Ancient ephemeral stream deposits: a local terminal fan model from the Bunter Sandstone Formation (L. Triassic) in the Tønder-3, -4 and -5 wells, Denmark

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Abstract
Summary: Facies studies of a c. 30 m thick fluvial sand body in the Tønder area, Denmark indicate that the sand body was formed by an ephemeral stream system comparable to modern terminal fans. The sediments are grouped into three genetic facies associations (A, B, C) reflecting two styles of channel flow (A and B) and overbank sheet flow (C). The sediments are dominated by parallel laminations (28%) and large-scale cross-bedding (27%) in association with small-scale cross-bedding (17%), massive bedding (14%) and intraformational conglomerates (5%). Mudstones and siltstones form only 9% of the deposits. Anhydrite (formerly gypsum) is associated with the fine-grained deposits which are mainly of overbank origin. It is commonly observed as intraformational clasts in the channel deposits. The inferred ephemeral stream complex probably formed a link between a more permanent braided stream system towards the N (the Skagerrak Formation) and a sabkha system towards the S (the Middle Buntsandstein in northern Germany).