Chapter 41 Correlation of Triassic Sandstones in the Strathmore Field, West of Shetland, Using Heavy Mineral Provenance Signatures

Chapter 41

CORRELATION OF TRIASSIC SANDSTONES IN THE STRATHMORE FIELD, WEST OF SHETLAND, USING HEAVY MINERAL PROVENANCE SIGNATURES ANDREW C. MORTONa,b, ROB HERRIESc AND MARK FANNINGd a

HM Research Associates, 2 Clive Road, Balsall Common, West Midlands CV7 7DW, UK b CASP, University of Cambridge, 181a Huntingdon Road, Cambridge CB3 0DH, UK c Amerada Hess Malaysia, Level 9, Menara Tan and Tan, 207 Jalan Tun Razak, Kuala Lumpur 50400, Malaysia d Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ABSTRACT Integrated heavy mineral, mineral–chemical and zircon age data show that Triassic sandstones in the Strathmore Field result from the interplay of sediment derived from eastern and western sources. The Early Triassic Otter Bank Formation is interpreted as having a source on the British margin of the Faeroe-Shetland rift. Two main provenance components (recycled Devonian-Carboniferous Upper Clair Group in conjunction with Lewisian orthogneiss) were involved. The overlying Foula Formation (Middle-Late Triassic) was derived from high-grade metasedimentary/charnockitic basement rocks, interpreted as lying in the Nagssuqtoqidian belt of southern East Greenland on the opposite side of the rift. Zircon age data from the Foula Formation also provide evidence for an important Permian igneous event along the proto-northeast Atlantic rift. The switch in sediment supply from easterly-sourced to westerly-sourced detritus is the most clearly defined correlative event in the Triassic succession of the Strathmore Field. Variable supply from a subordinate zircon-rich component (probably of granitic origin) provides a basis for intra-Foula subdivision and correlation. The upper part of the Otter Bank Formation is characterised by a relatively high apatite/tourmaline ratio, believed to indicate the initial appearance of sediment from East Greenland. The construction of the correlation framework for the Triassic succession in the Strathmore Field depends crucially on identification and quantification of parameters that are sensitive to changes in provenance and insensitive to other processes that operate during the sedimentation cycle. Developments in Sedimentology, Vol. 58, 1037–1072 r 2007 Elsevier B.V. All rights reserved. ISSN: 0070-4571/doi:10.1016/S0070-4571(07)58041-6 1037

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Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

This study demonstrates that ditch cuttings and core samples yield closely comparable heavy mineral data, indicating that construction of correlation frameworks can be readily achieved using ditch cuttings samples, although ideally cuttings data would benefit from calibration with core material. Keywords: Correlation; heavy minerals; provenance; Triassic; NE Atlantic; Strathmore Field

1. INTRODUCTION The provenance-sensitive nature of heavy mineral assemblages makes heavy mineral analysis one of the leading non-biostratigraphic methods for correlation of sandstones (Dunay and Hailwood, 1995). Traditionally, correlation is achieved using biostratigraphic methods, but this may be difficult if biostratigraphic control is poor or absent. This is frequently the case for sandstones deposited in non-marine or paralic environments, particularly in red-bed settings, but may also occur in marine depositional environments, if sedimentation rates are so rapid that biostratigraphic events lack sufficient resolution for detailed sand body correlation. In such circumstances, it is important that other methods of correlation are used, either in isolation or to complement biostratigraphic data. There are a number of non-biostratigraphic approaches that can be used for correlation, including sequence stratigraphic methods, direct or indirect dating and provenance studies (Dunay and Hailwood, 1995; Morton et al., 2002). Heavy mineral analysis belongs to the group of provenance-based methods, together with approaches such as clay mineral analysis (Jeans, 1995), whole-rock geochemistry (Pearce et al., 1999) and Sm-Nd isotope geochemistry (Dalland et al., 1995). Successful application of any provenance-based correlation method depends first on the existence of changes in sediment provenance or transport history during deposition, and second on the recognition of such events. Heavy mineral analysis is ideally suited for establishing a non-biostratigraphic correlation framework, since heavy minerals are sensitive indicators of provenance and because the mineralogical manifestation of changing provenance can be readily distinguished from the effects of other processes that operate during the sedimentation cycle. Heavy mineral analysis has been successfully applied to the correlation of sedimentary successions since the early part of the last century. The principles behind application of heavy mineral analysis in correlation were first laid out by Milner (1923), and although new approaches have been developed and analytical methods have become more sophisticated, the same principles still apply today. Heavy mineral correlation studies have been used to help map and delineate sedimentary units, for example in the Permian Karroo system of South Africa (Koen, 1955), the Tertiary of southern England (Blondeau and Pomerol, 1969) and the Tertiary of California (Tieh, 1973). More commonly, however, heavy minerals have been used to aid correlation of hydrocarbon-bearing sandstones, driven by the economic need to develop reservoir correlation schemes. One of the earliest studies in this context was that of Reed and Bailey (1927) in the San Joaquim Valley of California; other key

2. Correlation Using Heavy Minerals

1039

publications are those of Feo-Codecido (1956) on Venezuela, Rahmani and Lerbekmo (1975) on Alberta, Hurst and Morton (1988) on the North Sea, and Allen and Mange-Rajetzky (1992) on the Clair Field, west of Shetland. Heavy mineral correlation has been applied across a wide range of clastic reservoir facies, from aeolian, through fluvial, deltaic and shallow marine, to deep water, and from Palaeozoic to Mesozoic (Morton et al., 2002).

2. CORRELATION USING HEAVY MINERALS Although heavy minerals are generally volumetrically minor constituents of sandstones, assemblages are potentially diverse; Mange and Maurer (1992) illustrate around 50 minerals of relatively common occurrence, many of which have restricted parageneses that are diagnostic of particular source lithologies. Heavy mineral data therefore readily identify the changes in provenance required to construct a correlation framework. Changes in sand provenance may be relatively gradual, due to unroofing of fresh lithologies in a single source area through continued erosion. Such changes are manifested by evolutionary trends in heavy mineral parameters. By contrast, sudden changes in heavy mineral assemblages indicate switching of source lithologies, suggesting either tectonic evolution of the hinterland or changes in basin configuration. Provenance, however, is not the only factor that controls sediment composition. The processes that have the potential to overprint the provenance signal during the sedimentary cycle include: weathering at source, prior to incorporation in the transport system; mechanical breakdown during transport; weathering during periods of alluvial storage on the floodplain; hydraulic processes during transport and final deposition; diagenesis during deep burial; and weathering at outcrop (Morton and Hallsworth, 1999). It is crucial for reliable correlation that the parameters used to construct the correlation framework are sensitive to provenance but not to the other processes that operate during the sedimentary cycle. In this regard, heavy mineral analysis has one major advantage over other provenance-based correlation methods; the technique has a long history, and consequently there has been a considerable amount of research into the response of heavy mineral suites to the processes that act during the sedimentation cycle (Hubert, 1971; Morton, 1985; Mange and Maurer, 1992; Morton and Hallsworth, 1999). The knowledge gained during this research has made it possible to generate parameters that filter out overprinting effects and can be used for correlation because they accurately reflect changes in sediment provenance and transport history. As discussed by Morton and Hallsworth (1999), the most important of the overprinting processes are (1) hydrodynamics, which fractionate abundances of minerals with contrasting hydraulic behaviour during transport, and (2) diagenesis, which leads to dissolution of unstable minerals by pore fluids. Two complementary methods are used to counteract the effects of hydrodynamic and diagenetic processes on heavy mineral suites. One is to use ratios of the abundance of stable minerals with similar hydraulic behaviour (Morton and Hallsworth, 1994). Since hydraulic behaviour is controlled mostly by grain size and grain density, these

1040

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

provenance-sensitive ratios compare the abundances of minerals with similar densities, with the analysis confined to a limited size range (63–125 mm). Suitable ratios include apatite/tourmaline, rutile/zircon, monazite/zircon and chrome-spinel/zircon. Garnet/zircon is also a valid discriminator of provenance provided there is no evidence for garnet dissolution. The alternative approach is to undertake varietal studies, which concentrate on variations seen within one mineral group. This strongly diminishes the range of density and stability within the data set, resulting in data that reflect provenance characteristics without significant hydrodynamic or diagenetic modification. The classical approach to varietal studies is to distinguish types on the basis of their optical properties. Optically determined varietal data include tourmaline colour and morphology (Krynine, 1946; Mange-Rajetzky, 1995), zircon morphology (Poldervaart, 1955; Lihou and Mange-Rajetzky, 1996), apatite roundness (Allen and MangeRajetzky, 1992; Morton et al., 2003) and garnet and staurolite morphology (Van Loon and Mange, 2007—this volume). With the widespread availability of electron microprobe facilities, it has become commonplace to constrain provenance on the basis of the geochemical characteristics of a mineral population. Geochemical data from a wide variety of minerals have been successfully used to infer provenance, but many of these, such as pyroxene (Styles et al., 1989), amphibole (Von Eynatten and Gaupp, 1999) and epidote (Spiegel et al., 2002), are unstable during diagenesis, and therefore have limited application, especially in correlation of deeply buried sandstones. Of all mineral groups used in mineral–chemical studies, garnet has proved the most useful to date. This is because garnet has a wide potential compositional range, with solid solution between seven principal end-members, shows significant differences in composition between different types of garnet-bearing lithology (Wright, 1938; Sobolev, 1964; Rub et al., 1977), and is relatively stable during burial diagenesis (Morton and Hallsworth, 1999). In consequence, garnet geochemical studies have proved particularly useful in characterising, distinguishing and correlating sand bodies on the basis of their provenance (e.g. Morton, 1985; Haughton and Farrow, 1988; Tebbens et al., 1995; Hutchison and Oliver, 1998; Hallsworth and Chisholm, 2000). Tourmaline is also stable during diagenesis, but although it shows provenance-related geochemical variations (Henry and Guidotti, 1985), comparatively few studies have used tourmaline geochemistry for differentiation and correlation of sandstones (Jeans et al., 1993; Morton et al., 2005a). Although heavy mineral and mineral–chemical data provide important constraints on the lithological and mineralogical characteristics of the sediment sources, they do not supply information on the age of the source regions. Information on provenance age can be acquired by isotopic methods, including whole-rock methods such as Sm-Nd (Mearns et al., 1989), and single grain methods, such as U-Pb dating of zircon (e.g. Sircombe, 1999) or monazite (Evans et al., 2001). The rationale adopted in this paper follows that of Morton and Grant (1998), who used heavy mineral and mineral–chemical data to distinguish sandstones with different provenances, and then undertook a limited programme of single-grain U-Pb zircon age dating using the sensitive high-resolution ion microprobe (SHRIMP) to constrain the location of their respective sources.

