Origin of Giant Calcite-Cemented Concretions, Temple Member, Qasr El Sagha Formation (Eocene), Faiyum Depression, Egypt

Abstract

ABSTRACT The Temple Member of the Qasr El Sagha Formation contains spheroidal calcite-cemented concretions in sandstone ranging from 70 cm to 9 m in diameter, although most are 1 to 4 m in diameter. Most concretions are solitary, oblate spheroids, but some are prolate, coalesced (joined laterally), or are compound (enclose smaller concretions). Half the concretions have domains with a fairly uniform center-to-center spacing of 4-6 m, but half have domains of random spacing (both clusters and segregated concretions) or occur in parallel rows of coalesced concretions. Miocene marine carbonate shells, now preserved only inside concretions, were the source of calcite cement as further supported by carbon and strontium isotopic signatures of cement (13C = -2 to +1o/oo PDB; 87Sr/86Sr = 0.70765 to 0.70770). Concretions formed at shallow burial depths in meteoric pore water (most 18O values of calcite are from -4o/oo to -2o/oo PDB) at temperatures probably between 10 and 20°C. Spheroidal, uniformly spaced concretions formed where carbonate shells were uniformly distributed and where nucleation sites were influenced by low carbonate saturation levels and widely spaced sites, or at sites established by some self-organization process; concretions in orthogonal rows in the plane of bedding probably formed where shells were concentrated during deposition; randomly spaced concretions formed where shells were unevenly spaced or where nucleation of concretions was rapid. Some Temple concretions reached exceptional size in less than 20 My owing to a combination of conditions: large initial aragonite and Mg-calcite shell concentration, widely spaced concretion growth sites locally, presence of a stable hydrologic regime over a long period of time, and possibly bacterially mediated precipitation of calcite.