From marine bands to hybrid flows: Sedimentology of a Mississippian black shale

Organic‐rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic‐rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron‐scale sedimentological analysis reveals a heterogeneous succession of carbonate‐rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth‐order eustatic sea‐level fluctuation and localized block and basin tectonism. Marine transgressions deposited bioclastic ‘marine band’ (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens‐rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early‐cemented lenses and in plan‐view, and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin’ conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip‐up fragments in the down‐dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter.


INTRODUCTION
Syn-rift to early post-rift epicontinental seaways, such as the Mississippian Rheic-Tethys, were distinct from 'static' interior seaways such as the Cretaceous Western Interior Seaway (Schieber, 2016). These extensional epicontinental marine settings are often associated with the accumulation of organic-rich sediments (Ettensohn, 1997) and are therefore important sinks in terms of element cycling and hydrocarbon prospectivity. Yet the sedimentary processes operating in epicontinental rifted seaways are spatially and temporally transient (Cope et al., 1992) and typified by highly variable sediment supply rates, with potential for development of highly complex successions.
To investigate mechanisms of sediment input, transport and deposition, sedimentological observations at field/core to micron-scale were integrated with geochemical proxies for autochthonous [total organic carbon (TOC), carbonate and silica] and allochthonous components (Ti and Zr). This analysis was conducted for three time-equivalent sites in the Craven Basin, UK (Kirby et al., 2000), a basin with ongoing unconventional hydrocarbon exploration (DECC, 2016;Clarke et al., 2018). This study shows that organic-rich sediments accumulated in a dynamic environment host to pelagic to hemipelagic deposition and a variety of subaqueous density flows, moderated by fourth-order eustatic sea-level fluctuation, delta progradation and slope instability, and under variable bottom water salinity. This complexity means that sedimentary packages are interpreted in terms of changing basin accommodation rather than strictly within a sequence stratigraphic framework. Sea-level fall enhanced a shelf to basin bedload conveyor of sand to gravel-sized partially-consolidated mud clasts, as modelled by Schieber et al. (2010), which was remotely linked to a mud-rich delta system. The current study contributes to the debate (Schieber, 1994) that shows accumulation of organic-rich sediments is not necessarily coupled to the depositional 'energy' of bottom waters (sensu Tissot & Welte, 1978). These findings are important for understanding sedimentary processes and controls on organic carbon burial in epicontinental basins (Berner & Canfield, 1989;Schieber, 2016) and understanding hydrocarbon resource potential (Aplin & Macquaker, 2011).
In the earliest Pendleian, basinal highs and lows supported highly localized 'platform and ramp carbonate' and 'hemipelagic' regional facies belts (Fig. 1A), respectively (e.g. Dean et al., 2011). This represented a short-lived inheritance of the Lower Bowland Shale carbonate to mixed sedimentary system (Newport et al., 2018). Mud mounds, patch reefs and/or similar carbonate accumulations likely fringed the Craven Basin during this early stage of post-rift fill ( Fig. 2A; e.g. Cope et al., 1992;Waters et al., 2009;Dean et al., 2011). This included localized deposition of Wendsleydale Group crinoidal limestones, and brachiopod-mollusc-rich Sugar Loaf Shales and Sugar Loaf Limestone. These accumulated along the south-west margin of the Askrigg Block on a series of relay ramps across the Craven Fault Zone ( Fig. 1) (Arthurton et al., 1988;Fairbairn, 2001). The Bowland High, in addition to other intrabasinal highs such as the Ashnott High, also hosted localized carbonate accumulations (Riley, 1990;Brandon et al., 1998).
The Upper Bowland Shale includes four index marine bands; E 1a1 to E 1c1 (e.g. Brandon et al., 1998) spanning ca. 400 ka (Waters & Condon, 2012). Multiple flooding surfaces are recognized for E 1a1 (a, b and c) and E 1b2 (a and b) marine bands (Fig. 1C), potentially linked to sub-100 ka precession or obliquity forcing (Waters & Condon, 2012): E 1a1 and E 1c1 are also thought to represent peak highs associated with eccentricity (Maynard & Leeder, 1992;Waters & Condon, 2012). Marine band cyclicity in the Namurian was likely a response to far-field icesheet volume on Gondwana (Veevers & Powell, 1987). Marine band cycles are possibly superimposed onto 1Á1 to 1Á35 Ma duration Namurian 'mesothem' cycles (Ramsbottom, 1979), which  Lazar et al. (2015). Biostratigraphy from Riley (1988). See Materials and methods for a description of methods, including sedimentary classification scheme and derivation of excess Si. Approximate mud lens (intraclast) diameter (Ø) and abundance estimated for thin sectioned samples only. HB = high basin accommodation, DB = decreasing basin accommodation, LB = low basin accommodation, IB = increasing basin accommodation. See Discussion for further details and Fig. 3

The Pendle delta system
The inherited carbonate to mixed syn-rift system was gradually inundated by siliciclastic sediments ultimately attributed to the 'millstone grit' (deltaic) regional lithofacies (Fig. 1B). Siliciclastic sediments were supplied primarily from advancing fluvio-deltaic and turbiditic systems, linked to the Pendle delta system in the north to north-east (Pendleton Formation; Waters et al., 2009), across and around the Askrigg Block (Arthurton et al., 1988;Martinsen, 1993;Fraser & Gawthorpe, 2003;Kane, 2010). Presence of relatively large distributary channels towards the north of the Askrigg and Alston Blocks, such as the 50 m deep Rogerley Channel, suggest that the Pendle delta likely discharged a large volume of freshwater particularly during flood events (Dunham, 1990;Waters & Condon, 2012). Despite proximity to the Pendle delta, fault activity likely maintained sharp and steep basin margins (i.e. probably 2 to 10°) rather than ramps (Collinson, 1988;Martinsen et al., 1995), which were likely prone to failure (Talling, 2014).

