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

The Maevarano Formation of Northwestern Madagascar (Mahajanga Basin) is know for its spectacular assemblage of fossil vertebrates including fish, frogs, turtles, crocodyliforms, dinosaurs, birds, mammals and snakes. Correlation with the marine Berivotra Formation (which contains foraminifera of a Maastrichtian age) suggests that the Maevarano Formation is Campanian(?)-Maastrichtian in age. The first ever magnetic study of the Berivotra and all three members of the Maevarano (Anembalemba, Masorobe, and Midana) Formations was conducted using 24 sample horizons (with an average spacing between horizons of 4.4m) and analyzed using alternating field (AF) and thermal demagnetization methods. The study revealed a long normal polarity signal (horizons 1 through 22) capped by a single reversal (the top two sample horizons). Based on their position relative to the independently defined K/T boundary capping the interval, the normal and reversed intervals are interpreted to represent chrons 30N and 29R, respectively. This suggests a much younger age for the section under scrutiny and is consistent with its taphonomy (excellent preservation indicating rapid burial) and sedimentology (no indications of significant hiatus or erosion). A recent reappraisal of the biostratigraphy of the Berivotra Formation, which linked biostratigraphic zones to predicted magnetic signals for various horizons within the Berivotra Formation, contradicts some of this primary magnetic data. The resulting ambiguities hastened the recent collection of new samples from a number of previously sparsely sampled or disputed horizons and will yield a more precisely defined magnetic signal for the study interval as well as better age constraint for the diverse fauna it yields.