3. Triassic of the Strathmore Field

1041

3. TRIASSIC OF THE STRATHMORE FIELD Triassic sandstones in and around the British Isles almost invariably occur in redbed facies, deposited in broadly fluvial environments under arid to semi-arid conditions (Fisher and Mudge, 1998). Biostratigraphic controls are therefore scarce, and alternative approaches to correlation are required. Correlation is an especially important issue since Triassic sandstones are major hydrocarbon reservoirs across the entire region, including Wytch Farm in the Wessex Basin of southern Britain, Hewett in the southern North Sea, Marnock in the central North Sea, Beryl, Tern and Cormorant in the northern North Sea, Morecambe Bay in the Irish Sea, Corrib to the west of Ireland and Strathmore west of Shetland. Provenance-based correlation methods have been applied in several of these oil and gas fields. Heavy minerals (including tourmaline and apatite varieties) have been used to correlate Triassic reservoir sandstones in the Irish Sea area (Mange et al., 1999), tourmaline varieties in the Marnock Field and adjacent areas of the central North Sea (Mange-Rajetzky, 1995), and whole-rock geochemistry integrated with heavy minerals in the Beryl Field in the northern North Sea (Preston et al., 1998) and the Wessex Basin of southern Britain (Svendsen and Hartley, 2002). This contribution extends the application of heavy mineral methods in hydrocarbon reservoir correlation into the basins to the west of Britain. The Strathmore Field lies in the East Solan Basin, UK continental shelf Blocks 204/30 and 205/26 (Fig. 1), and contains hydrocarbons in the Triassic (Herries et al., 1999). As a result, the Triassic succession in the area has been drilled by several boreholes and extensively sampled by both core and ditch cuttings, providing a unique opportunity to characterise the provenance of Triassic sandstones in the rift systems to the northwest of Britain. The East Solan Basin is a Permo-Triassic rift basin that lies on the eastern edge of the Faeroe-Shetland Basin (Swiecicki et al., 1995). The Faeroe-Shetland Basin is contiguous with the Møre Basin to the northeast and the Rockall Trough to the southwest, all of which have a prolonged Mesozoic to Early Tertiary rift history that culminated with the separation of East Greenland from northwest Europe in the early Eocene (Roberts et al., 1999). The ultimate line of separation lies close to the East Greenland margin, as shown in Fig. 1. The sediment fill in the Permo-Triassic rift system to the northwest of Britain is interpreted as being entirely continental (Ziegler, 1990), but a narrow seaway was present further north along the rift system between northeast Greenland and mid-Norway (Seidler et al., 2004). Prospective source areas therefore occur on the Orkney-Shetland Platform (to the east), in central east and southeast Greenland (to the west), and the Rockall Plateau (to the southwest), as shown in Fig. 1. It should be borne in mind that Fig. 1 shows the reconstruction at end Palaeocene (immediately prior to the opening of the northeast Atlantic), and does not take into account the substantial stretching that occurred during the Mesozoic. Prospective sources to the west of Britain are therefore likely to have been considerably closer to the Strathmore area than the reconstruction in Fig. 1 suggests. The Triassic succession in the Strathmore Field (Fig. 2) is nearly 1000 m thick, as proved by well 204/30a-2, and comprises two sand-rich formations: the Otter Bank Fm. and the overlying Foula Fm. (Fig. 3). The Otter Bank Fm. is underlain by the

1042

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

Fig. 1. Best-fit pre-rift reconstruction of the northeast Atlantic (pre-magnetic anomaly 24), showing distribution of basement terrains on the British and Greenland margins, adapted from Scott (2000). Boundary between northern Rockall Plateau (NRP) and southern Rockall Plateau (SRP) is the inferred suture between Archaean crust in the north and Palaeoproterozoic crust, similar to the Ketilidian of south Greenland, in the south (Dickin, 1992). Box shows location of the map in Fig. 2. COB ¼ continent-ocean boundary.  ¼ location of Kulusuk sample described by Sørensen and Kalvig (2002).

4. Analytical Methods

1043

205/26

204/30 204/30-1

I D

R R

O

205/26-1

G

E

A

N

00

19

1900

20

00

205/26a-2 00

2000

202/5

0

2100

21

203/1

5 km

contour interval 50 ms TWT

OTT

ER

K BAN

LT FAU

Fig. 2. Location of the Strathmore Field wells discussed in this contribution. See Fig. 1 for regional location. Contours are on the base Cretaceous. Dashed lines show the 2D seismic grid used to define the prospect (Herries et al., 1999).

Otter Bank Shale Fm., dated as Griensbachian (basal Scythian) on the basis of a distinctive palynological assemblage (Swiecicki et al., 1995; Herries et al., 1999). The Otter Bank Fm. contains a very sparse palynological assemblage, most likely indicating a Scythian (Early Triassic) age (Swiecicki et al., 1995). Palaeomagnetic data (Swiecicki et al., 1995) indicate that the Otter Bank Fm. has predominantly reversed polarity, suggesting that it was deposited in the Diererian-Smithian (midScythian). The lower part of the Otter Bank Fm. comprises braided sandy fluvial deposits, with the upper part containing interbedded fluvial and aeolian sabkha deposits. Palynological constraints on the age of the Foula Fm. are also poor, but suggest that deposition began in the Ladinian and extended into the Carnian (Swiecicki et al., 1995). The Foula Fm. has also predominantly reversed polarity, consistent with a Ladinian age (Swiecicki et al., 1995). The lower part of the Foula Fm. also comprises interbedded fluvial and aeolian sabkha deposits, replaced by wholly fluvial deposits higher in the succession.

4. ANALYTICAL METHODS Core samples were gently disaggregated by use of a pestle and mortar, avoiding grinding action. Ditch cuttings samples were already disaggregated through the action of the drill bit. No chemical pretreatment was used, thus avoiding the possibility of modifying assemblages in the laboratory. Following disaggregation, the samples

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

1044

System Series

Stage

Substage

Lithostratigraphy

Rhaetian Late

Norian

Triassic

Carnian

?

Ladinian

? ? Foula Formation

Anisian

?

?

?

?

?

?

Middle

Scythian Early

205 Ma

Spathian Smithian

Otter Bank Formation

Dienerian Griesbachian

Otter Bank Shale Formation

248 Ma

Fig. 3. Triassic stratigraphy in the Strathmore Field area (from Swiecicki et al., 1995, and Herries et al., 1999).

were immersed in water and cleaned by ultrasonic probe to remove and disperse any clay that might have been adhering to grain surfaces. The 63–125 mm (very fine sand) fraction was separated by sieving, and placed in bromoform with a measured specific gravity of 2.8 to enable separation of heavy minerals by gravity settling. The heavy mineral residues were mounted under Canada balsam for optical study using a polarising microscope, with a split retained for microprobe study. Relative abundances of the non-opaque detrital component were estimated using a grain count of 200 (Table 1). Provenance-sensitive heavy mineral ratios (Table 2) were determined during the conventional petrographic analysis, using a count of at least 200 grains per mineral pair (where recovery allowed). The parameters used are as follows:
ATi (apatite/tourmaline index) ¼ % apatite in total apatite plus tourmaline
GZi (garnet/zircon index) ¼ % garnet in total garnet plus zircon
RuZi (rutile/zircon index) ¼ % rutile in total rutile plus zircon
MZi (monazite/zircon index) ¼ % monazite in total monazite plus zircon
CZi (chrome-spinel/zircon index) ¼ % chrome spinel in total chrome spinel plus zircon Mineral–chemical data were acquired from garnet using an AN10000 energydispersive X-ray analyser attached to a Cambridge Instruments Microscan V electron microprobe. Data reduction used the ZAF-4 programme. Garnet populations are plotted on ternary diagrams using molecular proportions of Fe+Mn, Mg and Ca as poles, as recommended by Droop and Harte (1995). The U-Pb analyses were undertaken using SHRIMP I at The Australian National University in Canberra. Zircon separates were obtained by standard density and magnetic separation techniques, avoiding further splitting on the basis of size or paramagnetic behaviour. An arbitrary, and presumed representative, fraction for

4. Analytical Methods

1045

Table 1. Detrital non-opaque heavy mineral abundances (frequency %) in the 63–125 mm fraction of Triassic sandstones from the Strathmore Field, on counts of 200 grains per sample Well 205/26a-3

205/26a-4

204/30a-2

Depth (m) Sample Formation At Ap Ca Cr Ep Gt Ky Mo Ru St To 2454.0 2485.1 2513.9 2544.3 2574.1 2610.0 2629.6 2630.8 2638.1 2648.4 2661.8 2671.6 2470.8 2488.8 2500.6 2534.4 2548.8 2563.5 2582.2 2596.5 2606.0 2619.5 2632.0 2647.5 2661.2 2702.5 2723.1 2739.0 2762.1 2778.7 2794.9 2809.9 2444.1 2464.0 2497.6 2530 2557 2591 2605.9 2635.9 2679 2713 2743 2774 2804 2835 2865 2896 2926 2957 2987 3018 3045 3079

Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core uwc uwc uwc Core Core uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc

Foula Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula Foula

R 0.5 R 0.5 1.0 0.5 0.5 R R 0.5 R 0.5 R R R R R R R R R 0.5 R R R 0.5 1.0 R 0.5 R 0.5 1.0 0.5 R R R 0.5 0.5 R R R R R R R R R R R R R R 1.0 R

15.0 82.0 11.5 36.0 15.5 38.5 13.0 8.0 3.0 21.5 11.5 27.5 14.5 16.0 17.5 4.5 9.5 6.5 11.5 6.0 7.5 14.0 5.0 4.0 11.0 15.5 15.5 12.0 10.0 5.5 3.5 8.0 42.0 9.0 12.5 4.0 23.0 6.0 7.5 6.0 1.0 1.0 3.5 6.0 3.5 10.0 7.5 16.0 4.5 3.0 3.5 R 2.0 5.0

R 0.5 R 0.5 R R

R

R

R 2.5 R

R 1.0 2.0 4.0 1.0 R 0.5 0.5 R R 3.5 0.5 R R R R R 1.0 R 0.5 2.0 R 0.5 0.5 R R R R R R 0.5 R R

84.0 2.5 82.5 52.5 68.0 34.0 78.0 83.5 93.0 66.0 81.5 46.0 83.0 80.0 75.5 93.5 87.0 91.5 86.0 90.0 89.5 81.0 92.0 94.5 86.0 79.0 74.5 78.5 78.0 85.5 89.5 84.0 53.5 90.0 85.0 93.5 66.0 88.5 85.0 88.5 96.0 93.5 89.5 91.0 93.0 R 86.0 81.5 75.0 91.0 92.0 0.5 93.0 97.5 91.0 R 92.5

R 0.5 R 0.5 0.5 R R R R R R 1.5 R R R R R R R R R 0.5 0.5 R R R R R 0.5 R R R R R R R

0.5 2.0 1.5 2.0 3.0 3.0 1.5 1.5 1.0 1.5 1.0 3.5 2.0 1.5 3.0 1.0 2.0 1.0 1.0 1.0 1.5 1.0 1.5 0.5 1.0 1.0 0.5 2.0 2.5 1.0 1.5 1.5 2.5 1.0 1.0 1.0 0.5 R 2.0 1.5 R 2.0 R 1.0 2.5 2.0 1.0 R 1.5 1.0 2.5 R 0.5 1.0 1.0 0.5 1.5 2.0 4.5 R 1.5