MATERIALS AND METHODS
Three localities in the Craven Basin were selected for analysis; the outcrop at Hind Clough (grid ref: 364430 453210, British National Grid projection) and boreholes Marl Hill 4 (MHD4) (367426 446752) and Cominco S9 (383090 463300) (Fig. 2B). Hind Clough and MHD4 are located on a basinal low and high, respectively. Cominco S9 is located on the north-east basin slope proximal to the Pendle palaeodelta. The stratigraphic framework was based on ammonoid biozones identified by Brandon et al. (1998), Riley (1988) and Arthurton (1972), respectively. The ca 124 m thick section at Hind Clough is exposed as a stream-cut and         Facies descriptions based on composition and texture. Grain size, including mud lenses, is denoted by square brackets [.
. .] (sensu. Lazar et al., 2015). weathered slope, located on the edge of the Ribblesdale Fold Belt (Fig. 2B). This is a set of north-east/south-west trending folds, thrust folds and monoclines that developed during Variscan inversion of the basin (Arthurton, 1984). True thicknesses were estimated using the dip and strike of bedding with an Abney level. Sampling was primarily conducted directly within the stream section, in order to minimize the effect of chemical weathering, as described by Emmings et al. (2017c). Where stream sections were poorly exposed, logging and sampling was conducted on the adjacent slope. Samples were screened for alteration using visual assessment and site-specific geochemical proxies for weathering, including cross-plots of Cs/Cu and oxygen index used as a local weathering proxy (Emmings et al., 2017c). 'Unaltered' samples are defined with respect to the sedimentological and geochemical analyses applied in this study. Field/borehole and sample descriptions, including estimates for grain size, composition and bioturbation index, was based on the Lazar et al. (2015) method. This included generation of a ternary compositional plot (Appendix S1). Several facies are 'lenticular'; this is defined as a mud lens-rich (intraclastic) texture within an argillaceous clay-sized matrix. Thus, in terms of grain size (sensu Lazar et al., 2015), several facies are described as 'sandy' mudstones to 'sandstones'. In terms of composition, these facies are primarily 'argillaceous mudstones'. Additional thresholds of 'weakly' and 'moderately' used to indicate mud lenses, carbonate and/or quartz abundances of >2Á5% and >10% of the bulk composition, respectively. 'Strongly' is used to indicate mud lens abundance of >50% of the bulk composition.
Sedimentary facies were defined at the bed scale, typically millimetres to centimetres in thickness. 'Mud-cap' laminae are defined as mud-rich layers, typically microns to millimetres-thick, which lack internal structure and exhibit uniform composition and texture. Strata between successive marine bands are not necessarily genetically linked (Hampson et al., 1997) and were also subject to local factors such as differential subsidence and changing sediment supply rate (Holdsworth & Collinson, 1988;Galloway, 1989). The bases of marine bands are the most prominent and laterally widespread surfaces, whereas erosion surfaces are laterally discontinuous and may be poorly defined in basinal settings. Therefore, packages are interpreted in terms of changing basin accommodation rather than strictly within a sequence stratigraphic framework (e.g. Posamentier et al., 1988). Sedimentary packages are defined on the basis of increasing, high, decreasing and low basin accommodation (IB, HB, DB and LB, respectively). These stacking patterns may equate to transgressive, highstand, falling stage and lowstand systems tracts of Posamentier et al. (1988), respectively. One hundred and ten samples were selected for geochemical analysis (including nine unaltered samples described by Emmings et al., 2017c), with 37 subsamples thin sectioned. Thus, the confidence of grain-size estimates is robust for samples calibrated with thin sections. Samples were selected in order to attain appropriate temporal and spatial coverage across all sedimentary facies. Whole rock powder X-ray diffraction (XRD) data were collected on a Bruker D8 Advance Powder Diffractometer equipped with a LynxEye Position Sensitive Detector (Bruker, Billerica, MA, USA) with a Bragg Brentano, flat plate h-h geometry using CuKa radiation (see Appendix S1 for data example). Scanning electron microscopy (SEM) was conducted on uncoated ultrathin sections using an S-3600N Hitachi microscope (Hitachi, Tokyo, Japan) with an Oxford INCA 350 energy dispersive spectrometer (EDS) (Oxford Instruments, Abingdon, UK). Electron microphotographs were acquired using backscattered electrons (BSE). False colour composite images were compiled using Fiji (ImageJ) software and are overlain on each corresponding BSE microphotograph. Element maps (using SEM-EDS) were mapped to red (R), green (G) or blue (B) channels. Total sulphur (S) was determined using a LECO CS 230 elemental analyzer (LECO Corporation, St. Joseph, MI, USA). X-ray fluorescence (XRF) data were acquired on fused beads (109 samples) and powder briquettes (108 samples) with a PANalytical Axios Advanced XRF spectrometer using default PANalytical SuperQ conditions (Malvern PANalytical, Malvern, UK). Pyrolysis was conducted using a Rock-Eval 6 TM apparatus (Vinci Technologies, Nanterre, France) at the British Geological Survey.
Titanium and zirconium are utilized as proxies for the detrital fraction, particularly heavy minerals (Hild & Brumsack, 1998). 'Excess silica' is defined as: Si excess = Si total À(2Á5 9 Al total ), sensu Sholkovitz & Price (1980), for samples where Si total /Al total > 2Á5 (i.e. ca 2Á5 is the local detrital Si total /Al total , using Facies G to I). Excess silica is used as a proxy for the biogenic (radiolarian) Si pool. Siliceous mudstones are silica-rich and contain abundant authigenic quartz cements. Siliciclastic mudstones contain quartz that is considered to be dominantly detrital rather than authigenic in origin. A full description of methods is provided in Appendix S2. Data are available via open access (Emmings et al., 2017a,b).

RESULTS
Sedimentary logs for each basin position indicate that the Bowland Shale comprises limestone, and carbonate-bearing, siliceous and argillaceous mudstones (on the basis of composition; Figs 3 to 7; Appendix S1). Ten sedimentary facies (A to J; Table 1) were defined using observations at field/core-scale through to scanning electron microscopy (Figs 8 to 15). Facies A to J are classified and ordered along a spectrum of decreasing carbonate and/or biogenic silica, and increasing siliciclastic content (Table 1; Fig. 3). Facies A to F lack bioturbation (bioturbation index = 0; based on Lazar et al., 2015). Facies A limestone is the carbonate end-member. Facies B and C are lenticular mudstones with an increasingly diminished carbonate component and moderate to high Si excess. Facies D and E exhibit highly variable, but typically low, carbonate content and moderate to high Si excess. Facies D deposits are typically wellbedded, well-sorted and moderately to highly carbonate-bearing. Facies E deposits are typically chaotic, poorly sorted and weakly carbonate-bearing. Facies F contains minimal carbonate and exhibits variable Si excess. Facies G lenticular mudstones contain trace carbonate and lack excess Si. Facies H to J represent the siliciclastic fine (H) to coarse (I) mudstone and sandstone (J) end-members. Figures 9 to 15 are ordered to match the Facies A to J spectrum.

Limestones
At outcrop, Facies A beds are competent and blocky, with slightly diffuse upper and lower contacts. Facies A comprises >90% calcite cement (Figs 4, 5 and 9A) present as discrete, poorly developed spherules, and is therefore defined as spherulitic limestone (Table 1). Calcite spherules are commonly fan-shaped and slightly elongate, and may radiate from a central nucleus, with individual spherule diameters (Ø) between 50 lm and 200 lm (Fig. 9A). Spherule long axes are typically perpendicular to discontinuous, wavy parallel organic-rich laminae. Widely spaced, sub-vertical wavy calcite veins (ca 10 to 20 lm thick) cross-cut calcite spherules and laminae. Extinction of each spherule under cross-polarized light is uniform. Facies A is interbedded with Facies B and C (Figs 4 and 5). Facies A samples exhibit very low total organic carbon (TOC), low silica excess, very high inorganic C and low Ti and Zr content (Fig. 4).

Carbonate-bearing and siliceous mudstones
Facies B and C are blocky to flaggy at outcrop (Fig. 8A), where moderately cemented beds are readily distinguished due to greater resistance to modern weathering ( Fig. 8B and C). Facies B (Figs 10A to F and 11A) and C ( Fig. 11A to F) exhibit a homogenous ( Fig. 10C and D) and rarely deformed (Figs 10E and 11A) sedimentary fabric at the centimetre-scale. Facies B and C are typically weakly to moderately lenticular (Table 1; Fig. 10C to E) at the micron-scale and are compositionally defined as weakly to moderately carbonate-bearing, moderately siliceous, argillaceous mudstones. Planar to slightly wavy, parallel, homogenous laminae comprising fine mud-sized grains (dominantly clay; 'mud-caps') are rare to moderately common in Facies B and Contrasting responses to weathering (competent and fissile) demonstrate interbedded carbonate-cemented (strataform dolomite) and poorly cemented lithologies from the E 1a marine band. (C) Despite compositions prone to weathering, stream sections expose unaltered material. The stream confluence (black arrow) was used as one of several anchor points during logging and sampling, at 32Á0 m above base. (D) Erosional base (white arrows) to hybrid mass transport deposit (Facies E), E 1a biozone at 48Á8 m above base. Sample 73A is from the underlying mudstone (mud-cap) and sample 75A is from the basal part of the bed. (E) Extensive slope exposure spanning the E 1a to probable E 1b1 biozones. Relatively planar, laterally extensive hybrid mass transport deposits crop out across the slope. Sedimentary logs with interpreted basin accommodation are superimposed. Length of pen and hammer are 0Á15 m and 0Á30 m, respectively. The true vertical thickness of the visible outcrop section in (C) is ca 10 m. Mixed hybrid beds and calcareous bioclastic lenticular mudstones~1 0 cm C ( Fig. 10A and B). Carbonate, quartz and clays are present in approximately equal proportions (Appendix S1). Facies B and C samples typically exhibit moderate to high TOC, moderate to high silica excess, moderate inorganic C and low to moderate Ti and Zr content (Figs 4 to 7). Quartz cement in Facies B and C (for example, Fig. 9B) is apparently widespread compared to detrital quartz grains. Bioclasts in Facies B include body fossils such as goniatites, trilobites, posidoniid bivalves, mollusc spat and fish debris (for example, Fig. 10F), and abundant mud to sand-sized shell fragments (for example, Fig. 10C). Bioclasts and shell fragments are often replaced and mantled by finely crystalline calcite cements (including scattered dolomite; Fig. 9C). Facies B also contains rare dispersed single calcite spherules (rounded, spherical, ca 30 lm Ø) with sweeping extinction. Phosphatic concretions and lenses in Facies B and C tend to exhibit a 'birds-eye' appearance (sensu Selley, 2000, after Illing, 1954 with long axis lengths ranging from 500 lm to several millimetres ( Fig. 9D to F). Phosphatic concretions comprise amorphous organic matter (OM) and bioclasts (including abundant radiolarian tests) rafted in a mixed phosphate and calcite cement (Fig. 9F). Phosphatic lenses contain mud lenses (Fig. 11B). Facies B mudstones in the E 1b2-b biozone also contain siderite nodules (typically 0Á1 to 0Á5 m Ø, which are spherical to sub-spherical; Fig. 6). Facies C lacks bioclasts but contains finely crystalline carbonate cement.
Facies D and E are blocky at outcrop, and where beds are sufficiently thick, these commonly form conspicuous beds on the slope ( Fig. 8D and E). Facies D and E are moderately to strongly carbonate-bearing, and weakly to moderately carbonate-bearing, respectively. Both are typically moderately siliceous, argillaceous, coarse mudstones (i.e. equivalent to siltstones; Lazar et al., 2015) and rarely sandstones (Table 1). Facies D is often wavy parallel-laminated towards the base of each bed and homogenous towards the top of each bed ( Fig. 10G and H). Laminae often exhibit erosive bases and 'load and flame' contacts with the underlying mudstone (Figs 10G and 15A). Laminae comprise calcareous bioclastic fragments (typically coarse mud to fine sand-sized; <200 lm Ø), detrital quartz and are cemented with carbonate and silica (Fig. 10H). Rarely laminae are cemented with euhedral pyrite. Quartz, carbonate and clays in Facies D and E account for approximately >40%, <20% and <40% of the bulk composition, respectively. Facies D and E samples typically exhibit low to moderate TOC, high silica excess, variable inorganic C and moderate to high Zr and Ti content (Fig. 5).
Facies E typically exhibits chaotic textures towards the base of beds, including deformed laminae (Fig. 12A). Towards the top of each bed, Facies E exhibits homogenous 'starry-night' texture (i.e. 'mud-caps', sensu Fonnesu et al., 2017) (Fig. 12B). The centre of Facies E beds exhibits high-angle discontinuous curved and climbing cross-laminae ( Fig. 12C and D), frayed fragments of OM ( Fig. 12D and E), and abundant gravel-sized carbonate and quartz cemented clasts and lenses ( Fig. 12C to G). This sequence of textures is not always present in every bed. The carbonate and silica-cemented clasts and

