Recognizing River Mouths that Produced Hyperpycnal Flows, Eocene Central Basin of Spitsbergen

Piret Plink-Björklund¹ and Ron Steel^2
1 Göteborg University, Gothenburg, Sweden
² The University of Texas at Austin, Austin, TX

Hyperpycnal flows have been documented to be a significant downslope sand transport agent in the Eocene Central Basin of Spitsbergen. This paper reports recognition of mouth-bar deposits from the river mouths that produced hyperpycnal flows. We have documented 2-7 m thick, dominantly plane-parallel-laminated sandstone units that stack vertically into 20 m thick successions, and occur on the dip-direction transect between fluvial channels and upper-slope canyons. Distinct grain-size changes occur in the plane-parallel-laminated sandstones (alternating laminae of coarse- and fine-grained sand), and are interpreted to indicate semi-continuous bypass of sands during the accumulation of these beds. “Walking out” the mouth bars to the shelf-edge canyons, shows that the intra-bed bypass features do not occur in the hyperpycnal-flow deposits much beyond the upper slope. The mouth-bar succession has a bi-partite stratigraphy. The lower stratigraphic unit, interpreted as FRST, consists of offlapping mouth-bar units, based either by erosion surfaces and clay-clast conglomerates, or by upwards-coarsening soft-sediment deformed deposits. The individual mouth-bar units accrete obliquely seawards, and can be “walked out” into their feeder channels landwards. The channels amalgamate landwards into an unconformity that was formed, as the older mouth bars were gradually eroded by the feeder channels of the younger mouth bars. The unconformity has been modified by waves, and is draped by a thin mudstone. The upper stratigraphic unit, interpreted as late-LST, is dominated by wave-reworked hyperpycnal-flow deposits, and occurs above the shelf edge. The maximum flooding surface is above the mouth-bar complex in the marine shales.