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ABSTRACT

Cycles recognised in the gamma-ray log of the Silurian Qalibah Formation have been time-series analysed. The periodogram lengths from the analysis have ratios coincident with the ratios of the Silurian precession, obliquity and eccentricity frequencies of the Milankovitch Band orbital periodicities. Most periodogram lengths are harmonics. The periodogram lengths for P1, P2, O1, O2 and E1 are 2.41, 2.82, 4.55, 5.47 and 15.62 feet respectively, for the Sharawra Member of the Qalibah Formation. These lengths can be used to estimate sedimentation rates and time durations. The organic matter within these cyclic sediments has been quantitatively studied in an attempt to relate the orbital signal to environmental change. The primary change within the organic matter appears to be increased preservation through increased organic productivity. These changes relate to increased gamma-ray log API and therefore cyclicity.

INTRODUCTION

Arguably one of the most significant recent advances in our understanding of the controls on sedimentation has been the recognition that climatic cyclicity within the Milankovitch Band occurs widely within the stratigraphic record (e.g. Fischer, 1986; Schwarzacher, 1993; House and Gale, 1995). The identification of this cyclicity through the proxy of carbon stable isotope variation within foraminifer tests has already become routine in the application of a high-resolution sub-division and time correlation for Neogene successions (Shackleton et al., 1995). It is now generally accepted that Milankovitch cyclicity provides a new stratigraphic tool for both correlation and the measurement of time since similar cycles have now been widely identified within much older parts of the geological column. These so called ‘deep-time’ intervals are beyond the ability of celestial mechanics to accurately predict the combined orbital signal. Therefore, for the Milankovitch method to work, the prerequisite is to establish a hierarchy of cyclicities using stratigraphic successions. Hence, it is important to be able to identify the cycles and their time hierarchies within stratigraphic successions. It is also equally significant to be able to link the process by which the sedimentological and palaeontological proxies for orbital cyclicity react to the changes in insolation. Palynological studies have a special role to play in making these links as the multi-component mix of organic matter within sediments (e.g. Waterhouse, 1995, 1999), in contrast to their mineral matrix, carries clues to both its environment of origin, the environment in which it was deposited and the diagenetic environment in which it was preserved. Hence it gives a method by which we can understand the link between cause and proxy within the sediment cycles.

Preliminary studies of palynofacies carried out during Phase I of the Saudi Aramco-Commission Internationale de Microflore du Paléozoïque (CIMP) project revealed the existence in the Nuayyim-2 (NYMM-2) well of palynofacies defined cycles with a repeat interval of some 50 to 70 feet (ft) (Marshall, 1992). These are defined by the relative abundance of marine and terrestrial palynomorphs. These cycles are significant in that they indicated the presence of climatically driven Milankovitch forcing which if present as a wireline log expression would provide the basis for such a high-resolution correlation within Saudi Arabia and ultimately to the Silurian stratotypes. Therefore as a contribution to the continuing project the cycles and their organic content were investigated in well Berri-84 (BRRI-84).

MATERIAL AND METHODS

The material investigated in this study was a wireline log, plus a series of closely spaced core samples of the Silurian Sharawra Member of the Qalibah Formation of Saudi Aramco well Berri-84 (Figure 1). The samples were demineralised using standard palynological procedures of an hydrochloric acid (HCl) leach to remove carbonates, decant washing to neutral, followed by an hydrofluoric acid (HF) mineral digest and further decant washing before sieving at 10 μm. At this stage Lycopodium tablets were added to provide an internal spike to determine the concentration of the various kerogen components (Stockmarr, 1971). Finally a short treatment in hot concentrated HCl was employed to remove neoformed fluorides. The resulting kerogen concentrate was vialed. Importantly no oxidative methods were employed. The vialed concentrate was next checked optically. If containing amorphous organic matter (AOM) the sample was split and half subjected to a short (5-second) treatment with a tunable ultrasonic probe followed by resieving. This technique very effectively removes the structured kerogen components (Marshall and Yule, 1999). The kerogen concentrate was then mounted using Elvacite 2044 and the various structured components counted against the exotic spike to a target number of Lycopodium spores. A Prior-Swift Model F point counter was used with all the individual structured components in each field of view being recorded.

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

In order to determine the organic matter richness the total organic carbon (TOC) and mineral carbon content of the rock samples was determined using a Carlo-Erba EA1108 elemental analyser. This was operated as per standard conditions. Decarbonation of the rock samples was by concentrated HCl with the samples being run as both total carbon (organic and mineral) and decarbonated. However recalculation to give calcite and TOC content showed that mineral carbonate content was minimal. Hence the decarbonated results are used in place of the corrected analyses. In addition, the atomic Hydrogen/Carbon ratio (H/C) of the kerogen isolates was also determined using the elemental analyser. However the hydrogen contents were generally low such that atomic H/C ratio was not a good discriminator of kerogen composition.

GAMMA-RAY LOG CYCLICITY

In order to better understand any causal link between cyclicity as evident in the wireline logs and the different palynofacies, a time-series analysis was made of the gamma-ray log. A good rationale for employing such methods is that they should not be used to detect unresolved cyclicity but rather enable the user to quantitatively analyse a visually recognisable but complex signal into individual sub-components. This can be seen in the Sharawra Member on the gamma-ray log (Figure 2) where the trace shows a characteristic composite signal of high and low, and thick and thin spikes. These result from the interference of different periodic signals. For the analysis, the gamma-ray log was manually digitised on an enlarged trace with a measurement taken every one foot. This gave some 1,400 data values in total but the analysis was carried out separately for the Sharawra and Qusaiba members. The gamma-ray log values were analysed using a periodogram program (Statgraphics 5) which uses a fast Fourier transform routine.

Figure 2:

Gamma-ray log trace for the Sharawra and Qusaiba members of the Silurian Qalibah Formation.

Figure 2:

Gamma-ray log trace for the Sharawra and Qusaiba members of the Silurian Qalibah Formation.