0.5

R

R R R

R

R

Zr Other (R)

R 0.5 4.0 8.5 1.0 3.5 3.0 5.5 2.5 9.5 6.0 18.0 1.5 4.5 0.5 5.0 2.0 1.0 4.0 6.5 5.5 0.5 4.0 17.0 R 0.5 0.5 2.0 R 4.0 0.5 0.5 R 1.5 R 1.0 R 1.5 0.5 2.5 0.5 1.0 0.5 2.5 R 1.0 R 1.0 R 2.0 R 4.0 1.5 7.0 4.0 3.5 2.5 6.0 R 8.0 1.5 3.5 0.5 5.0 0.5 1.0 R R R 1.5 R 1.5 2.0 4.5 R 1.0 0.5 1.5 0.5 2.0 R 1.5 R 2.5 R 1.0 R 1.5 0.5 1.5 0.5 1.5 5.5 0.5 7.5 R 3.0 R 3.5 R 1.5 R 0.5 R 1.0 R 1.0

Gh Gh

Sp

Br Br, Cp

Sp Cp

Cp

Cp Cp

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

1046 Table 1 (Continued ) Well

Depth (m) Sample Formation At Ap Ca Cr Ep Gt Ky Mo Ru St To 3109 3139 3170 3209.8 3228.7 3261 3292 3322 3353

uwc uwc uwc Core Core uwc uwc uwc uwc

Foula Foula Foula Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank

R 0.5 R R R 96.5 R R 2.0 0.5 R 94.0 R 4.0 R R 93.5 R 11.0 85.5 0.5 15.0 77.0 0.5 6.0 R 0.5 82.5 0.5 3.0 R R 1.0 86.0 R 1.5 R 85.0 R 5.0 1.0 84.5 R

R R R 0.5 R R R

1.5 R R 3.0 R 1.5 R R 0.5 1.0 2.0 1.0 2.5 4.0 1.5 R 2.5 3.0 2.0 2.0 1.5

Zr Other (R) 1.5 0.5 1.0 2.0 4.0 4.0 5.5 8.5 6.0

Cp Cp

Cp Cp

Note: At ¼ anatase, Ap ¼ apatite, Br ¼ brookite, Ca ¼ calcic amphibole, Cp ¼ clinopyroxene, Cr ¼ chrome spinel, Ep ¼ epidote, Gh ¼ gahnite, Gt ¼ garnet, Ky ¼ kyanite, Mo ¼ monazite, Ru ¼ rutile, Sp ¼ titanite, St ¼ staurolite, To ¼ – tourmaline, Zr ¼ zircon, R ¼ rare (o0.5%), uwc ¼ unwashed cuttings.

each sample was poured onto double-sided tape, cast into an epoxy disk, sectioned and polished. Transmitted and reflected light photomicrographs, and cathodoluminescence (CL) images were prepared for all grains. The procedures employed for zircon U-Pb dating followed Williams (1998) and references therein. The number of scans through the mass stations was limited to four, thereby achieving rapid data acquisition at the expense of some counting precision per analysis. In the first instance, an arbitrary group of 60 zircons were analysed from each sample, to ensure a 95% probability of identifying a component comprising 5% of the entire population (Dodson et al., 1988). Subjectivity in zircon dating was avoided by analysing all zircons encountered during the traverse of the mount, unless the grain showed evidence of being metamict or otherwise structurally compromised as determined from examination of the reflected and transmitted light photomicrographs and CL images. Normalisation of Pb/U isotopic ratios was achieved by reference to analyses of the AS3 reference zircon (1099 Ma; 206Pb/ 238U ¼ 0.1589; Paces and Miller, 1993). The raw SHRIMP data were processed using SQUID (Ludwig, 2001), with plots generated using Isoplot/Ex (Ludwig, 1999). For zircon areas that are older than approximately 800 Ma, the measured 206Pb/ 204Pb ratios have been used to correct for common Pb and the radiogenic 207Pb/ 206Pb ratio used to calculate the preferred age. For zircon areas that are younger than approximately 800 Ma, correction for common Pb was made using the measured 207Pb/ 206Pb and 238U/ 206Pb ratios, giving a radiogenic 206Pb/ 238U ratio and age following Tera and Wasserburg (1972) as described in Williams (1998, and references therein). When an analysis of Neoproterozoic and older zircons was 20% discordant, it was excluded from the relative probability plots. For the younger analyses, the validity of the radiogenic 206Pb/ 238U age has been determined on the basis of a number of factors, including the amount of common Pb (that is, if the total 207Pb/ 206Pb ratio deviates significantly from concordance on the Tera and Wasserburg plot), the relative concentrations of U and Th, the nature of the area analysed when examined post analysis and the abundance of a particular age grouping. The data from the two samples are given in Tables 3 and 4.

4. Analytical Methods

1047

Table 2. Provenance-sensitive heavy mineral ratios from Triassic sandstones of the Strathmore Field Well 205/26a-3

205/26a-4

204/30a-2

Depth (m) sample Formation 2454.0 2485.1 2513.9 2544.3 2574.1 2610.0 2629.6 2630.8 2638.1 2648.4 2661.8 2671.6 2470.8 2488.8 2500.6 2534.4 2548.8 2563.5 2582.2 2596.5 2606.0 2619.5 2632.0 2647.5 2661.2 2702.5 2723.1 2739.0 2762.1 2778.7 2794.9 2809.9 2444.1 2464.0 2497.6 2530 2557 2591 2605.9 2635.9 2679 2713 2743 2774 2804 2835 2865 2896 2926 2957 2987 3018 3045 3079

Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core uwc uwc uwc Core Core uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc uwc

ATi

Foula 98.5 Otter Bank 95.5 Otter Bank 88.5 Otter Bank 92.0 Otter Bank 84.5 Otter Bank 83.0 Otter Bank 82.5 Otter Bank 88.5 Otter Bank 64.0 Otter Bank 83.5 Otter Bank 65.6 Otter Bank 89.0 Foula 98.0 Foula 98.0 Foula 99.5 Foula 97.0 Foula 99.5 Foula 99.0 Foula 99.5 Foula 98.5 Foula 97.0 Foula 98.0 Foula 96.5 Foula 95.5 Foula 99.5 Otter Bank 97.5 Otter Bank 93.0 Otter Bank 81.5 Otter Bank 85.0 Otter Bank 91.0 Otter Bank 72.5 Otter Bank 82.5 Foula 99.0 Foula 97.5 Foula 97.0 Foula 92.0 Foula 96.5 Foula 97.8 Foula 95.5 Foula 98.0 Foula 95.0 Foula 98.1 Foula 99.5 Foula 98.3 Foula 98.0 Foula 98.0 Foula 100.0 Foula 98.0 Foula 97.2 Foula 97.4 Foula 96.9 Foula 94.9 Foula 89.6 Foula 96.0

Total GZi Total RuZi Total MZi Total CZi Total 200 200 200 200 200 200 200 200 200 200 186 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 185 200 200 179 107 200 59 200 200 73 200 179 116 128 79 48 200

99.5 24.1 95.7 90.5 87.7 59.5 94.8 94.8 98.5 90.9 98.0 70.7 99.5 97.1 96.2 99.5 98.0 99.0 98.5 97.6 99.0 97.1 99.0 99.0 98.0 95.2 89.7 96.2 93.5 91.3 96.2 94.3 97.6 99.5 98.0 98.5 93.5 99.0 98.5 97.6 99.5 96.2 99.0 98.5 98.5 99.0 94.3 91.3 97.1 96.6 97.6 99.5 99.0 97.6

203 108 209 221 228 247 211 211 203 220 204 283 201 206 208 201 204 202 203 205 202 206 202 202 204 210 223 208 214 219 208 212 205 201 204 203 214 202 203 205 201 208 202 203 203 202 212 219 206 207 205 201 202 205

69.7 24.8 25.5 20.3 27.8 18.4 27.4 28.3 26.5 25.1 38.5 18.0 52.8 43.6 42.1 61.5 58.7 62.1 59.7 59.5 63.5 60.2 60.0 60.8 57.8 17.0 30.1 30.8 27.5 14.9 21.3 24.5 56.1 58.9 64.0 51.0 18.6 54.4 66.9 58.2 48.9 51.2 64.3 48.0 52.4 40.3 27.2 13.4 38.9 64.8 57.3 69.7 68.0 59.2

66 109 106 251 277 245 197 258 272 267 39 244 212 266 259 260 242 264 248 247 274 251 250 255 237 241 286 289 276 235 254 265 228 243 278 204 43 90 163 146 235 215 176 50 210 124 62 172 175 196 96 188 147 245

4.7 2.4 1.3 2.0 2.0 1.5 3.4 2.1 5.7 0.5 4.0 5.7 3.4 4.3 5.2 9.2 7.0 6.5 9.0 3.4 7.1 8.3 11.7 6.4 1.5 2.0 5.7 3.8 3.8 3.8 2.9 2.9 6.7 9.5 9.9 3.3 0.0 4.7 0.0 1.6 1.5 0.0 0.0 0.0 0.6 0.0 0.0 0.7 0.0 0.0 2.7 0.0 0.0 0.7

21 84 80 204 204 203 148 189 212 201 25 212 207 209 211 206 215 214 158 207 127 109 197 109 203 204 212 208 208 208 206 206 195 221 222 182 35 43 54 62 134 105 62 26 178 74 45 150 107 69 37 57 47 137

0.0 0.0 0.0 0.5 0.0 0.0 0.7 0.5 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.6 0.0 0.0 1.3 2.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0

20 82 79 201 200 200 144 186 201 200 24 200 200 200 200 187 200 200 145 200 118 100 174 102 200 200 200 200 200 202 200 200 180 200 200 176 35 41 54 61 132 105 63 26 177 75 46 149 107 69 36 57 47 136

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

1048 Table 2 (Continued ) Well

Depth (m) sample Formation 3109 3139 3170 3209.8 3228.7 3261 3292 3322 3353

uwc uwc uwc Core Core uwc uwc uwc uwc

Foula Foula Foula Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank Otter Bank

ATi 93.2 96.8 91.4 88.5 91.0 60.0 60.5 54.0 66.7

Total GZi Total RuZi Total MZi Total CZi Total 117 156 186 200 200 200 200 100 117

98.5 99.0 99.0 98.0 93.5 93.0 92.2 90.9 92.6

203 202 202 204 214 215 217 220 216

69.4 58.3 48.0 23.5 26.4 29.5 26.8 25.9 26.4

327 240 244 238 148 190 205 270 208

0.0 1.5 0.8 2.2 5.2 0.0 1.1 1.0 0.7

106 137 128 186 115 134 191 202 154

0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

107 135 127 182 109 134 189 200 153

Note: ATi ¼ apatite/tourmaline index (% apatite in total apatite plus tourmaline); GZi ¼ garnet/zircon index (% garnet in total garnet plus zircon); RuZi ¼ rutile/zircon index (% rutile in total rutile plus zircon); MZi ¼ monazite/zircon index (% monazite in total monazite plus zircon); CZi ¼ chrome-spinel/ zircon index (% chrome-spinel in total chrome-spinel plus zircon). See Morton and Hallsworth (1994) for further information.