R = Si G = P B = Al R = Si G = P B = Al
lenses are typically rounded and partially deformed (Fig. 12G), and are often concentrated at the base of each cross-lamina (for example, Fig. 12C). These clasts commonly contain sandsized euhedral pyrite. Some clasts represent single bioclasts replaced by carbonate and silica cement. In plan-view, coarse mud-rich (silty) laminae are curved, and the carbonate and silica-cemented clasts and lenses are rounded, equant to elongate and aligned (Fig. 12F). These clasts occasionally exhibit tapered edges (Fig. 12G) At outcrop, weathering of Facies F generates highly fissile 'paper shale' in slope sections (Fig. 8C), so that individual beds are difficult to recognize. At Hind Clough, shoaling of the inclination of bedding by ca 3° (Fig. 8D) coincides with similar inclination observed in hand specimen and thin section at the contact ( Fig. 11C to E). Facies F is defined compositionally as weakly siliceous to siliceous argillaceous mudstone and is moderately to strongly lenticular (Table 1). Quartz and clay content is highly variable but typically <40% and >60% of the bulk composition, respectively. Facies F contains trace carbonate (Figs 5 and 6) and rarely contains carbonate, pyrite or silica-replaced bioclast casts or as moulds. Facies F samples typically exhibit moderate to high TOC, low to high silica excess, very low inorganic C and moderate to high Zr and Ti content (Figs 5 and 6). Facies F is rarely interbedded with Facies B to E. Planar to slightly wavy, parallel 'mud-cap' laminae are common in Facies F.
The clay to fine sand-sized 'matrix' of Facies B to F comprises framboidal pyrite (ca 5 lm Ø), rare pyrite euhedra and sphalerite, isolated and patchy macro-crystalline to micro-crystalline silica and/or calcite cement, abundant authigenic clay minerals (including illite and/or smectite-illite clays - Fig. 9C; and 'booky' kaolinite - Fig. 9G) and scattered dolomite that partially replaces calcite when present (Figs 9B to C and 12G). Unambiguous examples of detrital quartz in Facies B to F are rare and typically limited to Facies D basal laminae (Fig. 10A), although it is possible that the widespread silica cements (for example, Fig. 9B) are cored by detrital quartz (sensu Schieber et al., 2000). The matrix of Facies F lacks carbonate and contains abundant isolated silica cement, authigenic feldspar and authigenic and detrital clay minerals (Fig. 13A to G). Deformed, wavy to crinkled micron-scale to millimetre-scale elongate to platy fragments of OM are common in Facies E and rare to common in Facies B to F (Figs 10C, 12D, 12F and 13D). Spherical (ca 20 to 30 lm Ø) particles of OM are also present in Facies B and C and common in Facies F (Fig. 13E). The OM pores in Facies B to F contain silica cement, scattered pyrite framboids and microcrysts ( Fig. 13D and E) and locally 'books' of kaolinite in Facies F (Fig. 9G). Sulphate-bearing laminae in Facies F contain Ca-rich sulphate (anhydrite or gypsum) within OM pores (Fig. 11G) but otherwise the sedimentary matrix is similar to the bulk.   Fig. 14A to F). Facies G also contains organic-rich laminae and a matrix of authigenic and detrital clay minerals (Fig. 14C to G; including authigenic 'booky' kaolinite - Fig. 9H), pyrite euhedra and nodules (0Á5 to 20 mm Ø, rounded, spherical; Fig. 9H) and detrital quartz. Facies G is typically laminated with homogenous 'mud-caps' (for example, Fig. 14C). The Facies G bulk sediment comprises approximately 90% clays, 10% quartz and trace carbonate (Appendix S1). Facies G samples typically exhibit moderate TOC, very low inorganic C and high Ti and Zr content and lack excess silica (Fig. 6). Single pyritized burrows in Facies G (Fig. 14G) exhibit individual burrow Ø of ca 50 lm to 1 mm and burrow depth up to ca 2 mm (bioturbation index = 1; Lazar et al., 2015).
Facies H and I appear homogenous or exhibit normal grading with sharp bases (Fig. 15B) and are argillaceous fine to coarse, sandy mudstones and medium sandstones (i.e. siltstones; Table 1), respectively. Facies H is typically interbedded with Facies G (at Hind Clough). Facies H and I are also typically interbedded. Facies I also exhibits sand-filled scours along basal contacts (Fig. 15B) that are partially comparable to basal laminae in Facies D (Fig. 15A). The matrix of H and I comprises fine to coarse mud-sized detrital quartz, feldspar, chlorite, muscovite mica, other fine clay minerals (kaolinite, illite group), terrestrial OM and rare euhedral pyrite and is weakly siderite-cemented (Fig. 15C). Micas, clays and heavy minerals typically account for >60% of the bulk composition, with subordinate detrital quartz. Facies H and I also occasionally contain organic-rich laminae comparable to Facies G ( Fig. 15E and F). Facies H and I exhibit low to moderate TOC, low inorganic C and high Ti and Zr content (Fig. 7) and lack excess Si.

Lenticular fabrics and grain size
Mud lenses in Facies B, C and F typically lack the pyrite framboids and OM that are present in the matrix, and comprise abundant kaolinite and subordinate illite, mica, quartz and feldspar (for example, Fig. 13B to F). Mud lenses in Facies G also lack OM that is present in the matrix. Mud lenses in Facies B, C, F and G are flattened in bedding-perpendicular sections (Figs 10C to E, 11B, 13A, 13B, 13G, 14A and C to E) and equant and rounded in phosphatic lenses (Fig. 11F) and in plan-view (Figs 11F, 13H, 14B and 14F). Lenses also rarely exhibit imbrication and deformation textures (Fig. 13A). An early phosphate cement preserves mud lenses with pre-compaction geometry (Figs 11B and F) and suggests that the compacted thickness is ca 45% of the original thickness. Facies B and C mud lenses are rare to moderately common and exhibit 50 to 300 lm Ø (i.e. coarse mud to medium sand-sized; Figs 10C to E and 13H). Therefore, on the basis of grain size, moderately lenticular Facies B and C are medium to coarse sandy mudstones and sandstones. In   (Fig. 14B).

Sedimentary packages and geochemistry
The abundance of detrital elements (for example, Ti and Zr) strongly declines, whereas TOC, carbonate content and excess silica content increase, through each increasing basin accommodation (IB) package (Figs 4 to 7). In Cominco S9 a lower package (IBa) delineates onset of TOC/S < 10 (Fig. 7). An upper package (IBb) defines the increase in TOC, carbonate and excess silica content (Fig. 7). High basin accommodation (HB) packages exhibit initially relatively high TOC, carbonate and excess silica content that gradually decrease towards the top (Figs 4 to 7). Similarly, detrital elements gradually increase in abundance through each HB package. Decreasing basin accommodation (DB) packages generally exhibit stable TOC, decreasing or no excess silica content and increasing abundance of detrital elements towards the top of each package (Figs 4 to 7). Low basin accommodation (LB) packages tend to exhibit moderate to high detrital element content, whereas carbonate, TOC and excess silica content are highly variable (Figs 5 to 7).

