Rya Formation


The Rya Formation is a geologic formation in Skåne County, southern Sweden. It is Early to early Middle Jurassic in age. The Rya Formation comprises siltstones, claystones, sandstones, mudstones and rare coal beds. The formation overlies the Höganäs Formation and is overlain by the Vilhelmsfält and Mariedal Formations.
The formation was deposited in the Höganäs and Øresund Basins that formed in the earliest Jurassic as part of the break-up of Pangea. The thick formation comprises four members, from base to top the Döshult, Pankarp, Katslösa and Rydebäck Members. The depositional environment of the formation ranges from continental to open marine.
The Rya Formation has provided fossils of a number of sharks, ammonites, bivalves and ichnofossils. Coalified wood occurs as scattered pieces up to long and indeterminate belemnites, echinoids, serpulids, ostracods and nodosariid foraminifera were also recorded in the formation. Iron ooids containing erratic boulders, called Geschiebe in German, attributed to the Rya Formation were found in Holstein, northern Germany.

Description

The Rya Formation crops out in Colonus Shale Trough of western Scania.
The formation overlies the Höganäs Formation and is overlain by the Vilhelmsfält Formation in the Helsingborg area and by the Mariedal Formation in the area of Landskrona and Kävlinge. The Rya Formation is subdivided, from base to top, into the Döshult, Pankarp, Katslösa and Rydebäck Members. The formation is found in the Ängelholm, Helsingborg, Landskrona and Kävlinge areas. In southwest Skåne, the Rya Formation is missing or only
poorly developed. In the Øresund Basin between Sweden and Denmark, the formation is truncated by the Basal Middle Jurassic unconformity.
Erratic boulders, called Geschiebe in German, attributed to the Rya Formation and containing iron ooids were found in Holstein, northern Germany.

Subdivision

;Döshult Member
The early Sinemurian Döshult Member comprises coarse-grained cross-layered sandstones and siltstones in the lower part, and is dominated by dark clays and marls rich in marine fossils in the upper part. The member is up to thick in the Ängelholm, Helsingborg and Landskrona areas. Presently, the basal part of this member is exposed at three localities in the Helsingborg area. These contain mineralogically and texturally mature, trough cross-bedded sandstones, commonly with herringbone structures showing north and south oriented paleocurrent directions. The occurrence of herringbone structures in well-sorted sand suggests high energy foreshore to subtidal marine depositional conditions for the lower part of the member. In an abandoned quarry in northwest Skåne the upper part of the Döshult Member commences with bioturbated marine nearshore sands, including burrows, as well as abundant marine invertebrate body fossils. This is followed by a bioturbated shelf mudstone with storm-deposited sand and silt intercalations. A massive red mudstone with scarce marine body fossils and burrows follows, which is interpreted as having been deposited rapidly, in a low energy but oxidizing environment. The youngest part of the succession comprises siltstones and mudstones, with carbonate-rich beds, deposited in a shallow marine setting. Coaly detritus, muscovite, very small shells and shell fragments, and framboidal pyrite nodules are characteristic constituents of this member.
;Pankarp Member
The dark mudstones of the Döshult Member are overlain by the Pankarp Member with a sharp conglomeratic boundary. The late Sinemurian Pankarp Member has an estimated thickness of up to in the subsurface of the Ängelholm, Helsingborg and Landskrona areas. In the Kävlinge area, the thickness is about thick. In westernmost Skåne, the member has been observed in small diameter drill cores. There, the member is subdivided into a lower unit of variegated clays and shales, a middle, poorly sorted silty to sandy unit including a coal bed, and an upper monotonous mudstone unit which is silty and rich in organic matter at the base, and reddish–greenish at the top. It presents different coloration probably due to different degrees of oxidation of iron in the claystone. In the uppermost part of one core, the Pankarp Member comprises lenticular bedded heteroliths with Planolites burrows.
;Katslösa Member
The late Sinemurian to early Pliensbachian Katslösa Member is mainly known from the subsurface in westernmost Skåne, and it has a thickness of in the Ängelholm, Helsingborg and Landskrona areas. In the Kävlinge area, the thickness is about. Sedimentological interpretations are mainly based on the results of petrographical studies of museum collections. The Katslösa Member yields a rich marine microfauna and macrofauna, and it is dominated by homogeneous mudstone deposited in a marine low-energy environment. Is composed mostly by marine green, brown and dark gray claystones and sandstones. Thin beds of matrix-rich quartz wackes are common. They are typically mineralogically mature but texturally highly immature with abundant angular sand grains. The matrix comprises organic matter, micrite, mica and clay minerals. In thin section, the sandstones show evidence of intense burrowing, which has obliterated depositional structures. Scattered berthierine ooids, as well as authigenic siderite crystals have been observed in the member.
;Rydebäck Member
The late Pliensbachian to late Aalenian Rydebäck Member is up to thick in the Ängelholm, Helsingborg and Landskrona areas. It is only known from subsurface material in westernmost Skåne, and sedimentological analysis is based on observations from two wells. These layers were deposited in small bands during a sea regression, and consist of marine gray, black, green and reddish brown sand and siltstones. The member comprises a uniform succession of muddy arenites with a rich marine microfauna, and represents deposition in an offshore low-energy environment. The sediments are strongly burrowed, which has caused an effective mixing of sand and mud, resulting in the forming of quartz wackes. The sand is quartz-rich, and grains are typically well rounded. Berthierine ooids are common constituents of the sediment.