The output for the Sharawra Member is shown in Figure 3. These results show a number of clearly defined peaks representing periodicities (as cycles per foot, the reciprocal of periodicity lengths) present within the data sequence and their relative magnitude. As noted such peaks were already evident in the gamma-ray log trace but acquisition of the periodogram lengths permits a more rigorous analysis of the relationships between the periodicities and their attribution to the different cycles of the Milankovitch Band. The major technique employed for identifying the peaks is a comparison of the ratios of the periodicity lengths derived from the wells section with those predicted from known Milankovitch frequencies. Table 1 shows the time periodicity ratios for the different Milankovitch frequencies. The values of precession (P1 and P2) and obliquity (O1 and O2) used are as calculated for the Silurian Period by Berger et al. (1989a and b). The values for eccentricity (E1 and E2) being constant through geological time.

Figure 3:

Time-series analysis periodogram for the Sharawra Member. This output shows the different periodicities identified from the gamma-ray trace. The results are expressed in cycles per foot. Its reciprocal, the periodogram length in feet, is shown adjacent to each peak. The heights of each peak show their relative strength.

Figure 3:

Time-series analysis periodogram for the Sharawra Member. This output shows the different periodicities identified from the gamma-ray trace. The results are expressed in cycles per foot. Its reciprocal, the periodogram length in feet, is shown adjacent to each peak. The heights of each peak show their relative strength.

Table 1

Silurian Orbital Periodicities

Ratio table of Silurian Milankovitch Band periodicities as time durations in thousands of years (ky). Periodicity durations from Berger et al. (1989a and b). P1 and P2 precession, O1 and O2 obliquity, and E1 and E2 eccentricity.

Table 2 is the complementary similar set of length ratios calculated from all the significant peaks of the periodogram for the Sharawra Member. These two sets of data are then compared to recognise areas where the values and distribution pattern of these ratios are closely comparable. Such a comparison shows that the orbitally calculated ratios with values very close to 1.18, 1.58, 1.86 and 2.86 occur, some twice within the well data periodogram. These ratios are highlighted in red in both ratio tables. In addition, the first three ratios also occur in the same distribution pattern within both tables. Extracting these values gives:

Table 2

Ratio table of periodogram lengths in feet determined from the time-series analysis (Figure 3) of the Sharawra Member. Ratios common to Table 2 indicated in red and their preliminary orbital periodicities indicated. Integer values which show factorisation to higher orbital harmonics are in blue.

Periodogram length ratios Orbitally calculated ratios

18.22/9.65 = 1.891.86O1/P1
41.0/21.87 = 1.87
18.22/11.31 = 1.611.58O1/P2
41.0/25.23 = 1.63
11.31/9.65 = 1.171.18P2/P1
25.23/21.87 = 1.15
15.62/5.47 = 2.862.85E1/O2
18.22/9.65 = 1.891.86O1/P1
41.0/21.87 = 1.87
18.22/11.31 = 1.611.58O1/P2
41.0/25.23 = 1.63
11.31/9.65 = 1.171.18P2/P1
25.23/21.87 = 1.15
15.62/5.47 = 2.862.85E1/O2

The similarity of the ratios and their occurrence in the same hierarchy therefore enables the periodicity lengths from the well section to be identified with specific Milankovitch orbital bands. These are:

P1 9.65and21.87note that 21.87/9.65 = 2.27
P2 11.31and25.23note that 25.23/11.31 = 2.23
O1 18.22and41.0note that 41.0/18.22 = 2.25
O2 5.47   
E1 15.62   
P1 9.65and21.87note that 21.87/9.65 = 2.27
P2 11.31and25.23note that 25.23/11.31 = 2.23
O1 18.22and41.0note that 41.0/18.22 = 2.25
O2 5.47   
E1 15.62   

The presence of two different lengths for the same orbital periodicity value shows that neither are the true values but both are harmonics of a shorter periodicity. This is confirmed by the length of 2.86 for O2. It is also significant that when the related length pairs are divided larger over smaller then the constant value of 2.25 occurs. This value (as the ratio 9:4) is then used to determine new values for length which factorise with these values derived from the periodogram. These new length values (in decimal feet) are:

P12.41
P22.82
O14.55
O25.47
E115.62
P12.41
P22.82
O14.55
O25.47
E115.62

These almost certainly represent the true Milankovitch periodicity lengths for the P1, P2, O1, O2 and E1 periodicities. As such they give a more realistic value for sedimentation rate when comparing the time duration of the different periodicities with their lengths, (i.e. compacted sediment thickness) than do the much longer values derived from the periodogram. These values are short and given the limitation of sub-surface gamma-ray measurement, the speed of tool movement whilst logging and the one foot resolution on the digitisation they would have a low resolution on the periodogram. However, they can be confirmed as real values since inspection of the relevant section of the periodogram (Figure 4) shows minor but distinct peaks to occur at these predicted values.

Figure 4:

Time-series analysis periodogram for the higher frequency cycles from the Sharawra Member. This shows low power but distinct peaks at 4.55 ft (.220), 2.82 ft (.335) and 2.41 ft (.415). Numbers in parentheses are units in cycles per foot.

Figure 4:

Time-series analysis periodogram for the higher frequency cycles from the Sharawra Member. This shows low power but distinct peaks at 4.55 ft (.220), 2.82 ft (.335) and 2.41 ft (.415). Numbers in parentheses are units in cycles per foot.

Other features to note within the periodogram length ratio matrix are the integer values 2, 3, 4 and 7 (in blue, Table 2). These confirm this factorisation procedure to determine the true Milankovitch periodicity lengths. What they represent are further whole number combinations of the short true periodicities. For example 25.23/8.41 = 3. Here both numbers can be factorised by 2.82 to give the ratio 9:3 which signifies that the periodicity 8.41 is composed of three true Milankovitch periodicity lengths. It is also significant that each of these integer combinations in Table 2 occurs with a length value (e.g. 21.87, 12.62, 8.41, 5.86) which was not involved in identifying the orbital time duration ratio.

Table 3 shows the periodogram lengths ratios with the addition of the newly identified lengths. This has enabled further ratios to be identified (3.43, 5.54, 6.48) which result from the interaction of E1 (15.62) with O1, P2 and P1 (with calculated orbital ratios of 3.48, 5.49 and 6.46, respectively). This provides a significant internal check as the 15.62 length was not used in any of the factorisation calculations. In addition it emphasises the validity of the method because all the peaks with the periodogram length ratio matrix can now be identified with orbital ratios. There are no unassigned peaks and conversely no spurious ratios. In Table 3 the ten harmonics are also identified. This represents a number approaching that identified in Pleistocene isotope data and a significant finding within a dataset of Silurian age.

