T147 Terrestrial Proxies of Paleoclimate and Paleoenvironment in Deep Time

Tuesday, 6 November 2012
8:00 AM - 12:00 PM
202AB

GEOCHEMICAL SELF-ORGANIZATION AND MICROBIALLY-MEDIATED OXIDATION OF SIDERITE IN THE SHINARUMP MEMBER OF THE CHINLE FORMATION

Paper number: T147-6
Program: Technical Sessions
Time: 9:05 AM – 9:20 AM

Authors

Abstract

The Kanab Wonderstone is sandstone (Shinarump member; Chinle Formation) that is variably cemented and stained with iron oxide. Similar features occur in sedimentary rocks all over the world and have been referred to as Liesegang bands. Closer examination reveals that the rocks in each locality are sandstones that host one to 20 mm thick, undulatory bands of iron oxide cement (IOC). The rock between the IOC bands is alternately lightly stained and lacking iron oxide stain (IOS). Both the IOS and IOC crosscut and obscure sedimentary structures. Many of the bands of IOS are arcuate and appear to be truncated by the bands of IOC; mimicking the appearance of cross-stratification. Both the IOC and IOS are related spatially to vertical joints that cut the sandstone at regular intervals. The spacing of bands of IOS is variable but typically follows the Jablczynski spacing law whereas the width of the bands typically increases with distance from a band of IOC. Bands of IOC, on the other hand, exhibit more variable spacing and a weak but significant dependence of band thickness on distance between bands of IOC.

The IOS consists of mounds of acicular needles precipitated on quartz grains. The pore-filling IOC comprises both needles and hexagonal plates of iron oxide, but there are also bundles of twisted filaments. At least three filaments occur in each bundle. The bundles are approximately 1 μm in diameter and 10s of μm long. The ends of some filaments are anchored to quartz grains. These morphotypes are similar to those exhibited by the microaerophilic iron-oxidizing bacteria Gallionella.

We interpret the IOC to be produced by microbially-mediated dissolution of siderite and oxidation of ferrous iron. Iron-oxidizing bacteria colonized the redox interface between siderite-cemented sand and porous sandstone. Microbes oxidized aqueous Fe(II), generating acid that caused siderite dissolution. Bands of IOS largely retain the morphology of reaction fronts, presumably as a result of differential adsorption of aqueous Fe(II) during siderite dissolution. Variations in the width of bands of IOS and in the spacing between these bands is evidence that the IOS is a part of true Liesegang system. The IOC, however, is the product of the geochemical drive for organization combined with microbial activity.