Depositional environments

Tethys Sea transgression entailed formation of fossil-bearing marine deposits in Skane, also associated with an increased tectonic activity. Deposition of the Rya Formation began with nearshore coarse clastics, and continued with offshore mudstones with tempestites, followed by offshore muddy sediments with a brief non-marine interval, and ended with deposition of open marine low-energy deposits. The marine Rya Formation shows an overall fining-upwards trend, and an up-section bathymetric deepening of the depositional environment. The depositional environment in western Skåne was either physically protected from the storm energy due to basin topography, or deposition in Skåne took place below storm wavebase. Berthierine ooids occur scattered in the Katslösa
Member and are increasingly abundant up-section in the Rydebäck Member. There is a possibility that iron ooid formation was promoted by precipitation of iron and silica from volcanic fluids rising up through the substrate, as has been reported from modern marine sediments offshore Indonesia. This hypothesis has emerged with the publication of age data for the volcanic rocks in Skåne, which now appear to be comparable in age to the prominent iron ooid-bearing deposits, i.e. the Rydebäck Member and the Röddinge Formation.

Age

Based on foraminifers, ammonites and ostracods, the Döshult Member is dated to the early Sinemurian, the Pankarp Member to the late Sinemurian, the Katslösa Member to the late Sinemurian to early Pliensbachian and the Rydebäck Member to the late Pliensbachian to late Aalenian.
The formation is time-equivalent with the Röddinge Formation of the Vomb Trough, the Djupadal Formation in central Skane and the Sorthat Formation of Denmark, with which it shares the Spheripollenites–''Leptolepidites and Callialasporites–Perinopollenites'' Zones. The formation also correlates with the Fjerritslev Formation of the Danish Basin, and the Gassum Formation of the Øresund Basin. The storm-dominated, hummocky cross-stratified Hasle Formation on Bornholm is contemporaneous with the muddy Katslösa Member of the Rya Formation.

Basin history

;Basement
The basins where the Rya Formation was deposited form part of the Sorgenfrei-Tornquist Zone of the Trans-European Suture Zone, the boundary between Baltica to the northeast and Peri-Gondwana to the southwest. The orogeny was active in the Late Ordovician, or approximately 445 million years ago.
At the Carboniferous-Permian boundary around 300 Ma, the area was influenced by the Skagerrak-Centered Large Igneous Province, another large igneous province stretching across the North Sea, the eponymous Skagerrak between Denmark and Sweden and to the northwest up to northern England and Scotland.
;Break-up of Pangea
The basins of southern Sweden and eastern Denmark were formed during the latest Triassic and earliest Jurassic. During this time the Central Atlantic magmatic province, with an estimated the largest igneous province in Earth's history, was formed to the present southwest of the Danish-Swedish realm. In the Skåne area, the Central Skåne Volcanic Province was active during the time of deposition of the Rya Formation, commencing around the Sinemurian-Pliensbachian boundary. The earliest magmatism was partly emplaced into and across pre-existing extensional basin structures. The first and the main volcanic phase of this volcanic province occurred in the Early Jurassic at 191–178 Ma. Analysis of the volcanic rocks produced by this Jurassic volcanism suggests a continental Strombolian-type eruptive character close to the oceans of the Early Jurassic. No correlative pyroclastic beds have yet been identified in sedimentary basins surrounding central Skåne.
;Toarcian
During deposition of the Rydebäck Member, the Toarcian turnover happened. This event at the Pliensbachian-Toarcian boundary characterized by widespread anoxic conditions globally, led to the extinction of various groups of flora and fauna. Taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and belemnites. Epifaunal taxa adapted to low-oxygen conditions, such as the buchiids, posidoniids and inoceramids, flourished in the post-extinction environment during the survival interval.