5. HEAVY MINERAL STRATIGRAPHY Three wells were used in the study: 204/30a-2, 205/26a-3 and 205/26a-4 (Fig. 2). The sandstones in 205/26a-3 and 205/26a-4 were characterised exclusively using core material. By contrast, only three short cores were taken in the Triassic succession in 204/30a-2, and thus comprehensive coverage of this well was achieved using ditch cuttings, which were taken at 9 m intervals. 5.1. Provenance-Sensitive Ratio Data Of the provenance-sensitive mineral parameters determined in the study, the rutile/ zircon (RuZi), garnet/zircon (GZi) and apatite/tourmaline (ATi) indices show marked variations. Downhole variations in these parameters for the three wells are shown in Figs. 4–6. By contrast, variations in monazite/zircon (MZi) appear to lack stratigraphic significance. Finally, the chrome-spinel/zircon (CZi) values are low to very low throughout, and have no value in discriminating the formations in this example. The most dramatic feature shown by the downhole plots is the sudden increase in RuZi in all three wells, from a baseline value of approximately 20–30 in the lower part of each well, to a baseline value of approximately 60 in the upper part. The increase in RuZi occurs at the boundary between the Otter Bank and Foula Fms, coincident with an increase in gamma ray measurements (Figs. 4–6). The ATi and GZi also show changes across the Otter Bank-Foula Boundary. GZi is consistently extremely high in the Foula Fm., but is slightly lower and more variable in the Otter Bank Fm. ATi is also consistently high in the Foula Fm. and generally lower in the Otter Bank Fm. However, variations in ATi appear to define a two-fold subdivision of the Otter Bank Fm., with the upper part of the Otter Bank Fm. (subunit O2) having higher ATi than the lower part (subunit O1). Within the Foula Fm., GZi and ATi values remain uniformly high, but there are distinct variations in RuZi. These are best developed in the thickest Foula Fm.

Grain spot

U (ppm)

Th (ppm)

Th/U

Pb* (ppm)

204

Pb/ Pb

f206%

Total ratios 238 206

159 267 9 65 348 21 62 187 715 137 126 255 140 159 155 110 161 149 129 65 482 49 28 201 178 451 532 82 138 495 407 240 78 689 366 120 94

162 216 17 59 170 11 61 55 85 215 85 163 42 131 40 107 97 87 51 40 34 77 24 165 43 414 252 97 84 432 80 253 69 589 157 140 66

1.02 0.81 1.79 0.90 0.49 0.55 0.99 0.30 0.12 1.57 0.67 0.64 0.30 0.82 0.26 0.98 0.60 0.58 0.40 0.61 0.07 1.58 0.87 0.82 0.24 0.92 0.47 1.18 0.61 0.87 0.20 1.06 0.88 0.85 0.43 1.17 0.69

45.5 69.4 0.4 18.7 14.3 6.4 29.5 79.0 274.3 4.9 5.0 130.7 41.2 46.1 69.8 3.9 42.9 45.2 64.2 30.7 133.1 14.5 8.8 56.7 79.7 18.5 100.5 21.6 63.7 19.0 187.5 64.1 37.4 24.0 158.9 57.3 3.6

0.0000 0.0002 0.0076 0.0009 0.0006 0.0004 0.0002 0.0001 0.0001 0.0015 – 0.0000 0.0001 0.0001 0.0001 0.0003 0.0006 0.0000 0.0001 0.0002 0.0000 0.0003 0.0001 0.0000 – 0.0001 0.0002 0.0006 0.0001 0.0011 0.0000 0.0001 – 0.0002 0.0000 0.0001 0.0010

o0.01 0.29 12.85 1.34 0.15 0.54 0.26 0.13 0.12 0.27 0.56 0.05 0.16 0.10 0.08 0.34 0.87 0.04 0.13 0.21 0.06 0.42 0.13 0.07 o0.01 0.11 0.38 0.92 0.07 1.82 0.03 0.19 o0.01 0.45 0.05 0.07 0.68

U/ Pb

2.9974 3.3087 22.3772 2.9783 20.9445 2.7941 1.8039 2.0308 2.2382 24.0399 21.6772 1.6748 2.9112 2.9640 1.9063 24.1461 3.2218 2.8261 1.7230 1.8303 3.1104 2.8794 2.7318 3.0417 1.9182 20.9096 4.5478 3.2380 1.8604 22.3273 1.8647 3.2119 1.8009 24.6586 1.9800 1.8002 22.5232

7

207

Pb/ Pb

7

206

0.0400 0.0406 1.3152 0.0519 0.2834 0.0757 0.0283 0.0249 0.0239 0.3999 0.3618 0.0195 0.0361 0.0376 0.0241 0.4235 0.0410 0.0359 0.0222 0.0273 0.0340 0.0550 0.0548 0.0361 0.0230 0.2483 0.0495 0.0483 0.0236 0.2691 0.0205 0.0383 0.0273 0.2965 0.0218 0.0220 0.3956

0.1164 0.1244 0.1541 0.1172 0.0535 0.1435 0.1967 0.1884 0.1834 0.0536 0.0566 0.2318 0.1158 0.1162 0.1870 0.0541 0.1197 0.1167 0.1938 0.1897 0.1174 0.1174 0.1165 0.1159 0.1876 0.0532 0.1807 0.1231 0.1846 0.0664 0.1860 0.1152 0.1879 0.0549 0.1869 0.1956 0.0573

206

Pb/ U

7

238

0.0010 0.0008 0.0146 0.0029 0.0011 0.0030 0.0015 0.0009 0.0005 0.0017 0.0019 0.0008 0.0008 0.0008 0.0010 0.0019 0.0009 0.0009 0.0014 0.0014 0.0005 0.0015 0.0019 0.0007 0.0009 0.0008 0.0010 0.0013 0.0010 0.0066 0.0006 0.0007 0.0014 0.0009 0.0006 0.0009 0.0019

0.3336 0.3013 0.0389 0.3400 0.0477 0.3560 0.5529 0.4918 0.4462 0.0415 0.0459 0.5968 0.3443 0.3370 0.5242 0.0413 0.3077 0.3542 0.5806 0.5452 0.3213 0.3458 0.3656 0.3285 0.5216 0.0478 0.2190 0.3060 0.5371 0.0440 0.5361 0.3108 0.5554 0.0404 0.5048 0.5551 0.0441

207

Pb/ U

7

235

0.0044 0.0037 0.0026 0.0059 0.0007 0.0099 0.0087 0.0060 0.0048 0.0007 0.0008 0.0070 0.0043 0.0043 0.0066 0.0007 0.0040 0.0045 0.0075 0.0082 0.0035 0.0067 0.0074 0.0039 0.0063 0.0006 0.0024 0.0046 0.0068 0.0007 0.0059 0.0037 0.0084 0.0005 0.0056 0.0068 0.0008

207

Age (Ma) Pb/ Pb

7

r

206

206

Pb/ U

7

238

5.351 5.065

0.084 0.086

0.1163 0.1219

0.0010 0.0014

0.8509 0.7288

5.956

0.179

0.1271

0.0031

0.5787

6.817 14.814 12.697 11.221

0.345 0.286 0.176 0.125

0.1389 0.1943 0.1873 0.1824

0.0059 0.0022 0.0012 0.0006

0.5467 0.8174 0.8868 0.9608

19.042 5.590 5.358 13.466

0.233 0.080 0.078 0.190

0.2314 0.1177 0.1153 0.1863

0.0009 0.0008 0.0008 0.0012

0.9511 0.8654 0.8758 0.8977

4.760 5.738 15.540 14.118 5.177 5.421 5.814 5.224 13.525

0.111 0.084 0.229 0.259 0.063 0.198 0.174 0.075 0.175

0.1122 0.1175 0.1941 0.1878 0.1169 0.1137 0.1153 0.1153 0.1881

0.0022 0.0009 0.0014 0.0020 0.0006 0.0035 0.0026 0.0009 0.0009

0.5523 0.8647 0.8740 0.8165 0.8998 0.5315 0.6721 0.8247 0.9289

5.370 4.860 13.628

0.070 0.146 0.195

0.1778 0.1152 0.1840

0.0012 0.0030 0.0012

0.8419 0.5058 0.8876

13.732 4.864 14.404

0.158 0.072 0.242

0.1858 0.1135 0.1881

0.0006 0.0010 0.0014

0.9580 0.8035 0.9014

12.979 14.924

0.149 0.196

0.1865 0.1950

0.0006 0.0009

0.9553 0.9319

1856.0 1698.0 246.3 1887.0 300.2 1963.0 2837.0 2578.0 2379.0 262.0 289.1 3017.0 1908.0 1872.0 2717.0 260.7 1729.0 1955.0 2951.0 2805.0 1796.0 1915.0 2009.0 1831.0 2706.0 300.8 1277.0 1721.0 2772.0 277.4 2767.0 1744.0 2848.0 255.1 2634.0 2846.0 278.2

7

% Disc.

1901 1984

15 21

2 14

2058

43

8

2213 2779 2718 2675

74 18 11 5

11 2 5 11

3062 1922 1885 2710

6 13 13 10

1 1 1 0

1835 1918 2777 2723 1909 1859 1885 1885 2725

35 13 12 17 10 56 40 15 8

6 2 6 3 6 3 7 3 1

2633 1883 2689

12 47 11

51 9 3

2705 1856 2726

5 16 12

2 6 4

2711 2785

6 8

3 2

207

Pb/ Pb

206

22.0 18.0 15.8 32.0 4.0 47.0 36.0 26.0 21.0 4.3 4.8 28.0 21.0 21.0 28.0 4.6 20.0 23.0 32.0 34.0 17.0 32.0 35.0 19.0 27.0 3.5 13.0 23.0 29.0 4.1 25.0 18.0 35.0 3.0 24.0 28.0 4.9

1049

1.1 2.1 3.1 4.1 5.1 6.1 7.1 8.1 9.1 10.1 11.1 12.1 13.1 14.1 15.1 16.1 17.1 18.1 19.1 20.1 21.1 22.1 23.1 24.1 25.1 26.1 27.1 28.1 29.1 30.1 31.1 32.1 33.1 34.1 35.1 36.1 37.1

Radiogenic ratios

206

5. Heavy Mineral Stratigraphy

Table 3. U-Pb ages of detrital zircons in the Foula Fm. (well 205/26a-3, 2454.0 m) as determined by SHRIMP

1050

Table 3 (Continued ) Grain spot

U (ppm)

Th (ppm)

Th/U

Pb* (ppm)

204

Pb/ Pb

f206%

Total ratios 238 206

295 35 279 183 171 6 232 465 498 300 1280 73 80

71 17 99 98 114 2 326 149 240 155 1407 62 117

0.24 0.50 0.36 0.54 0.66 0.30 1.40 0.32 0.48 0.52 1.10 0.85 1.46

83.8 17.3 63.1 77.1 51.5 2.8 9.0 38.3 53.8 136.7 50.6 34.4 24.7

0.0000 – 0.0002 0.0000 0.0000 – 0.0005 – 0.0038 0.0000 0.0001 0.0001 0.0004

0.03 o0.01 0.32 0.05 0.02 o0.01 0.70 o0.01 6.69 0.02 0.07 0.16 0.60

U/ Pb

3.0222 1.7404 3.7973 2.0347 2.8529 1.7951 22.0705 10.4405 7.9513 1.8832 21.7134 1.8254 2.7793

