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In 2014 a geoarchaeological study was undertaken on two boreholes, reaching ~17.00m below current sea level, from the River Alna and Aker outlets into Oslo harbour at the head of Oslo Fjord, Norway. The study was prompted by development work which would make the area, situated just outside the waterfront of the medieval town of Oslo, inaccessible for many decades to come. The aim was therefore to establish and explore the archaeological potential of the buried sediment stratigraphy. Analysis of the core revealed, contrary to expectation and to sealevel curve data, that there was evidence for deep water anaerobic sediments, overlain by shallow water river delta sediments no later than the latest Nordic Iron Age (c. 1000 AD: pre-Picea). The latter were increasingly rich in anthropogenic inclusions moving up the sequence, with a topmost sample displaying a subaerial and Dark Earthlike character, with most radiocarbon dates showing an age of c. 1300–1400 AD. The best explanation for these sediment accumulations, types and dates, is an interpretation of Alna River delta front slumping. We suggest this process occurred as a series of contemporary medieval delta sediment accumulations that first slumped sideways gently down-slope, before sliding more steeply downwards, into deep water, with a set of similarly dated sedimentsbecoming a vertical sequence as a result. These medieval sediments in fact recorded probable pro-delta, delta front,channel, levee and marsh facies, which involved both shallow water and subaerial environments. Thus the investigation indicates that sedimentation in the innermost reaches of the Oslo Fjord in the High and Late Middle Ages was massive, with potential effect on harbour conditions. It also shows that the rapidly advancing river deltafront offered good conditions for the rapid embedding and subsequent preservation of organic materials in clayey sediments. This would be beneficial even for the preservation of large organic objects like ship hulls which,as a result of slumping events, may be found even at deeper levels than where they originally sunk. Lastly, the study shows how worthwhile and useful off-site coring can be for palaeoenvironmental and archaeological reconstruction where few on-site deposits are preserved, even in the difficult drilling conditions of Oslo Harbour.