The footprints of ancient CO2-driven flow systems: Ferrous carbonate concretions below bleached sandstone

Abstract

Iron-rich carbonates and the oxidized remains of former carbonates (iron-oxide concretions) underlie bleached Navajo Sandstone over large portions of southern Utah. Iron in the carbonates came from hematite rims on sand grains in the upper Navajo that were dissolved when small quantities of methane accumulated beneath the sealing Carmel Formation. As a second buoyant gas (CO 2 derived from Oligocene-Miocene magmas ) reached the seal and migrated up dip, it dissolved in the underlying water, enhancing the solution's density. This water carried the released ferrous iron and the methane downward. Carbonates precipitated when the descending, reducing water degassed along fractures. The distribution of a broad array of iron-rich features made recognition of the extent of the ancient flow systems possible. Although siderite is not preserved, dense, rhombic, mm-scale, iron-oxide pseudomorphs after ferrous carbonates are common. Distinctive patterns of iron oxide were also produced when large (cm-scale), poikilotopic carbonate crystals with multiple iron-rich zones dissolved in oxidizing waters. Rhombic pseudomorphs are found in the central cores of small spheroids and large (meter-scale), irregular concretions that are defined by thick, tightly cemented rinds of iron-oxide-cemented sandstone. The internal structure and distribution of these features reveal their origins as ironcarbonate concretions that formed within a large-scale flow system that was altered dramatically during Neogene uplift of the Colorado Plateau. With rise of the Plateau, the iron-carbonate concretions passed upward from reducing formation water to shallow, oxidizing groundwater flowing parallel to modern drainages. Finally they passed into the vadose zone. Absolute dating of different portions of these widespread concretions could thus reveal uplift rates for a large portion of the Plateau. Iron-rich masses in other sedimentary rocks may reveal flow systems with similar histories.

Bleached mudstone, iron concretions, and calcite veins: a natural analogue for the effects of reducing CO2‐bearing fluids on migration and mineralization of iron, sealing properties, and composition of mudstone cap rocks

“…However, Loope et al . (, , ), Loope & Kettler () and Weber et al . () consider that the spherical iron‐oxide concretions in the Navajo sandstone are not the original precipitates, but rather are the products of original cemented iron‐rich carbonate (mainly siderite) transformed by biological or nonbiological (decomposition or oxidization) reactions.…”

“…The formation of the ferrous carbonate precursors (siderite) of the iron‐oxide concretions and ferrous calcite in the Navajo sandstone was related to a pH increase that was caused by degassing of CO 2 from formation water (Loope et al . ; Loope & Kettler ). We consider that the origin of fracture‐filled calcite veins in the present study area was similar to that of the Navajo sandstone.…”

“…, ; Loope et al . , , ; Loope & Kettler ). The Fe derived from the bleaching of red mudstone was released by H 2 S in the fluids.…”

“…; Loope et al . , , ; Loope & Kettler ). Iron‐oxide concretions in the bleached Navajo sandstone (Utah, USA) were examined to reveal the changing composition, source, migration direction of fluids, and geochemical evolution of this ancient reservoir.…”

“…; Loope et al . ,, ; Loope & Kettler ). Moreover, spherical concretions in the Navajo sandstone were considered to be terrestrial analogues of hematite concretions on Mars (Chan et al .…”