7

207

Pb/ Pb

7

206

0.0340 0.0310 0.0457 0.0238 0.0345 0.0654 0.2917 0.1163 0.0910 0.0209 0.2320 0.0256 0.0399

0.1151 0.1942 0.1186 0.1915 0.1160 0.2047 0.0576 0.0587 0.1680 0.1862 0.0527 0.1909 0.1164

206

Pb/ U

7

238

0.0006 0.0018 0.0009 0.0006 0.0007 0.0045 0.0011 0.0006 0.0257 0.0006 0.0005 0.0012 0.0011

0.3308 0.5749 0.2625 0.4912 0.3505 0.5584 0.0450 0.0959 0.1174 0.5309 0.0460 0.5469 0.3579

207

Pb/ U

7

235

0.0037 0.0102 0.0032 0.0058 0.0042 0.0204 0.0006 0.0011 0.0058 0.0059 0.0005 0.0077 0.0051

207

Age (Ma) Pb/ Pb

7

r

206

206

Pb/ U

7

238

5.239 15.429 4.195 12.942 5.600 15.923

0.066 0.314 0.083 0.158 0.077 0.690

0.1149 0.1946 0.1159 0.1911 0.1159 0.2068

0.0007 0.0019 0.0018 0.0007 0.0007 0.0048

0.8895 0.8759 0.6172 0.9589 0.8818 0.8413

1.885 13.612

0.760 0.158

0.1165 0.1860

0.0466 0.0006

0.1224 0.9547

14.284 5.537

0.224 0.100

0.1894 0.1122

0.0013 0.0012

0.8954 0.7941

1842.0 2928.0 1503.0 2576.0 1937.0 2860.0 283.7 590.3 715.0 2745.0 290.0 2812.0 1972.0

7

% Disc.

1878 2782 1894 2752 1894 2881

10 16 28 6 12 38

2 5 21 6 2 1

1903 2707

719 6

62 1

2737 1835

12 20

3 7

207

Pb/ Pb

206

18.0 42.0 16.0 25.0 20.0 84.0 3.7 6.4 33.0 25.0 3.1 32.0 30.0

Note: Uncertainties given at the 1s level. Error in AS3 reference zircon calibration was 0.75% for the analytical session. f206% denotes the percentage of 206Pb that is common Pb. For zircon areas older than 800 Ma, correction for common Pb made using the measured 204Pb/ 206Pb ratio. For zircon areas younger than 800 Ma correction for common Pb made using the measured 238U/ 206Pb and 207Pb/ 206Pb ratios, following Tera and Wasserburg (1972) as outlined in Williams (1998). For % Disc., 0% denotes a concordant analysis.

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

38.1 39.1 40.1 41.1 42.1 43.1 44.1 45.1 46.1 47.1 48.1 49.1 50.1

Radiogenic ratios

206

Grain spot

U (ppm)

Th (ppm)

Th/U

Pb* (ppm)

204

Pb/ Pb

f206%

Total ratios 238 206

101 164 950 105 409 167 316 310 109 115 82 293 285 211 272 66 346 269 237 101 725 47 207 97 653 74 140 324 113 107 105 113 668 160 75

53 130 142 29 120 68 68 247 130 65 23 140 226 81 325 53 109 188 241 31 6 48 103 88 176 30 107 129 153 85 72 57 566 83 145

0.52 0.79 0.15 0.28 0.29 0.41 0.22 0.80 1.19 0.56 0.28 0.48 0.79 0.39 1.19 0.80 0.32 0.70 1.02 0.31 0.01 1.02 0.50 0.91 0.27 0.41 0.77 0.40 1.36 0.79 0.69 0.50 0.85 0.52 1.94

52.3 63.5 62.7 27.3 122.0 26.4 146.9 66.0 50.9 28.6 40.2 137.1 122.8 103.4 61.5 30.1 95.7 113.8 114.1 30.6 108.2 15.8 98.8 44.3 180.9 19.3 56.2 70.6 50.1 16.7 26.5 51.3 81.6 43.3 34.9

Age (Ma)

0.0003 0.0001 0.0000 0.0002 0.0000 0.0002 0.0001 0.0002 0.0001 0.0004 0.0000 0.0000 0.0000 0.0001 0.0006 0.0002 0.0000 0.0001 0.0001 0.0002 0.0001 0.0006 0.0002 – 0.0002 0.0011 0.0004 0.0001 0.0001 0.0002 0.0003 0.0002 0.0003 – 0.0003

0.36 0.14 0.43 0.25 0.04 0.37 0.07 0.37 0.14 0.67 0.05 0.04 0.02 0.08 0.98 0.26 0.04 0.12 0.10 0.26 0.14 0.90 0.28 o0.01 0.25 1.78 0.57 0.09 0.11 0.36 0.50 0.31 0.57 o0.01 0.38

U/ Pb

1.6551 2.2168 13.0168 3.2964 2.8802 5.4343 1.8474 4.0308 1.8381 3.4671 1.7503 1.8367 1.9971 1.7494 3.8049 1.8779 3.1046 2.0338 1.7866 2.8444 5.7539 2.5817 1.7978 1.8806 3.1002 3.2811 2.1397 3.9497 1.9394 5.5005 3.4080 1.8875 7.0268 3.1713 1.8421

7

207

Pb/ Pb

7

206

0.0272 0.0324 0.1508 0.0643 0.0352 0.0842 0.0250 0.0573 0.0301 0.0585 0.0311 0.0237 0.0268 0.0252 0.0547 0.0398 0.0398 0.0280 0.0252 0.0536 0.0749 0.0653 0.0284 0.0467 0.0391 0.0776 0.0383 0.0572 0.0381 0.1193 0.0702 0.0370 0.0867 0.0481 0.0341

0.2170 0.2003 0.0601 0.0993 0.1163 0.0794 0.2013 0.0963 0.1918 0.1104 0.2061 0.1998 0.1945 0.2051 0.1897 0.1940 0.1126 0.1934 0.2015 0.1257 0.0750 0.1821 0.2064 0.2079 0.1126 0.1047 0.1896 0.0919 0.1961 0.0848 0.1112 0.2016 0.1927 0.1105 0.1929

206

Pb/ U

7

238

0.0017 0.0014 0.0007 0.0025 0.0007 0.0013 0.0009 0.0016 0.0016 0.0015 0.0019 0.0010 0.0012 0.0013 0.0016 0.0024 0.0009 0.0012 0.0012 0.0018 0.0007 0.0031 0.0015 0.0031 0.0008 0.0036 0.0019 0.0011 0.0021 0.0022 0.0021 0.0037 0.0011 0.0012 0.0019

0.6022 0.4505 0.0765 0.3030 0.3471 0.1833 0.5409 0.2472 0.5433 0.2865 0.5710 0.5442 0.5006 0.5712 0.2603 0.5311 0.3220 0.4911 0.5591 0.3506 0.1736 0.3838 0.5547 0.5319 0.3218 0.3023 0.4647 0.2530 0.5151 0.1800 0.2920 0.5282 0.1415 0.3148 0.5412

207

Pb/ U

7

235

0.0099 0.0066 0.0009 0.0059 0.0042 0.0028 0.0073 0.0035 0.0089 0.0049 0.0101 0.0070 0.0067 0.0082 0.0038 0.0113 0.0041 0.0068 0.0079 0.0067 0.0023 0.0099 0.0088 0.0132 0.0041 0.0071 0.0084 0.0037 0.0101 0.0039 0.0061 0.0104 0.0018 0.0048 0.0100

207

Pb/ Pb

7

r

206

206

Pb/ U

7

238

17.820 12.367

0.325 0.208

0.2146 0.1991

0.0017 0.0016

0.9016 0.8703

4.101 5.546 1.927 14.967 3.174 14.278 4.133 16.189 14.965 13.411 16.101 6.538 14.037 4.981 13.019 15.462 5.968 1.767 9.236 15.597 15.260 4.899 4.085 11.830 3.180 13.858 1.901 4.305 14.485 3.680 4.742 14.232

0.131 0.078 0.053 0.215 0.081 0.275 0.144 0.324 0.209 0.198 0.256 0.132 0.355 0.076 0.201 0.242 0.178 0.035 0.381 0.285 0.440 0.079 0.184 0.289 0.062 0.315 0.073 0.154 0.406 0.056 0.089 0.300

0.0982 0.1159 0.0762 0.2007 0.0931 0.1906 0.1046 0.2056 0.1994 0.1943 0.2045 0.1822 0.1917 0.1122 0.1923 0.2006 0.1235 0.0739 0.1745 0.2039 0.2081 0.1104 0.0980 0.1846 0.0912 0.1951 0.0766 0.1069 0.1989 0.1886 0.1092 0.1907

0.0025 0.0008 0.0017 0.0009 0.0020 0.0019 0.0032 0.0019 0.0011 0.0012 0.0014 0.0025 0.0026 0.0009 0.0014 0.0013 0.0028 0.0011 0.0056 0.0019 0.0031 0.0011 0.0038 0.0030 0.0012 0.0022 0.0024 0.0031 0.0040 0.0016 0.0012 0.0019

0.6112 0.8683 0.5615 0.9444 0.5595 0.8521 0.4926 0.8862 0.9241 0.9096 0.9064 0.7227 0.8402 0.8360 0.8908 0.9035 0.6380 0.6528 0.6270 0.8648 0.8608 0.7881 0.5240 0.7394 0.7403 0.8653 0.5664 0.5818 0.7012 0.8228 0.8086 0.8793

3039 2397 475.1 1706 1921 1085 2787 1424 2797 1624 2912 2801 2616 2913 1491 2746 1799 2575 2863 1938 1032 2094 2845 2749 1798 1703 2460 1454 2678 1067 1651 2734 853 1764 2789

7

% Disc.

