PALEOMAGNETISM OF THE SIERRA MADERA IMPACT STRUCTURE, TEXAS

Monomict and polymict carbonate and siliciclastic breccia clasts, as well as deformed country rock, were sampled at Sierra Madera (Trans-Pecos Texas), an exhumed complex impact structure. Stepwise demagnetization reveals a characteristic remanent magnetization (ChRM) that is removed between 580-680°C. The ChRM has southerly declinations and steep up inclinations or northerly declinations with steep positive inclinations. The normal and reverse directions pass a reversal test. Results from multiple breccia clasts fail a conglomerate test, indicating a postdepositional remagnetization. The unblocking temperatures and rock magnetic studies indicate the ChRM resides in hematite. A second component is removed below 580°C and contains streaked directions between the ChRM and a southwesterly or northeasterly direction. Based on rock magnetic analysis this intermediate temperature component resides in both high and low coercivity phases. The origin of this component is under investigation. The pole for the ChRM falls near the 50-70 Ma part of the apparent polar wander path. Compared to the stratigraphic age range for the impact (40-100 Ma), the pole and the reversals suggest the impact is no younger than 50 Ma and no older than ~83 Ma. Petrographic observations show that flow banded glass is present in the breccia clasts and matrix, suggesting that the breccia was hot at deposition. The ChRM is tentatively interpreted as a thermal remanent magnetization (TRM) acquired during cooling of the breccia although a chemical remanent magnetization can not be ruled out without further analysis. If the ChRM is a TRM, it dates the impact to 50-70 Ma.