Chapter 7 Examples from the Carboniferous

107 Chapter 7

EXAMPLES FROM THE CARBONIFEROUS

The second part of this book will deal with specific examples of cyclic sedimentation and their cyclostratigraphic potential. The aim is not to give a comprehensive survey, but rather to choose examples which illustrate various aspects of cyclostratigraphic problems. Of prime importance will be the various arguments that can be used to demonstrate that the sedimentary cycles are Milankovitch cycles. This is particularly difficult for cycles from older formations, where absolute timing is much less accurate. It is also of interest to include cycles from different environments and especially from different environments of the same age. Too little is known about prePleistocene times for an accurate reconstruction of climatic conditions to be made, but it is known that most of the examples to be discussed, occurred during periods without major glaciations and therefore the mechanism of climatic variation must have been different from that of the Pleistocene. It is also hoped that the examples can be of value to stratigraphers and sedimentologists and that perhaps a more systematic review of known Milankovitch cyclicity in different formations will eventually lead to some valuable results. THE LOWER CARBONIFEROUS

Although the Carboniferous has been for many years the classical period for the study of cyclic sedimentation, direct evidence for Milankovitch cyclicity is poor. The reason for this is clearly the lack of chronometric dates for the individual stages in which cycles have been investigated. This difficulty is common to all the older formations and unless several orders of Milankovitch cycles can be found, the identification of the cycles will always be uncertain. A special point of interest, however, in studying the Carboniferous examples, is the wide variety of different environments in which cycles have been observed. The examples from the Lower Carboniferous (Mississippian) in the northwest of Ireland represent a hemipelagic environment of more or less continuous sedimentation. In contrast to this, the contemporaneous Carboniferous limestone of northern England developed on shallow carbonate platforms, where sedimentation was frequently interrupted and the record is therefore incomplete. The well-known

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108

Pennsylvanian cyclothems of North America and possibly some of the European coal measures, represent a very wide spectrum of facies and clearly indicate that sea level fluctuations must have played an important part in cycle formation. THE CARBONIFEROUS LIMESTONES OF NORTH-WEST IRELAND

An extensive study of cyclic sedimentation was carried out in the Lower Carboniferous of north-west Ireland in the counties of Sligo and Leitrim (Schwarzacher, 1964). Examples from this area will be treated in considerable detail to demonstrate how petrographical analysis can be used to determine the cyclic changes, which may have been controlled ultimately by orbital variation. The Visean epoch of the West European Carboniferous has been divided into five stages from the Chadian to the Brigantian (George et al., 1976). Unfortunately, this system is not yet well enough established in the Irish exposures for precise dating. The local stratigraphic divisions which are based on lithologies, indicate a major transgression which is followed by a regression (Fig. 7-1). The basal sandstone that has been recognised as being partly deltaic and partly tidal, is followed by a marl which contains thin beds of limestone and which is called Benbulbin shale.

Ma

Harland et al 1989 332.9 Ma bl

Brigantian

15-

Asbian

10

-

Ramsbottom 1977

-

Holkerian

Shgo T N George eta1 1976

Sligo

N.Walas

-7-- - -

Cyciostrat. L-p-p-

I

D5a

j

-

\

middle

-

(GI Girvanella band

hndian

5-

Settle

D. septosa bands

-

Meenymore formatn Daltry Imst.

n Glencar imst.

:hadian

U

Benbulbin shale

0-

349.5 Ma bp

349.5 Ma bp

Fig. 7-1. The Lower Carboniferous stratigraphy of Irish and British examples.

Mullaghmore sit

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This is followed by approximately 100 m of Glencar limestone and 200 m of Dartry limestone. The sequence is overlain by the Meenymore Formation, which is a very shallow water deposit and supratidal in parts. The basal sandstone is thought to be late Arundian or early Holkerian and the Meenymore Formation is almost certainly Brigantian. The sediments between these two are thought to be largely Asbian in age and therefore the whole sequence represents a time interval of about 7 to 9 Ma. The palaeogeographical boundaries of the area were well-defined by structural north-eastern and south-western trends which determined the shape of the original basin. The Sligo syncline formed a small embayment which was probably closed to the north-east and open towards the south-west. The exclusively marine sequence from the Benbulbin shale to the Dartry limestone, shows from the bottom upwards a continuously decreasing marl content and an increasing carbonate content. The term marl is used loosely as a field term to describe a lithology which has approximately 30% insoluble residue. This value is an average and values as high as 50% have been found. A grouping of beds into stratification cycles is present throughout the sequence although it becomes very indistinct in some intervals. THE BENBULBIN SHALE AND GLENCAR LIMESTONE

