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In the Triassic basins of England, facies associations (Figures 2.1b, 2.2, 2.3) show regional variations due to varying distances from the main sediment source (Armorican Massif), and tectonic and climatic variations (McKie and Williams, 2009; McKie and Shannon, 2011; Tyrrel et al., 2012).

In all the Triassic basins of England, the basal part of the Sherwood Sandstone Group is exclusively characterized by fluvial deposits, which are dominated by channelized architectural elements (Ambrose et al., 2014).

An overall northward decrease in mean grain-size and maximum clast size characterizes the fluvial deposits of the Sherwood Sandstone Group, reflecting the increasing distance from the sediment source (Smith, 1990;

Steel and Thompson, 1983; McKie and Williams, 2009). In fact, fluvial deposits of the lower Triassic pass from conglomerates with interbedded

pebbly sandstone in the Wessex, Worcester, Staffordshire, Needwood, Cheshire and southern eastern England Shelf basins to medium-fine grained sandstone in the northern part of the eastern England Shelf, eastern Irish Sea Basin, Vale of Eden and Carlisle basins (McKie and Williams, 2009; Ambrose et al., 2014). In this work these basal fluvial deposits are differentiated as A’ and A” units, which represent (i) conglomerate with pebbly sandstone and (ii) fine-medium grained sandstone, respectively (Figure 2.1b).

The Sherwood Sandstone Group passes upwards into sandy deposits which become progressively more abundant in aeolian facies (Jones and Ambrose, 1994; Mountney and Thompson, 2002; Ambrose et al., 2014).

Thus, two units which are named B and C are differentiated from the basal A and A” units by presence of both aeolian and fluvial deposits. However, the lower unit B and the upper unit C are distinguished based on the relatively lower and higher occurrence of aeolian facies associations, respectively (Figure 2.1b.). Arid climatic conditions were widespread across much of NW Europe during the Lower Triassic time, although episodically wetter episodes occurred (Brookfield, 2004, 2008; McKie et al., 2009; Mckie and Shannon, 2011). Thus, the upward increasing of aeolian facies content does not reflect increasing aridity, but it is likely due to a progressive switch-off or avulsion of the southerly fluvial system (Meadows and Beach 1993a, b; Jones and Ambrose, 1994). Notably, Mckie and Williams (2009) identify a general northward increase in the proportion of aeolian facies in a comparison of the upper part of the Sherwood Sandstone Group in the Wessex Basin (8% aeolian and 92% fluvial origin Otter Sandstone Formation) and in the eastern Irish Sea Basin, Vale of Eden and Carlisle basins (99% aeolian and 1% fluvial origin Ormskirk Sandstone Formation) further north (Jones and Ambrose, 1994; Gallois, 2004; Ambrose et al., 2014). This contrast in aeolian facies content can be interpreted to have arisen in response to a gradual downstream reduction in the discharge of the braided fluvial systems with increasing distance from the sediment entering the arid-climate linked basin system (Jones and Ambrose, 1994; McKie and Williams, 2009; McKie and Shannon, 2011).

Although this is consistent with the palaeogeographic scenario for the Triassic of NW Europe, extensional tectonics may also play a role in the preferential preservation of aeolian facies favouring wind deceleration and rapid burial of sediments below the water table (Gawthorpe and Leeder, 2000; Mountney, 2012). By contrast, aeolian deposits are absent throughout the entire succession of the eastern England Shelf Basin (Smith and Francis, 1967; West and Truss, 2006; Wakefield et al., 2015).

Preservation of aeolian facies in the eastern Irish Sea Basin (Calder and Ormskirk Sandstone formations) and their absence in the eastern England Shelf can also be related to the different rates of accommodation generation (Table 2.1). In fact, high rates of tectonic subsidence in the eastern Irish Sea Basin may (as in other tectonically active basins) have assisted long-term preservation of aeolian deposits by placing them beneath the local water table (cf. Howell and Mountney, 1997; Mountney, 2012; Rodríguez-López et al., 2014).

