Earle F. Mcbride2
(1) Tanta University, Kafr El-Sheikh, Egypt
(2) University of Texas
Abstract: Origin of giant calcite-cemented concretions, temple member, Quasar El Sagha Formation (Eocene), Faiyum Depression, Egypt
The Temple Member of the Quasar El Sagha Formation at three closely spaced localities exposes spheroidal concretions at the same stratigraphic level or within a 10-m interval. Concretions are mostly from 1 to 4 m in diameter, but range from 70 cm to 9 m in diameter. Most concretions are solitary, oblate spheroids, but some are prolate, joined (coallesced), or are compound (enclose smaller concretions). Half the concretions have a fairly uniform spacing of 5-6 m, but half have either random spacing or occur in parallel rows of coallesced concretions. Lower shoreface sandstones with 5 to 10% molluscs, now preserved only inside concretions, were the source of calcite cement as deduced from carbon isotopic signatures of cement and distribution of preserved fossils.
The spheroidal, uniformly spaced concretions formed by the diffusional supply of components derived from uniformly distributed carbonate shells. Either widely and uniformly spaced nucleation sites developed at the start or myiads of closely spaced sites evolved into fewer dominant sites by a maturation process that cannibalized early-formed concretions. Concretions in orthogonal rows probably formed where shells were concentrated in bedforms; randomly spaced concretions are attributed to unevenly spaced shells.
Oxygen isotopic composition of calcite cement does not yield a unique interpretation, but is most compatible with the scenario of growth of the concretions over a period of ~30 MY in dominantly meteoritic pore water.
Late-stage enlargement of spheroidal concretions in some places during advective fluid flow is suggested by some elongate concretions. Meteoric flushing dissolved all carbonate shells that were not protected inside concretions tightly cemented by calcite.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana