João A. Santos

Till fabric and grain-size analysis of glacial sequences in the Upper Sil River Basin, Cantabrian Mountains, NW Spain

Fabric and grain-size analysis of eight deposits present in the Upper Sil River Basin palaeoglacial system of northwest Spain were used to interpret till types and to reconstruct the glacial paleoprocess history of this region. They are represented by lodgement, deformation, and melt-out tills. The first, representative of glacial advance stages, present cluster fabrics and are generally composed of poorly sorted and finer sediments. Melt-out and deformation tills, representative of stages of glacial stability followed by fast recession, present transitional to girdle fabrics and are composed of poorly sorted coarser sediments. In any case, multiple criteria, including lithological, clast-shape, or structural data, support these directional and grain-size observations. These sequences, most of them located in the snout area of the Sil palaeoglacier, are important in the reconstruction of glacial dynamics due to poor preservation of landforms related to the maximum glacial advance. This palaeoglacial system was formed by multiple tributaries and had an extension of over 450 km2, with the Sil paleoglacier being 51 km long during its maximum glacial advance.

Till fabric analysis and origin of Portage Glacier Little Ice Age moraines, south-central Alaska

Abstract Observations of geomorphic and sedimentary processes occurring in modern glaciers allows interpretations of the genesis of older glacial deposits and landforms and has led glacial geomorphologists and geologists to place a greater emphasis on studies of glacial sedimentology. This research investigates the genesis of Little Ice Age (LIA) moraines present in Portage Glacier Valley, south-central Alaska using till fabric and grain size analysis. The present-day moraine morphology and sedimentology in Portage Glacier Valley reveals the presence of two types of till and moraines. The clast-rich medium to coarse sandy diamicton present in the AD 1852 moraine is interpreted to be a lodgment till indicating that this feature is primarily a Type A push moraine representing glacial advance of Portage Glacier prior to AD 1852. The clast-rich coarse to very coarse sandy diamicton present in the AD 1900 and AD 1922 moraines is interpreted to be a supraglacial melt-out till (ablation till) indicating that these features are ablation moraines representing glacier recession and moraine building of Portage Glacier in AD 1900 and AD 1922.

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

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.

Holocene glacial history of College Fjord, south-central Alaska

Abstract Holocene glacier fluctuations in College Fjord in south-central Alaska provide an integrated proxy record of climatic and related environmental changes through this critical geologic epoch. During the early Holocene around 10,000 yr BP, ice was receding from lower Port Wells but was still present in College Fjord. With the beginning of the Holocene Climatic Optimum, recession increased and the ice front reached the Golden Moraine site 1 by 7680 cal yr BP, starting its long early to middle Holocene retreat to the head of tidewater. An ice-free fjord existed between 7680 cal yr BP and about 5190 yr BP. A middle Holocene advance started between approximately 5190 and 4470 yr BP with ice arriving at the mouth of the fjord before 2470 cal yr BP. The late Holocene was characterized by various standstills and minor readvance positions as the glacier fell back and by significant Little Ice Age expansions and retreats of individual glaciers present in the fjord. During the late Holocene, the glacier built four large and distinctive moraines all representative of at least four standstill positions of College Fjord Glacier before 2470, 2060, 1330, and approximately 1120 cal yr BP.