Detached lowstand deposits illustrated through seismic data, lithology and clay mineralogy: an example from the Lower Miocene of Denmark

On seismic panels from southwestern Jutland a specific seismic facies, characterized by high amplitude, steeply dipping reflectors is recognized within a background of low-angle seismic reflection pattern. The prograding interval, interpreted as a prograding delta, records a distinct shift in facies during the deposition of offshore to shallow marine shelf deposits and is therefore interpreted as belonging to a lowstand systems tract. Correlative outcrops of nearshore sandy sediments in eastern Jutland also show a marked facies change characterized by a 1 m thick gravel layer sandwiched between fine-grained marine sands. This gravel is interpreted as having been deposited under subaerial conditions in a fluvial environment that was coeval with the lowstand delta in southwest Jutland. Clay mineral studies support this interpretation of the gravel layer. The marine sediments are characterized by having a smectite clay mineral association; in contrast, the gravel layer contains no smectite, but has a clay mineral suite that is characteristic of rivers draining the hinterland east of the study area.