The Jaramillo Subchron: New Magnetostratigraphy and 40Ar/39Ar Dating in the Death Valley Region, California

Abstract:

The Jaramillo subchron was the normal polarity period before the present Brunhes chron. However, the Jaramillo’s duration was only 10% as long as the Brunhes. Lasting only about 70 ka, the Jaramillo has frequently been missed (or is missing) from many magnetostratigraphic studies of the Early Pleistocene. We have examined two internally-drained basins with high sediment accumulation rates, that also contain Sanidine-bearing tephras. At Kit Fox Hills, in the Death Valley Basin, we sampled across 50m of tilted (45°) fine-grained sandstone/siltstone. A normal polarity magnetozone was delineated over ~12m of section (sedimentation rate of ~15cm/ka). We also collected 8 tephra beds, 4 of which are within the normal magnetozone, and 2 below and 2 above. To the east is the Tecopa Basin, which until the middle Pleistocene was internally-drained. We sampled through 36m of mudstones between the Tecopa Tuff (1.25 Ma) and the Bishop Tuff (0.77 Ma). A normal magnetozone was delineated over a 9m interval, ending 12.5m below the Bishop Tuff. A Sanidine-bearing tephra (previously unidentified) was found 4m above the base of the Jaramillo zone. There were 2 beds of reverse polarity within the Jaramillo zone, at 4.5m and 8m above the base. At least the lower one has been reported from other continental deposits (China, Spain). These sections should provide precisely calibrated chronostratigraphic tie points (approximately every 10-30 ka), providing sub-precessional level, direct geochronological control through the Jaramillo subchron. These sections are also being examined for paleo-environmental indicators (and minerals) that can be used to determine wetter or drier conditions (expanding saline lakes or dry playas). With all these event horizons (ash falls and polarity shifts), the Jaramillo has the potential to be used for high resolution, paleo-climate reconstructions on a regional scale.