A WHOLE-ROCK GEOCHEMICAL APPROACH TO THE RECOGNITION AND CORRELATION OF “ MARINE BANDS

Laterally extensive, thin, eustatically controlled, transgressive marine shale beds that occur within paralic sequences are generally regarded as reliable correlative markers. Such shale beds in the Carboniferous of NW Europe are referred to as marine bands and have been used extensively for stratigraphic correlations, particularly in the petroleum industry, where they are used to construct interwell correlations. True marine bands are represented by black anoxic shales (characterized by high U levels and high gamma API responses) that contain definitive ammonoid assemblages, i.e., demonstrably were deposited in a marine environment. However, not all black shales in the Carboniferous of NW Europe are the product of marine deposition, despite which they are still colloquially referred to as “marine bands” and are used for stratigraphic correlations. The problem of “marine band” recognition and correlation is exacerbated when dealing with well bores, where only wireline-log data and cuttings are available. This study demonstrates how inorganic geochemical data are used as a means to refine the identification of true marine bands and how these data can be used for enhanced stratigraphic correlations. “Marine-band chemostratigraphy” is established using core sections from the onshore Carboniferous Coal Measures sequences encountered in the West Midlands of England. Using variations in U, Mo, Zn, Cu, V, P2O5, Al2O3, Th, and Zr concentrations, a geochemically based facies classification scheme is erected, which allows the differentiation of mudstones deposited in marine, freshwater lacustrine, and floodplain environments, and which has been validated by palynological and sedimentological facies data. This scheme is successfully extended to a nearby well from which only cuttings are available. The general concept of marine-band chemostratigraphy can be applied to the sedimentary rocks deposited in any coastal-plain to marginal-marine setting. The methodology provides a robust technique for the identification and correlation of “marine bands” and also demonstrates the importance of inorganic geochemical data in the context of sequence stratigraphy.


INTRODUCTION
During late Carboniferous times extensive, low-lying coastal swamps were developed across much of what is now northwest Europe, resulting in the deposition of thick coal sequences and mudstone-dominated sequences.These successions became the focus of widespread coal mining in many countries, and ensured that their stratigraphy became closely studied.More recently, the associated sandstones have become the target for hydrocarbon exploration in the Southern North Sea.It was rapidly realized that, associated the nonmarine sandstones, coals, and gray rooted mudstone deposits, certain mudstone horizons possess marine fossils (goniatites and bivalves).Following the pioneering work of Ramsbottom (1977Ramsbottom ( , 1978) ) and Ramsbottom et al. (1978), the term "marine band" was adopted for such horizons.In the Carboniferous of the UK, marine bands correspond to thin (one to two meters) intervals of organic, pyritic, dark gray to black fissile shale containing goniatites and bivalves, along with frag-ments of degraded wood and biogenic kerogen.By using goniatite and bivalve zone fossils at outcrop, these horizons were found to display notable lateral persistence, and "marine band" correlations became widely used to constrain the stratigraphy of Upper Carboniferous sequences in the UK and across Europe (Ramsbottom, 1977(Ramsbottom, , 1978;;Ramsbottom et al,. 1978;Wilson, 1980;Eager et al., 1991;Owen, 1998;Wilson and Chisholm, 2004).
The identification, and particularly the correlation, of marine bands in boreholes and wells is more difficult due to the scarcity of macrofossils.However, petroleum exploration in Southern North Sea has increased the need for stratigraphic correlations in the Upper Carboniferous.Commonly, the recognition and correlation of marine bands in the southern North Sea has relied on gamma logs and palynological data.In many instances, marine bands have high gamma API values, due to their high levels of uranium (U).The U is concentrated mostly in organic matter, which is preserved in the shales owing to their deposition under anoxic conditions (Adams and Weaver, 1958;Archard and Trice, 1990).Leeder et al. (1990), Davies andMcLean (1996), andO'Mara andTurner (1997) have all reported that marine bands commonly coincide with "hot shales", i.e., shales with gamma API values two to five times greater than the surrounding mudstones.Therefore, through time, "marine band" in the Carboniferous of Northern Europe has become a generic term synonymous with black fissile shales that have high gamma values, whether they have demonstrably marine faunas or not.Subsequently, when correlating these sequences, there has been a tendency to accept all high-gamma shale horizons as "marine bands" without further corroboration from biostratigraphic and sedimentological data.This approach is flawed, since high gamma responses in the Carboniferous can be the result of heavy-mineral lags or tonsteins and not all marine bands exhibit high-gamma API responses.
When dealing with non-cored well-bore sequences, palynological analysis of cuttings is the most reliable method for the correlation of Upper Carboniferous paralic sequences (McLean and Murray, 1996;McLean and Davies 1999).However, marine bands in the Upper Carboniferous often do not exhibit a diagnostic marine palynoassemblage, their position being inferred from the occurrence of abundant amorphous organic material, bleached palynomorphs, high numbers of Potonieisporites spp., and possibly the arenaceous foraminifera.While these criteria can help prove the marine nature of a thin, high-gamma shale, they are not necessarily diagnostic of a particular marine band, making identification and correlation of individual marine bands difficult.Generally, the identity of marine bands is constrained by the background biostratigraphic zonation of the associated nonmarine mudstones.Typically, one or two marine bands may occur within one biostratigraphic zone which may be equivalent to 30-60 m intervals.However, in instances where the biostratigraphic zonation of the nonmarine mudstones is of low resolution, or the palynological elements are facies-dependent, there is potential for the misidentification and potential miscorrelation of marine bands.
This study focuses on two extensively cored intervals through the Coal Measures from the West Midlands of England (Besly and Cleal, 1997;Pearce et al., 1999) (Fig. 1).The sections have been sedimentologically logged and correlated using palynology and chemostratigraphy by the authors cited above.The purpose of this paper is to demonstrate that inorganic geochemical data can be used to differentiate between true marine bands, nonmarine mudstones, heavy-mineral lags, coals, and tonsteins in two cored sections.The geochemical criteria established from the cored intervals are then applied to cuttings from a nearby well.The resulting high-resolution correlation, when placed within a sequence stratigraphic context, can greatly improve reservoir-scale and subregional correlation schemes of paralic sequences.

DATA SET AND STUDY INTERVALS
The study focuses on onshore Upper Carboniferous (Duckmantian) coal measures sections from the Staffordshire Coal Field, West Midlands (U.K.), as encountered by two coal exploration boreholes (Little Paddocks and The Rowe) and one oil exploration well (North Stafford No-1 (NS-1) (Fig. 1).For this study, 436.9 m of core is available from The Rowe borehole, 149.4 m of core is available from the Little Paddocks borehole, and cuttings samples covering c.400 m of section are available from well NS-1 (Fig. 2).
Detailed sedimentological logs are available for the two core intervals, along with detailed palynological analyses.The study intervals include a total of eight thin shales that have been called marine bands and have been used for regional correlation.Each of these "marine bands" has been assigned a name: Vanderbeckei (VMB), Maltby (MMB), Clown (ClMB), Haughton (HMB), Sutton (SuMB), Aegiranum (AMB), Shafton (SMB), Edmondia (EMB), and Cambriense (CMB).The positions of the marine bands in the Little Paddocks and The Rowe boreholes are inferred from core descriptions and E-logs, as well as with reference to nearby boreholes Allotment-1 and Harts (Fig. 1).Their positions in well NS-1 are based on gamma responses and coal-seam stratigraphy (Besly and Cleal, 1997;Besly, personal communication), along with reference to borehole Allotment-1 (Pearce et al., 1999); see Figure 2.

MATERIALS AND METHODS
The analyzed samples consist primarily of mudstones and occasional coals.The size of the core samples ranges from 2 to 3 cm 3 , whereas between 300 and 400 chips, making up about 1 g in weight, are hand-picked from each cuttings sample of well NS-1.Where chips of black and medium to dark gray mudstone occur in the same cuttings sample, both lithologies are picked and separately analyzed, just in case each lithology represents a different facies, (i.e., the black mudstones might be marine in origin, but the lighter gray ones could be nonmarine).With respect to well NS-1, 37 cuttings from the Etruria Formation and the Halesowen Formation, plus 106 cuttings from the Coal Measures, were analyzed geochemically, the spacing between the analyzed samples ranging from 1 m to 15 m.A total of 140 core samples from the two boreholes were analyzed geochemically, the spacing of these samples ranging from 1 m to 30 m (Fig. 2) with closely spaced sampling (0.5-1 m) across potential marine bands and other transgressive mudstone horizons.All of the samples were prepared for geochemical analysis by alkali fusion (Pearce et al., 1999) and then analyzed using inductively coupled plasmaoptical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS), with data being acquired for 48 elements and the methods outlined in Jarvis and Jarvis (1992) and Pearce et al. (1999).

