Hoanib River flood deposits of Namib Desert interdunes asanalogues for thin permeability barrier mudstone layers inaeolianite reservoirs

Abstract

The ephemeral braided Hoanib River of NW Namibia flows for a few days a year, and only high discharges enable the river to pass through interdunal depressions within the northern Namib Desert dune field to the Atlantic. The dune field comprises mainly large transverse dunes resulting from predominant SSW winds. River flood deposits between aeolian dunes are analogous to mudstone layers conformably interbedded with ancient aeolianite dune foresets. Deep floods pond laterally to considerable depths (metres to >10 m) in adjacent interdunes, depositing mud layers (1-50 cm) to considerable heights on avalanche and stoss faces of bounding dunes. Fairly passive flooding only disturbs aeolian stratification minimally. Floodwater clay infiltrates and settles as an impermeable seal, with a flood pond on top, perched, above regional groundwater. Flood ponds evaporate slowly for long periods (>3 years). Early emergence desiccates higher parts of a mud layer. Subsequent floods can refill a predecessor pond, benefiting from the existing impervious seal. Potential preservation of such mud layers is lower on the stoss face, but high on the avalanche face after burial by subsequent dune reactivation and migration. The leeward (right) Hoanib bank, a dune stoss face, is river and wind eroded to exhume fossil interdune pond mud layers of an earlier Hoanib channel. The highly inclined layers are interbedded with dune avalanche foresets and represent the edges of two successive fossil ponds exposed in plan. Ancient flood pond mudstones occur in the Permian-Triassic hydrocarbon reservoir, the Sherwood Sandstone Group of the Cheshire Basin (Kinnerton Formation) and Irish Sea Basin and were previously used erroneously to argue against the aeolian origin of cross-bed sets. Hoanib studies show that primary river interaction with a dune field might preserve only localized erosional omission surfaces in ancient aeolianites, with little sandy barform preservation, prone to aeolian reworking. Around the main fluvial channel locus, however, flood pond mudstone layers should form a predictable halo, within which fluid permeability will decrease.

Architectural analysis of a Triassic fluvial system: The Sherwood Sandstone of the East Midlands Shelf, UK

“…This element is interpreted as overbank deposition on a sandy floodplain (Sadler and Kelly, 1993;Bristow et al, 1999;Stanistreet and Stollhofen, 2002;Fisher et al, 2007). The individual erosive surface and upward-fining units represent individual discrete flood events.…”

Stratigraphic architecture of back-filled incised-valley systems: Pennsylvanian–Permian lower Cutler beds, Utah, USA

“…Aeolian interdunes composed exclusively of aeolian wind-ripple strata had dry substrates, whereas those composed of massive sandstone record episodes of short-lived flash flood events along interdune corridors (cf. Langford and Chan, 1989;1993;Bullard and Livingstone, 2002;Mountney and Jagger, 2004;Cain, 2009) in a style similar to that in many present-day dune fields (Lancaster, 1983;Langford, 1989; Wakefield and Mountney Tooth, 1999; Stanistreet and Stollhofen, 2002;Krapf et al, 2003;2005). These massive sandstone sets are of fluvial origin and are differentiated from similar sets of aeolian origin by virtue of their diagnostic red-brown colour, occurrences of rare intraformational rip-up clasts and, in some cases, increased mica content.…”

“…Langford and Chan, 1988;1989;Stanistreet and Stollhofen, 2002;Mountney and Jagger, 2004), relatively few have examined tripartite styles of interaction involving coastal settings where aeolian, fluvial and shallow-marine processes all operated coevally (e.g. Simpson and Eriksson, 1993;Blakey et al, 1996).…”

Alluvial-eolian interaction in a Cambrian rift margin: the Pedra das Torrinhas and Pedra Pintada formations (Guaritas Group, RS)

“…ANDRÉ MARCONATO et al 1989, Tooth 2000, Stanistreet and Stollhofen 2002, Stromback et al 2005. The interaction between eolian systems and alluvial fans is less documented (Clemmensen andAbrahamsen 1983, Gardner et al 2006), and even less documented are the interactions of such systems before the evolution of land plants (Hadlari et al 2006, Paim andScherer 2007).…”

“…Brookfield 1992, Kocurek 1996 with an increasing interest in the particularities of the interactions among both systems (e.g. Langford 1989, Langford and Chan 1989, Stanistreet and Stollhofen 2002, Paim and Scherer 2007, with descriptions of the resulting depositional architecture (e.g. Herries 1993, Fischer et al 2007).…”