DISTINCTIVE FEATURES THAT DEVELOP IN SIDERITE-CEMENTED SANDSTONES WHEN (DURING EXHUMATION) FLOW SYSTEMS BECOME OXYGENATED

Siderite (FeCO₃) is a common cement in sandstones. Because siderite is unstable in the presence of O₂, it is typically completely lost from uplifted, permeable sandstones when they enter oxidizing shallow groundwater or soil water. Iron oxides like hematite, limonite, and ferrihydrite are insoluble in near-neutral, oxidizing waters, so these minerals usually accumulate near the site of siderite dissolution. We have found iron-oxide-rich sandstone originally cemented by siderite in: Umm Ishrin and Ammar Fms (Cambrian and Ordovician of southern Jordan); Shinarump Member of the Chinle Fm (Triassic, UT & AZ); Navajo Sandstone (Jurassic, UT); and Dakota Sandstone (Cretaceous, NE & KS). Rhombic iron-oxide pseudomorphs (after euhedral siderite crystals) and mm-scale spheroids (replacements of sphaerosiderite) are distinctive and common. Like calcite and dolomite cements, siderite cements commonly replace quartz (both detrital grains and overgrowths). Notch-like pits (5-20 microns in diameter) that were generated during penetrative growth of euhedral siderite crystals into quartz grains become fully exposed on grain surfaces when ferrous iron migrates away upon siderite dissolution. If iron oxide instead accumulates to form a pseudomorph of the penetrative siderite crystal, notches partially filled with broken, in-situ, iron-oxide pseudomorphs are visible with SEM.

Spheroidal to oblate, iron-oxide-cemented concretions (2 mm-2 m diameter) commonly develop dense, impermeable rinds underlain by iron-poor “moats”. In small concretions, all ferrous iron dissolves and migrates to the perimeter where it is oxidized by iron-oxidizing microbes. Large (and a few small) concretions also have a central, iron-rich core beneath the “moat”. “Moats” and rinds grow and cores shrink when iron progressively dissolves and migrates to concretion perimeters. Pseudomorphs are found only in cores. Ironstones and concretions that preserve rhombic pseudomorphs and notch-like pits are not primary iron-oxide precipitates, and concretions with dense rinds, iron-free moats, and iron-rich cores did not originate via buffering of iron-rich, acidic water by iron-free, calcite-cemented concretions.

Large, iron-oxide-rich, cylindrical concretions (present in Umm Ishrin, Ammar, Shinarump, Navajo) form when O₂ first enters aquifers bearing small, scattered crystals of siderite. Cylinders (datable by (U-Th)/He) grow parallel to groundwater flow direction, and thereby provide valuable information on the evolution of paleoflow systems.