Depositional Origin of an Adverse Delta Surface
Slope at Altermarka, northern Norway
Kevin P. Blake, Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Lars Olsen, Geological Survey of Norway, Leiv Eirikssons vei 39, P. O. Box 3006,
Lade, Trondheim, Norway
The surface of a glaciomarine delta in northern
Norway slopes gently in the opposite direction of the dip of the foreset beds.
Based on grain-size analysis of sediment samples, Lassila (1984) identified two
tills on the delta surface and suggested that a glacier planed off the delta
topset beds, creating the adverse surface slope. In a reinvestigation of the
delta using principles of sedimentology, stratigraphy, soil science, and
geomorphology, there was no evidence that a glacier overrode the surface: the
topset beds remain intact and no till was found. Sediments resembling the
"till" samples were located, but soil processes and other sedimentary
processes easily account for the wide grain-size distributions of these
sediments. As a result, this study warns against using grain-size analysis, on
its own, to interpret the genesis of sediments. Other processes potentially
responsible for the adverse surface slope include differential isostatic
rebound, recent differential slip along ancient thrust faults, and failure of
the delta to build up to sea level. Based on the sedimentology of the topset
beds and the local relative sea level history, the delta is determined to have
built up below sea level with an adverse surface slope.