Lithification of Peritidal Carbonates by Continental Brines at Fisherman Bay, South Australia, to Form a Megapolygon/Spelean Limestone Association

Abstract

ABSTRACT Groundwater springs in the supratidal zone at Fisherman Bay, South Australia, are the sites of aragonite precipitation and cementation of Holocene marine carbonates. Lithification, which commenced less than 3000 years BP and is still active, has formed a cavernous limestone containing megapolygons, tepees, and speleothems including pisoliths, floe aragonite, and aragonitic pool deposits. The emerging waters are from a continental source and have evolved from low alkalinity waters of Pleistocene sand and clay coastal plain aquifers which have passed through an underlying Tertiary marine carbonate aquifer. The resulting waters have high PCO2, total carbonate. Ca, and sulphate concentrations. They are close to saturation with respect to aragonite, and their mMg2+/mCa2+ ratios approach or exceed the critical aragonite precipitation value. Aragonite is precipitated from these waters when they approach the surface and evolve CO2. Features which, taken together. may diagnose ancient examples of this process include: 1) primary aragonitic cements with high mSr2+/mCa2+ values; 2) nonmarine 34S values in associated gypsum; 3) two superimposed networks of surface polygons, one delineated by extensional boundaries, the other by tepees; 4) high-water vadose-zone isopachous grain cements; 5) interconnected, speleothem-lined cavities; and 6) the presence of evaporites only in surface sediments. Possible ancient examples of peritidal carbonate lithification by saline spring waters rising from confined aquifers are recognized in descriptions of carbonates from West Texas, Lombardy, and the Atlas Mountains. The areal extent of each of these deposits suggests that the process may be a geologically important feature, and its products may be diagnostic of semi-arid or arid-zone paralic sedimentation.