Rhythmic Sedimentation in Glacial Lake Hitchcock, Massachusetts-Connecticut
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.
Figures & Tables
Glaciofluvial and Glaciolacustrine Sedimentation
This publication is the outgrowth of a symposium on Glacial Sedimentology that was held in Buffalo, New York, March 1972. The great interest generated in glacial phenomena during the nineteenth century had important implications and repercussions for the infant field of sedimentology. It provided its fair share of the background stimulus necessary to establish sedimentology as a separate branch of the earth sciences in the twentieth century. The time for reciprocity is now at hand; feedback from the expertise gained in the burgeoning field of sedimentology can greatly help the Quaternary specialist solve particular field problems. The last decade has witnessed a growing interest in the sedimentology of the Quaternary, and it seems appropriate now to summarize progress in the study of stratified drift, to present results of some recent studies, and to focus attention on avenues of research that should be explored in the near future.