Sedimentary and structural evidence of a new early Campanian rift phase in the Nuussuaq Basin, West Greenland

Abstract

A new early Campanian rift phase related to the opening of the Labrador Sea has been recognised in the Nuussuaq Basin, West Greenland. A major angular and erosional unconformity separates deltaic deposits of the upper Albian-lowermost Campanian Atane Formation from fully marine gravity flow deposits of the lower-middle Campanian Aaffarsuaq member of the Itilli formation. Dating of this unconformity was achieved by new field work in the classic Agatdalen area and neighbouring valleys which placed known ammonite localities in an overall stratigraphic scheme and integrated the ammonite dating with a re-evaluation of old and recently collected palynological samples. On the basis of a sedimentological analysis the sediments of the Atane Formation can be classified as pre-rift with respect to Campanian rifting, and the Aaffarsuaq member can be divided into a rift-initiation and main-rift succession. The pre-rift sediments of the Atane Formation are deltaic. The uppermost preserved Atane cycle is 84m thick and is by far the thickest cycle recognised in this formation, suggesting a relative sea-level rise in late Santonian time that was possibly a result of tensional sagging preceding the rift phase. The rift-initiation phase is marked by the major, in places angular, unconformity, locally succeeded by fluvial deposits of the Itivnera beds. During the time of maximum rate of fault displacement sedimentation was outpaced by subsidence, creating relief in the basin. Sedimentation during the main-rift phase was chaotic. Deposition took place mainly from gravity flows in a channelized footwall fan system. During tectonically quiet periods heterolithic sediments were dominantly deposited from low-density turbidity currents in small shallow channels, whereas sandstones and conglomerates were deposited from high-density turbidity currents and debris flows in major channels following tectonically more active periods. Angular unconformities and major erosional surfaces are frequently present underneath the major coarse-grained turbidite channels and several syn-depositional faults are present in the succession, suggesting that these channels formed in response to continued structural movements in the early Campanian.