Abstract
The drilling of hydrocarbon exploration wells in the Faroe–Shetland Basin has provided an expanding sample resource that provides material for testing recently developed palynology-based sediment transport analysis. This technique has been verified by comparison with heavy mineral analysis; both approaches have been used to identify sediment sources and input points along the strike of the Palaeocene West Shetland Platform. Integration of heavy mineral and palynological data has provided a basis for understanding arenaceous and argillaceous sediment distribution and sourcing. In addition to a source from the western, Greenland side of the basin, four argillaceous and four arenaceous sedimentary sources have been identified along the strike of the West Shetland Platform. These vary in temporal and spatial distribution, and thus provide a history of sediment source evolution. This analysis supports a persistent difference in source between the Corona Basin and the Flett and Judd Sub-basins. Although source variation and overlap between basins is evident, transfer zones represent both conduits for and barriers to effective sediment transport. Both palynological and heavy mineral evidence identifies the former presence of Late Namurian–Westphalian strata on the West Shetland Platform, which were removed by subsequent erosion.
Continued interest in hydrocarbon exploration in the Faroe–Shetland and Corona Basins, NE Atlantic (Fig. 1) has provided an expanding resource of sample material from exploration wells. Analysis of these samples has supplemented our knowledge of the geological evolution of this part of the NE Atlantic during the periods prior to and during the onset of Palaeocene rifting (Smallwood & White 2002). Because of the relatively narrow, restricted marine basin during this phase of development, there is potential for Palaeogene sediment derived from landmasses to the west (Greenland) and the east (Hebrides–Shetland) to reach the basin centre (Smallwood 2005). Transport of arenaceous and argillaceous sediment from both sides of the basin has been demonstrated by the use of geochemical, heavy mineral and palynofloral analysis (Carter et al. 2002; Whitham et al. 2004; Jolley et al. 2005). Sediment input from multiple West Shetland Platform sediment sources to the Foinaven Sub-basin (Lamers & Carmichael 1999) was demonstrated by Morton et al. (2002) using heavy mineral analysis alone. To refine the understanding of sediment transport pathways into the Faroe–Shetland Basin, we have expanded the heavy mineral and pollen–spore analytical programmes to derive a more rigorous model for Palaeocene sediment transport pathways in the NE Atlantic.
Map of the NE Atlantic showing major structural elements and basins in their pre-rift position. The well locations discussed in the study are marked by an asterisk.
Stratigraphical framework
In this paper, we concentrate on the Palaeocene sediments of the Sullom, Vaila and Lamba formations (Knox et al. 1997) of the Faroe–Shetland and Corona basins (Figs 1 and 2). These formations comprise marine mudstones and sandstones deposited by turbidites and debris flows. Towards the margins of the basins, shelf facies and littoral deposits are recorded. Regional mudstone horizons deposited during flooding events are seen across the basin, and help to define a sequence stratigraphy. Here, we examine the interval spanning depositional sequences T10–T38 of Ebdon et al. (1995), although because of difficulties in identifying some sequence boundaries, we use a subdivision based on maximum flooding surfaces (Fig. 2; see also Jolley et al. 2005).
Stratigraphical framework showing the comparison of the current nomenclature, based on flooding surfaces (after Jolley et al. 2005), with sequences of Ebdon et al. (1995). For brevity, the prefix TPa is omitted from the flooding surface names in the text.
Source area characterization using terrigenous palynofloras
Plant pollen and spores are abundant in the Palaeogene sediments of the Faroe–Shetland Basin (Jolley 1997; Ellis et al. 2002). Terrestrially derived palynomorphs fall into the size range of 5–60 μm, and have hydrodynamic properties comparable with silt-size sedimentary particles. Pollen and spores preserved in marine basins are derived from vegetation growing within catchment areas at the basin margins, with transportation into the marine basin by dominantly fluvial vectors (Muller 1959). In marine basins with salinity stratification, or where there is major input from large fluvial systems, the pollen and spore load may be transported for considerable distances offshore. This has been shown to occur in the North Sea Basin during the deposition of the Rogaland Group, which contains abundant pollen and spores (Schroder 1992; Jolley 1996). Similar conditions occurred in the NE Atlantic during the Palaeocene and Early Eocene, resulting in the offshore transfer of large volumes of terrigenous plant debris (Naylor et al. 1999).
Palaeocene pollen and spore floras from wells in the Faroe–Shetland and Corona basins (Jolley 1997; Jolley et al. 2005) are dominated by Pityosporites (Piniaceae) and Inaperturopollenites hiatus (Metasequoia or Taxodium relatives). Piniacaceae pollen is dominant in the interval deposited during the highest relative sea levels by virtue of its buoyant hydrodynamic qualities and resistance to decay. The remainder of the pollen assemblages through the Vaila and Lamba formations were derived from lowland mire and riparian communities.
