Cyclicity and Facies Relationships at the Interaction Between Aeolian, Fluvial, and Playa Depositional Environments in the Upper Rotliegend: Regional Correlation Across Uk (Sole Pit Basin), the Netherlands, and Germany
Published:January 01, 2011
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Matteo Minervini, Massimo Rossi, Donatella Mellere, 2011. "Cyclicity and Facies Relationships at the Interaction Between Aeolian, Fluvial, and Playa Depositional Environments in the Upper Rotliegend: Regional Correlation Across Uk (Sole Pit Basin), the Netherlands, and Germany", The Permian Rotliegend of the Netherlands, Jürgen Grötsch, Reinhard Gaupp
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The Permian Upper Rotliegend Group in the UKCS Quads 48-49 was deposited in a mixed aeolian–fluvial–playa– lacustrine environment which displays different orders of internal cyclicity.
A low-frequency backstepping–forestepping depositional sequence, which encompasses the whole Rotliegend succession in the study area, was probably influenced by a long-term tectonic control. This depositional sequence can be subdivided into five cycle sets, 30–70 m thick, designated Unit U1 to Unit U5. These units are defined by the recognition of marked shifts in the evolution of depositional systems. The cycle sets are in turn subdivided into 16 elementary cycles (15–20 m thick), bounded by regionally widespread surfaces picked at the point of lower aridity. These high-frequency cycles show drying-upward and drying–wetting-upward trends.
The Base Permian Unconformity shows a consistent topographic relief, and it represents a major sequence boundary. The lower part of the sedimentary succession, dominated by the deposits of semipermanent braided streams and catastrophic floods, was deposited in relatively confined sub-basins controlled by extensional WNW–ESE-trending faults. The fluvial system merged basinward into playa and lacustrine facies associations (U1).
Vertically, the succession records a climatic change from more humid conditions (U1) to a first aridity peak (U2) marked by erg expansion and a change in fluvial style, with ephemeral streams between the erg and the inherited structural highs. The aridity peak was combined with a smoother palaeotopography.
The middle to upper (U3–U5) part of the succession was deposited during a phase of tectonic quiescence in which the initial pronounced palaeotopography was almost completely leveled. Following a dramatic climatic shift toward more humid conditions, an erosional surface cut deeply into the underlying erg complex. This wetting phase was responsible for sudden deactivation of erg expansion between U2 and U3, abrupt reorganisation of depositional environments, and overall backstepping of facies belts (U3).
As a consequence of the maximum Silverpit Lake expansion, the depositional setting was characterised by strong lateral uniformity in the study area. Relatively confined fluvial systems fed thin and isolated sandstone lobes interbedded with lacustrine mudstones, which alternated with anhydrite-rich mudstones. These latter deposits, testifying to dry maxima, were correlative toward the margin of the basin with aeolian sandstones, highlighting the contraction of the playa lake (U4).
Following this prolonged stage, the sedimentary environment was characterised by deposition of gravity-flow-dominated delta-front lobes in the study area. This depositional change suggests active progradation under relatively humid conditions which characterised the uppermost interval of the Upper Rotliegend Group (U5).
The proposed hierarchy of the sedimentary succession, located at the interaction between fluvio–aeolian and playa– lacustrine depositional environments, provides a tool for the understanding of their mutual relationships in the UK sector (Rotliegend feather edge) as well as in the Dutch and German regions.
The Permian Rotliegend Group is one of the most prolific gas provinces of northern Europe, extending from the Polish–Russian border into the UK. Even though it is largely considered to be a mature hydrocarbon province, it displays a high degree of reservoir and structural complexity that allows subtle structural and stratigraphic traps and stratigraphic tongues to continue to be discovered. This stratigraphic interval has been the focus of investigation by industry and academia also for its complex relationships between aeolian, fluvial, and playa-lake sedimentary facies and their interaction with synsedimentary tectonics and climatic overprints (Glennie, 1972; Glennie, 1984b; Ziegler, 1990a; Gast, 1991; George and Berry, 1993; Yang and Nio, 1993; George and Berry, 1994, 1997; Plein, 1993; Howell and Mountney, 1997; Glennie, 1998; Sweet, 1999; Gaupp et al., 2000; Van Wees et al., 2000; Rieke et al., 2001; Moscariello, 2005; Börmann et al., 2006). High-resolution stratigraphic frameworks based on recognition of subaerial unconformities and lacustrine flooding horizons overlain by laterally extensive drying-upward and drying–wetting-upward units have been demonstrated to be the most viable way for correlation (Gast, 1991, and Börmann et al., 2006, in Germany; Yang and Nio, 1993, offshore the Netherlands; George and Berry, 1993, Howell and Mountney, 1997, Maynard and Gibson, 2001, Strömbäck and Howell, 2002, offshore UK). These elementary cycles are driven primarily by climatic fluctuations in lake level and water table.
This study focuses on the Rotliegend deposits localised in the western sector of the Southern Permian Basin (UK Continental Shelf Quads 48–49). The sedimentary succession is chronostratigraphically and environmentally analogous to the Rotliegend reservoirs of the Netherlands and Germany.
The objectives of this paper are threefold: (1) to provide a description of the interactions between aeolian, fluvial, and playa environments in the study area; (2) to develop a new stratigraphic framework of the Upper Rotliegend 2 Unit (UR2); and (3) to propose a regional correlation with coeval reservoir deposits in the northern Netherlands and Germany.
The structural configuration at the beginning of Rotliegend sedimentation was a basin stretching in an east–west direction, some 1500 km long and in certain areas 500 km wide (Fig. 1). The basin was bounded on the south by the remnant Variscan Highlands, to the west by the Midland Pediment, Pennines, and London–Brabant Massif, and on the north by the Mid North Sea High. During late Carboniferous, these structures were subjected to a complex transtensional tectonic regime that resulted in the generation of N–S, NW–SE, and NE–SW oriented grabens and half grabens organised in an en echelon pattern (Glennie, 1990; Ziegler, 1990b; Coward, 1995; Glennie and Underhill, 1998).
The study area is located in one of these en echelon troughs, the Sole Pit Basin. The bounding inherited Late Carboniferous structures (Fig. 1A) were partially reactivated during the Permian (Glennie, 1984a) and controlled the sedimentary evolution of the basin. Particularly, the Dowsing–South Hewett faults and the Swarte Bank Hinge were active throughout the sedimentation of the Rotliegend.
Throughout northwestern Europe, the Rotliegend Group rests unconformably on Carboniferous deposits (Ziegler, 1990a). The basal unconformity has a considerable relief, further accentuated by Late Permian subsidence and testified by the broad difference in thickness of the overlying Rotliegend deposits, which varies between 0 to over 300 m in the UK offshore.
The Rotliegend Group can be subdivided into Lower and Upper (Glennie, 1998). The Lower Rotliegend Group contains a mixture of aeolian sandstones, fluvial conglomerates and sandstones, and volcaniclastic deposits. It was confined in separate structural depressions in the Netherlands, northern Germany, and the Polish Trough (Ziegler, 1990b; Schneider and Gebhardt, 1993; Rieke et al., 2001; Börmann et al., 2006).
The Upper Rotliegend Group, mainly Tatarian in age (Menning et al., 1988; Menning, 1995), can be traced across the UK, the Netherlands, Germany, and parts of Poland (Glennie, 1984b; Gralla, 1988; Ziegler, 1990a; Gast, 1991; Burri et al., 1993; George and Berry; 1993; Schröder et al., 1995; Glennie, 1998; Gaupp et al., 2000). In the UK, the Upper Rotliegend Group is lithostratigraphically divided into three diachronous formations (Fig. 2): Lower Leman, Silverpit, and Upper Leman formations. These lithologic units are broadly similar to the Lower Slochteren, Upper Slochteren, and Ten Boer formations of the Dutch sector. The Lower and Upper Leman formations encompass sandstones of aeolian and fluvial to playa origin. The Silverpit formation was mostly deposited in a lacustrine and playa environment.