The best stratification cycles are found in the middle Glencar limestone. Particularly in the north-east part of the basin, limestone beds occur in well-defined groups of frequently five well-developed beds, which are separated from the next group by a thick marl layer (Fig. 7-2). The limestone beds are between 10 cm to 15 cm thick and are separated from each other by 2 cm to 5 cm thick marls. Only the basal marl is considerably thicker, usually between 40 cm to 60 cm thick. The marls are often well laminated and rich in fossils, predominantly fenestelid bryozoans. Elongated skeletal particles and crinoid stems often show a preferred orientation that is due to currents and some laminae have been truncated by erosion, which possibly indicates the formation of shallow ripples. The limestone in contrast, contains very little internal bedding and only some indistinct layers of slightly coarser fossil material. In the thicker limestone beds there are thin zones with skeletal particles that are aligned roughly parallel and which subdivide the bed without forming clearly defined bedding planes. Such divisions will be called sub-bedding planes. There is no preferred fossil orientation between such horizons and skeletal particles occur in all positions and give the impression of a sediment that has been thoroughly mixed. Much of this is probably due to bioturbation and burrows of the type Diplocraterion are often recognisable. Such burrows originate usually at bedding planes or sub-bedding planes and so they are often filled with a slightly more dolomitic sediment. The interpretation which has been given to the formation of the limestone beds is the following. Each bed consists of between one to several layers which

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CYCLOSTRATIGRAPHYAND THE MILANKOVITCH THEORY

Fig. 7-2. The Middle Olencar limestone at lake Glencar, county Sligo, Ireland.

have been deposited very rapidly. The internal sub-bedding planes, which have an average spacing of about 5 cm, could be the remains of such layers. At possibly regular time intervals, the sedimentation conditions changed to reduced carbonate sedimentation and increased current activity. The supporting evidence for rapid sedimentation is found in selectively preserved areas, which contain groups of bryozoans and calcisponges which are covered with encrusting foraminifera in their original growth positions. The groups must have been covered rapidly to preserve them. A possible mechanism for such sedimentation is the occasional stirring up of carbonate muds, possibly by storms. If one examines the regional distribution of the marl and limestone thicknesses in a cycle, a very clear pattern emerges (Schwarzacher, 1968). For most of the cycles, the marl thicknesses increase towards the south-west but some cycles, which generally contain less marl, show a marl increase towards the south-east. The limestone thickness increases consistently towards the south-west which must be regarded as having been the more open part of the basin. It is likely but difficult to prove, that a certain part of the carbonate mud was brought into the Sligo bay from the more open part of the basin. The distribution of the marl thicknesses on the other hand, leaves no doubt that the clays are derived from the known land masses corresponding to the present day Donegal mountains in the north-west and the Precambrian Ox mountains in the south-east. Therefore the cyclicity of the Glencar

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limestone is to a large extent, a cyclicity in the supply of terrestrial material since it is impossible to explain the distribution pattern in any other way. It has been claimed that the stratification pattern of the Glencar limestone is of diagenetic origin (Walther, 1982) but such an interpretation bears no relation to any of the observable facts. Indeed the distribution of marl and limestones in this type of cycle is a fairly convincing demonstration that the sometimes proposed migration of carbonate from marls into limestones, could not have taken place. A decalcification of the 50 cm to 80 cm thick marls which is found at the base of the cycles, should clearly have released considerably more carbonate than the decalcification of the 2 cm to 5 cm thick marls that are between the limestone beds that are higher up in the cycle. However, both the amount of cementation and the amount of the insoluble residue of all the limestones is the same, irrespective of their relative positions to the basal marl. Furthermore, the carbonate content of most of the basal marls is only 40%, which is considerably lower than the carbonate content of the marls between the limestone beds, where it is usually around 60 to 70%. In most of the cycles, the marl layers decrease in thickness and the limestone beds increase in thickness in an upward direction. However, this apparent thickening of limestone beds towards the top of the cycle is partly caused by the fusing of two or more beds, a process which can be revealed by tracing individual beds laterally. The south-western exposures in the Sligo syncline have more beds per cycle than the area with higher marl sedimentation in the south-east. This is at first unexpected, as one normally assumes that a lower clay influx would encourage the fusion of beds. The most likely explanation of the phenomenon, however, is that the beds in the south-west exposures contain more sub-bedding planes as a result of rapid sedimentation steps. If the hypothesis that the steps represent storm deposits that are derived from the south-west is true, then it is possible that the layers became individual beds in this direction. The mechanism of bed formation raises an important question about the time that is involved in this process. It is very clear from the petrographic evidence, that a limestone bed in this sequence does not represent sediment which has steadily accumulated and which measures time by its continuous growth. On the other hand, with a few exceptions, one cannot classify the beds as event stratification, since they represent several events in most cases. This complex relationship between the thickness of beds and time, can be studied by computer simulation (Schwarzacher, 1975; 1976). lb obtain a first model, one makes the following assumptions. Sedimentation is discontinuous and sediment arrives in the form of discrete layers which for simplicity, we call storm layers. Such “storms” are randomly distributed in time but they occur with constant density. The amount of material that is deposited by each event is again random but its mean and variance can be estimated from actual measurements. It is further assumed that this process operates over a constant but unknown time interval, representing a bed. Based on these assumptions, distributions of bed thicknesses are calculated and a

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CYCLOSTRATIGRAPHY AND THE MILANKOVITCH THEORY

comparison with the observed distributions permits an approximate estimation of the storm densities to be made. In the second stage of the model experiment, the assumption of a constant time interval for each bed is relaxed and distributions for variable time intervals are calculated. The results of such calculations showed that the bed formation of the Glencar limestone beds can be explained by a few randomly distributed sedimentation bursts which occurred over time intervals which were variable and had a coefficient of variation of 50%. If one assumes that the beds represent the 21 ka precession period, then the time error (standard deviation) that is associated with such a unit is 10.5 ka, which is a very large error indeed. The estimate of this variation depends on the assumption that the sedimentation events were randomly distributed. This is very unlikely and the large variation of 50% must be regarded as being an upper limit. A very important factor that contributes to the uncertainty of estimating the time which is represented by a single bed, is the difficulty of defining “beds”. The problem has been discussed in Chapter 6 , where it was pointed out that the formation of a bedding plane often depends on the marl percentage reaching a critical level and that sometimes, several subsidiary bedding planes may develop near this level. This is particularly the case with the Glencar limestone, where each bed contains a number of sub-bedding planes caused by the stepwise sedimentation. Lateral correlation has shown that the cycles can be traced throughout the basin but this is not the case for individual bedding planes (see Fig. 7-3). As was shown in the previous chapter, it is only possible to predict a likely position in which a bedding plane will develop. The diagram that is shown in the previous chapter (Fig. 6-4) is constructed from identical cycles in the middle Glencar limestone which

Fig. 7-3. Selected correlated sections of the middle Glencar limestone.

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were measured in different outcrops. The cycles are reduced to unit thickness and the positions of the bedding planes relative to the cycle boundaries, are given as frequencies. Because of the strong regional variation, the marl-limestone couplets are treated as single beds and this accounts for the well-defined maxima near the base of the cycle. The diagram suggests that the average cycle can be divided into five fairly evenly spaced beds. However, there are always additional bedding planes which increase the number of beds per cycle to between seven and nine. The number depends on the locality and is higher in the south-west part of the syncline, as mentioned earlier. In most of the Glencar limestone cycles, the bed thickness increases towards the top of the cycle and the top limestone is followed immediately by the basal marl of the next cycle. Occasionally, however, the top limestone can be split into several thinner layers and the pronounced asymmetry of the cycles is lost, particularly in the lower and upper parts of the sequence and the cycle boundaries are more difficult to determine. It is unfortunately a feature of many sections, not only in the Carboniferous, that cyclicity is well-developed in some parts of a sequence but very indistinct in other parts and may even disappear completely. This makes the counting of cycles over long stratigraphic intervals very uncertain and the misinterpretation of cycle boundaries is the most likely source of error. A relatively simple method of obtaining a quantitative description of the sequence, is to determine the limestone percentages in equal intervals of the section. In the original work on the Sligo successions (Schwarzacher, 1965), limestone percentages were calculated for every 20 cm of the measured sections. A more direct method can be used by coding the two lithologies, limestone and marl, with plus and minus one. Such a coding can be done if necessary at every centimetre and is therefore a very accurate representation of the sequence. In fact, the coded series with positive limestones and negative marl resembles the original outcrop with its protruding limestones and receding marls. If the coding is performed at very close intervals, then the series may be too detailed for cycles to be recognised and in this case, a filter can be used to remove the very short fluctuations. Walsh filter methods are particularly useful for marl - limestone coded data and they produce results which can be much more easily interpreted than the original series. A composite section through the Benbulbin shale and Glencar limestone (Fig. 7-4) gives an example of a filtered sequence. In this case, the cut-off frequency for the filter was taken at 250 cycles and the 2048 data points were obtained by coding the section at 8.5 cm intervals. The filtered section clearly shows some stretches with very well-developed cyclicity. In others, cycles are quite difficult to recognise. Well-developed cycles are seen particularly clearly in the Glencar limestone at 30 m to 50 m. From 80 m upwards a distinct grouping of about four cycles into a higher-order cycle seems to develop. In attempting to count the cycles, one comes across two difficulties. In some stretches of the section, such as 10 m to 20 m, the cyclicity is hardly developed at all and

114

CYCLOSTRATIGRAPHY AND THE MlLANKOVlTCH THEORY

1

DO

E

1i o

150

130

137 metres

Fig. 7-4. Filtered bed thickness data of the Benbulbin shale and Glencar limestone sequence (County Sligo, Ireland).