Palynofloras characteristic of the East Greenland source
Analysis of data from Corona Basin and Faroe–Shetland Basin wells has demonstrated that diagnostic pollen–spore and heavy mineral assemblages are derived from the western (Greenland) side of the Faroe–Shetland Basin (Jolley et al. 2005). First recorded in the Kettla Member and underlying Vaila Formation Unit V4 sands (Fig. 2) of some Faroe–Shetland Basin wells, this westerly source of both arenaceous and argillaceous sediment is now also recognized in sediments as old as the Sullom Formation, beneath MFS60. Other strong pulses are seen in the MFS90–MFS95 interval of the Vaila Formation (Fig. 2), and between MFS75 and MFS80.
Palynofloras derived from the westerly source are characterized by commonly occurring Cupuliferoipollenites and Cupuliferoidaepollenties (Fagaceae), co-occurring with Momipites species (Juglandaceae). This flora is associated with the degraded volcanic ash and volcanogenic lithoclasts that characterize the Kettla Member. This flora is particularly evident from the Kettla Member core in well 205/9-1. To date, the source of this volcanogenic sediment has not been identified, although Ellis et al. (2004) suggested a local source for Vaila Formation volcaniclastic sediments in the Corona Basin.
Palynofloras characteristic of Hebrides–Shetland sources
Empirical inspection of palynofloral data derived from wells along the axis of the Faroe–Shetland Basin indicates that the Hebrides–Shetland flora changed little during deposition of the Sullom to Vaila formations. However, it is equally evident that the floras exhibit geographical variation in their composition, this being particularly noticeable in wells along the strike of the Faroe–Shetland Basin. (Supplementary data are available online at http://www.geolsoc.org.uk/SUP18266. A hard copy can be obtained from the Society Library.) To quantify the nature of this variation, statistical analysis of these floras was undertaken. This analysis concentrated on the sub-dominant taxa in the assemblages recorded, excluding the dominant genera Pityosporites and Inaperturopollenites (the ‘facies components’ of Boulter & Hubbard 1982), and taxa forming less than 1% of the assemblages. This approach removed the masking dominance of the ‘facies taxa’ and eliminated the noise caused by records of persistently rare and sporadic species in the counts, but removed <50% of the total dataset. However, those species remaining exhibited distribution patterns not related to facies or scarcity. Correspondence analysis of this refined dataset (with species expressed as a percentage of sub-dominant taxa), contained 64% of the variation in the first two axes. This allowed the comparison of the first two axes alone in data plots while maintaining a high degree of confidence in these data. Similarly, cluster analysis of the same dataset was undertaken using Ward's linkage and Euclidean distance, for both the variables (taxa) and cases (wells).
The dendrogram (Fig. 3b) and correspondence analysis graphs (Fig. 3a) for the study wells show close linkages between the wells south of the Brendan's Dome area (218/28-2, 220/26-1 and 219/27-1). There are also similarities between wells in the south of the area (205/9-1, 205/14-1 and 204/20-3), and between wells in the centre of the basin (208/17-1 and 214/28-1), whereas Foinaven Field well 204/193A has a flora of contrasting composition. From the correspondence analysis (Fig. 3c) and cluster analysis (Fig. 3d) of the palynofloras, it is possible to identify separate co-occurring groups of sub-dominant species in the dataset (see supplementary data; see p. 000). These groups form the basis of four ecological–geographical divisions.
Correspondence analysis plots and cluster analysis dendrograms of the sub-dominant taxa in the study wells, plotted for well number (a, b) and species (c, d). (a) and (b) show the clear regional clustering of wells between the Judd and Clair transfer zones, Clair and Erlend transfer zones, and north of the Erlend Transfer Zone. The separation of the Foinaven flora (F) in well 204/193A should be noted. In the species analysis (c, d), the central flora (C) is displayed as a cluster of Tricolpites hians, Momipites tenuipolus and Laevigatosporites haardtii, whereas the northern flora (N) is represented by the cluster of Alnipollenites verus and Liquidambarpollenites stigmosus. The southern flora (S) is characterized by Momipites tenuipolus, M. anellus and Deltoidospora adriennis.
