Laminated sediments in prodeltaic deposits, Glacier Bay, Alaska

In Glacier Bay, southeastern Alaska, meltwater streams carry high sediment loads into Qords. Sea-floor sediments in the proximal marine areas of glacier-fed deltas exhibit prominent layering predominantly composed of: (1) couplets of coarse laminae grading to fine laminae, (2) solitary massive to graded layers, and (3) microlaminae. Sediment layers form from pulsations in sediment influx. Our investigations have shown that sedimentation rates vary as a result of cyclic and noncyclic processes on several time scales. Semi-diurnal and diurnal tidal stages, diurnal discharge fluctuations, fortnightly neap-spring cycles, and a distinct melt season may result in a temporal hierarchy of sea-floor sediment stratification. During the melt season, graded couplets are produced on the order of 1 to 2 per day by suspension settling from sediment pulsations due to reentrainment of delta plain sediment and encroachment of stream channel mouths during ebb of the macrotidal prism. Diurnal discharge fluctuations appear to play a lesser role in sediment layer production. Sequences of relatively thin fine and thick coarse layers are deposited during neap and spring tides, respectively. Sediment gravity flows from the delta front occur mainly during lower low spring tides when delta plains are completely exposed and fluvial bed load is transported directly to the delta plain edge. Massive to graded layers result, as well as graded couplets from small sediment gravity flows. Microlaminae may be produced in short intervals by several processes such as minor transport fluctuations in plumes and other transporting currents. The processes and sedimentation products described here are probably not restricted to temperate glacial marine deltas, but could be expected in other regions characterized by high tidal amplitudes and sediment influxes.