Journal ofthe Geological Society, London, Vol. 148, 1991, pp. 379-390, 7 figs. 1 table. Printed in Northern Ireland

The Lower Carboniferous Stainmore Basin, N. : extensional basin tectonics and sedimentation

R. E. L1. COLLIER Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

Abstra& Recently released industry seismic data image a 6 km thick Dinantian succession in the Stainmore Basin, and are used to interpret deposition in response to extensional subsidence. The basinis bounded tothe north by the Lunedale-Wigglesworth-Butterknowle enechelon suite of normal faults. An early (?Courceyan-?Arundian) syn-rift sequence more than 4 km thick includes footwall-derivedclastic fans. A basinalterrigenous sequence deposited through the Arundian, Holkerian and early Asbian contrasts with coeval carbonates on the Ravenstonedale Shelf and the Askrigg Block. Deposition continued in the basin whilst the Alston block and northern parts of the basin were emergent during early Asbian times. A late Asbian relative base level rise and the possible cessation of active rifting allowed both basin and structural highs to be inundated by shallow marine carbonates. Rhythmic ‘Yoredale’ sedimentation followed. Dinantian depositional patterns reflect the interplay between active extensional faulting, a susp- ected local inversion event, changes in sediment influx rates, sea level variations and differential rates ofcompaction between basement highs and the basin. However, error marginson the available biostratigraphy and timing of active faulting prevent the detailed separation of these variables and their impact on sedimentation in the Dinantian.

The Stainmore Basin forms one of a number of NE-SW to the contrasts in sedimentary facies around the basin to be E-W trending sedimentary basins in northern Britain that established for much of the Dinantian. A model has been formed in response to lithospheric extension in the Lower constructed of three-dimensional facies variation in response Carboniferous(Dewey 1982; Leeder 1982, 1987~).The to rifting. The significance of establishing the areal regional tectonic context of the basins has been related by dimension is that areal variance in facies is to be expected these authors to crustal stretching (in a back-arc position) to around the margins of any rift basin, in accordance with the thenorth of the Rheno-Hercynian collisional zone.An differences in fault activity around the margins of the basin. easterly structural tilt across Stianmore exposes lower parts Three further themes are introduced, the first of which of the Carboniferous succession in the west of the basin. In arises out of the inferredrelationship between facies theeast, Silesian depositssubcrop the base-Permian patterns and rift histories. This explores the degree to which unconformity in thearea (Fig. 1). Footwall highs tothe sedimentary featuresand sequencegeometries within an north and south of the basin comprise relatively low density overall rift ‘megasequence’ can be used to distinguish more basement blocks buoyed by the Weardale and Wensleydale detailed pulses of rift activity from ‘passive’ infilling phases granites respectively (Dunham et al. 1965; Bott 1967; Wilson when subsidencecontinues dueto lithoshperic cooling & Cornwell 1982). (McKenzie 1978). Despite this problem,the secondpoint Recent work hasled theto characterization of emphasizes that seismic facies analysis within a depositional sedimentary facies patterns expectedin active rift basins sequenceframework, and constrained by well andtor (e.g. Leeder & Gawthorpe 1987; Dodd & Gawthorpe 1991). outcrop data, can improve the interpretation of a basin fill This has led to the process of deriving information about a andits tectonic significance. The third theme is the basin’s rift history fromthe sedimentary fill. Such an distangling of threethe variables of tectonic approach isnow being applied to Carboniferous basins in subsidenceJuplift rates, rates of eustatic sea level changes Britain, and sedimentary and sequencegeometries related and sediment supply rates, which can then beaddressed to active faultinghave been described by Williams et al. within the depositional sequence framework. (1989) in the Munster and South Munster Basins, Wilson et Previous work (Bott 1967; Burgess & Mitchell 1976; al. (1988) in the SE Wales area, Ebdon et al. (1990) in the Burgess & Holliday 1979; Dunham & Wilson 1985) based East Midlands, Gawthorpe (1986) in the Bowland Basin and on gravity, outcrop studies and shallow boreholes,has Ord et al. (1988) in the Solway Basin. This paper follows a established thatthe Stainmore Basin had an extensional similar approach in relation to the Lower Carboniferous of half-graben or asymmetric graben morphology during the the StainmoreBasin, inferring details of its rift evolution Lower Carboniferous, being one of anumber of such from the character and variation in basin-fill deposits. The Carboniferousstructures in northern Britain (reviewed in study has used newly available seismic data tied to well logs Gawthorpe et al. 1989). The principal structural elements of and outcrop, to extend our understanding of the Stainmore the basin are outlined on Fig. 1. The basin is inferred to be succession. an asymmetric graben with its major bounding structures in 300 km of seismic sections from the north-central area of thenorth, the en echelonLunedale, Wigglesworth and Stainmore (Fig. l), together with outcropto the north, ButterknowleFaults (Fig. 2). Thesouthern basin margin south and west, supported by borehole data, have allowed must befault-bounded, probably along the line of the