2941 2819

13 14

3 15

1590 1894 1101 2832 1491 2747 1708 2871 2822 2779 2862 2673 2757 1835 2762 2831 2007 1037 2601 2858 2891 1806 1587 2695 1450 2786 1111 1748 2817 2730 1787 2748

47 13 46 8 40 17 56 15 9 10 11 23 23 15 12 11 41 31 54 15 24 18 72 27 25 19 63 53 33 14 20 17

7 1 1 2 4 2 5 1 1 6 2 44 0 2 7 1 3 1 19 0 5 0 7 9 0 4 4 6 3 69 1 1

207

Pb/ Pb

206

42 29 5.4 30 20 16 31 18 37 25 42 29 29 34 19 48 20 29 33 32 12 46 36 56 20 38 37 19 43 25 30 44 10 25 54

1051

1.1 2.1 3.1 4.1 5.1 6.1 7.1 8.1 9.1 10.1 11.1 12.1 13.1 14.1 15.1 16.1 17.1 18.1 19.1 20.1 21.1 22.1 23.1 24.1 25.1 26.1 27.1 28.1 29.1 30.1 31.1 32.1 33.1 34.1 35.1

Radiogenic ratios

206

5. Heavy Mineral Stratigraphy

Table 4. U-Pb ages of detrital zircons in the Otter Bank Fm. (well 205/26a-3, 2648.4 m) as determined by SHRIMP

1052

Table 4 (Continued ) Grain spot

U (ppm)

Th (ppm)

Th/U

Pb* (ppm)

204

Pb/ Pb

f206%

Total ratios 238 206

103 159 562 635 320 405 294 282 178 119 643 610 761 904 579

106 130 161 338 276 114 95 134 405 38 141 21 69 66 237

1.03 0.82 0.29 0.53 0.86 0.28 0.32 0.47 2.28 0.32 0.22 0.03 0.09 0.07 0.41

45.1 53.8 253.3 133.5 84.7 77.0 123.6 130.9 81.2 55.1 241.5 297.5 86.0 276.7 251.4

0.0002 0.0000 0.0001 0.0003 0.0003 0.0004 0.0001 0.0001 0.0003 0.0004 0.0002 0.0001 0.0003 0.0001 0.0000

0.25 0.01 0.07 0.43 0.51 0.63 0.17 0.13 0.40 0.49 0.27 0.11 0.48 0.18 0.06

U/ Pb

1.9579 2.5334 1.9071 4.0852 3.2499 4.5177 2.0461 1.8525 1.8829 1.8522 2.2883 1.7613 7.6013 2.8065 1.9787

7

207

Pb/ Pb

7

206

0.0327 0.0376 0.0223 0.0485 0.0428 0.0578 0.0270 0.0247 0.0277 0.0309 0.0282 0.0207 0.0903 0.0319 0.0252

0.1841 0.1864 0.1974 0.1861 0.1916 0.1160 0.1895 0.2068 0.1923 0.1939 0.1951 0.2188 0.0718 0.1625 0.1820

206

Pb/ U

7

238

0.0016 0.0015 0.0007 0.0010 0.0012 0.0010 0.0010 0.0011 0.0013 0.0016 0.0011 0.0009 0.0007 0.0007 0.0007

0.5095 0.3947 0.5240 0.2437 0.3061 0.2200 0.4879 0.5391 0.5290 0.5373 0.4358 0.5672 0.1309 0.3557 0.5051

207

Pb/ U

7

235

0.0085 0.0059 0.0061 0.0029 0.0041 0.0028 0.0064 0.0072 0.0078 0.0090 0.0054 0.0067 0.0016 0.0040 0.0064

12.777 10.143 14.219 6.144 7.920 3.360 12.652 15.285 13.765 14.038 11.588 17.035 1.223 7.897 12.635

207

Age (Ma) Pb/ Pb

7

r

206

0.253 0.176 0.176 0.088 0.129 0.071 0.184 0.223 0.239 0.283 0.161 0.213 0.032 0.098 0.170

0.1819 0.1864 0.1968 0.1828 0.1876 0.1108 0.1881 0.2056 0.1887 0.1895 0.1928 0.2178 0.0678 0.1610 0.1814

206

Pb/ U

7

238

0.0019 0.0017 0.0008 0.0014 0.0018 0.0018 0.0011 0.0012 0.0017 0.0021 0.0012 0.0010 0.0016 0.0008 0.0008

0.8458 0.8587 0.9472 0.8363 0.8108 0.6141 0.9088 0.9171 0.8494 0.8302 0.8861 0.9371 0.4524 0.9174 0.9459

2654 2145 2716 1406 1722 1282 2562 2780 2737 2772 2332 2896 793 1962 2636

207

Pb/ Pb

7

% Disc.

17 15 6 13 16 30 10 9 15 18 11 7 49 8 7

1 21 3 48 37 29 6 3 0 1 16 2 8 20 1

206

36 27 26 15 20 15 28 30 33 38 24 27 9 19 28

2670 2710 2800 2679 2722 1812 2725 2871 2731 2738 2767 2965 862 2466 2666

Note: Justification of columns (according to decimal point) everywhere correct! Uncertainties given at the 1s level. Error in AS3 reference zircon calibration was 0.73% for the analytical session. f206% denotes the percentage of 206Pb that is common Pb. For zircon areas older than 800 Ma, correction for common Pb made using the measured 204Pb/ 206Pb ratio. For zircon areas younger than 800 Ma correction for common Pb made using the measured 238U/ 206Pb and 207Pb/ 206Pb ratios, following Tera and Wasserburg (1972) as outlined in Williams (1998). For % Disc., 0% denotes a concordant analysis.

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

36.1 37.1 38.1 39.1 40.1 41.1 42.1 43.1 44.1 45.1 46.1 47.1 48.1 49.1 50.1

Radiogenic ratios

206

5. Heavy Mineral Stratigraphy

2500

F5 F4

2600

2700

F3

depth (m)

2800

Foula Formation F2

2900

3000 F1 3100

3200

O2

3300

O1

Otter Bank Formation

Otter Bank Shale Formation

3400 0

150 50 gamma ray (API)

100 0 ATi

100 0 GZi

80 RuZi

1053

Fig. 4. Downhole variations in key heavy mineral ratio parameters for well 204/30a-2, showing subdivision into heavy mineral units O1-O2 and F1F5. ATi ¼ apatite/tourmaline index, GZi ¼ garnet/zircon index, RuZi ¼ rutile/zircon index (for definitions, see text).

1054

F3 F2

2500

depth (m)

F1

2600

2700 O2

Otter Bank Formation

O1

2800 0

150 50 gamma ray (API)

100 0 ATi

100 0 GZi

80 RuZi

Fig. 5. Downhole variations in key heavy mineral ratio parameters for well 205/26a-4, showing subdivision into heavy mineral units O1-O2 and F1-F3. ATi ¼ apatite/tourmaline index, GZi ¼ garnet/zircon index, RuZi ¼ rutile/zircon index (for definitions, see text).

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

Foula Formation

5. Heavy Mineral Stratigraphy

F1 Foula Fm. depth (m)

2500

O2 Otter Bank Fm.

2600 O1 2700 0

150 50 gamma ray (API)

100 0 ATi

100 0 GZi

80

Otter Bank Shale Formation

RuZi

Fig. 6. Downhole variations in key heavy mineral ratio parameters for well 205/26a-3, showing subdivision into heavy mineral units O1-O2 and F1. ATi ¼ apatite/tourmaline index, GZi ¼ garnet/zircon index, RuZi ¼ rutile/zircon index (for definitions, see text).

1055

1056

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

succession, present in 204/30a-2 (Fig. 4), but are also manifested by the cored succession in 205/26a-4 (Fig. 5). The variations in RuZi enable subdivision of the Foula Fm. into five subunits, three with high RuZi (F1, F3 and F5) and two with low RuZi (F2 and F4). The two subunits with low RuZi are also characterised by a slight lowering of GZi. The clear distinction between the Foula and Otter Bank assemblages is also manifested by cross-plots of the ratio parameters (Fig. 7). Foula Fm. samples show well-defined clustering, with very high ATi and GZi and generally high RuZi. Otter Bank samples show greater variation, but display little overlap with the Foula Fm. data. They have uniformly low RuZi, and generally lower ATi and GZi than the Foula samples. The only area of overlap is between the Otter Bank and the low-RuZi subunits in the Foula Fm. (subunits F2 and F4), which have similar RuZi, GZi and ATi values. The Foula and Otter Bank assemblages have been characterised using both core and ditch cuttings samples. Data from core samples are reliable since their position within the well is accurately known and they cannot be contaminated or otherwise altered during the drilling process. Ditch cuttings, however, represent a range of depths (dependent on the sampling frequency), may be contaminated by downhole caving, and may have been modified by the mechanical action of the drill bit. The conventional heavy mineral data (Table 1) show evidence for contamination of cuttings samples. In core samples, the unstable minerals epidote, calcic amphibole and clinopyroxene are extremely scarce (epidote) or entirely absent (calcic amphibole, clinopyroxene), but occur in small amounts in most of the cuttings samples. The almost complete absence of these minerals in the core samples indicates that minor contamination of the cuttings has occurred. This is most likely to be related to caving of lithologies from higher in the well bore, although it is possible that the unstable minerals have been present in low-porosity siltstone and mudstone intercalations within the Triassic succession. Use of provenance-sensitive ratios helps to eliminate the problems caused by caving or other forms of contamination. Plots of ratio data from core and ditch cuttings (Fig. 8) show that there is remarkable consistency between the two types of sample, with little deviation between the ratios determined from core and cuttings. GZi and ATi values in core and cuttings are virtually identical, the only slight deviation being that some cuttings samples in the Otter Bank of 204/30a-2 have marginally lower ATi than Otter Bank core samples. A more pronounced deviation in ATi between core and cuttings was identified in the Clair Field, west of Shetland (Morton et al., 2003), where it was ascribed to loss of apatite by the mechanical action of the drill bit. Apatite is more susceptible to mechanical loss than the other heavy minerals because of its lower hardness, and is preferentially lost through the grinding action of the drill bit. However, in the Otter Bank case, it is possible that the slight difference in ATi values between core and cuttings is due to provenance heterogeneity. RuZi values in core and cuttings samples from the Otter Bank Fm. are closely comparable, but some cuttings samples from the Foula Fm. have distinctly lower RuZi than the core. These occur in 204/30a-2, and represent the samples defining subunits F2 and F4. The deviation in this case is ascribed to genuine differences in sediment provenance, since mechanical abrasion would cause cuttings values to be

1057

100

100

80

90

60

80 ATi

RuZi

5. Heavy Mineral Stratigraphy

40

70

20

60 50

0 0

20

40 60 GZi

80

100

0 100

90

80 % Type A garnet

100

ATi

80 70

40

60 GZi

80

100

60 40 20

60

0

50 0

20

40 RuZi

60

80

0

100

100

80

80 % Type A garnet

% Type A garnet

20

60 40

20

40 RuZi

60

80

60 40 20

20

0

0 50

60

70

80 GZi

90

50

60

70

80

90

100

ATi

Otter Bank O2 Foula F3

100

Otter Bank O1 + Foula F5

Foula F2

Foula F4

Foula F1

Fig. 7. Binary plots of key heavy ratio and garnet geochemical parameters, displaying the marked difference in provenance characteristics between the Foula and Otter Bank Fms and the intraformational variations enabling the distinction of subunits O1-O2 and F1-F5.

