The Hageland Hills, Legacies of the Depositional Architecture of the Miocene Diest Sands

Abstract

The Hageland area is characterized by long, a few tens of metres high hills that form a remarkable array of parallel topographic features. The existing model stating that the Hageland hills are remnants of offshore tidal sandbanks formed during deposition of the Miocene Diest Formation should be abandoned. Also the role of ironstone beds as the primary cause of the hills is questioned. Instead, the hills are thought to be determined by elongated parallel coarser grained sand bodies that were embedded in a more abundant facies of slightly clayey, bioturbated fine sand inside the Miocene Diest Formation. A sedimentary model is presented that interprets the elongate coarse sand bodies as the fill of a series of parallel, linear depressions, each time scoured by locally constricted flow during the continuous lateral fill of a tidal marine embayment. The coarse sand bodies are thought to have been sculpted out as positive relief features much later, during Pleistocene uplift of the area. Their fabric and higher permeability are put forward as important causes of primary preservation during the landscape morphogenesis. The ironstones can only have formed after initial appearance of the long hills. Their presence may have contributed to preserve some hills during further uplift and erosion.