A modern sequence stratigraphic approach based on the recognition of key basinwide surfaces (lacustrine flooding and subaerial unconformities; see Talbot, 1985; Kocurek and Havholm, 1993; Shanley and McCabe, 1994; Howell and Mountney, 1997; Sweet, 1999; Bailey, 2001) overrun the classical lithostratigraphic subdivisions. Sequence stratigraphy provided a qualitative tool for correlating the different sectors of the Southern Permian Basin, from UK across the Dutch region to western Germany (Fig. 2). Five regionally widespread cycle sets, from U1 to U5, were regionally mapped, which constituted the bulk of the present study.
The Rotliegend Group is overlain by thin black shales (Kupferschiefer; Copper Shale) that mark the onset of marine conditions of the Zechstein Group (Fig. 2). (Glennie, 1990; Bailey et al., 1993; Burri et al., 1993; George and Berry, 1994; Sweet et al., 1996; Glennie, 1997; Geluk, 2007).
Database and Methods
This study is based on cores from seven wells (up to 900 m), wireline log (sonic, density-neutron, and gamma ray) correlation from 20 wells, and seismic interpretation. Most of the dataset is concentrated in the six offshore blocks across the Sole Pit Basin (Fig. 1A, B). Both released wells of Dutch and northwestern Germany sectors used as input for regional correlation (Gast, 1991; Geluk, 2007) and palaeo-geographic maps (George and Berry, 1993; Howell and Mountney, 1997; Maynard and Gibson, 2001) applied to the reconstruction of facies association maps (Fig. 16) were integrated in this study.
The calibration between core and logs (Fig. 15) provided a powerful tool for the identification of key surfaces in the sedimentary succession. In particular, the classification of stratigraphic surfaces by Sweet (1999) was used to build a sedimentological correlation between wells. Finally, the lithofacies associations described in the cored wells have been extended to the uncored wells (Figs. 15, 18).
Following Shanley and McCabe (1994), Howell and Mountney (1997), Sweet (1999), Bailey (2001), and Maynard and Gibson (2001), well-log correlation was based on application of high-resolution sequence stratigraphy to continental deposits. Elementary depositional sequences were mainly bounded by surfaces picked in correspondence of high-GR peaks (see Figs. 15, 18). These basinwide surfaces, linked to lacustrine flooding events, were recognised and correlated across the study area, and they lead to the identification of 16 allostratigraphic units, showing variable drying-upward and drying–wetting-upward trends. The 3D expression of each depositional unit in different parts of the basin is considered to have been the response to different conditions of subsidence and sediment supply in a context of climatically controlled cyclicity (see Scherer et al., 2007, and Bourquin et al., 2009, for further details about this stratigraphic approach).
Most of the study area is covered by three-dimensional seismic. The low seismic resolution prevented the observation of facies details, but the seismic was used to map the fault distribution and geometry, as well as to define the gross thickness of the Rotliegend succession.
The facies observed in cores are typical of the aeolian, fluvial, playa, and lacustrine depositional environments, described by numerous other studies on the Rotliegend (e.g., Glennie, 1972; Heward, 1991, George and Berry, 1993; Yang and Nio, 1993; Sweet et al., 1996; Howell and Mountney, 1997; Sweet, 1999). The classification approach used here (Table 1) partly follows that of George and Berry (1993) with local modifications.
Erg-Centre Facies Association
The most representative facies in this association is composed of well- sorted, poorly to moderately cemented, fine-to medium-grained sandstone, containing a high proportion of well-rounded quartz grains (A1). These sandstones are generally red-orange in colour, although yellowish grey, olive grey, and brownish grey colours are also common. The sandstones are organised in planar cross-stratified co-sets, up to 8 m thick. Individual sets are 0.4–2.5 m thick with angular or tangential bases (Fig. 3A). Planar foresets dip at up to 35°, with average dip of 15–20°. Foreset laminae are from 3–4 cm thick, reversely to occasionally normally graded, to less than 1 cm thick, darker, more cemented, and slightly argillaceous. Contorted and even overturned laminae or massive beds are occasionally present.
Horizontally or nearly horizontally laminated sandstone are a subordinate facies which constitutes 5–10% of the association. The facies is represented either by well-sorted, fine- to medium-grained, slightly argillaceous sandstone (A2) or by bimodally sorted, generally well-cemented, fine- to very coarse-grained sandstone (A3) (Fig. 3A, B).
Two facies, identified on the basis of shale content, are closely connected with aeolian dune deposits. Facies A4 is composed of horizontally to subhorizontally thinly laminated, moderately sorted, fine-grained argillaceous sandstones containing abundant well-rounded coarse sand and fine granule grains (Fig. 3C). Textural variations emphasise a variety of minor sedimentary features, including discontinuous to lenticular laminae, wind ripples, graded bedding, and small scour-and-fill structures.
The muddier facies (A5) is composed of very finegrained sandy mudstones. Discontinuous, irregular argillaceous adhesion ripples are common and are accompanied by irregular to aligned concentrations of coarse sand (aeolian lag; Fig. 3D). When this facies is dominated by bright-red mudstones, the horizontal lamination is characterised by desiccation cracks and minor mud curls (Fig. 3C).
This facies association, represented by the vertical sedi-mentological profile of Figure 4, is representative of an aeolian dune system dominated by grain-fall and grain-flow processes active on the slip faces of aeolian dunes. Reversely graded laminae were deposited as a result of grain-flow processes occurring on steep avalanche faces, whereas the finer-grained well-laminated foresets were deposited from grain-fall processes by vortices developed on the lee sides of the dunes (cf. Hunter, 1977). Set boundaries were created as dunes migrate on top of each other (Kocurek, 1988). Contorted and overturned laminae were triggered by saturation during heavy rainfall or by flooding of low-relief dunes. Facies A2 characterises the base of migrating dunes, whereas facies A3 is interpreted as aeolian sand sheets, formed by traction and saltation that generate wind-ripple trains (Kocurek and Nielson, 1986; Trewin, 1993). Aeolian sand sheets are usually located at the margins of dune fields. They are commonly extensive and are associated with formation of supersurfaces. Supersurfaces form mostly during wetter climatic conditions when the erg becomes starved of sediment for a long time (Kocurek, 1988; Kocurek and Havholm, 1993). The area is covered then by an extensive sand sheet with only a few, poorly developed dunes.
Facies A4 and A5 are representative of dry and wet interdune areas, respectively. Dry interdunes are characterised by a slow sedimentation rate because of vertical rather than lateral accretion, with frequent periods of deflation. Saltation forms wind-rippled sandstones when sand is blown over dry surfaces. The sharp base is generated by erosion associated with bedform migration. Thick units of facies A4 develop on the upwind (proximal) or downwind (distal) margins of dune fields during extended periods of strong winds and/or when rates of sand supply are limited (Fryberger et al., 1983; Kocurek, 1988). In the damp interdune areas, fine to very fine sands accrete to produce a variety of adhesion ripples and more irregular adhesion warts (cf. Olsen et al., 1989). Facies A5 occurs in wetter interdune areas, where the water table is periodically at the sediment surface (damp interdune) or constantly at or above this surface (wet interdune), enhancing soft-sediment deformations. These two facies are usually thin and areally restricted in the erg-centre facies association.
Erg-Margin Facies Association
Description. This facies association is the most widespread in the study area. It is composed of stacking of beds of fluvial and aeolian deposits 1–3 m thick.
Fluvial deposits are usually organised in fining-upward cycles composed of massive, poorly to moderately sorted, very fine- to medium-grained clay-rich sandstones (F4: cf. facies Sm of Olsen, 1987), with scoured basal erosional surfaces usually delineated by enrichment of thin and elongated mud clasts (Fig. 5A, C). Locally, the scoured sandstone units are coarser, granule-grade, massive, and characterised by dewatering dish and pillar structures.
Nevertheless, the association is dominated by horizontally laminated, closely spaced millimetres-thick laminae, marked by differences in grain size and grain packing, very fine- to medium-grained sandstone (facies F3). Individual beds are usually less than 1 m thick (Fig. 5A, B, C). Planar lamination may eventually grade upward into small-scale planar cross-lamination and mudstone drapes (facies F5).
The fluvial facies are usually interfingered with planar-cross-laminated, moderately to well-sorted, fine- to medium-grained sandstone (A1), organised in individual units less than 2–3 m thick. The planar-laminated sandstone belonging both to facies A2 and to facies A3 (Fig. 5B) is more common than in the erg-centre facies association, as well as for facies A4 and A5 (cf. Fig. 6).