(1) A ‘northern flora’ (N) is recorded in wells between the Erlend Volcanic Complex and Brendan's Dome Volcanic complex. This group has sub-dominants comprising Alnipollenites verus (Betulaceae), Tricolpites cf. hians (Platanaceae or Cercidiphylaceae), Laevigatosporites haardtii (an early successional polypodiaceous fern; Collinson 2002), Nyssapollenties kruschi (Nyssaceae, ‘black gum’), and Liquidambarpollenties stigmosus (Altingiaceae). No co-dominant is significantly more common than any other species. This dominance of wetland plants (alder type, black gum type and a fern taxon) indicates that this palynoflora is derived from lowland floodplain mires.
(2) A ‘central flora’ (C) is recorded in the area south of the Erlend Volcanic Complex as far as the Clair Transfer Zone. This flora is dominated by species associated with riparian (i.e. fluvial margin) environments, including the sub-dominant taxa Tricolpites hians, Momipites tenuipolus and Laevigatosporites haardtii. Less common sub-dominants include Nyssapollenites kruschii and the cyathacean fern spore Deltoidospora adriennis. Specimens of T. hians co-occurring with early successional ferns and mid-successional hickories indicate derivatiom from plants growing on a floodplain that experienced frequent disturbance. This contrasts with the stable wetland mires that sourced the ‘northern flora’, suggesting more rapid transfer of argillaceous sediment from source to basin.
(3) A ‘southern flora’ (S) is recorded between the Clair Transfer Zone and the Schiehallion field. This is the most diverse flora of those encountered. It has a larger group of sub-dominant taxa, the most significant being the primary colonizing fern spore Laevigatosporites haardtii (see Collinson 2002), associated with the highest frequencies of Momipites tenuipolus recorded in any of the floras. Common specimens of M. anellus with M. tenuipolus indicate that the ‘southern flora’ is derived from extensive juglandaceous mire communities growing on wet lowland floodplains.
(4) A ‘Foinaven flora’ (F) is recorded in the Foinaven Field (e.g. 204/24a-2). This flora is similar to the southern flora, but has a different platanaceous (plane type) species as its most significant sub-dominant taxon. The dominance of juglandaceous species points to the existence of extensive floodplain mires from the margin of the Shetland Platform to the Judd Transfer Zone.
Hebrides–Shetland Platform sediment source areas
Subsequent sea-level fall and erosion has removed most marginal Palaeocene sediments, but the remnants contain well-preserved palynofloras. These provide a record of the source area of the Faroe–Shetland Basin easterly derived floras.
Terrestrial sediments, equivalent in age to the Vaila Formation, occur in BGS borehole 82/15 (Inner Moray Firth Basin), comprising lignite overlain by a thin tuff bed. The tuff (Jolley & Morton 1992) has comparable composition to the Fairy Bridge Basalts of Skye and to the Balmoral Tuffite (Knox & Holloway 1992) of the Andrew Formation, North Sea Basin. The flora in the lignite was deposited in a lowland coastal mire, and the species composition changes up section reflecting changing mire ecology. The oldest sample contains a mid-succession Taxodiaceae–Juglandaceae mire with Momipites anellus forming 24% of the total palynoflora. In succeeding samples, Laevigatosporites haardtii increases in dominance to over 40%. This primary colonizing fern indicates that the section experienced increasing environmental stress, consistent with a rise in relative sea level and flooding of the mire. The dominance of Momipites anellus in the lignite is directly comparable with the presence of this taxon as one of the sub-dominant species in the Hebrides–Shetland southern flora. The location of the 82/15 borehole in the Inner Moray Firth Basin and the source of the southern flora confirms a comparable mire ecology on coastal floodplains from east to west across the Scottish landmass.
The lava fields of the Hebrides are 1° further south than the southern end of the Faroe–Shetland Basin. Despite this latitudinal difference, a comparison with some of the low-altitude floras from these lava fields is informative. The Skye Main Lava Series lava field contains a number of floras deposited in lowland environments. Of particular interest are the floras of the clastic facies and coal beds at Glen Osdale, Tungdal River and Allt Mor Carbostbeg (for location details, see Jolley 1997). These sites are dominated by Metasequoia type pollen, with a contribution from pines growing outside the immediate area of deposition. The sub-dominant taxa include the early successional fern Laevigatosporites haardtii, together with Momipites tenuipolus, Tricolpites hians and bryophyte spores. The Allt Mor Carbostbeg site shows a later successional flora, which includes a greater dominance of Inaperturopollenites hiatus (Taxodiaceae), common occurrences of Nyssapollenites kruschii and Sequoiapollenites polyformosus (Sequoia type). Although they are from a complex mosaic of environments, these sections contain similar palynofloras to those seen in the Faroe–Shetland Basin. They compare best with the Hebrides–Shetland ‘northern flora’, suggesting that the Skye Lava Field did not contribute sediment to the southern end of the study area. Rather than geographical proximity, the Skye Main Lava Series floras appear to be indicative of volcanic disturbance, both on Skye and in the north of the Faroe–Shetland Basin around the Erlend and Brendan's Dome complexes.
