Tertiary sediments were deposited extensively on the Cretaceous east of the Lebombo Mountains across the coastal plain of Zululand and southern Mozambique. These sediments are mainly shallow marine or estuarine with thin transgressive, regressive, and continental phases. Minor deposits of the oldest Tertiary (Eocene) remain in the southern part of the area; more extensive preservation is found in the north. Oligocene has not been identified and lower Miocene has limited distribution. Most of the sediments are middle to late Miocene and early Pliocene in age.
The Tertiary sediments reach a thickness of 300 ft along the Zululand coast east of the Lebombo Mountains. The sequence on land thickens northeastward from Lourenço Marques to about 4,400 ft near Morrumbene.
The almost horizontal attitude of the base of the Tertiary sediments on land implies practically no seaward tilting and that these sediments form a thin cover on the narrow continental shelf between the Umfolosi River mouth and Lourenço Marques. There is a thickening of the Tertiary over the wider continental shelf from Lourenço Marques toward Inhambane.
The nature and thickness of the Tertiary sediments in Zululand and southernmost Mozambique do not favor the presence of oil. Deep drillholes along the continental shelf east-northeast of Lourenço Marques and toward Inharrime are dry. These results offer no encouragement in an area earlier considered to have some oil and gas potentialities.