Lower Cretaceous Magnetostratigraphy and Paleolatitudes off Northwest Australia, ODP Site 765 and DSDP Site 261, Argo Abyssal Plain, and ODP Site 766, Gascoyne Abyssal Plain

Abstract

Lower Cretaceous sediments were sampled for magnetostratigraphy at three sites. ODP Site 765 and DSDP Site 261, in the Argo Abyssal Plain, consist primarily of brownish-red to gray claystone having hematite and magnetite carriers of characteristic magnetization. ODP Site 766, in the Gascoyne Abyssal Plain, consists mainly of dark greenish-gray volcaniclastic turbidites with magnetite as the carrier of characteristic magnetization. Progressive thermal demagnetization (Sites 765 and 261) or alternating field demagnetization (Site 766) yielded well-defined polarity zones and a set of reliable paleolatitudes.Magnetic polarity chrons were assigned to polarity zones using biostratigraphic correlations. Late Aptian chron M"-l"r, a brief reversed-polarity chron younger than MOr, is a narrow, 40-cm feature delimited in Hole 765C. Early Aptian reversed-polarity chron MOr is also present in Hole 765C. Polarity chrons Mir through M3r were observed in the Barremian of all three sites. Valanginian and Hauterivian polarity chrons can be tentatively assigned to polarity zones only in Hole 766A.The paleolatitude of this region remained at 35° to 37°S from the Berriasian through the Aptian. During this interval, there was approximately 16° of clockwise rotation, with the oriented sample suites of Site 765 displaying a Berriasian declination of 307° to an Aptian declination of 323°. These results are consistent with the interpolated Early Cretaceous apparent polar wander for Australia, but indicate that this region was approximately 5° farther north than predicted.

Defining the Limits of Greater India

“…The Exmouth Plateau northwest of Australia contains Early Cretaceous sandstone and siltstone interbedded with mafic sills/flows that highly resembles the facies from Cuona (Gradstein & Ludden, ). Lower Cretaceous (125–136 Ma) rocks from the Gascoyne and Argo abyssal plains, near the Exmouth Plateau, yielded paleolatitudes of 36.6 ± 2.1°S and 36.7 ± 3.4°S (Ogg et al, ), respectively, that are quite compatible with Cuona. The Cape Range Fracture Zone, Wallaby‐Zenith Fracture Zone, and Exmouth Plateau are all compatible with the paleogeography of Greater India shown in Figure c.…”

“…Such highly elongated distributions suggest either a significant amount of flattening of the paleomagnetic directions (Tauxe & Kent, ) or, more likely for lava flows, that secular variation was inadequately averaged. Moreover, the presence of limestone at such high latitudes seems unlikely (Roberts et al, ), is incompatible with the paleomagnetic results from the Gascoyne and Argo abyssal plains (Ogg et al, ), and goes against Early Cretaceous bivalve fauna distributions from the Tethyan Himalaya (Rao et al, ).…”

Paleomagnetism of Cretaceous Basalts from the East Mariana Basin, Western Pacific Ocean .

“…4). The resultant inclinations were weighted using a method similar to that described by Ogg et al (1991) and the mean inclination was computed for the sample suite. Since Ocean Drilling Program (ODP) drilling samples are azimuthally unoriented, the mean inclinations were calculated using the method of Kono (1980), a method which assumes that the statistics of inclination data mirrors a Fisherian distribution to magnetic directions.…”