Lower Cook Inlet is a forearc embayment associated with the northeast extension of the Aleutian volcanic arc. This highly active tectonic setting is the dominant influence on the nature of the depositional systems and the sediment composition along the inlet's shoreline. The 1216 km of shoreline of Lower Cook Inlet was subdivided into erosional (45%), neutral (38%), and depositional (17%) classes. Fan deltas and depositional spits are the most common depositional features in the mid- to upper-intertidal zone. Wide tidal flats and wave-cut rock platforms occur in many of the lower intertidal areas. The sand and gravel fractions of the coastal sediments show a wide range in size and composition. On erosional and neutral shorelines, the sediments are derived locally from eroding scarps in the sedimentary rocks that border most of the inlet. The deltaic areas, on the other hand, show a wide range in sediment composition, depending upon which source terrain their feeder streams drain. The northwestern source area, the volcanic arc, produces arkosic and volcanic debris; the southeastern source area, a metamorphic highland (Paleozoic and Mesozoic), produces mostly metamorphic rock fragments; and the northeastern source area, a sedimentary lowland (Tertiary), produces mostly sedimentary rock fragments and quartz. The shoreline depositional systems reflect primarily the interaction of sediment supply and wave action, with high-gradient streams producing lobate fan deltas in sheltered areas and arcuate-cuspate fan deltas in areas exposed to storm waves. Although the tides are macrotidal, they have relatively little influence in shaping the morphology of the mid- to upper intertidal depositional systems in Lower Cook Inlet.

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