Lacustrine and palustrine carbonates in a Brigantian (late Dinantian) intrashelf basin in the Derbyshire carbonate platform

The Brigantian (late Dinantian) Headstone Laminite provides a record of freshwater carbonate deposition in an intrashelf basin when the surrounding Derbyshire carbonate platform was emergent. The laminated grainstone/carbonate mudstone facies contains calcified plant material, peloids, ostracods and lenses of calcite tufa deposited in a lake margin and sublittoral part of a freshwater lake in a humid palaeoclimate. Precipitation of tufa may have taken place along water courses or resurgences fed by meteoric run‐off and seepage from the karsted carbonate platform. The fenestral carbonate mudstone with calcrete and fenestrae represent deposition in well‐drained parts of the lake margin. A shift to more arid conditions is represented by the laminated dolomudstone facies with an allochem assemblage of calcified plants and micropeloidal dolomicrospar grains and occasional alginite in restricted a lake basin setting. Gypsum crystals and desiccation curls are also present in evaporitic lake margin settings. A return to more humid conditions is indicated by deposition of the laminated grainstone/carbonate mudstone facies with organic‐rich mudstone with thin coal seams and carbonized roots deposited in a poorly drained lake margin setting. This may be a reflection of a gradual transgression after emergence. The eastern margin of the intrashelf basin may have been formed by the Edensor Anticline; however, it is not known if a further complex of lacustrine, palustrine and lagoonal facies was present to the east. Deposition of graded beds containing lithoclasts derived from lacustrine facies were associated with slumping generated by reactivation of structures controlling the intrashelf basin margins.


| INTRODUCTION
During the Brigantian, an intrashelf basin was present in the Derbyshire carbonate platform in which reworked bioclastic limestones sourced from the surrounding carbonate platform were deposited in a marine environment (Butcher & Ford, 1973;Gutteridge, 1987Gutteridge, , 1989Gutteridge, , 1990;;Walkden, 1977) (Figure 1a).However, the intrashelf basin succession also contains at least two thin intervals of laminated dolomitic limestones with unusual bioclast assemblages (Figure 1b).These are the metre-thick 'Rosewood Marble' within the Monsal Dale Limestones Formation interpreted by Adams and Cossey (1978) as a slope or basin facies (without specifying deposition in fresh or marine conditions) and the 'Headstone Laminite' that occurs at the boundary between the Monsal Dale and Eyam Limestones formations (Figure 1b) (Aitkenhead et al., 1985;Butcher & Ford, 1973;Fowles, 1989;Gutteridge, 1989Gutteridge, , 1991)).The boundary between the Monsal Dale and Eyam Limestones formations is marked by calcrete, karst and vadose cementation over the shelf areas of the Derbyshire carbonate platform; however, there is no evidence of subaerial exposure at the base of the Headstone Laminite, which was deposited in the intrashelf basin while the surrounding Derbyshire carbonate platform was exposed (Adams, 1980;Gutteridge, 1983Gutteridge, , 1989Gutteridge, , 1991)).Brown (1973) interpreted the Headstone Laminite as a lacustrine deposit while Fowles (1989) and Gutteridge (1983Gutteridge ( , 1989) inferred a marine-influenced peritidal environment based on the bioclast assemblage originally interpreted as marine.These bioclasts and other allochems are re-interpreted in this study and the depositional model of the Headstone Laminite has been re-evaluated.Similarly, advances in the recognition and understanding of lacustrine and palustrine carbonate systems have extended the available facies models and case histories against which these laminated limestones and dolomites can be compared (Bertani & Carozzi, 1985;Mercedes-Martín et al., 2017;Paz & Rossetti, 2006;Tucker & Wright, 1990;Wright, 2012;Wright & Barnett, 2015).Lakes are well known from modern carbonate platform top settings such as the Bahamas and the Turks and Caicos, some of which are evaporitic (Hubert et al., 2018;Rasmussen & Neumann, 1988); however, there are few descriptions of platform top lacustrine systems from the geological record (Azerêdo et al., 2015).
Much research has also been done on the significance of pedogenesis and karstification during periods of exposure and flooding of Carboniferous carbonate platforms (Alekseeva et al., 2016;Davies, 1991;Vanstone, 1996Vanstone, , 1998;;Wright et al., 1997).This paper focuses on the microfacies and depositional environment of the F I G U R E 1 (a) Brigantian palaeogeography of the Derbyshire carbonate platform during deposition of the Eyam Limestone Formation (from Gutteridge, 1987) showing the intrashelf basin; reactivated structures that influenced sedimentation include the Longstone Edge Monocline (LEM) the Taddington-Bakewell Anticline (TBA), Edensor Anticline (EA) and the Bonsall Fault (BF).The study area is in the red box, and details of the localities are given in Figure 2. (b) Summary of the Asbian and Brigantian stratigraphy of the intrashelf basin from Butcher and Ford (1973), Aitkenhead et al. (1985) and Gutteridge (1989).Line of section is show on panel (a).
Headstone Laminite that provides an example of evolving carbonate deposition in an intrashelf basin during exposure and subsequent transgression during a period of high-frequency, high-amplitude sea level fluctuations (Horbury, 1989).
The Headstone Laminite is exposed at localities shown by Figure 2; it is in sharp contact with the under-and overlying intrashelf basinal marine bioclastic limestones and shales and comprises four facies (Figure 3):