in other parts, some fluctuations which look like a cycle which is split into two or more and so counting becomes ambiguous. With different interpretations of the cycles and their boundaries, one will find between 60 to 90 cycles in this part of the section. Estimates of cycle numbers can be improved in two ways. A n approach which has been found very useful, is to interpret " cycle boundaries" in the field, rather than from collected records. Such interpretations are often more reliable if it is possible to see large exposures, where the persistency of certain beds and cycle boundaries can be judged much better than from a single record. Similar results can be obtained by interpreting two or more measured parallel sections simultaneously. A second and more objective approach is to determine the number of cycles in a given interval by spectral analysis. This method, as has been discussed, cannot find cycle boundaries but gives a statistical resuit in the form of a preferred frequency, which can be interpreted as being the average cycle that describes the section. The power spectrum of the combined Benbulbin shale and Glencar limestone section is given in Fig. 7-5. The section is 136 m long and has a strong peak at a wavelength of 209.2 cm. This indicates a total of 65 cycles for the complete interval. Significant peaks are also found at 4533.3 cm, 146.2 cm and 138.7 cm. A further peak at 824.2 cm is not significant when tested against the AR1 model (see Chapter 5 ) but it is nevertheless very clearly defined and can be found in a similar position in sub sections. The ratios of the low frequency maxima divided by the predominant frequency maximum are 21.6 and 3.9 and for the high frequencies, one obtains 0.66 and 0.53. These ratios are very important in the interpretation of the Glencar limestone cycles, since a ratio of 1 to 4 and 1 to 20 could indicate that one is dealing with the 100 ka, 400 ka and 2 Ma eccentricity cycles. Under this hypothesis, the higher frequency peaks would correspond to cycles of 70 ka to 53 ka and there is no direct explanation for such a cyclicity.

EXAMPLES FROM THE CARBONIFEROUS

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01 E

.-

L

3

1

2

4

3 x

5 cycleslcm

6

7

8

Fig. 7-5. Power spectrum of the unfiltered Benbulbin shale and Glencar limestone section given in Fig. 7-4.

A study of Fig. 7-4 will show that the series cannot be regarded as being stationary and even without analysis, it can be seen that in the lower Glencar limestone cycles are developed more thickly than the cycles towards the middle and top of this sequence. To obtain further data for this change in Sedimentation rates, spectra1,analysis of short subsections were calculated and displayed as a contoured three-dimensional spectrum (see Fig. 7-6). The diagram is based on the spectra from a 20 m wide window which was moved in steps of 2 m along the section. The horizontal scale gives the wavelengths of the cycles and the vertical scale gives the stratigraphical position of the lower margin of the window. In reading this diagram, emphasis should be given to the occurrence of maxima since the general increase of power towards the low frequencies is largely due to the red noise which is part of every stratigraphic sequence. The diagram shows a very clear division into a lower (0-50 m), a middle (50-80 m) and an upper (80-120 m) Glencar limestone. There is a distinct shortening of the predominant cycle lengths in the middle Glencar limestone. This cycle seems to return to its original length in the higher part of the section. The noise and the shortness of the sections make it impossible to get any further information on the low frequency maxima. The resolution decreases with increasing length because of the non-stationary behaviour of the section. On the other hand, long sections are needed to resolve low frequency maxima. This is an unavoidable situation and it cannot be overcome by any more sophisticated method of analysis. Further progress could only be made by making definite assumptions about the sequence which, of necessity, are partly subjective. For example, tuning the section can be done by adjusting the cycle length to a constant thickness and in this way, compensating for changes in sedimentation

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CYCLOSTRATIGRAPHY AND THE MILANKOVITCH THEORY

Fig. 7-6. Contoured spectrum of the Benbulbin shale and Glencar limestone section. Note that the horizontal scale gives wavelengths and not frequencies.

rates. In a similar way, complex demodulation needs a predetermined frequency which is to be demodulated (Schwarzacher, 1989). Also one has to make a priori assumptions about the analytical nature of the series, for instance the hypothesis that it is composed of sine waves. THE DARTRY LIMESTONE

The marl layers in the Dartry limestone are reduced to thin films between the limestone beds and it is therefore impossible to measure marl percentages, as has been done for the lower part of the section. Since no other directly measurable variable was available, bed thicknesses were plotted at regular intervals. A useful series was obtained by recording the thicknesses at 20 cm intervals. The use of bed thickness for obtaining data on the cyclicity relies on there being systematic thickness changes for beds within a cycle. The Dartry limestone is similar to the Glencar limestone and often shows a thickening of the beds towards the top of a cycle. This should result in an asymmetric sawtooth like curve, when the