Economic geology

A study on the geothermal potential of reservoirs in the Øresund Basin published in 2018 by Erlström et al. gave results of the formation together with the Gassum and Höganäs Formations, giving the following characteristics of the three Early Jurassic formations:
  • Net sand thickness -
  • Porosity - 18 to 34%
  • Permeability - 50 to 1500 mD
  • Cl− concentration - 120 to 190 gram/liter
  • Productivity index - 7.0 m3/hr/bar
A study published in the same year analyzing the CO2 storage potential of the Rya and Höganäs Formations concluded a storage capacity of 543 megatons of carbon dioxide.
The organic content of the Jurassic strata in Skåne is typically dominated by gas-prone kerogen, which is below, or at the onset of, thermal maturity.

Paleoenvironment

The sedimentological evolution of the Jurassic in southwestern Skåne, specially the Rya Formation, has provided depth diverse data about its different environments, specially on those samples recovered on the Höllviken-2 core and sidewall cores from the FFC-1 well. The unit was linked on all its sedimentological history with the main Fennoscandinavian land. The local succession in the Höllviken Halfgraben and Barsebäck Platform has measured continue during most of the Triassic and Early Jurassic times. Inte western part of the Höllviken Halfgraben during the Hettangian–Pliensbachian succession there is evidence of higher subsidence rate along the Öresund Fault, indicating tectonically controlled deposition and the presence of a submarine high at the west of this fault. In the east is the Skarup Platform, interpreted as a Jurassic high based on its incomplete or missing Rhaetian–Lower Jurassic strata, along redeposited spores and pollen in the Höllviken Halfgraben and increased sand output, that point to this zone as the major freshwater terrestrial source. This deposits shows several nearshore environments, from offshore marine environment probably below wave base on the sinemurian level, as proven by the presence of well cemented fine sand and 2 m thick clay dominated heterolites, that change to more marine influenced flaser bedded heterolites with some strongly bioturbated horizons and convolute beds in the Pliensbachian, and end in the Pliensbachian–Lower Toarcian with a sandy seashore setting with high energy lenticular beds. Some sections are suggested to be tidal flat zones and/or distal bar deltaic deposits. Coeval with the Rydebäck member, at Anholt where studied several levels that point to a connected fluvial system. Concretely the Upper Pliensbachian-Toarcian boundary marks the beginning of a major regression, which continued through the Toarcian and Aalenian. The discovery of the foraminifera Eoguttuliiia liassica, Bony Fishes and Scolecodonts, points that the Pliens-Toar boundary has a shallow water environment, possibly of reduced salinity. The Pollen and spores, identical of those found on the Rydebäck member, increase, and the dinoflagellate cysts decrease, suggesting more proximal shoreline. This layers, on both Anholt and the Rydebäck member, are considered to represent a regression from marine inner shelf environments to near lagoonal or deltaic conditions during the late Pliensbachian and early Toarcian, but with light marine influence, as show the presence of Dactylioceras on the Rydebäck member. The sequence points concretely than in both units The Late Pliensbachian sediments at were deposited in a storm-influenced Inner Shelf setting, that changed on the Lower Toarcian into a more restricted, marginal-marine conditions with delta progradation. Then in the late Toarcian increased brackish conditions, to end on a short marine encroachment during the Early Aalenian.
The study of the Jurassic sediments has allowed to know that the basement rocks of southern Sweden were deeply weathered in Late Triassic-Cretaceous times, with formed saprolites on the sub-mesozoic basement and high amount of Kaolinite on the stronger weathered profiles, opposed to smectite on the less weathered ones. Thus, fluvial currents released smectite-rich weathering material to the Late Triassic–Jurassic receiving basins. The members of the Rya Formation recover a transition from deltaic to paralic coast and shallow marine, dominated by kaolinite, along with peaks of illite and smectite. The record of this minerals showed that at the time of deposition of the Rya Formation, mid-latitude warmth and pronounced humidity to drier pseudomediterranean climates allowed on the denuded bedrock in the Fennoscandian Shield, composed of Gneisses or Granites, at places intersected by Dolerite dykes, fracturation and active erosion/weathering. The coeval Djupadal Formation volcanic ash falls at the east Skarup Platform-Cenntral Skane Volcanic Province, may have diluted the marine sediments, with raised smectite content. The pronounced mineralogical maturity of most Swedish post-Norian Mesozoic arenites confirms widespread feldspar destruction in the weathering profiles of the Paleozoic crystalline basement at the north, as a consequence of the increased humidity. A fish tooth recovered from the lower Toarcian of the North West German Basin presents a radiogenic seawater value of −6 ε-units, which is quite counter-intuitive to the idea of massive unradiogenic crustal-derived inputs from Laurasia. Clastic fractions found on the same layer suggest brief radiogenic Nd influxes from the Skåne flood basalts erupted at this time.
;Pliensbachian
Lower-late Pliensbachian Layers at Katslösa are similar in faunal composition. The Katslösa Member composition suggest deposition on a Low Energy Marginal marine environment with absence of changes in the salinity, with active bottoms filled by traces of diverse invertebrates and abundance of Bivalves. The Upper Pliensbachian section that mark the appearance of the Rydebäck Member continues the stable salinity environment with increased presence of Echinoderms, but lack of a diverse Bivalve fauna as in the older section. This unit was deposited likely on a marine setting more proximal to the shore and low-energy, more strongly burrowed than older layers. The presence of Selachian fauna can be more likely derived from a more suitable depositional setting for this remains than a biota turnover. Palynology in this section is dominated by spores in the Karindal bore no. 1, suggesting a humid climate on nearby emerged lands.
;Toarcian
Toarcian layers of the formation where influenced by the ongoing vulcanism located in the Central Skåne Volcanic Province, as, marine water influence is observed in the main outcrop of the last unit, where enriched Zeolite by Barium is suggested to derive from oceanic water, that may have circulated as hydrothermal flows in the lapilli tuff and facilitating diagenetic changes. Is also known by palynological analysis on the Bonnarp Cone, where saltwater/brackish acritarchs like Leiosphaera and Leiofusa or Dinoflajellates like Nannoceratopsis where recovered. This section also recovers the increased influence of Terrestrial weathering measured in the layers after the Toarcian AOE, as seen in the Vilhelmsfalt borehole, where plant remains increase their presence, suggesting a regression of the coast influenced by the increased volcanic-derived materials. The Palynology in this section is dominated by the Pollen of Chasmatosporites, with at least six species recovered, playing more than 50% of the total palynological samples, implicating a clear dominant role of the producer of this Pollen and suggesting along the increased amount of Cheirolepidiaceae pollen and general decrease of spores a shif towards a more arid climate on nearby settings.