Table 3

The periodogram length data from Table 2 with the newly recognised single orbital frequencies added. This allows more periodicity time duration ratios to be recognised as indicated in red. The higher harmonic orders are identified.

Figure 5 shows the periodogram data for the Qusaiba Member. In comparison to the Sharawra Member the periodogram shows peaks which are both less clear and have lower ordinate values. This reflects the less obvious cyclicity which can be seen in Figure 2 when both sections are compared. However comparison of the periodograms for both sections shows that the significant peaks in the Sharawra periodogram can be picked within the Qusaiba Member. These periodogram lengths of these peaks are cross-plotted in Figure 6 and show a very clear correlation.

Figure 5:

Time-series analysis periodogram for the Qusaiba Member. The labelled peaks are periodicities identified from the Sharawra Member which can be picked in the Qusaiba Member periodogram. They occur with slightly different values.

Figure 5:

Time-series analysis periodogram for the Qusaiba Member. The labelled peaks are periodicities identified from the Sharawra Member which can be picked in the Qusaiba Member periodogram. They occur with slightly different values.

Figure 6:

Periodogram length determined from the Sharawra Member plotted against those picked from the Qusaiba periodogram. Note the excellent correlation (R2 = 99.8, gradient = 1.02).

Figure 6:

Periodogram length determined from the Sharawra Member plotted against those picked from the Qusaiba periodogram. Note the excellent correlation (R2 = 99.8, gradient = 1.02).

The significance of this gamma-ray log analysis is that it enables the identification of the primary cycles within the succession. These cycles are otherwise unidentifiable from the gamma-ray log and the widely spaced palynological samples. However, the identification of the higher order harmonics might be possible from within the data.

It is probably significant that during Silurian times the Arabian Plate was situated at high mid latitude (Scotese and Barrett, 1990). This places it within a sensitive location as regards orbitally varying insolation and as such the climatic variation will have had greater potential to leave its expression within the sedimentary environment. The recognition of the periodicity is also an important but untested method for correlation using cycles with other Qalibah Formation sections from the sub-surface of Saudi Arabia. It also gives the ability to measure time within sedimentary sections. For example, the 2.41 foot P1 cycle has a duration of 16.4 ky and therefore a compacted sedimentation rate of .00015 ft per year or .0045 cm per year.

It is also possible to use these orbital cyclicities to determine the time duration of the Sharawra and Qusaiba members for the Berri-84 well. This will represent a minimum age for the Sharawra Member as its upper boundary is an unconformity beneath the Khuff Formation of Permian age. Using the 25.23 foot and 24.59 foot periodogram lengths to represent nine P2 cycles in the Sharawra (365 ft) and Qusaiba (1,016 ft) members gives time durations of 2.5 and 7.1 million years (My) respectively for the two units. The Silurian series/stage boundaries are not known within the Berri-84 well but elsewhere within Saudi Arabia, the base of the Qusaiba Member is basal Rhuddanian (acuminatus graptolite zone) whilst the top is late mid Telychian (griestoniensis graptolite zone) in age (Paris et al., 1995). High precision U/Pb zircon ages give dates of 438.7 ± 2.1 million years before present (Ma) for the uppermost graptolite zone of the Rhuddanian and 430.1 ± 2.4 Ma for the uppermost zone of the Telychian Series. This indicates a Llandovery Series duration of some 9 My (but note the 15 My duration in Tucker and McKerrow (1995) determined by plotting geochronological dates against sediment thickness and faunal zones) which is comparable with the 7 My estimated here using orbital cyclicity. Clearly there is much scope for integrating these complementary methods to give a unified time-scale.

Such methods will give an enhanced precision to models for burial and petroleum generation. In particular, it will permit an independent test of Silurian thermal history. This is significant because existing burial models based on chitinozoan reflectivity studies show a high heat flow coincident with the deposition of the Qusaiba Member source rocks within an extensional rift setting (Marshall, 1995). The sedimentation rate (decompacted) derived from orbital cycle thicknesses will give a second estimate of heat flow by back modelling extension and accommodation space within the rift system.

PALYNOFACIES

Table 4 details the abundance and ratios of the main components within the palynofacies samples. The TOC results and the gamma-ray values are also given. The results are also shown graphically in Figure 7. The different depth scales for the wireline logs and core samples have been registered using the top of the prominent sand at 15,321 ft (core), 15,376 ft (wireline) which is clearly recognisable in both sets of data.

Table 4

Palynological abundance, organic richness and size data as determined from the Berri-84 core samples. The plotted ratio values are also shown. The two depth scales are shown together with gamma-ray API measured from the composite log. The abbreviations are: TOC, total organic carbon; Ac, acritarchs; Sp, spores; Lei, leiospheres; Chit, chitinozoans; Phyt, phytoclasts; AOM, amorphous organic matter; palyn, total number of palynomorphs; Lyco, number of Lycopodium spores counted; SP diam, mean spore diameter for 30 spores; skew, skewness of the spore diameter population.