Recycled palynomorphs
Reworked palynomorphs occur frequently in the Palaeocene sections examined for this study. They originate by reworking of older formations, and provide information about source terrains. In the absence of oxidative weathering, these palynomorphs can survive within the sedimentary system to become a component of newly deposited strata. Their frequency is in part controlled by the hydrodynamics of the depositional system, but they form <1% of the total palynoflora. Accordingly, it is the age range of individual species, not their abundance, that is of importance.
In the majority of the offshore sections analysed, reworking of Mesozoic taxa is ubiquitous. These taxa include Toarcian marine floras, Bajocian–Bathonian terrigenous floras and Oxfordian–Kimmeridgian marine floras. Early Cretaceous floras are also well represented, and in contrast to more southerly latitudes, Late Cretaceous clastic sediments yield recycled taxa.
Recycling of Permian–Triassic sediments is difficult to determine, because of the paucity of parent terrigenous floras. Many of the studied sections contain sporadic recycled Permo-Triassic taxa. However, these taxa are more abundant in the Vaila Formation of the Foinaven–Schiehallion fields; this finding implies a different source component for this area. On the basis of recycled Permo-Triassic taxa, the source region delivering the ‘southern flora’ may be further divisible.
Recycling of Carboniferous palynomorphs is also geographically and stratigraphically restricted. The Carboniferous spores concerned (mainly Lycospora pusilla and Densosporites annulatus, both highly abundant in Carboniferous rocks), have a restricted stratigraphical Namurian–Westphalian range. Sedimentary rocks of this age have been reported from north of Inninmore Bay, Morven, but only late Devonian–Viséan sediments have been reported from the West Shetland Platform (Stoker et al. 1993). However, Namurian non-marine sediments occur in well 213/23-1 on the Corona Ridge. Although recycled Carboniferous spores occur between the Clair Transfer Zone and the Erlend Centre (Fig. 1) adjacent to the Corona Ridge, it is thought more likely that the source of the reworked material was Namurian–Westphalian sediments on the West Shetland Platform.
Argillaceous source areas from palynomorph data
Palynofloras with a westerly derived component have so far been recognized in MFS45–60, MFS75–80, MFS90–95 and MFS100. Although the dataset is of variable quality in the older interval, it is possible to map out the potential sources of the four Hebrides–Shetland floras and the Greenland flora (Fig. 4).
Distribution of phytogeographical groups and heavy mineral sources through the Early to Late Palaeocene. It should be noted that the maximum transfer of the westerly derived ‘Greenland flora’ phytogeographical group occurs during the MFS90–95 interval. Also, the switch from heavy mineral Source I to an argillaceous sediment input into the Foinaven area should be noted. Heavy mineral data for the MFS100–125 interval are lacking from the Flett Basin because the mud-prone sedimentation of this period precludes analysis.
Brendan's Dome–Erlend source area
The area south of Brendan's Dome accrued sediment from the north of the Shetland Platform (Fig. 4b–d), and probably also from the uplifted Brendan's Dome area to the north (Fig. 4d). Palynofloral analysis of samples from well 219/27-1 (Fig. 1) has shown that volcanism occurred during the Late Palaeocene from above MFS90 to above MFS100. This would have provided a further source of volcaniclastic material into the region around wells 219/27-1 to 220/26-1. Although volcanism effectively isolated this area from the north, it was also structurally isolated from the Flett Sub-basin by the Erlend Platform. Although volcanism on the Erlend Platform did not commence until the latest Palaeocene (Jolley & Bell 2002), evidence from wells 209/3-1 and 209/4-1 indicates that this was an uplifted zone throughout the Palaeocene.
Clair Transfer Zone–Erlend Platform source area
This area broadly equates to the Flett Sub-basin, being separated from the northern portion of the study area by the Erlend Platform, with a western margin defined by the Corona Ridge. Although some minor argillaceous sediment input from the west penetrated through the area of the Victory Transfer Zone during MFS45 to MFS60 (Fig. 1 and see Jolley et al. 2005), this is not recorded in the Late Palaeocene (Fig. 4). Palaeocene argillaceous sediment in the north of the Faroe–Shetland Basin was mostly derived from the West Shetland Platform, and palynofloral evidence suggests that the coastal belt was dominated by fluvial channels and floodplains, possibly with some mires. It is apparent that the coastal belt was restricted in extent, and subject to disturbance, which probably resulted from changes in floodplain morphology.
