Why Are There Seashells In My Alluvial Valley? – The coastal geologist’s perspective of valley-fill sequences.

For engineers tasked with placing infrastructure across alluvial valleys in eastern Australia the mélange of unconsolidated sand and silt/clay deposits encountered beneath modern floodplains must often appear baffling, particularly the juxtaposition of river derived and marine influenced deposits. However, it is this apparent anomaly that immediately alerts coastal geologists, for whom modelling these valley-fill sequences is their stock and trade, that they are dealing with an ancient ‘drowned-valley estuarine’ depositional environment.

Valleys that ultimately connect to the coast have experienced numerous cycles of erosion and deposition as a consequence of major sea level fluctuations through geological time. The last major phase of fill has occurred in response to marine inundation of coastal valleys by rising sea level which stabilized at approximately its present level some 6,500 years ago.

With the exception of the alluvial capping layer that supports human habitation of these valleys, the greater proportion of silt/clay sediments deposited during this last phase have remained beyond the influence of pedogenic processes and are therefore ‘unripe’. This immaturity provides a number of challenging geomechanical and environmental aspects to working with these sediments.

Although local complexity may occur, stratigraphic models of ‘drowned-valley estuarine’ deposition provide a good general framework for understanding the distribution of both geomechanical and environmental properties of the valley- fill sequences.