2003. Journal of Vertebrate Paleontology, Abstracts of Papers, 23(3):39.

MAGNETIC STRATIGRAPHY OF THE UPPER CRETACEOUS MAEVARANO FORMATION (CAMPANIAN(?)-MAASTRICHTIAN), NORTHWESTERN MADAGASCAR

CASEY Michelle M., ROGERS, Raymond R., Geology Dept., Macalester College, St Paul, MN, JACKSON, Michael J., Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455; BUCKLEY Gregory A., Roosevelt University, Evelyn T. Stone University College, Chicago, IL 60605.

The Maevarano Formation (Mahajanga Basin, Madagascar) is well known for its assemblage of fossil vertebrates, including fishes, frogs, turtles, snakes, crocodyliforms, dinosaurs, birds, and mammals. The unit is presently considered Campanian(?)-Maastrichtian based on correlation with portions of the marine Berivotra Formation, which yield foraminifers that are Maastrichtian in age. Here we report results of the first paleomagnetic study of the Maevarano Formation. Samples (n=24) were collected from all three members of the formation (Masorobe, Anembalemba, Miadana), and the Berivotra Formation, and analyzed using both AF (alternating field) and thermal demagnetization at the Institute for Rock Magnetism. The average spacing between sampled horizons is 4.4 m. Three analyses were conducted on each sample. The magnetic carrier was identified as magnetite except in the Masorobe Member, which contains both hematite and magnetite.

All samples derived from the Maevarano Formation are normal in polarity. Seven of the nine samples collected from the ~30 m thick Berivotra Formation are also normal in polarity. Only the top two samples in the Berivotra Formation, which are situated within a few meters of the overlying Betsiboka limestone and the Cretaceous-Tertiary boundary (as defined by independent biostratigraphic data), are reversed in polarity. Based on the position of the K-T boundary, the two samples with reversed polarity at the top of the Berivotra Formation are interpreted to represent Chron C29R. Underlying samples in both the Berivotra and Maevarano formations with normal polarity are interpreted to represent Chron C30N. This interpretation, which suggests that much of the section under scrutiny is younger than previously surmised, is consistent with the sedimentology and taphonomy of the study interval in that there are no indications of significant hiatus or erosion, and fossils are exquisitely preserved (indicative of rapid burial). Our results are somewhat contrary to a recent reappraisal of the biostratigraphy of the Berivotra Formation, and thus more sampling is planned, especially in lower parts of the section.