This facies association is often characterised by collapse features with related steep surfaces. The morphological scour is filled by very fine-grained sandstones of the overlying depositional event, which is draped by thin muddy layers. These layers are characterised by the presence of sand-filled mud cracks (Fig. 5D). In some places thin packages of low-angle to planar-cross-laminated fine-grained sandstone (F2) have been found (Fig. 5B–D).
The distinction between fluvial and aeolian deposits in the erg-margin depositional environment is not always obvious, mainly in relation to their reciprocal erosional effects. Nevertheless, these sedimentary processes are clearly selective on reworked sediments, and they are characterised by strongly different transportation mechanisms. As a consequence, a relevant number of sedimentological features (grain-size range, degree of sorting, sedimentary structures, colours) allows their macroscopic recognition (cf. Glennie, 1983a; George and Berry, 1993; Sweet et al., 1996; Sweet, 1999; Strömbäck and Howell, 2002; Fischer et al., 2007). Moreover, the lateral and vertical juxtaposition of fluvial and aeolian deposits provides a useful tool for their characterisation (cf. Langford and Chan, 1989; Herries, 1993; Veiga et al., 2002; Minervini, 2004; Taggart et al., 2010).
The fluvial deposits are dominated by facies F3 and F4 (Fig. 6) which are variably stacked with facies F1 (at the bases of individual fluvial cycle) and facies F5 (usually capping the facies sequence). These sediments are interpreted as the product of rapid deposition from gently confined to unconfined high-density, turbulent, high-discharge flows in ephemeral streams (Picard and High, 1973; Rust, 1978; Ward, 1988; Clemmensen et al., 1989; Herries, 1993; Miall, 1996; North and Taylor, 1996). In fact, the massive and sometimes dewatered sandstones (F4) were generated during rapid deposition, which hindered the development of bedforms. Mudstone caps are interpreted to record suspension settling in abandonment ponds formed when the flooded area was cut off from the main flow. The upward transition from massive sandstones to laminated finer-grained sediments (F2) probably reflects a change to normal traction sedimentation in the dilute residual flows. The well planar- to ripple-laminated sandstones were deposited as a result of sheet-flood traction transport during waning unconfined flow.
Flash-Flood Facies Association
This facies association is made up of sandstone bodies 1–4 m thick. These sandy units are bounded by a flat to concave-up lower surface and dominated by concentrations of intraformational clasts (F1; Fig. 7). Mud clasts are up to 15 cm (Fig. 7A: a single mud clast), subrounded to angular (Fig. 7B), sometimes displaying a natural imbrication (Fig. 7D). These clasts pave or are located several centimetres above the lower boundary. This clast-rich fa-cies usually consists of poorly sorted, massive to faintly laminated, fine- to coarse-grained sandstone.
These very coarse-grained clast-rich sandstone beds occur in fining-upward sequences. Facies F1 is usually overlain, through abrupt facies shifts, by granule-rich, massive, fine- to medium-grained sandstone (F4) (Fig. 7A, C). This facies can be also characterised by fluid-escape structures (Fig. 7C, D). Alternatively, facies F1 is associated with very fine- to fine-grained planar-cross-bedded sandstone sets 20–50 cm thick (facies F2) and/or horizontally laminated medium-grained sandstone (beds less than 30 cm thick) (facies F3) (Fig. 8C).
This facies association is dominated by intraformational clast-rich massive sandstone (facies F1), overlain by massive to dewatered (facies F4) and/or horizontally laminated (facies F3) medium-grained sandstone (Fig. 8). It is interpreted to represent poorly confined sheet-flood deposits (Miall, 1985; Olsen, 1987; George and Berry, 1993) (Fig. 8).
The common occurrence of both massive clast-rich sandstone (hyperconcentrated flows; Pierson and Costa, 1987; Benvenuti and Martini, 2002) and horizontal lamination (upper flow regime; Picard and High, 1973; Frostick and Reid, 1977; Reid and Frostick, 1997) indicates fast and intermittent high-capacity streams. Intraformational mud-stone clasts, at the bases of the facies sequences, represent reworking of floodplain deposits.
The development of fine-grained sand bodies composed of planar and trough cross-sets (facies F2) above thicker beds of intraformational conglomerates (facies F1) is interpreted as the result of low-sinuosity stream flows (Miall, 1978, 1985).
Semipermanent Fluvial Facies Association
This facies association is largely dominated by moderately sorted, medium-scale planar- and trough-cross-bedded, very fine- to medium-grained sandstones (facies F2; Fig. 9 A, C). This facies is commonly present above a thin basal intraformational clast lag, in sets 10–20 cm thick in which the reactivation surfaces are usually marked by flat and rounded very fine pebble-grade shale rip-up clasts (Fig. 9A, B).
This facies is often interbedded with horizontally (to low-angle cross-laminated) fine-grained sandstones (facies F3; Fig. 9B, C). The upper part of the facies sequence usually consists of poorly sorted, very fine-grained sandstones with small-scale cross-bedded and ripple sets (facies F5; Fig. 9D), capped by thin mudstone horizons up to 10 cm thick. Soft-sediment deformation is common. Ripple profiles are locally preserved by mud drapes.
This facies association is dominated by medium-scale cross-laminated, generally fine-grained sandstones (facies F2). Small-scale cross-laminated sandstones (facies F5) are a minor constituent of this facies association, although they are common. The sediments are organised in fining-upward sequences 30 cm to 250 cm thick (Fig. 10). This facies sequence probably reflects an initial channel scouring followed by migration of fluvial bars or megaripples (Miall, 1978; Best and Bristow, 1993). Small-scale cross-lamination, usually associated with decreasing grain size, indicates falling-stage conditions followed by cessation of flow and fallout of suspended mud (Miall, 1978; Allen, 1982; Baas, 1993).
These depositional elements represent low-sinuosity channel-fill deposits (cf. Miall, 1996), probably characterised by a semipermanent regime (Minervini, 2004). This interpretation is supported by the relative difference with the previously described fluvial facies associations and by the vertical and lateral juxtaposition of these depositional elements with the wet-playa-dominated facies sequence. These sedimentological and architectural features lean towards a more humid climatic regime characterised by a higher water table and relatively constant rainfall.
Playa Facies Association
This association consists of two facies that grade from argillaceous sandstones (S1) to sandy siltstones (S2). The coarse-grained playa facies is represented by poorly to moderately sorted, clay-rich sandstones irregularly inter-bedded with mudstone and siltstone laminae and associated with incipient gypsum nodules and salt-ridge structures (S1). The sandstone is coarse- to medium-grained, with the finest grain sizes occurring in association with mudstone layers, and displays irregular, crinkly lamination. Mudstone layers typically have irregular bases and undulating tops, deformed with flame structures. Ball-and-pillow structures are common at the bases of the overlying sandstones (Fig. 11A, D). A high proportion of rounded and frosted quartz clasts is very common (Fig. 11A).
The fine-grained playa facies is composed of reddish sandy siltstones, massive to undulating laminated (S2). Lamination is invariably disrupted because of the combined effects of desiccation and displacement by growth of evapor-ite minerals. Bioturbation in the form of insect burrows is a possible process in disrupting the primary sedimentation (cf. George and Berry, 1993; Sweet, 1997). Nodular anhydrite is rare. Mud cracks, sandstone dykes (a few centimetres long), and bedding deformation (flame structures and ball-and-pillow structures) are common (Fig. 11B, D).
Facies S2 is in some cases interbedded with planar-cross-laminated and rippled-laminated very fine- to finegrained sandstones (F2–F5). These intervals, 10–20 cm thick, are bounded by sharp, locally erosional surfaces and characterised by the stacking of thin (less than 5 cm thick) cross-laminated sets (Fig. 11C).
The deposits are organised into fining- and thinning-upward units, up to 6–8 m thick, marked by upsection increase of mudstone content and deformation structures (Fig. 12).
