Paleomagnetism of the Valle Group, Baja California, Mexico and environmental magnetism of White Lake, New Jersey, United States of America.

摘要

As part of continued efforts to investigate the causes of anomalously shallow paleomagnetic inclinations in the western North America, a detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian-age strata of the Valle Group (∼95 Ma) at El Campito, Baja California. Sixteen of 18 sites yield characteristic remanent magnetizations (ChRM) with a mean of Dec = 342.6°, Inc = 19.1°, alpha95 = 3.4° in stratigraphic coordinates. The inclination is 38.7° shallower than that predicted by the reference paleopole for North America. Fold tests failed to provide tight constraints on the remanence's age. An interpretation of a primary remanence would require a significant southerly, then northerly paleolatitudinal offset, which is not supported by geologic data. A more likely scenario is a Late Cenozoic low-temperature remagnetization, which may have resulted from burial and/or Miocene volcanism.; To examine thermal effects on magnetic fabrics, particularly remanence fabrics that test the accuracy of paleomagnetic records, the pre- and post-heating magnetic fabrics of synthetic (magnetite + CaF2) and natural samples were compared. The natural samples were collected from a thermally remagnetized, baked zone of the Triassic Passaic Fm. in southeastern Pennsylvania, USA. Comparison of magnetic fabrics before and after heating indicates that the synthetic samples appear to inherit their pre-heating oblate fabrics, whereas the natural samples apparently lost their initial AMS fabrics but inherited their initial depositional remanence fabrics during thermal remagnetization. Thus, the presence of oblate fabrics may not uniquely indicate a primary remanence.; The environmental magnetic study of the calcareous, organic-rich White Lake sediments from northwestern New Jersey aims at (1) reconstructing the environmental change in response to climate change since the last glaciation; and (2) exploring the mineral magnetic - climate link in organic rich sediments. A >14,000-year mineral-magnetic record was recovered. The mineral magnetic records documented (1) the initial deglaciation processes and development of the lake and its watershed during the late Pleistocene; and (2) the lake level fluctuations in the mid-late Holocene, with low lake levels occurring at ∼1.3 ka, ∼3.0 ka, ∼4.4 ka, and ∼6.1 ka. The quasi-1500 year lake level fluctuations are likely part of the regional response to broad-scale, millennial-scale climate variations in the Holocene.