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The use of trend surfaces in palaeoenvironmental reconstructions
Pkl3 0-
tcataG~Y
ELSEVIER
Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
The use of trend surfaces in palaeoenvironmental reconstructions J . P . le R o u x
Department of Geology, University of Stellenbosch, Stellenbosch 7600, South Africa Received 9 November 1993; revised and accepted 21 March 1994
Abstract Trend surfaces can be produced by a number of mathematical techniques and have been used widely to depict the regional variation of various geological parameters. In this paper, it is demonstrated that trend surfaces can be constructed by a simple graphical method and may be applied successfully to palaeoenvironmental reconstructions using elevation contours of tilted or folded contacts. Two case studies are discussed to illustrate different aspects of the technique. In the first example, the upper contact of a shallow marine sandstone outcropping over an area of roughly 4000 km 2 in the southwestern part of the Sydney Basin (Australia), is restored by means of a first order trend surface. This reveals the palaeotopography as a sinuous shoal trending towards the northnortheast. The second example, from the Beaufort Group of the Karoo Basin (South Africa), restores the basal contact of a fluvial sandstone by means of a second order (quadratic) trend surface. The restored surface portrays the individual thalwegs and islands of a braided stream system over an area of about 4 km 2.
I. Introduction The application of mathematical surfaces to solve problems in the earth sciences has increased markedly since 1954. Trend surfaces have proved especially useful in calculating the average composition and variability of intrusive masses, the evaluation of ore reserves, and geophysics (e.g. Dawson and Whitten, 1962; Hewlett, 1964; H o r t o n et al., 1964). In sedimentology, trend surfaces have been employed mainly to investigate spatial variations in sediment-size, thickness, mineral content and facies (e.g. Miller, 1956; Allen and Krumbein, 1962; Duff and Walton, 1963; Agterberg et al., 1967; Wermund and Jenkins, 1970; Potter and Pettijohn, 1977). The purpose of this communication is to discuss the use of trend surfaces for palaeoenvironmental reconstructions based specifically on contact elevation maps. Case studies from the Sydney Basin of 0031-0182/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-0182(94)00027 6
Australia and the K a r o o Basin of South Africa are examined to illustrate the advantages of the method.
2. Construction method Various techniques have been developed to produce trend surfaces, including least squares regression (Simpson, 1954), orthogonal polynomials (Oldham and Sutherland, 1955), two-dimensional moving averages (Krige and Ueckermann, 1963) and Fourier analysis (Whitten, 1967). In areas of tilting or gentle folding, however, it is possible to construct first or second order trend surfaces without resorting to mathematical methods. First, a normal structure contour m a p of the relevant contact is prepared. This usually reveals broad trends which are commonly due to the
J.P. le Roux l Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
186
effects of tilting or folding rather than the original configuration of the contoured surface. In such cases, the general strike of the contours can often be determined fairly accurately by eye. Alternatively, the well-known "three point solution" used in structural geology (Bennison, 1985) can be applied to a number of widely-spaced point elevations to calculate a mean strike for the trend surface. In Fig. la, for example, two "three point solutions" yield strikes of 097 ° and 083 ° respectively, giving a mean of 090 °. The study area is subsequently covered with a grid, which is orientated with one axis parallel to the calculated mean strike of the folds (Fig. lb). The mean elevation of the surface is then determined along each of the strike-parallel grid lines by averaging their elevations at intersections with the complementary grid lines, using interpolation to find the elevation of the relevant surface at these localities. In Fig. lb, the southernmost grid line has an average elevation of 512 m ([530+495+490+510+530+515]/6). The mean elevations of the strike-parallel grid lines are subsequently plotted against their distance (normal to the strike) from the relevant margin of the study area. A line of best fit through these
points defines the trend surface in two dimensions (Fig. lc). The strike lines of the trend surface can now be superimposed on the base map with the original structure contours, and the differences in elevation between the two surfaces are determined by interpolation at convenient points. Contouring these residual values (Fig. ld) records the original configuration of the contact before tilting or folding, as it has the same effect as restoring the trend surface to the horizontal position. The rationale for restoring trend surfaces to the horizontal is that local differences in elevation, which may be due to erosional features or other topographic elements, are often masked by the larger contour intervals required to represent tilted or folded strata. This effect can be considerable even at very low regional dips (less than 1°), depending on the size of the study area and the relative variation in elevation of the original topography. Substantial errors can arise in the palaeoenvironmental interpretation of contact elevation maps if the effects of tilting and folding are not eliminated first, as illustrated in the case studies below.
