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Soft-sediment deformation structures from the Marinoan glacial succession, Adelaide foldbelt: implications for the palaeoaltitude of late Neoproterozoic glaciation
Sedimentary
Geology Sedimentary Geology 106 (1996) 165- 175
Expressed
Soft-sediment deformation structures from the Marinoan glacial succession, Adelaide foldbelt: implications for the palaeolatitude of late Neoproterozoic glaciation George E. Williams * Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia
Received 23 April 1996; accepted 1 August 1996
Abstract deformation structures occurs in tidal rhythmites of fine sand to silt grade in the late Ma) Elatina Formation, which is part of the Marinoan glaciogenic succession in the Adelaide
A suite of soft-sediment
Neoproterozoic (-600
foldbelt, South Australia. The structures include: (1) sets of asymmetrical cuspate ridges (h = 13-50 cm, h = 3-5 cm) formed on bed surfaces and underlain by folds affecting as much as 60 cm thickness of strata; (2) symmetrical and interference ripple forms (h = 3-15 cm, h = ~1.5 cm) mostly confined to the troughs between the cuspate ridges and which are underlain by folds, involving up to 20 cm thickness of strata, that commonly parallel the undulations of the bed surface but in places have steepened limbs; (3) rill marks on the flanks of cuspate ridges and some ripple forms. The crests of the cuspate ridges and ripple forms commonly were draped and locally eroded and truncated during overall vertical accretion. The cuspate structures are interpreted as gravity slide deposits that formed after transformation of surficial sediment to a hydroplastic state, possibly by the cyclic stresses generated by storm waves, and its sliding on tidal-delta slopes. The ripple forms resulted from continuing wave activity and were maintained by draping and vertical accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. The further deformation of the cuspate folds, as revealed by palaeomagnetic analyses of the structures, implies additional sliding and/or the differential loading of hydroplastic sediment in the troughs between the cuspate ridges. This study confirms that positive palaeomagnetic fold-tests on several cuspate folds indicate a primary origin for the shallow palaeomagnetic inclination (-5.3“) of the Elatina Formation and hence the equatorial palaeolatitude of late Neoproterozoic glaciation in South Australia. Keywords:
Neoproterozoic; Glacial deposits; Soft-sediment transformation; Paleomagnetism; Paleolatitude
1. Introduction The sedimentary structures described here occur in tidal rhythmites of the Elatina Formation ‘Tel.: +61 (8)8303 5843; Fax: +61 (8)8303 4347.
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the principal formation of the late Neoproterozoic (-600 Ma) Marinoan glaciogenic succession in South Australia (Preiss, 1987) - at Pichi Richi Pass and Warren Gorge in the western Adelaide foldbelt (Fig. 1) where the formation is 100-150 m thick. The rhythmites at Pichi Richi Pass form
0037-0738/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PI1 SOO37-0738(96)00062-O
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Pichi Richi Pass (PRP) and Warren Gorge (WC) near the faultbounded western margin of the foldbelt. Major faults are shown as heavy lines. Adapted from Preiss (1987).
a lo-m-thick unit comprising lamina-cycles up to 2 cm in thickness, interpreted as fortnightly groupings of thin (52 mm), diurnal and semidiurnal laminae (Williams, 1989. 1991). The laminae usually fine upward from very fine-grained sandstone to siltstone, and successive lamina-cycles are separated by thin clayey bands deposited during neap tides. The planarity of laminae is locally disturbed by undulations and cross-lamination that record periodic unidirectional bottom currents, and by small-scale folds evidently of soft-sediment origin. At Warren Gorge, 25 km to the north, an 18-m-thick rhythmite unit of very fine- to fine-grained sandstone also displays lamina-cycles. The Elatina rhythmites were deposited from tidal plumes and density underhows on distal ebb-tidal deltas near the western margin of the gulf-like Adelaide ‘geosyncline’ (Williams,
Geology 106 (1996) 165-175
1989, 1991). Rare, poorly laminated interbeds of fine-grained sandstone up to 10 cm thick that have erosional lower contacts and are weakly graded evidently record episodic density flows and slumping on the delta slopes. The rhythmites at Pichi Richi Pass have provided geophysical information of wide significance. Palaeotidal data recorded by the rhythmites have illuminated the Earth’s palaeorotational history (Williams, 1989, 1990, 1991), and palaeomagnetic analyses of the rhythmites. together with data for the Elatina Formation elsewhere in the Adelaide foldbelt, have provided strong evidence for the low palaeolatitude of late Neoproterozoic glaciation (Embleton and Williams, 1986; Schmidt et al.. 1991; Schmidt and Williams, 1995). The small-scale folds in the rhythmites at Pichi Richi Pass permitted positive palaeomagnetic fold-tests at a high level of confidence, with the interpretation of the folds as soft-sediment, not tectonic, in origin being vital in confirming the early timing of magnetic remanence in the Elatina Formation (Sumner et al., 1987; Schmidt et al., 1991; Schmidt and Williams, 1995). Meert and Van der Voo (1994), however, without having seen the structures, queried their proposed soft-sediment origin, suggesting instead that they may be of tectonic origin related to the Delamerian Orogeny that formed the Adelaide foldbelt in Cambro-Ordovician time (Preiss. 1987). Firmly establishing the nature and origin of the small-scale folds in the Elatina rhythmites therefore will have important implications for the palaeomagnetic interpretation of the Marinoan glaciogenic succession in South Australia and, more widely, for late Neoproterozoic palaeogeography. 2. Deformational
structures
2.1. Cuspate ridges and folds Bed surfaces of the rhythmite unit at Warren Gorge display slightly sinuous, discontinuous cuspate ridges arranged in near-parallel sets (Fig. 2). Typically the ridges have wavelengths of 30-50 cm and locally as little as 13-20 cm, and heights of 3-5 cm measured from trough to crest. Sets exposed on successive bed surfaces have different orientations, with dip-corrected strikes of sets lying be-
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167
Fig. 2. Slightly sinuous, discontinuous cuspate ridges 30-50 cm apart on a bed surface of the Elatina Formation at Warren Gorge. The hollows between the ridges are covered by symmetrical ripple forms arranged approximately at right angles to the cuspate ridges. Hammer 33 cm long.
tween 110” and 170”. Nine successive sets have a mean dip-corrected strike of 149” (range 130-163”) and a mean stratigraphic spacing of 58 cm (1 l-190 cm). Most ridges are asymmetrical in cross-section: steeper flanks near ridge crests slope at 11-17” with respect to mean bed surfaces, compared with slopes of 7-12” for the opposite flanks. The steeper flanks typically face northeastward (dip-corrected). The ridges are underlain by fold structures (Fig. 3) spanning thicknesses of up to 50-60 cm and as many as 30 lamina-cycles representing more than one year’s deposition. The amplitude of the folds gradually increases upward from undeformed strata and the crests of some folds are erosionally truncated by overlying strata. Cuspate ridges on bed surfaces at Pichi Richi Pass are mostly smaller than those at Warren Gorge, with wavelengths of 14-40 cm and heights of up to 34 cm. The mean dip-corrected strike is about 115”
(range 62-128”). Some ridges have slightly steeper northern flanks and others are symmetrical in crosssection. As at Warren Gorge, the ridges are underlain by folded lamina-cycles spanning thicknesses of up to 30-40 cm and several tens of lamina-cycles. The trends of the cuspate ridges at both localities are unrelated to the meridional (strike 175-180”) tectonic fold trends in this area. Fig. 4 shows the detailed structure of the cuspate fold at Pichi Richi Pass that was studied palaeomagnetically by Schmidt et al. (1991). During deposition from ebb-tidal plumes (see Williams, 1989, 1991), delicate erosional truncations of laminae occurred at the fold crest and lenses of fine sand accumulated on the downcurrent flank of the fold. Such unidirectional current action during deposition of the three topmost lamina-cycles, which accumulated at the peak of the yearly cycle when ebb-tidal currents were strongest, caused the thinning of one cycle on
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Geology
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165-l
75
Fig. 3. Transverse vertical section of a cuspate ridge at Warren Gorge, showing sharply folded tidal lamina-cycles neap-tidal bands spaced -1 cm apart. The crest of the fold is eroded and draped by lamina-cycles. Scale 15 cm.
the upcurrent side of the fold and the crest to migrate in the current direction. Other lamina-cycles usually are thinnest on the crest and thickest on the downcurrent side of the fold. although laminae typically persist across the fold apart from the modifications near the crest. The same erosional and depositional modifications of fold crests and flanks occur in other cuspate structures at Pichi Richi Pass. Micro-faults with variable displacements of 11 mm, identifiable as soft-sediment features by the flexing of some bands across irregular and impersistent lines of failure, occur on the flank of the fold shown in Fig. 4. Such soft-sediment micro-faults, and small-scale contemporaneous scours and erosional truncations of laminae, have also been illustrated by Williams ( 1991) in the rhythmites from Pichi Richi Pass and coeval rhythmites from Hallett Cove near Adelaide. 2.2. Ripple forms Sets of smaller ridges with the appearance of wave-generated symmetrical or oscillation ripple marks, and less commonly of interference ripple
marked
by darker,
marks, cover the bed surfaces between the cuspate ridges at Warren Gorge and occur between some cuspate ridges at Pichi Richi Pass. The symmetrical ripple forms (Fig. 2) usually are spaced 3-15 cm apart and have heights ranging from a few millimetres up to 1.5 cm. Bifurcations occur in places. Typically the symmetrical ripple forms are arranged at wide angles (75-90”) to the trend of cuspate ridges on the same bed surface and rarely extend over ridge crests, usually petering out and locally being deflected slightly near the crests (Figs. 5a and 7). Patterns of symmetrical ripple forms are dissimilar on opposite sides of a cuspate ridge. The interference patterns (Fig. 5b). which extend over some cuspate ridges, commonly show ‘triple junctions’ of crests that form depressions 10-20 cm across and as much as 1.5 cm deep. The ripple forms are underlain by folds, involving up to 20 cm thickness of strata, that commonly parallel the undulations of the bed surface (Fig. 6a) but in places have steepened limbs (Fig. 6c; Schmidt and Williams, 1995, their fig. 5) suggesting some later deformation. Locally, the bedding is analogous
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Geology 106 (1996) 165-175
169
Fig. 4. Thin-section (plane light) cut perpendicular to an ESE-WNW-trending cuspate fold in the Elatina rhythmites at Pichi Richi Pass. Scale bar 1 cm. The photograph spans nine lamina-cycles of graded laminae of fine-grained sandstone (pale) and siltstone; the dark, clayey bands (IV) bounding the cycles were deposited at neaps. During deposition by ebb-tidal currents, erosional truncation of laminae occurred at the fold crest and lenses of fine sand accumulated on the downcurrent (right-hand) side of the crest, causing the crest at the top of the photograph to migrate in the current direction. Lamina-cycles usually are thinnest on the crest and thickest on the downcurrent side, consistent with draping and vertical accretion in the presence of unidirectional currents. A soft-sediment micro-fault (arrows) with variable displacement of (1 mm occurs on a flank of the fold. The thin-section was cut from the upper part of a block sample that gave a positive palaeomagnetic fold-test (Schmidt et al., 1991).
supercritical cross-lamination (Fig. 6b; see Allen, 1982), which caused the crests of the ripple forms to migrate laterally during deposition of lamina-cycles. This type of lamination is common beneath the interference ripple forms, although the general lack of low-angle curved lamina intersections contrasts with hummocky and swaley cross-stratification (see Walker, 1982). In addition, delicate erosional truncations and wedge-outs of laminae, and thinning of lamina-cycles, occur on the crests of ripple forms (see Schmidt and Williams, 1995) and some ripple forms are erosionally truncated by overlying strata (Fig. 6~). Draped and truncated ripple structures also are shown by Williams (1991) from the rhythmites at Pichi Richi Pass. to
2.3. Associated
rill marks
A distinctive feature of the cuspate ridges and some symmetrical ripple forms at Warren Gorge is the presence of rill marks on several upper bedsurfaces (Figs. 5a and 7). These features are elongate, curved crenulations of sandstone about 1 mm in relief and as much as 29 cm long and 5 cm across; they are widest at ridge and ripple-form crests and taper, singly or in coalescing groups, toward the deepest parts of adjacent hollows in the bed surface. The rill marks occur only on the steeper, northeastern flanks of the cuspate ridges and on the northeastern sides of ripple forms, and their orientation is unrelated to the present vertical.
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Geology 106 (I 996) 165-l 75
Fig 5. B ed surfaces at Warren Gorge. (a) Symmetrical ripple forms locally deflected by, and terminating at, a cuspate ridge, Ripple fort ns G3” opposite sides of the ridge crest are not collinear. Rill marks extend to the right from the ridge crest, below and to the right of the 3-c m scale. (b) Interference ripple forms. Scale 15 cm.
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Geology 106 (1996) 165-175
Fig. 6. Transverse sections through ripple forms, Warren Gorge. (a) Twelve lamina-cycles (about six months’ deposition) displaying near-symmetrical folds above and below symmetrical ripple forms on adjacent bed surfaces. The thickness of the lamina-cycles does not vary appreciably across the folds. Scale 15 cm. (b) Eight lamina-cycles of variable thickness with the structure of supercritical cross-lamination which progressively displaced the crests of the ripple forms to the left during deposition. Scale 3 cm. (c) Folded lamina-cycles associated with ripple forms erosionally truncated by a sandstone bed. Scale 3 cm.
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Geology 106 (1996) 165-175
Fig. 7. Two parallel rill marks on a bed surface near the crest of a cuspate ridge at Warren Gorge. The rill marks extend down the Rlank of th e ridge between symmetrical ripple forms, which terminate just below the ridge crest. Scale 3 cm.
3. Interpretation of the deformational structures 3. I. Soft-sediment
origin
Much evidence indicates that the small-scale deformational structures in the Elatina Formation at Warren Gorge and Pichi Richi Pass are of softsediment origin and were modified in places by penecontemporaneous erosion and deposition. The erosional truncation of the crests of some cuspate ridges by overlying beds, the delicate erosion and deposition on crests and flanks, and the downward passage of related folds into undeformed strata demonstrate that the cuspate ridges were initiated in surficial sediments and were modified during overall vertical accretion by periodic unidirectional bottom currents. The cuspate fold shown in Fig. 4 exemplifies such modification: the structure formed a low ridge on the sea floor, and the resulting altered flow in the ebb-tidal currents carrying fine elastic material
in suspension caused delicate erosional truncations of laminae and thinning of lamina-cycles at the ridge crest, while flow separation over the crest led to the deposition of sand lenses and cross-laminae on the down-current side of the ridge. The internal structure of the cuspate ridges and their various orientations on different bedding surfaces thus militate against a tectonic origin. Detailed palaeomagnetic study of two different block samples of cuspate folds from the same site at Pichi Richi Pass (Schmidt et al., 1991; Schmidt and Williams, 1995), each showing erosional and depositional modification of fold crestal areas (Fig. 4), indicates that the samples examined acquired their magnetisation when the folds were only 33% formed (i.e., the tightest clustering and the least correlation between structure and magnetisation are attained at 66-68% unfolding). This finding implies that the cuspate folds formed in several stages: (1) the formation of low ridges on the sea floor; (2) continued
G.E. Williams/Sedimentary Geology 106 (1996) 165-175
near-uniform vertical accretion of the tidal sediments, thus maintaining the ridges on the sea floor except for the modifications to crestal areas and flanks discussed above; and (3) further soft-sediment deformation that substantially increased the amplitude of the folds. The occurrence of cross-lamination and erosional truncations at the crests of some ripple forms indicates that the latter features also formed on the sea floor and were modified by current action, The confinement of most ripple forms to the troughs between the cuspate ridges, and the termination and deflection of numerous ripple forms adjacent to ridge crests, indicate that the ripple forms developed after the ridges. Continued deposition maintained near-uniform thicknesses of laminae and lamina-cycles over the cuspate ridges and ripple forms apart from the local truncation, modification or migration of crestal areas caused by bottom currents. Such vertical accretion over bedforms has been recorded in other laminites: Donaldson and Munro (1982) observed the vertical accretion of laminites over rippled lenses in the Palaeoproterozoic Gowganda Formation in Ontario, and Leithold et al. (1989) noted the draping of distal tidal rhythmites over wave-rippled sandstone lenses in the Eocene Elkton Formation in Oregon. The rill marks on the surface of cuspate ridges and ripple forms may record the flow of dense or turbid water that escaped from ridge and ripple-form crests, or bottom currents. 3.2. Possible causes of the soft-sediment
deformation
The cuspate structures are interpreted as gravity slide deposits (see Allen, 1982; Myrow and Hiscott, 1991) that formed on tidal-delta slopes. The orientation and asymmetry of these structures suggest a regional northeastward palaeoslope, which is consistent with the envisaged deposition of the Elatina rhythmites on eastward-prograding ebb-tidal deltas (Williams, 1989, 1991). The fairly sharp flexures of some folds (Figs. 3 and 4), the presence of soft-sediment micro-faults, and the well-defined laminae normally showing minimal thickness variation imply that the sediments were in a hydroplastic state, and not liquidised, when deformed. The Elatina rhythmites were susceptible
173
to transformation from a solid-like to a much weakened state with the operation of a suitable trigger (see Allen, 1986): they have a dominant grain size in the lower part of the sand range and a paucity of clayey material apart from the thin bands deposited at neaps, and were rapidly deposited from ebb-tidal plumes and density underflows. Of the main triggers for such transformation of sediments (Allen, 1982, 1986), only cyclic stresses resulting from earthquakes and/or vigorous wave action seem applicable to the Elatina rhythmite palaeoenvironment. Seismites are widely recognised in the stratigraphic record (Allen, 1982, 1986; Mohindra and Bagati, 1996), with their expected stratigraphic frequency increasing exponentially toward major faults at the margins of sedimentary basins (Allen, 1986). In view of the proximity of Warren Gorge and Pichi Richi Pass to a major fault at the western margin of the Adelaide foldbelt (Fig. 1) that was active during Neoproterozoic deposition (Preiss, 1987), an earthquake trigger for the gravity slides should be considered. However, the Elatina rhythmites were deformed when in a hydroplastic state and show no evidence of liquidisation or such deformation structures as diapiric flame-like intrusions, recumbent folding, dish and ball-and-pillow structures, and sand dykes that commonly are associated with seismic events (see Mohindra and Bagati, 1996). The lack of such features in the rhythmites argues against an earthquake influence. The soft-sediment deformation structures in the Elatina rhythmites therefore may best be ascribed to wave action; the similarity of the ripple forms to wave-induced ripple marks is consistent with a strong influence of wave activity during rhythmite deposition. Stresses generated by wind waves, particularly of storm origin (see Allen, 1986), may have changed the surficial sediments to a hydroplastic state, triggering mass movement and the formation of the cuspate ridges by gravity sliding on delta slopes. The superimposed ripple forms are attributable to continued wave activity, and were maintained by draping and vertical accretion from sediment-laden unidirectional currents and locally by deposition of supercritical cross-lamination. The steepened fold limbs associated with some of the ripple forms also may have been caused by wave-generated stresses and fluid shear. The further deformation of the
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cuspate folds, as indicated by detailed palaeomagnetic analyses (see Sect. 4), suggests their additional down-slope sliding and/or the differential loading of a hydroplastic substrate in the troughs between the ridges during disturbance by wave action. It is noteworthy that the symmetrical ripple forms at Warren Gorge trend ENE-WSW on average (dipcorrected), which is perpendicular to the southsoutheasterly palaeowind direction determined from cross-strata1 attitudes for the coeval periglacialaeolian Whyalla Sandstone on the adjacent Stuart Shelf (Williams, 1994, 1996). The wide angles between cuspate ridges and associated symmetrical ripple forms, that are maintained over a range of ridge trends, suggests that the troughs between cuspate ridges also may have influenced the direction of oscillatory currents on the sea floor. The larger size and greater abundance of deformational structures at Warren Gorge may reflect a more proximal, shallower palaeoenvironment than that at Pichi Richi Pass. The late Neoproterozoic Chapel Island Formation in Newfoundland displays deformational and depositional features analogous to those described here. Myrow and Hiscott (1991) interpreted cuspate soft-sediment anticlines in thinly bedded sandstones and siltstones as a slide feature resulting from mass movement initiated by stresses generated by a major storm, and overlying hummocky cross-bedded sandstone as a storm-wave deposited sand. 4. Implications for late Neoproterozoic palaeogeography Independent palaeomagnetic fold-tests have been conducted on three different folds from the same site in the Elatina rhythmites at Pichi Richi Pass: a small-scale fold probably of cuspate type (Sumner et al., 1987), and two different cuspate folds (Schmidt et al., 1991; Schmidt and Williams, 1995) that show the same erosional and depositional modifications of crests and flanks. In each instance the authors ascribed the folds to soft-sediment deformation and the fold test was positive at a high level of confidence (95-99%). All authors concluded that the stable remanent magnetisation of the Elatina Formation at this site was acquired early. Indeed, the most detailed studies (Schmidt et al., 1991; Schmidt and Williams, 1995) indicated that the magnetisation