379

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\ Permian-Mesozoic

Carboniferous: --- Westphalian

...... Namurian

AlstonGroup ...... m H...... Dinantian

Lower .-. 'I' Basin-bounding Palaeozoics normalfault Fault 7Thrust fault

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a

NORTH 0 1 km SOUTH b U

Fig. 2. (a) Migrated seismic section across the northern marginof the Stainmore Basin from the UK86 survey. (b) Structural and depositional sequence interpretation of the seismic line in (a). Numbers 1-6 denote depositional se- quences within the Lower Carboniferous basin-fill. Lithostratigraphic divisions are approximate equivalents of these deposi- tional sequences.'A highlights clino- forms in depositional sequence1. Approximate depth scale is converted from seismic stacking velocities. LA-6000

Stockdale Disturbance, because the northwardsextrapola- faults and occurrences of syn-sedimentary deformation. tion of sedimentarydips onthe Askrigg Block does not Both lines of evidence are used to constrain- the timing of accommodate the thickness of Dinantian sediment imaged on syn-rift fault movements, the latter after Leeder (19876). the seismic to the north. At the western end of Stainmore, the succession exposedin the Ravenstonedale area is Stratigraphic evidence around the basin margins interpreted as a shelf sequence, starved of the clastic material that dominated deposition in the basin proper for Alston Block much of theDinantian, as discussed below. Critical The Lower Carboniferous succession on the Alston Block is relationshipsstudied in the field include facies and well documented dueto its widespreadexposure (e.g. sedimentary thickness changesacross surface outcrops of Burgess & Holliday 1979) and its penetration by a number

Fig. 1. (a) Location map for the Stainmore Basin, onshore north-east England. Open circles indicate boreholes and filled circles wells that were used to constrain seismic interpretation and stratigraphies. 1, Brafferton-l; 2, Seal Sandsno. 1; 3, Rookhope; 4, Collier's Law; 5, Roddymore; 6, Woodland; 7, New Shildon no. 146; 8, Randolph; 9, Mount Pleasant; 10, Coldsides; 11, Beckermonds Scar;12, Raydale. Detail (b) outlines structures at the western endof the Stainmore Basin.A and B locate sections logged and illustrated in Figure3.

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Table 1. Stratigraphic correlation between the Lower Carboniferous of the Stainmore Basin and surrounding areas

~~~~ ~~~ ~ ~ RAVENSTONEDALE 4SKRIGGBLOCK Stage ALSTONBLOCK STAINMOREBASIN SHELF (Northernarea)

P P a a BRIGANTIAN 0 2 L 0 0 C C 0 0 c c -U) -U) 4 4 DannyBridge GreatScar KnipeScar Limestone I - Limestone Limestone ASBIAN PottsBeck Garsdale Limestone Limestone

Ashfell Fawes Wood Hillbeck HOLKERIAN Hillbeck Limestone Limestone Limestone Limestones P Formation a a a ~~ 2 2 Ashfell 0 0 Ashfell Ashfell Sandstones C C Sandstone Sandstone 0 \ Ashfell 0 c c L L taven avenstonedalc 0 \ 0 TomCroft Limestone Limestones Formation / -- 301./Sst.'

CHADIAN D .- I RomanFell Beds 1 Sandstones P P m a a 0 0 Tebay L L I0 0 Conglomerates RomanFell c c C c ! m m Shales E I m COURCEYAN U) PinskeyGill m m m m Beds Basal Congloms.

LL

Sources include Barraclough (1983), George et al. (1976), Burgess & Holliday (1979), Higgins & Varker (1982), Strank (1982) and Wilson & Cornwell (1982).

of boreholes(Woolacott 1923; Dunham et al. 1965). Here Sandstones may have extended into the HolkerianI (Higgins the pre-Arundian Dinantian succession (Table 1) is limited & Varker 1982). to 50-200 m in thickness. The petrographic content of basal The overlying Holkerian Hillbeck Limestone Formation conglomerates(Burgess & Harrison 1967) indicates local varies in thickness across normalfaults which bound the derivation fromthe Alston block. The Ashfell Sandstone northern margin of the Stainmore Basin. It is only about Formation outcropping along the Pennine escarpment (Fig. 30 m thick onthe Alston Block where decimetre-scale 1) comprises 5-55 m of trough cross-stratified sandstones wackestone beds characteristically show evidence, in the with dominantly westerly palaeocurrentdirections. The form of symmetric, mud-draped ripples and hummocky Ashfell Sandstone is ascribed to the Arundian according to cross-stratification, of wave and storm influence (Fig. 3). the lithostratigraphy of George et al. (1976). However, The Hillbeck Limestones thicken southwards to 80m in the basinal equivalents of the Ashfell Sandstone incorporate the hangingwall of the Swinedale Beck Fault (Fig. 1). Here the Pu Zone miospore Lycosporapusilla (Dunham & Wilson Formationincludes storm-reworked carbonates, corallifer- 1985) which had a Chadianto Holkerian agerange. The ous limestones and intercalations of trough cross-bedded presence of Cavusgnathus unicornis in the upper part of the and ripple-laminated fluvio-deltaic sandstones. The Swine- sandstone in Ravenstonedale also suggests that the Ashfell dale Beck Fault is therefore interpreted to have been active

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B SOUTH

;OU 0m= -3x _.

Dolomitised limestones

Hummocky Z cross-stratification Shoreface d carbonates

Wave- d influenced Q carbonates withstorm beds

Q 0 Q - -\ \ S" Coralliferous Q, limestones

\ 0 Limestone(dolomitised) 0 \ Sandstone \ \ Shale \ Fluviatile Wave-modifiedripples 10 m sandstones

Currentripples GO 0 5 Q IPalaeocurrent reading %e 00 Corals(colonial)

49 Brachiopods

at this time, exerting an influence on water depth and recorded.Although this may bedue to environmental consequently sedimentary facies, and also on sedimentary control, it does concur with the early Asbian being a period thicknesses within the Hillbeck Formation. There is no field of non-deposition andlor erosion over the northern footwall evidence of an emergent fault scarp, so that the structure of the Stainmore Basin. was probably moving syn-deposition of theFormation, The Melmerby Scar Limestone is 35-52 m thick over the rather than the Formation forming a post-faulting infill to a Alston Block, thickening to90m southwards across the fault-induced topography. Swinedale Beck Fault.Because of the absence of any The early Asbian is apparently absent onthe Alston marked facies change across this boundary, it is not clear Block, with the lateAsbian Melmerby Scar Limestone whether this thickness change was accommodated by active resting directly on Hillbeck Limestonebeds (Burgess & faulting or was produced byinfilling a pre-existing Holliday 1979). The early Asbian Potts Beck Limestone of southwardsincrease in water depth(and/or due to any the Ravenstonedale area(George et al. 1976) is not compaction-induced differential subsidence, Collier 1989). represented onthe Alston footwall high, wherethe The late Asbian and Brigantian 'Yoredales' range from definitive early Asbian brachiopod Daviesiella llangollensis 240 m to 273 m in thickness on theAlston Block (Dunham et found in Ravenstonedale(Strank 1982) has not been al. 1965; Burgess & Holliday 1979).

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Ravenstonedale Shelf carried out by Ashton (1971) and Barraclough (1983). The first Carboniferous deposits to have been laid down were the The 1500 m of Dinantiansediments outcroppingin the Pinskey Gill Beds(Table l), consisting of mudstones and Ravenstonedale area (George et al. 1976) were deposited in dolomitized limestones of Courceyanage which were a gulf which formed an embayment at the western end of deposited across a tidal flat and conglomerates which infilled the Stainmore Basin (Barraclough 1983). Lack of outcrop an erosionaltopograpy onthe eastern flank of theLake and well penetrationprevents detailed conclusions being District Lower Palaeozoicbasement massif. The Pinskey drawn on whether thegulf linked northwestwards through a Gill Beds were overlain by conglomerates and sandstones of proto-Vale of Eden Basin (Fig. 1) intothe Solway Basin the