No.Core depth (feet)Log depth (feet)GR APITOC (%)AcSpLeiChitPhytAOMRatio Ac/Ac+SppalynPhyt (%)LycoSP diamskew
115,266.5015,320.501610.563,2464,841165033,8731,3030.408,2517834230.60.0
215,273.2015,327.501820.625,0056,9902596906,1831930.4212,9453221831.80.1
315,276.9015,330.901610.6519,32117,16176389019,42714,3900.5338,134277429.50.5
415,289.0015,343.001700.595,2056,44614314350,2616280.4511,9378019731.40.3
515,299.1015,353.101580.343,8845,8797481435,99300.4010,6543626432.21.0
615,314.4015,368.401620.521,2577,4642285714,9801460.149,5213424734.21.8
715,320.1015,374.101580.446642,04755283,02700.242,7945234031.50.5
815,333.0315,387.001220.522,6304,8581559310,68300.357,7365830435.21.0
915,342.8315,396.801180.4911,98215,90111207,89600.4327,996228432.71.8
1015,366.6615,410.701060.437,74212,819166838,50000.3820,8112911336.00.5
1115,365.1915,419.20990.343,72810,49657577,72100.2614,3393516434.41.5
1215,384.9015,438.901060.282612,109004,81000.112,3696739734.8-0.1
1315,409.6015,463.60890.367642,707009,38500.223,4717327134.90.54
1415,426.7015,480.70950.645,24313,95607712,85100.2719,2764012232.40.3
1515,431.0015,485.001320.6511,75833,3937841,0979,63300.2647,033176029.21.0
No.Core depth (feet)Log depth (feet)GR APITOC (%)AcSpLeiChitPhytAOMRatio Ac/Ac+SppalynPhyt (%)LycoSP diamskew
115,266.5015,320.501610.563,2464,841165033,8731,3030.408,2517834230.60.0
215,273.2015,327.501820.625,0056,9902596906,1831930.4212,9453221831.80.1
315,276.9015,330.901610.6519,32117,16176389019,42714,3900.5338,134277429.50.5
415,289.0015,343.001700.595,2056,44614314350,2616280.4511,9378019731.40.3
515,299.1015,353.101580.343,8845,8797481435,99300.4010,6543626432.21.0
615,314.4015,368.401620.521,2577,4642285714,9801460.149,5213424734.21.8
715,320.1015,374.101580.446642,04755283,02700.242,7945234031.50.5
815,333.0315,387.001220.522,6304,8581559310,68300.357,7365830435.21.0
915,342.8315,396.801180.4911,98215,90111207,89600.4327,996228432.71.8
1015,366.6615,410.701060.437,74212,819166838,50000.3820,8112911336.00.5
1115,365.1915,419.20990.343,72810,49657577,72100.2614,3393516434.41.5
1215,384.9015,438.901060.282612,109004,81000.112,3696739734.8-0.1
1315,409.6015,463.60890.367642,707009,38500.223,4717327134.90.54
1415,426.7015,480.70950.645,24313,95607712,85100.2719,2764012232.40.3
1515,431.0015,485.001320.6511,75833,3937841,0979,63300.2647,033176029.21.0
Figure 7:

The abundance of the main palynomorph groups from the Berri-84 core samples. The two scales (ft) represent wireline log (left) and core (right, core samples 1-15) depths. Note the correlation of increased relative percentage of acritarchs with acritarch and spore abundance. Spore diameter and skewness are also shown. The spore diameter shows minima at maximum acritarch abundance.

Figure 7:

The abundance of the main palynomorph groups from the Berri-84 core samples. The two scales (ft) represent wireline log (left) and core (right, core samples 1-15) depths. Note the correlation of increased relative percentage of acritarchs with acritarch and spore abundance. Spore diameter and skewness are also shown. The spore diameter shows minima at maximum acritarch abundance.

The first observation from the data is the simplicity of the sedimentary system and its organic matter. There is a negligible carbonate content, a virtual absence of AOM in all but one sample and a small number of kerogen components which make up the bulk of the organic matter. The second is that the higher frequency cyclicity as defined by the gamma-ray log analysis has a shorter wavelength than the sample spacing. Hence, it is not possible to directly compare the progressive evolution of changing organic matter composition through the individual cycles as recognised from the time-series analysis of the gamma-ray log trace. However there is certainly a periodicity within the data subject to the constraints of the short core run and the significant influx of sands below 15,736 ft. The periodicity within the three part cycles seen in the palynofacies abundance and TOC data is approaching 100 ft and may relate to the 109.29 ft (20 O2) periodogram length (Figure 3). Given the limitations of the sample spacing and core length a more rigorous analysis of the grouped palynofacies data has been attempted to investigate any link between cyclicity and variation in kerogen content.

Figure 8 is a cross-plot of TOC % versus the total number of palynomorphs within the samples. This plot was chosen as it is a common procedure to count proportions and ratios of specific palynomorphs rather than the entire kerogen population. Two trends are clearly shown by the total palynomorph data. The difference is the number of palynomorphs in samples which have the same TOC content. The upper trend is palynomorph rich, the lower trend palynomorph poor. Accepting that on average all palynomorphs contribute equally to TOC there must be an additional component in samples from the lower trend line which adds organic matter to the sample. Inspection of Table 4 shows that the difference between the two groups of samples is in the abundance of phytoclasts, in this case sheet-like material of land plant origin. This variability in phytoclast content is shown in Figure 9 where TOC % is plotted against the relative % of phytoclasts versus spores and acritarchs; i.e. the majority palynomorph group. This demonstrates that the group of spore-rich low TOC samples from Figure 8 have the lowest phytoclast content across the range of TOC in all the samples.

Figure 8:

Cross-plot of TOC % against the abundance of all palynomorphs (spores, acritarchs, chitinozoan, leiosphere). Two trends are present showing that a significant third component adds to the TOC % in the lower trend line.

Figure 8:

Cross-plot of TOC % against the abundance of all palynomorphs (spores, acritarchs, chitinozoan, leiosphere). Two trends are present showing that a significant third component adds to the TOC % in the lower trend line.

Figure 9:

Cross-plot of the ratio of phytoclasts to spores plus acritarchs against TOC %. The samples which comprised the upper trend line in Figure 8 (5, 11, 10, 9, 3, 15) are those with the least proportion of phytoclasts.

Figure 9:

Cross-plot of the ratio of phytoclasts to spores plus acritarchs against TOC %. The samples which comprised the upper trend line in Figure 8 (5, 11, 10, 9, 3, 15) are those with the least proportion of phytoclasts.

A commonly applied measure of ‘distality’ is the relative ratio of acritarchs to spores. The rationale being that the marine sourced component will relatively increase as spore numbers decrease offshore. Figures 7 and 10 show the relationship between the spore and acritarch abundance. Figure 7 gives results that are counter intuitive. As the relative percentage of acritarchs increases, in other words the samples would be interpreted as becoming more marine, then the actual number of spores in the sample in fact increases. It does not decrease as would be anticipated. The ratio changing to a greater proportion of acritarchs is because the acritarch abundance is increasing faster than the spore abundance. Figure 10 shows two clear trends where this occurs. The lower trend being generally within the samples which are rich in phytoclasts, the upper trend being those poor in phytoclasts. Accepting that sample 15 has an anomalously high abundance of spores then the upper trend appears to show a tail-off in increasing spore abundance. Samples 10, 9 and 3 being towards the upper limit of spore abundance, having reached the limit of their concentration by differential transport into the marine environment.

