Concretions, bleaching, and CO2-driven fl ow The footprints of ancient CO2-driven fl ow systems: Ferrous carbonate concretions below bleached sandstone David B. Loope* and Richard M. Kettler Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68588, USA ABSTRACT the vadose zone. Absolute dating of differ- In this paper, we present a conceptual model ent portions of these widespread concretions for bleaching of the sandstone and the down- Iron-rich carbonates and the oxidized could thus reveal uplift rates for a large por- ward transport of iron from the source rock remains of former carbonates (iron-oxide tion of the Plateau. Iron-rich masses in other to the geochemical trap where concretions concretions) underlie bleached Navajo Sand- sedimentary rocks may reveal fl ow systems nucleated and grew. First, we will describe stone over large portions of southern Utah. with similar histories. and interpret a variety of iron-rich concre- Iron in the carbonates came from hematite tions. Some still contain carbonate minerals rims on sand grains in the upper Navajo INTRODUCTION and some do not, but we argue below that all that were dissolved when small quantities originated as reduced-iron carbonates. The of methane accumulated beneath the seal- Over a broad portion of the Colorado Pla- bleached rocks are striking evidence for a ing Carmel Formation. As a second buoyant teau of southwestern United States, the Jurassic large-scale fl ow system (Beitler et al., 2005; gas (CO2 derived from Oligocene–Miocene Navajo Sandstone contains a wide variety of Chan et al., 2005). We claim that the concre- magmas ) reached the seal and migrated up iron-oxide–cemented masses. Here we explain tions reveal the changing composition, source, dip, it dissolved in the underlying water, why these structures are found down section oxidation state, and migration direction of the enhancing the solution’s density. This water and down dip from bleached sandstone. When fl uids in that system. carried the released ferrous iron and the carbon dioxide dissolves in an aqueous solu- In the Ladbroke Grove Gas Field (Otway methane downward. Carbonates precipi- tion, the density of that solution increases. This Basin, South Australia), the Cretaceous sand- tated when the descending, reducing water fact has huge implications for subsurface car- stone reservoir contains high levels of CO2 that degassed along fractures. The distribution of bon sequestration (Lindeberg and Wessel-Burg, was derived from a magmatic source (Watson a broad array of iron-rich features made rec- 1997; Ennis-King and Paterson, 2005; Mac- et al., 2004). In a nearby gas fi eld, the same ognition of the extent of the ancient fl ow sys- Minn and Juanes, 2013), and here we show that sandstone is not exposed to CO2-enhanced pore tems possible. Although siderite is not pre- it is essential to understanding iron diagenesis in waters. Analysis of cores shows that iron-rich served, dense, rhombic, mm-scale, iron-oxide the Navajo Sandstone. carbonate minerals are much more abundant pseudomorphs after ferrous carbonates are North of the Grand Canyon, in southern in the former reservoir than in the latter. These common. Distinctive patterns of iron oxide Utah’s Grand Staircase, the upper part of the pore-fi lling carbonates occur mostly as con- were also produced when large (cm-scale), Navajo Sandstone forms the White Cliffs and cretions 2–10 cm in diameter. The Ladbroke poikilotopic carbonate crystals with multi- the lower Navajo forms the upper part of the Grove Field is an excellent analog for geologi- ple iron-rich zones dissolved in oxidizing Vermillion Cliffs (Fig. 1). Mudrocks and evapo- cal storage of CO2 (Watson et al., 2004), and we waters. Rhombic pseudomorphs are found rites of the Middle Jurassic Carmel Formation argue here that—before it was exhumed during in the central cores of small spheroids and form an effective seal at the top of the Navajo. the Neogene—the Navajo Sandstone contained large (meter-scale), irregular concretions Previous workers have shown that the upper CO2, methane, and abundant, iron-rich carbon- that are defi ned by thick, tightly cemented Navajo was originally red and that the iron in ate concretions. rinds of iron-oxide–cemented sandstone. the concretions came from iron liberated dur- Ferrous iron, carbon dioxide, methane, The internal structure and distribution of ing the bleaching of the overlying rocks (Beitler and siderite have been widespread in the pore these features reveal their origins as iron- et al., 2005; Chan et al., 2005). We agree with waters of Earth’s sediments and sedimentary carbonate concretions that formed within a these conclusions, but they raise two previ- rocks for billions of years, but iron oxides have large-scale fl ow system that was altered dra- ously