Isolation of sedimentary transport into the Flett Sub-basin is further indicated by the reworked Carboniferous sporomorphs found in the Palaeocene units throughout the MFS80–100 interval north of the Clair Transfer Zone. Throughout this interval, Carboniferous recycled palynomorphs remain restricted to the area between the Clair and Erlend transfer zones (Fig. 4), except during MFS90–95, when they extended south of the Clair Transfer Zone (Fig. 4c); this latter finding suggests that sediment mixing occurred along the boundary of the southern and northern Hebrides–Shetland flora sediment transport paths. These data indicate that the source for this argillaceous material was the West Shetland Platform.
Judd Transfer Zone–Clair Transfer Zone source area
In the southern part of the Faroe–Shetland Basin, sedimentation was more rapid, with thicker sections developed, with interplay between western and eastern sources. The West Shetland Platform argillaceous sediment was mostly derived from between the Clair and Judd transfer zones (Fig. 4), and the palynofloras indicate that this region was fringed by a complex of mires and floodplains.
From the upper part of MFS95–100 (Fig. 4d) to the lower part of MFS100–125, there was a minor resurgence of argillaceous sediment transported from the west. This influx was limited to the south of the Faroe–Shetland Basin, with the Clair and Westray transfer zones acting as conduits (Fig. 4d). A clear division remained between southern and central Hebrides–Shetland floras. Data from recycled Carboniferous sporomorphs indicates a continued lack of mixing between the southern and central Hebrides–Shetland floras along the Clair Transfer Zone.
Foinaven source area
A southeasterly source, the Foinaven flora, is recorded from wells south of the Westray Transfer Zone (Fig. 4b and c) in MFS90–100.
Source area characterization using heavy minerals
Heavy mineral analysis on the Palaeocene succession in the Foinaven Sub-basin has demonstrated the effectiveness of the method in identifying the interplay between two distinct sediment sources (Morton et al. 2002). Discrimination between the two source regions was achieved using a combination of provenance-sensitive ratio parameters and garnet geochemistry; further information on this approach has been given by Morton & Hallsworth (1994, 1999). The critical parameters in the Foinaven Sub-basin are garnet:zircon (GZi), apatite:tourmaline (ATi) and abundance of low-Ca, high-Mg garnets (termed Type A by Morton et al. 2004).
In this paper, heavy mineral characterization of Palaeocene sandstones is extended to the north as far as the Erlend Transfer Zone (Fig. 1). There are no data from north of the Erlend Transfer Zone owing to the scarcity of sandstones in this region.
Parameters useful for discriminating sandstone provenance between the Judd and Erlend transfer zones are GZi, ATi, rutile:zircon (RuZi) and garnet compositions. Garnet assemblages vary in abundance of Type A garnet (Fig. 5), and in the relative abundance of Type Ai (XMg >30%) and Type Aii (XMg between 20% and 30%) garnets (Fig. 5). Variations in sandstone provenance characteristics are shown using five key wells: 204/19-3A, 204/20-1Z and 204/24a-3, located south of the Westray Transfer Zone, and 205/9-1 and 214/27-1, located between the Westray and Erlend transfer zones (Fig. 6).
Representative garnet ternary diagrams from Palaeocene sandstones in the Faroe–Shetland Basin, displaying evidence for marked differences in garnet assemblages. XFe, XMn, XMg and XCa are ionic proportions of Fe + Mn, Mg and Ca, with all Fe determined as Fe. •, XMn <5%; ○, XMn >5%.
Stratigraphic variations in key heavy mineral parameters in Palaeocene sandstones from wells 204/19-3A, 204/20-1Z, 204/24a-3, 205/9-1 and 214/27-1, depth in mbsl on vertical axis. For clarity, an expanded depth scale is used in more condensed sequences. ATi, apatite:tourmaline index (percent apatite in total apatite plus tourmaline); GZi, garnet:zircon index (percent garnet in total garnet plus zircon); RuZi, rutile:zircon index (percent rutile in total rutile plus zircon).
Stratigraphic variations in mineralogy
Well 204/193A
Well 204/19-3A contains sands at several stratigraphic levels through the interval of interest (Fig. 6). The profile shows the presence of marked changes in provenance characteristics. These are most clearly manifested by ATi and garnet geochemistry, with GZi showing more subdued variations and RuZi being comparatively uniform. The earliest sandstones (up to MFS75) are characterized by relatively low ATi values, and have garnet assemblages with comparatively low Type A garnet abundances. Within MFS75, there is a marked and sudden increase in abundance of Type A garnet, which remains high through the interval from MFS80 to MFS90. From MFS80 onwards, ATi values are consistently high. Type A garnet abundances progressively decrease during MFS95-MFS125.
