West, Ian M. 2013. A tidal flat evaporite facies in the Visean of Ireland. Revised web page based on original paper. Internet site: www.southampton.ac.uk/~imw/Visean-Evaporites.htm. By Ian West, Romsey and Southampton University. Version: 20th December 2010.

Ian West,
Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,
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This is not specifically related to the Wessex Coast, except that the evaporites resemble in some respects those in the Purbeck Formation of Dorset. This is revised but based on a paper published in 1968 (West, Brandon and Smith, 1968). New references have been added giving details of papers published since the original version of this work was published. A connection to the Wessex Coast is that Carboniferous evaporites are present in rock armour at Barton-on-Sea, Hampshire, but they are from the eastern Mendips, not from Ireland.

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A sequence of thinly bedded limestones, dolomites, shales and sandstones contains evaporitic beds in County Leitrim and Cavan, Republic of Ireland. This sequence constitutes the Aghagrania Formation (new name) of B2 to P1c age, (Upper Visean - upper Asbian to basal Brigantian), with a type section east of Drumshanbo, County Leitrim. The evaporitic beds which have not previously been recorded from this horizon or locality, are mostly unfossiliferous laminated limestones and dolomites with evaporite nodules and with pseudomorphs after gypsum, anhydrite, and halite. This facies of the Aghagrania Formation [now regarded as consisting of the Meenymore Formation (bottom), Glenade Sandstone Formation and the Bellavally Formation of the Leitrim Group] also includes evaporitic breccias, and celestite and carbonate replacements of calcium sulphate. Blocks of gypsum in boulder clay, on the shore of Lough Allen, are probably derived from these beds. The evaporitic strata alternate with shales and limestones containing marine faunas, and with unfossiliferous sandstones. All the facies show evidence of shallow water deposition and were probably formed in an area of low relief subjected to transgressions and regressions of a shallow sea. The evaporitic beds may be compared to the dolomite and gypsum deposits of present day tidal flats and associated shallow lagoons. They also resemble certain other occurrences of ancient laminated dolomite and limestone beds which been recently described and attributed to a tidal flat origin.

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Original Research Material

The purpose of this work was to determine the genesis and environment of deposition of evaporitic beds and associated strata discovered in the Upper Visean (Mississipian, Lower Carboniferous) of Counties Leitrim and Cavan, Republic of Ireland. Twenty-four exposures, mostly in stream beds, were measured in detail and systematically sampled. More than 100 specimens were tested for mineral content by X-ray diffraction, using a Philips diffractometer with Cu K-alpha radiation. Polished surfaces of the handspecimens were examined, and about 60 representative thin-sections were studied, chemical stains having been applied to many of these.

[These strata, referred to in 1968 as the "Aghagrania Formation" were subsequently classified by Brandon (1977) as the lower part of the Leitrim Group, which consists of the Meenymore Formation, the Glenade (Sandstone) Formation above it and the Bellavally Formation at the top. Thus For "Aghagrania Formation" now read "Meenymore Formation, Glenade Formation and Bellavally Formations". The recent work of Cozar et al. (2006) provides more data on the ages of these strata. The Aghagrania Formation is mostly upper Asbian, extending at the top just into the basal part of the Brigantian. In goniatite terms the formation corresponds to the B2a (upper part), B2b, P1a and P1b.]

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The Upper Visean strata of several areas of Leitrim and Cava, Republic of Ireland, have been described by Padget (1953), Oswald (1955), Caldwell (1959), Yates (1962), and Hodson and Moore (unpublished). Full references to earlier work, which commenced with that of Griffith in 1818, will be found in Yates (1962); these are not repeated here.
The upper part of the Visean strata of this region can be sub-divided into two major groups, differing in facies. Limestones with marine faunas comprise most of the lower group, approximately 3000 feet (915m) thick. The upper, however, consists predominantly of shales with some sandstones and thin limestones, and this group forms the Roscunnish Shales of Caldwell (1959), approximately 400 feet (122m) thick. The lower half of the Roscunnish Shales is a transitional sequence, comprising alternating beds of shale, limestone, dolomite, and sandstone, most of which have now been found to exhibit shallow-water features. In the course of field-mapping of these beds by two of the authors (Brandon and Smith), evidence for evaporites wasw found. The evaporitic beds outcrop within part of an outlier of Namurian and Upper Visean strata and this area is shown on the one-inch-to-one-mile, Irish Ordnance Survey sheets 44, 55, 66, and 67.

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The Aghagrania Formation - Localities and Exposures

The Visean beds with evaporites form a distinct unit of strata which is named in this paper [first named in the original version of the paper - 1968] the Aghagrania Formation. It is 71.6m thick in the type section in the bed of the Aghagrania River, east of Drumshanbo. (The rules of the American Commission on Stratigraphic Nomenclature (1961) and the Report of the Stratigraphical Code Subcommittee of the Geological Society (George and others, 1967) were applied when naming stratal units.

Fig 1 - A location map for the Visean Evaporite exposures of the Aghagrania Formation, County Leitrim, Ireland

Caldwell (1959) published a simplified horizontal section of the river exposure, and Yates (1962) provided faunal information. A vertical section obtained by detailed measurement is shown here. The base of the Aghagrania Formation corresponds approximately with the base of Caldwell's Roscunnish Shales but the top, taken as the highest evaporitic bed, much lower than the top of the shales. The thinly bedded heterogeneous strata of the Aghagrania Formation are underlain by massive coral-brachiopod limestones with "knoll-reefs". These form the Cavetown Limestone of Caldwell (1959). Dark shales with goniatites and lamellibranch faunas overlie the Aghagrania Formation.

F. Hodson, on the basis of studies of the goniatite faunas on Dough Mountain, County Leitrim, considered (personal communication 1967) "that the Aghagrania Formation includes beds of the upper part of B2, the whole of P1a and P1b and the basal part of P1c." Goniatites found in other parts of the outlier confirm this view.

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STRATIGRAPHY: The Aghagrania Formation

Vertical Sequence and Members

Vertical sections, Aghagrania Formation, Visean Evaporites, Leitrim, Ireland, Fig 2

Included below is a diagram modified after Brandon (1972), showing hypersaline facies in County Fermanagh. This is for comparison with the diagram above.

Hypersaline strata in the Asbian-Brigantian, Visean, Lower Carboniferous, Doagh Outlier, Northern Ireland, modified after Brandon, 1972

The formation can be sub-divided into several small stratigraphical units of distinctive lithofacies, which are here named as members. The Meenymore Member is the lowest of these and is named after a townland near the hamlet of Glenboy. Here, several stream exposures provide partial sections through the member, which is approximately 18.3 m thick. Poorly laminated carbonates with chert occur in the lower part, but the upper part is more finely laminated and consists almost entirely of unfossiliferous dolomite weathering to a yellowish grey color. At Aghagrania, further south, the same member is about 38.4 m thick and contains laminated dolomites, limestones, and shales. Carbonate replacements of anhydrite occur in association with breccias, and some chert is present. At this locality the member is apparently unfossiliferous except for a bed of shale, 1.4 m thick, with marine fossils.

At Aghagrania, the Meenymore member is overlain by a 3.6 m thick, unfossiliferous sandstone. This corresponds in position with the Glenade Member, a thick sandstone at other localities, and it probably represents a thin lateral margin. The sandstone member attains about 30 m. in thickness at Lannanerriagh, and it increases to about 76 m. near Carraun (fig. 1). The name "Glenade Sandstone" was applied by Oswald (1955), but it was not given member status. At Larkfield, above this sandstone, a thin unfossiliferous laminated limestone constitutes the Tullyskeherny Member. It is, in turn, overlain by the Larkfield Member, consisting of 3 m. of shales and limestones with fossils. The fauna indicates the return of marine conditions.

The Sraduffy Member follows and varies in thickness from about 7.6 m. to about 30 m . It is unfossiliferous throughout. The lower part contains laminated dolomites, breccias, and unfossiliferous grey shales. A thicker middle division consists predominantly of greenish grey mudstones and flaggy sandstones, and the upper part is mainly composed of grey shales and laminated carbonates. It is succeeded by about 6 m of fossiliferous shales and limestones belonging to the Lugasnaghta Member, and then by about 3 m of unfossiliferous laminated carbonates of the Drummangarvagh Member. A useful marker horizon is provided by the Doobally Member above, a medium-grained sandstone varying in thickness from 1.5 m to 4.9 m, and containing a bed of fine pebbles at Doobally, near Dowra. Continuing upwards in the stratigraphical sequence, unfossiliferous carbonates and shales of the Glenkeel Member follow. This member attains only 2.4 m in thickness and is limited to the northern part of the area studied. It is overlain by the Sheena Member (3.7 m in thickness) consisting mainly of shales with marine fossils. The uppermost member of the Aghagrania Formation is the Corry Member which varies in thickness from 1.5 m to 9.2 m. It is characterized by thin beds of unfossiliferous laminated dolomite and limestone separated by calcareous shales. The base is well defined by a bed with former calcium sulphate nodules.

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Fig 5 - An inclined block of gypsum with satin spar veins, in boulder clay, from Visean Evaporites, Dinantian, Mississipian, Leitrim, Ireland

Gypsum on the shores of Lough Allen was reported in 1857 by Kincaid, who believed it to be in place, underlain by limestone (of Carboniferous age), and formed by the alteration of such limestone. Wilkinson and Cruise later (1886) described it as consisting of drifted blocks in boulder clay. Cole and Hallissy (1924) also mentioned the gypsum, and they attributed it to the alteration of limestones by the decomposition products of iron pyrites [a rather unlikely theory for gypsum deposits of this thickness].

The gypsum could be seen as blocks in boulder clay in the west bank of Lough Allen. These occur for a total distance of about 1 mile where the shore borders Gubb, Lecarrow, and Drummans Lower Townlands. The blocks of gypsum are water-worn and weathered to a crumbly condition on the surface, but one large, very inclined block in the boulder clay, at a small promontory in Lecarrow Townland, attains a thickness of 2.2 m. A thin band of black shale, near the center of this, contains veins of satin spar. The gypsum sometimes shows a poorly defined laminated appearance. In thin-section, microscopic relics of anhydrite suggest that the rock is an hydration product of anhydrite and not, of course, a result of the alteration of limestone by the products of pyrite decomposition. No gypsum was found in place, although, with changes in level of the water in Lough Allen, previous observers may have seen exposures that are now submerged. Field mapping in the area suggests that the Aghagrania Formation lies beneath the boulder clay with gypsum. As it is unlikely that the blocks have been transported far in view of their size and their soft, soluble nature, they have probably been derived from these Visean, Carboniferous, beds.

Evaporites, in fact, are not as rare in the Lower Carboniferous of the British Isles as were once thought, although they had, when the paper was originally written, never been recorded from this horizon. Here are some other examples:

1. Anhydrite occurs in the Tournasian of Northern Ireland (Sheridan and others, 1967).
2. Gypsum is present in the Carboniferous in the Midland Valley of Scotland (Clough and others, 1925).
3. Anhydrite is present in the Hathern borehole near Nottingham, England (Falcon and Kent, 1960).
4. Remains of evaporites occur in the Carboniferous Main Limestone of the South Wales Coalfield. Bhatt, 1975. This is of C2S1 - Chadian to Holkerian age, and therefore rather older than that in the Leitrim area.
5. Evaporites facies in the Lower Carboniferous Cementstone Group of the Tweed Embayment, Berwickshire ( Scott, 1987).
6. ( Llewellyn and Stabbins, 1970;
7. Chadian evaporites in County Carlow, Ireland Nagy et al. 2005. These are significantly older than the Asbian-Lower Brigantian evaporite of Leitrim.

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Hollow moulds and pseudomorphs after halite in Visean dolomite, County Leitrim, Ireland

Clastic Casts of Halite Crystals, Lecarrow Shore, Visean, Carboniferous, County Leitrim Ireland

Evidence for the original presence of halite in the Aghagrania Formation has been found at several localities. Small calcite pseudomorphs after halite occur in laminated unfossiliferous dolomites and limestones in the Corry and Sraduffy Members of the Carraun section. Microscopic forms are present in a limestone breccia of the Sraduffy Member at Briscloonagh, and they occur at Larkfield at exactly the same horizon. Displacement structures suggest that the halite formed within the sediment. In Glenboy Townland, poorly defined calcite and fluorite pseudomorphs after halite are present in the Meenymore Member.

Clastic casts of halite crystals were found on the shore at Lecarrow, and although these were not in place, they were closely associated with debris from the Aghagrania Formation that can be matched in other exposures. The hopper faces of the casts attain a width of more than 6 mm, and they lie on the underside of fine sand laminae, sometimes rippled marked, which alternate with compact limestone. Thin bands of the latter show desiccation cracks.

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Macroscopic pseudomorphs of celestite after euhedral gypsum crystals reach a size of 3 cm in a dolomite of the Corry Member at Carraun. Microscopic pseudomorphs, however, are more frequent, particularly those of calcite after gypsum. These are sublenticular in form, and each is composed of several large sparry calcite crystals in a matrix of micrite. Examples occur in a laminated dolomite from the Corry Member at Bellavally Lower and in a breccia from the same member at Corry. Similar pseudomorphs have been found in the lower part of the Sraduffy Member at Briscloonagh. At Lecarrow, pseudomorphs occur in dolomite, and they consist of secondary anhedral gypsum after euhedral crystals of earlier, perhaps primary, gypsum. Pseudomorphs after a prismatic mineral, probably anhydrite, are also frequently found in thin-sections of the unfossiliferous Aghagrania carbonate rocks. They occur, for example, in the Drummangarvagh Member at Knockagullion.

Nodular (or macrocell structure of West, 1965) is a feature well developed in the Visean gypsum of Nova Scotia (Goodman, 1952) and in the Carboniferous gypsum and anhydrite of Spitsbergen (Holliday, 1966). In the Lecarrow gypsum it forms a coarse network (chicken-wire structure), when seen in section, with individual nodules of about 5 mm diameter. Isolated nodules of gypsum occur in laminated dolomite associated with this rock, and these are rimmed with pyrite, perhaps due to the action of sulphate-reducing bacteria.

Similar isolated nodules were encountered in laminated dolomite throughout the formation. At localities other than Lecarrow, however, the pyrite rims are present, but, instead of calcium sulphate, the evaporite nodules are occupied by large secondary crystals of celestite or calcite. Such nodules are best developed in rocks in which pseudomorphs after gypsum crystals are common, suggesting that, as in the Purbeck Beds of Dorset, the celestite or calcite is a replacement of calcium sulphate.

Replacement by calcium carbonate has sometimes occurred on a larger scale, and thin beds of secondary limestone have been formed by the diagenetic alteration of anhydrite. These rocks, which can be recognized by palimpsest fabrics and associated minerals, are considered in more detail below.

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Original Fig. 4 - Celestite replacement of sabkha-type evaporite nodules, associated with microbial mat dolomite, Visean Evaporites, County Leitrim, Ireland

Displacive gypsum forming incipient enterolithic veins, sabkha of the First Depression, El Hammam, between Alexandria and El Alamein, northern Egypt, 1978

Nodular celestite replacing calcium sulphate, Sruduffy Member, Aghagrania Formation, Visean evaporites, Cornamen, County Leitrim, Ireland

The replacement of calcium sulphate by the less soluble strontium sulphate is a common phenomenon; celestite is thus an ubiquitous accessory mineral in gypsum and anhydrite deposits. In the Aghagrania Formation it is found at most major exposures. Well developed clear euhedral crystals of celestite, with a bluish tinge, occur in evaporite nodules in the laminated dolomites of the Sraduffy Member at Cornaman. At this locality the culmination of celestite development is a rock which consists entirely of tile mineral, forming a bed 3.8 cm in thickness. The porous, coarsely crystalline texture of the Cornaman celestite, its chicken-wire or nodular structure, and the presence of microscopic relics of anhydrite are characteristic features of rocks that have resulted from the replacement of anhydrite. Celestite also occurs in the Sraduffy Member at Briscloonagh and in evaporite nodules of the Corry Member at Corry and Carraun. An unusual habit of celestite in the Aghagrania Formation is a fibrous variety resembling satin spar, of which it may be a replacement. This occurs in the Drummangarvagh Member at Glenkeel and in the base of the Sraduffy Member at Briseloonagh. The identity of both specimens was confirmed by X-ray diffraction. The Briscloonagh samples consist of bluish white veins from 1 to 7 mm in thickness, of which the individual fibres are curved and slightly oblique to the bedding.