Geology 106 ( 1996) 165-I 75
was acquired during deposition (detrital remanent magnetisation) when the folds were only 33% developed. These fold tests were vital in demonstrating the early acquisition of the stable remanent magnetisation and shallow palaeomagnetic inclination characterising the Elatina Formation. As observed above, the attitude of the softsediment cuspate folds in the Elatina rhythmites differs from the meridional tectonic fold trends in the Pichi Richi Pass-Warren Gorge area. Indeed, all the soft-sediment folds selected for palaeomagnetic study trend ESE-WNW, because such an attitude is transverse to the mean palaeomagnetic declination of 191.9” for the Elatina rhythmites at Pichi Richi Pass (Embleton and Williams, 1986) and so is essential to permit conclusive fold-tests. It should be noted that the consistency of palaeomagnetic declinations determined for the soft-sediment folds, for associated rhythmites at Pichi Richi Pass, and also for other facies of the Elatina Formation with a wide range of structural attitudes elsewhere in the Adelaide foldbelt (Embleton and Williams, 1986; Schmidt et al., 1991; Schmidt and Williams, 1995), indicates that any rotation about a vertical axis possibly caused by soft-sediment or tectonic deformation of these rocks is unimportant. Confirmation of the soft-sediment origin of the cuspate folds, and hence the early acquisition of the stable remanent magnetisation and shallow palaeomagnetic inclination of the Elatina Formation, carries important implications for late Neoproterozoic palaeomagnetism and palaeogeography. Palaeomagnetic studies that concluded the Elatina rhythmites were deposited in low palaeolatitudes (Embleton and Williams, 1986; Schmidt et al., 1991) are upheld, and palaeomagnetic data for the complete Elatina Formation, which indicate a mean dip-corrected inclination of -5.3” and a palaeolatitude of 2.7 f 3.7” (Schmidt and Williams, 1995), provide the strongest evidence yet for the equatorial palaeolatitude of late Neoproterozoic glaciation. 5. Conclusions Small-scale folds are common in fine-grained tidal rhythmites of the late Neoproterozoic (-600 Ma), glaciogenic Elatina Formation in the Adelaide foldbelt. A soft-sediment origin for the folds and
G.E. Williams/Sedimentary
their contemporaneity with deposition are shown. Cuspate ridges and associated folds formed by the episodic transformation of surficial sediments on tidal deltas to a hydroplastic state, possibly through stresses caused by storm wave activity, followed by downslope sliding of the sediments. Continued wave activity during deposition produced ripple forms on the cuspate ridges, and may have contributed to further deformation of the cuspate folds as indicated by palaeomagnetic analyses. The bedforms and fold structures were maintained by draping and vertical accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. Confirmation of the soft-sediment origin of the cuspate folds carries important implications for the palaeomagnetic interpretation of the Marinoan glaciogenic succession in South Australia and for late Neoproterozoic palaeogeography. Positive palaeomagnetic fold tests previously conducted on these structures do indeed show that the low palaeomagnetic inclination of the Elatina Formation is primary, and hence confirm that the Marinoan glaciation occurred in equatorial palaeolatitudes. Acknowledgements I thank Phil Schmidt and cussions on the Neoproterozoic the Adelaide foldbelt, and the Council for the provision of a lowship. The paper benefited Van Loon.
Dave Clark for dispalaeomagnetism of Australian Research Senior Research Felfrom review by A.J.
References Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis, Vols. I and II. Elsevier, Amsterdam, 593 and 663 pp. Allen, J.R.L., 1986. Earthquake magnitude-frequency, epicentral distance, and soft-sediment deformation in sedimentary basins. Sediment. Geol., 46: 67-75. Donaldson, J.A. and Munro, I., 1982. Precambrian geology of the Cobalt area, northern Ontario. 1 lth Int. Congr. Sedimentology, Hamilton, Ont. Field Excursion Guide Book 16B, 72 pp. Embleton, B.J.J. and Williams, GE., 1986. Low palaeolatitude of deposition for late Precambrian periglacial varvites in South
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Australia: implications for palaeoclimatology. Earth Planet. Sci. Lett., 79: 419-430. Leithold, E.L., Bourgeois, J. and Kreisa, R.D., 1989. Genesis of tidal rhythmites of an Eocene delta slope, S.W. Oregon, U.S.A. 2nd Int. Symp. Clastic Tidal Deposits, Alberta, August, Abstr., p. 55. Meert, J.G. and Van der Voo, R., 1994. The Neoproterozoic (100&540 Ma) glacial intervals: No more snowball earth? Earth Planet. Sci. Lett., 123: l-13. Mohindra, R. and Bagati, T.N., 1996. Seismically induced softsediment deformation structures (seismites) around Sumdo in the lower Spiti valley (Tethys Himalaya). Sediment. Geol., 101: 69-83. Myrow, PM. and Hiscott, R.N., 1991. Shallow-water gravityflow deposits, Chapel Island Formation, southeast Newfoundland, Canada. Sedimentology, 38: 935-959. Preiss, W.V. (Compiler), 1987. The Adelaide Geosyncline. Geol. Surv. S. Aust. Bull. 53, 438 pp. Schmidt, PW. and Williams, G.E., 1995. The Neoproterozoic climatic paradox: Equatorial palaeolatitude for Marinoan glaciation near sea level in South Australia. Earth Planet. Sci. Len., 134: 107-124. Schmidt, P.W., Williams, G.E. and Embleton, B.J.J., 1991. Low palaeolatitude of Late Proterozoic glaciation: early timing of remanence in haematite of the Elatina Formation, South Australia. Earth Planet. Sci. Lett., 105: 355-367. Sumner, D.Y., Kirschvink, J.L. and Runnegar, B.N., 1987. Softsediment paleomagnetic field tests of late Precambrian glaciogenie sediments (abstr.). Eos, 68: 1251. Walker, R.G., 1982. Hummocky and swaley cross-stratification. In: R.G. Walker (Editor), Clastic Units of the Front Ranges, Foothills and Plains in the Area between Field, B.C. and Drumheller, Alberta. Int. Assoc. Sedimentol. Excursion 21A Guidebook, 160 pp. Williams, G.E., 1989. Late Precambrian tidal rhythmites in South Australia and the history of the Earth’s rotation. J. Geol. Sot. London, 146: 97-l 11. Williams, G.E., 1990. Tidal rhythmites: key to the history of the Earth’s rotation and the lunar orbit. J. Phys. Earth, 38: 475&9 1. Williams, G.E., 1991. Upper Proterozoic tidal rhythmites, South Australia: sedimentary features, deposition, and implications for the Earth’s paleorotation. In: D.G. Smith, G.E. Reinson, B.A. Zaitlin and R.A. Rahmani (Editors), Clastic Tidal Sedimentology. Can. Sot. Pet. Geol. Mem., 16: 161-178. Williams, G.E., 1994. The enigmatic late Proterozoic glacial climate: an Australian perspective. In: M. Deynoux, J.M.G. Miller, E.W. Domack, N. Eyles, I.J. Fairchild and G.M. Young (Editors), Earth’s Glacial Record. Cambridge University Press, Cambridge, pp. 146-164. Williams, G.E., 1996. Late Neoproterozoic periglacial-aeolian sand sheet in equatorial palaeolatitudes, Stuart Shelf, South Australia (unpublished).
