Upper Devonian Research Articles

Microstructure of the Valves of the Genus Praeoehlertella Mergl (Family Discinidae) from the Upper Devonian Deposits of Kazakhstan

The shell microstructure of rarely discovered lingulate genus Praeoehlertella Mergl, 2001 was studied in detail. It was obtained in Famennian stage of Kazakhstan. The peculiarity of the microstructure the secondary granular layer was discovered. The prints of the cells of the inner epithelium of mantle were observed.

Ordovician and Devonian microfacies of Sierra Agua Verde, central Sonora, Mexico: A new locality of the Nuia Province from western Laurentia

The Lower Ordovician carbonate rocks exposed in the Tuntunudé hill north of the Sierra Agua Verde, northeast of the town of Mátape, in central Sonora, consists of intraformational breccia, conglomeratic limestone, sandy limestone and limestone, which are overlain by a mixed succession of quartz sandstone and dolomitic sandstone. This succession is in turn unconformably overlain by an Upper Devonian sequence of dolomitic limestone, marl, conglomeratic limestone and sandy dolomite. The Lower Ordovician rocks contain abundant bioclasts of gastropods, brachiopods, trilobites, echinoderms, and sponges. Their microfacies show a marine medium to low energy environment, with sedimentation dominated by distal tempestites in laminar stages and intraclast debris formation during deposition. The depositional paleoenvironment corresponds to a marine middle-to-outer ramp. These strata are characterized by conodonts belonging to the Oepikodus communis and Reutterodus andinus biozones, as well as cyanobacteria incertae sedis Nuia sibirica. Devonian rocks overlying the Ordovician strata contain bioturbation and abundant bioclasts of echinoderms, brachiopods, bryozoans, gastropods, chaetetiform sponges, stromatoporoids; and also, colonial corals like Hexagonaria attenuata, Phillipsastrea hennahii and Phillipsastrea jachowiczi, monothalamous foraminifera like Vicinosphaera sp., monoserials such as Tikhinella measpis and planispiral such as Nanicella gallowayi. The microfacies show a high-energy environment in marine tropical shallow open-sea lagoonal waters, with ooids and reef building organisms.

Brachiopods as indicators of the Upper Devonian Frasnian sedimentation in the Timan-Pechora oil and gas province

Brachiopods as indicators of the Upper Devonian Frasnian sedimentation in the Timan-Pechora oil and gas province

An overlooked contribution to Devonian studiesin the Holy Cross Mts: rediscovering Alexei Doronin’s 1893 article on the Kadzielnia Limestone

The unused Kadzielnia quarry in Kielce is the most publicly known geological site in the Holy Cross Mts and its geotouristic landmark (now the Kadzielnia Park and Reserve). The ‘rocky’ (coral) Kadzielnia Limestone, corresponding to a stromatoporoid-microbial mud mound, now treated as the Kadzielnia Massive Limestone Member of the Kowala Formation, has been studied for almost 200 years. This large bioherm is famous for its exceptionally diverse fossil inventory. The history of its geological exploration is insufficiently known, as exemplified by the overlooked contribution of Alexei Doronin, a master’s student at the Imperial University of Warsaw (IUW). In 1893, under the guidance of Professor Vladimir Amalitsky, he published a note on the age of the Kadzielnia Limestone. The paleontological basis for this discussion was the IUW student’s fossil collection, in which Doronin identified 28 taxa, mainly brachiopods (19 species). He was the first to recognize the occurrence of gastropods (e.g., Loxonema) and crinoids, and at least four species of brachiopods (e.g., Dielasma sacculus). In a comprehensive brachiopod-based stratigraphic discussion, Doronin emphasized the great similarity of the Kadzielnia assemblage to the Middle Devonian fauna of Western Europe, but also including the typical Upper Devonian species Rhynchonella cuboides. Ultimately, he advocated assigning a terminal Middle Devonian age to the Kadzielnia Limestone. Despite this erroneous dating, Doronin’s noteworthy but unnoticed contribution holds significant implications not only for further study of Kadzielnia but also for understanding the development of geosciences in the late 19th century in the Congress Poland (then part of partitioned Poland within the Russian Empire), including the academic role of the IUW. Unfortunately, for reasons unknown, the scientific career of Amalitsky’s talented pupil never truly began.