Both facies were deposited on a low-relief continental playa bordering a desert lake (Briere, 2000). The sedimentation of coarse-grained facies was subjected to variable degrees of aeolian input and subaerial desiccation and deflation. In fact, the playa areas were frequently inundated by fluvial sheetfloods, and during periods of strong winds both the sheetflood units and the playa sediments were often reworked into aeolian sheet sands. These deposits, typically coarse grained and characterised by irregular bedding, are successively deformed by water-table-controlled processes (facies S1). In fact, irregular bedding was enhanced by occurrence of gypsum nodules generating salt-ridge structures. These structures are related to continued evaporation, and ground-water recharge resulted in precipitation and dissolution of salts and formation of crinkly lamination and salt ridges (Glennie, 1970; Ahlbrandt and Fryberger, 1982; Glennie, 1984b; George and Berry, 1993).
Desiccation cracks, distorted bedding, and flame and load structures in the fining-upward, fine-grained playa deposits are indicative of wet-playa conditions (Fryberger et al., 1983; Herries and Cowan, 1997; Goodall et al., 2000). The undulating argillaceous laminae probably originally formed as adhesion ripples. Deformation is highly indicative of the cohesiveness of the wet sediments. It was further promoted by density contrasts and possibly by loading effects accompanying the migration of sand dunes across the loosely packed, water-saturated surface, or by rapid fluvial deposition. Also the abundance of water-laid mud-stone layers is further proof of a predominantly wet, frequently flooded environment, subject to occasional drying and evaporite precipitation.
Lacustrine Facies Association
The lacustrine association consists of three superimposed facies, L1, L2, and L3. L1 and L2 are predominantly argillaceous; L3 is evaporitic.
Facies L1 consists of thick, mottled grey, green, and/or red mudstones and silty mudstones with rare desiccation structures. This facies shows various degrees of bioturbation (cf. George and Berry, 1993; Sweet, 1999). Sometimes, sand-rich intervals are deeply deformed by intense bioturbation generating vertical to horizontal meniscate burrows (Fig. 13A) (cf. Melchor et al., 2006). Intervals characterised by lower content of the sand fraction are intensely reddish-greenish colour-mottled (Fig. 13B). This muddy facies rarely contains irregular rows of well-rounded sand grains. Soft-sediment deformation, including load-casted ripples and small ball-and-pillow structures, locally occurs (Fig. 13A, B).
This facies is often interbedded with generally massive and/or graded, moderately sorted, fine-grained, (occasionally medium-grained) sandstone (Minervini, 2004). The lower boundary is generally sharp and sometimes erosional (Fig. 14A) (cf. shelfal lobes of Mutti et al., 1996).
The L1 facies is usually found in relation to red-brown to green-grey dolomite mudstones of facies L2 (Fig. 13C) (Turner et al., 1993; Valero-Garcés et al., 2000). Desiccation cracks and small injection structures (dykes) are ubiquitous, and thin clay-curl breccias are locally present. Silt and very fine-grained sandstones occur as irregular streaks and isolated ripples. This facies is generally restricted to packages less than 2 m thick. Anhydrite nodules become more common as L2 merges into L3 (Fig. 13D). The nodules may laterally coalesce to form continuous layers (Fig. 13E). Adhesion ripples are common and are associated with nodular anhydrite. Their amplitude is usually up to 5 cm or more, and they look more deformed than those of the almost anhydrite-free interdune playas.
These facies are interpreted to represent respectively permanent (L1), playa (L2), and evaporative desert-lake (L3) deposits (facies scheme slightly changed from George and Berry, 1993). The development of thick sequences of facies L1 characterised by a general absence of desiccation features and well-developed bioturbation (testifying oxy-genation in the lower part of water column), suggests deposition in a permanent lake (Fig. 14A; cf. Maynard and Gibson, 2001). Nevertheless, water depths are considered to have been quite shallow, with occasional emergence in relation to a semiarid climate, as highlighted by the rare presence of alignments of quartz clasts.
Facies L2 was deposited in a mud flat periodically subject to drying (cf. Last, 1990; Reinhardt and Ricken, 2000). Red siltstones represent periods of prolonged regional aridity with the complete desiccation of the lake. Intermittent ephemeral increased stream discharge led to the inundation of the mudflat, and green-grey dolomite-bearing siltstone accumulated (Minervini, 2004).
Finally, during periods characterised by a falling water table, the lacustrine area was subjected to precipitation of nodular anhydrite (facies L3) (Fig. 14B; Glennie and Provan, 1990; George and Berry, 1993). During prolonged periods of aridity, thick units of pure halite precipitated in supersaline pools, likely located in the centre of the lake, outside of the study area (Gast, 1991; Glennie, 1997; Geluk, 2007).
The detailed sedimentological facies description of aeolian, fluvial, playa, and lacustrine deposits (up to 900 m of cored section), interpretation of their depositional environments, reconstruction of their lateral and vertical relationships through a core-to-log calibration, lead to the recognition of the cyclic nature of the sedimentary record.
This cyclicity has been envisaged by many authors, and it is related mainly to two controlling factors: (1) creation and destruction of accommodation space related to tectonism and (2) sediment supply, introduced into the area mostly by aeolian and fluvial processes through different sedimentation rate in relation to climatic trends (cf. George and Berry, 1997; Howell and Mountney, 1997; Bailey, 2001; Bailey and Lloyd, 2001; Maynard and Gibson, 2001).
In order to investigate the various scales of cyclicity, the sedimentary record has tentatively been analysed from a chronostratigraphic point of view (Minervini, 2004), following a cyclostratigraphic approach (Algeo and Wilkinson, 1988; Berger and Loutre, 1994; De Boer and Smith, 1994). In particular, spectral analysis of lacustrine sediments (GR log calibrated with cores) shows a signal which is in agreement with a combination of precessional and obliquity cycles. The frequency peaks have been reviewed through a statistical approach, highlighting a consistency between 95% and 99% (Preto, 2004, personal communication). Moreover, this analysis shows that the lacustrine facies are clearly characterised by a cyclical behaviour which is progressively missed moving into the aeolian and fluvial domains (Minervini, 2004). This change can be attributed to reduced preservation of the succession, mutual erosional processes, and autocyclic sedimentary mechanisms (cf. Bourquin et al., 2009). In accordance with these results, it is possible to preliminarily demonstrate a climatic influence on sedimentation driven by Milankovitch forcing (cf. Yang and Baumfalk, 1994; Reinhardt and Ricken, 2000; Bailey, 2001; Maynard and Gibson, 2001).
The main correlation surfaces between wells in the studied area appear to be geologically synchronous, and they can have a significant potential for a chronostrati-graphic analysis of the sedimentary record (as proposed by George and Berry, 1993, Howell and Mountney, 1997, and Maynard and Gibson, 2001). In fact, aeolian and fluvial sequence boundaries sensu Sweet (1999) are picked by evidence of a marked basinward shift of facies belts and of reorganisation of depositional environments (e.g., migration of aeolian dune fields; reorientation of fluvial palaeoflows). In the case of increasing aridity these surfaces are associated with rapid fall of lake level and groundwater table, followed by a period of lowstand of the lake (Howell and Mountney, 1997). On the contrary, a sudden shift toward a wet climate can generate fluvial sequence boundaries highlighted by deep erosion of the underlying deposits and penecontemporaneous lacustrine expansion. Maximum-flooding surfaces represent the maximum rate of increasing humidity causing rapid rise of the lake level and the groundwater table (Sweet, 1999), followed by a period of relative highstand of the lake.
A significant feature of the studied succession is related to the preserved thickness within each cycle, which is rather constant moving across different depositional environments from marginal aeolian systems to basinal lacustrine deposits (see Howell and Mountney, 1997, for a detailed discussion). This consideration provides a powerful tool to extend chronostratigraphic evaluation in the Rotliegend feather edge (George and Berry, 1997; Howell and Mountney, 1997; Maynard and Gibson, 2001) and eventually across the Silverpit Basin.
The time interval corresponding to the Upper Rotlie-gend Group is approximately between 262 Ma and 258 Ma (Fig. 2) (Menning et al., 1988; Menning, 1995; Glennie, 1997). In particular, considering that the Ameland Ingression is at 260 Ma (cf. Legler and Schneider, 2008), the fully lacustrine succession of the Silverpit Formation in the study area probably had a duration of less than 2 Myr.