Figure 2.2: Outcropping Sherwood Sandstone Group in England (see Figure 2.1b for location of sedimentary basins). (a) Geological limit between the (1) conglomerate of the Buldeigh Saltertion Pebble Beds and the (2) aeolian Otter Sandstone formations in the Wessex Basin, (b) conglomerate and interbedded sandstone sheets in the Kidderminster Sandstone Formation of the Staffordshire Basin, (c) Fault core with granulation seams in the Kidderminster Sandstone Formation of the Staffordshire Basin, (d) Fault core in the altered Kidderminster Sandstone Formation of the Staffordshire Basin, (e) Normal fault with granulation seams in the Sherwood Sandstone Group of the eastern England Shelf.

Figure 2.3: Outcropping Sherwood Sandstone Group in England (see Figure 2.1b for location of sedimentary basin). (a) Occurrence of granulation seams in conjugate sets in the aeolian dunes of the Helsby Sandstone Formation in the Cheshire Basin, (b) Swarms of granulation seams (Gs) in the in the aeolian deposits of the Helsby Sandstone Formation of the Cheshire Basin (2), (c) Amalgamated fluvial channels in the St Bees Sandstone Formation of the eastern Irish Sea Basin, (d) Normal fault in St Bees Sandstone Formation of the eastern Irish Sea Basin: (1) open fracture vs. (2) granulation seams, (e) Stratabound joints in the Ormskirk Sandstone Formation of the Cheshire Basin (Halliday et al., 2008), (f) (1) Mudstone deposited by unconfined flow and (2) fluvial channels interbedded in the basal St Bees Sandstone Formation of the Vale of Eden (from Millward et al., 2010).

2.2.1 Fluvial conglomerate and pebbly-sandstone facies association This facies association (Figure 2.2 a-c) includes conglomerate and interfingered pebbly-sandstone (Unit A’) forming successions up to hundreds of metres thick (Warrington et al., 1980; Steel and Thompson, 1983) characterizing the relatively southern Triassic grabens of the Wessex, Worcester, Staffordshire, Needwood and Cheshire basins. Also, this coarse grained facies association represents a productive hydrocarbon lithology at the Wytch Farm in Dorset and has been penetrated by wells in the western English Channel during hydrocarbon exploration (Mckie and Williams, 2009). This facies association also dominates in the southern part of the eastern England Shelf but progressively disappears northward at increasing distance from the potential sediment source represented by the Armorican and London-Brabant massifs (McKie and Williams, 2009;

Wackefield et al., 2015). Notably, conglomerates interbedded with pebbly-sandstone are absent in the eastern Irish Sea Basin, Vale of Eden and Carlisle basins (Jones and Ambrose, 1994; Brookfield, 2004; Holliday et al., 2008).

These coarse-grained fluvial deposits outcropping in the Wessex, Worcester, Staffordshire, Cheshire and southern England Shelf basins typically comprise laterally extensive and amalgamated sheets of cross-bedded and parallel laminated conglomerates, together with sand-prone interbedded lenses (Figure 2.2b). The depositional environment of the conglomerate was interpreted as braided, bedload dominated and confined streams (Steel and Thompson, 1983; Smith, 1990). However, sheets of pebbly sandstone lying between the conglomerates largely represent deposition from sandwaves and dunes (Steel and Thompson, 1983).

2.2.2 Sand-prone channel and floodplain facies association

Sand-prone fluvial channels (Unit A”) represent the most typical deposits in the Sherwood Sandstone Group, also forming the principal reservoir lithology for several hydrocarbon fields in the Central and Viking graben of

the North Sea and the Corrib Field in the western Irish Sea (Fisher and Mudge, 1998; Schmid et al 2004). In the subsurface as well as in outcrop, these sand-prone channels are characterized by erosively based, fining upward sandbodies up to 6 m thick, dominated by cross-bedded and parallel laminated fine to medium grained sandstone beds which are typically related to braided river systems (Jones and Ambrose, 1994;

Schmid et al. 2004; Mckie and Williams, 2009; Wakefield et al., 2015;

Olivarious, 2016). Sandstone-prone channel elements (Figures 2.2e, 2.3c) occur at the base of the fluvial succession in the eastern Irish Sea, Carlisle and Vale of Eden basins, where they in places occur interbedded with mudstone and sheet-like sandstone which were deposited by non-confined flood events in fluvial floodplain settings. However, the sandy channels occurring interbedded with mudstone and sheet-like sandstone represent crevasse channels (Figure 2.3f) feeding mudstone and sheet-like sandstone characterizing the floodplain (Jones and Ambrose, 1994;

Brookfield, 2004; Holliday et al., 2008).

