The start of the Phanerozoic at the northern edge of the East Sahara Craton (Libya): stratigraphy, facies, and process-correlation along the W-Gondwanan shelf
Final Report Abstract
Based on the detailed mapping of 15 sections of the siliciclastic Hasawnah Fm. of the northern, central, and southern Al Qarqaf Arch area (AQA, Jebel Hasawnah resp. Jebel Fezzan, Central west Libya) and on the documentation of sedimentary structures and fossil remains, as well as based on succeeding petrographic, analytical, and geochronological work, the Hasawnah Fm. has been identified to represent the transition facies from fluvial/deltaic to tidally influenced, and finally to shorttime shallow subtidal deposition. For the first time, rich fossil content has been identified from this unit. These traces represent the first biostratigraphic proofs indicating Cambrian age for the Hasawnah Fm. At the present state of investigation, the assemblage consists of: Rusophycus aegypticus, Cruziana problematica, C. isp., Diplichnites isp., Skolithos isp., Diplocraterion isp., and some non-identifiable arthropod traces. The assemblage is very similar to associations known from stratigraphic comparable strata of the Eastern Desert and the Sinai Peninsula (Egypt), as well as of the Dead Sea Valley (Jordan and Israel). The Libyan ichno-fauna indicates a biostratigraphic position around the traditional early-middle Cambrian boundary (boundary interval of Cambrian series 2 to 3). Detrital zircons from various stratigraphic levels of the Hasawnah Fm. were geochronologically investigated for the first time by LA-ICP-MS techniques. About 60% of the U–Pb zircon ages are Neoproterozoic and earliest Cambrian. These zircon populations are interpreted as detrital material derived from the Pan-African and possibly to a smaller proportion from the Cadomian Orogeny situated marginal to NW Gondwana. A few slightly older Neoproterozoic ages point to rifting events related to the dispersal of the Rodinia supercontinent. A minority of zircons became formed during the configuration of Rodinia (Mesoproterozoic–Neoproterozoic boundary). Further, some early Mesoproterozoic zircon ages had been found, too. The potential source area for the Mesoproterozoic zircons seems to have been far distant from the Al Qarqaf Arch (Arabian–Nubian Shield, Chad, Congo, Tanzania cratons). A considerable proportion (28%) of zircons represents Palaeoproterozoic populations. Less than 5% of all zircons are Archaean in age. A potential source area for Palaeoproterozoic and Archaean zircon grains is the West African Craton and the western part of the Saharan Metacraton. According to the zircon data, the best candidates for the main source region for the sandstones of the Hasawnah Fm. in the Al Qarqaf Arch type area are the Neoproterozoic–early Cambrian orogens of the Pan-African cycle in the Trans-Saharan Belt and of the peri-Gondwanan terranes (Cadomia). Input from the Palaeoproterozoic and Archaean sources of the cratonic basement (Saharan Metacraton and West African craton) is limited. The exact source of the exotic Mesoproterozoic zircons remains problematic. The data lead to the conclusion that the centre of early Palaeozoic thermal subsidence in central-northern Africa has to be located in the region of the Saharan Metacraton. Palaeogeographically, the ichno-fauna shows clear relations to the Middle East (northern Arabian–Nubian Shield). It seems that the Rusophycus aegypticus, Cruziana jordanica and C. salomonis ichno-assemblage represents a more-or-less contemporaneous stratigraphic interval in this palaeogeographic area (so-called “salomonis-aegypticus-level”). The always present and distinct unconformity at the base of the Cambrian Hasawnah Fm. indicates major uplift and considerable denudation in the latest Neoproterozoic to early Cambrian time interval, which is typic for the whole NE-Africa/Middle East area. Because of the conspicuous maturity of the Cambrian siliciclastic deposits of this whole area, a coeval intense chemical weathering under warm to humid climatic conditions in low to moderate southern latitudes and the formation of a large Gondwanan peneplain is indicated. The new fossil findings from Libya together with the distinctly improved knowledge on the trace and body fossil content from Egypt and Jordan/Israel opens the opportunity to reconstruct, to fix (stratigraphically) and to correlate the main transgression pulse of this palaeogeographic region and to fit them into the international stratigraphic scheme with some precision. These regions have been closely situated on the same shelf of Gondwana under quite similar palecological conditions. The unconformity between basement and Hasawnah Fm. sandstone indicates strong uplift and deep erosion during the latest Neoproterozoic and early Cambrian. According to the geochronological data, there is no evidence for deposition of sediments in the AQA in the first 10–12 Ma of the Cambrian. This suggests that relatively stable continental conditions may have existed for this time in this part of Gondwana. The high maturity of the sandstones implies intervals of coeval intense chemical weathering under warm to humid climatic conditions, resulting in denudation and prevalent peneplanation. The maximum of the Cambrian transgression in the today south-eastern Mediterranean (NE-Africa and the Middle East) can be defined close to the base of Cambrian stage 5 (series 3). A correlation of marine carbonate and sandstone successions is roughly possible because of the co-occurrences of body fossils and trace fossils in Jordan. A further correlation of the Libyan ichno-taxa seems to be reasonable with those from Sinai Peninsula, Eastern Desert (Egypt), and Jordan. The salomonis-aegypticus level (basal series 3) seems to represent the maximum flooding of this segment of Perigondwana by the Cambrian sea.
Publications
- (2010): Cambrian trace fossils from the north-eastern Africa – Middle East segment 3 of Perigondwana. Annual Meeting of the Palaeontological Society of Germany, Munich, October, 5-8, 2010. Zitteliana, series B, 29: 31-32
Elicki, O., Altumi, M.M., Leuschke, A., Farouk, S. & Hofmann, R.
- (2011): Correlating the Cambrian in the south-eastern Mediterranean regions of Gondwana. XVI. International Conference of the Cambrian Stage Subdivision Working Group, Flagstaff, Arizona/Nevada, 12.-20.6.2011. Museum of Northern Arizona Bulletin, 67: 280-281
Elicki, O.
- (2012): Stratigraphy and palaeogeography of the Hasawnah Formation (Cambrian, Al Qarqaf Arch, central-western Libya): data from detrital zircons and trace fossils. International Conference on the Paleozoic of Northern Gondwana and its Petroleum Potential. Kayseri, Turkey, 9.- 14.9.2012
Altumi, M.M., Elicki, O., Linnemann, U., Hofmann, M. & Göncüoğlu, M.C.
- (2013): U–Pb LA-ICP-MS detrital zircon ages from the Cambrian of Al Qarqaf Arch, central-western Libya: Provenance of the West Gondwanan sand sea at the dawn of the early Palaeozoic. Journal of African Earth Sciences, 79: 74-97
Altumi, M.M., Elicki, O., Linnemann, U., Hofmann, M., Sagawe, A. & Gärtner, A.
(See online at https://doi.org/10.1016/j.jafrearsci.2012.11.007)
