In this paper, a new interpretation of the ironstone crusts of the Bahariya Formation as late diagenetic
products is provided. The siliciclastic Lower Cenomanian Bahariya Formation outcropping in the northern
part of the Bahariya Depression (Western Desert, Egypt) is subdivided into three informal units that are
mainly composed of thinly laminated siltstone, cross-bedded and massive sandstone, fossiliferous sandstone/
sandy limestone and variegated shale. Abundant ironstone crusts occur preferentially within its
lower and upper units but are absent in the middle unit. The ironstone crusts show selective replacement
of carbonate components, including calcretes, by iron oxyhydroxides. More permeable parts of the terrigenous
beds such as burrow traces, subaerial exposure surfaces, concretionary features and soft-sediment
deformation structures led to heterogeneous distribution of the iron oxyhydroxides.
A variety of diagenetic minerals, where goethite and hematite are the main end-products, were
characterized by mineralogical analysis (XRD), petrography and SEM observation, and geochemical determinations
(EMPA). Other diagenetic minerals include Fe-dolomite/ankerite, siderite, manganese minerals,
barite, silica, illite/smectite mixed-layer, and bitumen. These minerals are interpreted to be formed
in different diagenetic stages. Some minerals, especially those formed during eodiagenesis, show features
indicative of biogenic activity. During burial, dolomite and ankerite replaced preferentially the depositional
carbonates and infilled secondary porosity as well. Also during mesodiagenesis, the decomposition
of organic matter resulted in the formation of bitumen and created reducing conditions favorable for the
mobilization of iron-rich fluids in divalent stage. Telodiagenesis of the Cenomanian Bahariya deposits
took place during the Turonian–Santonian uplift of the region. This resulted in partial or total dissolution
of Fe-dolomite and ankerite which was concomitant to iron oxyhydroxide precipitation upon mixing
with shallow oxygenated water.
Circulation of reducing iron-rich fluids through fractures and inter and intrastratal discontinuities is
proposed as an alternative model to explain the controversial source of iron for the ironstone crusts of
the Bahariya Formation. The origin of iron-rich fluids is probably related to the basement rocks. The
provided model relates the fluid movements through fractures and discontinuities with the preferential
replacement of carbonates. This combination of processes is consistent with the heterogeneous geometries
and the wide distribution of the ironstones. |
