The Agardhfjellet Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in Svalbard,

Norway Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...

. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

dating back to the Oxfordian to

Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years a ...

stages, spanning the

Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the ...

Early Cretaceous The Early Cretaceous (geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pr ...

boundary. The formation contains the Slottsmøya Member, a highly fossiliferous unit (

Lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

) where many

ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, altho ...

and plesiosaur fossils have been found, as well as abundant and well preserved fossils of invertebrates.Delsett et al., 2019, p.741Agardhfjellet Formation
at

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.org

Description

The formation overlies the Knorringfjellet Formation and is overlain by the Rurikfjellet Formation. The formation comprises the lower Oppdalsåta and upper Slottsmøya Members. The Slottsmøya Member, which averages in thickness in the study area, consists of dark-grey to black silty mudstone, often weathering to paper shale, and discontinuous silty beds with local occurrences of red to yellowish sideritic concretions as well as siderite and dolomite interbeds. It consists of a mix of

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

s and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...

s which were deposited in a shallow marine environment, near a patch of deeper

marine sediment Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mai ...

. The seafloor, which was located about below the surface, seems to have been relatively dysoxic, or oxygen-poor, although the diversity of the benthic fauna suggest that these environments were likely not truly anoxic and that oxygen-depletion may have been a seasonal phenomenon. Although direct evidence from Slottsmøya is currently lacking, the high latitude of this site and relatively cool global climate of the Tithonian suggest that some sea ice may have been present at least in the winter. These sites represent shallow-water

methane seep A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature of the seep ...

s which were spread over a relatively large geographic area, and like modern day seeps, they supported high biodiversity. Near the top of the member, various assemblages of invertebrates have been discovered; these include

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

, lingulate brachiopods, rhynchonellate brachiopods,

tubeworms A tubeworm is any worm-like sessile invertebrate that anchors its tail to an underwater surface and secretes around its body a mineral tube, into which it can withdraw its entire body. Tubeworms are found among the following taxa: * Annelida, the ...

belemnoids Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms o ...

, tusk shells,

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...

crinoid Crinoids are marine animals that make up the Class (biology), class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or coma ...

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) ...

brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locom ...

s, starfish,

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

s, and

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. ...

, numbering 54 taxa in total. The most common and abundant of these taxa were bivalves and brachiopods, each of which make up 27.8% of the known seep fauna. Several chemosymbiotic species are known, such as the bivalve '' Nucinella''. Outside of the cold seeps, several non-seep-restricted invertebrates were also present in abundance. In addition to the invertebrates, the Slottsmøya Member has also revealed a diverse assemblage of marine reptiles, including several taxa of

s and plesiosaurs. Many of these specimens are relatively complete and in articulation, which is rare among Jurassic marine fossil sites. It thus provides a unique and detailed glimpse into the boreal seas of the Late Jurassic. As it spans the Jurassic-Cretaceous boundary, it is also important to understanding how marine ecosystems changed going into the Cretaceous.

Paleobiota

The formation, especially the Slottsmøya Member, has provided an abundance of marine fossils dating to the Late Jurassic-Early Cretaceous. The timeline below follows stratigraphic data provided in Delsett et al. 2018 and Roberts et al. 2020. ImageSize = width:1000px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px Period = from:-152.1 till:-141 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:1 start:-152.1 ScaleMinor = unit:year increment:1 start:-152.1 TimeAxis = orientation:hor AlignBars = justify Colors = #legends id:CAR value:blue id:ANK value:rgb(0.4,0.3,0.196) id:HER value:green id:HAD value:blue id:black value:black id:white value:white id:tithonian value:rgb(0.675,0.675,0.35) id:berriasian value:rgb(0.753,0.753,0.486) BarData= bar:eratop bar:space bar:periodtop bar:space bar:NAM1 bar:NAM2 bar:NAM3 bar:NAM4 bar:NAM5 bar:NAM6 bar:NAM7 bar:NAM8 bar:NAM9 bar:NAM10 bar:NAM11 bar:space bar:period bar:space bar:era PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 shift:(7,-4) bar:periodtop from: -145 till: -141 color:berriasian text:Berriasian from: -152.1 till: -145 color:tithonian text:Tithonian bar:eratop from: -152.1 till: -141 color:CAR text:Vertebrate Biostratigraphy of the Slottsmøya Member PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left color:HAD bar:NAM1 from:-148.5 till:-147.5 text:

Colymbosaurus svalbardensis ''Colymbosaurus'' is a genus of cryptoclidid plesiosaur from the Late Jurassic ( Callovian-Tithonian) of the UK and Svalbard, Norway. There are two currently recognized species, ''C. megadeirus'' and ''C. svalbardensis''. Both species are relat ...

color:HAD bar:NAM2 from:-149.3 till:-148.3 text: Djupedalia engeri color:HAD bar:NAM3 from:-148 till:-147 text:

Spitrasaurus larseni ''Spitrasaurus'' is an extinct genus of cryptoclidid plesiosauroid plesiosaur known from the uppermost Jurassic of central Spitsbergen, Norway

color:HAD bar:NAM4 from:-146.4 till:-145.8 text: Spitrasaurus wensaasi color:HAD bar:NAM5 from:-145.3 till:-144.7 text: Ophthalmothule cryostea color:HAD bar:NAM6 from:-148.5 till:-146.3 text: Pliosaurus funkei color:HER bar:NAM7 from:-147.3 till:-146.3 text: Undorosaurus gorodischensis color:HER bar:NAM8 from:-151.5 till:-150.5 text: Arthropterygius lundi color:HER bar:NAM9 from:-148.5 till:-147.5 text: Arthropterygius hoybergeti color:HER bar:NAM10 from:-148 till:-144.5 text:

Arthropterygius chrisorum ''Arthropterygius'' is a widespread genus of ophthalmosaurid ichthyosaur which existed in Canada, Norway, Russia, and Argentina from the late Jurassic period and possibly to the earliest Cretaceous. Description ''Arthropterygius'' appea ...

color:HER bar:NAM11 from:-145.2 till:-144.5 text: Nannopterygius borealis PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:era from: -152.1 till: -141 color:CAR text:Vertebrate Biostratigraphy of the Slottsmøya Member

Vertebrates

Bony fish

Plesiosaurs

Ichthyosaurs

Invertebrates

An unnamed galatheid

squat lobster Squat lobsters are dorsoventrally flattened crustaceans with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the decapod infraorder Anomura, alongsi ...

is known from a single, poorly preserved fragment.

Cephalopods

Large

belemnoid Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of r ...

arm hooks are known.

Echinoderms

Bivalves

Remains of possible anomiid clams have been found which are as yet undescribed.

Serpulids

Brachiopods

Gastropods

References

Bibliography

* * * * * * * * * {{cite LSA , last=Knutsen , first=E.M. , first2=P.S. , last2=Druckenmiller , first3=J.H. , last3=Hurum , year=2012c , title=Redescription and taxonomic clarification of Tricleidus' svalbardensis'' based on new material from the Agardhfjellet Formation (Middle Volgian) , journal=

Norwegian Journal of Geology ''Norwegian Journal of Geology'' ( no, Norsk Geologisk Tidsskrift) is a quarterly peer-reviewed scientific journal In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually b ...

, volume=92 , pages=175-186 Geologic formations of Norway Jurassic Norway Lower Cretaceous Series of Europe Cretaceous Norway Mudstone formations Dolomite formations Deep marine deposits Open marine deposits Fossiliferous stratigraphic units of Europe Paleontology in Norway Geology of Svalbard Tithonian Stage Berriasian Stage Jurassic System of Europe