D
J f 200
330
f
430
B
~
~"
200
230
260
2 ~
298
346
326
~15
420
40~
398
400
510
530
SlS
--298
f
160
315
i
~5 O0
630
---
~
575
!b)
530
~
~
50O 400 300 200
----I . . . . . . . . . I I I iI II A B
100 •
C)
4-I I I II C
,-, I I I II D
O
Fig. 1. Graphical method of constructing trend surfaces. See text for explanation.
J. P. le Roux/ Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
187
3. Case studies 3.1. Nowra Formation The Nowra Formation forms part of the Shoalhaven Group, a sequence of marine sandstones, siltstones and conglomerates exposed in the southwestern part of the Sydney Basin in New South Wales. As the sedimentology of the Nowra Formation has been described in detail by Le Roux and Jones (1994), only a short summary is presented here. The formation consists of medium-grained sandstone up to 140 m thick, deposited in foreshore to middle/upper shoreface environments during a regressive-transgressive episode of Early to Middle Permian age. Cross-bedding directions are consistently towards the northnortheast, reflecting strong longshore currents. The elevation of the upper contact of the Nowra Sandstone was determined from field data plotted on 1:25,000 topographic maps. Structural contours drawn on this contact portray a gentle homoclinal flexure with an easterly dip of about 1° (Fig. 2). The contour interval of 50 m completely masks local topographic effects, except for a deviation in the contours defining a linear feature transecting the area from northwest to southeast. This reflects the presence of a submarine channel coinciding with a fluvial palaeovalley in the Palaeozoic rocks at the base of the Shoalhaven Group (Le Roux, 1994). To reconstruct the original topography, a first order trend surface with a strike of 346 ° and a dip of 0.96°E was superimposed on the structural contours (Fig. 2). Residual elevations between this surface and the structure contours were then plotted and contoured, effectively untilting the homocline to a horizontal datum level. The restored configuration of the Nowra Sandstone upper contact now indicates a sinuous shoal or ridge separated from the western shore of the Sydney Basin by deeper water (Fig. 3). Although the weighted mean grain-size of the sandstone becomes finer along the landward side of the shoal (Le Roux, 1992a), a proper lagoon is unlikely to have existed in this area. An independent restoration of the basal contact
Fig. 2. Structural contours on upper contact of Nowra Formation, showing a NE-dipping homoclinal flexure. Ulladulla is located at 35°21'S/150°29'E.
5c
Fig. 3. Restored elevation contours of Nowra Formation upper contact, showing a sinuous, NNE-trending shoal.
188
J. P. le Roux/Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
of the Nowra Formation, obtained by subtracting the sandstone thickness from the restored elevation of the upper contact, reveals a remarkably similar palaeotopography (Le Roux, 1994). This geomorphological interpretation is supported by the distribution of conglomerates and other lithofacies along the shoal (Le Roux and Jones, 1994). 3.2. Beaufort Group
The Matjieskloof deposit is located in the southwestern Karoo Basin in the lower part of the Permo-Triassic Beaufort Group. Extensive drilling for uranium during the late 1970's outlined a tabular sandstone with a maximum thickness of 34 m, consisting of two major arenaceous subunits normally separated by a thin (0-5 m) mudstone. Smaller mesocycles within the sandstone are bounded by erosion surfaces and occasional mudstone lenses. A study of 48 measured sections in both subunits showed up to 10 of these smaller mesocycles, which have a maximum and mean thickness of 3.6 and 1.11 m, respectively. Sedimentary structures are dominated by upper-phase plane beds (38%), followed by massive (structureless) sandstone (22%), ripple-laminated sandstone (22%), lowangle cross-bedded sandstone (12%), trough crossbedded sandstone (3%) and tabular cross-bedded sandstone (0.5%). Mud-pebble conglomerate and intraformational mudstone lenses (excluding the mudstone separating the two sandstone sub-units) together comprise less than 1% of the Matjieskloof deposit. A Markov analysis based on the measured sections failed to reveal any vertical ordering of facies. Statistical analysis of more than 600 palaeocurrent measurements in the sandstone indicates a vector mean azimuth of 045 ° and a sinuosity value of 1.36 (See Le Roux, 1991, 1992b). A generalized flow pattern map of the palaeocurrent directions is presented in Fig. 4. The study area is characterized by a dominantly northeasterly flow direction, but in the central and southeastern parts of the area a more or less northerly trending system is apparent. The confluence of the two flow systems coincides with a zone of coalescence of the two major arenaceous sub-units, indicating deeper ero-
