Abstract
The St. Elias orogen and the Surveyor Fan in the adjacent Gulf of Alaska are a coupled source to sink system recording the interplay of tectonics and variable degrees of glaciation during the collision of the Yakutat terrane with the southern Alaska margin since the Miocene. The Miocene to Holocene sediments of the Surveyor Fan were drilled during IODP expedition 341. The recovered material is used to constrain information on changes in erosion centers during the last 10 Ma to study the impact of climatic and tectonic processes on orogen evolution. Point counting of sand- and silt-sized light framework components and geochemical single grain analysis of heavy mineral groups epidote and pyroxene is applied to analyze patterns of sedimentary provenance of two sites on the distal and proximal Surveyor Fan (Site U1417 and U1418, respectively). The studied sands and silts of Miocene to Pleistocene age are slightly enriched in feldspar (plag >> kf) at the proximal site, compositions at both sites do not show systematical changes with time of deposition. Framework component spectra uniformly reflect the expected active margin provenance. Epidote and pyroxene compositions are very consistent and show no change with time of deposition. Associations of epidote and pyroxene with albite, titanite and pumpellyite are in line with near-shore sources in the Chugach Metamorphic Complex and the metabasite belt at its southern border, and in units of recycled detritus exposed in the fold and thrust belt on the western Yakutat Terrane, respectively. Rock fragments indicate input from mainly metamorphic sources during the Miocene and Pliocene and an increase of input from low-grade metamorphic and sedimentary rocks in the Pleistocene, a finding also indicated by the abundance of epidote and pyroxene. This implies increasing erosion of the near-shore areas of the fold and thrust belt with advance of glaciers to the shore since the Miocene, being enhanced by the onset of the Northern Hemisphere glaciation at the beginning of the Pleistocene. Climate changes connected to the mid-Pleistocene transition did not result in appreciable changes in the petrographic compositions. Glaciers seem to have remained nested in their topographically predefined positions, continuously feeding material with uniform characteristics into the fan.
Highlights
The detritus constituting clastic sedimentary rocks is commonly derived from a multitude of source rocks and from several source regions (e.g. Garzanti and Andò 2007)
The non-clayey deposits encountered at Sites U1417 and U1418 are fine to medium sands and silts formed as turbidite overbank sediments (Expedition 341 Scientists 2014)
The sands and silts recovered at Sites U1417 and U1418 are relatively well-sorted quartz-intermediate deposits of turbidity currents building up the Surveyor deep-sea fan on the Gulf of Alaska sea floor (Plafker et al 1994; Gulick et al 2015)
Summary
The detritus constituting clastic sedimentary rocks is commonly derived from a multitude of source rocks and from several source regions (e.g. Garzanti and Andò 2007). The erosion of mountain ranges by glaciers varies with changes in average temperature resulting in potentially lower rates effected by cold-based glaciers during glacial intervals, and increased rates connected to Alpine glaciers during interglacial periods. These variations depend on elevation and latitudinal position (Yanites & Ehlers 2012). Examples include the southern Andes and the adjacent overfilled trench (Thornburg et al 1990; Contreras-Reyes et al 2010), the Southern Alps and adjacent basins of New Zealand (Griffiths 1979; Jiao et al 2018,), the HimalayaBengal Fan system (Bergmann et al 2020; Pickering et al 2020b; Reilly et al 2020), and the St. Elias orogen in southern Alaska and the adjacent Surveyor Fan (Plafker et al 1994; Reece et al 2011; Gulick et al 2015; Huber et al 2018a,b; Bootes et al 2019)
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