Fossil content

The formation has provided fossils of typically marine fauna. With the exception of a continental coal bed, the formation is marine in character. Shark teeth were reported from the Rydebäck Member.

Annelida

Eklexibella johanniSerpula quinquesulcataSerpula terquemi

Echinodermata

  • ?Acrosaleniidae IndeterminateBalanocrinus subteroidesHispidocrinus scalarisIsocrinus ranae
  • ''Pentacrinus basaltiformis''

Ammonites

Agassiceras nodulatumAmaltheus margaritatusAmauroceras ferrugineumArnioceras cf. falcariesArnioceras sp. indet.Asteroceras obtusumCoroniceras rejnesiCymbites striariesDactylioceras tenuicostatumDactylioceras cf. semicelatumEchioceras raricostatumEleganticeras elegantulumEparietites sp. indet.Euagassiceras resupinatumEuagassiceras spinariesEuagassiceras lundgreniEuagassiceras cf. lundgreniMegarietites meridionalisOxynoticeras oxynotumOxynoticeras? sp. indet.Paracoroniceras crossiPleuroceras spinatumPleuroceras hawskerensePolymorphites angustusProdactylioceras davoeiPromicroceras planicostatumPromicroceras sp. juv.Tragophylloceras ibex

Belemnites

"Belemnites" elongatus"Belemnites" milleriPassaloteuthis alveolataPassaloteuthis apicicurvataPassaloteuthis cf. virgata