Cliff
Fig. 4. Generalized palaeocurrent directions and location of boreholes at Matjieskloof(32°15'S/21°40'E). sion of the channel that deposited the upper sand in this area. The Matjieskloof sandstone is interpreted as a braided river deposit on account of its tabular shape, dominantly upper flow-regime sedimentary structures and lack of Markov property. Fig. 5 is a composite map of the Matjieskloof sandstone constructed according to the method of Le Roux and Rust (1989). It integrates isopach values of the mudstone separating the two major sub-units (which indirectly reflects the depth of erosion of the current that deposited the upper sand), the percentage of sandstone in a 25 m section above the base of the lower sub-unit, the number of minor sandstones within the major sub-
Fig. 5. Compositemap of Matjieskloofsandstone,incorporating isopach, sandstonepercentage, number of sandstonesand contact elevationdata. The palaeotopographyshows a major NNE-trending channelwith islandsand secondarythalwegs.
189
J.P. le Roux/Palaeogeography, Palaeoclimatology, Palaeoecology l l l (1994) 185-190
units, as well as a restored elevation contour m a p of the basal contact of the lower sandstone. The lowest composite values reflect thalweg zones within the Matjieskloof deposit, depicting a major, NNE-trending channel in the center of the study area, secondary thalwegs joining this channel from the southwest and southeast, and other minor channels branching off to the northwest and northeast. There is a good correspondence with palaeocurrent directions recorded on surface, and the overall interpretation of a braided river system with stable, more argillaceous island areas is confirmed. Fig. 6 portrays elevation contours on the basal contact of the lower sandstone sub-unit. Due to a tectonic dip of about 2 °, there is a difference of 35 m between the southern and northern parts of the study area, which conceals the effect of local palaeotopographic elements. To correct the effect of tilting and flexure, a second order trend surface with parallel, straight-line contours (a special case of the general quadratic form) was constructed. Fig. 7 shows the line of best fit through the mean elevations of the basal contact along the strikeparallel grid lines. A slight curvature of the trend surface is apparent. The residual values between the trend surface and the actual elevations of the contact (Fig. 8), effectively depict the unfolded, pre-tilted surface. Comparison with the composite m a p (Fig. 5) reveals a striking similarity, which suggests that the restored contact elevation m a p
1080
,08° uJ
1151500,oo N - S Grid Line S p a c i n g
Fig. 7. Quadratic curve fitted to mean E-W base elevation values of Matjieskloof sandstone.
iiiii!ii?iii:, i +()o
ii! i!iii! i::!:iiii:
',i!iii i!iiiii . . . . . . . . . . . . . . . .
iiiiii!iiiii
500m
'::i?i~J!i!::" Fig. 8. Restored elevation contours of Matjieskloof sandstone (basal contact). The palaeotopography is similar to that depicted in Fig. 5.
kJ
in this case would have been sufficient reconstruct the sedimentary environment.
•
~
~ ~
~
1
0
-,~__,~
~ . . . ~
,.,.~
6
0
to
4. Conclusions
10 6 5
1070
1075
t
500
m
i
Fig. 6. Structural contours on the base of the Matjieskloof sandstone. Dip towards the north is about 2°.
In the past, contact elevation maps have been used mainly to portray regional structures or to delineate local structural elements such as brachianticlines and domes, which form traps for oil and gas. Removing the effects of folding or tilting by means of trend surfaces allows the use of smaller
190
J.P. le Roux/Palaeogeography, Palaeoclimatology, Palaeoecology111 (1994) 185-190
contour intervals, which often unmasks the original surface topography. This method can therefore be a useful tool in palaeoenvironmental reconstructions.
