Paleomagnetism of the Cretaceous Galula Formation and Implications for Vertebrate Paleobiogeography
This study uses magnetostratigraphy to help constrain the age of the paleontologically important Galula Formation (Rukwa Rift Basin, southwestern Tanzania).The formation preserves a Cretaceous vertebrate fauna, including saurischian dinosaurs, a gondwanatherian mammal, and notosuchian crocodyliforms, that helps fill a temporal gap in our understanding of vertebrate evolution in continental Africa and could help test competing paleobiogeographic hypotheses concerning faunal exchange throughout Gondwana during the Cretaceous. Paleomagnetic samples were collected from the upper Namba and lower Mtuka members of the Galula Formation and underwent stepwise thermal demagnetization. All samples displayed a strong normal magnetic polarity overprint, and maximum unblocking temperatures at approximately 690°C. Three short reversed intervals were identified in the Namba Member, whereas the Mtuka Member lacked any clear reversals. Given the relatively limited existing age constraint, one interpretation correlates the Namba Member to Chron C32. An alternative correlation assigns reversals in the Namba Member to recently proposed short reversals near the end of the Cretaceous Normal Superchron (Chron C34), a time that is traditionally interpreted as having stable normal polarity. The lack of reversals in the Mtuka Member supports deposition in the lower part of Chron C34. These data allow the Namba Member to be interpreted as being no older than mid-Late Cretaceous (Turonian-Campanian), with the Mtuka Member less well constrained to the middle Cretaceous (Aptian-Cenomanian). The paleomagnetic results are supported by the application of the fold and reversal tests for paleomagnetic stability, and the paleomagnetic poles for the Namba (246.4°/77.9°, α95 5.9°) and Mtuka (217.1°/ 72.2°, α95 11.1°) members that closely match the apparent polar wander path for Africa during the Late Cretaceous. Given these new age constraints, the Galula fauna likely represents the general diversification of several vertebrate clades in western Gondwana during the middle – Late Cretaceous.