Africa was still far south in the Late Ypresian: Paleomagnetic study on the early Eocene ‘Minia’ formation in central Egypt

Abstract

The paleomagnetic study was carried out on three sections of the Late Ypresian “Minia” formation limestone, in order to shed light on the paleolatitude of northeast Africa upon the end of the Early Eocene. The initial study on the anisotropy of magnetic susceptibility [AMS] helped in confining the paleomagnetic sampling to the virtually isotropic limestone beds. The subsequent stepwise thermal demagnetization of the three-axis isothermal remanence acquired in one sample of each sampled site, revealed the limited contribution of goethite and hematite with the main remanence carrier magnetite in most samples.The assessment of the natural remanence during the progressive stepwise thermal demagnetization [15–17 steps] of all samples, elucidated the early decay [<150 °C] of a present- day field [PDF] overprint component scattered around the magnetic field direction of the study area. Despite that, the anchored component in most samples was carried in magnetite, yet hematite was recorded in few sites. The visual inspection of the decay spectra in the orthogonal projections, followed by the determination of the best-fit line of every component using the principal component analysis [PCA] technique, differentiated between the magnetite- and hematite- remanence components:1.The magnetite-anchored component, which was overwhelming in most samples, was antipodal with shallow to medium inclination, yielding a paleomagnetic North poles at 73.8°N/197.5°E. This component, which successfully passed the reversal test at 95% confidence level, was considered as the characteristic primary remanent direction of the “Minia” formation. This paleomagnetic pole was, consequently, considered as representing the African Plate in the Late Ypresian.2.On the other hand, the hematite component had normal north-direction clustered around the present-day field [PDF] in the study area. The hematite paleomagnetic pole at 88.7°N/78.8°E, was considered as a PDF overprint acquired in few samples upon exposure to desert weathering and partial oxidation of their original magnetite to hematite.The reasonable consistence of the obtained pole of the “Minia” formation with synchronous poles of the main tectonic units rotated to the African coordinates, point to its reliability. This pole revealed that upon the end of the Lower Eocene, Northeast Africa was still at a far south paleolatitude with respect to its present-day position. The location of Cairo which is now at latitude 30°N, was at 14.2°N paleolatitude during the late Ypresion. This means that Africa was about 16° of latitude south to its current latitude.