Geology Sedimentary Geology 106 (1996) 165- 175
Expressed
Soft-sediment deformation structures from the Marinoan glacial succession, Adelaide foldbelt: implications for the palaeolatitude of late Neoproterozoic glaciation George E. Williams * Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia
Received 23 April 1996; accepted 1 August 1996
Abstract deformation structures occurs in tidal rhythmites of fine sand to silt grade in the late Ma) Elatina Formation, which is part of the Marinoan glaciogenic succession in the Adelaide
A suite of soft-sediment
Neoproterozoic (-600
foldbelt, South Australia. The structures include: (1) sets of asymmetrical cuspate ridges (h = 13-50 cm, h = 3-5 cm) formed on bed surfaces and underlain by folds affecting as much as 60 cm thickness of strata; (2) symmetrical and interference ripple forms (h = 3-15 cm, h = ~1.5 cm) mostly confined to the troughs between the cuspate ridges and which are underlain by folds, involving up to 20 cm thickness of strata, that commonly parallel the undulations of the bed surface but in places have steepened limbs; (3) rill marks on the flanks of cuspate ridges and some ripple forms. The crests of the cuspate ridges and ripple forms commonly were draped and locally eroded and truncated during overall vertical accretion. The cuspate structures are interpreted as gravity slide deposits that formed after transformation of surficial sediment to a hydroplastic state, possibly by the cyclic stresses generated by storm waves, and its sliding on tidal-delta slopes. The ripple forms resulted from continuing wave activity and were maintained by draping and vertical accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. The further deformation of the cuspate folds, as revealed by palaeomagnetic analyses of the structures, implies additional sliding and/or the differential loading of hydroplastic sediment in the troughs between the cuspate ridges. This study confirms that positive palaeomagnetic fold-tests on several cuspate folds indicate a primary origin for the shallow palaeomagnetic inclination (-5.3“) of the Elatina Formation and hence the equatorial palaeolatitude of late Neoproterozoic glaciation in South Australia. Keywords:
Neoproterozoic; Glacial deposits; Soft-sediment transformation; Paleomagnetism; Paleolatitude
1. Introduction The sedimentary structures described here occur in tidal rhythmites of the Elatina Formation ‘Tel.: +61 (8)8303 5843; Fax: +61 (8)8303 4347.
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the principal formation of the late Neoproterozoic (-600 Ma) Marinoan glaciogenic succession in South Australia (Preiss, 1987) - at Pichi Richi Pass and Warren Gorge in the western Adelaide foldbelt (Fig. 1) where the formation is 100-150 m thick. The rhythmites at Pichi Richi Pass form
0037-0738/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PI1 SOO37-0738(96)00062-O
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Pichi Richi Pass (PRP) and Warren Gorge (WC) near the faultbounded western margin of the foldbelt. Major faults are shown as heavy lines. Adapted from Preiss (1987).
a lo-m-thick unit comprising lamina-cycles up to 2 cm in thickness, interpreted as fortnightly groupings of thin (52 mm), diurnal and semidiurnal laminae (Williams, 1989. 1991). The laminae usually fine upward from very fine-grained sandstone to siltstone, and successive lamina-cycles are separated by thin clayey bands deposited during neap tides. The planarity of laminae is locally disturbed by undulations and cross-lamination that record periodic unidirectional bottom currents, and by small-scale folds evidently of soft-sediment origin. At Warren Gorge, 25 km to the north, an 18-m-thick rhythmite unit of very fine- to fine-grained sandstone also displays lamina-cycles. The Elatina rhythmites were deposited from tidal plumes and density underhows on distal ebb-tidal deltas near the western margin of the gulf-like Adelaide ‘geosyncline’ (Williams,
Geology 106 (1996) 165-175
1989, 1991). Rare, poorly laminated interbeds of fine-grained sandstone up to 10 cm thick that have erosional lower contacts and are weakly graded evidently record episodic density flows and slumping on the delta slopes. The rhythmites at Pichi Richi Pass have provided geophysical information of wide significance. Palaeotidal data recorded by the rhythmites have illuminated the Earth’s palaeorotational history (Williams, 1989, 1990, 1991), and palaeomagnetic analyses of the rhythmites. together with data for the Elatina Formation elsewhere in the Adelaide foldbelt, have provided strong evidence for the low palaeolatitude of late Neoproterozoic glaciation (Embleton and Williams, 1986; Schmidt et al.. 1991; Schmidt and Williams, 1995). The small-scale folds in the rhythmites at Pichi Richi Pass permitted positive palaeomagnetic fold-tests at a high level of confidence, with the interpretation of the folds as soft-sediment, not tectonic, in origin being vital in confirming the early timing of magnetic remanence in the Elatina Formation (Sumner et al., 1987; Schmidt et al., 1991; Schmidt and Williams, 1995). Meert and Van der Voo (1994), however, without having seen the structures, queried their proposed soft-sediment origin, suggesting instead that they may be of tectonic origin related to the Delamerian Orogeny that formed the Adelaide foldbelt in Cambro-Ordovician time (Preiss. 1987). Firmly establishing the nature and origin of the small-scale folds in the Elatina rhythmites therefore will have important implications for the palaeomagnetic interpretation of the Marinoan glaciogenic succession in South Australia and, more widely, for late Neoproterozoic palaeogeography. 2. Deformational
structures
2.1. Cuspate ridges and folds Bed surfaces of the rhythmite unit at Warren Gorge display slightly sinuous, discontinuous cuspate ridges arranged in near-parallel sets (Fig. 2). Typically the ridges have wavelengths of 30-50 cm and locally as little as 13-20 cm, and heights of 3-5 cm measured from trough to crest. Sets exposed on successive bed surfaces have different orientations, with dip-corrected strikes of sets lying be-
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Fig. 2. Slightly sinuous, discontinuous cuspate ridges 30-50 cm apart on a bed surface of the Elatina Formation at Warren Gorge. The hollows between the ridges are covered by symmetrical ripple forms arranged approximately at right angles to the cuspate ridges. Hammer 33 cm long.