Figure 10:

Cross-plot of spore versus acritarch abundance. Again two trends are apparent and are determined by the abundance of phytoclasts. Excepting sample 15 the upper trend line appears to tail-off as the limit of spore enrichment through differential transport in the marine environment is reached.

Figure 10:

Cross-plot of spore versus acritarch abundance. Again two trends are apparent and are determined by the abundance of phytoclasts. Excepting sample 15 the upper trend line appears to tail-off as the limit of spore enrichment through differential transport in the marine environment is reached.

However, this double trend, although appearing to demonstrate a distinct relationship between spores and acritarchs merely highlights the limitations of proportionate data. For, given a simple three component system there merely needs to be a change in the abundance of spores or phytoclasts to change the relative proportion of acritarchs. This difficulty can be mitigated by using quantitative data proportionate to a weight of sediment rather than relative the other kerogen components.

Figure 11 shows the absolute abundances of the three main kerogen components and TOC % versus depth. What the figure shows is clear peaks in abundance of organic matter when measured both as kerogen particles and TOC %. The components which increase in these peaks are acritarchs and spores, the abundance of phytoclasts being reduced. Two explanations appear possible. Firstly that the water depth has deepened or the shoreline has retreated and that there is a differential transport of spores relative to phytoclasts. This increase in spores being reinforced by an increase in acritarchs which reflects increased plankton productivity within this more marine environment. However, this does not explain how the abundance of phytoclasts has also increased when it should be expected to decline.

Figure 11:

Cumulative abundance of acritarchs, spores and phytoclasts versus depth. The TOC % is also plotted. Note the coincident increases in acritarch and spore abundance which suggest a mechanism of increased preservation through increased water column productivity.

Figure 11:

Cumulative abundance of acritarchs, spores and phytoclasts versus depth. The TOC % is also plotted. Note the coincident increases in acritarch and spore abundance which suggest a mechanism of increased preservation through increased water column productivity.

The second and preferred explanation is that the sedimentary environment has subtly altered such that the terrestrial organic matter has been input into a more marine influenced environment with a relative increase in spore abundance through differential transport. The latter could equally be the result of increased distance for the shoreline as much as depth. Importantly the overall organic input into this environment has increased with a significant increase in planktonic algae as shown by acritarch abundance and the increased leiosphere abundance in sample numbers 3 and 15 (Table 4) and the presence of AOM (no. 3). This increased organic input, largely stimulated by increased water column productivity, leads to increased organic matter preservation and the apparent increase in terrestrially sourced organic matter within a more marine environment. The water depth change may be very minor as the controls on increased organic matter preservation equally result from subtle differences in rates of organic matter and sediment supply and the character and cohesion of the sediment (Calvert and Pedersen, 1992). Preservation alone cannot be the mechanism on account of the changes in abundances of the different kerogen components.

In an attempt to test the significance of transport processes in producing the increased spore abundances, the mean spore diameter was measured (Table 4, Figure 7). Thirty spores were measured from each sample, this number being determined from a rarefaction curve of running average versus the cumulative number of measurements. The application of spore diameter was of particular interest in trying to find a robust method for determining relative transport without recourse to ratios or abundances. The results of both diameter (Table 4 and Figure 7) and sorting about the mean as measured by skewness (Table 4) are equivocal but do show local reductions, although not trends, when comparing the three most spore-rich samples (numbers 3, 9 and 15).

Now that the differences in kerogen composition can be explained in terms of a model for increased preservation, an attempt can be made to relate these changes to the higher frequency cycles recognised from the gamma-ray log analysis.

Figure 12 shows a cross-plot of TOC % versus gamma-ray API measured from the composite log. Such a conversion is not without error (?sample 14) given the lack of a core gamma-ray scan and the uncertainty over precise sample depth and core to wireline log depth registration. However, two distinct depth related trends are shown. A group with greater API values all come from the upper part of the Sharawra Member. The second group has lower API values and is from the lower part of the Sharawra Member which is characterised by an influx of sand. Establishing this relationship between TOC % and gamma-ray API suggests that, in common with the well established association between organic content and formation radioactivity, all the gamma-ray peaks on the composite log (Figure 2) will have increased TOC % with compositional variation as shown in Figure 11. Each composite log gamma-ray peak will be a log expression of a minor climatically induced productivity-linked organic matter high. This successfully demonstrates a link between the kerogen variation within the widely spaced sample set with higher frequency log-based cycles.

Figure 12:

Cross-plot showing two different relationships for TOC % and gamma-ray API for the upper and lower (sand-rich) parts of the Sharawra Member. This links the gamma-ray log response and cyclicity to kerogen composition and environmental change.

Figure 12:

Cross-plot showing two different relationships for TOC % and gamma-ray API for the upper and lower (sand-rich) parts of the Sharawra Member. This links the gamma-ray log response and cyclicity to kerogen composition and environmental change.

ACKNOWLEDGEMENT

The author acknowledges with gratitude the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company for permission to publish this study. Shir Akbari (SOES, SOC, Southampton) is thanked for preparing the samples and operating the elemental analyser.

