Australia: The Land Where Time Began
Elatina Formation - Cyclicity: Tidal or Solar Signature
According to the author1 new observations from 2 sedimentary rhythmites in South Australia dating to about 800-650 Ma, Reynella Siltstone and Chambers Bluff Tillite, have shown cycles that are structurally similar to the about 12-laminae cycles of the Elatina Formation, about 650 Ma, that is comprised of 1-26, or more, laminae. Models for the Elatina Formation regard the laminae of the formation as annular, and the cyclicity has been ascribed to solar variability (Williams & Sonett, 1985), or to the influence of the sunspot cycle combined with the cyclicity of the lunar nodal tide (Zahnle & Walker, 1987). It has been found to be difficult to accommodate the new data in the models for the Elatina Formation because of the discrepancy in the number of laminae. The author1 suggests that if the long-term 'Elatina Cycle', rather than the individual lamina, is taken as a yearly climatic signal, the Elatina, as well as other rhythmites, that have been studied may be interpreted as deposits formed in marine ebb-tide deltas recording variability of velocity and range of palaeo ebb-tides, the basic laminae-cycles representing lunar fortnightly cycles of diurnal-and/or semidiurnal laminae, these commonly being truncated as a result of deposition not taking place at neap tides. All of the 3 rhythmites that have been studied can be accounted for plausibly by the ebb-tidal model, which is therefore preferred, though there are many empirical similarities between the Elatina and sunspot series.
During the Marinoan Glaciation in the late Precambrian, about 650 Ma, sandstones and siltstones that comprise the Elatina Formation were widely deposited in the gulf-like Adelaide Geosyncline [Adelaide Rift Complex] in South Australia. According to the author1 at Pichie Richi Pass in the Flinders Ranges a laminated unit about 10 m thick is of interest to astrophysicists and palaeoclimatologists because of the complexity of periodic signals that are encoded by the variations in thickness of the delicate laminae (Williams, 1981, 1985). A number of interpretations of the cyclicity have been proposed, though the signals are apparently clear. It has been proposed that the periods recorded in the Elatina Formation are proxy for the solar cyclicity, based on the assumption that the laminae are each increments deposited in a single year (Williams, 1981, 1985; Williams & Sonett, 1985). It has been argued that the Elatina periods reflect a combination of solar and tidal cycles, though the laminae are accepted as having a yearly origin (Zahnle & Walker, 1987). As an alternative it has been pointed out that if instead of the individual laminae representing a yearly signal the long-term 'Elatina Cycle' displayed by the Elatina Series is taken to be a yearly signal, the Elatina periods may be interpreted as palaeotidal rhythms.
The author1 suggests the interpretation accepted will depend heavily on the time values attributed to the constituent periods. Periglacial structures within a penecontemporaneous fossil permafrost horizon northwest of Pichi Richi Pass (Williams & Tonkin, 1985; Williams, 1986) is good evidence of a periglacial climate that is strongly seasonal during the deposition of the Elatina Formation, accordingly, a regular sedimentary feature or repetitive signal in the sequence should be sought that may represent a seasonal signature.
The author1 suggests an evident interpretation, give that the setting is periglacial, is that the laminae represent varves, annual increments, recording the non-random discharge of meltwater into a periglacial lake that is deep (Williams, 1981, 1985). The laminae resemble 'intermediate distal facies' in the classification of (Smith, 1987). According to this interpretation the the thickest laminae would reflect warm summers that result in a large runoff of meltwater that would carry a lot of sediment to the lake. The thinnest laminae would then represent cold times when there is little runoff of meltwater. The Elatina record may be read as an interweaving of climatic cycles.
The hypothesis that the periodicities observed in the Elatina Formation, that is based on the varve interpretation, ultimately reflect variability of solar activity (Williams, 1981, 1985; Williams & Sonett, 1985). According to the author1 periods in the Elatina Formation are 12 ± 4, 22-about 25, and about 103 varve years and these may be equated with a sunspot cycle record comprising a basic cycle of about 11± 3 years, the Hale magnetic cycle of about 22 years, and a longer period of 90-110 years. Basic cycles of variable length are the basis of the Elatina and sunspot series, though there are commonly alternative cycles of relatively high and low amplitude. Positive skewness is shown by the individual cycles, the high cycles tending to have high crests and the low cycles broad crests, strong cycles tending to be of shirt duration.