A related sulphate mineral, barite, occurs in the Sraduffy Member at Corleckagh Upper and at Carraun. Narrow veins of this were also found in laminated carbonates of the Drummangarvagh Member at Aghagrania and in the Corry Member at Corry. This mineral has been reported from many evaporite deposits (for example, Goodman, 1952; Fisher, 1856), but, of course, unlike celestite, it is also common in non-evaporite strata.

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Samples collected from the Aghagrania Formation have been classified into a number of lithotypes. This classification was based on field observations, hand specimen studies, thin-section petrography, and by mineralogy as determined by X-ray diffraction.

The lithotypes fall into three main groups:- (1) those with evidence of evaporites, (2) lithotypes with marine fossils, and (3) other lithotypes. Most attention has been given in this paper to the evaporitic rocks. The colors, stated below, refer to the Rock-Color Chart of the Geological Society of America.

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Microbial Mat Dolomites and Limestones

Microbial map, laminated dolomite, Corry Member at Bellavally Gap, Visean Evaporites, County Leitrim, Ireland

Microbial mat lamination in unfossiliferous limestone, Corry Member, Carraun, Lower Brigantian, Visean Evaporites, Leitrim, Ireland

Thin beds, usually less than 1.22 m thick, of medium light grey, unfossiliferous carbonates with a fine lamination are common in the Aghagrania Formation . In thin-section, bioclastic debris is absent, and the laminae are usually thin, ruckled, carbonaceous layers separating bands of dolomite or limestone of micrite texture. They are apparently of microbial mat origin, and other microbial limestone textures are sometimes well developed. The lamination is sometimes marked by a variation in dolomite grain size, by an alternation of dolomite and calcite, or by layers of pseudomorphs after evaporites. The type of slightly ruckled lamination, the lack of marine fauna, and the association with pseudomorphs stongly indicates an origin in an intertidal sabkha environment, of the type often referred to as "algal mats".

A narrow zone of polygonally cracked, microbial mats, intertidal zone, Umm Said, Sabkha, Qatar

A narrow zone of microbial mats at the Umm Said Sabkha Qatar is shown above. This is in proximity to halite, which can be seen beyond. These particular microbial mats are not in a high carbonate environment. The well-known modern examples of Abu Dhabi are associated with carbonates and are thus better modern analogues for the Carboniferous environments.

Desiccation cracks are common in the Lower Carboniferous Aghagrania Formation. They have been found in such rocks in the Corry Member at Sheena, the Sraduffy at Carraun and Aghagrania, and the Meenymore Member at Glenboy. In the latter case they are of an unusually small size. Desiccation cracks have been produced experimently in subaqueous conditions by Burst (1965), but, as discussed by Matter (1967), these only formed if more than 2 percent of swelling clay minerals were present. Here the cracks often occur in fairly pure dolomites, and they are not better developed in the more argillaceous beds. It is very likely, therefore, that subaerial exposure of the laminated carbonates often took place.

Evidence of hypersaline conditions exists in the laminated carbonates as pseudomorphs of calcite, after halite, gypsum, and anhydrite. Celestite, quartz, and the lutecite variety of chalcedony are minerals found in these rocks that are frequently associated with evaporites elsewhere (Hutchins, 1962; Lacroix, 1897). Small-scale folding is a conspicuous feature of the laminated carbonates, and, indeed, it is rarely present in other rock types of the Aghagrania Formation. Series of small concentric folds occur within individual beds. Strata above and below are relatively unaffected. The folds have a wavelength of about 0.15 m to 0.6 m , vary in their acuteness, and are not consistently overturned in any particular direction. The fold axes. however, are nearly parallel at several different localities, and they trend predominantly NNE-SSW. A fine example of these contortions was seen in the Meenymore Member at Lannanerriagh where laminated dolomites are tightly folded.

Folding seems to have taken place while the sediments were relatively plastic since fracturing is found only in very tight folds. The constant association of the folding with the laminated carbonate suggests that these beds were once tectonically incompetent. Such a condition may have resulted from the original content of evaporites. Recent sediments from South Australia consisting of dolomite and calcite with halite are so plastic that locally they have been known as "pipe clay" (Alderman and Skinner, 1957). A plane of weakness produced by evaporites was an explanation suggested by Hollingworth (1938) for similar contortions in laminated limestones of the Purbeck Beds of Dorset, England. This theory has been supported by later work (West, 1964).

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Laminated Carbonates with Evaporite Nodules

The laminated carbonates pass into similar rocks containing nodules (fig. 4). These are predominantly dolomites rather than limestones, and the macrocells contain gypsum, as at Lecarrow, or replacements of gypsum such as celestite or calcite. These rocks are medium light grey in color and weather to a yellowish grey. Algal structures are frequently developed, and poorly defined algal tubules have been seen, but other fossils have not been found. In thin-section, the dolomite has a micritic texture, and sublenticular calcite pseudomorphs after gypsum are common.

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Porphyrotopic Gypsum with Nodular or Chicken-Wire Structure

Polished surface of Visean gypsum from Lecarrow, Lough Allen, Leitrim, Ireland

The gypsum at Lecarrow varies in color from very light grey to medium dark grey. It contains marl or shale impurities and large euhedral porphyrotopes (terminology of Friedman, 1965) sometimes with radiating orientations. The gypsum has a well developed nodular or chicken-wire structure, marked by the displaced impurities.

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Replacements of Calcium Sulfate

Several unusual rocks show structures and textures testifying to an origin by replacement of gypsum or anhydrite. They include the celestite rock from Cornaman, mentioned above , and a soft crumbly porous limestone, devoid of fossils and associated with breccia in the Meenymore Member at Aghagrania. It closely resembles limestone replacements of anhydrite described from the Purbeck Beds of Dorset (West, 1964). Two beds of a lutecite, quartz, and calcite rock occur in the Meenymore Member of the Aghagrania section. These are crumbly and porous with macrocell structure and resemble a weathered bed of gypsum in appearance. Both beds are completely unfossiliferous, and the lower one is associated with a breccia. Like similar strata in the Purbeck Formation of Dorset they appear to be partial replacements of anhydrite from which the remaining sulphate has been removed in solution.

At Bellavally Lower, a thin red limestone witll green deoxidation markings lies between a green mudstone, beneath, and a sandstone above. In thin section a palimpsest texture after elongated crystals of either gypsum or anhydrite is visible. This was the only rock of a red bed facies detected in the Aghagrania Formation, and oxidizing conditions were probably rare in those sediments.

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Unfossiliferous Carbonate and Shale Breccias

These breccias frequently display evidence of evaporites in the form of pseudomorphs after gypsum, anhydrite, and halite. Often cavernous with hollow macrocells, these rocks sometimes contain small spherulites of luteeite. An example in the Meenymore Member at Aghagrania consists of angular fragments of marl and shale in a matrix of calcite. The clearly laminated fragments of shale are randomly orientated, which implies that the clay sediment had first been converted into shale and then brecciated. These breccias are associated with other evaporitic rocks. At Cornaman, for example, a breccia passes laterally into dolomites with celestite. The breccias could be wholly, or partly, of collapse origin owing to the solution of evaporites. It is more probable that tectonic activity caused brecciation, however, while the calcium sulphate was still present and providing a plane of weakness. If the latter theory is correct, the brecciation may represent a related, but more extreme form of tectonic activity to that which merely folded certain of the laminated carbonates.

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Limestones with Marine Fossils

Thin-bedded grey compact limestones with fossils are also encountered in the Aghagrania Formation. These nnlaminated beds, usually less than 0.3 m in thickness, contain remains of goniatites and brachiopods. Rolled micrite-coated specimens suggest an origin in shallow water conditions.

