This study discusses the role of smectitic clays in the formation of an abiotic (physio-chemical) primary dolomite
within an evaporative alkaline-saline marginal lake system, in the absence of carbonate precursor and microbes.
The present work has been achieved in terms of textural, mineralogical, and geochemical characteristics of
dolostones in theMid-Eocene (Bartonian) lacustrine succession cropping out at Gebel El-Goza El-Hamra (Shabrawet
area, NEEgypt). This lacustrine succession is15–16mthick, and made up of alternating horizontal beds of dolostone,
marlstone and mudrock that show some pedogenic and subaerial exposure features. The dolostones are composed
mainly of dolomite (60–90%), smectite (20–30%) and quartz grains (5–10%). The dolomite comprises finecrystalline
rhombs to micro-spherical crystals with no obvious relics of microbial activity and/or carbonate precursor.
It is, ordered, nearly stoichiometric (with 46–50% mole of MgCO3) and has δ18O and δ13C values ranging from
+0.44 to +2.96 VPDB ‰, and 0.93 to −8.95 VPDB ‰, respectively. The smectite occurs as thin mats that are
commonly intergrown and associated with dolomite. Mineralogical, textural and stable isotopic results of the dolomite
indicated that the dolomite was formed as an abiotic primary precipitate in alkaline saline lacustrine systems.
In this respect, the gel-like highly viscous smectitic medium plus progressive CO2 degassing, elevated evaporation,
low sedimentation rate, low sulphates level and alkaline soil solution lowered the kinetic barriers of dolomite precipitation
from solution and promoted the incorporation of Mg2+ in the structure of dolomite. Consequently, the
presence ofsmectitic clays in evaporative saline lakes is significant for dolomite formation because they can generate
a gel-like highly viscousmediumand provide Mg2+ that can facilitate the physcio-chemical precipitation of primary
dolomite from solution at ambient temperatures. However, more work is needed to better characterize the role of
clays during dolomite formation in alkaline lacustrine environments at ambient temperatures. |