Tebay Conglomerate which onlap onto basement. These throughout the Dinantian. Themarked thickening of the weredeposited by northwards-flowing ephemeral streams, Dinantian succession off the Alston Block intothe according to Baraclough (1983). Ravenstonedale area suggests that the Pennine Faultsystem Thecarbonates of Chadian to Arundianage inthe may have downthrown to the west during the Courceyan to Ravenstonedale district include facies indicative of a variety mid-Asbian. of tidal flat sub-environments. These range from(a) The Courceyan to Asbian is represented by a current-influenced, cross-stratified bioclastic limestones and carbonate-dominated sequence in the Ravenstonedale area, brecciated algal laminites of a beach ridge association, reflecting its position on a relative structural high to the west through (b) micritic algal and pelletal limestones with a of the Stainmore Basin clastics depocentre (Fig. 4). restricted fauna of algal marsh/lagoonal origins, to (c) Sedimentary facies analyses of the succession have been dolomitized limestones which include calcretized karst

4170 5385 5 ALSTON BLOCK ,&

STAINMOREBASIN

RAVENSTONEDALE SHELF

rNlTE

+++++ ++++++++ +++++++ +++++++++ 0 5 10 km +++++++++ ++++++++ 1 ++++++l AS

Fig. 4. Structural and depositional sequencesummary diagram showing the variation in thickness of sequences, constrained by borehole data and inferred (dashed), between the Stainmore Basin and surrounding structural highs.B. F., Butterknowle Fault;W. F., Wigglesworth Fault. Numbered ticks on surface indicate position of boreholes locatedin Fig. 1. Sources include Woolacott(1923), Mills & Hull (1968), George et al. (1976) and Wilson & Cornwell (1982).

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surfaces recordingemergence of a coastalplain (Bar- Stainmore Basin fill. A seismic facies analysis has been raclough 1983). The Scandal Beck Limestone and Michelinia carried outon these data, within a newly constructed grandis Beds (Table 1) are however dominated by depositional sequence framework (outlined in Figs 2 and 4). packstones which are rich in colonial coralsand Seismic interpretationshave beenconstrained wherever brachiopods, and indicate moreopen marine shelf possible by cross-ties to well and outcrop reference points. conditions. Similar conditions returned in the Holkerian to These include the two deep wells in the basin, Seal Sands early Asbian, after a limited thickness (up to about17 m) of no. 1 and Brafferton-l (Fig. l), which provide important fluvio-deltaic and shallow marine Ashfell Sandstones were new stratigraphicevidence onthe basin fill. The main deposited in the area. inferences drawn from this exercise on the stratigraphy and Afterthe mid-Asbian regional transgressive event, facies distribution of the LowerCarboniferous basinal deposition overthe Ravenstonedale shelf followed the succession are presented below forcomparison with the distinctive northern history, with the accumulation successions alreadydescribed fromaround the basin of the Knipe Scar Limestone giving way toYoredale margins. shale-sandstone-limestone cyclothems. South of the Lunedale-Wigglesworth-Butterknowle faulted margin up to about 6 km of Dinantian sediments are Askrigg Block imaged on seismic sections (Figs 2 and 4). Thedepth is The Askrigg Block comprises a structural high of Lower converted directly from seismic stacking velocities. There Palaeozoic metasediments intruded by the Wensleydale is, however, no well penetrationbeneath the Chadian/ granite. The presence of the pluton was inferred from Arundian (GS, Zone).The timing of rift initiationcan gravity (Bott 1961) and confirmed by drilling of the therefore be estimatedonly from the regional context of BeckermondsScar and Raydaleboreholes (Wilson & the basin, and the age of the oldest basin-fill sediments is Cornwell 1982). The LowerCarboniferous succession taken to be latest Devonian or Courceyan, by analogy with onlapping southwards onto the Askrigg high is dominated neighbouring rift basins such as Northumberland (Leeder by carbonates, asdescribed by Wilson & Cornwell (and 1974) and Bowland (Gawthorpe 1986). Dunham & Wilson 1985). The succession is thusmore An early(?Courceyan-?Arundian) rift sequence is closely allied to that of the Ravenstonedale shelf area than picked from 1.3-2.5 two way travel time, which converts to that of the Stainmore Basin (see below). However, there is a sedimentary thickness of about 4100 m.Sequences are no evidence for the extended periods of restricted marine defined as packages of reflectors with internally consistent conditions which characterize the carbonates of tidal flats (but laterally variable) seismic character, with sequence origins in Ravenstonedale. boundaries being chosen, where possible, at widespread Chadian ?Arudianto dolomites interbedded with unconfonnity surfaces. The first sequence includes downlap- sandstones formthe basalCarboniferous strata in the ping clinoforms located against the northern basin-bounding Raydale boreholeand at outcrop in theGarsdale area. faults (e.g. A in Fig. 2). An important test that such features Thesedeposits indicate shallow waterdepositional condi- are not artefacts, for exampleproduced by side-swipe off tions (Burgess, in Dunham & Wilson 1985). Theyare faultsurfaces, is that they must be laterally consistent overlain by Arundiandark greylimestones which extend between adjacent lines. In the absence of well penetration, further south and were encountered in the Beckermonds interpretations of features observed only on single lines Scar borehole. Here, the 57m thick Tom Croft Limestone should notbe treated with a high degree of confidence. consists of calcarenitic grainstones with a variable peloidal Here, the clinoforms are imaged on neighbouring lines and and bioclastic content. Above a 9 m interval ascribed to the describe fanbodies with radii of upto 2 km. Theseare Ashfell Sandstone,about 100m of Holkerianto early interpreted as being footwall-derived alluvial fan, fan delta Asbian carbonates were penetrated. 