References Agterberg, F.P., Hills, L.V. and Trettin, H.P., 1967. Paleocurrent trend analysis of a delta in the Bjorne Formation (Lower Triassic) of northwestern Melville Island, Arctic Archipelago. J. Sediment. Petrol., 37: 852-862. Allen, P. and Krumbein, W.C., 1962. Secondary trend components in the Top Ashdown Pebble Bed: a case history. J. Geol., 70: 507-538. Bennison, G.M., 1985. An Introduction to Geological Structures and Maps. Edward Arnold, London, 4th ed., 65 pp. Dawson, K.R. and Whitten, E.H., 1962. Quantitative mineralogical composition and variation of the Lacorne, La Motte, and Preissac granitic complex, Quebec, Canada. J. Petrol., 3: 1-37. Duff, P.M. and Walton, E.K., 1963. Trend surface analysis of sedimentary features of the Modiolaris Zone, East Pennine Coalfield. In: L.M. Van Straaten (Editor), Deltaic and Shallow Marine Deposits (Devel. Sedimentol., 1). Elsevier, Amsterdam, pp. 114-122, Hewlett, R.F., 1964. Polynomial surface fitting using sample data from an underground copper deposit. Bur. Mines Rep. Invest., 6522. Horton, C.V., Hempkins, W.B. and Hoffman, A.A., 1964. A statistical analysis of some aeromagnetic maps from the Northwestern Canadian Shield. Geophysics, 29 (4): 582-601. Krige, D.G. and Ueckermann, H.J., 1963. Value contours and improved regression techniques for ore reserve valuations. J. S. Afr. Inst. Min. Metall., 63 (10): 429-452.
Le Roux, J.P., 1991. Paleocurrent analysis using Lotus 1-2-3. Comp. Geosci., 17 (10): 1465-1468. Le Roux, J.P., 1992a. Determining the sinuosity of ancient fluvial systems from paleocurrent data. J. Sediment. Petrol., 62 (2): 283-291. Le Roux, J.P., 1992b. Palaeogeographic reconstruction of sandstones using weighted mean grain-size maps, with examples from the Karoo Basin (South Africa) and the Sydney Basin (Australia). Sediment. Geol., 81: 173-180. Le Roux, J.P., 1994. Persistence of topographic features as a result of non-tectonic processes. Sediment. Geol., 89: 33-42. Le Roux, J.P. and Jones, B.G., 1991. Lithostratigraphy and depositional environment of the Nowra Sandstone in the southwestern Sydney Basin, Australia. Aust. J. Earth Sci., 41: 191-203. Le Roux, J.P. and Rust, I.C., 1989. Composite facies maps: a new aid to palaeo-environmental reconstruction. S. Afr. J. Geol., 92 (4): 436-443. Miller, R.L., 1956. Trend surfaces: their application to analysis and description of environments of sedimentation. J. Geol., 64: 425-446. Oldham, C.W. and Sutherland, D.B., 1955. Orthogonal polynomials: their use in estimating the regional effect. Geophysics, 20: 295-306. Potter, P.E. and Pettijohn, F.J., 1977. Paleocurrents and Basin Analysis. Springer, Berlin, 2nd ed., 425 pp. Simpson, S.M., 1954. Least squares polynomial fitting to gravitational data and density plotting by digital computers. Geophysics, 19 (2): 255-269. Wermund, E.G. and Jenkins Jr., W.A., 1970. Recognition of deltas by fitting trend surfaces to Upper Pennsylvanian sandstones in north-central Texas. In: J.P. Morgan (Editor), Deltaic Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Publ., 15: 256-269. Whitten, E.H.T., 1967. Fourier trend-surface analysis in the geometrical analysis of subsurface folds of the Michigan Basin. In: D.F. Merriam and N.C. Cocke (Editors), Computer Applications in the Earth Sciences: Colloquium on Trend Analysis, Comp. Contrib., 12. State Geol. Surv., Univ. Kansas, Lawrence, pp. 10-11.