CHEMOSTRATIGRAPHIC ZONATION
Chemostratigraphy, or chemical stratigraphy, uses majorelement and trace-element geochemistry to characterize and correlate sedimentary sequences.The elemental composition of sediments is highly variable due to source composition, facies, paleoclimate, and diagenesis (Ratcliffe et al., 2008, Pearce et al., this volume, and references cited therein).Even otherwise apparently homogeneous sequences show differences in their wholerock geochemistry, which has resulted in chemostratigraphy being extensively used in the petroleum industry to help define stratigraphic correlations between well bores (Ehrenberg and Siring, 1992;Racey et al., 1995;Pearce et al., 1999;Pearce et al., 2004;Wray, 1999;Ratcliffe et al., 2004;Pearce et al., 2005a;Pearce et al., 2005b;Ratcliffe et al., 2006).
Here, chemostratigraphy has been applied to the Upper Carboniferous sequences encountered in both boreholes and well NS-1 (Pearce et al., 1999).The existing lithostratigraphy and the resultant chemostratigraphic zonations together provide a stratigraphic correlation framework for the study sequences that have been used to constrain whether chemostratigraphy can be used for identification and correlation of marine bands.
Based on interpretations made from the graphical analysis of the mudstone geochemical data, a chemostratigraphic correlation has been established between well NS-1 and the Little Paddocks, Harts, and The Rowe boreholes (Fig. 2).Ideally, when applying chemostratigraphy to Upper Carboniferous sequences such as these, any correlations should be based on the lateral associations of the predominant facies, i.e., the nonmarine deposits, rather than depending on the recognition of thin marker beds such as marine bands, which are difficult to sample when dealing with cuttings.Once the broad chemostratigraphic zonations and correlations have been erected, they can then be refined by using marker beds, e.g., marine bands, coals, tonsteins, or paleosols (Pearce et al., this volume).
The chemostratigraphic zonation for the study intervals is founded on fluctuations in the abundance not only of those elements associated with changes in facies and clay mineralogy, e.g., CaO, Na 2 O, K 2 O, Fe 2 O 3 , MgO, P 2 O 5 , U, Mo, Cs, Sc, Sr, V, Rb, Ni, Cu, and Co, but also those elements associated with heavy minerals and which ultimately can be linked to variations in depositional environment and provenance, e.g., Zr, Hf, Ti, Nb, Ta, Th, and Cr.In well NS-1, four chemostratigraphic packages are recognized (Fig. 3): Package 1, the oldest package, is equivalent to the Coal Measures, Package 2 corresponds to the Etruria Formation, Package 3 is equivalent to the Halesowen Formation (Besly and Ceal, 1997), and Package 4 corresponds to Triassic sequences.The geochemical characteristics of each package are illustrated in Figure 3 and are summarized briefly below, as follows (also see Pearce et al., 1999):

Package P1: Coal Measures
This package is characterized by its lower Zr/Cr and Nb/ Al 2 O 3 values than Packages P2 and P3.Compared to Package P2, Package P1 has higher and upwardly decreasing K 2 O/Al 2 O 3 , and MgO/Al 2 O 3 ratios (Fig. 3).Package P1 is also marked by initially low and upwardly increasing Cs/Rb and TiO 2 /Al 2 O 3 ratios.The package is recognized in all three boreholes and in well NS-1.In well NS-1, Package P1 can be divided into two units: P1a and P1b.P1a is differentiated from P1b by having generally higher MgO/ Al 2 O 3 , K 2 O/Al 2 O 3 , and Cr/Al 2 O 3 , along with lower Cs/Rb and TiO 2 /Na 2 O ratios.A high gamma API spike at c. 780 m in well NS-1 marks the position of the Stafford Tonstein (Besly and Cleal, 1997), as do similar spikes in the boreholes, resulting in its confident correlation.However, only cuttings from the tonstein in well NS-1 have been analyzed geochemically, which show it to be characterized by high Be, Sn, Y, Pb, U, and Th concentrations (Spears and Rice, 1973).

Package P3: Halesowen Formation
This package is characterized by having higher MgO/Al 2 O 3 , K 2 O/Al 2 O 3 , TiO 2 /P 2 O 5 , and lower TiO 2 /Na 2 O ratios than Package P2.Package P3 also exhibits high Cr/Al 2 O 3 and Nb/Al 2 O 3 ratios comparable to Package P2 (Fig. 3).Package P3 can be further subdivided into two units, with the lower Unit 3A having high MgO/Al 2 O 3 ratios and Unit 3B having higher K 2 O/Al 2 O 3 and TiO 2 /P 2 O 5 values (Fig. 3).Package P3 is recognized in well NS-1 and the nearby Harts borehole (Pearce et al., 1999) and is believed to lie somewhere above the study intervals of The Rowe and Little Paddocks boreholes (Besly and Cleal, 1997).

Package 4: Triassic Sequences
The package corresponds to the Triassic sequences and is characterized by having much higher MgO/Al 2 O 3 , K 2 O/Al 2 O 3 , and Nb/Al 2 O 3 ratios than Package P3 (Fig. 3).The Triassic interval also exhibits significantly lower Cr/Al 2 O 3 and Cs/Rb ratios than Package P3.