56 m of shallow marine or possibly submarine fan coarse clastic bodies,probably Fawes Wood Limestonecalcarenites giveway to 41 m of onlapped by basinal fines. The onlap of these features by GarsdaleLimestone wackestones which include 10 por- several reflection wavelets suggests thatthe depositional cellanoushorizons, indicative of periodicemergence height (topset to bottomset) of some of the bodies may have (Wilson & Cornwell 1982). The upper part of the Garsdale beenas much as 300-500m. Active rifting is therefore Limestone is equated with the earlyAsbian Potts Beck inferred to have taken place, to generate thefault scarp and Limestone in Ravenstonedale by Strank (1982). basin topography necessary to accommodate fan geometries The late Asbian of the Askrigg Block is dominated by of this scale. carbonates, the initial‘Yoredale’ clastic pulses not having Within the basinal sediments of this suspected early rift reached south of the Stainmore Basin (Burgess & Mitchell package, a number of high amplitude reflectors are seen. 1976). TheDanny Bridge/KingsdaleLimestone does These may represent reflections off high velocity units such however include bioturbated or karstic surfaces suggesting as volcanics or evaporites, or possibly (dolomitized) cyclical fluctuations of relative base level and periodic carbonates or coals. Volcanic rocks, evaporites and coals emergence. The Brigantian stage is marked by ‘Yoredale’ have not beenrecorded at thisstratigraphic level in the cycles over the Askrigg high, clastic-starved tothe limited pre-Arundian correlatives on the Alston Block or on south-east, with shallow shelf sea limestones locally the basin periphery to the south or west. By analogy with including crinoidal ‘built-ups’ (Dunham & Wilson 1985). the Northumberland Basin, where basal (early rift) volcanic rocks are seen atoutcrop (Gawthorpe et al. 1989), the favoured interpretation of these strong events is that they New seismic and well evidence from the Stainmore may represent a limited input of rift volcanics tothe Basin basin. Newly available commercial seismic data have significantly The seismic character of the basinal equivalents of the expanded our knowledge of the scale and character of the Ashfell Sandstone and the upper parts of the Ravenstone-

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within the 600-700 m sequence which was deposited in the north-central part of the basin. This substantial thickness of Holkerian strata in the basin compares to only 30 m on the block to the north, supporting the structural observation of normal faulting continuing (or recommencing) during this depositional episode. Holkerian-Asbian siltstones with occasional limestones and coarsening up and ungradedsandstone bodies, up to 21 m thick, are recorded in the deep well Seal Sands No. 1 (Fig. 1). Coal shows are also recorded within the sequence, leading to these deposits being related to the similar facies association of the time-equivalent Scremerston Coal Group in the Northumberland Basin (Fowler 1926). In contrast, no terrigenous Scremerston equivalents have been identified on the highs aroundthe StainmoreBasin, this anomaly coinciding with the apparent absence of the early Asbian on the Alston Block (Burgess & Holliday 1979). The basinal m Shelf/shallowrnarine LowerPalaeozoic carbonates m basement t sequence reaches 300 m in thickness, and onlaps and pinches out northwards onto an erosional unconformity along the o 10 20 km U northern margin of the basin (Fig. 2). It is not evident whether the depositional discontinuity on the footwall was Fig. 5. Palaeogeographic summary map of the Stainmore Basin and produced by (a) a eustatic sea level fall, so that surrounding areas during depositionof the Ashfell Formation. sedimentation was intially limited tothe moresoutherly Arrows indicate measured and inferred palaeocurrentdirections. basinal low and the high tothe north was subjected to subaerialerosion, or whether (b) astructural inversion (tectonic uplift) of the Alston Block and most northerly daleLimestone (Table 1) show reflectors of varying parts of the basin produced the same effects. continuity and amplitude. Upto several hundredmetres The continuous Melmerby Scar/Great Scar Limestone (poorly-defined) of mudstones and sandstones, with with its southerly equivalents (Table 1; Burgess & Mitchell occasional thin carbonates, are ascribed to this stratigraphic 1976; Wilson & Cornwell 1982) marksa regional level in the basin. Thatthere was only minor onlap of transgression over basin and blocks alike. A regional Ashfell sandstones onto the Alston and Askrigg highs or the relative base-level rise is thus invoked,although whether Ravenstonedale shelf to the west, suggests that a subsidence this was eustatic in origin or due to thermal subsidence in a differential between the basin and its surrounding structural period of rift inactivity and low sediment influx, is not clear. margins continued through this episode of deposition, i.e. Figure 6represents the variationin seismic facies of the rifting was still active. Or, the sequence may represent a Melmerby/Great Scar reflector across the Alston Block period of relatively subdued