Following the described conceptual scheme, the characterisation of basinward vs. marginward shifts of strongly different depositional environments, and the characterisation of regional vs. local significance of physical surfaces, three orders of cyclicity have been recognised: (1) high-frequency cycles, 15–20 m thick; (2) medium-frequency cycles, 30–70 m thick, and (3) low-frequency cycles identified by major backstepping–forestepping units of regional and interregional importance.
Sixteen elementary cycles, 15 to 20 m thick on average, variably bounded by flooding surfaces or sequence boundaries (sensu Sweet, 1999) were recognised (cf. Fig. 15 and Fig. 16). These elementary cycles can be mapped in the Sole Pit Basin and along the entire Rotliegend feather edge, and they represent high-frequency depositional units. These elementary depositional units (physically equivalent to the subunits of George and Berry, 1993 and to the cycles of Howell and Mountney, 1997, and Maynard and Gibson, 2001) are largely controlled by climate, which in turn influences the water-table level in the marginal area and the lake-level location in the distal setting.
In particular, in case of a decrease in aridity or an increase in rainfall, an expansion of the lacustrine environment is recorded. This expansion is highlighted by a migration of playas and playa facies onto the marginal areas and a relative deactivation of dune expansion. The proximal sectors are dominated by the development of interdune facies and/or by fluvial systems which often truncate previously deposited aeolian deposits.
On the contrary, if the aridity increases, the proximal areas are dominated by the development of aeolian dunes which are expanding upon playas. At the same time, the fluvial systems are intermittently active and the distal sectors experience precipitation of evaporite sediments. During this stage, the basinal areas were characterised by the lowest sediment supply.
These stratigraphic relationships between the margin and the basin represent an ideal framework of a single cycle which could be modified by the interplay between inherited topography, the magnitude of climatic changes, and the behaviour of depositional processes.
The lowermost cycle is bounded at the base by a major erosional surface (Base Permian Unconformity; BPU), cutting deeply into the underlying Carboniferous deposits. The topographic relief generated by this unconformity has to be considered in the order of tens of metres, further enhanced by the presence of intrabasinal highs. These highs (e.g., the Inde High), bordered by high-angle normal faults, dissect the whole basin in narrow NW–SE-elongated minibasins which focus the distribution of fluvial systems (Fig 16.1). The cycle is terminated with a very thin and discontinuous fine-grained unit corresponding to a waning of fluvial processes. Sedimentation was controlled primarily by inherited topography and possibly by synsedimen-tary tectonics (Fig. 15).
This cycle is characterised by a partial peneplanation of the Inde High and by a related expansion of the playa depositional environment toward the south (Figs. 15, 16.2). Nevertheless, inherited structural elements exerted a tectonic control on location and type of depositional systems. The northwest-oriented fluvial drainage was still present and characterised by the stacking of fining-upward facies sequences, mostly confined in the Sole Pit area. The lower bounding erosional surface is unconformable in the Inde area and passes into a paraconformity farther to the northwest. To the south, erg-margin expansion was inhibited by oscillation of the water table, somehow connected to a lacustrine system present in the northeast (Fig. 16.2).
Cycle 3 records the first aeolian sedimentary input in the study area, which is regionally in agreement with the westward migration of a large erg present in the Dutch Sector (Yang and Kouwe, 1995). The lower surface corresponds to an aeolian sequence boundary (cf. Sweet, 1999), highlighted by a marked basinward shift of the marginal facies belt (Fig. 15). In fact, aeolian dunes eroded previously deposited fluvial sediments in the Sole Pit basin, while erg-margin and dry-playa facies directly overlie wet-playa sediments in the basinal setting (Fig. 16.3). Two phases of erg expansion have been described during this cycle in the marginal area, while the sedimentary succession shows a clear fining-upward trend in the distal sectors (Fig. 15). The fluvial deposits disappeared, as a consequence of drainage confinement and strongly reduced rainfall. Fa-cies distribution was still controlled by inherited structural elements that generated NW–SE-oriented facies belts. During this phase, the Indefatigable High underwent further marked erosion and strong retreat of the mountain ranges (Fig. 16.3).
The development of a peneplain over the former Indefatigable High characterises this cycle (Fig. 16.4). Despite low relief, facies distribution was still controlled by the inherited topography. A continuous facies-belt transition from erg-centre, erg-margin, dry-to-wet playas, and lacustrine areas can be recognised from southwest to northeast (Fig. 15). Cycle 4 displays a marked fining-upward trend, best recorded in the outer sectors by early lacustrine expansion. This flooding event is well known in the UK sector (cf. Sweet et al., 1996; Bailey, 2001) and is tentatively compared with the P Ingression (see Fig. 2), described in both the Dutch and the German sectors (Gast, 1991; Schneider and Gebhardt, 1993; Legler et al., 2005; Legler and Schneider, 2008; Stollhofen et al., 2008). This flooding event is somehow affected the erg system, causing a retreat toward the margin of the aeolian dunes and a related expansion of wet playa onto the erg-margin areas (Figs. 15, 16.4).
Cycles 5, 6, 7.—
This group of cycles is uniformly characterised by an overall eastwards expansion of the erg system onto the area previously occupied by the Inde High (Figs. 15, 16). Each of these cycles is bounded at the base by an aeolian sequence boundary (sensu Sweet, 1999), which is variably characterised by a change in grain size and/or erg sedimentary styles and a basinward shift of facies belts. Aeolian deposits were located within topographic lows and on the fringing slopes of surrounding highlands (Fig. 15). In the erg-margin area, the interaction between fluvial and aeolian systems was characterised by mutual erosive processes (Fig. 17), enhanced by the high intersection angle of flow directions. Aeolian damming of ephemeral streams enhanced catastrophic floods once the dune threshold was overtopped, favouring dune instability and their subsequent collapse (cf. Langford and Chan, 1989; Svendsen et al., 2003). Fluvial deposits, interfingering with and/or eroding aeolian sand dunes, were fed by ephemeral channels proceeding from the Inde High remnants. During prolonged dry phases, the aeolian dunes reworked previously deposited fluvial sediments. Lacustrine facies, particularly evaporites, developed to the north and east of the study area (Quadrant 44; cf. Maynard and Gibson, 2001).
Cycle 8 is easily recognisable in the study area through a sharp transition between fluvial and aeolian deposits highlighted by a fluvial sequence boundary (sensu Sweet, 1999). This surface was probably generated by a strong climatic change and associated with a regional flooding event. This depositional turnover produced a marked reorganisation in sedimentary regime both across the study area and generally on the whole Southern Permian Basin. North- and northeast-flowing ephemeral channels encroached and bypassed the aeolian dune fields, reaching the lacustrine sector (Fig. 16.8). Moreover, the playa lake expanded, aeolian processes became deactivated, and the fluvial systems were re-established. Due to an extremely high rate of accommodation-space creation during this phase, these thick and widespread fluvial systems experienced a rapid backstepping culminating in a maximum-flooding surface (sensu Sweet, 1999) (Fig. 15). This regional-scale flooding surface could be related to the Ameland Ingression (Fig. 2), which has a fundamental stratigraphic significance, corresponding with the boundary between the Dethlingen Fm. and the overlying Hannover Fm. in the German sector (Gast, 1991; Burri et al., 1993; Schröder et al., 1995; Geluk, 2007).
This cycle culminated with a shift toward a dry climate, testified by a generalised expansion of the erg over the flood plain. In the study area and offshore Germany (Figs. 15, 18, 21), relatively thin and isolated aeolian dunes were migrating over the previously deposited fluvial and wet playa facies.
This cycle records the same stacking pattern and depo-sitional behaviour of cycle 8 (Fig. 16.9). In particular, the lower half of the cycle is characterised by the development of fluvial systems and laterally equivalent wet-playa facies resting on previously deposited aeolian sediments. The upper half of the cycle shows the expansion of the erg-margin system, with isolated aeolian dunes reworking the underlying fluvial deposits (Figs. 15, 18).
This cycle is unequivocally characterised by a regional maximum-flooding surface (sensu Sweet, 1999), leading to a widespread lacustrine expansion (Figs. 15, 18). This event has tentatively been correlated throughout the Southern Permian Basin and identified with the U Ingression (cf. Legler, 2008; Stollhofen et al., 2008). At the base this cycle consists of backstepping wet-playa and bioturbated lacustrine deposits, passing toward the top into basinward-migrating interbedded fluvial and wet-playa deposits and rare aeolian dunes (Fig. 15). During this time, the erg system was confined farther to the south and southwest (cf. George and Berry, 1993; Howell and Mountney, 1997).
Cycles 11, 12, 13, 14.—
These cycles are uniformly characterised by a lower flooding surface and a following overall drying-upward trend. The lower, relatively wet interval records the expansion and thickening of fluvial deposits toward the margin, while wet playa and lacustrine facies developed toward the basin. Some thin and areally confined sandstone lobes (cf. Mutti et al., 1996), fed by ephemeral fluvial flash floods, were deposited in the playa sectors interbedded with the finer-grained facies (Minervini, 2004) (Figs. 15, 16, 18).
The upper parts of these cycles record a drying-upward trend which was characterised by the expansion of small isolated aeolian dunes reworking previously deposited fluvial bodies. It is known that time-equivalent aeolian draas are well developed over adjacent areas to the south (Glennie, 1990), due to an overall confinement of the erg in relation with previous fluctuating lake expansions. During these cycles, the lacustrine domain was characterised by alternating phases of lake contraction with the deposition of dolomite-bearing green-greyish shales and anhydrite-bearing mudstones (as nodules and/or coalescent streaks) and lake expansion with the deposition of bioturbated shales.
Cycles 15, 16.—
The last two cycles are linked to a major change in the climatic and sedimentary regimes, associated with a wet climatic peak. The aeolian systems were confined to the south, close to the London–Brabant Massif (George and Berry, 1997; Strömbäck and Mountney, 2002). In contrast, fluvial systems were extremely active and characterised by catastrophic processes. Braided streams prograded through river-dominated deltas, forming compensational stacking of relatively thick and laterally widespread river-mouth lobes (cf. Mutti et al., 1996) (Figs. 15, 16.15, 18), into the lacustrine basin, which was probably in connection with the open-marine domain (Legler and Schneider, 2008). In the uppermost part, the coarse clastic system progressively retreated, approaching the maximum fully marine flooding surface, characterised by the deposition of the blackish organic-rich Copper Shale.
The medium-frequency cyclicity is associated with marked palaeomorphological rearrangements which generated strong marginward and/or basinward shifts of different depositional environments. These depositional changes, related mainly to modification of geomorphic and climatic factors or to a combination of them, provide a tool for the identification of five cycle sets (units from U1 to U5), approximately equivalent, from a physical stratigraphic point of view, to units 1 to 5 of George and Berry (1993, 1997) and to supersequences of Yang and Baumfalk (1997).
The description of these basinwide modifications was supported by a detailed analysis of stacking patterns through a log-to-core calibration (Fig. 19), the correlation between wells across different depositional environments (Figs. 15, 18), and the generation of palaeogeographic maps (Fig. 16), integrated with bibliographic information (George and Berry, 1993, 1997; Sweet et al., 1996; Howell and Mountney, 1997; Sweet, 1999; Bailey and Lloyd, 2001; Maynard and Gibson, 2001).
Cycle Set 1 (Unit 1).—
The lower boundary of this set, which is composed of cycles 1 and 2, corresponds to the Base Permian Unconformity (Fig. 19). In this time interval, the basin was bordered by high-relief basement uplifts: the East Midland Shelf to the west, connected to the south with the London Brabant Massif and the Indefatigable High and the Swarte Bank Hinge mountains to the east (Fig. 20). These uplands bounded relatively narrow, SE–NW-elongate structural valleys, centred in the Sole Pit Basin (Figs. 16.1, 16.2). These valleys were characterised by the interference of different morphological processes. Winds blowing from the east and northeast (Glennie, 1984b) controlled the deposition of aeolian dunes, in turn dissected by semipermanent fluvial channels fed by intermittent heavy rainfalls (cf. George and Berry, 1997; Howell and Mountney, 1997). During periods of intense runoff, the active channels transported huge volumes of sediment; during dry phases, the fluvial plain was patched by small aeolian dunes. The lacustrine system was confined to the north and east by the mountain belt of the Swarte Bank Hinge (cf. George and Berry, 1997; Howell and Mountney, 1997).
Cycle Set 2 (Unit 2).—
This stratigraphic interval comprises cycles 3 to 7. The sedimentary succession recorded a period of overall aridity increase testified by an expansion of aeolian dunes toward the basin (Figs. 15, 16, 18, 19). Nevertheless, this trend was modulated by a higher frequency wetting phase responsible for a widespread lake expansion at the top of cycle 4 (Figs. 15, 16.4).
The general widening of the erg system caused confinement of braided streams, which were strongly reduced in extent and activity. In particular, the fluvial systems associated with the East Midland Shelf were virtually deactivated with a low discharge, a reduced floodplain, and a limited drainage area (cf. George and Berry, 1997; Howell and Mountney, 1997). On the other hand, the fluvial systems connected to the Inde High developed a drainage system interacting with the erg-margin system (cf. George and Berry, 1997). In this sector the aeolian and fluvial systems mutually overlapped through reciprocal erosional mechanisms. According to the dune-dammed model of Svendsen et al. (2003), during the dry phase the erg system could spread over the flood plain because the fluvial channels were completely deactivated. During relatively wet phases the fluvial system was characterised by spasmodic flash floods interacting with the previously migrating aeolian dunes. These aeolian depositional elements behaved as a dam, subject to progressive erosion and eventual breaching (Figs. 16.5, 16.6, 16.7).
Cycle Set 3 (Unit 3).—
This unit records strong physiographic and climatic modifications in the studied area. The morphological changes were related to the complete burial of the Inde High and of the Swarte Bank Hinge mountain chains and to a marked widening of the lacustrine domain (Figs. 16.8, 16.9, 20) The expansion of the lacustrine basin was probably associated with the combined effect of heavy rainfall and with the generation of semipermanent fluvial channels fed by the London–Brabant Massif (Fig. 20) (cf. George and Berry, 1997; Howell and Mountney, 1997). These fluvial channels completely bypassed the aeolian sand sea due to their large discharge and the related deactivation of the erg system. As a consequence, a high volume of sediment was transported to the lacustrine domain, probably with the deposition of sandstone lobes at the morphological break between playa and lacustrine areas. Sedimentation was no longer controlled by the inherited topography, due to the almost complete smoothing of the topography (Figs. 15, 16.8). As a consequence, creation of accommodation space was linked mainly to the combined effect of thermal subsidence and base-level oscillations induced by high-frequency climate-related contraction and expansion of the lacustrine system.
Cycle Set 4 (Unit 4).—
This cycle set, composed of cycles 10 to 14, recorded a steady geomorphic setting dominated by a playa-lake domain in the northeastern sector and a subaerial range in the southwestern zone (Figs. 16, 20).
Cycles 10 and 11 were dominated by a couple of lacustrine flooding surfaces developed at regional scale (Figs. 15, 18). In particular the flooding surface identified at the base of cycle 10 was recording the most widespread Silverpit Lake expansion (U ingression), recorded both in the Dutch and German sectors (cf. Geluk, 2007; Legler, 2008; Stollhofen et al., 2008). These two cycles terminated with the emplacement of ephemeral fluvial systems and the deposition of thin and small sandstone lobes in the lacustrine setting (Figs. 15, 18, 20).
Cycles 12 to 14 are characterised by an alternation of aeolian, playa, and fluvial deposits in the subaerial domain and an alternation of anhydrite-rich, dolomite-rich, and bioturbated mudstones in the lacustrine domain (Fig. 20). Detailed comparison between proximal and distal sectors shows the equivalence between aeolian deposits and anhydrite-rich mudstones (in turn probably time-equivalent with halite deposits located in the playa-lake centre). In fact, on a small scale, a high-frequency alternation of humid and arid climatic episodes can be recognised. During wet phases the subaerial sector was characterised by the presence of fluvial channels that could generate small and thin sandy lobes interfingered with lacustrine bioturbated shales. During dry phases, the aeolian processes reworked the previously deposited fluvial bodies, generating small and isolated dunes. In the lacustrine system, evaporative processes induced precipitation of anhydrite nodules and streaks (Fig. 15).