Well 204/20-1Z
Well 204/20-1Z contains sandstones at several stratigraphic levels, but unlike 204/19-3A, sand mineralogy remains relatively constant throughout (Fig. 6). ATi and GZi remain high, RuZi values track around 25 with occasional slightly higher values, and Type A garnets are abundant throughout.
Well 204/24a-3
The profile for 204/24a-3 (Fig. 6) demonstrates a change in provenance between MFS80 and MFS90. Type A garnet abundances are lower between MFS80 and MFS90 than between MFS90 and MFS95. This change coincides with a distinct, although subtle, increase in ATi.
Well 205/9-1
There are three main sandstone developments in 205/9-1. The MFS60–75 sandstone has high ATi and GZi, moderate RuZi, and high Type A garnet abundances (Fig. 6), similar in all aspects to the sandstones throughout the succession of 204/20-1Z, in MFS80–90 of 204/19-3A, and in MFS90–100 of 204/24a-3. By contrast, the MFS100–125 sandstone of 205/9-1 has a distinctive mineralogy, being characterized by relatively low RuZi values in conjunction with high ATi, high GZi and garnet assemblages containing relatively low numbers of Type A. Some of the MFS100–125 samples are also unusual in containing abundant clinopyroxene, which confirms the presence of basaltic detritus as recognized by Lamers & Carmichael (1999).
Sandstones in the MFS125–130 interval show a return to ‘normal’ RuZi values, and have low Type A garnet contents, closely comparable with those of coeval sandstones in 204/19-3A.
Well 214/27-1
Well 214/27-1 contains sandstones above the MFS60, MFS75, MFS90 and MFS95 surfaces (Fig. 6). GZi values are much lower than in the other wells studied in this paper, but this is not a provenance-related feature, as the grains display evidence of advanced corrosion. There is evidence for a change in provenance within the succession, with the earlier sandstones (MFS60–75) having high abundances of Type Ai garnet whereas MFS90–95 has high abundances of Type Aii. This cannot be a function of preferential chemistry-dependent garnet loss, as the main control on garnet stability during burial diagenesis is Ca content, not Mg (Morton 1987; Morton & Hallsworth 2007). The change to Type Aii-dominated assemblages appears to be gradual, beginning near the top of the MFS60 sequence. Sandstones in the lower part of the succession have similarities to those in MFS60 in 205/9-1 and to some sandstones south of the Judd Transfer Zone, but those in MFS90–95 have no counterparts further south.
Sand provenance
Four main sources of coarse clastic detritus can be identified on the basis of heavy mineral data. These are herein termed Sources I–IV. Two of these occur in the area south of the Westray Transfer Zone, as discussed by Morton et al. (2002), with two newly described sources between the Westray and Erlend transfer zones. The best discriminants are ATi, RuZi, abundance of Type A garnet, and abundance of Type Ai garnet, as shown in Figures 5, 6 and 7.
Source I
Source I has low abundances of Type A garnet, moderate RuZi and high GZi. ATi is high except in the early part of the analysed interval. The high abundance of Type B garnet indicates that a large proportion of the detritus was supplied from metamorphic basement, probably including Lewisian Gneiss and Moine or Dalradian metasediments, both of which are present on the West Shetland Platform margin (Morton et al. 2002) and have low contents of Type A garnets (Fig. 8). A minor contribution from the Triassic Foula Formation is inferred, to supply the small number of Type A garnets. The lower ATi in the early part of the analysed succession indicates the presence of a higher proportion of weathered detritus. In well 204/24a-7, there is a well-defined increasing ATi profile through MFS20–45 (Fig. 9), recording the erosion of the weathering profile on the Late Cretaceous land surface into fresh source material (Morton et al. 2002).
Source I is interpreted to lie in the south of the West Shetland Platform (Fig. 4). It supplied sediment to the Foinaven Field area in MFS20–45, MFS60, MFS75 and MFS80, but its influenced waned during MFS90. It reappears as a major source in MFS95–125, its influence being seen south of the Westray Transfer Zone (204/19-3A) and extending north to 205/9-1 and 214/27-1.
Source II
Source II is characterized by abundant Type A garnet, moderate RuZi, high GZi and high ATi. The abundance of Type Ai garnet indicates that the Triassic Foula Formation (Fig. 8) was widespread in the source region. Sediment derived from Source II has lower GZi than typical Foula Fm, therefore requiring additional input from a low-garnet source, such as Lewisian acidic gneisses or the Old Red Sandstone.