Fossilized soft tissues in tentaculitids from the Upper Devonian of Armenia: Towards solving the mystery of their phylogenetic affinities

The tentaculitids represent a significant and diverse group of mysterious fossils found in Palaeozoic strata. Their biology and evolutionary relationships remain poorly understood due to limited information about their soft body anatomy. The discovery of fossilized soft tissues in tentaculitids from the Upper Devonian of Armenia helps to resolve the old mystery of tentaculitid phylogenetic affinities. The thick bundles of phosphatic bars in the tentaculitid interior closely resemble the musculature of several tubicolous invertebrates, such as phoronids and bryozoans. Based on this similarity, we interpret these structures as fossilized muscles of the tentaculitid animal. The attachment of tentaculitid body muscles in a honeycomb pattern to the body wall is unlike that of any mollusc. Based on this muscle arrangement, molluscan affinities of tentaculitoids can be refuted. The muscle arrangement in tentaculitids is more akin to that of bryozoans than to phoronids. Presumably, tentaculitids and bryozoans shared a common ancestor. This ancestor was probably a solitary animal, with coloniality emerging later in the bryozoan branch of the phylogenetic tree. Alternatively, tentaculitoids might have evolved from a bryozoan-like ancestor by losing their coloniality.

The Frasnian stage of the Upper Devonian in Nakhichevan Autonomous Republic (Southern Transcaucasia)

Marine carbonate-terrigenous deposits of the Lower (partially), Middle, and Upper Devonian, rich in coral-brachiopod and other fauna, are widespread across the territory of the Nakhichevan Autonomous Republic. Various ore and non-metallic minerals are associated with these deposits: the Gyumushlug and Yaydzhinsky polymetallic deposits, etc. Further ex-pansion of the mineral resources of the study area, as well as the challenges associated with the establishment of large-scale geological mapping in this region, make the development of a detailed stratigraphic scheme crucial. An indispensable condition for the creation of such a scheme is the study of the most characteristic groups of ancient organisms, among which brachiopods play a dominant role in the organic world. Devonian deposits in the Southern Transcaucasus are distributed over a limited area – in the western part of the Na- khchivan Autonomous Republic – but they are widespread here and are represented by all three well-characterized faunal divisions. For the first time, a diagram has been compiled, reflecting the sequence of appearance and distribution over time of Devonian brachiopods. On this basis, a scheme for the zonal division of the Devonian deposits in the region has been developed, identifying 20 biostratigraphic zones and 16 stratigraphic units (formations). This study summarizes the scattered information contained in various sources on the paleontology and stratigraphy of the Devonian period in the region. The identified brachiopod zones are linked to standard conodont zones. The great importance of brachiopods is determined by the fact that they are one of the most effective groups of fauna not only for dating sediments in natural outcrops, but they also provide good results for broad interregional correlations. Based on the macrofaunal complex, the Frasnian age of the sediments within the formation in question is firmly established. A detailed biostratigraphic scheme with zonal division of the Devonian deposits of Transcaucasia was developed, based on the most important stages in the development of the brachiopod fauna. Biostratigraphic zones, formations, and subformations identified in the Devonian deposits of the Nakhchivan Autonomous Republic and adjacent areas, based on the study of the entire complex of organic remains, mainly brachiopods and cono- donts, can be compared with corresponding deposits of typical sections of Western Europe (Ardennes and Eifel Mountains) and many regions of Eurasia. The obtained data are of practical importance for theoretical geologists, prospectors and developers.

Mawson’s archival specimens from the birthplaces of petroleum exploration in Australia and North America: their historical and modern significance

ABSTRACT Sir Douglas Mawson, the eminent geologist-mineralogist and Antarctic explorer, was a professor and prominent researcher from 1921 to 1957 at the University of Adelaide where he began teaching in 1906. His little-known interest in the origins of crude oil is evident in two of his archived items with direct links to the birth places of petroleum exploration in the USA and Australia. The first, a sample of crude oil from Edwin Drake’s historic discovery well near Titusville, Pennsylvania, is unique for several reasons. Its saturated and aromatic hydrocarbon signatures, hitherto unreported, reveal it to be of sub-oxic marine origin and expelled from its source rock late in the conventional oil-generation window. Produced from the Upper Devonian Riceville Shale, it is the world’s first unconventional crude oil discovery. The second item is one of Mawson’s several specimens of coorongite recovered from Alfred Flat, South Australia, near the site of Australia’s first exploration well. It is a black rubbery material originally regarded as a product of local oil seepage. Later shown to be the desiccated remains of the freshwater alga Botryococcus braunii, its molecular signatures differ markedly from those of the three modern races of this alga. Coorongite is currently of interest as an intermediate in the production of biofuel.

Detrital Zircons From Lower Palaeozoic Metamorphic Complexes and Silurian–Devonian Strata in the South Kitakami Belt, Northeast Japan: Implications for the Northern Extension of the Terra Australis Orogen in Northeast Gondwana

ABSTRACTTo reconstruct the detailed location of proto‐Japan around northeastern Gondwana during the early Palaeozoic, this article presents detrital zircon U–Pb age spectra for lower Palaeozoic metamorphic complexes and Silurian–Devonian strata from the western margin of the South Kitakami belt in northeast Japan. Psammitic and siliceous schists have youngest age peaks ranging from the Ordovician (479–458 Ma) to early Silurian (ca. 439 Ma). Sandstones have youngest age peaks varying from the early Silurian (ca. 435 Ma) to Late Devonian (370–360 Ma). The U–Pb age spectra exhibit major peaks at 510–500, 480–470, and 450–440 Ma. Excluding the Upper Devonian sandstones, significant age peaks occur at 650–550 and 1300–900 Ma. These data can be interpreted as a Pacific Gondwana signature, indicating a tectonic association with the Terra Australis Orogen that developed along the northeastern Gondwanan margin and Paleo‐Pacific Ocean, extending from eastern Australia (i.e., the Thomson, Lachlan, and Delamerian orogens) to Antarctica (i.e., the Ross Orogen). In northeast Asia, the Pacific Gondwana detrital zircons and 480–470 Ma zircons occur in the Bainaimiao arc belt along the northern margin of the North China Craton and in the Jiangyu continental arc in the Jilin area. The magmatic arcs of proto‐Japan and these regions were part of the Terra Australis Orogen during the Ordovician–early Silurian. A decrease in the proportion of Pacific Gondwana detrital zircons in Upper Devonian strata may be due to multistage trench retreat, such as that recognized in the Tasmanides in eastern Australia, which corresponds to the northern Terra Australis Orogen.

The Silurian and the Devonian in the Svoge Unit, Western Bulgaria – a brief overview

Five stops, containing Silurian–Devonian strata of the Svoge Unit (Western Bulgaria) are described. At Stop 1 near the village of Tsarichina, Upper Devonian flysch (Katina Formation) can be observed, which ends the Paleozoic marine sedimentation in the area. The other two Devonian formations are observable at stop 2 near the village of Vlado Trichkov. These are the black shales of the Ogradishte Formation and the light shales of the Romcha Formation. At the village of Tseretsel (stop 3), all Silurian units are observed: the bedded cherts (lydites) of the Saltar Formation, the graptolite-bearing black shales of the Mala Reka Formation, and the grey to gray-green laminated shales of the Yabukov Dol Formation. The boundaries of the Silurian System, based on graptolites, were established at the next two stops, respectively at stop 4 the Silurian/ Devonian boundary and at stop 5 the Ordovician/Silurian boundary.

Middle and Upper Devonian depositional settings of the Moesian Terrane (northern Bulgaria)

The Devonian sediments from the Moesian Terrane (Moesian Platform, northern Bulgaria) are known only from oil and gas prospecting wells. Middle and Upper Devonian deposits are interpreted as formed in great variety of depositional settings. Thus, the Eifelian carbonates were regarded as inner- and mid ramp deposits developed in a shallowing-upward sequence. The shallowing tendency continued during the Givetian and Frasnian, when arid tidal-flat and lagoon settings predominated. At the beginning of the Famennian, a climate change occurred when the Moesian Terrane was removed from the Subtropical Arid to the Warm Temperate Climatic Zone. Then, carbonate deposition took place in various shallow- to deepermarine environments.

New ammonoid records and the definition of the base of the German Hemberg-Stufe (Famennian III, Upper Devonian)

In the revised ammonoid biostratigraphy of Becker (1993), the base of the German Hemberg-Stufe (Hembergian, middle Famennian, Upper Devonian III) was defined by the entry of Pernoceras dorsatum (Wedekind, 1918). However, this taxon is an invalid junior homonym of Tornoceras simplex var. dorsata Gürich, 1900 and Wedekind`s form was poorly defined and originally not illustrated. Based on a review of Wedekind material and unpublished collections from two trenches at the Enkeberg type section, the defining taxon of the Hembergian is re-assigned to Protornoceras genulobatum (Sobolew, 1914b). The species is discussed in the context of previous records of Tornoceras dorsatum and in the taxonomic framework of the Aulatornoceratinae, especially of the genera Protornoceras and Pernoceras. In the revised regional and international zonation scheme, UD III-A becomes a regional Prot. genulobatum Zone and international Protornoceras Genozone. Some other Protornoceras, questionable Falcitornoceras, and Pernoceras are described. These include Prot. rhenicum n. sp., Prot. umbilicatoides (Sobolew, 1914a), Prot. aff. delepinei (Petter, 1975), and Prot. aff. crebriseptum (Raymond, 1909) from the Enkeberg, Prot. aff. genulobatum from Beringhauser Tunnel, the Rhenish Perno. kochi (Wedekind, 1908), and the Russian Pernoceras kirsaense n. sp.

First palynological evidence from the Upper Devonian of Armenia (northern Gondwanan margin): biostratigraphic implications

Diverse and abundant terrestrial palynomorphs are recovered from the shales in the Frasnian–Famennian succession of the Ertych section situated in central Armenia. They represent the first discovery of spores from the Upper Devonian shallow marine sedimentary sequences of the South Armenian Block, which at that time was part of the northern Gondwanan continental margin. Their taxonomic assessment recognizes 37 spore taxa assigned to 22 genera. Notably, the presence of Teichertospora torquata allows us to assign the entire studied succession to the torquata–gracilis zone, indicating a late Frasnian to early Famennian age. In addition, the occurrence of Acinosporites lindlarensis, which corresponds to the spores of the herbaceous lycophyte Leclercqia, suggests that the age of the studied succession should be further restricted to the late Frasnian. The presence of chitinozoans (i.e., Angochitina mourai) in the studied interval argues for marine deposition. Our biostratigraphic results provide a more coherent picture for the sedimentary evolution of this Frasnian–Famennian sequence arguing for a rather uninterrupted sedimentation close to the critical boundary interval. The identified spore assemblage highlights the close affinity of the Frasnian vegetation preserved in Armenia with other parts of the Northern Gondwana and southern Laurentia. We provide the first evidence of the occurrence of Kraeuselisporites ollii outside Canada and extend its known age range to late Frasnian.

Don’t mind the “charcoal gap”: A reassessment of Devonian wildfire

Little evidence of macrofossil charcoal, a wildfire proxy, is recorded from upper Lower to lowermost Upper Devonian rocks. Coals of this age are few, and petrographic data indicate low volumes (<10% mineral-matter free) of charcoal. This paucity of data forms the basis of the “charcoal gap,” which is used to suggest an extended interval of abnormally low atmospheric oxygen (pO2). We reassess the current evidence for this hiatus using Emsian−Eifelian charcoal from the Trout Valley and St. Froid Lake Formations, Maine (northeastern United States), and integrate the microscopic charcoal record of dispersed organic matter. We conclude there is ample evidence of fire in the Middle Devonian. This interval is not innately of low pO2. Rather, it is one in which under-interpretation of available data has led to a perceived paucity of charcoal. This reconciliation indicates the Phanerozoic record of wildfire was substantially uninterrupted. Hence, we propose that pO2 achieved levels >16% and remained at such levels from the Silurian through the floral and faunal colonization of land and, from our current estimates, stayed as such until the present.

Sedimentary provenance of the Upper Devonian Old Red Sandstone of southern Ireland: an integrated multi-proxy detrital geochronology study

The Devonian Old Red Sandstone (ORS) magnafacies of southern Ireland is hosted in the Early Devonian Dingle Basin and the Late Devonian Munster Basin. Following the closure of the Iapetus Ocean during the Caledonian Orogeny, the Dingle Basin developed as a pull-apart structure before being deformed by Acadian tectonic activity. The Munster Basin developed as a half-graben structure in response to post-Acadian north–south extension in the region. Thus, the Irish ORS provides insights into the region's tectonic history owing to its temporal and spatial proximity to the Caledonian ( c . 475–425 Ma), Acadian ( c . 400–390 Ma) and Variscan orogenic events ( c . 390–290 Ma). This study presents the first detrital zircon and apatite U–Pb geochronological data for the Upper ORS (UORS) in southern Ireland in addition to detrital white mica 40 Ar/ 39 Ar geochronological data to help unravel the depositional history of the Irish UORS and to assess the possible role of sedimentary recycling in Late Devonian basin development. Most UORS samples contain few late Neoproterozoic detrital zircon grains and are instead dominated by early Paleozoic and c. 1.1 Ga zircons. These populations represent recycling of northerly-derived Ordovician to Silurian strata of the Southern Uplands–Longford–Down terrane, which are of Laurentian affinity, and not recycling of Lower ORS (which contains a significant number of late Neoproterozoic detrital zircons) as previously thought. Similar detrital zircon dates have been observed in Givetian–Frasnian quartzites of the Pulo do Lobo Zone on the Iberian Peninsula, providing a possible Rheic Ocean link with the UORS.

Bryozoan fauna from the Ferques Formation (Upper Devonian, Frasnian) of France

Bryozoan fauna from the Ferques Formation (Upper Devonian, Frasnian) of France

Assessing shale gas potential through classification of geochemical and mechanical properties of reservoir facies in the Upper Devonian shale succession, Liard Basin, Canada

Assessing shale gas potential through classification of geochemical and mechanical properties of reservoir facies in the Upper Devonian shale succession, Liard Basin, Canada

Early Frasnian (Upper Devonian) Genundewa Event in the Bolshaya Nadota River section of the Subpolar Urals (Russia)

Early Frasnian (Upper Devonian) Genundewa Event in the Bolshaya Nadota River section of the Subpolar Urals (Russia)

Morphogenesis of a new landform responsive to superimposed sulphate and carbonate buried karsts in western Canada

AbstractThis case study documents the morphogenesis of a new karstic landform consisting of a calcareous tufa mound with a gypsum caprock that developed at La Saline Lake, an oxbow of the Athabasca River in northeast Alberta, Canada. The development of this novel tiered carbonate‐sulphate mound architecture resulted from emplacement above the largest known buried salt dissolution collapse‐subsidence karst system, which was superimposed by an overlying buried carbonate karst. Dissolution of halite‐anhydrite beds of the Middle Devonian Prairie Evaporite, 200 m below, resulted in extensive subsidence and brecciation of the overlying Upper Devonian carbonate karst. The development of the calcareous tufa mound resulted from meteoric‐driven groundwater mixed with glacial meltwater directed along the shallowly buried karstic carbonate terrain of Middle‐Upper Devonian limestone, which floors the McMurray Formation, the main reservoir unit of the Lower Cretaceous Athabasca Oil Sands. The flow of the carbonate‐saturated groundwater responsible for the emplacement of the tufa mound at the surface was abruptly redirected along a deeper pathway in response to dissolution collapse‐subsidence adjustments within the buried salt karst of the Prairie Evaporite. The redirected groundwater flow deeper encountered an anhydrite‐dominated segment of the salt dissolution trend within the Prairie Evaporite Formation, 200 m below. The migration up‐section of sulphate‐saturated brine onto the surface of the previously formed tufa mound resulted in the emplacement of a gypsum caprock as a new landform.

Reservoir evaluation of dolomitized Devonian strata in the Western Canada Sedimentary Basin: implications for carbon capture, utilization, and storage

Abstract Differentially dolomitized carbonate strata in the Western Canada Sedimentary Basin (WCSB) are increasingly targeted for carbon capture, utilization, and storage (CCUS), yet few studies have evaluated the petrophysical characteristics of these conventional hydrocarbon reservoirs for this purpose. To address this, this study uses drill-core analysis (sedimentology, diagenesis, pore morphology, and distribution), together with core-plug and production data, to evaluate the properties of five depleted oil and gas fields in the Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group. The Swan Hills and Leduc formations are composed of reef, shoal, and lagoon deposits that are predominantly fossil-rich (e.g., stromatoporoid-dominated rudstones and boundstones). In contrast, the carbonate-ramp deposits of the Wabamun Group are fossil-poor, consisting instead of variably bioturbated carbonate mudstones, wackestones, and packstones. Replacement dolomitization is variable throughout each stratigraphic unit, but generally occurs within fossil-rich and/or heavily bioturbated intervals. Fracture densities are broadly comparable in limestone and dolostone. Porosity in the Swan Hills and Leduc formations is predominantly moldic and vuggy, occurring where fossils (e.g., stromatoporoids) are partially or fully dissolved. Pore space in the Wabamun Group is mostly restricted to intercrystalline porosity in burrows. In general, burial cements (e.g., calcite and dolomite) are volumetrically insignificant and only partially fill pores. Exceptions to this include porosity-occluding cements associated with fractures and breccias in the vicinity of faults. Dolomitization and depositional facies are found to exert a strong control on pore morphology, distribution, and interconnectivity. Porosity is principally controlled by the relative abundance of skeletal grains and by the presence of burrows. These highly porous facies acted as fluid pathways during burial diagenesis, resulting in their preferential dolomitization, solution enhancement of pre-existing pores, and creation of volume reduction-related porosity. The high CO2 storage capacity and low unplanned plume migration risk (due to depositional and/or diagenetic baffles) of dolomitized reefal reservoirs (e.g., Swan Hills and Leduc formations) make them more attractive targets for CCUS than those with limited capacity and/or potential migration pathways (e.g., fault-related fractures and breccias in the Wabamun Group). These results demonstrate that drill-core analysis, in combination with legacy data, can provide valuable insights into the factors that control reservoir CO2 injectivity, plume migration, and storage capacity.

OIL FAMILIES AND GEOCHEMICAL COMPOSITION OF DEVONIAN OILS AT THE RECHITSA FIELD, PRIPYAT BASIN, BELARUS

The sedimentary column at the Rechitsa oilfield in the Pripyat rift basin, Belarus, is dominated by an Upper Devonian synrift succession. The succession includes uppermost Frasnian and mid‐Famennian salt units which are about 1000 m and 2000 m thick respectively. Reservoir rocks consist of sandstones and carbonates in the intra‐, inter‐ and sub‐salt successions. In this paper, the geochemical analysis of 15 oil samples from different stratigraphic intervals at the Rechitsa field is used as a basis for reservoir characterisation. Geochemical studies included biomarker and stable C, N and S isotope analyses.Four genetic oil groups were identified and are referred to as Groups A to D. Oils in Group A came from upper intra‐ and inter‐salt reservoir rocks; the oils are early mature, enriched in heavy (C36+) hydrocarbons, heteroatoms, aryl‐isoprenoids and gammacerane, with low Pr/Ph = 0.6 and a sulphur isotope composition averaging 22.7‰ CDT. Oils in Group B were from sub‐salt reservoirs and are at peak maturity with Pr/Ph = 1, an increased proportion of C27 regular steranes, and a sulphur isotope composition of 8.1‰ CDT. The single oil sample in Group C was from a Proterozoic reservoir. The oil was overmature with a low content of heavy fractions, heteroatoms and steranes; its hopanes composition indicated that it was generated by the same source rock as the oils in Group B. Oils in Group D came from inter‐salt reservoir rocks and were composed of a mixture of Groups A and B oils in roughly equal proportions, as indicated by their average isotope, molecular and biomarker compositions.Observed differences in oil composition were explained in terms of contributions from at least two different source rocks together with variations in source rock maturity. Group A oils were interpreted to have been generated by Famennian carbonate‐rich source rocks containing dominantly marine and bacterial organic matter deposited in an anoxic evaporitic setting. Source rocks for Groups B and C oils were suggested to be composed of OM‐rich marine shales of Frasnian age or older.The geochemical characteristics of the Devonian oils from Rechitsa field, and the oil‐oil and oil‐ source rock correlations reported, will contribute to a better understanding of the petroleum system in the Pripyat Basin although direct oil‐ source rock correlations are not yet available. The presence of at least two source rocks for the Rechitsa oils has been suggested, respectively comprising carbonates in the inter‐salt succession and marine shales and/or carbonates in the sub‐salt succession. The main controls on oil composition in the Devonian reservoir units were the varying contributions from the different source rocks and differences in source rock thermal maturity associated with variations in burial depth and tectonics, together with the stratigraphic distribution of reservoir units which was in turn controlled by the presence of the thick Frasnian and Famennian salt units.