Process Regime Changes Across a Regressive to Transgressive Turnaround in a Shelf-Slope Basin, Eocene Central Basin of Spitsbergen

Abstract

Abstract The sedimentological, architectural, and stratigraphic database from one investigated shelf-margin clinothem complex in the Eocene Central Basin of Spitsbergen suggests that dominating process regimes and facies products may change several times during a sea-level cycle, even though the basin-wide processes are constant. Continuous mountainside exposures were investigated by a combination of “walking-out” of stratigraphic surfaces and geometrical elements, closely spaced detailed vertical sections, and tracing on helicopter photo mosaics. The clinothem complex represents a regressive to transgressive shoreline complex and consists of five clinothem sets, recognized by specific stacking patterns and sedimentary facies. The regressive part preserves wave- and river-dominated deltaic deposits in a progradational to aggradational stacking pattern. The delta clinothems consist of tide-influenced distributary channels, wave- and hyperpycnal-flow-dominated mouth bars that on the shelf sourced turbidite-filled slope channels, and slope lobes. The transgressive part of the clinothem complex preserves strongly tide-dominated estuarine deposits underlain by tidal ravinement surfaces that cut down into deposits of wave-dominated deltas. This change in facies product on the inner shelf from wave to tidal at the regressive to transgressive turnaround, was however not a basin-wide process change, inasmuch as the distributary channels also reflect tidal influence. This suggests that tidal currents were present only in areas protected from waves and amplified by the lateral confinement of the estuary. There was a process-regime change on the middle to outer shelf, from wave-dominated highstand and falling-stage delta complex to river-dominated (and wave-influenced) early-lowstand delta complex. The delta complex prograded across the shelf during highstand, and during falling stage the relative sea-level fall forced the deltas out to shelf edge. The early-lowstand delta mouth bars are volumetrically dominated by hyperpycnites, indicating dominance of riverine processes. The proportion of the hyperpycnal-flow deposits decreased during the late lowstand, and the delta complex became wave-dominated anew. The process-regime change from wave dominance to fluvial dominance and back to wave dominance is interpreted to have occurred due to the seaward, and then landward, shift of the feeding river system, whereas the tidal effects were dependent on shelter from waves and on the shoreline configuration.