REFERENCES

Berger
,
A.L.
,
M.F.
Loutre
and
V.
Dehant
1989a
.
Influence of the changing lunar orbit on the astronomical frequencies of pre-Quaternary insolation patterns
.
Paleoceanography
 , v.
4
, p.
555
-
564
.
Berger
,
A.L.
,
M.F.
Loutre
and
V.
Dehant
1989b
.
Pre-Quaternary Milankovitch frequencies
.
Nature
 , v.
323
, p.
133
.
Calvert
,
S.E.
and
T.F.
Pedersen
1992
. Organic carbon accumulation and preservation in marine sediments: how important is anoxia? In
J.
Whelan
and
J.W.
Farrington
(Eds.),
Organic matter: productivity, accumulation, and preservation in recent and ancient sediments
 .
Columbia University Press
,
New York
, p.
231
-
263
.
Fischer
,
A.G
.
1986
.
Climatic rhythms recorded in strata
.
Annual Review of Earth and Planetary Sciences
 , v.
14
, p.
351
-
376
.
House
,
M.R.
and
A.
Gale
(Eds.)
1995
. Orbital forcing timescales and cyclostratigraphy.
Geological Society Special Publication
 , v.
85
,
The Geological Society
,
London
,
210
p.
Marshall
,
J.E.A
.
1992
.
Palaeozoic palynofacies, hydrocarbon source rocks and burial history of Saudi Arabia
.
8th International Palynological Congress, Program and Abstracts
, p.
99
.
Marshall
,
J.E.A
.
1995
.
The Silurian of Saudi Arabia: thermal maturity, burial history and geotectonic environment
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
139
-
150
.
Marshall
,
J.E.A.
and
B.L.
Yule
1999
. Spore colour measurement. In
T.P.
Jones
and
N.P.
Rowe
(Eds.),
Fossil Plants and Spores: modern techniques
 .
Geological Society
,
London
, p.
165
-
168
.
Paris
,
F.
,
J.
Verniers
,
S.
Al-Hajri
and
H.
Al-Tayyar
1995
.
Biostratigraphy and palaeogeographic affinities of Early Silurian chitinozoans from central Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
75
-
90
.
Schwarzacher
,
W
.
1993
.
Cyclostratigraphy and the Milankovitch theory
 .
Developments in Sedimentology
 , v.
52
,
Elsevier
,
Amsterdam
,
225
p.
Scotese
,
C.R.
and
S.F.
Barrett
1990
. Gondwana’s movement over the South Pole during the Palaeozoic: evidence from lithological indicators of climate. In
W.S.
McKerrow
and
C.R.
Scotese
(Eds.),
Palaeozoic Palaeogeography and Biogeography
 .
Geological Society Memoir
, v.
12
, p.
75
-
85
.
Shackleton
,
N.J.
,
N.
Crowhurst
,
T.
Hagelberg
and
N.G.
Pisias
1995
.
A new Late Neogene timescale: Application to ODP Leg 138 Site 846
.
Proceedings of the Ocean Drilling Program Science Results
, v.
138
, p.
73
-
101
.
Stockmarr
,
J
.
1971
.
Tablets with spores used in absolute pollen analysis
.
Pollen et Spores
 , v.
13
, p.
615
-
621
.
Tucker
,
R.D.
and
W.S.
McKerrow
1995
.
Early Paleozoic chronology: a review in light of new U-Pb zircon ages from Newfoundland and Britain
.
Canadian Journal of Earth Sciences
 , v.
32
, p.
368
-
379
.
Waterhouse
,
H.K
.
1995
.
High-resolution palynofacies investigation of Kimmeridgian sedimentary cycles
 . In
M.R.
House
and
A.
Gale
(Eds.),
Orbital forcing timescales and cyclostratigraphy
 .
Geological Society Special Publication
, v.
85
, p.
75
-
114
.
Waterhouse
,
H.K
.
1999
.
Orbital forcing of palynofacies in the Jurassic of France and the United Kingdom
.
Geology
 , v.
27
, p.
511
-
514
.

ABOUT THE AUTHOR

John E.A. Marshall is a Senior Lecturer in the School of Ocean and Earth Science at Southampton Oceanography Centre. Prior to going to Southampton he worked as a Palynologist in the Strat Lab at Gearhart Geodata in Aberdeen and in the Department of Geology, University of Newcastle upon Tyne. John’s research interests are Devonian palynology including high latitude spore assemblages, recognition of large-scale climatic change in Devonian and Jurassic sediments, hydrocarbon source rocks and palynofacies and thermal maturity determination, especially in pre-Carboniferous rock sequences. John received a BA from the University of Cambridge in 1976 and a PhD from the University of Bristol in 1981.

Figures & Tables

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 2:

Gamma-ray log trace for the Sharawra and Qusaiba members of the Silurian Qalibah Formation.

Figure 2:

Gamma-ray log trace for the Sharawra and Qusaiba members of the Silurian Qalibah Formation.

Figure 3:

Time-series analysis periodogram for the Sharawra Member. This output shows the different periodicities identified from the gamma-ray trace. The results are expressed in cycles per foot. Its reciprocal, the periodogram length in feet, is shown adjacent to each peak. The heights of each peak show their relative strength.

Figure 3:

Time-series analysis periodogram for the Sharawra Member. This output shows the different periodicities identified from the gamma-ray trace. The results are expressed in cycles per foot. Its reciprocal, the periodogram length in feet, is shown adjacent to each peak. The heights of each peak show their relative strength.

Figure 4:

Time-series analysis periodogram for the higher frequency cycles from the Sharawra Member. This shows low power but distinct peaks at 4.55 ft (.220), 2.82 ft (.335) and 2.41 ft (.415). Numbers in parentheses are units in cycles per foot.

Figure 4:

Time-series analysis periodogram for the higher frequency cycles from the Sharawra Member. This shows low power but distinct peaks at 4.55 ft (.220), 2.82 ft (.335) and 2.41 ft (.415). Numbers in parentheses are units in cycles per foot.

Figure 5:

Time-series analysis periodogram for the Qusaiba Member. The labelled peaks are periodicities identified from the Sharawra Member which can be picked in the Qusaiba Member periodogram. They occur with slightly different values.

Figure 5:

Time-series analysis periodogram for the Qusaiba Member. The labelled peaks are periodicities identified from the Sharawra Member which can be picked in the Qusaiba Member periodogram. They occur with slightly different values.

Figure 6:

Periodogram length determined from the Sharawra Member plotted against those picked from the Qusaiba periodogram. Note the excellent correlation (R2 = 99.8, gradient = 1.02).

Figure 6:

Periodogram length determined from the Sharawra Member plotted against those picked from the Qusaiba periodogram. Note the excellent correlation (R2 = 99.8, gradient = 1.02).

