Palo Duro Canyon State Park Study of Conical Hematite Concretions, 20-R8003

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Principal Investigators
Cynthia L. Dinwiddie
James L. Myers
Ronald N. McGinnis

Inclusive Dates:  10/31/08 – 03/02/09

Background - Concretions commonly develop in preexisting sedimentary rock by precipitation of minerals from circulating fluids at mixing fronts. They are compositionally distinct from host rock because diagenetic mineral cement fills preexisting sediment porosity, making concretions significantly more dense and resistant to weathering than surrounding rock. In situ concretions are significant because they record both historical information about past processes that occurred within the geologic unit and present day information about the ability of the unit to conduct fluids. In effect, the mineralogy, morphology (shape), and spatial distribution of concretions can serve as controls on conceptual models for subsurface fluid flow. Unusual cone shaped, iron-oxide concretions within seven fluvial sandstone units of the basal member of the late Triassic Trujillo Formation at Palo Duro Canyon State Park were originally documented in an unpublished paper by William Holliday, who invited SwRI researchers to begin an in-depth study of controls on their occurrence.

Approach - To gain familiarity with the broader geological context of these unusual cone-shaped concretions, especially the host rock, and develop a working hypothesis for their occurrence, this four-month project funded preliminary field research and a supporting literature survey. Project team members made first-order observations of concretion orientations, shapes, aspect ratios and sizes, and conducted global positioning surveys to map their spatial density. The project team noted the presence of pyrite pseudomorphs associated with concretions, density characteristics, and lithology-dependent morphologies. Finally, the team interpreted the available data to formulate a working hypothesis for the development of cone-shaped concretions.

Accomplishments - In situ concretion orientation is cone-apex up, body radiating down and out, with long axis perpendicular to bedding. Concretions are densely mineralized at their apex, and become less densely mineralized with depth. Morphologies are associated with the sedimentary texture and primary bedding structure of the host rock and the corresponding hydrologic regime (i.e., advection vs. dispersion for Fe-transport behavior); three lithofacies exhibit different cone forms. The cone shape suggests downward and laterally spreading precipitation from mineralizing fluids in the unsaturated (vadose) zone (where partially filled pores would be subject to downward gravitational pull). Fluid flow in the vadose zone is generally gravity-dominated yet has a lateral component, consistent with both the apex-upright cone shape of these concretions and the depth-dependent density of their mineralization. The working hypothesis is that (1) iron pyrite cubes precipitated on organic-rich nuclei during saturated, reducing conditions; (2) hydrologic conditions shifted from saturated to unsaturated; (3) meteoric water infiltrating the vadose zone oxidized the pyrite; and (4) iron was reprecipitated as oxides to form vertically oriented cone-shaped concretions under the influence of gravity. This research strengthened a subsequent proposal to the National Science Foundation to conduct in situ minipermeametry, geochemical modeling, geological mapping, and laboratory analyses of these concretions and their host rock, and also enabled preparation of a documentary manuscript now in final revision for publication in the journal Geofluids.

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