Cycle Set 5 (Unit 5).—
This unit records a climatic shift toward more humid conditions than in unit 4. As a consequence, the drainage pattern was deeply affected by this wet climate, with huge volumes of water and sediments transferred from the mountain range to the lake via catastrophic flash flooding. These depositional processes generated relatively wide and thick sandy lobes in the lacustrine area (Figs. 15, 16.15, 18, 20). Lacustrine sandy lobes have been already recognised very close to the northwestern corner of the study area (see George and Berry, 1997). To the south and west, aeolian deposits are still present but they are characterised by deep reworking and homogenization associated with heavy rainfall, rising groundwater table, lake expansion, and related flooding. The combination of these factors generated the well-known Weissliegend formation. which is characterised mainly by soft-sediment-deformed tight deposits. These events were overall controlled by the catastrophic Zechstein Sea ingres-sion which affected the entire Southern Permian Basin (Glennie, 1983b; Strömbäck and Howell, 2002). In the lacustrine domain, the facies typical of dry climate phases (anhydrite-rich mudstone and halite) are completely absent.
The low-frequency cycle is characterised by a large backstepping–forestepping depositional sequence which approximately encompass the entire studied succession (Fig. 19). These opposite stacking patterns are generated by a different balance between rate of accommodation-space creation and sediment supply. Both of these two factors are in turn controlled by rate of tectonic subsidence and climatic changes (see Howell and Mountney, 1997, for a detailed discussion).
In particular, the lower part of sedimentary record (cycles 1 to 7) was characterised by a backstepping stacking pattern because the rate of accommodation-space creation (induced by tectonic subsidence) outpaced the rate of sediment supply. The tectonic subsidence was strictly connected to the activity of normal faults bordering the London–Brabant Massif (Dowsing Fault Zone) to the west and the Inde Pediment High and the Swarte Bank Hinge to the east (George and Berry, 1997). These structural elements bounded relatively small and NW–SE-elongated basins in which first fluvial and then aeolian successions were deposited (Fig. 20). During the aeolian sedimentation in the marginal setting, a number of halite cycles were deposited in the distal areas, from Quadrants 43 and 44 in the UK sectors to the western part of North German Basin (Gast, 1991; George and Berry, 1997; Yang and Baumfalk, 1997; Gaupp et al., 2000). These halite deposits showed limited lateral continuity and marked thickness changes following the available accommodation space (cf. Fig. 1.2 in Geluk, 2005). This stratigraphic interval is bounded at the base by the Base Permian Unconformity (Figs. 15, 18) and at the top by a climatically-induced erosional surface (base of cycle 8).
Cycles 8 to 9 recorded a further backstepping of depo-sitional systems (Fig. 15), controlled mainly by climatic change. A marked shift toward humid conditions forced a rapid lacustrine expansion, probably enhanced by fluvial runoff. This heavy rainfall triggered catastrophic fluvial flash floods, depositing huge volumes of sediments all over the study area (Fig. 20). Accommodation-space creation was enhanced by a number of basinwide lacustrine expansions culminating in a regional maximum-flooding surfaces (sensu Sweet, 1999), detected at the base of cycle 10 (Figs. 15, 18).
The upper portion of the Upper Rotliegend 2 succession (cycles 10 to 14) is equivalent to the forestepping interval of the low-frequency cycle. During this time, the rate of sediment supply overcame the rate of accommodation-space creation, which was generated by the combination of thermal subsidence and high-frequency climate-induced lake-level fluctuations. In fact, the previously active structural elements were buried and the associated topographic relief was completely smoothed (cf. Fig. 16 and Fig. 20). The sedimentary succession was dominated by a combination of ephemeral fluvial systems, erg-margin systems, and dry to wet playas in the marginal areas (Fig. 15). The basinal setting was characterised by deposition of relatively thin and widespread anhydrite cycles deposited during aridity peaks. In the most depocentral part of the SPB, the time-equivalent halite bodies experienced a progressive upward confinement toward the central portion of the basin following the forestepping of the marginal facies belts (cf. Fig. 1.2 in Geluk, 2005).
This stratigraphic interval is bounded at the base by a basin-wide lacustrine expansion (coincident with the U Ingression; Fig. 19) and by a marked basinward facies shift at the top. This sharp contact was highlighted by the deposition of huge volume of sediment through catastrophic fluvial flash floods feeding widespread and thick lacustrine sandstone lobes (cycles 15 to 16) (Figs. 15, 18, 20). This depositional event was linked to a shift toward more humid climatic conditions, leading to a rising of lake level. Suddenly after this wide lacustrine expansion, the Southern Permian Basin experienced a full connection with the open sea, as testified by deposition of marine black shales (the Copper Shale), corresponding to a third-order maximum flooding surface (Gast, 1991; Legler and Schneider, 2008; Stollhofen et al., 2008).
Regional Correlation of Upper Rotliegend Unit 2 (UR2) in the SPB from the UK to The Netherlands and Germany
Identification of several key surfaces and characterisation of a new depositional framework provide a powerful tool for the comparison between sectors located at opposite sides of the Southern Permian Basin. Recognition of regionally developed flooding surfaces in the Rotliegend feather edge allow their tentative correlation with the Dutch and German sectors. Gaupp et al. (1993) reported the stratigraphic framework of a well located in a rapidly subsiding sector of the German basin. General stratigraphy, stacking patterns, and lacustrine flooding surfaces show a strong similarity between this well and the wells located in the study area (Fig. 21).
In particular, two regionally widespread key surfaces, related to climatic forcing, were confidently correlated throughout the SPB. These surfaces corresponded to very rapid lake-level rise inducing a complete reorganisation of depositional systems in both the marginal and the basinal sectors.
At the top of cycle 8 this climate pulse was recorded by: (1) a sudden abandonment of the erg system expansion; (2) deposition of thick and widespread high-energy fluvial sandstones; and (3) associated expansion of wet-playa and lacustrine deposits (Figs. 15, 18, 19). This depositional event has been recorded in the German sector also and linked to the Ameland Ingression at the boundary between the Dethlingen Formation and the overlying Hannover Formation (Legler and Schneider, 2008; Stollhofen et al., 2008).
As in the UK sectors, also in the German sector, above this stratigraphic interval two well-defined coarsening-and drying-upward cycles were recognised (Figs. 15, 21). These two cycles culminated with an overall Silverpit Lake expansion, which is located at the base of cycle 10 in the UK sector and corresponds to the U Ingression in the rest of the SPB (Legler and Schneider, 2008; Stollhofen et al., 2008). From this event onward, lacustrine sedimentation was widespread on the Southern Permian Basin (Figs. 15, 18, 21).
Two more flooding surfaces have been reported in the German sectors: the P Ingression in the Dethlingen Fm., and the X Ingression in the topmost part of the Hannover Fm (Fig. 21). The P Ingression is not characterised by a noticeable depositional signature in the aeolian–fluvial-dominated succession of the German well. In contrast, the X Ingression is located in a succession dominated by high-frequency flooding events, as well as in the study area.
Although comparison of the lower part of the sedimentary record between the UK and German sectors is somehow questionable, some potential is still present in the studied succession. In fact, cycle 4, which was dominated by the interaction of aeolian and fluvial processes, recorded a sharp flooding surface at its top (Fig. 15). This event was expressed by deactivation of the erg system in the marginal sectors and a clear fining-upward sequence in the distal areas. This surface, corresponding to the first marked Silverpit lake expansion, reported by several authors for various areas of the UK SPB (cf. Sweet et al., 1996; Bailey, 2001), was tentatively correlated with the P Ingression.
