Massive upper Proterozoic diamictites of the Kapp Lyell Group, south of Bellsund on the west coast of Spitsbergen, constitute a sequence of distinctive units that can be differentiated on the basis of megaclast compositions. Within the group, ten lithostratigraphic units are distinguished from modal estimates on the outcrop of the abundances of the three most common types of megaclasts—quartzite, dolomite, and limestone. The total thickness of the diamictite sequence is ∼3,000 m. The clasts found in the Kapp Lyell Group are almost entirely local in origin and can be correlated with older rocks to the east that underlie the diamictites.

The diamictites were affected by two major periods of deformation. The first was either latest Precambrian (Jarlsbergian) or early Paleozoic (Caledonian) in age. This first episode of deformation (D1) is expressed in the diamictites primarily as a flattening of megaclasts. The degree of deformation due to D1 varies throughout the diamictite sequence. Part of this variation is due to differences in clast and matrix compositions and proportions, but some of it appears to be tectonic, suggesting that strain mechanisms were not uniform throughout the diamictite mass. Larger structures associated with D1 are not observed in the field owing to the massive nature of the unit and the strong overprint of the later period of deformation (D2).

The second phase of deformation (D2) is probably late Caledonian or Variscan in age, perhaps as young as early Carboniferous. Folds plunging gently to the west-northwest with an axial-planar foliation dipping southwest characterize this event. This axial-planar foliation occurs as an evenly spaced (0.5 to 2.0 cm) crenulation cleavage. The deformational intensity of D2 also varies throughout the diamictites. It is particularly strong, however, near the basal contact of the diamictites with the underlying phyllites, suggesting that this contact may be tectonic rather than depositional, or possibly both. A northeast-southwest principal horizontal compression is inferred from the foliation for the D2 event.

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