Figure 7:

The abundance of the main palynomorph groups from the Berri-84 core samples. The two scales (ft) represent wireline log (left) and core (right, core samples 1-15) depths. Note the correlation of increased relative percentage of acritarchs with acritarch and spore abundance. Spore diameter and skewness are also shown. The spore diameter shows minima at maximum acritarch abundance.

Figure 7:

The abundance of the main palynomorph groups from the Berri-84 core samples. The two scales (ft) represent wireline log (left) and core (right, core samples 1-15) depths. Note the correlation of increased relative percentage of acritarchs with acritarch and spore abundance. Spore diameter and skewness are also shown. The spore diameter shows minima at maximum acritarch abundance.

Figure 8:

Cross-plot of TOC % against the abundance of all palynomorphs (spores, acritarchs, chitinozoan, leiosphere). Two trends are present showing that a significant third component adds to the TOC % in the lower trend line.

Figure 8:

Cross-plot of TOC % against the abundance of all palynomorphs (spores, acritarchs, chitinozoan, leiosphere). Two trends are present showing that a significant third component adds to the TOC % in the lower trend line.

Figure 9:

Cross-plot of the ratio of phytoclasts to spores plus acritarchs against TOC %. The samples which comprised the upper trend line in Figure 8 (5, 11, 10, 9, 3, 15) are those with the least proportion of phytoclasts.

Figure 9:

Cross-plot of the ratio of phytoclasts to spores plus acritarchs against TOC %. The samples which comprised the upper trend line in Figure 8 (5, 11, 10, 9, 3, 15) are those with the least proportion of phytoclasts.

Figure 10:

Cross-plot of spore versus acritarch abundance. Again two trends are apparent and are determined by the abundance of phytoclasts. Excepting sample 15 the upper trend line appears to tail-off as the limit of spore enrichment through differential transport in the marine environment is reached.

Figure 10:

Cross-plot of spore versus acritarch abundance. Again two trends are apparent and are determined by the abundance of phytoclasts. Excepting sample 15 the upper trend line appears to tail-off as the limit of spore enrichment through differential transport in the marine environment is reached.

Figure 11:

Cumulative abundance of acritarchs, spores and phytoclasts versus depth. The TOC % is also plotted. Note the coincident increases in acritarch and spore abundance which suggest a mechanism of increased preservation through increased water column productivity.

Figure 11:

Cumulative abundance of acritarchs, spores and phytoclasts versus depth. The TOC % is also plotted. Note the coincident increases in acritarch and spore abundance which suggest a mechanism of increased preservation through increased water column productivity.

Figure 12:

Cross-plot showing two different relationships for TOC % and gamma-ray API for the upper and lower (sand-rich) parts of the Sharawra Member. This links the gamma-ray log response and cyclicity to kerogen composition and environmental change.

Figure 12:

Cross-plot showing two different relationships for TOC % and gamma-ray API for the upper and lower (sand-rich) parts of the Sharawra Member. This links the gamma-ray log response and cyclicity to kerogen composition and environmental change.

Table 1

Silurian Orbital Periodicities

Table 2

Ratio table of periodogram lengths in feet determined from the time-series analysis (Figure 3) of the Sharawra Member. Ratios common to Table 2 indicated in red and their preliminary orbital periodicities indicated. Integer values which show factorisation to higher orbital harmonics are in blue.

18.22/9.65 = 1.891.86O1/P1
41.0/21.87 = 1.87
18.22/11.31 = 1.611.58O1/P2
41.0/25.23 = 1.63
11.31/9.65 = 1.171.18P2/P1
25.23/21.87 = 1.15
15.62/5.47 = 2.862.85E1/O2
18.22/9.65 = 1.891.86O1/P1
41.0/21.87 = 1.87
18.22/11.31 = 1.611.58O1/P2
41.0/25.23 = 1.63
11.31/9.65 = 1.171.18P2/P1
25.23/21.87 = 1.15
15.62/5.47 = 2.862.85E1/O2
P1 9.65and21.87note that 21.87/9.65 = 2.27
P2 11.31and25.23note that 25.23/11.31 = 2.23
O1 18.22and41.0note that 41.0/18.22 = 2.25
O2 5.47   
E1 15.62   
P1 9.65and21.87note that 21.87/9.65 = 2.27
P2 11.31and25.23note that 25.23/11.31 = 2.23
O1 18.22and41.0note that 41.0/18.22 = 2.25
O2 5.47   
E1 15.62   
P12.41
P22.82
O14.55
O25.47
E115.62
P12.41
P22.82
O14.55
O25.47
E115.62
Table 3

The periodogram length data from Table 2 with the newly recognised single orbital frequencies added. This allows more periodicity time duration ratios to be recognised as indicated in red. The higher harmonic orders are identified.

Table 4

Palynological abundance, organic richness and size data as determined from the Berri-84 core samples. The plotted ratio values are also shown. The two depth scales are shown together with gamma-ray API measured from the composite log. The abbreviations are: TOC, total organic carbon; Ac, acritarchs; Sp, spores; Lei, leiospheres; Chit, chitinozoans; Phyt, phytoclasts; AOM, amorphous organic matter; palyn, total number of palynomorphs; Lyco, number of Lycopodium spores counted; SP diam, mean spore diameter for 30 spores; skew, skewness of the spore diameter population.