seismicity, the basinal sequence margin into the basin. The thick (35-52111) limestone over being essentially a passive infill of a pre-existing topography. theAlston Block, confirmed in outcropand inboreholes Seismic configurations describing sigmoidal clinoforms (Woolacott 1923; Dunham et al. 1965), is interpreted to thin are observedprograding southwards fromthe northern into the basin, south andeast of the Butterknowle, basin margin within the Ashfell-equivalent depositional Wigglesworth, Lunedale and Thornthwaite Faults (Fig. 1). sequence (Fig. 2). The high-amplitude top reflector would This is indicated by the subtle increase in frequency of the be consistent with these representinga carbonate rim reflection event signal to a high amplitude singlet. Over development, although a deltaic progradational clinoform is intrabasinal highs, repeated high amplitudeevents are notdiscounted. The possible relationshipbetween the interpreted as interbedded limestones and shales. The high depositionalsequence in the basin andthe carbonate- reflection coefficients of the limestone-shale contacts would dominated succession of ?Chadian/Arundian age in the explain the high amplitude signal character. These reflectors Ravenstonedale area is represented by the palaeogeographic onlap onto previously rotated tilt block surfaces, signifying model in Fig. 5. This proposes that a carbonate shelf margin an episode of (renewed) fault activity and basinal subsidence may have existed towards the western end of the Stainmore immediately prior to accumulation of theGreat Scar Basin, forming transitiona from the basinal shale- Limestone. dominatedsequence (which includessome sandstones), of The late Asbian-Brigantian Alston Group above the unknown water-depth, into the shallow marine carbonates Great Scar Limestone (Table 1) increases in thickness into of the RavenstonedaleLimestone. Such a facies division, the basin, from 450mto 700m across the Wigglesworth dueto an eastwardsdeepening of water depth, would Fault. These deposits are widely exposed in western parts of explain the absence of clastic rocks in the Ravenstonedale the study area and form the ‘Yoredale’ shallow marine to area through much of the Chadian to Arundian. fluviodeltaic cyclothems (Burgess & Mitchell 1976; Leeder Faulting produced tilting andonlap geometries onto & Strudwick 1987). Renewedtectonic subsidence may be intrabasinal fault blocks, imaged in seismic sections, within invoked to explain this southwards thickening, althougha the subsequent Holkerian HillbeckLimestone Formation proportion at least of the thickness variation will be due to (approximately equivalent to depositional sequence 3, Figs 2 differential rates of sedimentary compaction over blocks and and 4).Laterally extensive (basinwide) high amplitude basin. This is because there was a much thicker underlying reflectors characterize the sequence. These are interpreted column of sediment to compact in the basin compared with as carbonate units,a hypothesis confirmed by well data, over the blocks (Weller 1959; Collier 1989). Deeply incised

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LOWFREQUENCY SINGLET /DOUBLET HIGHAMPLITUDE MO DER ATE -HIGH AMPLITUDE SINGLETMODERATE-HIGHAMPLITUDE CONTINUOUSEVENTS CON TINUO US HIGH AMPLITUDE HIGH CONTINUOUS CONTINUOUS THICKCARBONATE THICKCARBONATES MOD. THICKNESSCARBONATE BASINAL INSHALES INBASINAL SHALES

Fig. 6. Diagrammatic summaryof seismic facies and inferred lithofaces variation of the late Asbian Melmerby Scar Limestone (on the Alston Block) and Great Scar Limestone (in the Stainmore Basin). Faults represent basin-bounding and intrabasinal normal faults. Additional high amplitude reflectors are interpretedas further thick carbonates accumulated over intrabasinal highs. Not to scale.

fluvial channels are found in late Asbian ‘Yoredales’ on the Discussion and regional implications Alston Block and also south of the Swinedale Beck Fault (See fig. 24 in Burgess & Holliday 1979). This implies that In summary, hitherto unavailableexploration seismic has the Swinedale Beck Fault was not an influence on facies at demonstrated6 km thick Dinantiansequence in the this time, but also that a marked relative fall in base level Stainmore Basin. About 4.1 km of ?Courceyan- must have occurred to induce the incision event onthe Chadian/Arundianstrata occur in the basin, whilst this footwall block. Metre-scalesoft-sediment contortions are interval is barely represented on the Alston Block to the seen within Late Asbian channel sandstones 2 km south of north or the Ravenstonedale shelf area to thewest. Arundian the Swinedale Beck Fault, suggesting active syn-depositional andHolkerian strata thicken by anorder of magnitude seismicity did continue into orrecommence during this stage into the basin. A sequence showing similarities tothe (after Leeder 1987). Scremerston Coal Group in Northumberland, unrepresented The late Asbian-Brigantian ‘Yoredale’sequence in- on the Alston Block, reaches up to300 m in thickness in the cludes a variety of fluvio-deltaic and shallow marine north-central Stainmore Basin. Equivalents on the Raven- sandstonebodies (Strudwick 1987). Individual ‘Yoredale’ stonedale shelf area to the west and the Askrigg Block to cyclothems are too thin to be resolved as separate reflections the south consist of carbonates deposited under a variety of or packets of reflections in the available seismic data. tidal flat to shallow marine shelf conditions. However, onthe basis of the lateralvariation in seismic Two important points arise from the seismic data. First, character within the sequence (Fig. 7), some inferences may that the Stainmore Basin contains a substantial rift-related be madeabout the lithofacies variation.A relatively Dinantiansequence which is distinct from the previously reflective package with moderatelycontinuous higher described 1.5 km thick succession in Ravenstonedale amplitude events is seen in the hangingwall tothe (George et al. 1976). Second, thatthe scale and style of Wigglesworth and ButterknowleFaults (Fig. 7). This may sedimentary thickness and facies changes into the basin vary be explained by ‘Yoredale’deltaic sandstone bodies being significantly around its northern, westernand southern located preferentially in the immediate hangingwall to basin margins. The Alston footwall to the north was onlapped by margin faults. The 40% thickening of the clastic members of only a limited thickness of Dinantian sediment prior to the the Smiddy and Grain Beck cycles southwardsacross the late Asbian, and includes a discontinuity which may signify a LunedaleFault (Fig. l), described by Burgess & Mitchell localized Holkerian and/or earlyAsbian inversion event. (1976), provides an outcrop example of this situation. The The Lower Palaeozoic basement in Ravenstonedale was preferential location of thesesandstone members against downfaulted in the ?Courceyan to early Asbian relative to basin margin faults indicates the existence of a structurally- the Alston Block, but remained a shelf area characterized controlled water-depth differential across the basin margin by shallow marine carbonate accumulation to the west of the when these fluvio-deltaic deposits prograded southwards basinal depocentre. ThisStainmore depocentre apparently into thearea. Within the basin itself, theopaque seismic captured the bulk of terrigenousmaterials reaching the facies of the sequenceindicates thatthe ‘Yoredales’ are Stainmore Basin fromsourcelands of Caledonian to probably shale-dominated over intrabasinal highs. mid-Proterozoic age to the north or north-east (Cliff et al.

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LOW-MOD.AMPLITUDE CONTINUOUSEVENTS VARIABLE AMPLITUDE OCCASIONAL LOW AMPLITUDE MODERATELY TO CONTINUOUS CONTINUOUS SINGLETSIDOUBLETS EVENTS

SUB-SEISMIC SCALE SHALE-DOMINATED ‘YOREDALE’ CYCLES THIN CARBONATES (SUB-SEISMIC) ‘YOREDALE’ TYPE LITHOFACIES

CONTINUOUS CARBONATES & LESS CONTINUOUS SST. BODIES (DELTAIC)

Fig. 7. Diagrammatic summary of seismic facies and inferred lithofacies variationof late Asbian to Brigantian ‘Yoredales’off the Alston Block into the Stainmore Basin and onto intrabasinal highs. Not to scale.

1991). The Askrigg Block was progressively onlapped from scale clinoforms against normal faults. Rift subsidence may thenorth, through theChadianIArundian, by shallow have been“instantaneous”, in which case the sequence marine carbonates which show affinities to the Ravenstone- would then be a passive infill, the internal geometry being dale succession. All areas, including the granite-cored determined by sediment influx rates versus base level in the Alston and Askrigg Blocks,were transgressed by late timeinterval. Or alternatively, rift subsidence rates may Asbian times, with consequentcarbonate deposition and have beenconstant in the Courceyan toArundian, with then rhythmic ‘Yoredale’ sedimentation across the region. accommodation space and internal geometries having been The differential subsidencebetween blocks and basins controlled by a different history of sediment influx rates and through theDinantian in northern England has been base level change. The occurrence of 300-500m high attributedto active rift subsidence (Leeder 1982). This clinoforms might thendenote the renewed influx of contrasts with a more regional thermal subsidence which sediment after a period of non-deposition, as opposed to an affected the area through the Namurian and much of the episode of enhanced rift activity. Clearly on geological and Westphalian. In detail, however, it is not a simple matter to geophysical grounds,some intermediate scenario is the specify the nature (variability) of active extensional faulting more likely. But without an independent line of evidence to within the Dinantian. Gawthorpe (1986), Gawthorpe et al. constrain one or more variables, caution should prevail in (1989) and Ebdon et al. (1990) explore means of assessing any attempt to evaluatesubsidence rates when only poor the relative intensity of rift activity through time in various chronostratigraphic constraint is available. Dinantian basins. But, as demonstrated by theStainmore The same problem arises when trying to establish the Basin example,ambiguities remain when dealing with a tectonic context of the Ashfell Formation and equivalents. multi-variate system in which depositionalsequence Differential subsidence clearly segregateda basinal de- geometries and thicknesses are afunction of changesin pocentre, which capturedthe bulk of any terrigenous sediment influx rates as well as tectonic subsidence rates and sediment input, from carbonate-dominated relative highs to eustacy. Todemonstrate this point, two end-member the west and south. The sediment influx rate into the basin explanations may describe the evolution of features may, however, have been largely determined by subsidence observed on seismic sections in the early rift sequence of the ratesin fluvial-dominated basins furtherup the drainage Stainmore Basin. The 4.1c. km of ?Courceyan to pathway, such asthe Northumberland Basin. When such ChadianIArundian sediment are interpreted toinclude large basins subsided more rapidly andcaptured clastics to the