| Description
The laminated grainstone/carbonate mudstone facies is between 0.5 and 3.5 m thick and occurs at the top and the base of the Headstone Laminite (Figure 3).It comprises millimetre-to centimetre-thick laminations of carbonate mudstone consisting of uniform micrite with no allochems that alternate with fine-grained, ungraded bioclast peloid grainstone (Figure 4a,b).Allochems within the grainstone layers are aligned parallel to the laminations with individual grainstone layers often dominated by a single allochem type such as silt-to fine sandsized micrite peloids with a structureless internal fabric.Ostracods with smooth, thin-shells, most of which have been disarticulated, are also present; however, it was not possible to determine the genera.
Other allochems include the following: • Unsegmented, tubular bioclasts 120-150 μm in diameter with walls 30-50 μm in thickness composed of micrite (Figure 4c).The tubes are mainly straight with no branches, but some are slightly curved or form entwined clusters (Figure 4d).
• Filamentous bioclasts that comprise tubular structures that are sometimes branched.These have a circular cross-section comprising an inner spar-filled tube with an internal diameter of 80-120 μm that has an internal micrite lining with an outer layer that consists of radial fibrous calcite crystals up to 200 μm in length (Figure 4e,f).
F I G U R E 3 Sedimentological logs and key of the Headstone Laminite at the localities shown by Figure 2.
The laminated grainstone/carbonate mudstone also contains tabular and domical bodies up to 1 cm in thickness and a few centimetres across (Figure 5a).These structures comprise an open framework of planar to undulatory layers of calcite crystals that occasionally also form tubular structures and botryoidal crusts (Figure 5b,c).Each calcite layer comprising elongate to bladed calcite crystals up to 200 μm in length aligned perpendicular to the layer (Figure 5d).The surface of some of these cement layers are micritized and are draped by and interbedded with layers of micrite, peloids and other reworked allochems up to 500 μm in thickness (Figure 5e,f).