PALYNOSTRATIGRAPHIC ZONATION
The core intervals from the Little Paddocks and The Rowe boreholes have also been subjected to extensive palynological analyses, with the recovered palynological assemblages being referred to the Langsettian to Westphalian D Biozones of the inhouse University of Sheffield biozonation for the Carboniferous (modified after McLean, 1995).With regards to the Little Paddocks borehole, the study section corresponds to Biozones W6a and W5b (McLean, 1995), of Bolsovian age.The top of Biozone W5b is placed at 1038 m by the top of Grumosisporites varioreticulatus, with the base of the X Biozone of Smith and Butterworth (1967) and the SL Biozone (Clayton et al., 1977) being placed at 1084.50 m (Fig. 2).The Cambriense MB is placed at 1063 m and the Shafton MB at 1079.6 m.
The study interval of The Rowe borehole is assigned to Biozones W2b-W5b (middle Langsettian to early Bolsovian) (Fig. 4).The top of Biozone W2b is placed at 1217.60 m to correspond with the top of Radiizonates aligerens, whereas the top of Biozone W3a, which coincides with the Vanderbeckei MB, is put at 1156.65 m, based on the occurrence of Schulzospara rara and Apiculatisporis variocorneus.Very few samples from the interval 993.25 m-1147.40m have been analyzed, though nevertheless it is assigned to Biozones W4a-W4b, due to the presence immediately above 993.25 m of Biozone W4c elements R. striatus, T. cf.sculptils, L. noctina, and common L. noctiuna.The Clown MB is placed at 951.5 m within Biozone W5a, with the Maltby MB being put in a sample gap between 975.4 m and 993.25 m.Samples from 892.1 m down to 924.8 m are assigned to Biozone W5a, with the top of Ahrensisporites guerickei and common Triquitrites sculptilis, Vestispora magna, Cristatisporites connexus, and Lycospora rotunda rotunda occurring at 892.1 m.The Aegiranum MB lies close to the top of this biozone at 894.2 m, with the Haughton MB being near its base at 921.9 m.The remainder of the study interval is assigned to Biozone W5b, with its top being placed just above 803.4m, based on the top of Grumosisporites varioreticulatus.The Cambriense MB is placed at 803.7 m and the Shafton MB is put at 819.3 m, which also coincides with the base of SL Biozone of Clayton et al. (1977).
The robust, integrated chemostratigraphic and palynostratigraphic correlation established for The Rowe and Little Paddocks is consistent with the correlation presented by Besly and Cleal (1997) and Besly (personal communication), which are based on detailed core logging and E-log interpretations, and provides a stratigraphic context into which to place the marine bands.

RECOGNITION OF MARINE BANDS USING GEOCHEMICAL EVIDENCE
Numerous candidate "marine bands" have been recognized following detailed logging of core sections through the Coal Measures sequences encountered in the Staffordshire Coal Field (Besly and Cleal, 1997;Besly, personal communication).They correspond to laminated, dark gray to black mudstones that often immediately overlie coal seams.The mudstones are generally organic and pyritic, and contain fish detritus, bivalves, and arenaceous foraminifera.Some of these intervals correspond to marine bands, but others reflect "freshwater" lacustrine flooding events: differentiating the two can be difficult where macrofossils, core material, or E-logs are absent (Besly and Cleal, 1997).

Geochemical Evidence Used to Recognize Marine Bands
Adams and Weaver (1958), Archard and Trice (1990), Leeder et al. (1990), Davies and McLean (1996), and O' Mara and Turner (1997) have shown that the marine and nonmarine mudstones of the Coal Measures differ geochemically.Marine black mudstones have more U than continental mudstones, because the latter were deposited in more oxidizing environments, where U forms highly soluble U 6+ .Marine bands were deposited during times of very low oxygen levels (anoxia) and little clastic input, which allowed the concentration of insoluble U 4+ .Sediments contain abundant organic matter and U can be remobilized, becoming fixed in amorphous organic matter and adsorbed onto plant detritus and clay minerals (Archard and Trice, 1990;Leeder et al., 1990).Th, on the other hand, is effectively insoluble in water and thus its abundance has little relationship to the oxidation state of the sediments.Consequently, the above workers have used low Th/U values as evidence for the presence of marine bands over the Coal Measures sections.Leeder et al. (1990) note that Coal Measures mudstones with > 6 ppm of U probably have marine affinities.Adams and Weaver (1958) suggest that Th/U values below 2 are diagnostic of marine depositional environments, whereas Hollywood and Whorlow (1993) use a Th/U value below 3 as an indicator for marine deposition.Davies and McLean (1996) suggest that Th/U values below 3.8 and Th/K 2 O values below 6 indicate marine bands, which are also frequently enriched in phosphatic, carbonaceous, and bituminous material.
O'Mara and Turner (1997) erected a fourfold classification of marine bands from the Namurian and Westphalian successions of the Pennine Basin and the Southern North Sea, based primarily on the gamma and spectral gamma responses of the bands, coupled with sedimentological and biostratigraphical data.(b) Vanderbeckei-type marine bands were laid down in shallower water than the Namurian marine bands, during times when seawater oxygen levels were somewhat reduced.They contain thin "ammonoid acme" intervals and abundant benthic marine fossils and have relatively reduced U levels.These marine bands have little terrestrial humic material, because their depositional environments lay some way from land.
(c) Duckmantian-Bolsovian marine bands have high U levels due to the presence of abundant terrestrial plant material onto which U has been adsorbed.U levels are also high in the phosphatic valves of the inarticulate bivalve Lingula, which are particularly common in these types of marine band.
(d) Brackish-water Lingula beds contain much terrigenous clastic material but little U.
In most instances, marine bands can be identified from their spectral gamma responses, though problems occur where marine bands lack high U levels.It should be noted that the current study focuses on Duckmantian sequences, and marine bands are similar to those described in category (c) above

