Digital Varves

Paleomagnetism and Varve Chronology in the Northeastern U.S.

Across the northeastern U.S. the paleomagnetism of varved glaciolacustrine sequences provides additional stratigraphic information that has been useful for regional correlation and testing the ages stratigraphic units. Parameters such as remanent declination and inclination have been measured along with remanence intensities, bulk susceptibility, and anisotropy of susceptibility from oriented samples (See Field Methods – Paleomagnetism) collected at many localities. Several areas have been the focus of this work including the Connecticut and Merrimack valleys in New England and the western Mohawk, Champlain, and St. Lawrence Valleys of New York and western Vermont.

The paleomagnetic work on varves in the Connecticut Valley was among the first on sedimentary deposits and was the first to use the NEVC time scale to obtain a detailed paleomagnetic record (McNish and Johnson, 1938; Johnson et al., 1948). A secular variation curve of remanent declination was produced as well as a few inclination values. This work was later verified for the lower Connecticut varve sequence by Verosub (1979a, 1979b) who applied laboratory treatments to natural remanent magnetization measurements. Later work in New England has focused on extending the records of the lower Connecticut varves and verifying and extending results for the upper Connecticut and Merrimack varve sequences (Ridge et al., 1996, 1999, 2001). Paleomagnetic records also provide information useful for identifying where to look for potential matches between outcrop records and existing varve chronologies. The New England paleomagnetic records have been correlated to records from New York and western Vermont. The correlation of remanent declination records across the region has allowed the correlation of ice-recessional positions and deglaciation events as part of a comprehensive reconstruction of the pattern of deglaciation (See NEVC and Deglaciation). These correlations are important because most varve sequences in New York have not been measured and there are presently few correlations between New England and New York based on the matching of varve sequences, although such correlations are likely in the future especially for areas of the Hudson and Champlain valleys. Regional paleomagnetic correlations have also allowed the application of calibrated ages of the NEVC (See NEVC Calibration) to events from central New York to Maine.

Remanent Declination

Shown below is a plot of remanent declination records from varves of the NEVC in the Connecticut and Merrimack valleys. Data from these New England records is from Johnson et al. (1948; small dots connected with line), Verosub (1979a; open circles and envelope), Ridge et al. (1999; Conn. Valley, solid circles with error bars) and Ridge et al. (2001; Merrimack Valley, squares with error bars). The NEVC declination record is correlated to records from western Vermont and New York. This correlation is expected give the close proximity of the two areas. Regional constraints on deglaciation dictate that the two declination records must significantly overlap in time. In addition, the correlation shows a match of an extreme westward maximum in declination (to 40-60° W) on both records (14.8-14.5 cal kyr BP) where there are similar radiocarbon measurements from both records. Data in New York is not plotted against a varve chronology and varves have not been matched to the NEVC. The New York records have been assembled according to relative age based on superposition in single exposures and stratigraphic sequences and the regional time constraints placed on glaciolacustrine sequences by the pattern of deglaciation and lake impoundment at various stages of ice recession. Data in New York are from Ridge et al. (1990; western Mohawk Valley’s pre-Hawthorne to post-Holland patent beds, solid circles), Brennan et al. (1984; Mohawk Valley and Ontario Basin, open circles), Pair et al. (1994; St. Lawrence Basin, open circles), and Ridge et al. (1999; Champlain Valley, solid circles). Data shown with error bars are vector mean values of 10-12 samples clustered within 20-30 varve years. The error bars represent 95% spherical confidence intervals (α95) about the means. Abbreviations and colored lines represent the times of the maximum extent of glacial readvances in New York: SA = Salisbury Readvance, HS = Hinckley/St. Johnsville Readvance, BL = Barneveld/Little Falls Readvance, and RM = Rome Readvance in the western Mohawk Valley, and BP = Bridport Readvance in the Champlain Valley. See Ridge (2004) for the geographic positions of these readvances.

Remanent declination records from varves of the NEVC. See text above for description. Click image for high resolution version (~2 MB JPEG).

Remanent Inclination

Unlike remanent declination, remanent inclination has not been a useful parameter for regional correlation. Remanent inclination values do not vary as much as declination and are subject to flattening by depositional and post-depositional processes. Using the alignment of the remanent declination records from New England and New York a plot of remanent inclination values is shown below for the comparison of inclination records in both regions. Data is shown with the same symbols and sources as on the declination record. There does not always appear to be a pattern in inclination variation that matches the record in New England, especially in the western Mohawk Valley record. This is likely due to flattening of inclination during deposition and then again under extreme compaction as most Mohawk Valley measurements were from sediment compressed by overriding ice and large lithostatic loads (Ridge et al., 1990). Up section steepening of inclination in several single outcrops is also suggestive of the influences of up section changes in lithology and compaction rather than solely a reflection of changes in geomagnetic inclination at the sites.

Remanent inclination records from varves of the NEVC. See text above for description. Click image for high resolution version (~2.25 MB JPEG).

References