Fluvially Incised Shelf-Edge Deltas and Linkage to Upper Slope Channels (Central Tertiary Basin, Spitsbergen)
Published:December 01, 2013
Donatella Mellere, Anna Breda, Ronald Steel, 2013. "Fluvially Incised Shelf-Edge Deltas and Linkage to Upper Slope Channels (Central Tertiary Basin, Spitsbergen)", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Sandy shelf-margin clinoforms in the Eocene strata of the Central Tertiary Basin of Spitsbergen are usually generated by river-dominated shelf deltas, or by wave-dominated shorelines, though these two regimes can also be strike-equivalent to each other. Clinoforms occur in series or sets that show both sub-horizontal and rising trajectories of shelf-edge accretion. Clinothems involved in the former style of margin growth, however, tend to be dominated by delta deposits. Shelf-edge deltas of such clinoforms are commonly severely eroded by their own distributary channels, and this is especially noticeable at (though not restricted to) shelf-edge locations. Fluvially incised shelf edges are commonly linked directly across the shelf break, to turbidite-filled channels, gullies and small canyons on the slope.
Examples of this type of shelf-edge situation are present on Brogniartfjellet in Van Keulenfjorden, where the outer-shelf segment of the clinothem contains shelf-edge deltaic units that are 20-30m thick deposited during falling base level and lowstands. The deltas have been cut by deep erosive channels (up to 12 m) paved by shale rip-up conglomerates. The channel infill is dominated by up to 3 m-thick, flat and low-angle laminated, medium-grained sandstone bedsets deposited from upper-flow-regime conditions in riverine and shallow sand flats. Multiple phases of erosion can be demonstrated, separated by phases of minor re-establishment of delta-front facies. At peak regression of the delta system, still during falling relative sea level, the channels have reached the shelf break and allowed the river system to feed sediment directly into slope channels that were turbidity-current conduits to the basin floor. These are incised more than 25m deep on the upper slope, appear to have originated from fluvial input and retrogressive slumping on the slope, and link back up to the shelf-edge incisions. The infill of the slope conduits strongly suggests repeated phases of erosion/bypass that alternated with phases of low-efficiency, hyperpycnal-flow deposition. The apparent off-lapping architecture within the slope conduits strongly suggests oblique or downslope accretion of infill during continued relative fall (forced regressive and lowstand conditions) of sea level, and probably during basin-floor growth of the fan.
In the latest stage of the lowstand, the shelf-edge deltas have re-established themselves onto the shelf, aggrading and prograding onto the underlying canyonized succession, thus forming a lowstand prograding wedge. Minor fluvial incision occurs, but overall the system is less sand prone. During the subsequent transgression of the shelf, when sea rose back up to and above the shelf edge, the slope is blanketed by mud, there is tidal re-working and infilling of the older shelf-edge channels and a transgressive barrier/lagoon or estuary system migrated landwards.