Deep‐water sand transfer by hyperpycnal flows, the Eocene of Spitsbergen, Arctic Norway

Abstract

Flood‐generated hyperpycnal flows are dense, sediment‐laden, turbulent flows that can form long‐lived, bottom‐hugging turbidity currents, which undoubtedly transport large volumes of fine‐grained sediments into the ocean. However, their ability in transferring sand into deep‐water basins is debated. This study presents sedimentological evidence of sandy hyperpycnal flow deposits (hyperpycnites) in a series of basin floor lobe complexes associated with a progradational shelf margin in the Eocene of Spitsbergen, Arctic Norway. Four coexisting types of sediment gravity flow deposits are recognized: (i) sandy hyperpycnites deposited by quasi‐steady hyperpycnal flows; (ii) turbidites deposited by waning, surge‐type turbidity currents; (iii) hybrid event beds deposited by transitional flows; and (iv) mass transport deposits emplaced during rare slope failures. The abundance of thick‐bedded massive sandstones, frequent bed amalgamation, the distribution of hyperpycnites across the lobes and the abundance and systematic occurrence of plant‐rich hybrid event beds and associated climbing ripple cross‐laminated beds in the lobe fringes, suggest that hyperpycnal flow was the most important mechanism driving lobe progradation. Shelf‐edge positioned fluvial channels linked to the basin floor lobe complexes via deeply incised, sandstone‐filled slope channels, suggest that rivers fed directly onto the slopes where their dense, sand‐laden discharges readily generated quasi‐steady hyperpycnal flows that regularly reached the basin floor. The composite architecture and complex waxing–waning flow facies configuration of the hyperpycnites is consistent with sustained and concomitant suspension and traction deposition under fluctuating subcritical to supercritical conditions. Similar sandstone beds occur on the clinoform slopes, indicating that the hyperpycnal flows operated likewise on the slope. Plant‐rich hybrid event beds indicate transformation of initially turbulent flows by relative enrichment of clay and plant material via progressive sand deposition to such an extent that it suppressed turbulence. The multi‐faceted character of the hyperpycnites reported here, challenges traditional beliefs that hyperpycnites assumingly preserve the waxing–waning signal of single‐peaked floods.

Sedimentology and Stratigraphy of the Shenandoah Formation, Western Fan, Jezero Crater, Mars

“…In a sub‐aqueous, pro‐delta setting, distal or off‐axis deposition in lobes created through hyperpycnal flows could have deposited laminated sandstones like those observed in FA1 (e.g., Grundvåg et al., 2023; Jackson & Johnson, 2009; Lamb et al., 2010; Plink‐Björklund & Steel, 2004; Zavala, 2020). Hyperpycnal flows can be instigated by, or as, any type of sediment gravity flow, including as river‐generated turbidity currents plunging down the front of a delta (e.g., Plink‐Björklund & Steel, 2004) or cohesive debris flows derived from a fan delta.…”

“…Hyperpycnal flows can be instigated by, or as, any type of sediment gravity flow, including as river‐generated turbidity currents plunging down the front of a delta (e.g., Plink‐Björklund & Steel, 2004) or cohesive debris flows derived from a fan delta. On Earth, quasi‐steady, sand‐rich, hyperpycnal flows are most commonly associated with river systems and often, specifically, with flooding events (e.g., Grundvåg et al., 2023). If FA1's planar laminated, soft sediment‐deformed sandstones were deposited by subaqueous sediment gravity flows, they are most characteristic of laminae deposited through rapid sediment fall‐out from a high‐density, sand‐rich, turbidity current under quasi‐steady (not surging) flow conditions (Bouma T B , Kneller & Branney, 1995; Talling et al., 2012).…”