Summary:
As Sahabi and Jabal Zaltan are well-known fossil vertebrate African localities
situated in Sirt Basin of Libya. The Neogene Sahabi Formation has not yet been
chronologically calibrated precisely with the Messinian Event. Dating Sahabi
Formation comes from member "U1" based on mammalian biochronology. A Late
Miocene age of the fauna is the most probable estimate. However, mammalian
biochronology cannot exclude the youngest Early Pliocene age, for at least the
top of member "U1.
Formation "M", the underlying pre-Sahabi marine rock unit, is the only key for
dating Sahabi Formation using microfossil biostratigraphy. Jabal Zaltan was
used in this study for correlative purposes.
The invertebrate micro/macrofossils data base examined from both areas, include
samples from eight surface profiles (P10, P25, P28, P51, P52, P53, P66, and
P96c) and two subsurface drilled shallow wells in As Sahabi area (WW1 A1-NC214
and As Sahabi Borehole 2); in addition, two sections from Maradah Formation at
Jabal Zaltan were sampled. A total of 305 rock samples were collected from both
areas. From those were prepared, 110 petrographic thin sections, 372
micropaleontological slides (Foraminifera, Ostracoda and Bryozoa), and 153
smear slides (calcareous nannofossils).
The Late Neogene rock units of As Sahabi area are stratigraphically and
sedimentlogically investigated and reviewed based on surface observations and
microscopic examinations. Formation "Z" (Quaternary) represents terrestrial
paleosol. It is up-ranked in this study to formation rather than member of the
Sahabi Formation, based on the recognition of an unconformity with the
underlying member "V" of Sahabi Formation.
Mineralogical and geochemical investigation, performed for first time, of
clayey sediments from three localities, P25, P28 and P96c, from vertebrate
fossiliferous member "U1 "of the As Sahabi area, robustly showed very mature
and re-processed sediments of continental origin, and felsic sources, with no
influence from Libyan volcanic (basaltic) rocks. According to our data, the
studied As Sahabi sediments can originate in Precambrian continental sources in
northeastern Chad.
Five macrofossils phyla (echinodermata, mollusks, corals, bryozoa and
arthropoda) have been described and taxonomically classified into species in As
Sahabi area. Thirty-five foraminifera species have been documented, mostly from
the pre-Sahabi formation "M"; two local assemblage biozones are established,
the older Globigerinoides obliquus obliquus - G. trilobus - Orbulina universa
assemblage Biozone, and the younger Borelis melo melo – B. melo curdica
Biozone. Seventeen ostracod species from formation "M" in As Sahabi area have
been described systematically, and in order to interpret the depositional
paleoenvironment of the investigated sediments.
Seventeen calcareous nannofossils taxa have been described for the first time
from formation "M" in WW1 A1-NC214 which is dated to Early Tortonian and is
assigned to NN8/ MNN8b of Martini (1971) and Fornaciari et al. (1996)
respectively. In addition, from study of As Sahabi borehole 2, a Late Tortonian
is assigned to formation "M" (NN10b- NN11a of Martini, 1971). The two studied
Libyan subsurface sequences are correlated by means of calcareous nannofossil
biostratigraphy with several eastern Mediterranean equivalent sites,
strengthening the regional chronological correlation framework.
Jabal Zaltan, Maradah Formation is also studied lithologically and
micro-paleontologically. Only the younger Ar Rahlah Member at Z138 has been
found to contain abundant and diverse bryozoan assemblages. Thirty-one species
have been described; four new species have been proposed in this work. They are
Smittipora maradaensis n. sp., Calpensia spinosa n. sp., Thalamoporella
zaltaniensis n. sp., and Celleporaria rahlaensis n. sp.
By taking under consideration all tools applied in this study for the surface
materials of As Sahabi area (locality P53, formation "M" type locality),
formation "M" is assigned to Late Miocene (Tortonian) and not to previously
suggested Middle Miocene age. This study, based on calcareous nannofossil
biostratigraphy, gives an age estimation for formation "M" ranging between at
least 10.40 - 8.52 Ma., correlative to the recently acquired 87Sr/86Sr absolute
age of 8.99-9.36 Ma.
Keywords:
Tortonian, Sahabi, Jabal Zaltan, Calcareous nannofossils, Libya