tween 110” and 170”. Nine successive sets have a mean dip-corrected strike of 149” (range 130-163”) and a mean stratigraphic spacing of 58 cm (1 l-190 cm). Most ridges are asymmetrical in cross-section: steeper flanks near ridge crests slope at 11-17” with respect to mean bed surfaces, compared with slopes of 7-12” for the opposite flanks. The steeper flanks typically face northeastward (dip-corrected). The ridges are underlain by fold structures (Fig. 3) spanning thicknesses of up to 50-60 cm and as many as 30 lamina-cycles representing more than one year’s deposition. The amplitude of the folds gradually increases upward from undeformed strata and the crests of some folds are erosionally truncated by overlying strata. Cuspate ridges on bed surfaces at Pichi Richi Pass are mostly smaller than those at Warren Gorge, with wavelengths of 14-40 cm and heights of up to 34 cm. The mean dip-corrected strike is about 115”
(range 62-128”). Some ridges have slightly steeper northern flanks and others are symmetrical in crosssection. As at Warren Gorge, the ridges are underlain by folded lamina-cycles spanning thicknesses of up to 30-40 cm and several tens of lamina-cycles. The trends of the cuspate ridges at both localities are unrelated to the meridional (strike 175-180”) tectonic fold trends in this area. Fig. 4 shows the detailed structure of the cuspate fold at Pichi Richi Pass that was studied palaeomagnetically by Schmidt et al. (1991). During deposition from ebb-tidal plumes (see Williams, 1989, 1991), delicate erosional truncations of laminae occurred at the fold crest and lenses of fine sand accumulated on the downcurrent flank of the fold. Such unidirectional current action during deposition of the three topmost lamina-cycles, which accumulated at the peak of the yearly cycle when ebb-tidal currents were strongest, caused the thinning of one cycle on
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Fig. 3. Transverse vertical section of a cuspate ridge at Warren Gorge, showing sharply folded tidal lamina-cycles neap-tidal bands spaced -1 cm apart. The crest of the fold is eroded and draped by lamina-cycles. Scale 15 cm.
the upcurrent side of the fold and the crest to migrate in the current direction. Other lamina-cycles usually are thinnest on the crest and thickest on the downcurrent side of the fold. although laminae typically persist across the fold apart from the modifications near the crest. The same erosional and depositional modifications of fold crests and flanks occur in other cuspate structures at Pichi Richi Pass. Micro-faults with variable displacements of 11 mm, identifiable as soft-sediment features by the flexing of some bands across irregular and impersistent lines of failure, occur on the flank of the fold shown in Fig. 4. Such soft-sediment micro-faults, and small-scale contemporaneous scours and erosional truncations of laminae, have also been illustrated by Williams ( 1991) in the rhythmites from Pichi Richi Pass and coeval rhythmites from Hallett Cove near Adelaide. 2.2. Ripple forms Sets of smaller ridges with the appearance of wave-generated symmetrical or oscillation ripple marks, and less commonly of interference ripple
marked
by darker,
marks, cover the bed surfaces between the cuspate ridges at Warren Gorge and occur between some cuspate ridges at Pichi Richi Pass. The symmetrical ripple forms (Fig. 2) usually are spaced 3-15 cm apart and have heights ranging from a few millimetres up to 1.5 cm. Bifurcations occur in places. Typically the symmetrical ripple forms are arranged at wide angles (75-90”) to the trend of cuspate ridges on the same bed surface and rarely extend over ridge crests, usually petering out and locally being deflected slightly near the crests (Figs. 5a and 7). Patterns of symmetrical ripple forms are dissimilar on opposite sides of a cuspate ridge. The interference patterns (Fig. 5b). which extend over some cuspate ridges, commonly show ‘triple junctions’ of crests that form depressions 10-20 cm across and as much as 1.5 cm deep. The ripple forms are underlain by folds, involving up to 20 cm thickness of strata, that commonly parallel the undulations of the bed surface (Fig. 6a) but in places have steepened limbs (Fig. 6c; Schmidt and Williams, 1995, their fig. 5) suggesting some later deformation. Locally, the bedding is analogous
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Geology 106 (1996) 165-175
169
Fig. 4. Thin-section (plane light) cut perpendicular to an ESE-WNW-trending cuspate fold in the Elatina rhythmites at Pichi Richi Pass. Scale bar 1 cm. The photograph spans nine lamina-cycles of graded laminae of fine-grained sandstone (pale) and siltstone; the dark, clayey bands (IV) bounding the cycles were deposited at neaps. During deposition by ebb-tidal currents, erosional truncation of laminae occurred at the fold crest and lenses of fine sand accumulated on the downcurrent (right-hand) side of the crest, causing the crest at the top of the photograph to migrate in the current direction. Lamina-cycles usually are thinnest on the crest and thickest on the downcurrent side, consistent with draping and vertical accretion in the presence of unidirectional currents. A soft-sediment micro-fault (arrows) with variable displacement of (1 mm occurs on a flank of the fold. The thin-section was cut from the upper part of a block sample that gave a positive palaeomagnetic fold-test (Schmidt et al., 1991).
supercritical cross-lamination (Fig. 6b; see Allen, 1982), which caused the crests of the ripple forms to migrate laterally during deposition of lamina-cycles. This type of lamination is common beneath the interference ripple forms, although the general lack of low-angle curved lamina intersections contrasts with hummocky and swaley cross-stratification (see Walker, 1982). In addition, delicate erosional truncations and wedge-outs of laminae, and thinning of lamina-cycles, occur on the crests of ripple forms (see Schmidt and Williams, 1995) and some ripple forms are erosionally truncated by overlying strata (Fig. 6~). Draped and truncated ripple structures also are shown by Williams (1991) from the rhythmites at Pichi Richi Pass. to
2.3. Associated
rill marks
A distinctive feature of the cuspate ridges and some symmetrical ripple forms at Warren Gorge is the presence of rill marks on several upper bedsurfaces (Figs. 5a and 7). These features are elongate, curved crenulations of sandstone about 1 mm in relief and as much as 29 cm long and 5 cm across; they are widest at ridge and ripple-form crests and taper, singly or in coalescing groups, toward the deepest parts of adjacent hollows in the bed surface. The rill marks occur only on the steeper, northeastern flanks of the cuspate ridges and on the northeastern sides of ripple forms, and their orientation is unrelated to the present vertical.
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Geology 106 (I 996) 165-l 75
Fig 5. B ed surfaces at Warren Gorge. (a) Symmetrical ripple forms locally deflected by, and terminating at, a cuspate ridge, Ripple fort ns G3” opposite sides of the ridge crest are not collinear. Rill marks extend to the right from the ridge crest, below and to the right of the 3-c m scale. (b) Interference ripple forms. Scale 15 cm.
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Geology 106 (1996) 165-175
Fig. 6. Transverse sections through ripple forms, Warren Gorge. (a) Twelve lamina-cycles (about six months’ deposition) displaying near-symmetrical folds above and below symmetrical ripple forms on adjacent bed surfaces. The thickness of the lamina-cycles does not vary appreciably across the folds. Scale 15 cm. (b) Eight lamina-cycles of variable thickness with the structure of supercritical cross-lamination which progressively displaced the crests of the ripple forms to the left during deposition. Scale 3 cm. (c) Folded lamina-cycles associated with ripple forms erosionally truncated by a sandstone bed. Scale 3 cm.
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Geology 106 (1996) 165-175
Fig. 7. Two parallel rill marks on a bed surface near the crest of a cuspate ridge at Warren Gorge. The rill marks extend down the Rlank of th e ridge between symmetrical ripple forms, which terminate just below the ridge crest. Scale 3 cm.
