USING C-ISOTOPES TO CONSTRAIN INTRABASINAL STRATIGRAPHIC CORRELATIONS: MESOPROTEROZOIC ATAR GROUP, MAURITANIA

The Mesoproterozoic (~ 1.2 Ga) Atar Group consists of >750 meters of mixed carbonate and siliciclastic strata that are exposed in a sinuous, SW-NE trending outcrop belt that spans the West African craton. Atar Group strata are best studied in the Adrar Mountains of western Mauritania and the Hoggar uplift in western Algeria, where carbonate strata consist predominantly of deep subtidal to intertidal stromatolitic facies and intertidal to supratidal intraclastic breccia and evaporite facies.

In November-December 2003, field work was performed in the unmapped regions of the central basin to detail stratigraphic relationships and help constrain mechanisms of basin development. With the exception of a single stromatolitic (Conophyton) horizon, no diagnostic facies carry over to the central basin where strata are composed predominantly of sub-mm to cm-thick laminated micrite, intraclastic breccia, and carbonate-cemented quartz sandstone. Extensive developent of Holocene dune-fields prohibited direct observation of facies transitions and correlation of these facies to deeper-water strata at the craton margins.

Secular change in marine C-isotope signatures provides a potential mechanism for correlating strata in these disparate parts of the basin. In the Adrar region, C-isotope stratigraphy shows distinct changes through time (Kah and Bartley, unpublished data) with values near +2‰ near the base of the Atar Group (unit I-5), followed by an abrupt shift to –2‰ (unit I-6), a gradual rise to values exceeding +3‰ (units I-7, I-8), a moderate fall to values near –1‰ (unit I-9), and a second gradual rise from –1‰ to +1‰ (unit I-11). In this study, we are developing a C-isotopic profile for a measured section of the Atar Group collected near Aguelt el Mabha, in central Mauritania. Samples have been examined petrographically and with cathodoluminescence to determine fabric relationships and extent of postdepositional diagenesis, and we are currently analyzing microdrilled samples for C- and O-isotopes as well as a suite of major and trace elements (Ca, Mg, Mn, Fe, Sr). Ultimately, establishing a C-isotopic profile for this section will permit direct time-correlation of facies across the west African craton.