EarthRef.org Reference Database (ERR) -- Banerjee et al. 1997

Lower Pennsylvania Belden Formation carbonate rocks from Colorado were subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of two components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM-250 ¿C) remanence that is interpreted to be a modern, viscous magnetization. An intermediate unblocking temperature (250--400 ¿C) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a thermoviscous remanent magnetization. Many limestones also contain a high unblocking temperature (400--570 ¿C) remanence component which is interpreted to be a CRM. Fold tests from different parts of the basin indicate that the CRM was acquired either before or during Laramide folding. This CRM is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite. Hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the CRM is carried by single-domain/pseudo single-domain magnetite. Although elevated 87Sr/86Sr values indicate passage of radiogenic fluids through the limestones, the results of contact vein tests do not support the hypothesis that these fluids were responsible for the CRM. The time of CRM acquisition, which varies from late Paleozoic to Cretaceous, coincides with the modeled time of organic matter maturation in different parts of the basin. This suggests that diagenetic reactions, that were triggered by low to moderate burial temperatures, may have caused the magnetite authigenesis and probably gave rise to the CRM.¿ 1997 American Geophysical Union Lower Pennsylvania Belden Formation carbonate rocks from Colorado were subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of two components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM-250 ¿C) remanence that is interpreted to be a modern, viscous magnetization. An intermediate unblocking temperature (250--400 ¿C) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a thermoviscous remanent magnetization. Many limestones also contain a high unblocking temperature (400--570 ¿C) remanence component which is interpreted to be a CRM. Fold tests from different parts of the basin indicate that the CRM was acquired either before or during Laramide folding. This CRM is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite.