The Tertiary rocks of Spitsbergen are clastic sediments ranging from coarse conglomerates to shales. They occur in the central basin, covering much of the southern half of Vestspitsbergen, and in four small areas along the west coast of Spitsbergen (at Kingsbay, Foreland Sound, Kapp Lyell, and Oyrlandet) where they are preserved as a result of downfaulting.

The sandstones and shales of the central basin are mainly marine and estuarine deposits, but at the base and top of the 6,000 foot sequence there are continental units with important coal seams. The rocks unconformably overlie tilted Cretaceous strata. A basal conglomerate a few feet thick occurs on the south side of Isfjord. Some of the sandstones in the lower half of the sequence grade eastward into conglomeratic beds; similarly shales become sandier toward the east and northeast, and indicate the source lay in that direction.

At Kingsbay the 700-foot sequence lies unconformably on Permo-Carboniferous rocks and consists of sandstones and shales with thick coal seams and small amounts of conglomerate. Lateral lithologic variations suggest a source area at the north, with Permo-Carboniferous, Devonian and pre-Devonian (Hecla Hoek) rocks undergoing erosion. The general character of the rocks suggests correlation with the lowest part of the sequence in the central basin.

The Tertiary rocks of Foreland Sound are more than 6,000 feet thick, with a basal conglomerate reaching 500 feet and containing the coarsest Tertiary conglomerates of Spitsbergen. Sandstones make up about 50 per cent of the total thickness, conglomerates 20 per cent, and siltstones and shales 30 per cent. Whereas the sandstones at Kingsbay contain felspar, suggesting derivation from the Devonian arkoses or the pre-Devonian granitic and high-grade metamorphic rocks of northern and northwestern Spitsbergen, the sandstones of Foreland Sound are low-rank lithic graywackes and were derived from low-grade metamorphic rocks a very short distance west. They were formed in a developing graben, from erosion of a fault block rising along the system of faults now forming the western boundary of t is Tertiary area. In sediment-type, tectonic setting, and direct of derivation, they are unlike the Tertiary rocks of Kingsbay and the central basin, and it seems very unlikely they were formed contemporaneously.

The Tertiary rocks at Kapp Lyell consist of about 1,000 feet of sandstones and shales with some conglomerates. It is uncertain whether their contact with pre-Devonian metamorphics is an unconformity or a fault. There is not sufficient information on the rocks themselves to indicate whether they are part of the central basin and Kingsbay sequence, or were deposited in a southerly extension of the Foreland Sound trough.

The Tertiary rocks at Oyrlandet are apparently in fault contact with Permian rocks at the southwest, and with Mesozoic or Carboniferous rocks at the northeast. They probably represent an outlier of the Tertiary of the central basin.

Molluscan faunas in the central basin indicate a Paleocene or Eocene age. Only plant remains have been identified from Kingsbay, Foreland Sound, and Kapp Lyell. Comparing these with floral collections from the continental units of the central basin suggests that the Kingsbay rocks are related to the lowermost of the central basin sequence. The paleobotanical and other evidence leads to the conclusion that the Foreland Sound and Kapp Lyell rocks are either equivalent to, or younger than, the uppermost unit of central basin.

Further evidence, showing they are younger than the entire central basin sequence, comes from a comparison of the gentle synclinal structure of the Tertiary rocks at Foreland Sound with the structures in the belt of Tertiary folding and faulting along the west coast of Spitsbergen. This deformation is younger than the Tertiary sediments in the central basin. It becomes more intense toward the west. At Kingsbay Permo-Carboniferous rocks have been thrust onto Tertiary beds, which were locally overfolded. Similarly intense effects of Tertiary deformation are seen in a thrust wedge of Carboniferous rocks east of Foreland Sound. The slight deformation of the Tertiary rocks at Foreland Sound indicates they were formed later than this strong deformation.

Previous workers have suggested the Tertiary rocks of Foreland Sound are equivalent to the lower part of the central basin sequence, and pointed out that the contrast between a thick basal conglomerate at Foreland Sound and a thin one in the central basin indicates a westerly source for all these rocks. They have also invoked pre-Tertiary uplift of the west coast belt to account for the absence of Upper Paleozoic and Mesozoic rocks below the Tertiary at Foreland Sound. The pattern of sub-outcrop below the Tertiary unconformity, however, does not support this, but indicates that Cretaceous deformation involved tilting of a degree or two about an east-northeast axis, with the north of Spitsbergen uplifted more than the south. Erosion cut down to pre-Devonian rocks in northern Spitsberge , to Permo-Carboniferous rocks at Kingsbay, and to different levels of the Lower Cretaceous in the central basin. It was not until later than--or the last stages of--the deformation of the central basin and the more intense folding and faulting along its western margin, that the west coast strip was uplifted, and the Upper Paleozoic and Mesozoic cover removed from the pre-Devonian metamorphics. Then, part of this west coast horst collapsed, and a trough

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of sedimentation developed along its eastern edge, in the area that is now Foreland Sound. The evolution of the Foreland Sound graben was the last phase of Tertiary diastrophism in Spitsbergen.