A SUBSURFACE LITHOSTRATIGRAPHIC DIVISION OF THE HAUTERIVIAN TO APTIAN, FURT (INFORMAL) AND QISHN FORMATIONS, YEMEN

The Mesozoic basins of Yemen have a multiphase history of rift develop ment. Sediments of the 'Furt Formation' were deposited as a result of a second, discrete phase of rift activity during the Hauterivian to Ba rremian and unconformably overlie older deposits (A. C. Ellis, H. M. K err, C. P. Cornwell and D. O. Williams, 1996, Petroleum Geoscience 2, 29-42). The name is taken from the suggested type well, Al Furt-1, whe re the unit reached its maximum drilled thickness to date, in the Jeza -Howarime Basin. The lithology in the type well predominantly comprise s calcareous mudstones with subordinate carbonates. In western Yemen a nd more marginal settings, the 'Furt Formation' is dominated by arenac eous facies. Biostratigraphic analysis suggests that the upper and low er limits of the 'Furt Formation' are bounded by stratigraphic breaks. Regional seismic lines suggest and biostratigraphy confirms the exten sion of the 'Furt Formation' into basin margin areas and other rift ba sins, supporting the case for regarding the Furt section as a formatio n in its own right. The Qishn Formation has been divided on a triparti te basis. The oldest units are the 'Clastic' and 'Lower Carbonate Memb ers' which are lateral facies and age equivalents. The 'Clastic Member ' is found in the west of Yemen while the 'Lower Carbonate Member' is best developed in the east. The transition between the two is seen in the Jeza-Howarime Basin. The middle unit, the 'Shale Member', is a reg ionally extensive mudstone facies thought to relate to a maximum flood ing event. The youngest unit, the 'Carbonate Member', comprises a lime stone sequence. The Qishn Formation is unconformably overlain by the c lastics of the Harshiyat Formation and in the extreme east of the Yeme n by the carbonates of the Fartaq Formation. The Qishn Formation repre sents a transition from a rift to a post rift-phase (Ellis et al., 199 6). (C) 1997 Elsevier Science Ltd.