Johan Svendsen

Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia

Channel-shaped deposits of well sorted sand with only 2–4% fine-grained material, being either massive and structureless or upward fining with basal lag, are found interbedded with aeolian sand in the Skeleton Coast Erg, Namibia. Detailed analyses of the channel-shaped deposits suggest that they are formed as hyperconcentrated flows within the erg. Grain-size analysis and whole rock geochemical modeling revealed that some of the fluvial sediments contain up to 70% aeolian sand, interpreted as a result of dune collapse into the fluvial system. In certain cases, this instantaneous supply of sand resulted in generation of sandy mass flows with laminar flow behaviour. The presence of smectite as dominant clay mineral proved to be of crucial importance in formation of mass flow deposits. These mass flows had no erosional capacity, and drape the palaeotopography. They are comparable to those generated by catastrophic collapse of dunes, described in the literature. This paper suggests that all these deposits should be termed intra-erg mass flows, as several of them carry little, if any debris. Based on their origin, intra-erg mass flows can be divided into two groups:

Comparison between outcrop-spectral gamma ray logging and whole rock geochemistry: implications for quantitative reservoir characterisation in continental sequences

Abstract A high-resolution spectral gamma-ray (SGR) log is compared to elemental whole rock geochemical data in a reservoir scale outcrop analogue, the Otter Sandstone Formation (Sherwood Sandstone Group), east Devon. SGR data compare closely to those generated for point samples by inductively coupled plasma (ICP) techniques, with disparities attributable to the different sample volumes. ICP data reveal separate mineralogical sources for radio-elements, which enable three lithotypes to be discriminated using Th/U ratio and K 2 O data. Once established, the discriminants are applied to the SGR log enabling three lithotypes to be dicriminated. These conclusions suggests that SGR logs have significant potential for real-time lithotype prediction in continental sequences, which in turn enable predictions of reservoir quality, and increase confidence in the interpretation of sedimentary architecture.