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

1058

80

100 90

60

ATi

RuZi

80 40

70 20

60

0

50 0

20

40

60

80

100

0

20

Foula Fm core

Foula Fm cuttings

40

60

80

100

GZi

GZi Otter Bank Fm core

Otter Bank Fm cuttings

Fig. 8. Comparison of ratio parameters acquired from different sample types (core and ditch cuttings) in all three wells.

uniformly lower than core, rather than confining the effect to distinct zones, and because core data in 205/26a-4 also show the presence of a low-RuZi excursion. In the case of the Strathmore Field Triassic, therefore, ratio data from cuttings are considered reliable, their only disadvantage being the imprecision associated with their depth in the drilled succession. Greater care is required when interpreting conventional data, since assemblages in cuttings contain minerals that are absent from core, and have therefore been contaminated during the drilling process. 5.2. Garnet Geochemical Data Garnet compositions also show marked stratigraphic variations (Fig. 9). The Foula Fm. is characterised by garnet assemblages dominated by high-Mg, low-Ca types, termed Type A (Morton et al., 2004). Low-Mg types (Type B) and high-Mg, high-Ca types (Type C) are scarce. By contrast, the Otter Bank Fm. has fewer Type A garnets, being dominated by low-Mg types (Type B) with small amounts of high-Mg, high-Ca garnets (Type C). The abundance of Type A garnets is therefore another effective discriminant of the Foula and Otter Bank Fms. As shown in Fig. 7, plots of Type A garnet vs. GZi, ATi and RuZi show the clear distinction between the two formations. The garnet data also enable discrimination between the low-RuZi units in the Foula Fm. (subunits F2 and F4) and the Otter Bank Fm., which show a degree of overlap on the ratio cross-plots. This is because the garnets in the low-RuZi parts of the Foula Fm. are identical to those in the high-RuZi parts, both having approximately 70–90% Type A garnet, whereas Type A garnet abundances in the Otter Bank samples reach only a maximum of approximately 50%. It appears from the cross-plots (Fig. 7) that Otter Bank subunit O2 has slightly higher proportions of Type A garnet compared with O1, and therefore plots slightly closer to the Foula Fm. data cluster. This is especially noticeable on the cross-plot of Type A garnet and ATi.

5. Heavy Mineral Stratigraphy

1059

XMg a

XFe+XMn

XMg

XMg

b

c

XMg e

XMg d

XMg

XMg

f

XCa

g

XFe+XMn

XCa

type A

type C

type B

Fig. 9. Garnet compositions in selected samples from Foula and Otter Bank sandstones in the Strathmore Field. XFe, XMg, XCa, XMn ¼ molecular proportion of Fe, Mg, Ca and Mn respectively, calculated on the basis of 24 oxygens, and normalised to total Fe+Mg+Ca+Mn, as recommended by Droop and Harte (1995). All Fe calculated as Fe2+. Small plot shows the differentiation into fields A, B and C (from Morton et al., 2004). K, XMno5%; J, XMn>5%; (a) 204/30a-2, 2464.0 m, Foula Fm., subunit F5 (core); (b) 204/30a-2, 2743 m, Foula Fm., subunit F3 (ditch cuttings); (c) 205/26a-3, 2454.0 m, Foula Fm., subunit F1 (core); (d) 205/26a4, 2596.5 m, Foula Fm., subunit F1 (core); (e) 204/30a-2, 3322 m, Otter Bank Fm., subunit O1 (ditch cuttings), (f) 205/26a-3, 2671.6 m, Otter Bank Fm., subunit O1 (core); (g) 205/26a-4, 2723.1 m, Otter Bank Fm., subunit O2 (core).

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

1060

5.3. Correlation The heavy mineral correlation of the three wells is shown in Fig. 10. The correlation is ultimately constrained by the top Triassic unconformity (marked by a major rise in gamma-ray readings) and by the underlying Griesbachian Otter Bank Shale Fm. (not penetrated in 205/26a-4). A particular feature of note is the coincidence of boundaries between the heavy mineral units and events on the gamma-ray log. The most notable of these is the boundary between units O2 and F1, which coincides with a small but distinct upward increase in gamma intensity in all three wells. In 204/30a2, the mineralogically defined subunits F2 and F4 coincide with intervals of elevated gamma-ray values. In 205/26a-4, subunit F2 is also characterised by a change in gamma-ray readings but in this case the subunit has slightly lower gamma activity than the underlying and overlying sediment. The change in gamma-ray readings suggests a difference in facies between the two wells, demonstrating the value of using an independent approach to correlation. The boundary between O1 and O2 does not appear to have any obvious manifestation on the gamma-ray log, again 204/30-2

205/26a-4

205/26a-3

top Triassic unconformity F5 F4

F3

Otter Bank Shale Fm.

Foula Fm. F2

200m F1 vertical scale O2 Otter Bank Fm.

O1

Otter Bank . Shale Fm

Fig. 10. Correlation of the Triassic succession in Strathmore Field wells 204/30a-2, 205/26a-3 and 205/26a-4, showing the heavy mineral units defined in Figs. 4–6. Location of wells shown in Fig. 2. Well 205/26a-4 was terminated before penetration of the Otter Bank Shale Formation.

6. Provenance

1061

showing that the use of heavy mineral data provides an added level of sophistication to the correlation. The most obvious feature of the correlation is the marked thinning of the Foula Fm. from 204/30a-2, through 206/26a-4, to 205/26a-3. Despite the overall thinning, the individual subunits F1 and F2 appear to have relatively similar thicknesses, suggesting that the thinning is largely due to erosion at the top Triassic unconformity, rather than to differences in subsidence during the Triassic. Consequently, a considerable amount of sediment must have been stripped off prior to deposition of the overlying Jurassic.

6. PROVENANCE The marked difference in heavy mineral characteristics between the Foula and Otter Bank Fms indicates that there was a pronounced shift in the nature of the sediment source. The relatively wide spread of ratio values and garnet compositions within the Otter Bank sandstones indicates that the Otter Bank source was relatively heterogeneous. By contrast, the Foula Fm. source appears to have been more homogeneous, given the strong clustering of ratio parameters and the consistent nature of the garnet assemblages. The only exception to the overall homogeneity of the Foula sediment source is the lower RuZi in subunits F2 and F4. The heavy mineral data indicate that the ultimate sources of both the Foula and Otter Bank sandstones were predominantly metamorphic, as indicated by the high abundance of garnet. The higher abundance of zircon in the Otter Bank Fm. suggests a greater involvement of granitic or other intermediate-acidic igneous rocks, although recycling from pre-existing sandstones may also have played a role, especially since ATi is generally lower in the Otter Bank Fm. than in the Foula Fm. Other metamorphic indicator minerals, such as kyanite and staurolite, are scarce, owing to dissolution during burial diagenesis. The Foula Fm. is characterised by high rutile contents (high RuZi), suggesting a high-grade metamorphic source (Force, 1980). Garnet data provide the most important mineralogical constraints on the provenance of the Foula and Otter Bank sandstones. Garnet assemblages in the Foula Fm. are dominated by Type A (high-Mg, low-Ca) garnet. Garnets of this type are believed to be derived from high-grade (granulite facies) metasedimentary or charnockitic rocks (Sabeen et al., 2002). As shown in the summary ternary diagram (Fig. 11), such garnets are exceptionally scarce in the basement rocks of Scotland. Garnets from metasedimentary rocks of northern Scotland, such as the Moine of the Northern Highlands, the Dalradian of the Grampian Highlands, the Moine/ Dalradian of Shetland and the supracrustal rocks of the Loch Maree Group (Lewisian), fall almost exclusively into the low-Mg group, Type B (Morton et al., 2004). Garnets from typical Lewisian orthogneisses are also rich in Type B, but also contain some Type C (high-Mg, high-Ca). The only region providing Type A garnets is South Harris (Outer Hebrides), but even there they are overwhelmed by Type C garnets derived from the metabasic rocks of the South Harris Igneous Complex. The small numbers of Type A garnets derived from the South Harris region are believed

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

1062

C South Harris

Lewisian orthogneiss

Moine, Dalradian, Shetland, Loch Maree

B

A ORS Orcadian Basin

Upper Clair Group

Fig. 11. Ternary diagram showing the marked differences between Foula and Otter Bank garnet assemblages in the Strathmore Field. Garnet compositions from Scottish basement terrains and the Old Red Sandstone of the Orcadian Basin (Morton et al., 2004) and from the Upper Clair Group (unpublished data) are shown for comparison. The diagram plots the relative abundances of Type A, Type B and Type C garnets, the compositional ranges of which are shown in the inset for Fig. 9. &, Otter Bank Fm. subunit O1; J, Otter Bank Fm. subunit O2; m, Foula Fm. subunit F1; E, Foula Fm. subunit F2; ’, Foula Fm. subunit F3; K, Foula Fm. subunit F5.

to represent sediment sourced from high-grade metasediments of the Leverburgh and Langavat belts (Morton et al., 2004). The scarcity of Type A garnets in the basement terrains of northern Scotland suggest that the sandstones of the Foula Fm. had an exotic source, which, given the location of the Strathmore Field, is likely to have lain to the west of Britain. The heavy mineral and garnet geochemical data indicate that the source terrain comprised high-grade (granulite facies) metasediments and/or charnockites. Analysis of garnets from modern beach sediments from Kulusuk, located in the eastern Nagssugtoqidian belt of East Greenland (Fig. 1), have revealed the presence of closely comparable assemblages rich in the Type A component (Sørensen and Kalvig, 2002), suggesting one possible source of the Foula sediments lay on the conjugate margin of the Faeroe-Shetland rift. Support for a source to the west is given independently by palaeocurrent evidence acquired from borehole imaging log data (Swiecicki et al., 1995), which shows that the Foula Fm. was primarily deposited by easterly-flowing currents. Zircon age data provide the most definitive evidence for the source of the Foula Fm. As shown by the relative probability plot (Fig. 12), the zircon age spectrum in the Foula Fm. has three clearly defined components (Archaean, Early Proterozoic and Permian). The Archaean group forms 32% of the zircon population, most of the

6. Provenance

1063

7 Foula Fm. 6 Relative probability

Number

5 4 3 2 1 0 0

500

1000

1500 2000 Age (Ma)

2500

3000

3500

4 Otter Bank Fm.

Number

Relative probability

3

2

1

0 0

500

1000

1500 2000 Age (Ma)

2500

3000

3500

Fig. 12. Detrital zircon ages from the Foula Fm., subunit F1 (205/26a-3, 2454.0 m) and the Otter Bank Fm., subunit O1 (205/26a-3, 2648.4 m), displayed on combined relative probability/histogram plots.

Archaean grains being dated between approximately 2650 and 2800 Ma, with two slightly older grains (2910, 3070 Ma). The Early Proterozoic group, which comprises 30% of the zircon population, has a relatively narrow age range (approximately 1800–1900 Ma), peaking at 1880 Ma. Permian zircons are dated between approximately 250 and 300 Ma, and form 26% of the population. The Archaean group could have been derived from Lewisian basement rocks of Scotland, although similar age basement rocks also occur on the Greenland margin. By contrast, the 1880 Ma event cannot be related to any basement terrain in Scotland. The main phase of Laxfordian granite magmatism, which affects the southern part of the Lewisian complex, is some 180 Ma younger at approximately 1700 Ma (Park et al., 1994). Moine and Dalradian metasediments have a range of Proterozoic zircons, but grains around 1880 Ma are scarce (Cawood et al., 2003, 2004; Friend et al., 2003).