Subaqueous sediment density flows
Facies D coarse mudstones (Figs 10G, 10H and 15A) are interpreted as mud-rich low density calciturbidites ( Fig. 16A to C; defined by Stow & Shanmugam, 1980) that were in some cases reworked during deposition of overlying lenticular muds (Facies B, C or F). Basal sandy to coarse muddy wavy laminae ( Fig. 10G and H) are interpreted as the T D division of Bouma (1962) that grade into T E-1 of Piper (1978). These laminae possibly formed via deposition of coarse mud in a turbulent boundary layer beneath a dilute suspension (Stow & Bowen, 1978) or in a shear zone beneath a relatively concentrated gelled fluid mud layer (McCave & Jones, 1988). The typically sharp grain-size break between T E-1 and T E-2 is a feature common to many mud-rich low density turbidites and likely relates to flow transition from non-cohesive to cohesive behaviour (Talling et al., 2012). Massive to weakly graded fine mud-rich laminae and beds overlying Facies D coarse mudstones (Figs 10G, 10H and 15A) are interpreted as the T E-2 and T E-3 turbidite divisions of Piper (1978). Homogenous 'mud-cap' laminae lack underlying coarse mud or sandy laminae so that turbulent, hybrid or laminar flows were all potentially viable depositional mechanisms (Fig. 16H). Facies B (Fig. 10A and B), Facies C (Fig. 11C), Facies F (Fig. 13A), Facies G (Fig. 14C), Facies H (Fig. 15E) and Facies I (Fig. 15F) all include examples of these 'mud-cap' laminae. A lack of intraclasts observed within the 'mud caps' suggests that these deposits did not form via reworking of flocs as bedload on the lee face of ripples beneath relatively fast flows (Schieber et al., 2007a;Talling et al., 2012).
A variety of sedimentary structures suggest that Facies E beds were deposited by hybrid Fig. 12. Facies E chaotic, weakly carbonate-bearing, siliceous mudstones. (A) Sample 147, E 1b1 (?) zone, 68Á32 m above base, IB package debrite, cut-sample photograph, including contorted basal shear zone overlain by mud containing many cemented and fluidized bioclastic coarse mud-rich chaotica (water escape) and/or bioturbation (black arrow). (B) Sample 73A, E 1a zone, LB package, 'starry-night' turbidite mud-cap, 48Á14 m above base, microphotograph. (C) to (F) Sample 75A, E 1a zone, 48Á82 m above base, LB package. (C) Cut sample photograph. (D) Thin section scan with climbing ripples and truncated cross-laminae, and (E) microphotograph, including mud-rich microbial mat rip-up clast (inset). (F) Plan-view equivalent of (D), with many aligned discontinuous laminae and lenses of calcite and silica-replaced bioclasts and fragments of organic matter. Truncated surfaces are curved in plan-view, whereas cross-laminae are linear. (G) Sample 89A, 52Á17 m above base, E 1a biozone, crosslaminated mudstone with many calcite and silica cemented lenses and fragments of organic matter. Lenses exhibit rounded edges, tapered geometry and partial fragmentation (inset, cross-polarized light).   5) and are therefore interpreted as the deposits from mud-rich turbidity currents (Talling et al., 2012). Above these mud-rich turbidites, Facies E beds exhibit the greatest diversity of sedimentary structures (Figs 8D and 12A to F) that compare best to sediment deposition from 'lower transitional plug flows' (Fig. 16G), characterized by moderate clay concentration and extensive top-down reworking by long-lived post-deceleration steady flows (Baas et al., 2009(Baas et al., , 2011. The complete lack of any homogenous basal sand is interpreted as deep scour (for example, Fig. 8D) and complete reworking by the flow. Younger flows were apparently less erosive, because erosive bases, mud clasts and possible microbial mat fragments become increasingly rare (Fig. 5). Hybrid beds gradually exhibit the characteristics of 'quasi-laminar plug flows' (Baas et al., 2009(Baas et al., , 2011. These beds are equivalent to the Type IV deposits of Sumner et al. (2009). Above the E 1b1 marine band, hybrid beds are interpreted as a variety of 'quasi-laminar plug flows' (Fig. 16D), linked-debrites (Fig. 16E) and ungraded mud-caps interpreted as the T E-3 divisions of Piper (1978). The high clay and organic content likely sufficiently damped flow turbulence in order to generate such hybrid beds (Baas et al., 2011).
Facies I scours are filled with homogenous sand (Fig. 15B), indicative of a high density (and likely strongly bypassing) element of turbulent flow (T A division) (Talling et al., 2012). Sand to coarse mud-rich normally-graded beds in Facies H and I (Fig. 15E to F) are interpreted as the T D to T E-1 divisions of Piper (1978). Facies H and I muds overlying sand-filled scours (Fig. 15B) are interpreted as the T D and/or T E-1 , T E-2 and T E-3 divisions. Given the thin bedded nature of these deposits, Facies I muds were likely deposited from dilute low density turbidity currents (Stow & Bowen, 1978).
Occasionally Facies H and I exhibit draped mudstone (T E ) and deformation of basal coarse mud (silty) laminae ( Fig. 15E and F). These textures are comparable to deposits interpreted as tempestites (Schieber, 1986(Schieber, , 2016. Rare symmetrical coarse mud (silty) laminae in Facies I (Fig. 15E) suggests deposition from waxing and waning of hyperpycnal flows (Li et al., 2015;Schieber, 2016). This is plausible given that a strong halocline probably existed at the basin margins, considering that Facies H and I at Cominco S9 were deposited under a freshwater column ( Fig. 7; Berner & Raiswell, 1984).

Origin of mud lenses
Mud lenses in Facies B, C, F and G (Figs 10C, 10F, 11A to G, 13A to H and 14A to H) are interpreted as clasts that were transported as partially consolidated muds in the bedload of bottom currents (Schieber et al., 2010). Rarity of pyrite framboids within partially consolidated clasts (Fig. 13C) suggests that the solutes required for pyrite precipitation were largely unavailable. A relatively low clast permeability compared to surrounding sediments potentially limited infiltration by syngenetic and/or diagenetic (sulphidic) pore-fluids. Figure 11F demonstrates that lenticular mudstones were clastsupported prior to compaction. A rip-up interpretation is supported by dominantly equant lens shape geometry in un-compacted (Fig. 11F) and plan-view (Figs 11F, 13H, 14B and 14F), increasing clast diameter and abundance The fine mud-rich, pyritic (dark) planar, parallel laminae contains few, relatively small lenses. Note also deformed lenses and multiple examples of imbricated lensesred arrows in (A). BSE microphotograph lens and 'matrix' comparison. The sediment matrix (D) and (E) comprises pyrite framboids (pf), organic matter, macrocrystalline silica cement (si), illite laths (white arrows), and mixed fine clays and quartz. Organic matter exhibits two geometries; sheets (D) and equant particles (E). Mud lenses (F) contain illite laths (white arrows), mica, kaolinite (black arrow), feldspar (albite, with alteration rim) and fine quartz and kaolinite, and typically lack organic matter and pyrite. (G) Sample DC04, 37Á37 m above base, DB package, microphotograph with strong lenticular texture (white arrows) and organic matter (red arrows). (H) Facies C, sample 31, 25Á52 m above base, HB package, plan-view microphotograph, including organic matter (organominerallic aggregates; white arrow), mud lenses (black arrow) and carbonate and quartz grains (red arrow).   (Figs 13A and 14H). Dominance of clay (kaolinite) and lack of OM and bioclasts in mud lenses (Fig. 13F) precludes a faecal-pellet origin (e.g. R€ ohl et al., 2001). In some cases, lenses are composites of several smaller lenses (Fig. 14B), which may represent clasts of Facies G. Thus, the dense accumulations of lenses in Facies G (Fig. 14C to F) were potentially intermittently scoured to produce larger, composite 'clasts of clasts' observed downslope in Facies F (Fig. 14A  and B).
Where Facies B, C or F overlie 'mud-cap' laminae, the contact is often sharp to slightly diffuse ( Fig. 10A and B), suggesting that bottom currents at the site were capable of depositing mud clasts without substantial scour of the underlying deposits. A sharp transition from turbidite laminae into overlying lenticular muds (Facies B; Fig. 10B) also demonstrates that mud lenses are unlikely to represent burrows, on the basis that upper contacts were not homogenised. Bioturbated upper turbidite contacts in Facies H and I (Fig. 15F) are dissimilar to the lenticular fabrics observed elsewhere.
Namurian deltas generally lack sedimentary features indicative of tidal currents and to some extent wave action, suggesting river-dominance (Collinson, 1988). The region was probably shielded from high-energy waves, and long, constricted and shallow connections between basins potentially damped tidal forcing and wave energy (Keulegan & Krumbein, 1949). Yet tidal and/or wind-driven currents are thought to generate mud-rich rip-up clasts (Schieber, 2016). Bottom currents are amplified by suitable bathymetric roughness and shape (Klein & Ryer, 1978;Sztan o & Boer, 1995;Schieber, 2016), and/or favourable water depth and stratification (Egbert et al., 2004). Thus, is it possible that bottom currents were locally amplified in the Craven Basin. This is supported by presence of tempestites in Facies H and I, suggestive of intermittently vigorous storm-driven bottom currents with potential for seabed scour. Microbial mats potentially also promoted biostabilization and consolidation of mud at the seabed (see below).