When laminae-cycle thickness is plotted, comparing it with that of its equivalent in the series of sunspots, the sum of yearly sunspot number for each sunspot cycle, more similarities are observed. The sunspot record has been kept since 1830 and when cross-correlations are run it shows that the sunspot record from 1830 to the present is comparable to the portion of the Elatina Cycle that includes the first- and second-order peaks and the second-order trough that intervenes the 2 peaks. The author1 suggests that the sunspot peak of 1957 may be equated with the first-order peak observed in the Elatina Cycle, with sawtooth patterns being displayed by both series, that reflected the high and low cycles, that are draped over the long-term oscillations, with spikes similarly situated in both saw-tooth patterns. According to the author1 it appears the sunspot sawtooth pattern forms part of an envelope that is now close to pinching out. Correlations coefficients 0.93-0.98 are shown by cross-correlations and respective elements in the Elatina and the sunspot curves.
There are many similarities observed between the Elatina and sunspot records that might be the result of a direct connection via a variability climatic temperature variability, between the 'varve' thickness from the Late Precambrian and solar activity. This suggests that solar activity increase may have caused a corresponding increase in climatic temperature, resulting in a greater annual runoff of meltwater and the deposition of thicker varves. In Skilak Lake, Alaska, a study of recent varves that were deposited in the lake has suggested that a very weak solar signal has been recorded by these varves (Sonett & Williams, 1985).
Several new ways of looking at the sunspot record have resulted from the hypothesis that proxy solar data has been encoded in the Elatina series. The total yearly sunspot numbers represents the sunspot-cycle magnitude, and this clearly shows the long-term cycle magnitude modulation as well as the sawtooth pattern with the pinching out of the envelope and phase reversal that is possibly imminent. New mathematical approaches have been developed for viewing sunspot series structure, as well as the shape and duration of individual sunspot cycles (Bracewell, 1985, 1986) have been stimulated by the data recovered from the Elatina Formation. According to the author1 an excellent simulation of the sunspot record has been developed by Bracewell (1986) as a result of the Elatina data and the prediction that a peak of about 125 annual sunspot number will be attained in about 1991.
In the Elatina cycle, if the first-order peak is equated with the sunspot maximum of 1957 several more predictions become possible (Williams, 1985). An overall decline in the amplitude of sunspot cycles to a deep trough over the next 6-8 cycles, sunspot activity being lower than at any time in the preceding 300-350 years by the mid-21st century. Also, in the solar sawtooth pattern a phase reversal may be imminent. The author1 suggests that such a prediction, that is possibly relevant to the prognosis of the climate, would be the ultimate test of the climate-solar hypothesis.
The solar interpretation also presents some difficulties, even though there are strong empirical correlations between the Elatina series and the sunspot series. It has yet to be demonstrated that there is a mechanism by which solar activity might have a strong influence on the climate of the Late Precambrian. About 650 Ma the mean solar cycle is suggested by stellar models to have probably been 3-10% shorter than at present (Noyes et al., 1984), though the mean cycle is suggested by the Elatina data to have been about 8% longer in the Late Precambrian, about 650 Ma. This apparent discrepancy has been stressed by (Zahnle & Walker, 1987), their argument was that all the Elatina series periodicities reflect a basic solar cycle of 10.8 years and its beating with a presumed lunar nodal cycle of 20.3 years, though these fundamental periods may both be regarded as tenuous. Systematic overcounts of the laminae by Williams & Sonett (1985) are required for a period of 10.8 years. The period of 20.3 years resulted from interpolation of a speculative datum at 2.5 Ga (Walker & Zahnle, 1986) and palaeontological data from the Phanerozoic.
Late Precambrian, South Australia - New data
Critical assessment of these different interpretations of the Elatina series have been made possible by new observations of other rhythmites of the same Late Precambrian age. One of these other sites is the Reynella Siltstone, a correlative of the Elatina Formation, the site containing a unit that is several tens of metre thick, being exposed about 300 km to the south, at Hallett Cove near Adelaide (Preiss, 1987). Laminae of fine sandstone and siltstone, that are varve-like, consisting of laminae-cycles similar to those in the Elatina Formation, though in places they are up to 6 times thicker. These laminae-cycles, that are much thicker, that were discovered by the author1 allow the determination of the detailed structure of individual cycles with a degree of accuracy that had not been possible previously. An important factor of these thicker laminae, which consistently give counts of 14-15 laminae per cycle, is that they are bounded by thin laminae, less than 0.4 mm thick, that are still resolvable by photographic enlargement. At Pichi Richi Pass the maximum count has been found to be 16 laminae per cycle accords well with the 14-15 laminae/cycle, though it is notably higher than the mean count of 12 laminae/cycle for that locality. In the Reynella Siltstone, in most cycles lamina often comprise 2 'semi-laminae', in places the semi-laminae in each pair are found to be about the same thickness, the cycles appearing to contain up to 26 laminae, or possibly more.
In the Chambers Bluff Tillite, northern South Australia, of Late Precambrian age, fine sandstones and clayey siltstones, believed to possibly be of glacimarine origin, which are thinly laminated, about 0.1-1.5 mm thick, display conspicuous cyclicity. It was assigned to the Sturtian Glaciation with a date of about 800 Ma (Preiss, 1987), though the stratigraphic position of this formation within the Precambrian is uncertain. In the Chambers Bluff Tillite, the Elatina Formation and the Reynella Siltstone the laminae cycles all share the same basic structure, notably, within the cycles an overall regular change in the thickness of the laminae and a pattern in which there is alternation of thick and thin cycles with phase reversals. The cycles have been modified by syn-sedimentary scouring, particularly affecting the very thin laminae at the tops of thin cycles. According to the author1 on large photos along a line, offset in places to minimise the scouring effects, a sequence of 99 laminae was measured. In the sequence of 9 cycles the laminae total was15-25/cycle, the overall mean value being 20 (± 3.2 s.d.). The author1 suggests this mean must be regarded as a minimum estimate, as some laminae have probably been removed by scouring. A better minimum estimate of the cycle mean is the value 22.4, the mean count of the 5 alternate cycles that are relatively thick, The thick laminae of the thick cycles being more easily resolved and likely to have been affected to a lesser extent by scouring.
The laminae cycles of the Chambers Bluff Tillite, the Reynella Siltstone and the Elatina Formation appear to have the same ultimate origin. In the Chambers Bluff Tillite there are up to 25 laminae/cycle, as well as a tendency for thick and thin laminae to alternate suggesting to the author1 an affinity with those cycles in the Reynella Siltstone that are comprised of 26 more more semi-laminae. In the Chambers Bluff Tillite the lower number of laminae per cycle is attributable, according to the author1 to contemporaneous scouring. In the Elatina Formation a lamina may be equated with a pair of semi-laminae in the Reynella Siltstone and with 2 laminae in the Chambers Bluff Tillite. In the Reynella Siltstone and the Chambers Bluff Tillite the large number of laminae or semi-laminae in cycles militate against the claim that a bias in laminae counting has resulted in an overestimate of counts for the laminae cycles in the Elatina Formation (Zahnle & Walker, 1987). The author1 questions whether at Pichi Richi Pass the laminae cycles are commonly truncated as the result of non-deposition of discrete laminae at the 'dark band' boundaries between cycles. The author1 suggests that as a result of these new findings a re-appraisal of the Elatina Formation is warranted.
Elatina Series - A tidal model
There are no known present-day counterparts of the rhythmites of the Elatina Formation, Reynella Siltstone or Chambers Bluff Tillite, such sedimentary cycles possibly being attributed to deposition on distal ebb-tidal deltas. Sediment, mainly in suspension, is carried offshore by ebb-tidal jets, according to this model (Ozsoy, 1986), settling on an ebb-tidal delta forming a deposit of graded strata. The velocity of the ebb tide and tidal range would determine the effectiveness of the tide as an agent of such deposition (see Fitzgerald & Nummedal, 1983; Boothroyd, 1985). As a result of this there would normally be an association between relatively thick clastic strata and strong ebb-tidal currents and high tidal ranges, as occur in the spring phase of the tidal cycle. During slack water between tides thin clayey caps might form on sandy strata. On the ebb-tidal delta deposition pauses of the sandy strata might occur only for low, neap-tidal ranges, much of the fine material being allowed to settle and distinct clayey bands to be deposited between fortnightly 'bundles' of strata. The tidal rhythms recorded in the Elatina Formation might be overprinted by a seasonal storminess annual signal or rapid turbid meltwaters from the bordering permafrost terrain, on the ebb-tidal delta, peaks of thicker strata might record temporary, seasonal abundances of sandy to silty material in suspension. In the rhythmites from the Late Precambrian average rates of deposition that are lower than approximately 0.5 m/year are comparable to those of fine-grained clastic deposits of the present (van den Berg et al., 1983).
According to the author1 when this tidal sedimentary model is applied to the Elatina series the question of the time-values that should be ascribed to the various cycles and sedimentary structures arises again. The author1 suggests that because of the prevailing periglacial climate, that was strongly seasonal, in the short summer it is likely there would have been maximum turbidity of near-shore marine waters, suggesting a regular pulse of sediments should be sought, other than the individual laminae, that could have recorded the peaks of rapid deposition during summer. The first- order peak of the Elatina Cycle stands out, according to these criteria, of all the oscillations it is the most regular, a strong, regular spectral peak being revealed by Fourier transform (Williams, 1985; Williams & Sonett, 1985), that was associated invariably with the thickest laminae, and therefore the most rapid deposition.
Periodicities of the Elatina series can readily be equated with tidal parameters, assuming the first-order peak is regarded as a yearly signal:
A year in the late Precambrian would be represented by 26.2 (± .09) laminae-cycles, the mean period of the Elatina Cycle, the lunar fortnightly cycle during the Late Precambrian being represented by the laminae cycle.
In the Elatina Formation and the Reynella Siltstone diurnal increments would be represented by the graded laminae, and in the Reynella Siltstone and Chambers Bluff Tillite semidiurnal increments would be represented by the semi-laminae of the Reynella Siltstone and the laminae of the Chambers Bluff Tillite. The characteristic tidal patterns for the 3 formations therefore being diurnal, mixed and semi-diurnal, respectively. A number of features that have been plotted for lamina thickness of the Elatina series (Williams, 1985; Williams & Sonett, 1985), including alternate cycles of relatively high and low amplitude, the tendency for cycles to be positively skewed, sharp crests of high cycles and broad crests of low cycles, and the tendency of strong cycles to be of short duration, have all been observed taking place at Townsville, Queensland, Australia, in the daily pattern of fortnightly tidal cycles. According to the author1 tidal heights are a relative measure of tidal range, the best comparison between tidal cycles of the present and the rhythms recorded in ebb-tidal deposits is provided by plots of them. In the Reynella Siltstone certain laminae cycles have a structure similar to the growth patterns seen in the shells of present-day bivalves living in a mixed fortnightly tidal environment (Evans, 1972).
If it assumed that a maximum value of about 16 tides per fortnightly tidal cycle in the Late Precambrian - equivalent to the laminae maximum count in a laminae cycle of the Elatina Formation - which indicates, according to the author1, in certain laminae cycles up to 50 % of laminae are missing as a result of non-deposition at low tidal ranges. The 'dark bands', composed of fine clayey material, that bounds the laminae cycles, must mark horizons where clastic deposition has been arrested. When fortnightly bundles of laminae that have been truncated it would have occurred at a particular tidal range threshold, thereby explaining the general similarity of lamina thickness at minima between laminae cycles.
A pair of laminae cycles of high and low amplitude characterise the laminae-cycle 'doublet', which would reflect the alternation of high and low spring tides, a doublet therefore representing the lunar monthly tidal cycle. In the Late Precambrian there would have been 13.1 ± 0.5 lunar months in a year. The 'sawtooth envelopes' pattern that is punctuated by phase reversals that is shown by a plot of the laminae-cycle thickness is similar to that shown by high and low spring tides at Townsville. As exemplified by 20 years of tidal data for Townsville, from January 1966 to December 1995, average 14.0 (s.d. = 1.4, range = 12-18) spring tides between phase reversals, compared with the mean value of 14.6 (s.d. = 2.2, range - 9-23) for the Late Precambrian. The similarities between features of the Elatina Cycles and annual tidal curves from Townsville, and respective saw-toothed patterns, are evident.
A period of 13.1 ± 0.5 fortnightly cycles that is the 2nd harmonic of the Elatina Cycle would represent a half-yearly tidal oscillation. Neap-tide pauses in the deposition of clastic laminae are modulated by this oscillation, thereby also modulating the number of preserved laminae in laminae- cycles (see Williams, 10985; Sonett & Williams, 1987).
It has been postulated that in the Elatina Cycle the first-order peak is a yearly signal, that is non-tidal, which allows the interpretation of the Elatina series as an ebb-tidal sequence that records a number of tidal rhythms. The author1 says the full spectrum of palaeotidal periods that have been recorded will be discussed elsewhere.
Empirical comparisons of different time series are the basis for the cyclicity of the Elatina Formation in both interpretations, the ebb-tidal and the solar, and testing that is independent is required for such hypotheses. The prediction of sunspot activity is the basis for the ultimate test of the solar hypothesis. Tidal delta deposits formed by ebb-tidal jets at the present should preserve tidal signals in graded strata according to the tidal hypothesis. The author1 supports an ebb-tidal deposition model, based on the evidence available to him, as it can explain the Elatina cyclicity and also other more recent observations of rhythmites at other sites in South Australia, despite the varve-like features of the laminated rocks that have been studied and the strong correlations between the Elatina and sunspot series. Also, the ebb-tidal model is the only one, at the time the article was written, that fits with rhythmic processes that operate strongly on the surface of the Earth at the Present time.
In the Elatina Cycle, the yearly first-order peak is recognised as climatic a signal that is significant, according to the tidal interpretation, as the author1 suggests it may illuminate the structure of the climatic year in South Australia in the Late Precambrian.
According to the author1, overall, the South Australian rhythmites from the Late Precambrian provide a set of data that is unique, possibly being the first firm evidence of palaeotidal values and palaeorotation in the Precambrian. The data are consistent with findings of palaeorotation from the Phanerozoic that was derived from growth increments of fossils if an average equivalent phase lag of about 3o is used since the Late Precambrian instead of the values of the present of 6o (see Lambeck, 1980). The author1 says the possible implications of the data from the Late Precambrian for the palaeorotation of the Earth will be addressed in a separate study.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|