EXAMPLES FROM THE CARBONIFEROUS

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thicknesses are plotted over several cycles. Unfortunately, the bed thicknesses again vary a great deal and the cycle boundaries are therefore not always well defined. In the original work on the Dartry limestone (Schwarzacher, 1964), bed thickness values were filtered by a running average of five and this gave twenty to twenty-one cycles with an average thickness of 300 cm. Power spectral analysis of the same data gives three frequency maxima at 1236 cm, 309 cm and 115 cm. All three frequency maxima are significant at 90% if tested against an AR1 model and the strongest maximum at 309 cm would give 22 cycles in the 69 m long section. This result is clearly in good agreement with the earlier findings. The ratios of the main maximum to the other maxima are 4.00 and 0.372; assuming that the main maximum represents the 100 ka eccentricity cycle, then the low frequency maximum fits the 400 ka maximum and the higher frequency would indicate a cycle of 37 ka. The latter happens to be precisely the value which Berger et al. (1989) calculated for the 54 ka obliquity cycle in the Lower Carboniferous. Despite this coincidence, one cannot attach too much importance to such figures because once again, the Dartry limestone sequence is not quite stationary and the cycle periods are average values for the complete section. THE CYCLOSTRATIGRAPHICINTERPRETATION OF THE SLIGO SEQUENCE

Although the cyclicity of the Sligo sequence clearly falls into the general frequency band of Milankovitch cycles, it is nevertheless necessary to discuss critically, the astronomical origin of the cyclicity. Possibly the strongest argument for astronomical control is the persistence of the cycle pattern, despite the considerable change in lithology and facies in the sequence. The Benbulbin shale, which has only some 15 to 20% limestone, has the same repetitive pattern at a metre scale as the Dartry limestone which has more than 95% limestone beds. The repetition which is proved by significant spectra, indicates a persistent oscillating system which is quite independent of the local facies development and which therefore must be independent from locally changing conditions in the basin. This, together with the fact that the oscillations persist through several million years, excludes any local autocyclic origin. The actual timing of the Sligo cycles is less straightforward unfortunately. This is largely because of the uncertain stratigraphic dating of the sequence, together with the uncertainty of the Lower Carboniferous chronometric scale. The Harland et al. (1989) scale for the upper Dinantian is indicated in Fig. 7-1. Using this scale and the stratigraphic divisions suggested by George et al. (1976), the Glencar and Dartry limestones would fall into the Asbian and represent 3.4 Ma and the Benbulbin shale together with the basal sandstone, would be Holkerian in age and again represent 3.4 Ma. The cyclostratigraphic analysis of the Sligo sequence, excluding the basal sand-

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CYCLOSTRATIG RAP HY AND THE MI LAN KOV ITCH THEORY

stone, found between 80 to 100 well-developed cycles which, if it is assumed that one is dealing with the 100 ka eccentricity cycle, would represent 10 Ma. This is nearly twice the time suggested by the above scheme. In judging this discrepancy, one has to consider the uncertainty of the stratigraphic ages of the lower formations in particular. The classification of George et al. (1976) gives a Holkerian age for the basal Mullaghmore sandstone but Sevastopulo (1981) suggested an Arundian age which would come nearer to the cyclostratigraphicscheme that is based on the hypothesis of 100 ka cycles. The age of the top of the sequence is regarded as being more certain and is generally thought to be in the uppermost Asbian. It is succeeded by the shallow water facies of the Meenymore Formation which is assumed to be at the base of the Brigantian. If this age is accepted, then the Benbulbin shale would coincide with the base of the Arundian. It is also important to consider the uncertainty attached to the ages of the stages. These have been determined by Harland et al. (1989) by chron interpolation based on two tie-points: the base of the Chadian (349.5 Ma) and the top of the Brigantian (332.9 Ma). Chrons have been allocated in the following way: Chadian 2.0, Arundian 1.0, Holkerian 1.5, Asbian 1.5, and Brigantian 2.0. This is clearly arbitrary to some extent. Ramsbottom (1977) divided the Dinantian into eleven major cycles or mesothems. In this scheme, cycle D2a corresponds to the lower Chadian and cycle D6b to the top of the Brigantian. According to Harland’s scale (1989) the average length of such mesothems is 2.2 Ma. Figure 7-1 uses a chronometric scale, which is based on the assumption that the length of Ramsbottom’s mesothems is 2.0 Ma and it can be seen that this leads to a reasonable agreement with the Harland scale. If the Sligo data is plotted using this scale, a very clear picture emerges. The top of the Mullaghmore sandstone coincides with the top of the Arundian (D3). The Benbulbin shale and the lower Glencar limestone that consists of twenty 100 ka cycles are Holkerian. The middle Glencar limestone with again 20 cycles, is lower Asbian. The upper Glencar limestone is upper Asbian (D5b) and the Dartry limestone is lower Brigantian (D6b). The fact that one needs somewhat more time to accommodate the cycles, might suggest that one is dealing with shorter cycles, for example the obliquity induced cycle. However, even if one assumes that this latter cycle was not shorter in the past when compared with present day values, it is still too short and would lead to greater discrepancies with the chronometric scale. The hypothesis that the predominant cycle is caused by the 100 ka eccentricity variation is considerably strengthened by finding sub-maxima for both the 400 ka and 2 Ma periods which are known eccentricity cycles. The maximum found at 3.7 ka in the Dartry limestone could be an obliquity induced period but this is difficult to prove.