| Interpretation
The unsegmented tubular bioclasts with micrite walls are interpreted as the superficial coatings of plant material by micritic calcite with later decay of the plant substrate.These may have formed either by direct precipitation of micrite onto the plant stem or by adhesion of detrital micrite to plant stems.The tubular filamentous bioclasts are interpreted as an original central tubular structure, now decayed, that formed a substrate around which the layer of radial fibrous calcite crystals were precipitated.The overall branching shape of these bioclasts suggests that they may represent precipitation of calcite around plant stems.Arenas et al. (2010), describe the accumulation of both micritic calcite and precipitation of radial fibrous cement around plant stems and leaves in palustrine and lake margin settings in which the plant material is ultimately preserved as moulds.
The tabular and domical lenses of layered calcite cement within the laminated grainstone/carbonate mudstone facies are interpreted as depositional structures because they have been draped by reworked allochems including ostracods and peloids.The presence of micritized surfaces indicates pauses of precipitation during growth.These structures and calcite fabrics resemble the travertine and 'dense to porous' tufa facies of Chafetz and Folk (1984) and Arenas et al. (2010) respectively comprising layers of coarse calcite spar encrusting surfaces or plant material alternating with layers of reworked bioclasts, and peloids and micrite.This microfacies is interpreted as calcite tufa that has been precipitated on the sediment substrate or around plant material that grew in fresh water supersaturated with calcium carbonate.In this context, the term tufa does not imply any inference about water temperature.These tufa fabrics are recorded from palustrine settings associated with ponded or flowing fresh water (Arenas et al., 2010;Pedley, 1990Pedley, , 2000;;Pedley & Rogerson, 2010).Deposition may have taken place along water courses in the emergent intrashelf basin or resurgences fed from the vadose zone in the surrounding exposed carbonate platform.There is no indication of desiccation or emergence suggesting that the lake margin was permanently wet.The laminated grainstone/carbonate mudstone without the inclusions of tufa may have been deposited in a deeper water lake basin setting.

| Description
The laminated dolomudstone facies is between 2.0 and 2.5 m in thickness.It is in sharp contact with the underlying basal laminated grainstone/carbonate mudstone abruptly and is overlain by the fenestral carbonate mudstone or the laminated grainstone/carbonate mudstone (Figure 3).At Locality 1, the laminated dolomudstone is overlain erosively by the lithoclast grainstone/packstone facies.The laminations consist of mm-to cm-thick pale cream-coloured layers of pure dolosiltite and dolomicrite that alternate with darker grey, more argillaceous dolomitic partings separated by dissolution seams along which organic matter and carbonized stems and leaves are concentrated (Figure 6a-c).Some bedding surfaces of the pale dolomudstone have straight or sinuous polygonal ridges a few mm across and in relief (Figure 6d).These ridges appear to be a linear zone of contorted dolomicrite laminations but with no corresponding downward penetrating syn-sedimentary fracture.Layered and nodular replacive chert is also common within the dolomudstone (Figures 6a,c and 7c).In thin section, the dolomudstone layers appear to have become separated along these organic-rich partings with gaps between the laminae cemented by dolospar (Figure 6e).Occasional spar-filled needle-and lath-shaped pseudomorphs with rectangular-to lozenge-shaped cross sections up • Occasional segmented, branched, tubular structures up to 750 μm in length and 300-350 μm in diameter with micrite walls (Figure 7c,d).Most of these structures consist of an inner hollow tube surrounded by an outer layer of polygonal cells with a smooth rounded outer surface.Occasional more irregular aggregates of polygonal cells with dolomicrite walls are also present.
• Rare tabular or elongate clasts of amorphous dark brown organic material that lie along the laminations (Figure 6b).An interval of chaotically brecciated laminated dolomudstone at locality 1 comprises compacted tabular clasts of up to 5 cm in size of pale-to dark grey laminated dolomudstone (Figure 7e).The clasts show interpenetrating and welded contacts on all sides.Clasts of paler dolomudstone contain spar-filled fenestrae and have angular margins with spar-filled fractures that terminate at the margins of each clast.
Clasts of more argillaceous darker dolomudstone often show strongly contorted bedding forming layers of complex folding; these clasts also appear to have been extruded between other clasts.

| Interpretation
The very fine lamination and very fine dolomudstone sediment with a  (Oschmann, 2000).
The ridges seen on bedding surfaces and fenestrae are interpreted as desiccation features indicting emergence.However, the lack of associated desiccation cracks suggest that the sediment may have been bound by microbial mats that inhibited the formation sub-vertical desiccation fractures but instead underwent shrinkage and curled up in response to desiccation (Shinn, 1983a).The dolomudstone layers may also have separated along organic-rich partings as a result of desiccation.Clasts of paler dolomudstone within the chaotic breccia contain spar-filled fenestrae and have angular margins with spar-filled fractures that terminate at the margins of each clast suggesting that these were deposited in emergent conditions and underwent early lithification.
Clasts of micropeloidal dolomicrospar are similar to fenestral dolomite that formed laminae, crusts and clasts in middle Devonian lacustrine carbonates from the Orcadian Basin (Janaway & Parnell, 1989).They interpreted the dolomitic lake sediments to form at lake margins or during periods of lake low stand when lake waters became concentrated by evaporation promoting dolomitization of the Argillaceous dolomudstone clast with complex internal folding that has been extruded between adjacent clasts (yellow arrow).Locality 1, scale in cm.lake sediments.In the Headstone Laminite, the morphology of spar-filled pseudomorphs (Figure 6f) are interpreted as synsedimentary growth of gypsum crystals within the dolomudstone, also supporting deposition in evaporitic conditions.
The laminated dolomudstone facies represents a range of emergent desiccated lake margin to subaqueous stratified lake basin depositional settings.