The uppermost part of the Upper Rotliegend 2 Unit was characterised by cyclic repetition of lacustrine contraction and expansion. During lake-level fall, thick halite units were deposited in the basin centre (cf. Geluk, 2005) and anhydrite-rich shales were deposited at the basin margin. During lake-level rise, probably amplified by periodic marine ingressions (Schneider and Gebhardt, 1993; Schneider et al., 2006; Legler and Schneider, 2008; Stollhofen et al., 2008), anhydrite-free bioturbated shales were distributed all over the basin. One of the most important flooding surfaces is located at the top of cycle 13 (Figs. 15, 19), which anticipated a contraction of lacustrine system due to an overall facies-belt forestepping. This surface has been tentatively compared with the X Ingression reported in the upper part of the Hannover Fm.
The Permian Upper Rotliegend Group in the study area provides the opportunity to analyse the behaviour of different and contrasting sedimentary processes and to understand the evolution and interaction of different depo-sitional systems. Furthermore, it provides some key concepts to correlate the sedimentary succession between both sides of the Southern Permian Basin
Many sedimentological studies have been carried out in the UK sector of the Southern Permian Basin, focussing on the spatial interaction between fluvial and aeolian depositional systems and their distribution through time. On the contrary, only few studies deal specifically with analysis of the lacustrine succession and their relationships with the subaerial environment (Howell and Mountney, 1997; Bailey and Lloyd, 2001; Maynard and Gibson, 2001). The study area is centred in a sector dominated by the intertonguing of aeolian, fluvial, playa, and lacustrine environments.
The aeolian and fluvial systems generally interact together through reciprocal erosional processes. These relationships are here amplified due to their dramatic differences in transport direction. In particular, the fluvial system was characterised by sand transportation toward the northwest while aeolian dunes were generated by winds blowing from the east and northeast. However, the progressive filling of the Sole Pit Basin by aeolian dunes generated a relative eastward expansion of the erg system over the playa environment. As a consequence, the aeolian damming of ephemeral rivers generates catastrophic flooding of the erg when the dune threshold was overtopped (Langford and Chan, 1989; Svendsen et al., 2003). During prolonged expansion of the erg system, aeolian sequence boundaries (sensu Sweet, 1999) developed, indicating deflation before aeolian accumulation.
Concerning the relationships between the subaerial and the lacustrine environments, low GR spikes are generally attributed to deposition of aeolian sheet sandstones. However, a detailed core description shows that these layers can be interpreted as wide and relatively thick sandy lobes (sensu Mutti et al., 1996), deposited by catastrophic fluvial flooding entering the lacustrine domain. In fact, these sandstones are typically massive (no bimodal sorting or planar lamination is developed), characterised by normal grading and by a moderate to good sorting linked to erosion of originally well-sorted sediments (Minervini, 2004). Moreover, these deposits have been correlated in a marginward direction to very thick and well-developed fluvial sequences characterised by a facies association indicative of high-energy catastrophic flooding (Minervini et al., 2003).
Many authors have suggested that two main factors control facies distribution: (1) the pre-Rotliegend palaeotopography, affecting location of the erg system and local facies development, and (2) climate, driving changes in lake level and in water table (George and Berry, 1993; Clemmensen et al., 1994; George and Berry, 1997; Howell and Mountney, 1997; Maynard and Gibson, 2001).
In particular, a tectonic control on sedimentation was probably active only in the lower portion of the succession, when the depositional elements were filling inherited structural lows and onlapping onto neighbouring structural highs. Afterwards, tectonic subsidence linked to the thermal cooling of the Late Carboniferous–Early Permian volcanic phase dominated the entire basin (Glennie, 1990; Coward, 1995; Glennie, 1997). During this time, creation of accommodation space was also modulated by lake-level changes, in turn influenced by climate. As a consequence, in this wide basin dominated by thermal subsidence, climate played a significant role in controlling the occurrence of key stratigraphic surfaces and the lateral and vertical evolution of facies associations evolution (cf. Howell and Mountney, 1997; Sweet, 1999; Bailey and Lloyd, 2001; Maynard and Gibson, 2001).
The correlation of these key surfaces in different parts of the basin allows the recognition of 16 depositional cycles. The varied expression of each depositional cycle is considered as a balance between accommodation-space creation (controlled by subsidence) and sediment supply (dependent on climatic changes) (cf. Bourquin et al., 2006; Bourquin et al., 2009). In particular, the high-frequency cyclicity is driven mainly by the superimposition of inherited palaeotopography modifications and/or climatically induced oscillations in water table and lacustrine level. From a practical point of view, these depositional cycles are identified through the inferred point of minimum aridity (cf. Howell and Mountney, 1997), and they occur with a variety of drying- and drying-wetting-upward patterns. These cycles represent allostratigraphic depositional units which can be used to trace the history of the Upper Rotliegend succession in the Southern Permian Basin.
The medium-frequency cyclicity records dramatic palaeomorphological rearrangements, highlighted by strong marginward and/or basinward shifts of different deposi-tional environments. In particular, a sharp aeolian sequence boundary (sensu Sweet, 1999) was identified between Unit 1 and Unit 2, highlighting the first region-wide expansion of the erg system over the study area. This evolutionary trend was modulated by a contrasting lacustrine expansion which induced a water-table rise and a related flooding of the marginal areas. This transgressive surface has tentatively been correlated with the P Ingression of the Dutch and German sectors (Schneider and Gebhardt, 1993; Geluk, 2007; Legler and Schneider, 2008). In contrast, between Unit 2 and Unit 3 a fluvial sequence boundary (sensu Sweet, 1999) is developed with a sharp truncation of fluvial sediments onto previously deposited aeolian dunes. This erosional surface records a marked rearrangement of depositional systems, e.g., the reorientation of the direction of fluvial streams (Minervini et al., 2003). Two more flooding surfaces of regional importance have been highlighted: in cycle 8 and at the top of cycle 13. In particular, the transgressive surface of cycle 8 has been correlated with the Ameland Ingression (cf. Gast, 1991; Geluk, 2007; Legler and Schneider, 2008; Stollhofen et al., 2008), which corresponds to the boundary between the Dethlingen Fm. and Hannover Fm.
The low-frequency cyclicity concerns the overall infilling of the Southern Permian Basin. In the lower half of the succession, creation of accommodation space outpaced sediment supply (highlighted by the high preservation potential of aeolian deposits). In the upper half of the succession, generation of accommodation space by thermal subsidence was continuously balanced by sediment supply via aeolian and fluvial processes. The lower boundary of the sedimentary succession corresponds to the Base Permian Unconformity (BPU), a marked angular unconformity of regional extent developed between Permian and Carboniferous deposits. The surface identified at the base of cycle 10 corresponds to a maximum-flooding surface (sensu Sweet, 1999). This surface records the most important Silverpit Lake expansion, and it is coincident with the U Ingression of the Dutch and German sectors (cf. Gast, 1991; Geluk, 2007; Legler and Schneider, 2008; Stollhofen et al., 2008).
The fluvial sequence boundary recorded between Unit 4 and Unit 5 corresponds to a sharp downward shift of depositional systems, characterised by fluvial catastrophic deposits sharply overlying ephemeral fluvial systems and aeolian dune deposits in the marginal sectors, and by a progressive confinement of salt layers in the basin setting (cf. Geluk, 2005). Rotliegend-type sedimentation was suddenly deactivated by a marine ingression (MFS) leading to the deposition of the Copper Shale.
The proposed hierarchy of the sedimentary succession provides a tool for the understanding of mutual relationships between different depositional systems (aeolian, fluvial, playa, lacustrine) in the UK sector (Rotliegend feather edge) and their correlation with the Dutch and German regions.
The authors gratefully acknowledge Eni E&P Management for permission to present this paper. The manuscript was greatly improved by comments from S. Bourquin and M. Sweet. J. Grötsch and R. Gaupp are thanked for the constructive suggestions and helpful support. This paper is a part of the Ph.D. study of MM at the University of Padova.
Figures & Tables
The Permian Rotliegend of the Netherlands
More than 50 years ago, the discovery of the giant Groningen Gas Field in the subsurface of the Netherlands by NAM B.V. marked a turning point inthe Dutch and European energy market initiating the replacement of coal by gas. Despite the fact that the Rotliegend dryland deposits in the Southern Permian Basin are one of Europe's most important georesources, no sedimentological overview is available to date for the subsurface of the Netherlands. This SEPM Special Publication presents for the first time such a summary of the present-day knowledge, including a comprehensive core atlas from on- and offshore wells.