Paleomagnetic transition records of the Cobb Mountain Event from sediments of the Celebes and Sulu Seas

The Cobb Mountain (CM) event is a short normal polarity interval within the Matuyama Chron just before the Jaramillo Subchron. This event was found in the sediments of Site 767 in the Celebes Sea and Site 768 in the Sulu Sea during ODP Leg 124. Reversal depth and age relation analysis gave ages of 1.11 and 1.10 m.y. ago for the onset and termination of this event. We used discrete samples and high-density pass-through measurements to study the behavior of the geomagnetic field crossing this event. Although the two sites are very close to each other, the records of the transitional fields were found to be quite different. They are also very different from another CM event record from the North Atlantic Ocean. Based on the transition data we conclude that the transitional fields for the CM event were non-axisymmetric. One brief reverse in direction, called rebound, was found within the onset reversal at Site 767. The absence of this feature at Site 768 raises an interesting question concerning the source for rebounds.