tcataG~Y
ELSEVIER
Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
The use of trend surfaces in palaeoenvironmental reconstructions J . P . le R o u x
Department of Geology, University of Stellenbosch, Stellenbosch 7600, South Africa Received 9 November 1993; revised and accepted 21 March 1994
Abstract Trend surfaces can be produced by a number of mathematical techniques and have been used widely to depict the regional variation of various geological parameters. In this paper, it is demonstrated that trend surfaces can be constructed by a simple graphical method and may be applied successfully to palaeoenvironmental reconstructions using elevation contours of tilted or folded contacts. Two case studies are discussed to illustrate different aspects of the technique. In the first example, the upper contact of a shallow marine sandstone outcropping over an area of roughly 4000 km 2 in the southwestern part of the Sydney Basin (Australia), is restored by means of a first order trend surface. This reveals the palaeotopography as a sinuous shoal trending towards the northnortheast. The second example, from the Beaufort Group of the Karoo Basin (South Africa), restores the basal contact of a fluvial sandstone by means of a second order (quadratic) trend surface. The restored surface portrays the individual thalwegs and islands of a braided stream system over an area of about 4 km 2.
I. Introduction The application of mathematical surfaces to solve problems in the earth sciences has increased markedly since 1954. Trend surfaces have proved especially useful in calculating the average composition and variability of intrusive masses, the evaluation of ore reserves, and geophysics (e.g. Dawson and Whitten, 1962; Hewlett, 1964; H o r t o n et al., 1964). In sedimentology, trend surfaces have been employed mainly to investigate spatial variations in sediment-size, thickness, mineral content and facies (e.g. Miller, 1956; Allen and Krumbein, 1962; Duff and Walton, 1963; Agterberg et al., 1967; Wermund and Jenkins, 1970; Potter and Pettijohn, 1977). The purpose of this communication is to discuss the use of trend surfaces for palaeoenvironmental reconstructions based specifically on contact elevation maps. Case studies from the Sydney Basin of 0031-0182/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-0182(94)00027 6
Australia and the K a r o o Basin of South Africa are examined to illustrate the advantages of the method.
2. Construction method Various techniques have been developed to produce trend surfaces, including least squares regression (Simpson, 1954), orthogonal polynomials (Oldham and Sutherland, 1955), two-dimensional moving averages (Krige and Ueckermann, 1963) and Fourier analysis (Whitten, 1967). In areas of tilting or gentle folding, however, it is possible to construct first or second order trend surfaces without resorting to mathematical methods. First, a normal structure contour m a p of the relevant contact is prepared. This usually reveals broad trends which are commonly due to the
J.P. le Roux l Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
186
effects of tilting or folding rather than the original configuration of the contoured surface. In such cases, the general strike of the contours can often be determined fairly accurately by eye. Alternatively, the well-known "three point solution" used in structural geology (Bennison, 1985) can be applied to a number of widely-spaced point elevations to calculate a mean strike for the trend surface. In Fig. la, for example, two "three point solutions" yield strikes of 097 ° and 083 ° respectively, giving a mean of 090 °. The study area is subsequently covered with a grid, which is orientated with one axis parallel to the calculated mean strike of the folds (Fig. lb). The mean elevation of the surface is then determined along each of the strike-parallel grid lines by averaging their elevations at intersections with the complementary grid lines, using interpolation to find the elevation of the relevant surface at these localities. In Fig. lb, the southernmost grid line has an average elevation of 512 m ([530+495+490+510+530+515]/6). The mean elevations of the strike-parallel grid lines are subsequently plotted against their distance (normal to the strike) from the relevant margin of the study area. A line of best fit through these
points defines the trend surface in two dimensions (Fig. lc). The strike lines of the trend surface can now be superimposed on the base map with the original structure contours, and the differences in elevation between the two surfaces are determined by interpolation at convenient points. Contouring these residual values (Fig. ld) records the original configuration of the contact before tilting or folding, as it has the same effect as restoring the trend surface to the horizontal position. The rationale for restoring trend surfaces to the horizontal is that local differences in elevation, which may be due to erosional features or other topographic elements, are often masked by the larger contour intervals required to represent tilted or folded strata. This effect can be considerable even at very low regional dips (less than 1°), depending on the size of the study area and the relative variation in elevation of the original topography. Substantial errors can arise in the palaeoenvironmental interpretation of contact elevation maps if the effects of tilting and folding are not eliminated first, as illustrated in the case studies below.