Source II dominated supply to the Schiehallion area during MFS20–95, and extended into the Foinaven Field area at times. Source II was the dominant sediment supplier to 204/19-3A during sequences MFS80–90, and to 204/24a-3 during sequence MFS90. The same source also operated further north, and influenced sequence MFS60 sandstones in 205/9-1 and 214/27-1. Source II is therefore interpreted as occurring along the central part of the West Shetland Platform, as far north as the Victory Transfer Zone (Fig. 4).
Source III
Source III is seen only in the Kettla Member in well 205/9-1. It is characterized by distinctively low RuZi, unlike any other sandstones along the Faroe–Shetland Basin margin. ATi and GZi are high, and the abundance of Type B garnet is relatively high (Fig. 7). These features suggest derivation from predominantly gneissic basement. In addition, there are strong indications of input from basaltic rocks, as indicated both by the petrography (Lamers & Carmichael 1999) and the presence of abundant clinopyroxene. The lack of basaltic rocks on the eastern margin of the Faeroe–Shetland Basin indicates that Source III lay elsewhere. Furthermore, there is no obvious sand entry point on the West Shetland Platform margin for the MFS100 sandstone in 205/9-1 (Smallwood et al. 2004). Source III therefore lay to the west, which was the locus for extensive basic magmatism during the Palaeocene–Eocene, including widespread flood basalts (Kiørboe 1999).
Discrimination of sandstones derived from Sources I–IV, on the basis of variations in ATi, RuZi and abundance of Type A garnet.
Possible locations of Source III include the East Greenland landmass, or more local uplifted blocks within the Faroe–Shetland Basin. The Kangerlussuaq region is known to have supplied low RuZi sand during the Late Cretaceous and Early Tertiary (Whitham et al. 2004), and is therefore a possible candidate for the location of Source III. However, derivation from local structural highs such as the Corona Ridge, which may include high-grade gneissic basement rocks that were exposed during the Palaeocene, cannot be ruled out.
Magnetic fabric measurements on the MFS100 sandstone in 205/9-1 suggested transport from the NE (Smallwood et al. 2004). It is likely that these fabrics relate to local basin-floor topography rather than to the overall transport direction, as there is no obvious source to the NE. We therefore consider that the sands were introduced from the NW, with transport diverted to the SW owing to the presence of the Flett Ridge.
Source IV
Source IV is characterized by high abundances of Type A garnet, moderate RuZi and high ATi, similar to Source II. GZi values are also interpreted as being originally high, but have been reduced through burial-related garnet dissolution. Source IV is discriminated from Source II by a difference in composition of the Type A garnet group (Fig. 6). In Source II, the Type A garnets mostly have XMg >30% (Type Ai), but in Source IV they mostly have XMg between 20% and 30% (Type Aii). Garnets similar to those characterizing Source IV occur in the Devonian–Carboniferous Upper Clair Group (Fig. 8) of the Clair Field, located on the Rona Ridge immediately to the SE.
Garnet geochemical characteristics of source materials on the West Shetland Platform, which is believed to have sourced Palaeocene sandstones in the Faroe–Shetland Basin. XFe, XMn, XMg and XCa are ionic proportions of Fe, Mn, Mg and Ca, with all Fe determined as Fe. •, XMn <5%; ○, XMn >5%. (a) Foula Formation (Triassic) sandstone in well 204/30a-2, 2464.0 m (from Morton et al. 2007); (b) Upper Clair Group sandstone, Clair Field (previously unpublished data); (c) sediment from rivers draining Lewisian gneisses of NW Scotland (Morton et al. 2004); (d) sediment from rivers draining Moine metasediments of northern Scotland (Morton et al. 2004).
Stratigraphic variation in ATi (apatite:tourmaline index) through the MFS20–45 interval in well 204/24a-7, showing the presence of a well-defined increasing ATi trend (Morton et al. 2002). Depth in mbsl on vertical axis.
The first appearance of sediment derived from Source IV occurs in the upper part of MFS60 in 214/27-1, replacing sand derived from Source II, and it continues up to and including MFS95. The same pattern, of Source II superseded by Source IV, is seen in several other wells in the area, including 206/1-3, 214/19-1, 214/27-2, 214/27-3 and 214/28-1. Source IV also supplied the MFS90–95 sands in 208/19-1, further to the NW, but here the preceding part of the Palaeocene is devoid of sand, presumably being distal to the main source operating at this time (Source II).