1064

Chapter 41: Correlation of Triassic Sandstones in the Strathmore Field

The combination of an Archaean group at 2650–2800 Ma and an Early Proterozoic group peaking at 1880 Ma can be matched with geological events in the Nagssugtoqidian belt of Greenland (Fig. 1). Most geochronological studies of the Nagssugtoqidian belt have been undertaken on its western part, but it extends under the inland ice to be continuous with the eastern Nagssugtoqidian belt (also known as the Ammassalik belt). Early Proterozoic zircon-forming events in the Nagssugtoqidian comprise a phase of arc magmatism at 1920–1870 Ma followed by granulite-facies peak metamorphism at 1860–1840 Ma (Van Gool et al., 2002). The belt also includes Archaean gneisses that were reworked during the early Proterozoic metamorphic event (Kalsbeek et al., 1984, 1993). The origin of the Permian group of zircons is more enigmatic. The zircons are clearly of igneous origin, as they have high Th/U ratios (Table 3), are euhedral, and show intricate zoning under cathodoluminescence. They cannot be interpreted as Caledonian zircons that have leaked Pb, since they are concordant, and there are no undoubted Caledonian grains in the sample. Consequently, they are most likely to represent Permian intermediate-acidic igneous activity. There is scant record of Permian volcanism in East Greenland, the only published evidence being that by Stemmerik and Sørensen (1980) who identified Permian lamprophyre dykes in Scoresby Land. However, Cretaceous sandstones in the vicinity of Geographical Society Ø, northern East Greenland, also contain common Permian zircons (Morton et al., 2005b), providing further evidence for igneous activity of this age in East Greenland. Speculatively, the Permian zircons found in the Foula Fm. (and those from the Cretaceous of northern East Greenland) may be related to igneous activity associated with Permian rifting along the proto-northeast Atlantic. Elsewhere in northwest Europe, Permian rifting is widely associated with volcanism (Ziegler, 1990; Glennie, 2002), although this is generally basic (and hence zircon-poor) in character. The only other possible source for the Foula Fm. is the Rockall Plateau, a large submerged fragment of continental crust that lies between southeast Greenland and the British Isles (Fig. 1). The only published information on the nature of the Rockall Plateau crust comes from a small number of samples acquired by drilling, dredging, and diving on Rockall Bank (Hitchen, 2004), which show that the basement comprises high-grade basic to acidic gneisses and granites. On the basis of whole-rock Rb-Sr, Sm-Nd and U-Pb data (Morton and Taylor, 1991), Dickin (1992) argued that the Rockall Bank crust is equivalent to the Ketilidian metamorphic belt of southern Greenland and to the Islay-Inistrahull terrane of Scotland and Ireland. U-Pb zircon crystallisation ages from the Ketilidian belt and the Islay-Inistrahull terrane are younger than the 1880 Ma peak in the Foula Fm. sample. There were two main phases of granitoid emplacement in the Ketilidian belt (Garde et al., 2002), the first phase comprising intrusion of the calc-alkaline Julieneha˚b batholith at 1795–1854 Ma, closely followed by high-temperature/low-pressure metamorphism and intrusion of I-type granites at 1785–1795 Ma. Subsequent emplacement of rapakivi granite sheets occurred between 1732 and 1755 Ma. Daly et al. (1995) reported a U-Pb zircon crystallisation age of approximately 1750 Ma for the Annagh Gneiss of north Mayo (Ireland), and imply that samples from Rockall Bank yielded similar U-Pb zircon ages. It therefore appears that the Rockall Plateau and the InishtrahullIslay block share a common geological history with the Ketilidian belt, as proposed by Dickin (1992).

6. Provenance

1065

The available evidence therefore precludes derivation of the Foula Fm. from the southern part of the Rockall Plateau. However, isotopic data from Tertiary basalts (Hitchen et al., 1997) indicate that the northern part of the Rockall Plateau region (George Bligh Bank region) is underlain by Archaean rocks. In view of the juxtaposition of the northern Rockall Plateau with the Nagssugtoqidian belt and the adjacent Archaean craton in southern East Greenland (Fig. 1), it may be that the northern part of the Rockall Plateau is a suitable hinterland to supply sediment with Foula Fm. characteristics. It is impossible to test this hypothesis at present, however, because of the absence of basement samples from George Bligh Bank and adjacent areas. The garnet assemblages in the Otter Bank Fm. are much richer in Type B than those in the Foula Fm. However, although they plot close to typical Lewisian orthogneiss on the summary ternary diagram (Fig. 11), they contain significantly more Type A garnets than the Lewisian orthogneiss end-member. There are two possible explanations for this: they could represent mixing between a Lewisian component and a Foula-type component, but alternatively they could have been derived entirely from a different source. The most likely alternatives are the Devonian Old Red Sandstone (ORS) of the Orcadian Basin and the Devonian-Carboniferous Clair Group that forms the reservoir for the Clair Field to the northeast (Allen and Mange-Rajetzky, 1992). The only published garnet data from ORS of the Orcadian Basin (Morton et al., 2004) indicates that assemblages are directly comparable to those from the Moine and Dalradian (Fig. 11). Recycling from the ORS therefore seems unlikely, although the ORS garnet database is very small, and this conclusion may require re-evaluation should further data become available. By contrast, the close match between garnet data from the Upper Clair Group and Otter Bank sandstones (Fig. 11) suggests that recycling of the Upper Clair Group is a viable model for generation of the Otter Bank garnet suites. Zircon age data from an Otter Bank sandstone sample (Table 4) further emphasises the strong contrast in provenance between the Otter Bank and Foula Fms. The Otter Bank zircon population is dominated by Archaean zircons (64% of the assemblage falling between approximately 2500 and 3000 Ma), with a subordinate Proterozoic group, most of which lie between approximately 1050 and 1850 Ma), and a single Early Palaeozoic grain. There is no evidence for either the major Early Proterozoic (1880 Ma) or Permian zircon groups found in the Foula Fm., and although both formations contain Archaean zircons, there is a distinct difference in the age of the main peak on the two spectra (Fig. 12). The Archaean zircons can be matched with the local Lewisian basement, which is believed to have originated between 2900 and 2700 Ma (Hamilton et al., 1979; Whitehouse, 1988, 1989) with a prolonged period of subsequent metamorphic reworking (Park et al., 1994). The Proterozoic zircons are broadly comparable to those seen in the Moine and Dalradian (Cawood et al., 2003, 2004; Friend et al., 2003). Although post-Grampian Group Dalradian metasediments include some Archaean zircons (Cawood et al., 2003), they are less abundant than in the Otter Bank Fm., indicating that recycled Dalradian cannot be invoked as the sole source of the Otter Bank. The single Palaeozoic zircon (473 Ma) corresponds to the Grampian orogenic event of northern Scotland, but the high Th/U ratio indicates it is of igneous, rather than metamorphic origin. Migmatites within the Moine, which have been dated as

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between 460 and 470 Ma (Kinny et al., 1999; Rogers et al., 2001), may therefore have sourced this zircon. Zircon age data are therefore consistent with derivation of the Otter Bank sandstones from local Scottish basement rocks, although they do not rule out recycling from either the ORS or the Clair Group, since published data on zircon ages in both are lacking. The most likely scenario is that Otter Bank sandstones were derived from a combination of Upper Clair Group and Lewisian basement, with possible additional supply from the ORS. Garnet data suggest that reworking of the Upper Clair Group played an important role in the deposition of the Otter Bank Fm. However, GZi values in the Upper Clair Group are much higher than in the Otter Bank Fm. (mean GZi in the Upper Clair group being approximately 96, compared with approximately 85 in the Otter Bank Fm.). A significant proportion of zirconrich sediment must therefore have diluted the Upper Clair Group in order to generate the lower GZi values present in the Otter Bank Fm. Given the large numbers of Archaean zircons in the Otter Bank Fm., the most likely source of the zircon-rich component is Lewisian orthogneiss. The extent of the role of the ORS cannot be readily determined given the scarcity of data on garnet compositions and detrital zircon ages; however, Sm-Nd data (Knudsen, 2000) suggests that Archaean sources played a minor role in ORS provenance, and therefore it is unlikely that there was extensive recycling of ORS into the Otter Bank Fm. The upper part of the Otter Bank Fm. (subunit O2) has characteristics that are intermediate between subunit O1 and the overlying Foula Fm., suggesting that minor amounts of sediment from the Greenland margin reached the Strathmore area during the later phase of Otter Bank deposition. The parameter that shows the strongest deviation from the typical Otter Bank character is ATi. One possible explanation for this phenomenon is that apatite and tourmaline, being the least dense of the heavy minerals, were transported more rapidly from the distal East Greenland source than the denser garnet, rutile and zircon. As a result, changes in ATi provide the earliest manifestation of supply from the East Greenland source, with changes in RuZi and GZi following subsequently.

7. CONCLUDING REMARKS This case study has provided further evidence for the value of heavy mineral analysis as a non-biostratigraphic method for correlation of sand-rich successions. The careful selection of parameters that are sensitive to changes in provenance and insensitive to other processes that operate during the sedimentation cycle is a key part of the development of a reliable heavy mineral correlation framework. Suitable parameters are (i) ratios of abundance of minerals with similar hydraulic and diagenetic behaviour and (ii) varietal data, both petrographic and geochemical. The heavy mineral correlation framework for the Triassic of the Strathmore Field relies on the interplay between inputs from two distinct sand source regions. The two sand types can be identified using conventional heavy mineral data, heavy mineral ratio data and garnet geochemical data. Integration of this information with detrital zircon age data has given powerful constraints on the location and lithological composition of the source regions.

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The Early Triassic Otter Bank Fm. is interpreted as having a source on the British margin of the Faeroe-Shetland rift. A two-component provenance, comprising recycled Devonian-Carboniferous Upper Clair Group in conjunction with Lewisian orthogneiss, is envisaged. By contrast, the Middle-Late Triassic Foula Fm. was derived from a high-grade metasedimentary/charnockitic basement source area, most likely to lie in the eastern Nagssuqtoqidian belt (also known as Ammassalik belt) of southern East Greenland, on the opposite side of the rift. Derivation from the northern part of the Rockall Plateau, a submerged continental crustal fragment between Greenland and the British Isles, cannot be entirely ruled out, since the age and nature of this region is poorly constrained. However, derivation from the southern part of the Rockall Plateau is precluded, since this region is characterised by younger Palaeoproterozoic U-Pb zircon ages (o1800 Ma). Zircon age data from the Foula Fm. also provide evidence for an important Permian igneous event along the proto-northeast Atlantic rift, confirming previous indications of Permian volcanism found during SHRIMP zircon age dating of Cretaceous sandstones in northern East Greenland (Morton et al., 2005b). The switch in sediment supply from easterly-sourced to westerly-sourced detritus is the most clearly defined correlative event in the Triassic succession of the Strathmore Field. Variable supply from a subordinate zircon-rich component (probably of granitic origin) resulted in changes in heavy mineral ratios within the Foula Fm., providing the basis for intra-Foula subdivision and correlation. The Otter Bank Fm. is subdivided on the basis of an upward increase in ATi, believed to indicate the initial appearance of sediment from East Greenland.

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