QUANTITATIVE STUDY OF THE DISTRIBUTARY BRAIDPLAIN OF THE PREBOREAL ICE-CONTACT GARDERMOEN DELTA COMPLEX, SOUTHEASTERN NORWAY

This raised delta structure is an ice-contact deltaic complex with a v olume of c. 4.4.10(9) m(3), deposited c. 9500 yr BP in a shallow wide 'fjord' during the retreat of the Scandinavian ice cap. The delta plai n lies at an altitude of 200-223 m. It aggraded c. 20 m above the cont emporaneous sea level during a regional marine regression. The braidpl ain palaeochannel characteristics indicate a peak meltwater discharge of 7-9.10(3) m(3)/s. Calculations based on a glacial ablation model in dicate a mid-summer discharge of c. 5.5.10(3) m(3)/s. However, the flu vial topset of the delta has an erosive base whose altitude decreases upstream and indicates stream incision by more the 6 m below the conte mporaneous sea level. The deep scour is ascribed to episodic floods ov er the relatively short delta plain, which exceeded direct ablation-as sociated discharges. The depositional time-span of the delta is assess ed to have been 70 years, calculated from coastal gradient and shoreli ne displacement curves. The average sedimentation rate of the delta is thereby inferred to have been extremely high, c. 6.10(7) m(3)/yr. The sedimentation is thought to reflect 'extreme' ice-margin conditions, where the sediment and water discharge was maximized by full-scale abl ation, with simultaneous subglacial, englacial and supraglacial sedime nt and water supply. These conditions might further coincide with an a bundant rainfall in the catchment area or the drainage of dammed water s, initiating episodic floods which eroded deep beneath sea level. As a whole, the study illustrates the hydrological conditions of proglaci al sedimentation at the front of the rapidly retreating last Scandinav ian ice cap.