Till-Fabric analysis and origin of late Quaternary moraines in the Serra da Peneda Mountains, NW Portugal

Abstract

The combined usage of till-fabric and grain-size analysis in the sedimentological interpretation of glacial diamictons allows the reconstruction of the processes responsible for the genesis of certain moraines. Till-fabric and grain-size data from several late Quaternary moraines present in three glacial valleys of the Serra da Peneda Mountains in northwestern Portugal revealed the presence of two types of till deposited during several stages. During the first stage, ice advanced into the lower Alto Vez and Branda da Junqueira valleys and deposited the lodgment tills present in the Alto Vez moraine 1 (unit 1) and Branda da Junqueira moraine; therefore, these features are interpreted to initially be lateral-terminal and lateral push moraines, respectively. Subsequent stages of glacial stability followed by recession in the Alto Vez and Rio Pomba lower valleys deposited the supraglacial melt-out tills (ablation tills) present in the Alto Vez moraines 1 (unit 2), 2, and 3 and the Rio Pomba moraine. These four landforms are interpreted to be lateral-terminal, lateral, lateral-recessional and recessional ablation moraines, respectively. The Alto Vez moraine 1 had two constructional events, starting most likely as a lateral-terminal push moraine and ending as a lateral-terminal ablation moraine.