CHEMICAL REMAGNETIZATION AND BURIAL DIAGENESIS - TESTING THE HYPOTHESIS IN THE PENNSYLVANIAN BELDEN FORMATION, COLORADO

Lower Pennsylvanian Belden Formation carbonate rocks from Colorado wer e subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical rema nent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of t wo components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM-250 degrees C) remanence that is in terpreted to be a modern, viscous magnetization, An intermediate unblo cking temperature (250-400 degrees C) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a therm oviscous remanent magnetization. Many limestones also contain a high u nblocking temperature (400-570 degrees 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 f olding, 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 th at the CRM is carried by single-domain/pseudo single-domain magnetite. Although elevated Sr-87/Sr-86 values indicate passage of radiogenic f luids 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 Cre taceous, coincides with the modeled time of organic matter maturation in different parts of the basin, This suggests that diagenetic reactio ns, that were triggered by low to moderate burial temperatures, may ha ve caused the magnetite authigenesis and probably gave rise to the CRM .