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THE CYCLICITY O F YORKSHIRE AND NORTH WALES

The repeated facies changes in the Lower Carboniferous of Scotland, England and Wales are due to the widespread transgressions and regressions which make up Ramsbottom’s major cycles and which are closely related to the stages of Carboniferous stratigraphy (George et al., 1976). The mesothems or major cycles can be subdivided by a number of minor cycles or cyclothems which may be used, at least locally, as stratigraphic markers. Cyclothems like this, have been known for a long time in northern England and also in Scotland where alternating sand, shale, and limestone sequences are known as the Yoredale facies and where individual members of the cyclothems have been used as stratigraphic markers. However, minor cycles that are equivalent to the Yoredale cyclothems, were also developed as platform limestones which were deposited on the shelves of Yorkshire, Derbyshire and North Wales. Shallow shelf areas surrounding the deeper basin, which was situated in the Craven lowlands, were only flooded during the later stage of the Dinantian and in Asbian and Brigantian times. Systematic studies of the cycles were carried out in the Settle district of Yorkshire (Schwarzacher, 1958) and in North Wales (Somerville, 1979). The Asbian cycles of the Great Scar limestone in Yorkshire, consist of massive, light grey limestones which are 9-15 m in thickness and these are separated from each other by a 50-100 cm thick layer, which can be slightly darker in colour and weathers more easily. The cycle boundary is determined by a sharp bedding plane. In most of the surface exposures, the bedding plane appears simply as a gap between the massive limestones but Waltham (1971) could demonstrate that a number of these cycle boundaries represent palaeokarst surfaces with traces of shale and in some cases, plant remains. The predominant lithologies of the limestones are biosparites and biomikrites. Skeletal particles often show intense micritization and in this way, they may become indistinguishable from the matrix. Counts of organic material in thin sections showed that micritization increases towards the cycle boundaries. The limestones are seen to be vaguely laminated towards the top of some of the cycles, showing current bedding and occasionally, a preferred orientation of crinoid stems and other elongated fossils. Each cyclothem contains a number of bedding planes which are less persistent than the cycle boundaries. Nevertheless, these bedding planes appear in the same position in different localities, where individual cycles can sometimes be recognised by the pattern that is formed by these bedding planes. The cyclothems of North Wales are slightly more differentiated. A typical cycle consists of calcareous marls which are followed by even or wavy bedded, dark grey biosparites, and then thickly bedded pale grey limestone. The latter makes up the main part of the cycle. Somerville (1979) found very clear evidence that each cycle represents a marine transgression, followed by an actual emersion. The criteria for an emersion are paleocarst surfaces, K-bentonites (representing palaeosols) and

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CYCLOSTRATIGRAPHY A N D THE MILANKOVITCH THEORY

laminated crusts that probably represent calcretes, which are typical of subaerial weathering. Similar paleocarst surfaces were also found in Derbyshire (Walkden, 1974). The Brigantian cyclothems of North Wales are generally more complex and the cycles that are between the emersion surfaces show evidence of internal cyclicity, which might suggest that such cycles can be subdivided further. In one example, a sequence of limestone, shale and coal is recorded and this approaches the type of cycle that is seen in the Yordale facies of northern England. There can be little doubt that the cycles of Yorkshire and North Wales represent time equivalent intervals, but a correlation between the two can only be tentative. In the Settle district, there is a distinct algal horizon, the Girvanella band, which is just above cycle nine and this is possibly the tenth or eleventh cycle from the base of the cyclic sequence. It is at the base of the second cycle in the Brigantian. On the other hand, if the cycles are counted upwards from the first well-developed cycle in both areas, then one finds fossiliferous bands containing the brachiopod Davidsonina septosa appearing at the same levels and there is also a certain agreement in the thicknesses of the cycles. For instance, cycle six is exceptionally thin in both areas. It is thought that each mesothem or major transgression brings new faunal elements and that this is the justification for a biostratigraphic classification. While this may be true, such a classification probably does not give a sufficiently clear resolution to make it possible to differentiate between the minor cycles. The wide distribution of the cyclothems, their unchanged persistence throughout Asbian as well as Brigantian times, together with the very convincing evidence for actual emersion, make eustatic sea level variations the most likely explanation for the cycle formation. Estimating the time interval that is represented by such cycles is more difficult because both the number of cycles and the time intervals are small. The development of cyclothems in the Settle district and North Wales is thought to have commenced during middle or late Asbian time. A similar development took place in Derbyshire, perhaps somewhat earlier. In Harland’s time scale (1989), the duration of the Asbian is given as 3.4 Ma and the subsequent Brigantian as 3.1 Ma. As already pointed out, this is based on “chrono interpolation” which must be arbitrary to some extent. There is also no real indication as to how much of the lower Asbian is missing in the cycle sequence. If it is assumed that eighteen cycles represent 5-6 Ma, then one obtains 277-333 ka per cycle, which is certainly not close to any Milankovitch frequency. The nearest period is the 400 ka eccentricity cycle but considering the uncertainty of the time scale, one might also consider the 100 ka period. In the latter case, one would have to assume a very large hiatus at the base of the limestones in Yorkshire which is about 73% of the Asbian, if one accepts the Harland time scale. If the 400 ka cycle is accepted, this is slightly longer than the Asbian and no space is left in the time scale for the supposed hiatus. Since it is impossible to estimate the time involved during the emersion of the cyclothems accurately, one can only estimate extreme

