Paleomagnetic dating of burial diagenesis in Mississippian carbonates, Utah

Abstract

The objective of this study is to test models for the origin of widespread remagnetizations in the Mississippian Deseret Limestone. The Delle Phosphatic Member of the Deseret Limestone is a source rock for hydrocarbons in other units in Utah and entered the oil window in the Early Cretaceous during the Sevier orogeny based on modeling studies. Paleomagnetic results from the Deseret Limestone and the stratigraphically equivalent Chainman Shale in central and western Utah indicate that the unit contains two dual polarity ancient magnetizations, interpreted to be chemical remanent magnetizations ( CRMs) based on low burial temperatures. The demagnetization characteristics and rock magnetic studies indicate that the CRMs reside in magnetite. Three fold tests from western Utah indicate the presence of a pre-folding Triassic to Jurassic CRM. Geochemical (87Sr86 Sr, 6 13 C, and 6 18 0) and petrographic analyses suggest that these rocks are unaltered. The age of the CRM is just prior to the modeled time for organic matter maturation in the unit. Therefore, this CRM is interpreted to be the result of burial diagenesis, such as an early stage of organic matter maturation within the source rock or clay alteration. A second younger CRM in western and central Utah is post-folding based on a regional fold test. This CRM is Late Cretaceous to Early Tertiary in age and the timing overlaps with the oil window based on the thermal modeling. These results are also consistent with a connection between organic matter maturation and remagnetization, and the geochemical ( 87Sr86Sr, 613 C, and 6.18 0) and petrographic results from rocks with this component show no evidence for alteration by externally derived fluids. The results of this study support the hypothesis that widespread burial diagenetic processes are associated with pervasive CRMs. In addition, this study indicates that paleomagnetism can be used to determine the timing of burial diagenetic processes, which can benefit hydrocarbon exploration efforts.