GEOCHEMICAL CHARACTERISTICS OF MARINE BANDS
As stated earlier in this paper, Th and U data can be used to recognize marine bands.In cases where these data are crosscorrelated with the data for other elements, e.g., Mo, P 2 O 5 , Cu, Zn, Ni, V, and Zr, it is possible to differentiate between mudstones deposited in freshwater and brackish to marginal marine environments from those mudstones that come from proven marine bands.
Metallic elements, particularly P 2 O 5 , Mo, Ni, and Cu, but also Cr, Co, and Zn, are common in certain black mudstones.P 2 O 5 levels are high in mudstones containing Lingula valves and/or fish scales (O'Mara and Turner, 1997), whereas Mo/Al 2 O 3 levels can be linked with the occurrence of amorphous organic matter (AOM; Wilde et al. 2004) (Fig. 5) and, to a lesser extent, the presence of algal detritus, as determined from palynofacies investigations.High levels of Ni, Co, Zn, V, Cu, and Cr are typically found together with high U levels and are all associated with AOM and sulfide minerals, such as pyrite.However, high U levels can be due to the presence of heavy minerals such as zircon, apatite, and monazite occurring as silt-grade grains even in extremely finegrained mudstones, so when attempting to identify marine bands, the possible presence of these heavy minerals and their contribution to the overall U content can be taken into account by employing U/Zr and U/Th values rather than absolute U contents.
To highlight the range of geochemical characteristics associated with a marine band, Figure 6 plots the data for selected elements acquired from the Aegiranum MB as encountered in The Rowe borehole.Apart from the Th and U data, which are plotted as Th/U values, the remainder of the data are plotted as Al 2 O 3 ratios, in order to compensate for the variable amounts of clay in the analyzed samples.Samples from the marine band are characterized by very high U/Al 2 O 3 , Mo/Al 2 O 3, and P 2 O 5 /Al 2 O 3 values, as well as high Al 2 O 3 ratio values for the metallic elements, i.e., Ni, V, Zn, and Cu.In addition, these samples have low Th/U values, along with high Mo/Th and U/Zr values (not shown).Figure 6 shows that two samples from the Aegiranum MB have high Mo/Al 2 O 3 values, which are due to the presence of abundant AOM (as determined by palynological investigations).Furthermore, high P 2 O 5 /Al 2 O 3 levels are recorded from the marine band and can be linked to the occurrence of Lingula valves and fish debris noted in the band during core logging.
Figures 7 to 9 relate to the three study intervals and plot the geochemical criteria used previously to characterize the Aegiranum MB. Figure 8 shows that by using the aforesaid criteria, several marine bands are recognized in the core from The Rowe borehole: the Cambriense and Shafton both occur in Unit C7, the Edmondia is found at the top of Unit C6, the Aegiranum, Haughton, and Clown all occur within Unit C5, and the Vanderbeckei is identified at the top of Unit C1.However, the position of the Maltby MB remains somewhat uncertain, though it probably lies in an interval from which no samples have been collected.In the cored interval from the Little Paddocks borehole the Cambriense and Shatfon marine bands are well defined (Fig. 9).The Vanderbeckei, Clown, Aegiranum, and Edmondia marine bands are all recognized in well NS-1 (Fig. 7), though the Shafton MB is poorly defined, with only high P 2 O 5 / Al 2 O 3 and Cu/Al 2 O 3 values being recorded.It probably is very thin in this well, and the characteristically subtle U enrichments and, to a lesser extent, the Mo enrichments have been "diluted" by the incidence of nonmarine, dark gray mudstone chips in the analyzed cuttings sample collected over the three-meter-thick interval that includes the marine band in question.Therefore, when attempting to recognize marine bands using geochemical data acquired from cuttings, attention should be paid to the data relating to U, U/Al 2 O 3 , P 2 O 5 /Al 2 O 3 , Mo/Al 2 O 3 , and the metallic elements, along with Th/U values, so that the bands can be identified with confidence.

Criteria Used for Geochemical Classification of Mudstones
By utilizing a combination of geochemical criteria, petrophysical data, and biostratigraphic data the potential for the recognition of marine bands can be confidently proposed.In the classification proposed below, each analyzed mudstone sample is given a weighted score that ultimately allows those mudstones deposited during marine flooding events, during brackish water or lacustrine flooding events and in floodplain settings, to be geochemically differentiated, with the facies of each sample being corroborated by sedimentological and palynological data.The classification also ensures that samples with either high U or high P 2 O 5 levels, due to the presence of abundant heavy minerals, can be disregarded.The interpretation of the scores obtained by applying the criteria listed in Table 1 are as follows: Score of 4 = floodplain mudstone Score of 7 = brackish water or lacustrine Score of 10 = marginal marine mudstone Score of 13 or 16 = marine mudstone (= marine band) If a sample has a very high Zr/U value (above 65), it is presumed to contain abundant heavy minerals and is awarded a final score of 2. If a sample comes from a coal, it is awarded a default score of 0.
Marine bands can also have high levels of Cu, V, Ni, Zn, Fe 2 O 3 , MnO, and CaO, due to the presence of macrofossils and sulfide minerals, though such high levels are not recorded exclusively from marine bands.