| Description
The fenestral carbonate mudstone facies is between 0.5 and 0.8 m in thickness; it occurs between the laminated dolomudstone and the uppermost laminated grainstone/carbonate mudstone facies, overlying the lithoclast grainstone/packstone facies at locality 1 (Figure 3).This is a non-laminated carbonate mudstone (Figure 8a,b); no allochems are present, but tubular and irregular fenestrae are often concentrated along layers.These include irregular fenestrae up to 1 cm in size that have a partial geopetal infill of micrite with the remaining pore space infilled by calcite spar (Figure 8c).Irregular fenestrae are sometimes surrounded by contiguous areas of pervasive jig-saw brecciation in which the fractures are infilled by geopetal sediment and spar (Figure 8d).Tubular fenestrae are 0.5 to 1.5 mm in diameter, most are sparfilled with a smooth internal surface (Figure 8e); however, some have an internal lining by darker micrite compared with the surrounding micrite (Figure 8f).The central part of the micrite lining is smooth and itself infilled by calcite spar.
At Locality 1, the fenestral carbonate mudstone is overlain by a 0.2-m thick bed of brown organic-rich siliciclastic mudstone grading upwards in to very dark grey to black organic-rich mudstone that contains several mm-thick layers of shiny coal (Figure 3).The underlying fenestral carbonate mudstone is penetrated by plant roots preserved as vertical traces of organic matter.

| Interpretation
The irregular fenestrae are interpreted as desiccation structures that formed as a result of episodic wetting and drying of carbonate mudstone that formed in an emergent setting that was episodically submerged (Shinn, 1968(Shinn, , 1983b)).Fenestrae with multiple fills may represent original tubular fenestrae formed by desiccation and then  (Shinn, 1983b).The micrite-coated tubular fenestrae are interpreted as rhizocretions that formed around plant roots and the pervasive jig-saw brecciation both of which represent calcrete features (Adams, 1980;Wright, 1982;Wright & Tucker, 1991), suggesting deposition in emergent well-drained lake margin settings.The thin coal and seat earth overlying a calcretised carbonate mudstone overlain by organic-rich mudstone may have formed in more poorly drained swampy conditions around the lake margin.Lignites have been recorded in palustrine carbonates deposited during the Holocene transgression of the Yucatán carbonate platform (Platt & Wright, 2019).

| Description
This facies consists of beds 0.25-1.0m in thickness that occur at various levels within the Headstone Laminite (Figure 3).It is thickest at Locality 1 where it forms an easterly thickening wedge, up to 3 m thick that comprises five graded beds of lithoclast grainstone/packstone that also thicken to the east (Figure 9a,b).The base of the facies is in erosional contact with the underlying laminated dolomudstone and each bed within the package truncates the underlying bed.The internal structure of beds comprises single or several normally graded units within erosional or abrupt internal contacts.The basal part of each bed consists of pebble-sized rounded lithoclasts that fine up into coarse-sand and finer lithoclasts.Most of the beds are grain-supported, although one ungraded layer has a dark muddy matrix and is matrix-supported.

