The genesis of laminated ice-proximal glacimarine sediments in Muir Inlet, Alaska
Mackiewicz, Nancy E.
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Muir Inlet in Glacier Bay, Alaska, is a glacial fjord receiving a tremendous volume of sediment annually: about 9 m/a ice-proximally and decreasing amounts away from the ice. The primary sediment source is meltwater streams discharging at subglacial and ice-marginal positions to form overflows, interflows, and underflows. (i) Overflows transport large volumes of rock flour far down fjord. Overflows and interflows interact with tidal currents and deposit cyclopels, which are cyclic layers of a thin fine-grained sand or silt lamina that grades normally to a thicker poorly sorted mud lamina. Cyclopels are ubiquitous in Muir Inlet, and when deposited close to a plume source have higher sand/mud ratios than those deposited more distally. (ii) Underflows occur in this glacimarine environment because of unique conditions in subglacial fluvial systems. Because a large volume of sediment is required to fora an underflow, it will either dissipate quickly because of low momentum, or be overtaken when momentum is great and become a turbidity current. Therefore, underflow deposits occur only in proximal positions (<0.5 km from an ice face). Underflow deposits are coarse-grained, reverse to normally graded, and exhibit an increase in sorting and sand content up layer. Iceberg rafted debris is ubiquitous, though minor (<5%), and occurs as isolated particles, frozen pellets, or as lenses that in cores may have a lamina appearance. Debris identified as ice-rafted generally describes coarse-grained (>250 urn), poorly sorted and/or isolated particles. Proximally, ice-rafted debris is difficult to identify because proximal sediment is normally coarse-grained. Deposited sediment may be reworked by (i) tidal currents at the fjord floor, or (ii) sediment gravity flows, from slumps off deltas, subaqueous fans, morainal banks, or fjord walls. Depositional processes operating in Muir Inlet produce laminated sand/silt/clay that characterizes sediment proximal to ice and fines seaward to mud. Sediment is classified into one of three sediment types: (i) Type I sediment is very fine-grained (mean 8.65-7.17 phi), low in sand (0.1-11.2%), very poorly to poorly sorted, and has a platy- to mesokurtic distribution. It is the dominant sediment type in Muir Inlet, and was transported by plumes and deposited by suspension settling. (ii) Type II sediment is fine- to coarse-grained (mean 6.70-3.12 phi), low to high in sand (5.1-86.6%), very poorly to moderately sorted, and has a platy- to leptokurtic distribution. It represents reworked sediment, proximal plume deposits, or coarse-grained laminae of cyclopels. (iii) Type III sediment is coarse-grained (mean 3.89-94.4 phi), high in sand, poorly to well sorted, and has a meso- to very leptokurtic distribution. It was deposited by sediment gravity flows or underflows.