TROUGH EVIDENCE ALONG THE SOUTHERN FOOTHILLS OF THE PRINCE ALEXANDER MOUNTAINS (SEPIK DISTRICT — NEW GUINEA)

Abstract

The presence of a thick sedimentary sequence of Mio-Pliocene and Pleistocene age has been established in the Screw-Amogu Rivers area of the southern foothills of the Prince Alexander Mountains. Field geology and aeromagnetic, gravity and seismic surveys show evidence of up to 10,000 ft. of sediments that thin to the south towards the Sepik Plain and remain thin over the Plain with no indication of major faulting.

During late Tertiary and possible Pleistocene, ancestral Prince Alexander Mountains are thought to have formed an island ridge separating depositional areas north and south of this barrier. Diverse sediment types derived from the complex island ridge, coupled with an apparent difference in tectonic activity, probably account for the lack of litho-stratigraphic correlation between sediments on either side of the basement core of the Prince Alexander Mountains.

Immature sediments observed in outcrop in the foothills of the Northern Ranges support the local "Island Ridge" hypothesis for the source of the thick Screw-Amogu River section in preference to the alternative possible source area some 40 mi. to the south where basement is exposed in a series of hills along the Sepik River. The Screw-Amogr River sediments are thought to have been deposited in restricted, narrow trough that subsided contemporaneously with deposition. This concept of local troughs of thick Upper Tertiary sediments but otherwise relatively thin epicontinental deposits in the Sepik District necessitates moving the southern limit of the Northern New Guinea Basin north of the Sepik District.