Little Ice Age Glacial Geomorphology and Sedimentology of Portage Glacier, Alaska.
Abstract
The study of glacial landforms and deposits is important, as it is difficult to observe processes under modern glaciers and ice-sheets. Thus landscapes and sediments that are the product of present glaciation can give insight into processes that occurred during Pleistocene times. This study investigates the genesis of glacial landforms present in Portage Glacier, South-Central Alaska. Portage Glacier is a valley glacier with an ice tongue of approximately 7 km long and 1,6 km wide. It is fed by ice from the Blackstone-Spencer ice complex and from several local cirques. LIA moraines present on both sides Portage Valley that were previously identified and dated by other authors (Crossen, 1992) were dug and sampled for grain size, roundness and till fabric analysis. Analysis of sedimentary facies present in these features revealed an interesting glacial paleoprocess history in this valley. The 1852 dated moraine is the LIA end moraine in Portage Valley and is mainly composed of a massive poorly sorted gray clast-rich sandy diamiction with bi-modal grain size distributions and subrounded to subangular well-oriented striated clasts. The 1901 moraine is composed of moderately sorted gray sandy boulder gravel with unimodal grain size distributions and subrounded to angular poorly oriented striated clasts. The 1922 and 1984 moraines are similar in sedimentology to the 1901 moraine. They are composed of moderately sorted gray sandy boulder gravel with unimodal grain size distributions and subrounded to angular poorly oriented striated clasts. The present day moraine sedimentology in Portage Glacier valley reveals the presence of two types of glacial tills and moraines. The clast-rich sandy diamiction present on the 1852 moraine is interpreted to be a basal till indicating this feature is a push moraine representing a small advance or a standstill position of Portage Glacier in 1852. The moderately sorted gray sandy boulder gravel present on the 1901,1922 and 1984 moraines is interpreted to be an ice-marginal deposit (ablation till) with a mixture of subglacial, supraglacial and glaciofluvial sediments deposited by glaciotectonic processes, slumping and stream sorting. All of these features are interpreted to be ablation moraines representing glacier retreat and moraine building in 1901,1922, and 1984.
