Stratigraphic range: 55.8–53.5 Ma
Coastal cliff on the Danish island Fur - Mo-clay and ash layers
|Thickness||60 metres (200 ft)|
|Named for||Fur (island)|
The Fur Formation is a marine geological formation of Ypresian (Lower Eocene Epoch, c. 56.0-54.5 Ma) age which crops out in the Limfjord region of Denmark from Silstrup via Mors and Fur to Ertebølle, and can be seen in many cliffs and quarries in the area. The Diatomite Cliffs (moler in Danish) is on the Danish list of tentative candidates for World Heritage and may become a world Heritage site.
The Fur Formation is a unit of diatomitic sediment approximately 60 meters thick consisting of diatoms and clay minerals with up to 180 layers of volcanic ash. In Danish literature the formation has informally been referred to as the moler (Ler means clay). The diatomite comprises 2/3 opal tests of diatoms and 1/3 clay, interbedded with layers of volcanic ash and a few limestone horizons (‘cementstones’), and has exceptionally complete fossil preservation.
It is known for its abundant fossil fish, insects, reptiles, birds and plants. The Fur Formation was deposited just above the Palaeocene-Eocene boundary, about 55 million years ago, and its tropical or sub-tropical flora indicate that the climate after the Paleocene-Eocene Thermal Maximum was moderately warm (approximately 4-8 degrees warmer than today).
Glacial activity has moved and folded all exposed moler in a complicated pattern which permits very precise mapping of glacial movement at the end of the last ice age, and has, due to the ash layers, created an extraordinary pedagogical case for studying tectonics.
The Fur Formation is divided into two members: The lower Knudeklint Member was named for a location on the island of Fur. The upper Silstrup Member was named for a location in Thy. The stratigraphy exposed at Knudeklint constitutes the unit that containing the Paleocene/Eocene boundary informally named Stolleklint Clay, which grades up into the Fur Formation.
Fossils of great diversity and unique preservation (only 10 my. after the ‘great extinction’ of dinosaurs, ammonites etc.) Most unusual, if not unique, diversity of life from both ocean and land with extremely good preservation of details rarely seen, therefore very reliable reconstruction of palaeobiology. By far most of the important ‘Danekræ’ fossils since 1990 have been found in the ‘Mo-clay area’.
The earliest Paleogene fauna of any diversity, over 30 species, including some near complete, some preserved in 3-D, and some excellent bird-fossils (even with feathers and chromatine). Most are earliest known representatives of their ‘orders’ (e.g. Trogons, Swifts, Ibises) and all are terrestrial birds.
Bird fossil with preserved feathers from Fur, at the Geological Museum, Copenhagen
Bird skeleton at Fur Museum, Denmark
Bird (Charadriiformes) from the Fur Formation, Denmark.
- Apodiformes (Swifts)
Several fossil sea turtles are known from the Fur Formation. In one of them, a large leatherback turtle (Eosphargis breineri) remains of soft tissue and skin pigmentation have been recovered  A number of well-preserved turtle specimens have been recovered from the Fur Formation, two of which have been recognized to be a completely new species of the genus Tasbacka 
Leatherback turtle Eosphargis breineri skull cast, at Geological Museum in Copenhagen.
Sea turtle Tasbacka danica Complete unique fossil baby sea turtle. Length 10.5 cm
Sea snake Palaeophis sp. in Early Eocene Stolleklint Clay, Fur Formation
Large teleostean fauna, oceanic, possibly including earliest truly deep water fish, a ‘whale-fish’; earliest members of many living families and Tertiary diversity preserved as complete skeletons; some rare and sensational large and complete specimens (two ‘bony tongues’, one tarpon.
Huge fauna from land, over 200 species, earliest Tertiary diversity known, many are oldest of their families; many with colour spots and eye lenses, some extraordinary preservation with stridulation (sound) apparatus in grasshoppers, and apparently migratory moth mass mortality.
Unidentified Scorpion fly (Mecoptera) Early Eocene Fur Formation at Fur Museum, Denmark
Shrimp Penaeus hamleti in Early Eocene Stolleklint Clay, Fur Formation.
Crab Portofuria enigmatica in Early Eocene Stolleklint Clay, Fur Formation
Shrimp Morscrangon acutus
Nucula sp., Mytilus roesnaesiensis, Mytilus sp., Gari sp.
Mussel at Geological Museum in Copenhagen
Some members of the ‘Arcto-Tertiary flora’; some with cuticle preservation and some flowers preserved. Large silicified trunks (up to 9 m) of redwood, and some very soft wood preserved. Some trunks with mussels and barnacles attached. Many seeds and fruits.
Great diversity of unicellular, marine algae with siliceous (opal) tests, 130 species.
More than 200 layers of volcanic ash of predominantly basaltic composition have been found within the Mo-clay of the Fur Formation. 179 of the most prominent ash layers have been numbered. Comparison with volcanic ash layers in oil wells in the North Sea indicates that the Mo-clay is coeval with the Sele Formation and Balder Formation in the North Sea. The ash layers have also been found at other sites in Denmark, England, Austria and the Bay of Biscay.
The total eruption volume of this series have been calculated as 21,000 km3, which occurred in 600,000 years. The most powerful single eruption of this series took place 54.0 million years ago (Ma) and ejected ca. 1,200 km3 of ash material, which makes it one of the largest basaltic pyroclastic eruptions in geological history. 
MoClay with layers of volcanic ash
- The Heritage Agency of Denmark
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