3. Interpretation of the deformational structures 3. I. Soft-sediment
origin
Much evidence indicates that the small-scale deformational structures in the Elatina Formation at Warren Gorge and Pichi Richi Pass are of softsediment origin and were modified in places by penecontemporaneous erosion and deposition. The erosional truncation of the crests of some cuspate ridges by overlying beds, the delicate erosion and deposition on crests and flanks, and the downward passage of related folds into undeformed strata demonstrate that the cuspate ridges were initiated in surficial sediments and were modified during overall vertical accretion by periodic unidirectional bottom currents. The cuspate fold shown in Fig. 4 exemplifies such modification: the structure formed a low ridge on the sea floor, and the resulting altered flow in the ebb-tidal currents carrying fine elastic material
in suspension caused delicate erosional truncations of laminae and thinning of lamina-cycles at the ridge crest, while flow separation over the crest led to the deposition of sand lenses and cross-laminae on the down-current side of the ridge. The internal structure of the cuspate ridges and their various orientations on different bedding surfaces thus militate against a tectonic origin. Detailed palaeomagnetic study of two different block samples of cuspate folds from the same site at Pichi Richi Pass (Schmidt et al., 1991; Schmidt and Williams, 1995), each showing erosional and depositional modification of fold crestal areas (Fig. 4), indicates that the samples examined acquired their magnetisation when the folds were only 33% formed (i.e., the tightest clustering and the least correlation between structure and magnetisation are attained at 66-68% unfolding). This finding implies that the cuspate folds formed in several stages: (1) the formation of low ridges on the sea floor; (2) continued
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near-uniform vertical accretion of the tidal sediments, thus maintaining the ridges on the sea floor except for the modifications to crestal areas and flanks discussed above; and (3) further soft-sediment deformation that substantially increased the amplitude of the folds. The occurrence of cross-lamination and erosional truncations at the crests of some ripple forms indicates that the latter features also formed on the sea floor and were modified by current action, The confinement of most ripple forms to the troughs between the cuspate ridges, and the termination and deflection of numerous ripple forms adjacent to ridge crests, indicate that the ripple forms developed after the ridges. Continued deposition maintained near-uniform thicknesses of laminae and lamina-cycles over the cuspate ridges and ripple forms apart from the local truncation, modification or migration of crestal areas caused by bottom currents. Such vertical accretion over bedforms has been recorded in other laminites: Donaldson and Munro (1982) observed the vertical accretion of laminites over rippled lenses in the Palaeoproterozoic Gowganda Formation in Ontario, and Leithold et al. (1989) noted the draping of distal tidal rhythmites over wave-rippled sandstone lenses in the Eocene Elkton Formation in Oregon. The rill marks on the surface of cuspate ridges and ripple forms may record the flow of dense or turbid water that escaped from ridge and ripple-form crests, or bottom currents. 3.2. Possible causes of the soft-sediment
deformation
The cuspate structures are interpreted as gravity slide deposits (see Allen, 1982; Myrow and Hiscott, 1991) that formed on tidal-delta slopes. The orientation and asymmetry of these structures suggest a regional northeastward palaeoslope, which is consistent with the envisaged deposition of the Elatina rhythmites on eastward-prograding ebb-tidal deltas (Williams, 1989, 1991). The fairly sharp flexures of some folds (Figs. 3 and 4), the presence of soft-sediment micro-faults, and the well-defined laminae normally showing minimal thickness variation imply that the sediments were in a hydroplastic state, and not liquidised, when deformed. The Elatina rhythmites were susceptible
173
to transformation from a solid-like to a much weakened state with the operation of a suitable trigger (see Allen, 1986): they have a dominant grain size in the lower part of the sand range and a paucity of clayey material apart from the thin bands deposited at neaps, and were rapidly deposited from ebb-tidal plumes and density underflows. Of the main triggers for such transformation of sediments (Allen, 1982, 1986), only cyclic stresses resulting from earthquakes and/or vigorous wave action seem applicable to the Elatina rhythmite palaeoenvironment. Seismites are widely recognised in the stratigraphic record (Allen, 1982, 1986; Mohindra and Bagati, 1996), with their expected stratigraphic frequency increasing exponentially toward major faults at the margins of sedimentary basins (Allen, 1986). In view of the proximity of Warren Gorge and Pichi Richi Pass to a major fault at the western margin of the Adelaide foldbelt (Fig. 1) that was active during Neoproterozoic deposition (Preiss, 1987), an earthquake trigger for the gravity slides should be considered. However, the Elatina rhythmites were deformed when in a hydroplastic state and show no evidence of liquidisation or such deformation structures as diapiric flame-like intrusions, recumbent folding, dish and ball-and-pillow structures, and sand dykes that commonly are associated with seismic events (see Mohindra and Bagati, 1996). The lack of such features in the rhythmites argues against an earthquake influence. The soft-sediment deformation structures in the Elatina rhythmites therefore may best be ascribed to wave action; the similarity of the ripple forms to wave-induced ripple marks is consistent with a strong influence of wave activity during rhythmite deposition. Stresses generated by wind waves, particularly of storm origin (see Allen, 1986), may have changed the surficial sediments to a hydroplastic state, triggering mass movement and the formation of the cuspate ridges by gravity sliding on delta slopes. The superimposed ripple forms are attributable to continued wave activity, and were maintained by draping and vertical accretion from sediment-laden unidirectional currents and locally by deposition of supercritical cross-lamination. The steepened fold limbs associated with some of the ripple forms also may have been caused by wave-generated stresses and fluid shear. The further deformation of the
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cuspate folds, as indicated by detailed palaeomagnetic analyses (see Sect. 4), suggests their additional down-slope sliding and/or the differential loading of a hydroplastic substrate in the troughs between the ridges during disturbance by wave action. It is noteworthy that the symmetrical ripple forms at Warren Gorge trend ENE-WSW on average (dipcorrected), which is perpendicular to the southsoutheasterly palaeowind direction determined from cross-strata1 attitudes for the coeval periglacialaeolian Whyalla Sandstone on the adjacent Stuart Shelf (Williams, 1994, 1996). The wide angles between cuspate ridges and associated symmetrical ripple forms, that are maintained over a range of ridge trends, suggests that the troughs between cuspate ridges also may have influenced the direction of oscillatory currents on the sea floor. The larger size and greater abundance of deformational structures at Warren Gorge may reflect a more proximal, shallower palaeoenvironment than that at Pichi Richi Pass. The late Neoproterozoic Chapel Island Formation in Newfoundland displays deformational and depositional features analogous to those described here. Myrow and Hiscott (1991) interpreted cuspate soft-sediment anticlines in thinly bedded sandstones and siltstones as a slide feature resulting from mass movement initiated by stresses generated by a major storm, and overlying hummocky cross-bedded sandstone as a storm-wave deposited sand. 4. Implications for late Neoproterozoic palaeogeography Independent palaeomagnetic fold-tests have been conducted on three different folds from the same site in the Elatina rhythmites at Pichi Richi Pass: a small-scale fold probably of cuspate type (Sumner et al., 1987), and two different cuspate folds (Schmidt et al., 1991; Schmidt and Williams, 1995) that show the same erosional and depositional modifications of crests and flanks. In each instance the authors ascribed the folds to soft-sediment deformation and the fold test was positive at a high level of confidence (95-99%). All authors concluded that the stable remanent magnetisation of the Elatina Formation at this site was acquired early. Indeed, the most detailed studies (Schmidt et al., 1991; Schmidt and Williams, 1995) indicated that the magnetisation