Brachiopods

Rhynchonella deffneriSpiriferina walcottiZeilleria numismalis
  • ''Zeilleria perforata''

Bivalves

Anomia pellucidaAnisocardia luggudensisArcomya decoraArcomya cf. elongataAstarte angeliniAstarte fructuumAstarte scanensisAstarte fortuna Astarte deltoideaAstarte oerbyensisAstarte ryensisAstarte sp.Barbatia pullaCardinia tolliniCardinia expansaCardinia ingelensisCardinia kullensisCardinia spp.Chlamys interpunctataChlamys textoriaChlamys janiformisChlamys subulataChlamys tullbergiChlamys textariaChlamys interpunctataDimyodon sp.Entolium sp.Entolium hehliEntolium calvumEntolium cingulatumEntolium lundgreniEotrapezium gerrnariEotrapezium pullastraEotrapezium hebertiEotrapezium menkeiGervillia angeliniGervillia scanicaGervillia hagenowiGervillia sjögreniGoniomya heteropleuraGrammatodon cypriniformisGrammatodon sinuatusGrammatodon subrhomboidalisHomomya sp.Homomya ovalisHomomya venulithusHomomya centralisHomomya librataIsognomon sp. Leda sp.Liastrea hisingeriLima duplicataLima pectinoidesLimea acuticostataLimea katsloesensisLiogryphaea sp.Liogryphaea arcuataLiogryphaea lataLioqryphaea regularisModiola hillanaModiola ruuthiModiola cf. tenuissimaModiola scalprumMyoconcha decorataMytilus cf. lamellosusNucula sp.Nucula distinguendaNuculana sp.Nuculana zieteniNuculana dorisOxytoma sinemurensisOxytoma inaequivalvisOxytoma scanicaPalaeoneilo sp.Palaeoneilo galateaPalaeoneilo bornholmiensisPalaeoneilo oviformisPlagiostoma succinctaPlatymya aquarumPleuromya sp.Pleuromya forchhammeriPleuromya corrugataPlicatula spinosaPlicatula orbiculoidesProtocardia philippianaProtocardia oxynotiProtocardia truncataPseudopecten aequivalvisPseudopis sp.Rollieria sp.Rollieria bronniSphaeriola kurremolinaeTaeniodon nathorstiTancredia arenaceaTancredia securiformisTancredia erdmanniTancredia johnstrupiTancredia lineataTerquemia arietisTrigonia primaevaTrigonia modestaTutcheria cingulata

Gastropods

Actaeonina nathorstiActaeonina cf. striataChemnitzia sp.Chrysostoma cf. solariumKalchreuthia frankeiPtychomphalus cf. expansusTrochus cf. imbricatus

Scaphopods

Baltodentalium weitschati"Dentalium" hexagonaleLaevidentalium elongatumLaevidentalium sp.Prodentalium bandeliProgadilina spaethi
  • ''Progadilina subtrigonalis''

Branchiopoda

  • ''Kuqaia scanicus''

Ostracods

Astacolus denticulina carinataBrizalina liassica amaltheaCitharina inaequistriataCristacythere crassireticulataMarginulina spinata spinataNanacythere simplexOgmoconchella danicaOgmoconchella mouhersensisOgmoconchella adenticulataOgmoconcha amalthei amalthei

Mites

  • ''Hydrozetes nsp.''

Ichnofossils

Chondrites sp.Diplocraterion sp.Planolites sp.Rhizocorallium sp.Skolithos sp.

Fish

Apart from a few teeth of the hybodont Hybodus reticulatus, the shark fauna from the Rya Formation is exclusively neoselachian.

Plant Remains

Coalified wood occurs as scattered pieces up to long and indeterminate belemnites, echinoids, serpulids, ostracods and nodosariid foraminifera were also recorded in the formation.
Ammonites where found on layers with plant fossils. Fragments of Dactylioceras were found at a depth of 170 m in the Vilhelmsfalt borehole, together with plant remains and
beautifully preserved Pelecypods. The plant material, probably derived from watercourses emptying in the neighborhood, consists mostly of small fragments that seem to have been intimately sedimented with the muddy material that flocculated on meeting the seawater. Other specimen, Arnioceras sp. indet. occurs on what appears to be a transitional environment, probably a back-beach with coalified fragments of plants, some of a large size. Probably due to the shell being washed to land due to a storm.