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Abstract

Most of the fine-grained bottom sediments of Glacial Lake Hitchcock are rhythmites composed of silt- clay couplets that occur in three structural groups :

Group I—clay thickness greater than silt thickness.

Group II—clay thickness approximately equal to silt thickness

Group III—clay thickness less than silt thickness .

Thin sections of impregnated sediments show flat bedding with as many as 40 graded laminae in one 5-cm layer. Erosional contacts and ripple cross-lamination are common in Group III, but rare in Groups I and II. The contact between silt and the overlying clay layer in any one rhythmite is gradational in less than 25 percent of the samples. Other sedimentary features include two distinct types of trace fossils. Mean grain size of the silt layers (5.50 to 8.50) depends up the environment of deposition of silt within the lake. Mean grain size of the clay layersis much the same everywhere (averaging 10.5ø).

Data from 34 localities suggest thatthe rhythmites are annual (that is, they arevarves), andthefol

lowing depositional mechanism is proposed. Sediment was transported by streams from the glacier and from nearby deglaciated uplands. Gravel andsand were deposited on deltas. Silt and claywere carriedoutinto

the lake, mainly by turbidity currents.Sediments coming into the lake contained asignificant amount of

clay that settled out continuously, but clay became the dominant deposit only during the winter when coarse material was less available.

Varves belonging to Groups I and II generally were formed in still water away from river mouths, where little sediment was received directly from density currents. Group III varves were formed relatively near delta fronts, in environments characterized by high sedimentation rates.

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