Rock Magnetic Characterization Of The Helderberg Group And Tonoloway Formation, West Virginia
Abstract
Rock magnetic and geochemical results will be reported from two stratigraphically adjacent units, the Devonian Helderberg Group and Silurian Tonoloway Formation, in the Appalachians in West Virginia. Both units have similar remagnetizations that reside in magnetite but fold test results indicate a difference in the timing of rema- nence acquisition. The remagnetizations are apparently synfolding in the Helderberg and synfolding to prefolding in the Tonoloway. One explanation for the differences is that the remagnetizations are different in the two units. To investigate this issue, rock magnetic and geochemical properties were compared. The hysteresis properties from both units are similar with high coercivity and magnetization ratios that plot plot close to the remagnetization line reported in other studies. The results of low temper- ature remanence measurements are variable with most curves containing evidence for superparamagnetic grains and some containing the Verwey transition. There are no differences in the character of the curves, however, from the two units. Comparisons of isothermal remanent magnetization acquisition and decay curves are also similar for both units. Geochemical results (fluid inclusion, stable isotope, and Sr ratios) are similar for both units and do no indicate any evidence for the introduction of exter- nally derived fluids. The rock magnetic and geochemical results are consistent with the interpretation that the both units contain the same remagnetization. A difference in magnetic properties, therefore, cannot explain the differences in the fold test results. The fold test results may indicate different responses to deformation by the generally coarser grained, thick-bedded Helderberg compared to the finer grained, thin-bedded Tonoloway. There is an apparent relationship with both total strain and strain parti- tioning with the Tonoloway generally exhibiting less total strain and in particular less pressure solution strain relative to grain boundary sliding and calcite twinning. While the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behavior can exist at a very local scale.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.4930E