The importance of coastal geomorphological setting as a controlling factor on microtextural...

Abstract

Quartz grains collected from Arauco and Mataquito (central Chile) after the 2010 Maule tsunami presented an overwhelming dominance of dissolution textures. The analysis of superficial imprints proved that some grains were mechanically impacted before deposition. However, the percentage of grains with fresh surfaces and percussion marks was significantly lower than average values from other tsunami deposits elsewhere in the world. In this work, we discuss the reasons for such results in the context of the geomorphological setting of the areas analyzed and its influence on the microtextural signatures observed. The data presented in this study evidences a geographic dependence in the type of microtextures in the areas analyzed. For example, in Arauco the abundance of dissolution textures decreases rapidly towards the center of the embayment and increases towards the rocky headlands of its westernmost sector. By contrast, an increase of mechanical marks (e.g. fresh surfaces) is observed in the central region of the Arauco’s embayment. Similarly, in Mataquito, dissolution features are more abundant in the headlands or small capes, while there is a higher presence of mechanical marks in sandy embayments. The results of this study demonstrate the importance of the geomorphological context as a controlling factor in the intensity of mechanical imprints on the surface of quartz grains transported by tsunamis and deposited in the inner shelf and coastal areas. Therefore, our results suggest that without a detailed geomorphological contextualization microtextural discrimination can lead to misleading interpretations. Hence, there is a need for more microtextural analysis on tsunami deposits in order to assess the variability in the geographic distribution and intensity of microtextures imprinted on the surface of quartz grains deposited during a tsunami event.