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north or northeast, the Stainmore depocentre would have -, SWINEURN,P. M. & LONG,R. E. 1984. Deep structure and origin of the been starved of sediment. Northumberland and Stainmore Troughs. Proceedings of the Yorkshire By late Asbiantimes, when the region was inundated Geological Society, 44, 479-495. BURGESS,I. C. & HARRISON,R. K. 1%7. Carboniferous Basement Beds in across blocks and basins alike, with accumulation of the the Roman Fell district, . Proceedings of the Yorkshire Great Scar and equivalent limestones, it is not possible to Geological Society, 36,203-225. distinguish between this being due toa eustatic sea level rise - & HOLLIDAY,D. W.1979. Geology of the country around or to the cessation of tectonic subsidence and the onset of a Brough-under-Stainmore. Memoir of the GeologicalSurvey of Great Britain, 31/25, 30. more regional thermal subsidence phase. A similar dilemma -& MITCHELL,M. 1976. Visean LowerYoredale limestones on the Alston exists as to whether the discontinuity on the Alston Block and Askrigg Blocks, and the base of the D, zone in northern England. coeval with Scremerston Coal Group deposition in the basin Proceedings of the Yorkshire Geological Society, 40, 613-630. is a response to a eustatic sea level fall or to a structural CLIFF,R. A., DREWERY,S. E. & LEEDER,M. R. 1991. Sourcelands for the inversion event. Coal shows in the Seal Sands no. 1 well at Carboniferous Pennine riversystem: Constraints from sedimentary evidenceand U-Pb geochronologyusing zircon and monazite. In: this level imply sedimentation(periodically) infilled the MORTON,A. C.,TODD, S. P. & HAUGHTON,P. D. W. (eds) basin to base level at this time. The c. 300 m magnitude of Developments in Sedimentary Provenance Studies. GeologicalSociety, the relative base level change, as implied by the magnitude London, Special Publication, 57, 137-159. of the erosional unconformity, leads this author to prefer an COLLIER,R. E. LI. 1989. Modelling the role of differential compaction and tectonicsupon Westphalian facies architecture in the Northumberland origin due to the short-lived uplift of the footwall and most Basin. In: GWITERIDGE,P., ARTHURTON,R. S. & NOLAN,S. C. (eds) The northerly areas of the Stainmore hangingwall. Role of Tectonics in Devonian and Carboniferous Sedimentation in the The competition between rates (of change) of tectonic British Isles. Yorkshire Geological Society, 189-199. subsidence, sea level and sediment influx in determining -, LEEDER,M. R. & MAYNARD,J. R.1990. Transgressions and regressions: a model for the iduence of tectonic subsidence, deposition depositionalsequence geometries is evaluated forthe andeustacy, with application to Quaternary and Carboniferous Carboniferous in Collier et al. (1990). Given the typical examples. Geological Magazine, 127, 117-128. transverse asymmetry of extensional basins andtheir DEWEY,J. F. 1982. Plate tectonicsand the evolution of the BritishIsles. along-axis variation in subsidence rates (fault displacements Journal of the Geological Society, London, 139, 371-412. DODD,C. D. & GAWTHORPE,R. L. 1991. Observations on sedimentation being offset en echelon or by transverse faults), it is to be patterns in arid to semi-arid rift basins. Geological Society of America expected that facies changesacross a basin’s margins will Bulletin, (in press). vary spatially,as well astemporally, as tectonic activity DUNHAM,K. C. & WILSON,A. A. 1985. Geology of the Northern Pennine evolves. Thecontrasts sedimentationin throughthe Orefield, vol. 2, Stainmore to . Economic Memoir of the British Geological Survey. Dinantian between the Ravenstonedale shelf and the Alston -, DUNHAM,A. C., HODGE,B. L. & JOHNSON,G. A. L. 1965. Granite and Askrigg Blocks are therefore likely to be typical of the beneath Visean sediments withmineralization at Rookhope, northern range of facies changes to be foundaround other Pennines. Quarterly Journal of the Geological Society of London, U1, extensional basins in the Carboniferous and elsewhere. 383-417. Simple two-dimensional models of facies variationin ELIDON, C.C., FRASER,A. J., HIGGINS,A. C., MIXHENER,B. C. & STRANK, A. R. E. 1990. The Dinantian stratigraphy of the EastMidlands: a extensional basins are therefore rarely going to be adequate seismostratigraphic approach. Journal of the Geological Society, London, representations of reality. 147, 519-536. The construction of depositional sequences, outlined in FOWLER,A. 1926. The Geology of Berwick-on-Tweed,Norham and Figs 2 and 4 for the Stainmore Basin, offers a manageable Scremerston. Memoir of the Geological Survey of Great Britain, 1, 2. GAWWORPE,R. L. 1986. Sedimentation during carbonate ramp-to-slope framework within which to describe the lateral variation in evolution in a tectonicallyactive area: BowlandBasin (Dinantian), facies distributionacross some time-slice. Theinterpreta- northern England. Sedimentology, 33, 185-206. tion of facies patterns within any depositionalsequence -, GUTIERIDGE,P. & LEEDER,M. R. 1989. Late Devonian and Dinantian ideally becomes an iterative process between seismic facies basin evolution in northern England and North Wales. In: GUTERIDGE, P., ARTHURTON,R. S. & NOLAN,S. C. (eds), The Role of Tectonics in analysis and any additional well data as and when it Devonian and Carboniferous Sedimentation in the British Isles. Yorkshire becomes available. Geological society, 1-23. GEORGE,T. N.,JOHNSON, G. A. 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Received 1 February 1990; revised typescript accepted 2 August 1990.

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