Abstract

Detrital remanent magnetization (DRM), recorded in laminated glaciolacustrine silt and clay of the western Mohawk Valley of central New York, was used to construct a secular variation record of geomagnetic declination for parts of the late Wisconsinan in the interval 12.5-18 ka B.P. Remanent declination has been used for time-stratigraphic testing of correlations between laminated fine-grained glaciolacustrine sediments at different exposures in the western Mohawk Valley. A discontinuity in the remanent-declination record supports the interpretation that an observed lithostratigraphic discontinuity in the region represents an unconformity. Diamicton units, composed of till and proximal proglacial diamictons, are overlain by laminated clay and silt units that have time-transgressive lower contacts. The clay and silt units exhibit stratigraphic onlap in the direction of ice recession, a characteristic that is recognizable in their paleo-magnetic records. The use of a secular variation record of remanent declination has an advantage over lithostratigraphic and morphostratigraphic correlation techniques in that it is time dependent. For a part of the late Pleistocene, it provides a powerful tool for correlating physically disjunct stratigraphic assemblages in New York and New England, where no other dating technique has provided interregional correlations with comparable resolution.

Sampling of different exposures of contemporaneous sediment was used to demonstrate the fidelity of remanent declination as a recorder of geomagnetic declination. The remanent inclination record was not useful for testing correlations because inclinations in strata known to be contemporaneous were different. As compared to remanent inclinations from other contemporaneous lacustrine sequences in the eastern United States, the average remanent inclination of the western Mohawk Valley sediments is at least 11 degrees too shallow. Shallow inclinations may be the result of errors acquired at the time of deposition or compaction resulting from drying of the sediment, consolidation by overriding ice, or lithostatic load of overlying sediment. Subglacial deformation, dewatering, and some outcrop conditions, especially ground-water seepage, were recognized as processes that may cause or facilitate postdepositional disturbance of remanent magnetization.

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