COMPARATIVE PALEOMAGNETISM AND DIAGENETIC ALTERATION OF IMPACT STRUCTURES THROUGHOUT THE UNITED STATES

Bolide impacts are a catastrophic geologic process that affect the target rocks on micro and macroscopic levels. The timing of impacts is often difficult to determine, particularly in the absence of a complete stratigraphic section. Paleomagnetism can be used as a tool to constrain the timing of deformation when other dating methods are not possible. We collected samples of impact structures in Missouri, Indiana, Nevada, and Tennessee, with the aim of discovering a shock remanent magnetization (SRM) created by the impacts. The target rocks did not hold SRMs but did hold multiple ancient remanent magnetizations; the instantaneous creation of potential fluid pathways following these impact events allowed diagenetic alteration in the form of (re)magnetizations and mineral formation.

The Weabeauleau and Decaturville impact craters in Missouri show a chemical remanent magnetization (CRM) in magnetite that are Mississippian and Late Paleozoic in age, respectively. These ages further constrained the impact ages from what was inferred stratigraphically. The magnetization was post-deformation in both impacts, with the CRM possibly related to hydrothermal flow from the impact process.

The paleomagnetic results from Kentland, Indiana, indicate a Jurassic magnetization in polymict breccias. The presence of possible tridymite and sylvite, and the alignment of minerals such as illite, silica, gypsum and apatite within the veins, suggest a flow texture associated with hydrothermal alteration.

Alamo Impact Breccia samples were collected throughout south central Nevada. Using conodonts, the age of this impact has been constrained to late Devonian. Paleomagnetic analysis revealed two CRMs: an intermediate temperature CRM held in pyrrhotite and interpreted as Tertiary in age; and a high temperature CRM held in magnetite and interpreted as Late Paleozoic. Sr isotope analysis values were elevated from coeval, indicating alteration by externally derived fluids.

The Flynn Creek and Wells Creek impact craters in north central Tennessee both contain a CRM in magnetite that is late Mississippian-early Permian. This magnetization is widespread and is likely related to fluid flow from the Alleghenian Orogeny.

The magnetizations found in Missouri and Indiana were not formed as a result of the impact directly, but by hydrothermal pathways made possible by deformation created during the impact event. In Nevada and Tennessee, the CRMs were likely related to large scale hydrothermal fluid movements during orogenic events.