Internship 5: Compilation and mapping of ancient flow data

Paleocurrent direction refers to the direction of water flow during the period of sediment deposition. The analysis and study of paleocurrent direction is an important part and one of the effective means to identify the depositional environment and reconstruct paleogeography. It helps to determine the edge of the paleobasin and the location of the provenance area, the direction of the paleoshoreline, and the layout of the basin sediment filling. , paleoslope direction, and the shape and extension direction of sandstone bodies. Therefore, in the study of sedimentary strata and their sedimentary minerals, the determination of paleocurrent directions has received widespread attention.

1. Measurement and data processing methods of paleoflow direction data

Paleoflow direction analysis is based on the analysis of the primary sedimentary structures and structures that are generated by the flow of sedimentary media and have the significance of indicating the direction of flow. Research on the directional arrangement of sub-particles, etc., to restore the flow direction of water flow during the sediment deposition period. Primary sedimentary structures that can indicate the direction of water flow include cross-bedding, bottom marks, ripple marks, water flow lineations, scour-fill structures, and directional arrangements of biological fossils and detrital particles. Among them, cross-bedding, bottom marks, and gravel orientations Sexual alignment is a good indicator of paleocurrents. This is because in practical applications, the pointing structures used as indicators for paleoflow direction analysis must be easy to measure and widely distributed, and must be related to the main flow direction. The following takes the most common cross-bedding in the field as an example to briefly introduce the measurement and data processing methods of paleoflow direction data.

(1) Measurement of cross-bedding

The foreset tendency of plate-shaped cross-bedding and wedge-shaped cross-bedding is generally consistent with the paleoflow direction. Measuring the paleoflow direction is actually Determine the tilt direction of the foreset layer. A wooden board (or cardboard, field record book, etc.) can be placed on the exposed layer to expand the measurement surface. Measuring the occurrence of the wooden board represents the occurrence of the foreset layer. The foreset layer often slows down downward or upward and downward at both ends, so attention should be paid to measuring in the middle of the fine layer. The measurement of the inclination of the foreset layer of trough-shaped cross-bedding should be done with caution, and attention must be paid to observe and determine the extension direction of its axis.

In addition to measuring the occurrence of foreset layers, the type of cross-bedding and other bedding elements should also be described and recorded on a special form (Table 5-1). When the rock formation inclination angle is greater than 10°, the occurrence of the rock formation where the cross-bedding is located should also be measured and recorded for indoor correction.

For field measurements, it is necessary to select as many measurement points as possible, and measure and collect as much data as possible for statistical analysis.

Table 5-1 Cross-bedding measurement registration form

(2) Correction of measurement data

Most of the rock formations with various pointing structures have been Later tectonic movements changed the original occurrences, thus changing the original occurrences of these pointing structures. Therefore, the measurement data must be corrected using the Chiping projection occurrence correction method in structural geology. The correction is usually done using Wu's website. Any point on the diagram represents the orientation of an angle line emanating from the origin. This angle line is the normal of a plane, so it can also represent the shape of a plane. The inclination can be obtained by extending the line from the center of Wu's network to this point and intersecting the circle. The inclination angle can be read by turning the center of the circle to the EW axis or SN axis of the network (Figure 5-1).

Figure 5-1 Using Wu’s Net to correct ancient flow direction data

When correcting, first cover the Wu’s Net with transparent paper and fix the center of the circle with a needle.

Project the occurrence of rock layers and cross-bedding (foreset layers) onto Wu's network (called poles). For example, the normal occurrence of rock formations is NE30° inclination and 60° inclination. Turn the place marked 30° on the transparent paper to the equatorial line of Wu's network and read 60° outward from the center of the circle. This is the formation of the rock formation. The projection point is represented by the symbol "□". The projection method of cross-bedding (foreset layer) is the same as that of the rock layer and is represented by the symbol "☉".

The steps for correction are as follows:

When neither the rock layer nor the cross-bedding is inverted (as shown in Figure 5-2a), transfer the rock layer projection point P to the EW of Wu's network axis to move it toward the center of the circle along the EW axis (meaning to restore the rock layer to a horizontal position). Move the cross-bedding projection point to A by the same angle along its latitude in the same direction as point P. Return the transparent paper to its original position. At this time, the position of point A represents the original position of the pointing structure.

When the rock layer is reversed and the cross-bedding tendency is not reversed (Figure 5-2b), the P point that has been moved to the EW axis moves away from the center of the circle to the zero position, making the cross-bedding projection point Move the same degree along the latitude line to A in the same direction as P. Return the transparent paper to its original position, and A is the original shape of the oblique layer.

When the rock layer is not inverted but the cross-bedding is inverted (Figure 5-2c), move point P directly to the center of the circle along the EW axis to the zero position, so that the cross-bedding projection point is directed along the latitude line. Move in the same direction as P by the same degree, and after reaching the periphery, move to A along the corresponding latitude line on the other side of the equator. Return the transparent paper to its original position, and A is the original shape of the oblique layer.

Figure 5-2 Correction of the original occurrence of cross-bedding

(According to Liu Baojun et al., 1985)

a—Neither the rock layer nor the cross-bedding is inverted. ;b—The rock layer is inverted but the cross-bedding tendency is not reversed; c—The rock layer is not inverted but the cross-bedding is inverted; d—The rock layer and the cross-bedding are both inverted

When the rock layer and the cross-bedding are both inverted ( Figure 5-2d), move point P to the center of the circle in the direction opposite to the center of the circle, move the cross-bedding projection point by the same degree along the latitude line in the same direction as the movement of P, and then move to the periphery along the other side of the equator. Move to A on the corresponding latitude line. Return the transparent paper to its original position, and A is the original shape of the oblique layer.

(3) Data sorting and plotting

There are many methods for sorting ancient flow direction data, including histograms, rose diagrams, pole diagrams, arithmetic average methods, and vector average methods. and matrix methods, etc. The first three methods mainly represent the paleoflow direction qualitatively and vividly, while the latter three methods quantitatively represent the average and standard deviation of the paleoflow direction.

Figure 5-3 Paleoflow direction histogram and corresponding rose diagram

(According to Porter et al., 1977)

The histogram and rose diagram are The most commonly used graphic representation method, it simply and vividly indicates the direction of ancient water flow. Generally, all paleoflow direction data are grouped using grouping intervals of 30°, 40° or 45°, and the number of observations and the percentage of observations in each group are calculated. Plot them on the graph according to the grouping intervals on the abscissa and the scale on the ordinate to get a histogram. The rose diagram actually turns the histogram into a circle, that is, using the azimuth intervals on the circumference to replace the grouping intervals on the abscissa. The difference between the two is just the drawing method and expression method.

(4) Environmental significance of paleoflow direction data

Different sedimentary environments have different hydrodynamic conditions and may also have different paleoflow patterns. There are three types of common paleoflow rose diagrams (Figure 5-4): unidirectional (or singular mode), bidirectional (double mode), multidirectional (multimodal) or indifferent directions. Obviously, they represent three different paleocurrent patterns. River sand is unidirectional, and its ancient flow direction changes mostly between 90° and 120°. This change may be related to the curvature of the river. The more complex the serpentine, the greater its variability. Most turbidite sands and many ancient eolian sands are also unidirectional. Most delta sands are unidirectional, and their water flow changes within a large range, generally 180° to 220°; however, very few delta sands are affected by tidal effects. And it's bidirectional. Beach sand and harbor sand are mostly two-way, but some can be one-way. The two-way flow pattern is speculated to be generated by tidal action; the one-way flow pattern may be related to shoreward, offshore or coastal currents. Because shallow marine shelf sand is less controlled by ancient slopes, its paleoflow direction changes greatly, and is generally bidirectional or non-directional. Table 5-2 summarizes the relationship between depositional environment and paleocurrent patterns. As for the relationship between paleocurrent directions and paleosedimentary slopes, various depositional environments differ. The paleocurrent directions of some of these depositional environments are controlled by paleoslopes, such as rivers, deltas, and most turbidity current deposits. In some depositional environments, such as aeolian sand flats and coasts, there is little relationship between the ancient flow direction and the ancient slope.

Figure 5-4 Types of flow rose diagrams

(According to Porter et al., 1977)

Table 5-2 Sedimentary environment and paleoflow patterns The relationship between the Figure and explain processes and methods.

(2) Use the corrected cross-bedded foreset occurrence data to make frequency histograms and rose diagrams.

(3) Explain the pattern of ancient currents reflected in the figure.

3. Internship Purpose and Requirements

(1) Master the methods of measuring and recording field measurement and description of structures.

(2) Master the drawing methods of frequency histograms and rose diagrams and their interpretation of ancient flow directions.

IV. Internship materials and assignments

(1) Internship materials

Measurement of the cross-bedding foreset layers of the ××× section in the southern margin of Junggar and horizontal correction data table, 60 measurement and correction data of known cross-bedding foreset layers (Table 5-3).

Table 5-3 Cross-bedding measurement and horizontal correction data table of ××× group of ××× section in the southern margin of Junggar

Continued table

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(2) Assignment

(1) Draw the cross-bedding foreset tendency histogram and rose diagram.

(2) Data interpretation, that is, the types of ancient flow rose diagrams produced by analysis are one-way (or single-mode), two-way (double-mode), and multiple. If the direction is directional (multimodal) or the direction is not obvious, its possible formation environment is inferred.

Internship Report 5: Compilation and Mapping of Paleoflow Data

Sedimentology and Paleogeography Internship Guide