Holocene freshwater carbonate structures in the
hyper-arid Gebel Uweinat region of the Sahara Desert (Southwestern Egypt)
by Margarita M. Marinova, A. Nele Meckler, Christopher P. McKay, in Journal
of African Earth Sciences 89 (2014) 50–55
preliminary exploration (in 2005) of the Gebel Uweinat region,
two valleys ("Karkur Talh") were discovered that contain morphologically
distinct carbonate structures and at one of the sites form a bench along
the valley wall. Both of these valleys are located in likely ancient valley
network channels. At Site 1, the carbonate structures form a distinct
bench along the sandstone valley wall; the bench is continuous for over
50 m and is about 1 m thick vertically (Site 1, N21°58' / E25°06' , elevation
715 m SL)
The carbonate bench is ‘‘pasted’’ onto the valley wall and is not part
of, or aligned with, the stratigraphic sequence. At this location the
valley is about 20 m deep. No apparent shorelines were observed.
Site 2 is located about 5 km away in a neighboring valley, and
at a slightly higher elevation than the first site (Site 2, N21°56' /
E25°04' , elevation 775 m SL). The height of the surrounding, broader
valley is about 10 m. The carbonates structures had a similar, distinct
morphology, but did not form a bench along the valley walls: they were
distributed both along the valley walls and along the valley floor.
Bulk XRD mineralogical analysis of the samples showed carbonates and quartz
as the main minerals, with other minor constituents (less than a few percent
total weight fraction). The carbonate is in the form of calcite, with
less than 5 mol% magnesium. The average total carbonate weight fraction
(as determined by acid digestion) is 46% for Site 1, with the three subsamples
giving 42.5%, 49.0%, and 46.8% by weight. For Site 2, the average total
calcite content was 28% by weight, with the three subsamples giving 29.2%,
28.2%, and 26.0% by weight. The quartz component was likely contributed
by the ubiquitous sand sheets and dunes in the region.
Energy dispersive spectrometer (EDS) analyses showed some of the minor
mineralogical components to be calcium sulphate, magnetite, ilmenite,
(Na, Al) pyroxenes, and orthoclase. The composition at both sites is similar,
consistent with their close proximity. The organic carbon content of
the samples is below the detection limit of the measurement: less
than 0.3 weight percent.
Microscopic examination of the samples from both sites shows a calcite
matrix primarily embedded with quartz, with other mineral grains as a
minor component. The quartz grains are angular with little rounding, similar
to the angular nature of the quartz grains in the surrounding sandstone
rocks. The persistent angular grains suggest short transport distances.
No fossils or pollen grains were observed.
The calcite structures were dated using radiocarbon dating of the bulk
sample. Two subsamples from Site 1 were dated, giving similar dates: 8190
calibrated years BP and 7970 cal. years BP. For Site 2, the carbonate
age is 9440 calibrated years BP." (BETA Analytic Inc., Miami)
|Fig. 3. Calcite matrix and composition.
SEM image of a thin-section of the calcite structures at Site 1.
The matrix is calcite (1), the most ubiquitous incorporated mineral
is quartz (2); iron oxides/ilmenite (3, bottom left), pyroxenes (Na,
Al pyroxenes in this image; 4), and orthoclase (5) are also present.
rocks are not present upstream of the study
sites. .... No active springs were found upstream, although one
spring is known on the other side of the mountain and carbon input from
groundwater cannot be fully ruled out. .... The level of the deposits
reported here is uniform across the valley wall consistent with formation
from lake water. ... The chemical and morphological similarity of these
formations to carbonate structures from modern lakes suggests that these
lakes contained fresh, standing water. ...
Microbial carbonate structures are reported from a diversity