Abstract
This work deals with field, petrographic, stable oxygen and carbon isotopic and elemental composition studies of pervasively calcite-cemented concretions within shallow marine and fluvial host sandstones of the Upper Eocene Birket Qarun Formation of El-Faiyum depression, Egypt in order to decipher their conditions of formation. The studied concretions have been classified into two types: type-1 and type-2. Type-1 concretions occur in the fluvial sandstones deposited at the northern scarp of Birket Qarun. Some of these concretions are confined within the host sandstones, while others appear as erosional remnants of the host sandstones. They show oblate-spheroidal to flattened ellipsoidal shapes and have variable diameters ranging from 0.5 to 3 m. Geometrically, they are bedding-parallel concretions with spacing ranges from 0.3 to 1 m. Petrographically, the rocks of type-1 concretions are attributed to quartzarenite cemented by sparry calcite that exhibits poikilitopic texture. These calcite cements are characterized by small ranges of delta (super 18) O (-8.36 to -9.73 ppmPDB) and delta (super 13) C (-11.01 to -15.04 ppmPDB) values. They were originated from light-root-respired carbon and media of meteoric pore water. Geometry of these concretions is interpreted to correspond to fluid flow during early diagenetic processes. Type-2 concretions are recorded in the shallow marine sandstones deposited at Gabal El-Lahun. They are confined within the host rocks. These concretions exhibit spheroidal to ellipsoidal shapes and range from 0.3 to 1 m in diameter. Geometrically, the individuals of these concretions occur as scattered, strata-bound and continuously cemented layers. Microscopically, the rocks of type-2 concretions are mainly belonged to quartzarenites cemented by micritic and microsparitic calcites. These calcites have small ranges of delta (super 18) O (-3.4 to -4.60 ppmPDB) and delta (super 13) C (-0.78 to -1.67 ppmPDB) values. The calcite cements of type-2 concretions are interpreted to be a result of dissolution of marine carbonate shells at shallow burial depth in slightly modified marine pore water. Geometry of these concretions was controlled by the spatial distribution of carbonate shells in their host sandstones. This study imports an idea that variation of depositional environments of the host clastic rocks of concretions plays an important role in the difference of modes of formation of their concretions. Also, caution should be made to simplify and generalize the conditions of formation of concretions in host clastic rocks deposited in one sedimentary basin of similar age. Abstract Copyright (2008) Elsevier, B.V.