un answered questions: How, if the iron- been widespread on Earth only since the Early matically during Neogene uplift of the Colo- oxide concretions came from the bleached rocks Protero zoic. Recognition that iron-oxide masses rado Plateau. With rise of the Plateau, the above, was the iron moved down section and in the Navajo Sandstone had ferrous carbon- iron-carbonate concretions passed upward down dip? And, if the concretions formed as pri- ate precursors (Loope et al., 2010, 2011, 2012; from reducing formation water to shallow, mary precipitates from the mixing of reducing Kettler et al., 2011; Weber et al., 2012) and that oxidizing groundwater fl owing parallel to and oxidizing waters (Beitler et al., 2005; Chan those carbonates were emplaced by CO2-driven modern drainages. Finally they passed into et al., 2005), how was oxidizing water intro- fl ow systems should aid interpretation of other duced and maintained below the widespread iron-oxide accumulations on Earth and on *Corresponding author email: [email protected] reducing waters? other planets. Geosphere; June 2015; v. 11; no. 3; p. 1–15; doi:10.1130/GES01094.1; 12 fi gures; 1 table. Received 26 June 2014 ♦ Revision received 20 January 2015 ♦ Accepted 25 March 2015 ♦ Published online ___ Month 2015 For permissionGeosphere, to copy, contact June [email protected] 2015 1 © 2015 Geological Society of America Loope and Kettler ′ Tmv Miocene volcanics 30 miles N 38° 28 50 km Jna Jc N Tov Oligocene volcanics Tov Te Eocene lake beds 24 Jc Jurassic Carmel Fm pre-J Jna Jurassic Navajo SS A DF* qu study site ar ius 12 with former Pla ′ teau 12 road or preserved carbonates Tmv Circle Cliffs Uplift Escalante Anticline Proposed,Jna subsurface paleoflow paths of: 2 89 W 110° 54 mantle-derived CO2 (buoyant; up dip) * t CC Te l u water3 carrying CO2 (aq) a – 2+ F HCO CH 4 and Fe t 3 n 12 (high density; down dip) u Tov g u t l a s u a n F u 12 E * e a P n e a Jna c n i i - l r r mJ-K c u o H n Te * o DF White M mJ-K b Cliffs a b i Cliffs Jna a White K RG Sevier Fault Vermillion * Jc ″ 9 MN* WY Cliffs ZNP * 30 ′ ZT White Cliffs UT NV study WC area * Vermillion Jna BG CO W 113° 12 Vermillion Cliffs 59 89 * PC pre-J Cliffs * N 37° 00′ AZ Figure 1. Simplifi ed geologic map of southwestern Utah showing distribution of Mesozoic and Cenozoic rocks, major structures, locations of study sites, and postulated, subsurface paleofl ow paths of buoyant, supercritical CO2 and, in the eastern part of study area, paths of dense water carrying ferrous iron and aqueous CO2 (alter- native explanation for the eastern paths [black arrows] is hydrodynamic fl ow; Loope et al., 2010). Flow paths of dense waters in the western area are unknown. SH—Sand Hollow; RG—Russell Gulch; ZT—Zion Tunnel; 3LC—Three Lakes Canyon; MN—Mollie’s Nipple; BG—Buckskin Gulch; PC—Paria Canyon; CC—Calf Creek; E—Egypt; DF—Dry Fork. ZNP near southwest corner is Zion National Park. For generalized cross section of the Grand Staircase: http:// en .wikipedia .org /wiki /Grand _Staircase -Escalante _National _Monument #mediaviewer /File: Grand _Staircase -big .jpg. Previous Work primary precipitates formed during the mixing Study Area and Geologic Setting of iron-bearing, reducing waters and oxygen- Chan et al. (2000, 2005), Beitler et al. ated meteoric waters. This hypothesis guided This paper is based on fi eld observations and (2005), and Nielsen et al. (2009) carried out fi eld-based work (Potter and Chan, 2011), analyses of four different types of samples from extensive studies on both the bleaching of the bench experiments (Chan et al., 2007; Barge the Navajo Sandstone at ten localities (Fig. 1): Navajo Sandstone and the iron-oxide concre- et al., 2011), and iron isotope studies (Chan (1) rinded, iron-oxide–cemented concretions; tions within it. Nielsen et al. (2009) showed et al., 2006; Busigny and Dauphas, 2007). (2) sandstone with poikilotopic, iron-zoned that although the upper Navajo Sandstone is Loope et al. (2010, 2011, 2012) and Kettler ferroan calcite cement; (3) calcite concretions bleached in the southern part of Zion National et al. (2011) reinterpreted the concretions as the containing large, rhombic, iron-oxide pseudo- Park (and continuously for 100 km eastward; altered (oxidized) remains of precursor concre- morphs or iron-oxide accumulations along Fig. 1), bleaching does not extend to the north- tions cemented by ferrous carbonate minerals. cleavage planes; and (4) iron-oxide patterns western portion of Zion, where the Navajo Weber et al. (2012) presented evidence that preserved in non-calcareous sandstone. Three retains its early diagenetic, red color. Although iron-oxidizing microbes mediated the oxida- of our sites (Fig. 1) are down dip from the this color-transition zone is only ~2 km wide, tion of the carbonates.