D
J f 200
330
f
430
B
~
~"
200
230
260
2 ~
298
346
326
~15
420
40~
398
400
510
530
SlS
--298
f
160
315
i
~5 O0
630
---
~
575
!b)
530
~
~
50O 400 300 200
----I . . . . . . . . . I I I iI II A B
100 •
C)
4-I I I II C
,-, I I I II D
O
Fig. 1. Graphical method of constructing trend surfaces. See text for explanation.
J. P. le Roux/ Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
187
3. Case studies 3.1. Nowra Formation The Nowra Formation forms part of the Shoalhaven Group, a sequence of marine sandstones, siltstones and conglomerates exposed in the southwestern part of the Sydney Basin in New South Wales. As the sedimentology of the Nowra Formation has been described in detail by Le Roux and Jones (1994), only a short summary is presented here. The formation consists of medium-grained sandstone up to 140 m thick, deposited in foreshore to middle/upper shoreface environments during a regressive-transgressive episode of Early to Middle Permian age. Cross-bedding directions are consistently towards the northnortheast, reflecting strong longshore currents. The elevation of the upper contact of the Nowra Sandstone was determined from field data plotted on 1:25,000 topographic maps. Structural contours drawn on this contact portray a gentle homoclinal flexure with an easterly dip of about 1° (Fig. 2). The contour interval of 50 m completely masks local topographic effects, except for a deviation in the contours defining a linear feature transecting the area from northwest to southeast. This reflects the presence of a submarine channel coinciding with a fluvial palaeovalley in the Palaeozoic rocks at the base of the Shoalhaven Group (Le Roux, 1994). To reconstruct the original topography, a first order trend surface with a strike of 346 ° and a dip of 0.96°E was superimposed on the structural contours (Fig. 2). Residual elevations between this surface and the structure contours were then plotted and contoured, effectively untilting the homocline to a horizontal datum level. The restored configuration of the Nowra Sandstone upper contact now indicates a sinuous shoal or ridge separated from the western shore of the Sydney Basin by deeper water (Fig. 3). Although the weighted mean grain-size of the sandstone becomes finer along the landward side of the shoal (Le Roux, 1992a), a proper lagoon is unlikely to have existed in this area. An independent restoration of the basal contact
Fig. 2. Structural contours on upper contact of Nowra Formation, showing a NE-dipping homoclinal flexure. Ulladulla is located at 35°21'S/150°29'E.
5c
Fig. 3. Restored elevation contours of Nowra Formation upper contact, showing a sinuous, NNE-trending shoal.
188
J. P. le Roux/Palaeogeography, Palaeoclimatology, Palaeoecology 111 (1994) 185-190
of the Nowra Formation, obtained by subtracting the sandstone thickness from the restored elevation of the upper contact, reveals a remarkably similar palaeotopography (Le Roux, 1994). This geomorphological interpretation is supported by the distribution of conglomerates and other lithofacies along the shoal (Le Roux and Jones, 1994). 3.2. Beaufort Group
The Matjieskloof deposit is located in the southwestern Karoo Basin in the lower part of the Permo-Triassic Beaufort Group. Extensive drilling for uranium during the late 1970's outlined a tabular sandstone with a maximum thickness of 34 m, consisting of two major arenaceous subunits normally separated by a thin (0-5 m) mudstone. Smaller mesocycles within the sandstone are bounded by erosion surfaces and occasional mudstone lenses. A study of 48 measured sections in both subunits showed up to 10 of these smaller mesocycles, which have a maximum and mean thickness of 3.6 and 1.11 m, respectively. Sedimentary structures are dominated by upper-phase plane beds (38%), followed by massive (structureless) sandstone (22%), ripple-laminated sandstone (22%), lowangle cross-bedded sandstone (12%), trough crossbedded sandstone (3%) and tabular cross-bedded sandstone (0.5%). Mud-pebble conglomerate and intraformational mudstone lenses (excluding the mudstone separating the two sandstone sub-units) together comprise less than 1% of the Matjieskloof deposit. A Markov analysis based on the measured sections failed to reveal any vertical ordering of facies. Statistical analysis of more than 600 palaeocurrent measurements in the sandstone indicates a vector mean azimuth of 045 ° and a sinuosity value of 1.36 (See Le Roux, 1991, 1992b). A generalized flow pattern map of the palaeocurrent directions is presented in Fig. 4. The study area is characterized by a dominantly northeasterly flow direction, but in the central and southeastern parts of the area a more or less northerly trending system is apparent. The confluence of the two flow systems coincides with a zone of coalescence of the two major arenaceous sub-units, indicating deeper ero-
