Abstract
Tertiary - Quaternary volcanic activity is conspicuous along the eastern scarp of Jabal Nafusah and comprises an old lava series (OLS), young lava series (YLS), volcanic cones, phonolite dome-shaped intrusions, and pyroclastic rocks. The OLS are the most voluminous and form a fairly continuous plateau in the central part of the region. In contrast to the older flows, the YLS is mostly restricted to the periphery of the OLS plateau, as is clearly shown on satellite images. In addition, parts of the YLS occupy an ancient buried wadi system. Both the OLS and YLS are petrographically similar, although field evidence revealed a number of separate eruptive events. The suggestion that the OLS may represent the first phase of igneous activity in the area is debatable because of suspect dating. The phonolites are exposed at only a few localities. Field evidence and K-Ar Age dating suggest that they are much older than the late volcanic centres (LVC). These centres, erupted from central volcanoes and dykes, are represented by very small volumes of the eruptive products throughout and outside the lava field. Black, massive, poorly-sorted deposits made of fragmented products of explosive eruptions and matrix-supported blocks are encountered in the area. These deposits are considered to have been formed initially by vent opening phreatic - phreatomagnetic explosions and part of which were later reworked. Based on field criteria they have been classified into vent facies, near-vent facies, and distal facies.