction must be made for this tilt. ^[1]

Field Measurement

The orientation of directional sedimentary structures is determined in the field with a Brunton compass, taking measurements from as many different outcrops and individual beds as possible. The orientation data from any particular bed or stratigraphic unit commonly shows considerable scatter. Directional data must therefore be treated statistically to reveal primary and secondary directional trends. ^[1]

Scatter is expected and does not reflect measurement error. A meandering river, for example, produces cross-beds whose foresets point in every direction along the meander belt, even though the overall flow is in one direction. Only by accumulating many measurements can the dominant trend emerge above the noise of local bends and secondary channels.

Flow Pattern Classification

When examining orientation data statistically, it is possible to identify whether ancient flow was unidirectional, bidirectional, or polydirectional. Cross-bed foresets in a meandering river may show considerable scatter but still indicate a single dominant flow direction - this is unidirectional flow. By contrast, cross-bed foresets in sandy deposits of marine tidal channels may display two opposing dip directions, formed during both incoming and outgoing tides - this is bidirectional flow. In some environments, such as the eolian environment, depositing currents may flow in several directions at various times - this is polydirectional flow. ^[1]

Tilt Correction and Rose Diagrams

Paleocurrent data from stratigraphic units that have not been tectonically tilted can be compiled and summarised directly. If the rocks have undergone considerable tilting, it is necessary to correct the measured orientations by restoring them to their original attitude before tilting. A stereogram procedure can be used to reorient directional data from tilted units. ^[1]

After any necessary reorientation, data are commonly plotted as a circular histogram - a rose diagram. Such diagrams show the principal direction of paleocurrent flow and any secondary or tertiary modes. If the paleocurrent flow shown in the rose diagram is dominantly in a single direction, the vector is said to be unimodal; if two principal directions of flow are indicated, it is bimodal; if three or more directions are revealed, it is polymodal. ^[1]

Environmental Significance

Local paleocurrent directions may have environmental significance. Sediments from alluvial and deltaic environments tend to have unimodal paleocurrent vector patterns, whereas bimodal paleocurrent patterns are more common in shoreline and shelf sediments. Paleocurrent data have their greatest usefulness when plotted on a regional scale to reveal regional paleocurrent patterns. ^[1]

The connection between flow pattern and environment is logical. Rivers and deltas funnel sediment in one prevailing direction, so their cross-beds and ripples consistently record that direction. Tidal systems are bidirectional by definition - the flood tide and ebb tide produce flows 180° apart. Wind systems in deserts are seasonally variable and can deposit dunes from multiple directions across the year, producing a polymodal signal. Regional compilation amplifies these differences: a unimodal pattern across hundreds of kilometres points to a major river system draining a distant mountain range; a bimodal pattern indicates a tidal seaway; a broadly dispersed polymodal pattern suggests an open ocean or a complex eolian system.

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