Lithoclasts include the following:
• Rounded, elongate clasts composed of layered and botryoidal calcite cement similar to the calcite tufa described above (Figure 9c,d, 1).

| Interpretation
The sharp, erosional bases and grading of the beds indicate repeated deposition from high energy but waning currents.The range of lithoclasts indicates reworking from contemporaneous lake basin and lake margin facies described above.The bioclast packstone lithoclast was derived from limestone deposited in open marine conditions and was presumably derived by erosion and reworking of the surrounding exposed carbonate platform.

| Description
Both the laminated dolomudstone and laminated grainstone/ packstone facies have been affected by slumping and soft-sediment deformation at four of the localities studied (Figures 3 and 11).Folds in the laminated grainstone/carbonate mudstone facies tend to be open and upright (Figure 10a) while the folds in the dolomudstone tend to be recumbent, near isoclinal and are often underlain by lowangle layer-parallel discontinuities or listric thrusts (Figure 10b,c).The laminated dolomudstone interval also shows broad upright folds with a wavelength of several metres that are eroded by the overlying graded beds of lithoclast packstone that infill the underlying undulatory topography (Figure 10d).Palaeoslope directions were determined using the methods of Woodcock (1976Woodcock ( , 1979) ) by fold vergence directions, the trend of fold axes and direction of thrusting (Figure 11).Aitkenhead et al. (1985)

| Interpretation
The erosion and depositional draping of folds and thrusts indicates that the deformation was the result of syn-sedimentary slumping.The vergence and trends of slump fold axes indicate the presence of inward dipping palaeoslopes and the trend and dip directions of the intrashelf basin margins in which the Headstone Laminite was deposited (Figure 11).The presence of westward-directed slumping at locality 6 and the facies change and unconformity across the Edensor Anticline suggests that it was active during sedimentation and formed at least a partial barrier to the east of the intrashelf basin.Gutteridge (1989) noted a consistent association of slumped carbonates overlain erosively by resedimented carbonates throughout the Monsal Dale and Eyam Limestones deposited in the intrashelf basin and proposed that the resedimented carbonates were generated by retrogressive slope failure immediately after slumping (Pickering, 1979).Gutteridge (1989) also showed that the slumps and palaeoslopes within the intrashelf basin were related to reactivation of the fault-controlled basin margins and intrabasinal structures that took place repeatedly throughout the history of the intrashelf basin.The slumping and redeposition of carbonates observed in the Headstone Laminite is interpreted as a response to the tectonic evolution of the intrashelf basin rather than being linked to lacustrine processes.

| DEPOSITIONAL MODEL OF THE HEADSTONE LAMINITE
The Headstone Laminite was deposited in an intrashelf basin surrounded by the karsted Derbyshire carbonate platform to the north, west and south west and by the syn-sedimentary Edensor Anticline to the east (Figures 1a and 12).The estimated depositional relief between the platform carbonates and the intrashelf basin is a minimum of 25 m based on the thickness of shallowing units that represent progradational episodes of the intrashelf basin margins (Gutteridge, 1989).The depositional relief of the Edensor Anticline is not known.
During emergence, ground water was supplied to the intrashelf basin by a combination of meteoric run-off and seepage driven by the F I G U R E 1 2 Ground water and facies model of the Headstone Laminite.1. Emergence and onset of freshwater sedimentation in the intrashelf basin with deposition of laminated grainstone/carbonate mudstone with tufa in vegetated lake margin areas with calcrete and fenestrae.Tufa is precipitated in resurgences or water courses fed from super-saturated water derived from the surrounding karsted carbonate platform.Laminated grainstone with ostracods and peloids deposited in freshwater lake basin.2. Palaeoclimatic shift to net evaporation in the intrashelf basin.Deposition of laminated dolomicrite in desiccated emergent lake margin settings.Precipitation of micropeloidal dolomicrospar grains and crusts and evaporites in lake margin.Lake basin is stratified with preservation of alginite.3. Resumption of freshwater deposition of laminated grainstone/carbonate mudstone with tufa in vegetated lake margin areas.Organic-rich mudstone with thin coal seams develop in poorly drained lake margins caused by rising water table during relative transgression.Notes on key to sedimentary components: tubular micritic structures, for example, Figure 4c,d; internal cellular dolomicritic structure, for example, Figure 7c,d; radial fibrous calcite encrustation, for example, Figure 4e,f; micropeloidal dolomicrospar, for example, Figure 7a,b.topographic head of the surrounding karsted carbonate platform.
Seepage may also have taken place through the barrier formed by the Edensor Anticline; however, it is not known if the Edensor Anticline separated the intrashelf basin from a marine environment or if a further complex of lacustrine, palustrine and lagoonal facies was present to the east.This mixing of ground water provided a range of fresh and brackish water to a series of lacustrine and palustrine environments within the intrashelf basin.
The laminated grainstone/carbonate mudstone facies was deposited in lake margin and basin settings in freshwater from which calcite was precipitated around plants and as tufa along water courses or around spring resurgences fed by meteoric water from the surrounding karsted carbonate platform.Better drained parts of the lake margin areas are represented by fenestral carbonate mudstone and calcrete.
The deposition of the laminated dolomicrite facies indicates a change to more restricted saline conditions with the formation of micropeloidal dolomicrospar grains and crusts in the lake basin and margins together with the precipitation of gypsum crystals.During arid periods, the lake margins became desiccated and the lake basin facies within intrashelf basin shrank becoming restricted and stratified.This may represent a change to a more arid palaeoclimate during exposure.Vanstone (1996) recorded shifts between arid and humid palaeoclimate during periods of exposure in Asbian and Brigantian platform carbonates from northern England and Wales.
The palaeoclimate reverted to more humid conditions with increased input of fresh meteoric water and further deposition of the laminated grainstone/carbonate mudstone facies in lake margin and basin setting.The presence of organic-rich mudstone with thin coal seams and carbonized roots at the boundary between the laminated dolomicrite and the overlying grainstone/carbonate mudstone may be a reflection of poorly drained lake margin conditions as a result of rising ground water during the subsequent transgression.Wright et al. (1997) note that Dinantian palaeosols with thin coal seams and carbonized rootlet horizons formed during gradual flooding of emergent carbonate platforms, while pyritized palaeosols formed during rapid flooding by marine waters of emergent surfaces.This suggests that the intrashelf basin may have been part of an embayment in the Derbyshire carbonate platform that would have undergone gradual flooding rather than an intrashelf basin entirely surrounded by a carbonate platform that would have flooded rapidly.
The Headstone Laminite is interpreted as a transgressive freshwater carbonate system that was deposited in a similar range of lacustrine, palustrine and lagoonal settings as the modern Sian Ka'an wetlands in the eastern margin of the Yucatán Platform (Platt & Wright, 2019).Here a shallow karstic aquifer in the carbonate platform supplies freshwater into a series of freshwater lake and palustrine environments and brackish water lagoons along the coastal areas.Sea-level rise results in the landward migration of freshwater and shallow water marine environments.In contrast with the Sian Ka'an wetlands, the Headstone Laminite was underlain by impermeable marine intrashelf basinal carbonates that were unlikely to contribute to ground water flow.
The intrashelf basin was eventually flooded during the subsequent transgression and deposition of marine periplatform carbonates reworked from the surrounding carbonate platform resumed.

| CONCLUSIONS
The Headstone Laminite was deposited in an intrashelf basin at the Monsal Dale/Eyam Limestone boundary when the surrounding Derbyshire carbonate platform was emergent.Initial deposition took place in freshwater lake margin or lake basin setting, with ground water derived from the surrounding exposed carbonate platform, represented by the laminated grainstone/carbonate mudstone facies calcified plant material, peloids and ostracods with layers and lenses of calcite tufa.Well-drained lake margin settings are represented by fenestral carbonate mudstone with calcrete and fenestrae.
A palaeoclimatic shift to net evaporation is represented by the laminated dolomudstone facies including calcified plants and micropeloidal dolomicrospar grains and crusts and occasional alginite deposited in a restricted lake basin.While gypsum crystals, fenestrae and desiccation curls represent evaporitic lake margin settings.
The palaeoclimate reverted to more humid conditions, indicated by further deposition of the laminated grainstone/carbonate mudstone facies.The presence of organic-rich mudstone with thin coal seams and carbonized roots seat were deposited in poorly drained lake margin conditions as a result of rising ground water during the subsequent transgression.This also suggests that the transgression was gradual and that the intrashelf basin may have formed an embayment, rather than an enclosed basin, in the Derbyshire carbonate platform.Palaeoslopes inferred from slump structures suggest that the Edensor Anticline may have formed a barrier at the eastern margin of the intrashelf basin; however, it is not known if the Edensor Anticline separated the intrashelf basin from a marine environment or if a further complex of lacustrine, palustrine and lagoonal facies was present to the east.Deposition of graded beds of lithoclasts derived from lacustrine and palustrine facies were associated with slumping generated by tectonic reactivation of structures controlling the basin margins.
and b) Macroscopic appearance of the laminated grainstone/carbonate mudstone showing fine undulatory alternations of carbonate mudstone and fine grainstone laminae.Both 0.4-m vertical field of view from locality 1. (c) Straight and curved tubular bioclasts composed of granular micrite infilled by calcite spar.(d) Tubular bioclasts composed of granular micrite forming clusters.(e) Longitudinal section through filamentous bioclast with internal micrite-lined tube (arrowed) encrusted by outer layer of radial fibrous calcite (double arrow).(f) Transverse section through filamentous bioclast with internal micrite-lined tube (arrow) encrusted by outer layer of radial fibrous calcite (double arrow).(c) through (f) from locality 1 with 500 μm scale bars.
Lens of layered calcite in the laminated grainstone/carbonate mudstone (arrowed), locality 1.(b) and (c) Open fabric comprising layers of calcite cement forming layered and tubular interbedded with layers of peloidal sediment.(d) Detailed structure of calcite with radial fibrous calcite with hollow centre.(e) Layers of detrital peloids and calcarenite (double arrow) draping botryoidal structure (B).(f) Layer of calcite cement crust (double arrow) draping botryoidal cement with enclosed peloids (b) and micritsed surface (arrow).All thin section images from locality 1, with 500 μm scale bars.to 400 μm Â 800 μm occur in clusters or are concentrated in layers (Figure 6f).Allochems are scarce but include the following: • Bushy, tabular and irregular-shaped clasts up to 300 μm in size that consist of micropeloidal dolomicrospar with a clotted fabric and internal fenestral-like pores that have been cemented by dolospar (Figure 7a,b).
low abundance and diversity bioclast assemblage indicate deposition in a very low energy, restricted environment.The segmented and branched tubular allochems are interpreted as internal or surficial encrustation or F I G U R E 6 (a) Macroscopic appearance of the laminated dolomudstone showing fine undulatory dolomudstone laminae separated by thin argillaceous partings.Occasional replacive chert nodules are present (arrow) vertical field of view 0.5 m.Locality 1.(b) Laminated dolomudstone with layers of dolosiltite and doloarenite with alginite fragment (arrowed).Locality 1. 500 μm scale bar.(c) Silicified dolomudstone (top part of image with blue microporosity) and dolomudstone laminae.Locality 1. 500 μm scale bar.(d) Bedding surface on laminated dolomudstone with linear ridges of rolled laminae interpreted as desiccation curls, Locality 1, scale in cm.(e) Layerparallel spar-filled fracture indicates separation along organic-rich parting between laminae (yellow arrow).Locality 1. 500 μm scale bar.(f) Dolomudstone layer with pseudomorphs infilled by dolospar Locality 1. 500 μm scale bar.calcification of plant material and other plant debris indicate terrestrial conditions.The clasts of darker brownish more argillaceous dolomite within the chaotic breccia show evidence of plastic deformation but with no internal evidence of desiccation.The tabular and elongate clasts of amorphous dark brown organic material preserved along partings of the dolomicrite are interpreted as alginite formed by microbial alteration of organic matter in a water column
also mapped a change to shallower water facies and an intra-Brigantian unconformity over the crest of the N-S trending Edensor Anticline.F I G U R E 1 0 (a) Upright fold (arrow) in the laminated grainstone/carbonate mudstone.Locality 1. (b and c) Recumbent slump folds in laminated dolomudstone in layer at the top of the dolomudstone interval; fold axis arrowed.Locality 1.(d) Largescale low-amplitude slump folds sheet that are eroded by overlying beds of resedimented packstone/grainstone. Locality 1. F I G U R E 1 1 Palaeoslopes inferred from slump fold vergence and trend of axes; number of fold axes measured at each locality are shown.