IRON PRECIPITATION IN THE BRUSHY BASIN MEMBER OF THE JURASSIC MORRISON FORMATION: CLUES TO DIAGENETIC BIOGEOCHEMICAL CYCLES

The upper part of the Brushy Basin Member of the Jurassic Morrison Formation has been interpreted as a large, ephemeral, alkaline saline lake system centered in the Four Corners region of the Colorado Plateau. Copious input of volcanic ash combined with alkaline groundwater resulted in a unit composed of reactive, fine-grained sediments. New research characterizes concretions in the Brushy Basin Member and documents their relationships to host rock as well as to biological influences. The purpose is to better understand subsurface iron mobilization and precipitation and related biota-water-rock interactions in fine-grained rock. This research has applications to the characterization of potential seal rocks for CO₂ sequestration on the Colorado Plateau.

Field studies in southeastern Utah and southwestern Colorado indicate two common categories of concretions based on size: 1. microconcretions < 5mm in diameter. 2. macroconcretions 5 mm to ~15 cm in diameter. Rare examples are ~ 40 to 80 cm in diameter. Laboratory analyses including whole rock analysis, x-ray diffraction, Vnir reflectance spectroscopy and petrography indicate a variety of concretion cements such as iron (oxyhydr)oxides, siderite, pyrite, calcite and clays. This range of cement mineralogies is likely due to the geochemically reactive host rock sediments. Some concretions line burrows and root traces and some clay-rich concretions are highly bioturbated with preferential iron oxyhydroxide and pyrite precipitation along the burrows/root traces. These suggest a biogenic component to iron cycling in the subsurface during early diagenesis.

The fine-grained nature of sediments affected pathways of fluid flow. Concretions typically form near boundaries between red and green siltstone/claystone facies and suggest iron is mobilized and re-precipitated via diffusion at local redox fronts. Local paleotopographic elevations and depressions (meter-scale) controlled water table movement and ponding. Channel splay siltstones and sandstones created minor conduits for preferential subsurface fluid flow and occasionally contain spheroidal concretions concentrated along more permeable layers. This research documents important post-depositional relationships of iron cycling and biogeochemical alterations in mudrocks.