The Rya Formation (Swedish: Ryaformationen) is a geologic formation in Skåne County, southern Sweden. It is Early to early Middle Jurassic (early Sinemurian to late Aalenian) 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 295 metres (968 ft) 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 6 centimetres (2.4 in) 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 (Swedish Döshultsledet) 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 80 metres (260 ft) 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 (Gantofta brickpit in the Helsingborg area, outcrop very limited at present) 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 (tempestites). 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 (0.1–0.3 mm in diameter) are characteristic constituents of this member.
Pankarp Member The dark mudstones of the Döshult Member are overlain by the Pankarp Member (Swedish Pankarpsledet) with a sharp conglomeratic boundary. The late Sinemurian Pankarp Member has an estimated thickness of up to 70 metres (230 ft) in the subsurface of the Ängelholm, Helsingborg and Landskrona areas. In the Kävlinge area, the thickness is about 20 metres (66 ft) 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 (Swedish Katslösaledet) is mainly known from the subsurface in westernmost Skåne, and it has a thickness of 30 to 40 metres (98 to 131 ft) in the Ängelholm, Helsingborg and Landskrona areas. In the Kävlinge area, the thickness is about 75 metres (246 ft). 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.
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