Particulate organic matter and candidate microbial mats
Most micron to millimetre-scale OM particles in Facies A to F (for example, Figs 13D, 13E and 14H) are interpreted as organominerallic aggregates (Tyson, 1995;Macquaker et al., 2010). Lack of compaction of the spherical OM particles (Fig. 13E) indicates very early cementation around and/or within the particles. Juxtaposition of spherical and elongate forms of OM is important (Fig. 13C to E). This suggests that elongate particles are not compressed versions of originally spherical particles and supports a distinctive origin for micron-scale elongate forms of OM.
Organominerallic aggregates likely represent phytodetritus, a type of 'marine snow' (e.g. Alldredge & Silver, 1988;Macquaker et al., 2010). This is common beneath productive water columns (Riley, 1971) where OM aggregates with detritus through grain collision and clumping with extracellular polysaccharides (Alldredge & Silver, 1988), precipitates from dissolved organic matter (Bowen, 1984;Velimirov, 1987;Mann, 1988) or are the faecal pellets of zooplankton (e.g. Porter & Robbins, 1981). Although the microscopic techniques utilized could not fully resolve nanoscale structures, Facies B to F contain finely (i.e. less than micron-sized) disseminated OM associated with the clay matrix ( Fig. 13F) (Salmon et al., 2000;Kennedy et al. 2014). Organic matter in Facies B to F is   Fig. 15C) is interpreted primarily as terrigenous to mixed in origin.
Organic-rich clasts in Facies E exhibit frayed edges (Fig. 12E), a feature that suggests these are fragments of microbial mats (Schieber, 1986(Schieber, , 2004Schieber et al., 2007b;Davies et al., 2016). Relatively large elongate OM in Facies A to F also potentially represent mat fragments (for example Fig. 10C) or sheet-like organominerallic aggregates (e.g., Riley, 1971;Macquaker et al., 2010; see also review by Turner, 2015; Fig. 13D). The organic laminae in Facies G (Fig. 14C to G) are interpreted as in situ microbial mats, similar to those described by Schieber (2004). The anastomosing nature of laminae could also support the existence of mats, although differential compaction around the mud lenses could also produce this texture (Schieber et al., 2007b). Facies G is comparable to modern sedimentary and microbial textures associated with oxygen and salinity restricted conditions in similar water depth (Virtasalo et al., 2011). Rare 'roll-over' of organic-rich laminae (Fig. 14E) suggests that the mats were intermittently locally re-worked. Organic-rich clasts in Facies E resemble Facies G (Figs 5, 6, 12D and 12E), and are therefore potentially down-dip, transported fragments scoured from Facies G during passage of density flows. Organic-rich laminae in Facies A are difficult to interpret due to distortion by calcite spherules but could also represent microbial mats, which were possibly a catalyst for spherule growth (Dupraz et al., 2004).
Upslope colonization by microbial mats could explain the origin and relatively high abundance of mud lenses observed in Facies B to F and G.
Microbial mats bind sediment, increasing the sediment tensile strength, limiting short-term erodibility and therefore initially protect muds from bottom currents that would otherwise induce scour (Neumann et al., 1970). This is supported by the presence of dewatering structures (Figs 11A and 14H) and microfaulting (Figs 4 to 7, 12A and 14H) in underlying Facies B to F, and early diagenetic nodular (pyrite and kaolinite) cements (Figs 9H and 14C) and pyritized burrows (Fig. 14G) in Facies G. These observations suggest that sulphidic porewaters gently advected through, or stagnated in, relatively cohesive sediment (e.g. Rickard, 2012) beneath the candidate microbial mats in Facies G muds. Inherent instability of large microbial mats (Vignaga et al., 2013), coupled with periodically stronger currents, would trigger periodic catastrophic failure of the mat and re-suspension of mat fragments and semi-consolidated (biostabilized) mud clasts. Thus, mud clasts and microbial mats were potentially genetically linked. Rare coupling of mud lenses and organic fragments 'stuck' together as single larger rip-up clasts supports this hypothesis (Fig. 14H).
Organic matter is enriched in muds via seasonal or persistent bottom water anoxia (Demaison & Moore, 1980;Tyson & Pearson, 1991), high organic loadings under productive water columns (Calvert et al., 1992), sorption onto clay mineral surfaces (Keil et al., 1994) and/or low sediment accumulation rate (Tyson, 2001). The presence of phosphatic fish faecal pellets and radiolaria is suggestive of at least moderate rates of primary productivity, perhaps triggered by nutrient upwelling or loading at the front of the Pendle delta. Presence of phosphate and chert nodules on the Askrigg shelf edge suggests that these conditions were relatively widespread (Davies et al., 1993;Fairbairn, 2001). Facies D and E tend to exhibit relatively high Ti and Zr content compared to Facies B and C, and high excess Si content compared to Facies F. This

Facies H
suggests that the reduced TOC in this interval is primarily a function of dilution by siliciclastic sediment and silica cement. Abundant framboidal pyrite and lack of bioturbation in Facies A to F is consistent with the interpretation that sediment porewaters at seabed (at least) were likely euxinic and therefore toxic to infaunal organisms (Wang & Chapman, 1999). Emmings et al. (2017c) suggested that bottom waters were also (at least intermittently) anoxic during deposition of Facies F muds (above the E 1a1 marine band; see Fig. 5). Several redox-sensitive trace elements (e.g. Tribovillard et al., 2006), such as Mo, are enriched in Facies F and were likely fixed in association with solid sulphides (Emmings et al., 2017c). These conditions apparently persisted despite relatively vigorous bottom current conditions, and were likely inhospitable to all aerobic, benthic (including infaunal, epifaunal and nektobenthonic) organisms. Clearly the relatively high TOC in Facies A to F is linked to at least moderate rates of OM export into anoxic bottom waters. Yet these processes are interlinked and often coupled. Interpretation is further complicated by the effects of changing sediment accumulation rate, autodilution (for example, radiolarian Si) and remineralization of OM during diagenesis.
Facies H and I exhibit lower TOC concentration than any other facies (excluding Facies A). Bioturbation in these facies suggests that bottom waters were at least intermittently oxygenated and supported benthic fauna (Eagar et al., 1985). Thus, oxygenated bottom waters, increased bioturbation, dilution by detrital sediment, weaker organic loadings, change in mineralogy, change in sediment properties (for example, porosity) and/or change in OM type and distribution are all viable mechanisms for the relatively low TOC in Facies H and I.