Geology 106 ( 1996) 165-I 75
was acquired during deposition (detrital remanent magnetisation) when the folds were only 33% developed. These fold tests were vital in demonstrating the early acquisition of the stable remanent magnetisation and shallow palaeomagnetic inclination characterising the Elatina Formation. As observed above, the attitude of the softsediment cuspate folds in the Elatina rhythmites differs from the meridional tectonic fold trends in the Pichi Richi Pass-Warren Gorge area. Indeed, all the soft-sediment folds selected for palaeomagnetic study trend ESE-WNW, because such an attitude is transverse to the mean palaeomagnetic declination of 191.9” for the Elatina rhythmites at Pichi Richi Pass (Embleton and Williams, 1986) and so is essential to permit conclusive fold-tests. It should be noted that the consistency of palaeomagnetic declinations determined for the soft-sediment folds, for associated rhythmites at Pichi Richi Pass, and also for other facies of the Elatina Formation with a wide range of structural attitudes elsewhere in the Adelaide foldbelt (Embleton and Williams, 1986; Schmidt et al., 1991; Schmidt and Williams, 1995), indicates that any rotation about a vertical axis possibly caused by soft-sediment or tectonic deformation of these rocks is unimportant. Confirmation of the soft-sediment origin of the cuspate folds, and hence the early acquisition of the stable remanent magnetisation and shallow palaeomagnetic inclination of the Elatina Formation, carries important implications for late Neoproterozoic palaeomagnetism and palaeogeography. Palaeomagnetic studies that concluded the Elatina rhythmites were deposited in low palaeolatitudes (Embleton and Williams, 1986; Schmidt et al., 1991) are upheld, and palaeomagnetic data for the complete Elatina Formation, which indicate a mean dip-corrected inclination of -5.3” and a palaeolatitude of 2.7 f 3.7” (Schmidt and Williams, 1995), provide the strongest evidence yet for the equatorial palaeolatitude of late Neoproterozoic glaciation. 5. Conclusions Small-scale folds are common in fine-grained tidal rhythmites of the late Neoproterozoic (-600 Ma), glaciogenic Elatina Formation in the Adelaide foldbelt. A soft-sediment origin for the folds and
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their contemporaneity with deposition are shown. Cuspate ridges and associated folds formed by the episodic transformation of surficial sediments on tidal deltas to a hydroplastic state, possibly through stresses caused by storm wave activity, followed by downslope sliding of the sediments. Continued wave activity during deposition produced ripple forms on the cuspate ridges, and may have contributed to further deformation of the cuspate folds as indicated by palaeomagnetic analyses. The bedforms and fold structures were maintained by draping and vertical accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. Confirmation of the soft-sediment origin of the cuspate folds carries important implications for the palaeomagnetic interpretation of the Marinoan glaciogenic succession in South Australia and for late Neoproterozoic palaeogeography. Positive palaeomagnetic fold tests previously conducted on these structures do indeed show that the low palaeomagnetic inclination of the Elatina Formation is primary, and hence confirm that the Marinoan glaciation occurred in equatorial palaeolatitudes. Acknowledgements I thank Phil Schmidt and cussions on the Neoproterozoic the Adelaide foldbelt, and the Council for the provision of a lowship. The paper benefited Van Loon.
Dave Clark for dispalaeomagnetism of Australian Research Senior Research Felfrom review by A.J.
References Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis, Vols. I and II. Elsevier, Amsterdam, 593 and 663 pp. Allen, J.R.L., 1986. Earthquake magnitude-frequency, epicentral distance, and soft-sediment deformation in sedimentary basins. Sediment. Geol., 46: 67-75. Donaldson, J.A. and Munro, I., 1982. Precambrian geology of the Cobalt area, northern Ontario. 1 lth Int. Congr. Sedimentology, Hamilton, Ont. Field Excursion Guide Book 16B, 72 pp. Embleton, B.J.J. and Williams, GE., 1986. Low palaeolatitude of deposition for late Precambrian periglacial varvites in South
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Australia: implications for palaeoclimatology. Earth Planet. Sci. Lett., 79: 419-430. Leithold, E.L., Bourgeois, J. and Kreisa, R.D., 1989. Genesis of tidal rhythmites of an Eocene delta slope, S.W. Oregon, U.S.A. 2nd Int. Symp. Clastic Tidal Deposits, Alberta, August, Abstr., p. 55. Meert, J.G. and Van der Voo, R., 1994. The Neoproterozoic (100&540 Ma) glacial intervals: No more snowball earth? Earth Planet. Sci. Lett., 123: l-13. Mohindra, R. and Bagati, T.N., 1996. Seismically induced softsediment deformation structures (seismites) around Sumdo in the lower Spiti valley (Tethys Himalaya). Sediment. Geol., 101: 69-83. Myrow, PM. and Hiscott, R.N., 1991. Shallow-water gravityflow deposits, Chapel Island Formation, southeast Newfoundland, Canada. Sedimentology, 38: 935-959. Preiss, W.V. (Compiler), 1987. The Adelaide Geosyncline. Geol. Surv. S. Aust. Bull. 53, 438 pp. Schmidt, PW. and Williams, G.E., 1995. The Neoproterozoic climatic paradox: Equatorial palaeolatitude for Marinoan glaciation near sea level in South Australia. Earth Planet. Sci. Len., 134: 107-124. Schmidt, P.W., Williams, G.E. and Embleton, B.J.J., 1991. Low palaeolatitude of Late Proterozoic glaciation: early timing of remanence in haematite of the Elatina Formation, South Australia. Earth Planet. Sci. Lett., 105: 355-367. Sumner, D.Y., Kirschvink, J.L. and Runnegar, B.N., 1987. Softsediment paleomagnetic field tests of late Precambrian glaciogenie sediments (abstr.). Eos, 68: 1251. Walker, R.G., 1982. Hummocky and swaley cross-stratification. In: R.G. Walker (Editor), Clastic Units of the Front Ranges, Foothills and Plains in the Area between Field, B.C. and Drumheller, Alberta. Int. Assoc. Sedimentol. Excursion 21A Guidebook, 160 pp. Williams, G.E., 1989. Late Precambrian tidal rhythmites in South Australia and the history of the Earth’s rotation. J. Geol. Sot. London, 146: 97-l 11. Williams, G.E., 1990. Tidal rhythmites: key to the history of the Earth’s rotation and the lunar orbit. J. Phys. Earth, 38: 475&9 1. Williams, G.E., 1991. Upper Proterozoic tidal rhythmites, South Australia: sedimentary features, deposition, and implications for the Earth’s paleorotation. In: D.G. Smith, G.E. Reinson, B.A. Zaitlin and R.A. Rahmani (Editors), Clastic Tidal Sedimentology. Can. Sot. Pet. Geol. Mem., 16: 161-178. Williams, G.E., 1994. The enigmatic late Proterozoic glacial climate: an Australian perspective. In: M. Deynoux, J.M.G. Miller, E.W. Domack, N. Eyles, I.J. Fairchild and G.M. Young (Editors), Earth’s Glacial Record. Cambridge University Press, Cambridge, pp. 146-164. Williams, G.E., 1996. Late Neoproterozoic periglacial-aeolian sand sheet in equatorial palaeolatitudes, Stuart Shelf, South Australia (unpublished).