Development of an incised-valley fill under the influence of tectonism and glacio-eustatic sea-level change: valley morphology, fluvial style, and lithology

Sequence stratigraphic concepts for making predictive lithologic models are based on relative sea-level changes, which for lower-rank cycles of 1 Ma or 400 ka are considered to be driven by eustasy. This study of an incised-valley fill of the lower Miocene Addit Member, Billund Formation, Denmark, shows that this is an oversimplification and that the distribution of lithology differs from that commonly used in sequence stratigraphic models. The incised-valley fill is composed of up to 60 m of fluvial deposits with intercalated lacustrine and marine sand and mud. A regional subaerial erosional surface separates the incised-valley fill into two distinct units that together constitute a compound fill. This second boundary clearly shows a stepped erosional pattern characteristic for a regional composite scoured surface (RCS). The scouring shows a systematic migration towards the east. The main incision and the twofold development of the valley fill are interpreted to be strongly influenced by the eustatic sea-level changes that occurred during the end of the Aquitanian (early Miocene), the so-called Mi1a glacial event. The development of systems tracts also follows the standard sequence stratigraphic model. However, inversion tectonism and continued uplift of the hinterland resulted in the upstream widening of the valley morphology and the systematic eastward migration of incision of at least the second regional composite scour surface. The presence of gravel clasts (< 5 cm) indicates a relatively steep dip for the fluvial system. As only minor local climatic changes have been detected during the incision and filling of the valley, local climate is not considered to have been important.