Discussion
We have used the example of the Faroe–Shetland region to demonstrate the compatibility of heavy mineral and phytogeographical analysis. However, it is apparent that although the source areas of sediments indicated by these contrasting approaches are closely comparable, they are not identical. These differences provide us with additional insights into the environmental factors controlling sediment input into basins, and from this, opportunities for improved understanding of sediment transport systems.
Conflicting distribution patterns of pollen (argillaceous sedimentary particles) and heavy minerals (arenaceous fraction) can be explained in terms of differing transportation energies, but care must be exercised to avoid making oversimplified conclusions. In the MFS75–80 interval of the Judd–Westray and Westray–Clair blocks in the Faroe–Shetland Basin (Fig. 4b), two different heavy mineral sources, one dominantly Lewisian–Dalradian, the other dominantly Triassic, conflict with a derivation from a single phytogeographical group. We interpret this as reflecting sediment input from two major feeder channel zones, each with distinct source mineralogy, cutting through one extensive lowland mire ecosystem (Hebrides–Shetland southern flora). Differences in the heavy mineral assemblages reflect different sources within the hinterland catchment of the Hebrides–Shetland zone, whereas the single phytogeographical group reflects input into these fluvial systems as they passed through the extensive coastal mire.
Combining heavy mineral, in situ and reworked palynomorph data can help elucidate sediment sources. Within the Flett Sub-basin, the appearance of heavy mineral assemblages with affinities to those of the Upper Clair Group (Devonian–Viséan) corresponds to occurrences of recycled Namurian–Westphalian miospores. Reconciliation of the age disparity between these two pieces of evidence is possible, if it is accepted that Namurian–Westphalian sands (now eroded from the Hebrides–Shetland landmass) represent continued supply from the area supplying the Upper Clair Group. Regional evidence supports this hypothesis, as Namurian sandstones in the Pennine Basin, which were derived from a source area to the north of the British Isles, have similar garnet assemblages to those of the Clair Group (Hallsworth et al. 2000).
Evidence of the role of transfer zones in controlling sediment distribution in the Faroe–Shetland region is provided by the concurrence of heavy mineral and phytogeographical data in the Flett Sub-basin. The termination of deposition against the Clair Transfer Zone highlights the effective barrier it created through much of the late Palaeocene. Furthermore, the composition of the Hebrides–Shetland central flora and the increasing influx of recycled taxa in the Flett Sub-basin attest to greater environmental instability, and more rapid erosion of the adjacent West Shetland Platform (Fig. 4c and d). This is in direct contrast to the neighbouring Clair–Judd transfer zones.
Integration of provenance techniques has also provided evidence of source shut-off and rejuvenation. During MFS100–125, the palynofloras provide evidence of an argillaceous input from the SE of the Foinaven field (the Foinaven flora), which is not mimicked by the heavy mineral data. Instead, this phytogeographical trend parallels a discrete source seen in heavy mineral assemblages in the 5 Ma older, Danian strata. These data suggest that this more southerly source was a pervasive component of sedimentation in the Foinaven field area. This source initially delivered arenaceous sediment, but the waning energy of the source resulted in a shift to argillaceous input.
Where heavy mineral and phytogeographical evidence for a sediment transport pathway concur, they do not necessarily indicate the same source area. Phytogeographical evidence for a westerly source in the Kettla Member of well 205/9-1 marks the final phase of a pervasive input from the westerly, Greenland flank of the region (Jolley et al. 2005). This westerly sourced Greenland flora component is accompanied by Source III heavy mineral assemblages, which were probably derived locally from the Corona Ridge (Fig. 4). This more local source provided the abundant volcanic lithoclasts and degraded ash that characterize the Kettla Member.
Finally, although we have noted the importance of transfer zones in compartmentalizing the Faroe–Shetland region into NW–SE-trending blocks with contrasting sediment input pathways, the role of volcanism in controlling sediment distribution is also displayed by our results. Late Palaeocene volcanism has been recorded in MFS100–125 in well 219/23-1, and is evident on the Corona Ridge from the volcanogenic sediments of the Kettla Member. Further activity during sequence MFS135–140 is evident at the Erlend Centre (Jolley & Bell 2002) and probably again on the Corona Ridge. Although a pulsed record of the Greenland flora within the Corona Basin has already been attributed to thermal uplift across the nascent North Atlantic rift zone (Jolley et al. 2005), thermal uplift at the Corona Ridge and Erlend Complex would assist in isolating the sub-basins in the east from Greenland-sourced sediment.
- © 2007 The Geological Society of London
References
Understanding basin sedimentary provenance: evidence from allied phytogeographic and heavy mineral analysis of the Palaeocene of the NE Atlantic
