Scanning electron microscopical observations on weakly cemented Miocene sands

Abstract

The cementation that has led to the local formation of slightly indurated layers in the white Miocene sand deposits, is due to authigenic quartz growth. Scanning electron microscopical information reveals that the secondary quartz is not only found at the grain contacts. A rather unexpected finding is its presence all around very small as well as large grains. The former ones show the most advanced idiomorphic habit and seem to play an actual part in the cementation. Presolved surfaces could not be found, so that the used silica does not appear to be produced by pressure solution. It is concluded that the grain surfaces must have been clean and highly disturbed before cementation started. This is, because only such surfaces seem to provide numerous suitable sites for nucleation, which may result in a comparatively rapid and full-scale overgrowing. However, since silica under certain conditions also dissolves very readily from disturbed surfaces, it is still possible that a part of the crystallized silica has come from the deposit itself. This particular type of early stage of cementation suggests strongly that the nature of the detrital quartz grain surfaces may be a very important factor in the kind of silica cement being formed. It is suggested that the origin of these weakly indurated layers must have been associated with seasonally rising and falling silica-bearing groundwater