Abstract

Scanning electron microscope fabric analysis of laminated Pleistocene glacial silty clay and clay from New York State provides evidence of two sedimentary processes: suspension settling of flocculated aggregates, followed by bioturbation of the upper parts of silty clay and clay laminae. The glacial rhythmites in two Pleistocene glacial lakes in New York State formed dominantly from episodic sedimentation of flocculated sediment. Graded bedding and random particle orientation indicate continuous deposition of flocculated sediment during the deposition of a typical glaciolacustrine couplet. However, during the interlude between rhythmite deposition of the flocculated silty clay and clay, a second (but more subordinate) sedimentary process of bioturbation dominated. Nematode burrows found in the upper part of the clay portion of couplets are associated with preferred orientation of clay flakes. This fabric found around the burrows was produced as the soft, plastic, moist, flocculated clay sediment was sheared and reoriented by nematode movement between sedimentation events. The preferred orientation between burrows in the upper part of the clay laminae resulted from postdepositional compaction due to the weight of overburden. Reorientation was limited, however, to only the upper part of the clay laminae. Random orientation present in the original flocculated sediment was preserved in the lower part of the couplet. The study presents a model of how clay fabric can be used in determining sedimentary processes of glaciolacustrine deposits.

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