No.Core depth (feet)Log depth (feet)GR APITOC (%)AcSpLeiChitPhytAOMRatio Ac/Ac+SppalynPhyt (%)LycoSP diamskew
115,266.5015,320.501610.563,2464,841165033,8731,3030.408,2517834230.60.0
215,273.2015,327.501820.625,0056,9902596906,1831930.4212,9453221831.80.1
315,276.9015,330.901610.6519,32117,16176389019,42714,3900.5338,134277429.50.5
415,289.0015,343.001700.595,2056,44614314350,2616280.4511,9378019731.40.3
515,299.1015,353.101580.343,8845,8797481435,99300.4010,6543626432.21.0
615,314.4015,368.401620.521,2577,4642285714,9801460.149,5213424734.21.8
715,320.1015,374.101580.446642,04755283,02700.242,7945234031.50.5
815,333.0315,387.001220.522,6304,8581559310,68300.357,7365830435.21.0
915,342.8315,396.801180.4911,98215,90111207,89600.4327,996228432.71.8
1015,366.6615,410.701060.437,74212,819166838,50000.3820,8112911336.00.5
1115,365.1915,419.20990.343,72810,49657577,72100.2614,3393516434.41.5
1215,384.9015,438.901060.282612,109004,81000.112,3696739734.8-0.1
1315,409.6015,463.60890.367642,707009,38500.223,4717327134.90.54
1415,426.7015,480.70950.645,24313,95607712,85100.2719,2764012232.40.3
1515,431.0015,485.001320.6511,75833,3937841,0979,63300.2647,033176029.21.0
No.Core depth (feet)Log depth (feet)GR APITOC (%)AcSpLeiChitPhytAOMRatio Ac/Ac+SppalynPhyt (%)LycoSP diamskew
115,266.5015,320.501610.563,2464,841165033,8731,3030.408,2517834230.60.0
215,273.2015,327.501820.625,0056,9902596906,1831930.4212,9453221831.80.1
315,276.9015,330.901610.6519,32117,16176389019,42714,3900.5338,134277429.50.5
415,289.0015,343.001700.595,2056,44614314350,2616280.4511,9378019731.40.3
515,299.1015,353.101580.343,8845,8797481435,99300.4010,6543626432.21.0
615,314.4015,368.401620.521,2577,4642285714,9801460.149,5213424734.21.8
715,320.1015,374.101580.446642,04755283,02700.242,7945234031.50.5
815,333.0315,387.001220.522,6304,8581559310,68300.357,7365830435.21.0
915,342.8315,396.801180.4911,98215,90111207,89600.4327,996228432.71.8
1015,366.6615,410.701060.437,74212,819166838,50000.3820,8112911336.00.5
1115,365.1915,419.20990.343,72810,49657577,72100.2614,3393516434.41.5
1215,384.9015,438.901060.282612,109004,81000.112,3696739734.8-0.1
1315,409.6015,463.60890.367642,707009,38500.223,4717327134.90.54
1415,426.7015,480.70950.645,24313,95607712,85100.2719,2764012232.40.3
1515,431.0015,485.001320.6511,75833,3937841,0979,63300.2647,033176029.21.0

Contents

GeoRef

References

REFERENCES

Berger
,
A.L.
,
M.F.
Loutre
and
V.
Dehant
1989a
.
Influence of the changing lunar orbit on the astronomical frequencies of pre-Quaternary insolation patterns
.
Paleoceanography
 , v.
4
, p.
555
-
564
.
Berger
,
A.L.
,
M.F.
Loutre
and
V.
Dehant
1989b
.
Pre-Quaternary Milankovitch frequencies
.
Nature
 , v.
323
, p.
133
.
Calvert
,
S.E.
and
T.F.
Pedersen
1992
. Organic carbon accumulation and preservation in marine sediments: how important is anoxia? In
J.
Whelan
and
J.W.
Farrington
(Eds.),
Organic matter: productivity, accumulation, and preservation in recent and ancient sediments
 .
Columbia University Press
,
New York
, p.
231
-
263
.
Fischer
,
A.G
.
1986
.
Climatic rhythms recorded in strata
.
Annual Review of Earth and Planetary Sciences
 , v.
14
, p.
351
-
376
.
House
,
M.R.
and
A.
Gale
(Eds.)
1995
. Orbital forcing timescales and cyclostratigraphy.
Geological Society Special Publication
 , v.
85
,
The Geological Society
,
London
,
210
p.
Marshall
,
J.E.A
.
1992
.
Palaeozoic palynofacies, hydrocarbon source rocks and burial history of Saudi Arabia
.
8th International Palynological Congress, Program and Abstracts
, p.
99
.
Marshall
,
J.E.A
.
1995
.
The Silurian of Saudi Arabia: thermal maturity, burial history and geotectonic environment
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
139
-
150
.
Marshall
,
J.E.A.
and
B.L.
Yule
1999
. Spore colour measurement. In
T.P.
Jones
and
N.P.
Rowe
(Eds.),
Fossil Plants and Spores: modern techniques
 .
Geological Society
,
London
, p.
165
-
168
.
Paris
,
F.
,
J.
Verniers
,
S.
Al-Hajri
and
H.
Al-Tayyar
1995
.
Biostratigraphy and palaeogeographic affinities of Early Silurian chitinozoans from central Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
75
-
90
.
Schwarzacher
,
W
.
1993
.
Cyclostratigraphy and the Milankovitch theory
 .
Developments in Sedimentology
 , v.
52
,
Elsevier
,
Amsterdam
,
225
p.
Scotese
,
C.R.
and
S.F.
Barrett
1990
. Gondwana’s movement over the South Pole during the Palaeozoic: evidence from lithological indicators of climate. In
W.S.
McKerrow
and
C.R.
Scotese
(Eds.),
Palaeozoic Palaeogeography and Biogeography
 .
Geological Society Memoir
, v.
12
, p.
75
-
85
.
Shackleton
,
N.J.
,
N.
Crowhurst
,
T.
Hagelberg
and
N.G.
Pisias
1995
.
A new Late Neogene timescale: Application to ODP Leg 138 Site 846
.
Proceedings of the Ocean Drilling Program Science Results
, v.
138
, p.
73
-
101
.
Stockmarr
,
J
.
1971
.
Tablets with spores used in absolute pollen analysis
.
Pollen et Spores
 , v.
13
, p.
615
-
621
.
Tucker
,
R.D.
and
W.S.
McKerrow
1995
.
Early Paleozoic chronology: a review in light of new U-Pb zircon ages from Newfoundland and Britain
.
Canadian Journal of Earth Sciences
 , v.
32
, p.
368
-
379
.
Waterhouse
,
H.K
.
1995
.
High-resolution palynofacies investigation of Kimmeridgian sedimentary cycles
 . In
M.R.
House
and
A.
Gale
(Eds.),
Orbital forcing timescales and cyclostratigraphy
 .
Geological Society Special Publication
, v.
85
, p.
75
-
114
.
Waterhouse
,
H.K
.
1999
.
Orbital forcing of palynofacies in the Jurassic of France and the United Kingdom
.
Geology
 , v.
27
, p.
511
-
514
.

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