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minimum sedimentation rates of 10 cm/ka and 25 cm/ka, for the hypotheses of 100 ka and 400 ka cycles. Using the previously introduced chronometric scale, which is based on the assumption that Ramsbottom’s mesothems represent 2 Ma intervals and accepting the 400 ka cycle hypothesis, this would allow for a short hiatus at the base of the Asbian in Settle. It would not allow sufficient time to accommodate 115 m of Tynant limestone which, according to Somerville (1979), is thought to be of Lower Asbian age and which lies below the cyclic sequence which he described. Alternatively, if the 100 ka hypothesis is adopted, then the hiatus between the Holkerian and the Asbian would be excessively long. For this reason, the 400 ka cycle hypothesis is suggested to be the cause of the cyclothem formation and this solution is therefore shown in Fig. 7-1. The eight cyclothems which are recorded for the Brigantian, would allow sufficient space for the overlying sand passage beds in North Wales, as well as time for a short hiatus at the top of the Asbian. It should be noted that in all the areas where hiatuses have been stipulated, sedimentation is always conformable, apart from paleocarst surfaces, and nowhere is there evidence of the erosion of a complete cycle or part of a cycle. This may indicate that hiatuses may not have been as extensive as has sometimes been assumed. It is obvious that the tentative hypothesis of interpreting the Asbian and Brigantian cyclothems as 400 ka cycles, will need further proof and one particular approach could be to search for evidence of the 100 ka eccentricity cycle and perhaps even shorter cycles. Indications of shorter cycles within the cyclothems have been recorded in the Brigantian of North Wales and a number of Great Scar limestones show a distinct division into four approximately equal beds with a thickness of 1 to 2 m. If the 400 ka hypothesis is true, then this 100 ka signal is very much subdued when compared with the situation in Ireland. A fully complete stratigraphic record in the English examples can only be assumed at the 400 ka level, compared with the Irish examples which seem to be complete at the 100 ka level at least. At the same time, the English cyclothems, which look similar to the Irish stratification cycles, must serve as a warning not to judge simply by appearance. The regularity of the cyclothem thicknesses in the Yorkshire cycles for example, yielded an average thickness of 9.3 m with a coefficient of variation of 30% and this must be regarded as being a fairly large variation for an orbitally controlled cycle. As will be discussed in considerably more detail in the next chapter, the preservation of widespread transgression and regression cycles often requires a complimentary tectonic subsidence, otherwise the general sea level would have to rise indefinitely. Sinking movements are likely to be relatively continuous but regional differences will occur and even eustatically controlled cycles will therefore show regional deviations. The Lower Carboniferous can be interpreted in terms of Milankovitch cyclicity, as has been attempted in the previous section, and then one may reasonably