Results of the Geochemical Classification
Table 2 summarizes the geochemical classification scores for the analyzed mudstones from The Rowe borehole and shows that all the samples from known marine bands have scores above 10, i.e., marginal marine mudstones-marine mudstones.Furthermore, the brackish-water or lacustrine flooding-event mudstones can be differentiated from the proven marine mudstones, inasmuch as the former have high P 2 O 5 /Al 2 O 3 values, occasional moderate enrichments in the metallic elements, and no U or Mo enrichments, so do not exhibit the low Th/U values and high Mo/Th values that characterize the marine bands.For example, Table 2 shows that the 827.1 m sample has a score of 7 and is interpreted as a brackish-water/lacustrine mudstone.The core descriptions report the presence of fish scales from around this depth, which presumably are the source for the high P 2 O 5 level recorded from this sample.In well NS-1 and The Rowe and Little Paddocks boreholes, application of the aforesaid geochemical criteria allow the marine bands and those mudstones deposited by lacustrine flooding events to be differentiated with confidence.
When applied to the thicker marine bands from which several samples have been analyzed, the geochemical classification reveals that they record fluctuating anoxic events related to localized periods of transgression and regression that are associated with increases in the rates of input of terrigenous material (Boyd et al., 2000;Wadsworth et al., this volume).For example, the Aegiranum MB in The Rowe borehole was deposited initially in anoxic conditions, which then became progressively more brackish before floodplain environments became established (Fig. 6).
With respect to individual marine bands, the geochemical classification scheme can also highlight lateral changes in depositional settings.For instance, the Shafton MB encountered in the Little Paddocks borehole is classified as marginal marine (Fig. 9), whereas the mudstone samples from the same marine band in The Rowe borehole are classified as fully marine (Fig. 8, Table 2).In addition, the sample from the base of the above marine band in the Little Paddocks borehole is classified as brackish/lacustrine, whereas the overlying mudstone sample is classified as marginal marine.These variations in the geochemistry and thickness of the Shafton MB show that this band, as penetrated by The Rowe borehole, was deposited by a marine flooding event into a floodplain area, whereas the same band encountered in the Little Paddocks borehole was deposited by a similar event but into a lagoonal environment.
High Fe 2 O 3 , MgO, MnO, and CaO levels are often recorded from samples associated with the marine bands, though similar   (Coveney et al., 1991).** Samples with high U and P 2 O 5 levels may come from horizons containing exceptionally abundant heavy minerals; these high element levels would lead to the samples being classified as "marine".To compensate for this, Zr/U values are used to identify those samples containing abundant heavy minerals.
concentrations of these elements are found in the other mudstones due to the presence of calcareous macrofossils (Lingula), siderite, ankerite, and ferroan calcite.Particularly high levels of these four elements are recorded from mudstones lying above the CMB in the Little Paddocks and Harts boreholes, the mudstones probably having been deposited during brackish-water/lacustrine flooding events.
In well NS-1, low Th/U values and high levels of the metallic elements confirm the presence of the Edmondia and Aegiranum marine bands, whereas high levels for P 2 O 5 , Mo, and the metallic elements, e.g., Cu, indicate the occurrence of the Shafton MB (Fig. 7).Furthermore, the Edmondia MB is placed at 770 m, with the 820 m sample being classified as a marine mudstone and thus could be from one of the minor marine bands such as either  Although marine mudstones can often be identified from just their color (Besly and Cleal, 1997), this study demonstrates that such an approach is flawed, inasmuch as those geochemically classified as marine mudstones exhibit a variety of colors.For example, mudstones from the Edmondia and Maltby marine bands are black (N1-from the Rock Color Chart), whereas those from the Aegiranum MB are dark gray (N3) and the mudstones from the Vanderbeckei, Shafton, and Cambriense marine bands are medium gray (N5).Therefore, with respect to cuttings, the selective picking of black mudstone chips in preference to other colors offers no confident guarantee that material from marine bands has been selected for geochemical analysis.

Integration of the Geochemical, Sedimentological, Microfossil, and Palynofacies Data
With reference to The Rowe and Little Paddocks boreholes, the geochemical classification of the mudstones is supplemented by sedimentological data, palynofacies data, and information regarding flora, macrofossils, and microfossils.The marine bands correspond to laminated, dark gray to black mudstones that mostly immediately overlie coal seams.Core inspection reveals that these mudstones are typically pyritic and rich in organic matter, with some containing fish detritus, bivalves, and arenaceous foraminifera, though no goniatites were observed.There are instances, especially if macrofossils are absent, where core examination alone cannot differentiate between these different flooding events.One then needs to turn to palynology to achieve this.Table 2 lists the interpreted palynofacies for the flooding events identified in the above boreholes, and the criteria employed to discriminate the various palynofacies are as follows (McLean, 1995): • Marine facies: in general, "typical" marine palynofacies indicators, e.g., abundant amorphous organic material, bleached palynomorphs, high numbers of Potonieisporites spp., relatively high numbers of hinterland forms sensu Davies and McLean (1994), and striate and non-striate bisaccate gymnosperm pollen, appear restricted to the lower parts of the marine bands.Distinct palynofacies variations are noted within individual marine bands, although they have not, as yet, been studied in detail.
• Brackish-water facies: the remains of scolecodonts, which are the jaw apparatuses of brackish to marine annelid worms, are relatively common in the marine band samples.Their presence may signify deposition in brackish lacustrine settings.
• Freshwater facies: the relatively common occurrence of the freshwater alga Botryococcus indicates that the depositional environments had high water tables, with deposition occurring in either interdistributary-marsh or interdistributarylake settings.Freshwater influences are recognized in some of the marine-band samples in this study, thus suggesting that marine flooding events invaded freshwater settings, though sometimes this makes positive discrimination between freshwater and marine facies difficult.
• The abundance and distribution of fish detritus in the samples are recorded, but high abundances of such detritus can occur in mudstones deposited in marine and freshwater environments.
Palynological assemblages were subjected to palynofacies analyses, i.e., subjective and not quantified, in order to generate paleoenvironmental information that could be used to supplement the chemostratigraphical and sedimentological information relating to the two boreholes but not well NS-1.Consequently, certain strata have thus been identified as being of either marine or freshwater origin (Table 2, Fig. 10).
There is good correspondence between the placement of marine bands based on geochemical evidence and their recognition using palynology (Fig. 11).In addition, Table 2 highlights the intervals classified geochemically as lacustrine or brackish water, floodplain, and vegetated swamp (= coal), and once again, there is a strong correlation between these interpretations and the palynofacies.
Where possible, any facies interpreted from geochemical evidence should always be corroborated by sedimentological data and palynofacies data.However, when dealing with cuttings, inorganic geochemical data could be used, preferably in combination with palynological data (and along with E-log data), to interpret facies.

Chemostratigraphy and Correlation of Marine Bands
The study has demonstrated that geochemical data can lead to better recognition and correlation of the mudstones previously defined as solely "marine bands" occurring in the onshore Upper Carboniferous Coal Measures sequences of the West Midlands (UK).However, the approach utilized in this study can be applied equally well to any coastal-plain to marginal-marine sedimentary rocks.
The recognition and correlation of marine transgressive deposits, coals, and unconformities in paralic sequences of all ages are of vital importance to sequence stratigraphy (Flint et al., 1995;Bohacs and Suter, 1997, Hampson et al., 1997) Furthermore, Hampson et al. (1997) argue that coal-measures facies architecture is dependent on variations not only in the rate of sediment supply but also in the rate of change in accommodation space (Fig. 12) and put forward a detailed discussion on the application of sequence stratigraphy in paralic sequences, and the recognition of sequence boundaries, incised-valley fills, and interfluves, along with the significance of coals and marine bands.In lowaccommodation settings during development of lowstand systems tracts (LSTs), incised valleys are developed above subregionally correlative sequence boundaries (SB), whereas marshy areas develop as accommodation increases during development of a transgressive systems tract (TST).Marine bands represent the periods of most condensed sedimentation and are developed at the point of maximum rate of base-level rise (the R inflection point, not at the eustatic sea-level highstand; Fig. 12).Flint et al. (1995) and Hampson et al. (1997) suggest that differences in the lateral extent and faunal content ("marineness") of marine bands, coupled with the amount of base-level rise, allows a hierarchy of flooding surfaces, depending on whether the marine bands are either marginal marine or open marine in origin.It is suggested that with further investigation the geochemical classification of marine-band facies into brackish/lacustrine, marginal marine, and marine may provide a means to distinguish and improve the recognition hierarchy of marine band surfaces in a sequence stratigraphic context, especially in sequences where only cuttings are available.For example, it is speculated that marine bands in this study with high scores may correspond to true open-marine deposits equivalent to third-or fourth-order maximum-flooding events, whereas bands with marginal marine and brackish or lacustrine scores may correspond to fifth-order parasequence flooding events.