Moving upward in the fluvial successions of the basins, the sand prone-channel elements become progressively more amalgamated in the Triassic fill of the eastern Irish Sea Basin and the Carlisle and Vale of Eden basins, although mudstone elements of fluvial overbank origin remain interbedded with and, in places, occur within the channelized architectural elements (Jones and Ambrose, 1994; Nirex, 1997; Holliday et al., 2008). Sand prone-channel elements also occur in the eastern England Shelf (Figure 2.2e), where they are characterised by a progressive reduction in grain-size to the north, with increasing distance from the principal source of sediment which is represented by the Armorican Massif (Tyrrel et al., 2012). Despite this general trend, the eastern England Shelf although relatively high latitude and hence distant from the sediment source (Figure 2.1; Doncaster area) is characterized, according to core and quarry stratigraphic logs, by more abundance in pebbly-lithofacies than the eastern Irish Sea Basin.

Pebbly lithofacies represent 5% and 46% in the eastern Irish Sea Basin in West Cumbria and in the eastern England Shelf in the Doncaster area, respectively (Gaunt et al., 1992; Gaunt, 1994; Nirex, 1997; Pokar et al.,

2006; West and Truss, 2006; Wakefield et al., 2015). Notably, the fluvial deposits of the eastern England Shelf are also characterized by significantly lower amount of mudstone (interlayered with sand-prone channels) compared with those of the eastern Irish Sea Basin, 2% vs. 8%, respectively (Gaunt, 1994; Nirex, 1997; Pokar et al., 2006; West and Truss, 2006; Wackfield et al., 2015). The role of tectonic subsidence as a control on fluvial lithofacies distribution in the eastern England Shelf and the eastern Irish Sea Basin represent a specific research objective of this study.

2.2.3 Aeolian facies association

Aeolian deposits (Figures. 2.2a, 2.3a, b, e) occur in the Sherwood Sandstone Group either interbedded on a metre-scale with fluvial channels or as vertically continuous intervals up to 200 m thick (Jones and Ambrose, 1994; Mountney and Thompson, 2002, McKie and Williams, 2009).

Examples of the latter case are common in the eastern Irish Sea Basin since aeolian facies characterize 80% and 99% of the successions in the Calder (Unit B) and Ormskirk Sandstone (Unit C) formations, respectively (Jones and Ambrose, 1994). Aeolian deposits in the Sherwood Sandstone Group are generally dominated by cross-bedded sandy dunes, although very fine-grained sandstone of damp interdune origin also occurs (e.g., Thompson, 1970a, Mountney and Thompson, 2002). Rarely, siltstone laminae representing wet interdune deposits also characterize aeolian deposition in the Sherwood Sandstone Group (Mountney and Thompson, 2002). Notably, aeolian facies are described in all the Triassic basins in England with the exception of the eastern England Shelf (Ambrose et al., 2014). Analogous Triassic aeolian facies have been recognized in the producing hydrocarbon fields of Morecambe in the Irish Sea and Wytch Farm in Dorset (Meadows and Beach, 1993a, b; Katternhorn and Pollard, 2001). Aeolian deposits have also been intercepted in exploration wells on-shore in the North German Basin as well as off-on-shore in the Danish North Sea (Clemmensen, 1985; Goldsmith et al., 1995; Olivarious et al., 2015,

2016). Additionally, although the Lower Triassic in the UK and Norwegian sector of the North Sea is dominated by fluvial deposits, wind ripple laminations and small dunes have been recoded in cores (McKie et al., 2005, 2010; McKie and Williams, 2009).