Penecontemporaneous carbonate breccias with marine fossils were found at two localities in the beds immediately above the Aghagrania Formation. They appear to be of erosional type and provide further evidence for the shallow-water origin for the marine limestones. At Bellavally Lower such a breccia contains blocks of laminated dolomite, similar to the underlying bed, in a matrix of limestone. This suggests that both penecontemporaneous dolomitization and penecontemporaneous lithification took place. At Aghagrania an erosional breccia occurs in a similar stratigraphical position.

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Fossiliferous Shales

The fossiliferous shales are medium light grey in color and they are usually calcareous and pass into shaly mudstone. The fauna of these shales consists entirely of marine fossils, and there are no evaporites.

The occurrence of ammonoids and the pelecypod Posidonia has been observed by Newell and others (1953) to usually indicate stagnant basin conditions. Both are common in the fossiliferous shales of the Aghagrania Formation, but in view of the alternation of these shales with shallow water strata, it is unlikely that the shales were deposited in very deep water conditions. Moreover, the presence of a benthonic fauna in these beds suggests that the bottom conditions were not appreciably stagnant.

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Micaceous Sandstones

The sandstones in the Aghagrania Formation contain plant debris, sometimes in considerable quantities, but no other fossils. Ripple marks and ripple cross bedding have been found at many localities. Desiccation cracks and clay gall breccias [mud clast or rip-up clast breccias] are equally common, and discordant bedding often characterizes the bases of the sandstones, as in the Sraduffy and Doobally Members at several localities. All these sedimentary structures suggest shallow-water conditions. The lack of marine fossils and the common occurrence of plant debris may imply a fluviatile or deltaic origin.

These sandstones generally lack evidence of evaporites, although there is an exception in the Meenymore Member at Aghagrania. Here a thin bed of sandstone contains lenticular hollow moulds with horizontal long axes. It lies just above the insoluble residue of a bed of calcium sulphate, and the cavities may represent gypsum sand crystals, since removed in solution.

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Greenish-Grey Mudstones

Greenish-grey mudstones, resembling the Tea-Green Marls of the British Trias, characterize the Sraduffy Member at many localities. They also occur, to a lesser extent, in the Doobally Member at several exposures and in the Glenade Member at Aghagrania. These mudstones are always unfossiliferous and are sometimes micaceous and silty. They are interbedded with sandstones and frequently exhibit dessication cracks, suggesting that they were occasionally subjected to subaerial exposure.

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Unfossiliferous Calcareous Nodules

Calcareous nodules of a medium grey color often occur in the greenish-grey mudstones. The nodules frequently show septarian cracks and contain veins of barite with a feathery habit.

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Unfossiliferous Shales

Such shales are common, but definite evidence of their origin is lacking. Many of the beds probably originated in hypersaline water which prevented the development of a marine fauna, although some could have been laid down in fresh or brackish water.

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Unlaminated or Poorly Laminated Carbonate Rocks

The Aghagrania Formation contains some beds of unlaminated or poorly laminated limestones and dolomite. They are usually fine grained, compact rocks without fossils, and some of these, too, may be of hypersaline origin,but no positive evidence for this has been detected.

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Certain Aghagrania carbonate rocks appear to contain the mineral dolomite in thin section. Due, however, to the very fine grain size, acid and staining tests did not always provide satisfactory distinctions, and, furthermore, these rocks differ surprisingly little in appearance whether they are composed mainly of dolomite or of calcite. It was therefore necessary to test an appreciable number by X-ray diffraction using random powder samples in (Durofix) plastic glue. Rock chips and sedimented preparations provided additional information.

The results of the study show that a large proportion of the rocks contain both the minerals calcite and dolomite. Rocks with dolomite as the predominant mineral occur in greatest number among the evaporitic lithotypes, and those with calcite predominate among the lithotypes with marine fossils.

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Depositional Conditions Evidence for the conditions under which the Aghagrania sediments were deposited is provided by comparison with Recent sediments. The sedimentological study of the Visean, late Asbian and very early Brigantian, evaporitic beds has revealed certain significant features that would be expected in a Recent analogue.
They include:

1) The common occurrence of desiccation cracks.
2) The development of gypsum and halite.
3) The frequent occurrence of laminated, fine-grained dolomites, without fossils.
4) The presence of microbial stromatolites and the remains of microbial mats.
.5) The alternation of the evaporitic beds with marine shales and limestones.
6) The thin-bedded nature of the strata. Most of these characteristics could result from sediment deposition near mean sea level, with minor fluctuations above and below this datum. Dessication cracks, algal features, and the presence of evaporites could be due to very shallow water deposition in arid or semi-arid climatic conditions.

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Comparable Recent Deposits

The association of the Aghagrania evaporites with fine-grained dolomites is of interest. Recent dolomite has been found at appreciable depths in the sea (Fischer and Garrison, 1967), but it is present only in small quantities. Friedman and Sanders (1967) considered that most dolomite deposits in the geological record owe their origin to hypersaline brines and should be regarded as evaporite deposits. Dolomite indeed occurs at the present day in inland salt lakes, and these often display features present in the Aghagrania Formation.

Eardley (1938) reported the precipitation of dolomite in Great Salt Lake, Utah, and gypsum is present in the Recent deposits (Eardley and Stringham, 1952). Dolomite has also been found in the sediments of Lake Eyre in Australia (Bonython, 1956). The alternation of the Aghagrania evaporites with marine rocks suggests, however, that the evaporites originated in an environment nearer to the sea. In South Australia Recent dolomite has, in fact, been found in lagoons nearer to the coast such as the Coorong Lagoon and nearby Kingston Lake (Alderman and Skinner, 1957). Kingston Lake is only 0.3 to 0.6 m deep and becomes hypersaline, drying out in the summer. There is seasonal alternation in deposition of dolomite and halite which, after diagenesis, presumably could give rise to laminated carbonate rocks. Algal stromatolites are present (Mawson, 1929), and Alderman and Skinner attributed the deposition of dolomite to a high pH produced by the rich growth of water plants.In addition to these plants, copepods and small molluscs live in the water, at least during part of the year. The Aghagrania evaporitic carbonates, however, differ in that they contain no such faunal remains.

Another environment, one in which perhaps most Recent dolomite and gypsum has been found, is that of tidal flats. (The term "tidal flat" is used in this paper in a broad sense to include the intertidal fiats, supratidal flats, and coastal sabkhas of various authors.) Recent dolomite has been described from the shores of the Persian Gulf by Wells (1962), by Curtis and others (1963), and by Illing, Wells, and Taylor (1965). The tidal flats or sabkhas in which it is found appear to be best developed adjacent to embayments or lagoons. The dolomite has been attributed to penecontemporaneous alteration of aragonite sediments by hypersaline groundwaters, and Illing and others found that the content of the former mineral increased landwards in the supratidal sediments.

Gypsum also occurs in both the intertidal and supratidal parts of the Persian Gulf or Arabian Gulf Sabkhas (Shearman, 1966). In the intertidal areas it is associated with algal mats, and those cover a belt nearly half a mile wide. Anhydrite with macrocell or nodular structure is found in the supratidal sabkha (Shearman, 1966), although it is still uncertain whether these structures are entirely limited to such sediments (Stewart, 1966). Gypsum deposition also takes place subaqueously in shallow lagoons which border the Persian Gulf. The sulphate is deposited just offshore from lime mud, bound by algae (Bramkamp and Powers, 1955).

On the island of Bonaire, in the Netherlands Antilles, fine-grained dolomite is being formed on extensive supratldal flats which cover 13 square miles (Deffeyes and others, 1965). Its origin is, again, believed to be the replacement of lime sediments. Desiccation cracks are common, and occasionally, where erosion has occured, lime mud and sand form a matrix between clasts of dolomite. This provides an interesting analogue to the Asbian-Brigantian limestone with lithoclasts of dolomite, described above. Comparison can also be made between the Aghagrania evaporites and the Recent gypsum of Bonaire which is deposited in shallow hypersaline lakes within the area of the tidal flats.

