The Neogene stratigraphy of the glaciated European margin from Lofoten to Porcupine

The study of the Neogene (Miocene to Holocene) stratigraphic record on the glaciated Atlantic margin of NW Europe has, to date, largely been undertaken on an ad-hoc basis. Whereas a systematic approach to understanding the stratigraphic development of Palaeogene and older strata has been undertaken in areas such as the North Sea, West of Shetland and Norway, the problem of establishing a Neogene framework has been only partly addressed by academia and the oil industry. In most cases where a Neogene stratigraphy has been constructed, this has been largely in response to problem solving and risk assessment in a restricted area. Nevertheless, in the past few years it has become increasingly apparent that there is a common history in the Neogene development of the passive Atlantic margin of NW Europe, between mid-Norway and SW Ireland (Fig. 1). The inspection and interpretation of an extensive geophysical and geological database (Fig. 2) has identified several regionally significant and correlatable unconformities along this continental margin. Thus, a regional approach to the stratigraphical development of the Neogene succession on the glaciated European Atlantic margin is undertaken in this volume.

Structural and physiographic setting

The continental margin between mid-Norway and SW Ireland is an area of complex bathymetry bordering the NE Atlantic Ocean (Fig. 1). This structural configuration of shallow platforms, shelves, ridges and banks separated by deeper-water basins largely reflects late Mesozoic–early Cenozoic continental rift events that ultimately led to the opening of the NE Atlantic Ocean in the early Eocene (Knott et al. 1993; Doré et al. 1999; Roberts et al. 1999). This configuration may have become accentuated by compression, extension, and broad uplifts and subsidence that variably affected the continental margin during the mid- to late Cenozoic interval (Lundin & Doré 2002). Of particular significance is the widespread development of prograding wedges that fundamentally shaped the continental shelves between mid-Norway and NW Ireland, in response to late Neogene epeirogenic uplift. These depocentres have been the focus for episodic mass failure of the continental margin (Fig. 3).

The main physiographic elements of the study area are indicated in Fig. 1. Offshore Norway, the width of the continental margin — as delineated by the continent–ocean boundary (COB) — ranges from 200–500km. This margin is widest on the Vøring Plateau, where a significant left lateral offset of the COB along a major NW-trending fracture zone has occurred along the southern edge of the plateau (Fig. 3). Farther south, the width of the continental margin generally ranges from 500–800km, being greatest off NW Britain and Ireland. In contrast to the Norwegian margin, which gradually slopes and deepens into the oceanic Lofoten and Norwegian basins, the Faroe–Shetland and Rockall–Porcupine margins include the deep-water intra-continental basins of the Faroe-Shetland Channel, Rockall Trough and Porcupine Seabight. Although the crust beneath these basins is continental in nature, it is highly attenuated and stretched (cf. Stoker et al. 1993 and Corfield et al. 1999 and references therein). Consequently, the bathymetric range is high across the study area, from less than 200m on the shelves and platforms to between 1000 and 4000m in the basins (Fig. 1). One unique aspect of the Rockall Trough is that the continuity of the basin is interrupted by the late Cretaceous–early Palaeogene seamounts of Rosemary Bank, Anton Dohrn and Hebrides Terrace.

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Controls on sedimentation

The Neogene sedimentary history of the NW European Atlantic margin has been strongly influenced by the interaction of tectonics, oceanographic circulation and climate change. Changes in plate configuration and rates of spreading have not only resulted in localised compressional doming (Doré et al. 1999; Lundin & Doré 2002), but also impacted on the oceanographic circulation pattern (Andersen & Boldreel 1995; Stoker 1997, 2002; Stoker et al. 2001). The onset of Norwegian Sea Deep Water (NDSW) overflow is one of the most significant events in the study area, linked to the submergence of the Greenland-Scotland Ridge, which includes the Iceland-Faroe and Wyville-Thomson ridges (Fig. 1). The Neogene sedimentary response to these changes is preserved in the deep-water sediment fills of the Norwegian Basin, Faroe-Shetland Channel, Rockall Trough and Porcupine Seabight, expressed as regional unconformities and by changing styles of deposition. Perhaps the most distinct aspect of Neogene sedimentation is the Plio-Pleistocene development of major prograding shelf-margin wedges. Epeirogenic uplift coupled with climatic deterioration and widespread glaciation has resulted in significant prograding wedges building out and shaping the NW European margin all the way from the mid-Norwegian to the NW Irish margins, including the Faroese margin (Fig. 3) (Andersen et al. 2000; King et al. 1996; Stoker 1995, 1999, 2002; Vorren & Laberg 1997; Vorren et al. 1998). Further details on all of these aspects are presented in Fig. 3.

The combined effects of tectonics, deep-ocean currents and climate change resulted in a physically dynamic sedimentary environment along the Atlantic margin of NW Europe throughout the Neogene interval. Consequently, the preserved record of sedimentation documents the interaction of various depositional processes, notably downslope, alongslope and hemipelagic (vertical flux) processes. The main characteristics of each of these groups of processes are detailed in Fig. 4, and the resulting distribution and thickness pattern of the Neogene succession is shown in Fig. 5.

Stratigraphy

In this study, the term Neogene is that as defined by Berggren et al. (1995) who treat it as a period/system subdivision of the Cenozoic Era/Erathem, incorporating the Miocene, Pliocene, Pleistocene and Holocene. Together with Palaeogene, these two terms replace the antiquated terms Tertiary and Quaternary.

The stratigraphic scheme for the Neogene succession is indicated in Table 1 and Fig. 6. Essentially, a two-fold subdivision of the Neogene succession is proposed, which represent the Miocene–early Pliocene and early Pliocene–Holocene intervals. However, some problems still persist in the Rockall–Porcupine area in terms of the seismic definition of base Neogene. Table 1 compares this scheme with some of the previous frameworks that have been employed in the various Work Package (WP) areas. The previous classifications represent a mix of lithostratigraphic (Deegan & Scull 1977; Dalland et al. 1988; Knox et al. 1997) and seismic-stratigraphic (Shannon et al. 1993; Stoker 1999; Andersen et al. 2000; Stoker et al. 2001) approaches to subdivision. The latter combine geophysical and geological data and represent a more integrated methodology. Whilst this methodology underpins the present study it is further enhanced by sequence-stratigraphic concepts that bring together units that constitute important genetic packages. By adopting a regional approach to Neogene stratigraphy it is essential to recognise the correct level of mapping and correlation in order for a unified framework to be developed. Thus, emphasis is placed on the establishment of megasequences whose development tends to reflect major changes in continental margin evolution. Such changes are commonly manifest by regional unconformities that form megasequence boundaries. It is these boundaries that form the basis of our regional correlations (Fig. 6). 

Inspection of Table 1 shows that the proposed Neogene units in the North Sea Fan–Vøring region coincide with the pre-existing lithostratigraphic units of the Kai and Naust formations. Thus these terms are retained in order to avoid confusion, albeit with their definitions broadened in light of the seismic-sequence stratigraphic approach. In the Faroe–Shetland and Rockall–Porcupine regions new terminology's are introduced. In the Faroe–Shetland region this reflects the first ever integration of data from both the Faroes and West Shetland margins, thereby prompting a new unified notation. The upper-case letters (FSN) refer to Faroe-Shetland Neogene, whilst the numbers (1, 2) refer to the specific megasequences. In the Rockall–Porcupine region, the notation expands upon the recently published scheme for the Rockall Trough (Stoker et al. 2001) by incorporating the Porcupine region. Thus, RT (Rockall Trough) of Stoker et al. (2001) is replaced by RP (Rockall–Porcupine) with the lower-case letters (a-d) representing specific megasequences. In this study, emphasis is placed on RPa and RPb. 

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On several parts of the NW European Atlantic margin, more detailed subdivision of the Neogene exists, especially for the Plio-Pleistocene succession (e.g. Stoker et al. 1993; Rokoengen et al. 1995; King et al. 1996; McNeill et al. 1998; Andersen et al. 2000; Evans et al. 2002). This has largely resulted in a confusing plethora of stratigraphic terms. This new scheme is intended to greatly simplify the stratigraphic nomenclature, thereby improving inter-regional correlation (WP's 1–3) and providing a context for future, more detailed and meaningful correlation of higher-order sequences. A full description of the proposed unified stratigraphic scheme is given on subsequent pages.

Mapping Work Procedures

Data sources: Bathymetric data was derived from the National Oceanic and Atmospheric Administration, from Worldwide Digital Terrain Data, and from TerrainBase, a global 5-minute Digital Terrain Model. Sample locations and mapped surfaces were provided by the WP1–3 Partners.

Table 1. Comparison of stratigraphic schemes on the Atlantic margin of NW Europe

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Data Mapping and Compilation:    Two different projections have been used throughout this project. The European overview maps (e.g. Fig. 1) are defined using Lambert's Conformal Conic projection with two standard parallels. The individual WP maps (e.g. Fig. 8, Fig. 9, and Fig. 10) have been defined on the Universal Transverse Mercator Projection, Zones 29, 30 and 31. These were chosen based on their ability to keep the data as true as possible with the minimum of distortion. The compilation and mapping was carried out in Geosoft Oasis-montaj v 5.1.2. The thickness data supplied from the partners was imported into a database in preparation for gridding. The data was gridded within the system with a cell size of 1000m using a search radius of 40,000m. The bathymetry data from the NOAA and TerrainBase data was imported into a database and gridded using a cell size of 1000m and a search radius as defined automatically by the software. The data was then masked to the area of particular interest using relevant boundaries supplied by the partners. The colourwash outputs for the two-way travel time (TWTT) maps are displayed at intervals of 100 milliseconds (ms), with the contour lines varying depending on the output grid, but they are typically at 100ms and subsequent 200ms intervals. The bathymetry colourwash outputs are displayed at intervals of 500m, with contour lines at 200, 500 and subsequent 1000m intervals. The data exports from the system were DXF and Georeference Tiffs. DXF and Tiff data was imported into Microstation J, where individual maps were created for the atlas. These maps were then exported and manipulated in Corel Draw, where the data was ready for inclusion in the Atlas. It should be noted that all of the enclosed maps have been compiled using the data supplied by the individual WP groups. The WP maps show gridded raw data received from the relevant WP, whereas the overview maps are an amalgamation of all of the raw data supplied from all WP's. Where there is a data overlap, an average value is taken. Consequently, whilst the Neogene overview map (Fig. 5) is fully integrated similar overview maps were not produced for individual megasequences (e.g. Naust–FSN-1–RPa); megasequences are described separately within each WP area. Thus, there may appear to be discrepancies at overlaps between the individual WP maps (e.g. Fig. 22 and  Fig. 23). However, megasequence overview maps will be included as part of the Margin Evolution Model report (STRATAGEM Partners 2003).

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