CONCLUSIONS OF THE CASE STUDY
• Geochemical data from mudstones are used to differentiate between marine and nonmarine mudstones, thereby enabling true marine bands to be differentiated from shales with features such as high gamma values that do not represent a period of marine flooding.• Mudstone geochemical data are used to recognize marine bands in the core sections from The Rowe and Little Paddocks boreholes (onshore West Midlands (UK)) that penetrate continental coal-measures sequences.The locations of the marine bands, as deduced from geochemical evidence, match closely with the locations of the same bands as deduced from palynofacies and sedimentological information.Geochemical data acquired from cuttings samples in well North Stafford-1 (NS-1) have likewise been employed to recognize marine bands.
• A confident correlation of the geochemically defined marine bands between The Rowe and Little Paddock boreholes and well NS-1 has been established, and does not conflict with the chemostratigraphic correlation proposed for the nonmarine mudstones.Combining the two correlations produces a robust and detailed correlation for the Upper Carboniferous sequences in question.
• With respect to well NS-1, previous E-log interpretations suggested that the Maltby marine band was absent, probably due to faulting, though geochemical evidence points to its possible presence at 847.34 m.In the same well, E-log interpretations implied the presence of the Vanderbeckei marine band at 1003.55 m.There is no geochemical evidence to support this, though evidence suggests that this marine band could be present at 990.6 m, with the strongest geochemical evidence, however, for the occurrence of the Vanderbeckei marine band coming from the mudstone sample at 1018.03 m.
• With respect to the core sections from the two boreholes, calibration of the whole-rock mudstone geochemical data with the sedimentological and paleontological information allows a facies classification scheme based upon the wholerock geochemistry to be developed.The scheme allows the differentiation of mudstones deposited in floodplain, brackish-water or lacustrine, marginal-marine, and anoxic marine environments, and can also be successfully applied to the mudstone geochemical data acquired from the cuttings of well NS-1.

GENERAL CONCLUSIONS
• Although the geochemically based facies classification scheme is at present specific only to the Coal Measures sequences described in this paper, the general concepts involved in its definition are equally applicable to any successions of marginal-marine sedimentary rocks.By following the procedures outlined herein, one should be able to identify marine bands and nonmarine mudstones with increased confidence and thereby better correlate the true marine bands.
• Once the geochemical data have been calibrated against sedimentological information derived from core or outcrop, they can be employed to differentiate facies when dealing with well bores from which only wireline logs and cuttings are available.This is a particularly powerful tool when dealing with samples derived from a well bore that has been drilled with a fixed cutter bit, which tends to obliterate the primary lithological nature of the sedimentary rocks.
• Once true marine bands have been identified, the ability to confidently differentiate one marine band from another greatly improves the resolution of stratigraphic correlations in marginal-marine or coastal sequences.Furthermore, inasmuch as marine bands are laterally extensive, coeval surfaces associ-FIG.
12.-A basic accommodation envelope of an unconformity-bounded sequence (from Van Wagoner et al., 1990), and a conceptual sea-level curve showing the positions of systems tracts as a function of rate of creation and destruction of accommodation space necessary to preserve sediment (Flint et al., 1995).LST, lowstand systems tract; TST, transgressive systems tract; HST, highstand systems tract; SB, sequence boundary; MFS, maximum flooding surface.
ated with eustatic fluctuations, their improved chemostratigraphic correlation in sequences represented just by cuttings strengthens their role in the sequence stratigraphy of paralic sequences
FIG. 2.-A summary of the study intervals in boreholes (Little Paddocks and The Rowe) and well North Stafford No-1 (NS-1).The depths of samples analyzed by FIG. 4.-A comparison between the chemostratigraphic zonation and palynostratigraphic scheme in The Rowe and Little Paddocks boreholes.No palynostratigraphic

FIG. 6
FIG. 6.-Geochemical characteristics and geochemical facies classification associated with the Aegiranum MB in The Rowe borehole.

FIG. 9
FIG. 9.-Geochemical characteristics associated with the marine bands in the Little Paddocks borehole.
11.-Chemostratigraphic correlation of the Coal Measures sections combined with the correlation of the marine bands: The Rowe and Little Paddocks boreholes and well NS-1.

TABLE 1 .
-Criteria used to produce a weighted score from whole-rock geochemical data.
*Mo is present in pyrite and amorphous organic matter, therefore Mo levels typically are very high in nearshore (marine) environments close to vegetated swamps

TABLE 2 .
-A summary the of geochemical and palynological facies classification scheme for the silty claystones from the Rowe Borehole.

Stratigraphical Position Score Geochemical Classification Plant Detritus Lingula / bivalves Forams Fish Scales Palynofacies the
Sutton or Haughton marine bands.Previous E-log interpretations have suggested that the Vanderbeckei MB is at 1003.55 m, but the sample from this depth is not classified as a marine mudstone.However, the 990.6 m sample has high Mo/Al 2 O 3 and P 2 O 5 /Al 2 O 3 levels, whereas the sample from 1018.03 m is classified as a marine mudstone on the basis of a low Th/U value, high U and P 2 O 5 levels, and a moderately high Mo/Al 2 O 3 level.It is suggested that the Vanderbeckei MB lies at c. 990 m and the 1018.03m sample corresponds to an older marine band.