Palynology of Middle Tertiary lacustrine deposits

186 K. TAKAHASHI & U. lux
dated in general terms: Eocene to Miocene.
The fluviatile Kerri-Kerri Formation, mentioned already above, rests unconformably
on folded Cretaceous sediments (Matheis, 1975). Fossils are missing in the
gritty of sandy sequence which includes only few whitish or purplish clay beds.
Nevertheless, in a borehole, which penetrated the formation in northeastern
Nigeria (Duku), some coal was obtained and palynologically examined (Shell-B. P.
Petroleuem Development Co.)
to display a doubtful Paleocene age at least (Carter
et al., 1963). Hampered by the arguments of some field geologists (Carter, 1963;
Burke, 1970; Dessauvagie, 1975) favouring a much younger age (Miocene to
Pliocene), palynologic reinvestigation of carbonaceous intercalations (Adegoke et al,
1978) maintained and supported a Paleocene age for the lower Kerri-Kerri Formation.
Among the assemblage of palynomorphs, consisting of monocolpates (20-40
%), monoporate (1 - 4 %), tricolpates and tricolporates (30 - 50%), inaperturates
(5 -10%) as well as spores (5 -10%), certain forms were registered as
reliable index fossils: Mauritiidites crassibaculatus v. H. - Kl. (1 -12%),
Monocolpites marginatus v. d. Hammen (0 - 4 %), Proxapertites operculatus (0 ­
10%), Costatricolporites reticulatus Wymstra (5 -10%). Other forms mentioned
(Adegoke et al., 1978 in Kogbe, 1981) are Cranwellipollis gombeensis v. d. Chene &
Adegoke and Syncolporites corrugatus v. d. Chene & Adegoke. The lateritic crust on
top of the Kerri-Keri Formation is likely to be synchronous with the ferruginous
horizons capping or replacing the fluvio-volcanic series of the Jos Plateau. As it
deals with a regional phenomenon the lower and upper stratigraphic boundaries of
the "Ironstone Formation" (=Siderolithique du Tertiaire, Continental Terminale,
main laterite crust) may differ somewhat from place to place. Therefore the assignment
of a Neogene, end-Tertiary (Durotoye, 1975) or even post-Eocene to pre­
Quaternary age respectively (Lang et al., 1983), which all may be reasonably explained
by both epeirogenetic and paleoclimatic events (Boudouresque et al., 1982),
need further biostratigraphic support.
Sedimentary sequence in Sabon Gida mine
The biotite granites of the Jos-Bukuru complex are well known to be tin bearing. Due
to the shallow, recently unroofed structures and the advanced alluvial concentration
of ores within the conine of the massif (Buchanan et al., 1971). Therefore, abundant
placers of cassiterite and columbite were mined along the river, the N'gell draining
the Sabon Gida area included (fig. 2). Present production of tin, although considerably
reduced is performed by open-cast methods from beneath a rather thick
detrital overburden and follows lacustrine depressions or buried stream channels of