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ask whether a similar interpretation is possible for the remaining part of the system. No discussion of this problem in any detail is intended but some of the difficulties that are involved can be discussed in very general terms. It was seen in both the Irish as well as the English-Welsh example, that in order to construct a cyclostratigraphic scheme, it was necessary to alter the intervals in the current chronometric scale. In the English-Welsh examples, it was additionally necessary to ignore some biostratigraphic evidence, which suggests that the cyclic sequence should be restricted to the upper part of the Asbian. There are no real discrepancies with the biostratigraphic evidence in Ireland, largely one suspects, because the biostratigraphy is less firmly established there. The case for Milankovitch cyclicity in the Irish examples is considerably better than for the English-Welsh examples, simply because a much larger number of cycles is observable in the Irish ones and because there is reasonable evidence for at least three orders of frequency which approximately fit the 100 ka, 400 ka and 2 Ma cycles of eccentricity. The argument in favour of Milankovitch cyclicity in the shallow water deposits of England and North Wales relies partly on the knowledge of the probably orbitally controlled cyclicity in the deep Irish basins, at the same time and with the approximate timing which can be obtained by assuming that the Asbian and Brigantian stages have a length of 2 Ma. It is tempting to associate this 2 Ma interval, which is in fact quite close to the average duration of a large number of the Carboniferous stages, with the long eccentricity period. The twenty five stages of the Carboniferous chronostratic scale (Harland, 1989) have an average length of 2.92 Ma. The seven stages of the Namurian have an average length of 2.08 Ma. Although many of the stages are directly determined by a transgression-regression cycle, others contain more than one (Ramsbottom, 1977). It is clear that there are at least two good reasons why the number of cycles that is recorded or mapped in a given interval can be wrong. If a land surface cannot be reached by a transgression because of its vertical position, no sediment is deposited. Alternatively, the evidence of a transgressive cycle could have been removed subsequently by erosion. Of course, additional errors can also be introduced by misinterpretation. THE PENNSYLVANIAN (UPPER CARBONIFEROUS) CYCLES

The Pennsylvanian (Upper Carboniferous) cyclicity of the central parts of North America have been regarded for many years as one of the prime examples of cyclic sedimentation. In the Kansas cyclothems for example (Moore, 1936), the sequence consists of a basal shale which sometimes contains coal fragments, followed by marine limestone and black shale, which may again be followed by a limestone. There is no doubt that each cyclothem represents a transgression and it is now thought that the black shale represents the maximum depth and the top limestone represents a regressive phase (Heckel, 1986). A problem which was discussed

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extensively in the 1940’s concerned the origin of such transgessions, which were almost exclusively attributed to tectonic movements that were more or less local (Weller, 1930). However, Wanless and Shephard proposed as early as 1936 that the cyclothems resulted from glacio-eustatic sea level fluctuations and that the cause of the transgressions and regressions was climatic and not tectonic. This idea was very much in advance of its time and was difficult to accept because it suggested the occurrence of at east fifty glacial and interglacial stages, when it was still believed that the Pleistocene was represented by only four glaciations. World-wide sea level fluctuations are now generally accepted and it is most likely that these are controlled by the Gondwana glaciation, which was a major glaciation. Whether the cyclicity can be attributed to Milankovitch cyclicity, is more difficult to prove and relies on two lines of evidence. The most direct way is to establish the length of the cycles by estimating their absolute time duration. The second argument relies on the frequency structure of the cycles. It was noted by R.C. Moore (1950) that the cyclothems of the Upper Pennsylvanian in Kansas show a regular grouping, which is repeated several times. Between major transgressions which are characterised by the appearance of black shales, three to four limestoneshale cycles are found that represent minor transgressions. Moore called such groups megacyclothems. Heckel (1986, 1990) attempted to construct a sea level curve by tracing the extent of the transgressions and regressions and he classified the marine cycles into three categories. The major cycles are cycles in which the transgression proceeded far enough on to the shelf to form conodont-rich shales at the northern limit of the outcrops in Iowa. Minor cycles typically lack conodont-rich shales and only cover the lower shelf. Intermediate cycles are transgressions which only reach the lower shelves of Iowa. Heckel’s major cycles correspond more or less to Moore’s megacyclothems. In Heckel’s sea level curve, eighteen major cycles were found in the interval from the top of the Cherokee to the base of the Wabaunse group and a total of 51 cycles was found altogether. Interpreting from Harland’s 1986 time scale, gives a time interval of 9.5 Ma. This gives a duration of 527 ka for the major cycles and 186 ka for the minor cycles. Using a somewhat arbitrary method of estimation, Heckel obtained values of between 235 ka and 393 ka for the major cycles and values of 44 ka to 118 ka for the minor cycles. Using yet another time scale, de V. Klein (1990) obtained values of 24 ka to 64 ka for the minor cycles and 129 ka to 216 ka for the major cycles. However, it is likely that these values are too short. These discrepancies clearly indicate the uncertainty which is involved in estimating the length of the Pennsylvanian cycles. Although Heckel’s results and the above estimate (which is based on Harland’s 1989 time scale) seem to indicate that the major cycles could be related to the 400 ka eccentricity cycle, estimates for the minor cycles do not confirm a frequency structure of one to four, which is what one would expect if both eccentricity cycles were present. In conclusion, one cannot produce any positive proof for Milankovitch cyclicity in the Pennsylvanian system of North America except that the ’cycles are of the

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order of orbital cycles and that no satisfactory alternative explanation has yet been given. It is perhaps of interest to note that both the Carboniferous limestones of England and the Pennsylvanian cyclothems seem to suggest periodicities which are around 400 ka, whereas the shorter eccentricity cycles are much less obvious.