Cliff
Fig. 4. Generalized palaeocurrent directions and location of boreholes at Matjieskloof(32°15'S/21°40'E). sion of the channel that deposited the upper sand in this area. The Matjieskloof sandstone is interpreted as a braided river deposit on account of its tabular shape, dominantly upper flow-regime sedimentary structures and lack of Markov property. Fig. 5 is a composite map of the Matjieskloof sandstone constructed according to the method of Le Roux and Rust (1989). It integrates isopach values of the mudstone separating the two major sub-units (which indirectly reflects the depth of erosion of the current that deposited the upper sand), the percentage of sandstone in a 25 m section above the base of the lower sub-unit, the number of minor sandstones within the major sub-
Fig. 5. Compositemap of Matjieskloofsandstone,incorporating isopach, sandstonepercentage, number of sandstonesand contact elevationdata. The palaeotopographyshows a major NNE-trending channelwith islandsand secondarythalwegs.
189
J.P. le Roux/Palaeogeography, Palaeoclimatology, Palaeoecology l l l (1994) 185-190
units, as well as a restored elevation contour m a p of the basal contact of the lower sandstone. The lowest composite values reflect thalweg zones within the Matjieskloof deposit, depicting a major, NNE-trending channel in the center of the study area, secondary thalwegs joining this channel from the southwest and southeast, and other minor channels branching off to the northwest and northeast. There is a good correspondence with palaeocurrent directions recorded on surface, and the overall interpretation of a braided river system with stable, more argillaceous island areas is confirmed. Fig. 6 portrays elevation contours on the basal contact of the lower sandstone sub-unit. Due to a tectonic dip of about 2 °, there is a difference of 35 m between the southern and northern parts of the study area, which conceals the effect of local palaeotopographic elements. To correct the effect of tilting and flexure, a second order trend surface with parallel, straight-line contours (a special case of the general quadratic form) was constructed. Fig. 7 shows the line of best fit through the mean elevations of the basal contact along the strikeparallel grid lines. A slight curvature of the trend surface is apparent. The residual values between the trend surface and the actual elevations of the contact (Fig. 8), effectively depict the unfolded, pre-tilted surface. Comparison with the composite m a p (Fig. 5) reveals a striking similarity, which suggests that the restored contact elevation m a p
1080
,08° uJ
1151500,oo N - S Grid Line S p a c i n g
Fig. 7. Quadratic curve fitted to mean E-W base elevation values of Matjieskloof sandstone.
iiiii!ii?iii:, i +()o
ii! i!iii! i::!:iiii:
',i!iii i!iiiii . . . . . . . . . . . . . . . .
iiiiii!iiiii
500m
'::i?i~J!i!::" Fig. 8. Restored elevation contours of Matjieskloof sandstone (basal contact). The palaeotopography is similar to that depicted in Fig. 5.
kJ
in this case would have been sufficient reconstruct the sedimentary environment.
•
~
~ ~
~
1
0
-,~__,~
~ . . . ~
,.,.~
6
0
to
4. Conclusions
10 6 5
1070
1075
t
500
m
i
Fig. 6. Structural contours on the base of the Matjieskloof sandstone. Dip towards the north is about 2°.
In the past, contact elevation maps have been used mainly to portray regional structures or to delineate local structural elements such as brachianticlines and domes, which form traps for oil and gas. Removing the effects of folding or tilting by means of trend surfaces allows the use of smaller
190
J.P. le Roux/Palaeogeography, Palaeoclimatology, Palaeoecology111 (1994) 185-190
contour intervals, which often unmasks the original surface topography. This method can therefore be a useful tool in palaeoenvironmental reconstructions.
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