The paleomagnetism and potassium-argon ages of ten basaltic lavas from the island of Madeira have been investigated.
Eight of the lavas are in a single sequence, which also contains three distinct sedimentary horizons up to 4 m thick. The section ranges in age from 0.74 ± 0.4 m.y. to 1.76 ± 0.07 m.y. Both the Jaramillo and Gilsa geomagnetic polarity events are represented. Of the two other lavas examined, a reversed polarity basalt with an age of 3.05 ± 0.15 m.y. records the Mammoth event, and a normal polarity basalt may represent the Jaramillo, or a previously suggested short event at t = 1.07 m.y.
These data provide strong confirmation of the reality of the Gilsa event, and show that the lower boundary of the event can be no older than 1.83 m.y., could be younger than 1.69 m.y., but not younger than 1.61 m.y.