SYNTILTING ORIGIN OF THE CHEMICAL REMANENT MAGNETIZATION OF THE PHOSPHORIA FORMATION AT SHEEP MOUNTAIN AND SPENCE DOME, WYOMING: REVISITED

A previous paleomagnetic investigation in the 1980s on the Phosphoria Formation at Sheep Mountain anticline in Wyoming revealed a syntilting chemical remanent magnetization (CRM) residing in magnetite based on thermal and alternating field demagnetization. However, this study did not employ low temperature demagnetization which is now routinely used on carbonates to remove viscous magnetizations in multidomain magnetite that could contaminate the characteristic magnetization. Recently, we resampled the dolomites at the Sheep Mountain anticline and an adjacent anticlinal fold as well as tilted beds in a small synsedimentary tepee structure at Spence Dome. All samples were subjected to low-temperature demagnetization protocol prior to isolation of CRM using thermal step-demagnetization. Almost all samples from the three different areas isolated a CRM with maximum unblocking temperatures up to 540°C. Rock magnetic studies confirm that the magnetite is the most prominent remanence carrier. The Sheep Mountain anticline and its smaller subsidiary anticlinal fold contain a pre-tilting northerly and steep-down magnetization, suggesting that the Phosphoria Formation was remagnetized prior to Laramide deformation. The synsedimentary tepee structure at Spence Dome contains a post-tilting northerly and steep-down CRM. These results suggest that the previous syntilting result was false and probably caused by the incomplete removal of a Modern overprint. The results from the Phosphoria Formation indicate that the unit was remagnetized after deposition and prior to Laramide deformation in the Early to Middle Tertiary. Although the origin of the CRM is still under investigation, we note that the Phosphoria at Sheep Mountain contains abundant hydrocarbons in dolomite porosity and the CRM could be related to hydrocarbon migration into the unit.