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Timing and Origins of Magnetizations in Impact-related Carbonate Breccias and their Effects on Reservoir Potential

Devin Dennie

University of Oklahoma School of Geology and Geophysics

Norman, Oklahoma, USA

[email protected]


Buried craters such as the Ames (Oklahoma) and Red Wing (North Dakota) craters can be important hydrocarbon reservoirs via the structural deformation and brecciation that form as a result of impacts. Impacts may also enhance reservoir potential through the mobilization of hot fluids. This paleomagnetic study tests the timing and origins of an impact-related breccia modification within the Alamo MegaBreccia (Devonian Guilmette Fm., Nevada). The unit is interpreted as a tsunami deposit developed after an impact on a marine shelf. A paleomagnetic conglomerate test was conducted to investigate the origin of the breccia. Individual clasts from the upper part of the megabreccia from most locations contain random magnetization directions suggesting a pre-depositional magnetization consistent with an impact-generated tsunami origin. Tsunami reworked clasts would presumably produce deposition of cold clasts with random directions versus hot clasts containing grouped directions, as expected from impact melt breccias or hot alteration fluids. A capping fine grained breccia is interpreted as having a detrital remanent magnetization formed during the last stages of breccia deposition after impact. In contrast, analysis of lapilli clasts from multiple localities within the member suggest that a potential chemical remanent magnetization residing in magnetite dating to the late Paleozoic/early Mesozoic exists. This remagnetization may be caused by post-impact migration of alteration fluids, influencing the rock character and by proxy the reservoir potential of the unit. Geochemical studies and further analysis are planned to help clarify the potential fluid/rock interactions in the impact ejecta and their significance with reservoir potential.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid