Pseudospherulitic fibrous calcite from the Quaternary shallow lacustrine carbonates of the Farafra Oasis, Western Desert, Egypt: A primary precipitate with possible bacterial influence

Abstract

Pseudospherulitic fibrous calcite (PFC) has been found as a major constituent (85–90%) within thin massive limestone beds of the Quaternary mudflat-shallow lacustrine facies association (1.5–2m thick) that forms part of combined facies associations of the Quaternary clastic-carbonate unit (25–30m thick) at Bir-Karawein area in the Farafra Oasis, Western Desert, Egypt. The thin massive limestone beds (2–5cm thick) are devoid of pedogenic features and marine fossils. They form a rhythmic cyclic succession with thin massive mudrocks (5–10cm thick). The mudflat-shallow lacustrine facies association herein occurs within a depositional sequence of distal alluvial-floodplain (6–12m thick) and palustrine (1.5–4.5m thick) facies associations.The PFC is a composed of loosely packed rounded to sub-rounded single low-Mg-calcite crystals (150–250μm-sized) with intracrystalline fibrous microfabric marked by fibers (150–250μm long and 10–20μm wide) radiating from the center of the individual crystals and displaying irregular internal growth with lobate pattern. The PFC crystals show non-planar to highly irregular intercrystalline boundaries. Under SEM, the individual crystal fibers group of PFC form ellipsoid to sub-globular bodies. Each PFC crystal exhibits successive zones of thick non-luminescence and thin brightly orange to dull luminescence. The matrix (10–15%) between the PFC crystals is mainly a honeycomb-like smectite.The PFC is postulated to be a primary precipitate. This concept is reached because the PFC: (i) does not display the criteria of typical Microcodium structures, root-calcification, speleothem structures, calcite spherulites of laminar calcretes, and calcitization of precursor dolomite or aragonite, (ii) possesses homogenous compositional and textural characteristics, and (iii) occurs within limestone beds that lie in between impermeable massive mudrock beds that dampen diagenesis. A role for possible bacterial contribution in crystallization of the PFC is assumed in the view of its internal microfabric characteristics (pseudospherulitic and lobate internal growth patterns), and morphological features (ellipsoidal to subglobular appearance), in addition to its slightly negative values of δ13C (−0.51‰ to −2.19‰). The low concentration of Na (0.11–0.20%), Sr (70–110ppm) and Mn (0.04–0.31%), in addition to the negative values of δ18O (−4.65‰ to −5.96‰) in the PFC reflect its deposition from oxygenated freshwater. In addition, the absence of covariance between δ13C and δ18O values (r=−0.202) of the PFC indicates precipitation in a hydrologically-open, short-lived lake setting. In summary, the PFC is of low-Mg type and formed in a hydrologically-open, short-lived, freshwater lake as a primary precipitate with possible bacterial contribution.