Marine bands
Discrete and repeated packages of carbonate-rich macrofauna-bearing Facies B interbedded with  These are slightly graded to ungraded deposits likely from dilute expanded suspensions (Stow & Bowen, 1978) or dense fluid mud layers (McCave & Jones, 1988;Talling et al., 2012).
Facies A, C and D fit with definitions of marine bands (Ramsbottom, 1977;Waters & Condon, 2012). Facies B and C (Figs 9B to F, 10C to E and 11B to F) are interpreted as mixed pelagic and hemipelagic sediments (Table 1), interbedded with calciturbidites (Facies D), deposited during periods of increasing to high basin (IB to HB) accommodation (Posamentier et al., 1988;Martinsen et al., 1995). Facies B body fossils are primarily euhaline ('normal marine') and pelagic (e.g. Fl€ ugel, 2004), equivalent to the faunal phases 4 to 6 of Ramsbottom et al. (1962). Goniatites were nektonic and probably relatively intolerant to basin desalination events (Ramsbottom & Saunders, 1985). The Pendle delta system likely induced a strong salinity gradient (Collinson, 1988) with potential for euhaline faunal occlusion. This explains why Facies H and I, deposited under a freshwater column (TOC/S > 10), are 'barren' (Fig. 7). Freshwater likely prograded as a plume from the Pendle delta, and was not basin-wide. All other mudstones were likely deposited in marine conditions, on the basis of low TOC/S (Berner & Raiswell, 1984). Lack of bioclasts in Facies C (Fig. 11B and F) suggests either that conditions in the water column were unfavourable for colonization, perhaps due to reduced salinity (Ramsbottom et al., 1962), or that the aragonitic bioclasts (e.g. Fl€ ugel, 2004) were not preserved at the seabed (Ku et al., 1999).
Mississippian trilobites (Fig. 10E) were primarily benthic organisms (e.g. Fl€ ugel, 2004). This suggests that bottom water anoxia, as a possible explanation for the relatively high TOC, was transient or that the trilobites were tolerant to sub-oxic conditions (Fortey & Wilmot, 1991). Nyhuis et al. (2015) favoured this explanation for the presence of trilobites in concomitant rocks. Alternatively, trilobites and mud lenses were potentially transported together, from a nearby oxic and euhaline basinal high, downslope, into anoxic bottom waters. This is the favoured explanation for presence of benthic fauna in the contemporaneous US Barnett Shale (Loucks & Ruppel, 2007).
Relatively small mud lenses in marine bands (Figs 3 to 7) possibly indicates that the provenance of mud lenses shifted further away onto adjacent shelves (Figs 2A and 10C to E) and that clasts were fractionated over a longer run-out distance during marine transgressions. The basal Facies B package in the E 1a1 marine band is an exception to this pattern, however, because it contains relatively large mud lenses similar to Facies F (Fig. 5). Perhaps these large clasts were shed initially from relatively steep and mud-rich (syn-rift) slopes on adjacent highs ( Fig. 2A) during the E 1a1 marine transgression (Fig. 1). Subsequent marine bands possibly lack these relatively large lenses because local highs lacked available mud, perhaps because the inherited syn-rift structures became progressively infilled and smoothed during progradation of the Pendle delta. Relatively abundant and large clasts in Facies F, likely deposited during periods of reduced basin accommodation (Figs 13A, 13B, 14A and 14B), suggests that clasts were sourced nearby or that bottom currents were enhanced.
In Cominco S9 mudstones in the E 1c marine band are finely lenticular, whereas the underlying and overlying Facies H and I lack mud lenses and instead comprise coarse mud (silt) and fine sand ( Fig. 7; Fig. 10D versus Fig. 15B and C). The switch from marine band fine mudstone to coarse mudstone, is indicative of the movement of a pseudo-'mudline' in response to eustatic sea-level fluctuation (Weaver, 1989). During periods of high sea-level, mud-rich sediments accumulated close to Cominco S9 (i.e. on the Askrigg Block or adjacent slope) and were intermittently scoured and transported as rip-up clasts to be deposited as part of the E 1c marine band. In more distal and older marine bands, pelagic components tend to dominate over mud lenses, possibly because bottom currents transporting lenses had run-out. During the subsequent sea-level fall, coarse mud (silt) was deposited primarily via turbidity currents at Cominco S9, whereas mud likely bypassed this proximal site.

Phosphate concretions
Phosphatic concretions in Facies B (Fig. 9D to F) are interpreted as early-cemented faecal pellets, possibly from fish (Saba & Steinberg, 2012;Zato n & Rakoci nski, 2014). These are compositionally similar to phosphatic bromolites described by Hunt et al. (2012) and geometrically comparable to siderite-cemented coprolites in the Mississippian Lower Oil Shale Group (Bojanowski & Clarkson, 2012). Preservation of contents, such as radiolarian tests (mostly spumellarian entactinids; Casey, 1993), skeletal material and amorphous OM (Fig. 9F; e.g. Tyson, 1995) in unflattened geometry indicates that cementation occurred very early. Radiolarian tests are preserved within the phosphate concretions as thin organic linings rafted in phosphate cement (Fig. 9F). Radiolarian silica dissolved and locally precipitated within each concretion as patchy quartz cements (Fig. 9F). Modern radiolaria often host symbionts as mixotrophs (Mitra et al., 2016). This means that radiolaria are important primary producers in some marine environments (e.g. Caron et al., 1995) and that radiolarian OM could have contributed significantly to the organic fraction in these rocks. The presence of radiolaria is further evidence for at least intermittently euhaline upper water column conditions (e.g. Fl€ ugel, 2004).
Absence of radiolarian tests outside concretions suggests that the relatively labile silica tests completely dissolved and, in the absence of the protective early phosphate cement, silicarich fluids then re-precipitated in pores as clay and/or quartz cement during early diagenesis (e.g. . The high abundances of radiolarian linings preserved in the concretions and pervasive quartz cements elsewhere is important. Interpreting concretion contents as an approximation for the local pelagic fraction deposited at each site, this supports a pelagic and biogenic origin for the excess silica, and for many or all of the patchy quartz and clay mineral cements in the matrix (Figs 9B,9C and 13D). It is beyond the scope of this paper to further discuss diagenetic phases (e.g. Curtis, 1977;Kastner et al., 1977;Moore et al., 2004;Macquaker et al., 2014). These phases include spherulitic calcite (Facies A; Fig. 9A), patchy calcite (Fig. 9C), fine framboidal ( Fig. 13A to E) and euhedral and nodular (Fig. 14C to G) pyrite, sulphate (Fig. 11G), scattered dolomite replacing calcite (Figs 9B, 9C and 12G), quartz (Fig. 9B) and clay mineral cements ( Fig. 9G and H).

Controls on Craven Basin infill
Field to micron-scale analysis of the Upper Bowland Shale demonstrates that the epicontinental Craven Basin received sediment from three provenances. Firstly, detrital clay, coarse mud (silt) and sand supplied via turbulent and hybrid flows, including direct supply from the Pendle delta system; secondly, clay-rich mud clasts scoured from nearby mud-rich, potentially biostabilized, slopes; and, finally, pelagic and hemipelagic sediment, rich in clays, OM and biogenic carbonate and, silica. Changing basin accommodation, moderated by eustatic sea-level fluctuation, delta progradation and fault instability at the basin margins moderated the supply of sediment from these provenances.

Changing basin accommodation
At Hind Clough relatively thin packages of marine band Facies B and/or C, interbedded with low density turbidites of Facies D, are interpreted as deposition during periods of increasing basin (IB) accommodation (Figs 5 to 7). Siliciclastic turbidites in IB packages could represent the final flows through shelf to basin conduits (for example, canyons), such as via the Dent Fault between the Southern Lake District High and Askrigg Block (Fig. 2). Such flows were potentially cut-off by rising sea-level (Piper & Normark, 2009;Talling, 2014). Turbidites could also represent failure of sediments that accumulated at canyon heads by longshore drift (Covault & Graham, 2010). Calciturbidity currents were also potentially sourced by slope failure of the flank of the Askrigg Block and local highs, by loading via carbonate accumulation on slopes, or possibly thicker and/or warmer water column (e.g. Maslin et al., 2004;Talling, 2014).
Increasing basin (IB) packages (Figs 4 to 7) record the increasing and decreasing input of (hemi)pelagic mud and turbidity currents with time, respectively. This is consistent with increasing rarity of thin calciturbidite beds (Facies D; Figs 10H and 15A) and progressive dominance of calcareous lenticular mudstones (Facies B and C). Juxtaposition with overlying marine bands suggests that IB packages were deposited during rising eustatic sea-level driven by onset of deglaciation on Gondwana (Veevers & Powell, 1987). The IB packages may therefore correspond to the transgressive systems tract (Posamentier et al., 1988). At MHD4, an equivalent IB package is highly condensed (Fig. 4) likely because the isolated basinal high position further limited sediment supply.
The contact between the IB packages and overlying carbonate-rich packages is relatively sharp at Hind Clough and MHD4, and is attributed to the transition from increasing to high basin (HB) accommodation. The HB packages best fit the definition of marine bands. The HB packages were almost certainly deposited during periods of high eustatic sea-level (e.g. Ramsbottom, 1979;Fig. 17A) and therefore represent deposition initially during the maximum rate of transgression and the subsequent highstand systems tract (high to slightly decreasing basin accommodation) (Posamentier et al., 1988). Relatively high TOC, carbonate and excess silica content in HB packages (Figs 4 to 7) is consistent with a dominance of calcareous, weakly lenticular mudstones (Facies B and C; for example, Fig. 10E). Calciturbidity currents (of Facies D; for example, Fig. 10G and H) were probably delivered by shedding from local carbonate-rich highs (Fig. 17A). These calciturbidites are more abundant in older marine bands (E 1a and E 1b1 ) possibly because fringe reefs/carbonate accumulations on the shelf or slope were relatively widespread ( Fig. 2A).
At Hind Clough, lenticular Facies F and G commonly overlie the HB packages and are collectively interpreted as deposition during periods of decreasing basin (DB) accommodation. Persistence of DB packages above HB packages suggests a common driver. Decreasing basin packages may therefore form part or all of the falling stage systems tract (Posamentier et al., 1988). Accommodation in shallow waters reduced sufficiently to permit scour of previously deposited (and biostabilized) muds and transport as clasts into the basin. Above the E 1a1-b marine band, a conformable package (DBa) is overlain by an inclined package (DBb) (Figs 5 and 11C to E). The majority of scoured sediments were possibly initially trapped in proximal positions because clasts are typically small in DBa (Fig. 17B). An increasing supply of mud lenses and thin 'mud-cap' laminae (Facies F; for example, Figs 11C to E and 13A to G) during falling sea-level is consistent with an increasing abundance of detrital elements towards the top of each DB package (Figs 4 to 7).
Given that the primary source of sediment was likely from the north-east (e.g. Collinson, 1988), the inferred angle of palaeodip (towards the north-east) of package DBb is the reverse of the expected direction for progradational clinoforms (e.g. Hampson, 2010) from the Pendle delta (north-east). Lack of deformation structures probably also discounts a slope failure origin for this structural change. This geometry could represent an aggradational and onlapping package of lenticular sediments that accumulated on a gently sloping seabed. Bottom currents transporting the mud clasts rapidly decelerated, and aggraded, on the relatively low-gradient seabed at Hind Clough. Given that lenses are typically larger and more abundant than the conformable DBa package beneath, this implies a phase of more significant sea-level fall, increased sediment supply and/or tectonic uplift (Fig. 17C). Intermittent presence of goniatite moulds, cemented lenses, increased excess silica and Facies D interbeds (Fig. 5) in package DBb suggests a pulse of increased basin accommodation (possibly the E 1a1-c flooding event; Fig. 1C, perhaps local rather than regional). This increase in basin accommodation was insufficient to isolate the basin from the primary source of rip-up clasts.
Above the DBb package at Hind Clough, the succession of density flow deposits (Facies E) (Figs 5 and 16) is interpreted as deposition during a period of decreasing to low basin (LB) accommodation (Fig. 17D). Delta progradation coupled to falling sea-level and/or slope instability at the fault-bound basin margins could have triggered these mass transport processes (Piper & Normark, 2009;Talling, 2014). This is consistent with the relatively high abundance of detrital elements in LB packages. Beneath the E 1a1 marine band in MHD4, a discrete package of Facies B and C with slump structures is also interpreted as a LB package (Fig. 4).
The transition between types of hybrid flows in Facies E (Figs 5 and 6) potentially relates to changing sea floor geometry or the type of sediment entrained in the flow. At the onset of deposition of the E 1a LB package at Hind Clough, flows potentially passed over a low-gradient and/or irregular seabed relief defined by the underlying onlapping DBb package (Figs 11C to E and 17D). This relief probably promoted rapid deceleration of the flow and scour of underlying sediment. Following this initial deceleration, steady flows were apparently long-lived. Gradual infill and/or smoothing of the basin sea floor could therefore explain the transition between deposit types. A reduction in clay content, possibly due to increased input of coarse mud and sand from the Pendle delta, changing seabed geometry, a change in the type of failed sediment and/or reduced entrainment of clays and OM during passage, promoted sediment deposition from more turbulent flows above the E 1b1 marine band. The succession between E 1b1 to E 1b2-b marine bands (Figs 6, 17D and 17E) is interpreted as a combination of changing basin accommodation and mass transport processes potentially triggered by slope failure at the fault-bound margins of the basin ( Fig. 2A).
The E 1b2-b DB package is associated with colonization and biostabilization of the seabed by candidate microbial mats at Hind Clough (Facies G). Such mats potentially occupied a niche environment linked to delta progradation (Fig. 17F), perhaps associated with a high redox gradient at seabed (Grunke et al., 2011). Overlying the E 1b2-b DB package at Hind Clough, Facies H to J ( Fig. 3A) are collectively interpreted as a LB package. This package represents a significant step-change in the basin evolution, however, with the introduction of coarse siliciclastic fill as the turbidite-fronted Pendle delta (e.g. Collinson, 1988). This explains the enrichment in detrital elements through the E 1c1 LB package (Fig. 7). Shelfal and basinal barriers had become sufficiently infilled and/or breached, ultimately permitting development of the Pendle Grit toe of slope fan in the Craven Basin (Martinsen et al., 1995). This was potentially fed by a lowstanddominated river-fed canyon system (Covault & Graham, 2010), perhaps as a breach along the Dent Line between the Askrigg Block and Southern Lake District High ( Fig. 2A). Thus, the E 1c1 DB package was potentially originally much thicker in Cominco S9 but was scoured during deposition of the overlying LB package (Figs 7 and 15B). Associated with this increased siliciclastic input, a plume of freshwater extended from the Pendle delta outflow across the adjacent Askrigg Block to (at least) the proximal margins of the Craven Basin.

Delta progradation and slope instability
The potential for complex sediment routing and sediment lock-up in more proximal basins towards the north-east of the Askrigg Block (for example, Stainmore Trough) meant that the Pendle delta system was poorly (but increasingly) connected to the Craven Basin during deposition of the Bowland Shale. Siliciclastic turbidity currents supplied directly from river-fed canyons likely initially followed circuitous routes first through adjacent basins (for example, the Cleveland Basin to the east), around the Askrigg Block (perhaps through the Dent Line; Fig. 2A), before deposition into the Craven Basin (Brandon et al., 1998;Fraser & Gawthorpe, 2003). The entrance points in adjacent basins were likely poorly connected to the Pendle river system, with potential for stranding of canyons at the shelf break (for example, lowstand-dominated; Covault & Graham, 2010).
Mud lenses typically increase in size and abundance in Facies F through the section at Hind Clough, because adjacent intrabasinal mud traps, such as on the South Craven Fault, were progressively infilled with mud sourced from the Pendle delta (for example, Fig. 17E). Therefore, a greater area of (potentially biostabilized; Fig. 17B) mud-rich seabed was likely exposed to erosion by wave shear and/or storm-driven currents. Thus, biogenic input (delivered via bedload or pelagic settling) was increasingly diluted by the escalating input of rip-up clasts and hemipelagic settling of detrital clays. Whilst the main basin margin with the Askrigg Block likely retained the sharp 'block-edge' geometry ( Fig. 2A) into the E 2c biozone (Martinsen et al., 1995), mud was likely available on the Askrigg Block (Hudson, 1940) as a source for mud clasts.
Unconformities north of the Middle and South Craven faults suggest periods of inversion, especially within the E 1a biozone (Hudson, 1940;Rowell & Scanlon, 1957;Arthurton et al., 1988). 'Limestone boulders' in the footwall of the South Craven Fault (Arthurton et al., 1988) are broadly contemporaneous with the Facies E beds observed at Hind Clough. Therefore at least some of the hybrid flows were likely fed by slope failure on the scarps of the Middle & South and/or North Craven faults (for example, Fig. 17D) or other fault-bound highs. Both Middle and South and North Craven faults bounding the Craven Basin were intermittently active and/ or the basin flexed at these points, in order to accommodate the basin subsidence required for accumulation of the several kilometre-thick Millstone Grit Group succession on top of the Bowland Shale in the Craven Basin (e.g. Fraser & Gawthorpe, 2003). Figure 18 is a conceptual cross-section across the Craven Basin ( Fig. 2A) which integrates findings across the three sites and stratigraphic relationships on the Askrigg Block and slope. It simplifies the expected heterogeneities in sediment package geometries, particularly across intrabasinal highs and lows. The strong asymmetry of packages is characteristic of sediments deposited under the combined influence of eustatic sea-level fluctuation and basin subsidence (e.g. Martinsen et al., 1995). Initially the basin exhibited steep bounding slopes ('blockedge' geometry) and fringing reefs on both South and North Craven faults (e.g. Wensleydale Group; Arthurton et al., 1988). Thus the Lower Bowland Shale in the Craven Basin represents a carbonate-dominated succession fed by unstable basin margins (Newport et al., 2018). Linkeddebrites of Facies E are similar, although not equivalent, to Facies 5 of Newport et al. (2018). Shedding of carbonate from reefs (e.g. Riley, 1990) contributed to the comparatively large carbonate component, with potential for basinal thickening, in the E 1a1 marine band. These calciturbidites (Facies D) are partially equivalent to the homogenous and lag-containing Facies 1 in the contemporaneous Morridge Formation, in the Widmerpool Gulf (K€ onitzer et al., 2014) and Facies 4 of Newport et al. (2018).

Synthesis
Progressive basin infill and cessation of rifting (Leeder, 1982) smoothed and infilled intrabasinal structures. This regional shift in intraplate deformation style promoted deposition of discrete marine band packages (Facies A to C) in the Upper Bowland Shale, driven by fourthorder eustatic sea-level cyclicity. Equivalents are observed in many other Namurian successions (see Waters & Condon, 2012)  several intervals in the Lower Bowland Shale (Facies 1 and/or 2; Newport et al., 2018), which are relatively diffuse, likely due to the combined effects of active rifting and diminished glacioeustasy (Veevers & Powell, 1987). Packages are likely to be most spatially complex in the E 1a1 biozone, and progressively simplify as the basin evolved towards a muddy ramp-type system. Lenticular muds are present in both the Upper (Facies F) and Lower Bowland Shale (Facies 3 of Newport et al., 2018), and are common in many epicontinental successions (for example, Facies 3, 4a and 4c of K€ onitzer et al., 2014). Highly lenticular Facies G may represent a clast-rich version of Facies 3, or Facies 4a, of K€ onitzer et al. (2014). Abundant and relatively large rip-up clasts in DB packages suggest that scour of shelfal mud-rich successions is the most likely provenance for such clasts. Linkage between highly lenticular, microbial-mat-bearing Facies G deposited during delta progradation (Fig. 17F) suggests that clasts were sourced specifically from prodeltaic and biostabilized muds trapped on shelves and slopes. Under a thermal sag regime, proximal basins, such as the Stainmore Trough (e.g. Waters et al., 2009), were infilled by the Pendle delta system and ultimately connected the Craven Basin with the Pendle prodelta. Unconfined delta progradation triggered the development of a toe of slope fan system in the Craven Basin, recognized as Facies H to J. These facies are partially comparable to Facies 4a and 5 in the Morridge Formation (K€ onitzer et al., 2014).

Organic-laminated strongly lenticular fine mudstone (G)
Micaceous coarse mudstone to medium sandstone (I)

Lithofacies (B to J) at Hind Clough
Assuming deposition over ca 1Á8 Myr (using the chronostratigraphy of Waters et al., 2011), an uncompacted sediment thickness of 100 m (assuming 55% compaction), the Lower Bowland Shale basinal high succession (P 2 biozone) described by Newport et al. (2018) exhibits a SAR of ca 0Á055 mm year À1 . This compares with ca 0Á080 mm year À1 SAR calculated for an average Lower Bowland Shale thickness in the Lancaster Fells sub-basin (after subtraction of the Pendleside Sandstone member; Brandon et al., 1998). Similarly, SARs of 0Á013 mm year À1 and 0Á002 to 0Á008 mm year À1 are estimated for the contemporaneous Barnett Shale (Loucks & Ruppel, 2007) and North American Late Pennsylvanian Midcontinent Seaway cyclothems (Algeo et al., 1997(Algeo et al., , 2008, respectively (Fig. 3B).
Attaining an appropriate estimate for SARs across different settings is clearly problematic and depends on an appropriate assignment and understanding of sedimentary facies. Lenticular mudstones are perhaps best interpreted as deposits on a continuum of processes (e.g. Rebesco et al., 2014) between mud densite (Talling et al., 2012) and (hemi)pelagite end-members. Weakly to moderately lenticular 'marine band' mudstones (Facies B and C) are dominated by hemipelagic and pelagic components and are therefore interpreted as (hemi)pelagites with subordinate densite mud. Strongly lenticular mudstones (Facies F and G) are interpreted primarily as mud densites mixed with subordinate hemipelagite.
The estimated SAR calculated for the Bowland Shale is an order of magnitude higher than the Barnett Shale. This cannot be explained solely by different timespans (i.e. frequency of hiatuses). Clearly the export of mud from the Pendle delta system was large and relatively fast, despite shielding by the Askrigg Block and intrabasinal complexity. The SARs estimated for the Bowland Shale and contemporaneous Morridge Formation are closest to the 'deltaic' median SAR (Fig 3B; after Sadler, 1999). This high SAR is linked to the evidence for widespread and sustained deposition both from bedload and suspension. All other the organic-rich mudstone successions are closest to abyssal plain, abyssal rise, continental slope and/or turbidite fan median SARs (Fig. 3B). Considering that the Bowland Shale is up to 500 to 700 m thick into the deeper basin (Aitkenhead et al., 1992;Brandon et al., 1998;Kirby et al., 2000;Clarke et al., 2018), a relatively large volume of sediment likely also bypassed the studied sites. This is consistent with the interpretation of deposition of rip-up clasts by laminar flows, and passage of hybrid flows with variable degrees of turbulence damping. 'Allogenic' megaflutes in the Hind Sandstone (Kane et al., 2009) also support this interpretation.
The Bowland Shale is heterogeneous, with a significant clay component, compared to the Barnett Shale (Loucks & Ruppel, 2007). Whilst the marine bands of the Upper Bowland Shale are likely the most prospective horizons (high TOC, carbonate and quartz cementation and widespread occurrence; Jarvie et al., 2007), the composition of each marine band varies considerably depending on basin position and age. Marine bands deposited during the early stages of basin infill, within inherited rift structures, are likely to be more complex than younger marine bands. Hybrid event beds exhibit lower TOC but are compositionally varied, due to their probable slope failure origin. Given the extensive cementation in these units, hybrid event beds may be considered prospective for unconventional hydrocarbon extraction. However, the geometry of such deposits is likely to be complex. Hind Clough is located in a relatively deep and confined part of the basin ( Fig. 2A; see also Kane, 2010), which likely explains the relatively abundant hybrid deposits.

CONCLUSIONS
The Bowland Shale is an organic-rich mudstone that exhibits substantial compositional heterogeneity. Geochemical and sedimentological analyses at the bed, hand specimen and thin section scale from three localities in the Craven Basin demonstrate: 1 The epicontinental Craven Basin was supplied by three sediment provenances. Firstly, detrital clay, coarse mud (silt) and sand supplied via turbulent and hybrid flows, including direct supply from the Pendle delta system; secondly, clay-rich mud clasts scoured from nearby mud-rich, potentially biostabilized slopes; and finally, pelagic and hemipelagic sediment, rich in clays, organic material (OM) and carbonate. 2 A variety of laminar, turbulent and hybrid flows developed during periods of reduced basin accommodation and/or fault activity. 3 Lenticular fabrics indicate persistent deposition of mud clasts from bedload. Mud clasts are interpreted as rip-up clasts, generated by bottom current scouring of partially consolidated mud. Lenticular mudstones represent mixtures of (hemi)pelagic and densite muds. Mud clasts were potentially biostabilized by microbial mats. The Pendle prodelta was likely the primary source for these mud clasts, demonstrating the far-reaching effects of the delta system. 4 Marine transgressions ('marine bands') promoted pelagic and hemipelagic settling, including fixation of abundant biogenic silica, and diminished mud density flows. 5 Lack of bioturbation and benthic faunal tests during the early phases of basin infill and in all marine bands suggests that bottom waters were at least intermittently anoxic. 6 Abundant organominerallic aggregates, silica enrichment, phosphatic faecal pellets containing radiolaria and pelagic macrofauna suggests that the water column was productive. 7 Candidate in situ microbial mats, and as ripup clasts in a variety of down-dip hybrid event beds, were potentially important consolidators of mud and burial of OM. 8 The Bowland Shale accumulated an order of magnitude faster than other epicontinental mudstones, such as the Barnett Shale, and the Lower Bowland Shale unit.
Epicontinental basins remotely linked to delta systems, such as the Craven Basin, were capable of rapidly accumulating both sediment and OM. This rapid accumulation has implications for understanding the role of epicontinental seaways as a carbon sink and the present day hydrocarbon prospectivity of these mudstones.