There are many other Recent tidal flat sediments with features similar to those of the Aghagrania evaporitic carbonates. On Andros Island, in the Bahamas, Recent dolomite has been formed on tidal flats and is associated with laminations, stromatolites, and desiccation cracks (Shinn, Ginsburg, and Lloyd, 1965). On the mud flats of the Laguna Madre of Texas, gypsum and halite deposition takes place beneath the surface algal mat (Masson, 1955; Rusnak, 1960) and gives rise to algal laminated sediments. Algal laminated carbonate sediments are also well developed in the intertidal zone of Shark Bay, Western Australia (Logan, 1961) which, like the Laguna Madre, is a partially enclosed lagoon.

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It is thus apparent that a tidal flat environment would best account for the features of the Aghagrania laminated carbonates. Similar laminated unfossiliferous dolomites and limestones, often associated with algal textures and desiccation cracks, occur in strata of various ages, and have frequently been ascribed to tidal flat origins. Examples occur in the Ordovician of Western Maryland (Sarin, 1962; Matter, 1967), the Lower Devonian of New York State (Laport, 1967), in the Silurian of Ohio (Textoris and Carozzi, 1966), and, of particular interest, in the Visean of the Canadian Maritime Provinces (Schenck, 1967).

A significant difference between the Aghagrania evaporitic carbonates and the Recent analogues, discussed above, is that the latter usually contain shell fragments, washed or blown into the area. In thin section, the fine grain size and the clearly defined laminated fabric of the Aghagrania carbonates suggests that shell remains have not been obscured by recrystallization but were originally absent. There would be less likelihood of shells being drifted into the area, if the Aghagrania tidal flats were very extensive. Small hypersaline lagoons, in which the high salinity would prevent the colonization of the water by molluscs could account for the thicker gypsum deposits. Much calcium sulphate and halite, however, appears to have been developed on the tidal flats by evaporation of very shallow standing hypersaline water or by evaporation of groundwater when the flats were subaerially exposed. All the Aghagrania lithotypes, not only the laminated carbonates, probably originated in conditions near sea level. A slight rise in relative sea level would produce the marine beds, whereas a fall might account for the sandstones of presumably fluviatile or deltaic origin. The probable environments or formation of these various facies are shown diagrammatically in figure 14. Studies of goniatite faunas indicate that there is no major diachronism of the laminated carbonates and that certain thin beds can be identified throughout the area studied. Thus, the original extent of each depositional environment was probably very large, perhaps many kilometres. The sandstones, however, provide an exception, and they are less regular in their occurrence.

In the environment of low relief that would have followed a long phase of Lower Carboniferous sedimentation, the effect of minor sea level changes would be very great, This would have been particularly so at a time of shallow water conditions when minor transgressions and regressions would produce a cyclical succession of facies in any one section (fig. 2). Thus the Aghagrania Formation, in common with other shallow water deposits, is thin-bedded in character and varied in lithology.

[This Lower Carboniferous, Visean evaporitic facies is very similar, although on a smaller scale, to the evaporitic facies of Nova Scotia. It is also approximately of the same age. For more information on the equivalent across the Atlantic see: Calder (1998) and note, in particular the following:

Relevant extract on the Visean of Nova Scotia, from Calder (1998) page 279:

"The Visean has long been held to represent a semiarid palaeoclimate (Bell 1929; Schenk 1967a, b) in which strandline algal stromatolite carbonates, playa salt flats, halite, anhydrite and potash formed. Laminated carbonate of the Macumber Formation, basal Windsor Group, has been ascribed to semiarid seasonality (Schenk et al. 1994). Considerable evidence exists for the persistence of semiaridity during the late Visean following retreat of the Windsor Sea: playa lakes, deep desiccation cracks, gypsum casts, calcareous 'cementstone' beds (Belt et al. 1967: Neves and Belt 1970; McCabe & Schenk 1982; Crawford, 1995) and a conchostracan fauna (Copeland 1957)."

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The late Professor F. Hodson provided valuable help and advice, many years ago. Two of the authors, Brandon and Smith were at the time of the early research in receipt of grants from the Natural Environment Research Council.

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Alderman A. R. and Skinner, H. C. W., 1957, Dolomite sedimentation in the south-east of South Australia. American Journal of Science, vol. 255, pp. 561-567.
Bhatt, J. 1975. Evidence of evaporite deposition in the Lower Carboniferous Main Limestone Series of South Wales, U.K. Sedimentary Geology, vol. 13, issue 1, March 1975, pp. 65-70.
Abstract: Petrographic examination of the dolomitized Main Limestone Series cropping out in the south, east and northeast corners of the South Wales Coalfield Basin shows evidence of the earlier presence of evaporite minerals. However, it is believed that lack of extensive evaporite deposits in these rocks may be due to the active diagenetic dissolution and oncoming humid coal conditions of post Main Limestone time. The evaporite minerals in the Main Limestone rocks seem to be overwhelmingly early diagenetic in origin in the light of the following observations: (1) calcite or dolomite pseudomorphs after gypsum crystals associated with a fine pelmicrite matrix; (2) association of such pseudomorphs with oolitic pelsparite; and (3) evaporite solution breccia texture.
[The relics of evaporites occur in the Main Limestone in strata of C2S1 Zone (Chadian to Holkerian, 349.5-339.4 my). These are older than the evaporites in the Carboniferous of the Leitrim area, Republic of Ireland, which are Asbian to Brigantian.]
Bonython, C. W., 1956, The salt of Lake Eyre - its occurrence in Madigan Gulf and its possible origin. Royal Society South Australia, Transactions, vol. 79, pp. 66-90.
Bramkamp, N. A., and Powers, R. W., 1955, Two Persian Gulf lagoons (abstract). Journal of Sedimentary Petrology, vol. 25, pp. 139-140.
Brandon , A. 1968. The Geology of the Carboniferous Strata (Visean-Namurian) in parts of counties Leitrim and Cavan. Unpublished Ph.D. thesis, Southampton University.

Brandon, A. 1972. The Upper Visean and Namurian Shales of the Doagh, County Fermanagh, Northern Ireland. Irish Naturalist's Journal, vol. 17, No.5, January, 1972. pp. 159-170.
An account is given of the stratigraphy, palaeontology and distribution of some 240 to 260 metres of shale preserved by downfaulting in in an area of slightly older Visean sandstones. The outlier lies, at its nearest, about 11.3 km. north-north-east of Thur and Dough Mountains, Co. Leitrim, which are the northernmost hills in the Connaught Coalfield. These are the nearest areas where similar shales are preserved. The stratigraphical scheme formulated for the Lacklagh Hills, Dough and Thur Mountain area have been applied to the strata of the Dough Outlier and it is shown that signficant changes in the thickness and nature of the sediments occur in going southward towards the Dough Outlier.
[Evaporite features, including pseudomorphs after halite and gypsum are recorded in the strata above the Glenade Sandstone. See diagram.]

Brandon, A. 1977. The Meenymore Formation - an extensive intertidal evaporitic formation in the Visean (B2) of north-west Ireland. Report No. 77/23. Natural Environment Research Council, Institute of Geological Sciences, Her Majesty's Stationery Office, London. 14 pp. with folded insert of Figs 2 and 3 (vertical sections). By Dr. Alan Brandon. [the Meenymore Formation corresponds to the lowest part of the Aghagrania Formation of West, Brandon and Smith, 1968].
Summary: The stratigraphy, palaeontology and extent of the dominantly shallow water Meenymore Formation in the Upper Cracoean (B2) substage of the Visean of northwest Ireland is discussed. The formation, 0 to 244 m thick consists of a complex succession of thin-bedded lithological units occurring in various proportions in different areas. Deposition is cyclical though complete cycles are seldom developed. The main lithologies occur in the following sequence:
(e) Sandstones and siltstones, thin-bedded, ripple-marked, micaceous fluviatile.
(d) Shales, micaceous and silty, unfossiliferous.
(c) Shales, medium grey, fissile, unfossiliferous.
(b) Micrites, calcareous and dolomitic, yellowish grey, finely laminated, stromatolitic, associated with medium dark grey, unfossiliferous, fissile shales and with beds of replaced or residual gypsum and anhydrite.
(a) Shales and shaly mudstone, calcareous, marine fossiliferous, medium grey and with thin compact micrites.
With one exception, correlation of individual units within the formation has not been found possible. The formation is present throughout an area of remnant clastic outliers from Glenade in the west to Slieve Beagh in the east, and from the Fermanagh Highlands in the north to Slieve Anierin in the south (Figs. 1 and 6). It is absent only locally where 'reef knolls' protrude above the top of the Dartry Limestone. The formation contains characteristically, a fauna equally nektonic and benthonic in composition. The goniatites date the formation as B2 in age.
[Note the nomenclatural changes. The Aghagrania Formation of West, Brandon and Smith (1968), the paper that is revised in this webpage, contains the Meenymore Member at the base, the Glenade Member above, and then nine more members above (Brandon's Bellavally Formation). It is only the Meenymore Member, upgraded to the Meenymore Formation, that is discussed in this 1977 paper of Brandon, but with regard to a wider area. Brandon did not in 1977 use the name Aghagrania Formation when dealing with the whole region of northwest Ireland.]

Brandon, A. and Hodson, F. 1984. The stratigraphy and palaeontology of the late Visean and early Namurian rocks of north-east Ireland. Geological Survey of Ireland, Special Paper, No. 6, 1-54 pp. Published under the authority of the Minister for Energy by the Geological Survey of Ireland. ISBN 0085-1019. By Dr. Alan Brandon and the late Professor Frank Hodson.
The stratigraphy of approximately 500 square km of the Connaught Coalfield in Leitrim and Cavan including mainly the Lackagh Hills, Dough Mountain, Thur Mountain and the Slieve Anierin range is described. The stratigraphical succession reaches 520m in thickness. The beds range in age from Asbian to Arnsbergian. The succession, formally designated the Leitrim Group, can be subdivided into nine formations, all mutually conformable:-
Leitrim Group
Bencroy Shale Formation - 58m.
Lackagh Sandstone Formation - 60-90m.
Gowlaun Shale Formation - 55-64m.
Briscloonagh Sandstone Formation - 0-76m.
Dergvone Shale Formation - 85-168m.
Carraun Shale Formation - 46-52m.
Bellavally Formation - 30-46m.
Glenade Sandstone Formation - 4-60m.
Meenymore Formation - 0-38m.
The Meenymore Formation, usually resting conformably on the Dartry Limestone, consists mainly of an alternation of shallow-water, unfossiliferous, grey shales and laminated micritic dolomites and limestones, sometimes with evaporites or relic evaporitic textures. Occasionally the formation lies with a slight non-sequence on reef limestones of the Dartry Limestone. The Glenade Sandstone Formation is a southward - thinning wedge of thick-bedded, medium-grained, orthoquartzitic deltaic sandstone. It usually overlies the Meenymore Formation but is banked directly against reef knolls in the Dartry Limestone in places. It is overlain conformably by the interbedded, intertidal, cyclical deposits of the Bellavally Formation which range from thin fluviatile sandstones, through hypersaiine laminated carbonates with relic evaporite textures, to shallow-water marine shales and mud stones. The succeeding Carraun Shale Formation is marine throughout and minor cyclicity is shown by thin beds of micritic carbonate. At least four widespread thin K-bentonite beds occur near the top. The higher part of the Leitrim Group is markedly different: carbonate rocks are almost absent; the claystones are entirely shaly and the sandstones are much more thickly developed. The Dergvone Shale, Gowlaun Shale and Bencroy Shale Formations are lithologically similar - thin goniatite-bearing shales separated by thicker layers without fossils. The Briscloonagh Sandstone Formation consists of turbidites passing up into delta-top sandstones with a soil horizon at the top. The Lackagh Sandstone Formation is shallow-water throughout. It consists of coarse-grained, cross-bedded, delta-top sandstones and includes impersistent coals, some of which have been worked.
The faunas of the Leitrim Group also change upwards, shallow-water sessile benthos - coral, brachiopod, crinoid faunas - are present in the Bellavally Formation and the lower part of the Carraun Shale Formation but almost absent above. Mobile molluscs - cephalopods and pectinacean bivalves - commonly occur with the sessile benthos and abound in the higher beds of the group. Goniatite faunas representative of all the standard zones of the North of England Carboniferous from Upper Cracoean (top B2) through to Arnsbergian (E2b) are present, excepting those characteristic of the lowest Bollondian zone (Pla).
The main structural feature of the area is the Belhavel Fault zone - a north-east striking, reverse fault with monocline flexure. It had little known effect on sedimentation but slight movements on it may have been responsible for the abundant sandstone dykes in the lower part of the Dergvone Shale Formation and minor changes in the thickness of sediments towards the top of the Carraun Shale Formation.
Burst, J. F., 1965, Subaqueously formed shrinkage cracks in clay. Journal of Sedimentary Petrology, vol. 35, p. 348-353.
Caldwell, W. G. E., 1959, The Lower Carboniferous rocks of the Carrick-on-Shannon syncline: Quarterly Journal of the Geological Society, London, vol. 115, p. 163-187.
Calder, J. H. 1998. The Carboniferous evolution of Nova Scotia. Geological Society, London, Special Publications, vol. 143, Lyell: The Past is the Key to the Present. pp. 261-302. The paper is available online as a pdf file at the Lyell Collection, Geological Society, London. By J. H. Calder, Nova Scotia Department of Natural Resources, PO Box 698, Halifax, Nova Scotia, Canada B3J 2T9.
Abstract: Nova Scotia during the Carboniferous lay at the heart of palaeoequatorial Euramerica in a broadly intermontane palaeoequatorial setting, the Maritimes-West-European province; to the west rose the orographic barrier imposed by the Appalachian Mountains, and to the south and east the Mauritanide-Hercynide belt. The geological affinity of Nova Scotia to Europe, reflected in elements of the Carboniferous flora and fauna, was mirrored in the evolution of geological thought even before the epochal visits of Sir Charles Lyell. The Maritimes Basin of eastern Canada, born of the Acadian-Caledonian orogeny that witnessed the suture of Iapetus in the Devonian, and shaped thereafter by the inexorable closing of Gondwana and Laurasia, comprises a near complete stratal sequence as great as 12 km thick which spans the Middle Devonian to the Lower Permian. Across the southern Maritimes Basin, in northern Nova Scotia, deep depocentres developed en echelon adjacent to a transform platelet boundary between terranes of Avalon and Gondwanan affinity. The subsequent history of the basins can be summarized as distension and rifting attended by bimodal volcanism waning through the Dinantian, with marked transpression in the Namurian and subsequent persistence of transcurrent movement linking Variscan deformation with Mauritainide-Appalachian convergence and Alleghenian thrusting. This Mid-Carboniferous event is pivotal in the Carboniferous evolution of Nova Scotia. Rapid subsidence adjacent to transcurrent faults in the early Westphalian was succeeded by thermal sag in the later Westphalian and ultimately by basin inversion and unroofing after the early Permian as equatorial Pangaea finally assembled and subsequently rifted again in the Triassic. The component Carboniferous basins have provided Nova Scotia with its most important source of mineral and energy resources for three centuries. Their combined basin-fill sequence preserves an exceptional record of the Carboniferous terrestrial ecosystems of palaeoequatorial Euramerica, interrupted only in the mid-late Visean by the widespread marine deposits of the hypersaline Windsor gulf; their fossil record is here compiled for the first time. Stratal cycles in the marine Windsor, schizohaline Mabou and coastal plain to piedmont coal measures 'cyclothems' record Nova Scotia's palaeogeographic evolution and progressively waning marine influence. The semiarid palaeoclimate of the late Dinantian grew abruptly more seasonally humid after the Namurian and gradually recurred by the Lower Permian, mimicking a general Euramerican trend. Generally more continental and seasonal conditions prevailed than in contemporary basins to the west of the Appalachians and, until the mid-Westphalian, to the east in Europe. Palaeogeographic, paleoflow and faunal trends point to the existence of a Mid- Euramerican Sea between the Maritimes and Europe which persisted through the Carboniferous. The faunal record suggests that cryptic expressions of its most landward transgressions can be recognized within the predominantly continental strata of Nova Scotia. [relevant to Carboniferous evaporites in Ireland because of discussion of the Carboniferous Windson evaporites of the Nova Scotia, and the the late Dinantian semi-arid palaeoclimate.
Clarke, E. de D. and Teicher, C. (Curt Teichert), 1946. Algal structures in a West Australian salt lake. American Journal of Science, vol. 224, p. 271-276.
Clough, C. T., Hinxman, B. A., and others (Revised Edition by MacGreger, M., Dinham, C. H., Bailey, E. B. and Anderson, E. M.), 1925. The Geology of the Glasgow District. Memoirs of the Geological Survey of Scotland, 299 pp.
Cole, G. A. J. and Hallissy, T. 1924, Handbook of the geology of Ireland. London, 82 pp.
Cozar, P., Somerville, I.D., Mitchell, W.I. and Medina-Varea, P. 2006. Correlation of Mississippian (Upper Visean) foraminiferan, conodont, miospore and ammonoid zonal schemes, and correlation with the Asbian-Brigantian boundary in northwest Ireland. Geological Journal, vol. 41, pp. 221-241. Available online as a pdf file.
Abstract: The microbiota of the upper Visean (Asbian-Brigantian) rocks in the Lough Allen Basin in northwest Ireland is analysed. The Middle Mississippian sequence studied extends from the upper part of the Dartry Limestone/Bricklieve Limestone formations of the Tyrone Group to the Carraun Shale Formation of the Leitrim Group. The rocks have been traditionally dated by ammonoid faunas representing the B2a to P2c subzones. The Meenymore Formation (base of the Leitrim Group) also contains conodont faunas of the informal partial-range Mestognathus bipluti zone. The upper Brigantian Lochriea nodosa Conodont Zone was recognized by previous authors in the middle of the Carraun Shale Formation (Ardvarney Limestone Member), where it coincides with upper Brigantian ammonoids of the Lusitanoceras granosus Subzone (P2a). Foraminifera and algae in the top of the Dartry Limestone Formation are assigned to the upper Cf6 delta Foraminifera Subzone (highest Asbian), whereas those in the Meenymore Formation belong to the lower Cf6 delta Foraminifera Subzone (lower Brigantian). The Dartry Limestone Formation. Meenymore Formation boundary is thus correlated with the Asbian-Brigantian boundary in northwest Ireland. For the first time, based on new data, a correlation between the ammonoid, miospore, foraminiferan and conodont zonal schemes is demonstrated. The foraminiferans and algae, conodonts and ammonoids are compared with those from other basins in Ireland, northern England, and the German Rhenish Massif. Historically, the Asbian-Brigantian boundary has been correlated with several levels within the P1a Ammonoid Subzone. However, the new integrated iostratigraphical data indicate that the Asbian-Brigantian boundary in northwest Ireland is probably located within the B2a Ammonoid Subzone and the NM Miospore Zone, but the scarcity of ammonoids in the Tyrone Group precludes an accurate placement of that boundary within this subzone.
Curtis, R, Evans, G, Kinsman, D. J. J., and Shearman, D. J., 1963, Association of dolomite and anhydrite in the Recent sediments of the Persian Gulf. Nature, vol. 197, p. 679-680.
Deffeyes, K. S., Lucia, F. J., and Weyl, P. K., 1965, Dolomitisation of Recent and Plio-Pleistocene Sediments by Marine Evaporite Waters on Bonaire, Netherlands Antilles, p. 71-88 in Pray, L. C., and Murray, R. C., editors., Dolomitization and limestone diagenesis, a symposium. Soc. Econ. Paleontologists and Mineralogists, Special Publication 13, 180 p.
Dixon , O.A. 1972. Lower Carboniferous rocks between the Curlew and Ox Mountains Northwestern Ireland. Journal of the Geological Society, London, vol. 128, 71-101.
Eardley, A. J., 1938, Sediments of Great Salt Lake, Utah: American Association of Petroleum Geologists Bulletin, vol. 22, p. 1305-1411.

Eardley, A. J. and Stringham, B, 1952, Selenite crystals in the clays of Great Salt Lake. Journal of Sedimentary Petrology, vol. 22, p. 234-235.
Falcon, N. L., and Kent, P. E., 1960, Geological Results of Petroleum Exploration in Britain, 1945-1957. Geological Society of London, Memoir No. 2, 56 pp.
Fischer, A. G., and Garrison, R. F., 1967, Carbonate lithification on the sea floor. Journal of Geology, vol. 75, pp. 488-496.
Fisher, O. 1856, On the Purbeck Strata of Dorsetshire. Cambridge Philosophical Society Transations, vol. 9, p. 555-581. By the Rev. Osmond Fisher.
Friedman, G. M., 1965, Terminology of crystallization textures and fabrics in sedimentary rocks: Journal of Sedimentary Petrology, vol. 35, p. 643-655.

Friedman, G. M., and Sanders, J. E., 1967, Origin and occurrence of dolostones, p. 267-348 in Chilingar, G. V., Bissell, H. J., and Fairbridge, R. W., 1967, Developments in sedimentology, 9a, Carbonate Rocks. Elsevier, Amsterdam, 471 p.
Gallagher , S.J. 1996. The stratigraphy and cyclicity of late Dinantian platform carbonates in parts of southern and western Ireland. In: Recent Advances in Lower Carboniferous Geology, by Strogen, P., Somerville, I.D., Jones, GLI (editors) Special Publication 107, Geological Society, London, 239-251.
Goodman, N. R., 1952, Gypsum and auhydrite in Nova Scotia. Nova Scotia Department of Mines, Memoir No. 1, 75 pp.
Griffith, R. J., 1818, Geological and Mining Survey of the Connaught Coal District in Ireland. Dublin, 108 pp.
Harney , S.J., Long, C.B., MacDermot, C.V. 1996. Geology of Sligo - Leitrim. 1:100,000 scale map series, Sheet 7, Geological Survey of Ireland, Dublin.
Holliday, D. W., 1966, Nodular gypsum and anhydrite rocks in the Billefjordan region, Spitzbergen. Norsk Polarinstitutt, Arbok, 1965, pp. 65-73.
Hollingworth, S. E., 1938, The Purbeck Broken Beds: Geological Magazine, vol. 75, p. 33~332.
Hutchins, P. F., 1962, Authigenic minerals in Carboniferous sediments from Central Vestspitsbergen: Geological Magazine, vol. 99, 63-68.
Illing, L. V., Wells, A. J., and Taylor, J. C. M., 1965. Penecontemporary dolomite in the Persian Gulf, pp. 89-111 in Pray, L. C. and Murray, R. C., editors, Dolomitisation and Limestone Diagenesis, a Symposium. Society of Economic Paleontologists and Mineralogists, Special Publication No. 13, 180 pp.
Kelly, J.G. 1989. The late Chadian to Brigantian Geology of the Carrick-on-Shannon and Lough Allen Synclines, N.W. Ireland. Ph.D. Thesis, University College, Dublin.

Kelly, J.G. 1996. Initiation, growth and decline of a tectonically controlled Asbian carbonate ramp: Cuilcagh Mountain area, NW Ireland. In: Recent Advances in Lower Carboniferous Geology, by Strogen, P., Somerville, I.D., Jones, GLI (editors) Special Publication 107, Geological Society, London, 253-262.
Kincaid, J. 1857, A Section across the Coal Beds of Leitrim: Journal of the Geological Society of Dublin, vol. 7, p. 301-302. By Joseph Kincaid, Jr.
Lacroix, A, 1897, Le gypse de Paris et les mineraux qui l'accompagnent [The Gypsum of Paris and the Accompanying Minerals]: Nouvelle Archives du Museum D'Histoire Naturelle de Paris, [New Archives of the Natural History Museum of Paris] vol. 9, p. 201-296. By Alfred Lacroix. [This is good, detailed and well-illustrated work that shows the various crystal forms and crystal habits of gypsum, particularly lenticular gypsum. The gypsum discussed is the Eocene gypsum of Paris which has retained many early crystal habits which have not been destroyed by diagenesis such as change to anhydrite.]
Laporte, L. F., 1967, Carbonate deposition near mean sea-level and resultant facies mosaic: Manlius Formation (Lower Devonian) of New York State: American Association of Petroleum Geologists, Bulletin, vol. 51, p. 73-101.
Llewellyn , P.G. and Stabbins, R. 1970. The Hathern Anhydrite Series, Lower Carboniferous, Leicestershire, England, Institute of Mining and Mineralogy, 79 (1970), pp. B1-B15.
Logan, B. W., 1961, Cryptozoon and associate stromatolites from the Recent, Shark Bay, Western Australia: Journal of Geology, vol. 69, p. 517-533.
Masson, P. H., 1955, An occurrence of gypsum in South-West Texas: Journal of Sedimentary Petrology, vol. 25, pp. 72-77.
Matter, A. 1967, Tidal flat deposits in the Ordovician of Western Maryland: Journal of Sedimentary Petrology, vol. 37, pp. 601.-609. By Albert Matter.
Mawson, D. 1929, Some South Australian algal limestones in process of formation: Quarterly Journal of the Geological Society, London. vol. 85, p. 613-621. By Sir Douglas Mawson.
MacDermot , C.V., Long, C.B. and Harney, S.J. 1996. Geology of Sligo - Leitrim. A Geological Description of Sligo, Leitrim and adjoining parts of Cavan, Fermanagh, Mayo, Roscommon, to accompany bedrock geology 1:100,000 scale map series, Sheet 7, Sligo-Leitrim. Geological Survey of Ireland, Dublin.
Nagy , Z.R., Somerville, I.D., Gregg, J.M, Becker, S.D. and Shelton, K.L. 2005. Lower Carboniferous peritidal carbonates and associated evaporites adjacent to the Leinster Massif, southeast Irish Midlands. Geological Journal, vol. 40, Issue 2, pp. 173-192. By Zsolt R. Nagy, Ian D. Somerville, Jay M. Gregg, Stephen P. Becker, and Kevin L. Shelton.
Abstract: Analysis of a 275 m-thick section in the Milford Borehole, GSI-91-25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?-late Chadian to late Holkerian (Viséan, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68 m-thick, well-developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian–Holkerian strata, transgressive–regressive carbonate units are recognized. These comprise high-energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite-bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif. The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base-metal sulphide deposits.
Newell, N. D., Rigby, J. K., Fischer, A. G., Whiteman, A. J., Hickox, J. E. and Bradley, J. S., 1953. The Permian Reef Complex of the Guadelupe Mountain Region, Texas and New Mexico; A Study in Paleoecology. W. H. Freeman, San Francisco. 236 pp.
Oswald, D. H. 1955, The Carboniferous rocks between the Ox Mountains, and Donegal Bay. Quarterly Journal of the Geological Society, London. vol. 111, p. 167-186.
Padget, P. 1953, The stratigraphy of Cuilcagh, Ireland. Geological Magazine, vol. 90, pp. 17-24. By Peter Padget.
Ramsbottom, W.H.C. Transgressions and regressions in the Dinantian: a new synthesis of British Dinantian stratigraphy. Proceedings of Yorkshire Geological Society, vol. 39, 567-607.
Rusnak, G. A., 1960, Sediments of the Laguna Madre, Texas. pp. 153-196 in: Shepard F. P., Phleger, F. B., and van Andel, T. H., 1960. Recent sediments, Northwest Gulf of Mexico. American Association of Petroleum Geologists, Tulsa, Oklahoma, 394 ppp.
Sarin, D. D., 1962, Cyclic sedimentation of primary dolomite and limestone. Journal of Sedimentary Petrology, vol. 32, p. 451-471.
Schenck, P. E., 1967, The Macumber Formation of the Maritime Provinces, Canada - A Mississippian analogue to Recent strand-line Carbonates of the Persian Gulf. Journal of Sedimentary Petrology, vol. 37, p. 365-376.
Scott, W.B. 1987. Nodular carbonates in the Lower Carboniferous, Cementstone Group of the Tweed Embayment, Berwickshire: evidence for a former sulphate evaporite facies. Scottish Journal of Geology, vol. 22. pp. 325-345.
Abstract: A small number of coarsely crystalline nodular carbonate beds of calcite and dolomite are intercalated within lacustrine sediments in the Lower Carboniferous Cementstone Group of the Tweed area. Textural and mineralogical features indicate that they represent former nodular anhydrite beds which developed in lake marginal sabkhas. The close association of these replaced sulphates with beds of cementstone (finely crystalline dolomites) favours a sabkha model for cementstone dolomitisation in which sulphate precipitation helps to promote the requisite Mg/Ca ratios in the dolomitising brines. Two main stages of diagenesis, each dominated by mineral replacement, appear to have affected the sulphate facies subsequent to its formation: initially the anhydrite was subject to partial gypsification and then, at a later stage of diagenesis, dolomite and calcite replacements of sulphate occurred.
Shearman, D. J., 1966, Origin of marine evaporites by diagenesis. Instution of Mining and Metallurgy, Transactions. Section B, vol. 75, pp, B208 - B215.
Sheridan, D. J. R., Hubbard, W. F. and Oldroyd, R. W., 1967, Tournaisian Strata in Northern Ireland. Scientific Proceedings of the Royal Dublin Society. vol. 3, pp. 33-37.
Shinn, E.A., Ginsberg, R.N. and Lloyd, R.M. 1965. Recent supratidal dolomite from Andros Island, Bahamas, pp. 112-123 in: Pray, L. C., and Murray, R. C., Dolomitization and Limestone Diagenesis, a Symposium. Society of Economic Paleontologists and Mineralogists, Special Publication, 13, 180 pp.
Stewart, F. H., 1966, Discussion of paper by Shearman. Transactions of the Instution of Mining and Metallurgy, Section B. vol. 76, p. B 83.
Textoris, D. A. and Carozzi, A. V., 1966, Petrography of a Cayugan (Silurian) stromatolite mound and associated facies, Ohio. American Association of Petroleum Geologists, Bulletin, vol. 50, p. 1375-1388.
von Bitter, E H. and Austin, R. L. 1984. The Dinantian Taphrognathus transatlanticus conodont range zone of Great Britain and Atlantic Canada. Palaeontology, vol. 27, pp. 95-111.
Wells, A. J., 1962, Recent dolomite in the Persian Gulf. Nature, vol. 194, p. 274-275.
West, I. M., 1964, Evaporite diagenesis in the Lower Purbeck Beds of Dorset: Proceedings of the Yorkshire Geological Society, vol. 34, p. 315-330.

West, I.M. 1965, Macrocell Structures and euterolithie veins in British Purbeck gypsum and anhydrite. Proceedings of the Yorkshire Geological Society, vol. 35, p. 47-58.

West, Ian M. 2010. Geology of the Mendip Hills. Webpage in preparation. Associated with Geology of the Wessex Coast Webpages. Discussion of evaporites in Carboniferous Limestone blocks, that have originated in the Torr Works Quarry (formerly Merehead Quarry) near Shepton Mallet, Somerset.

West , I.M., Brandon, A. and Smith, M. 1968. (Revised 2010). A tidal flat evaporitic facies in the Visean of Ireland. Journal of Sedimentary Petrology, vol. 38, No. 4, pp. 1079-1093, Figs 1-14. December, 1968. For the original, as a pdf file online , go to: West et al. 1968. A tidal flat evaporitic facies in the Visean of Ireland.
Wilkinson, S.B. and Cruise, R.J. 1886. The District around Swanlinbar, Florencecourt and Dowra. Memoirs of the Geological Survey of Ireland (for sheet 56), Dublin, 23 pp.
Yates, P. J., 1962, The palaeontology of the Namurian Rocks of Slieve Anierin, County Leitrim, Eire. Palaeontology, vol. 5, p. 355-443.

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