Diagenetic Advective and Diffusive Mass-Transfer Rates of Selective Solutes in Hematite Concretion Formation

Marjorie A. Chan¹, William T. Parry¹, and Anthony Park^2
1 University of Utah, Salt Lake City, UT
² Sienna Geodynamics & Consulting, Inc, Bloomington, IN

Spherical iron oxide concretions are common in the Jurassic Navajo Sandstone of Utah-Arizona, and also at Meridiani Planum, Mars. Terrestrial and Mars concretions share similar physical features of insitu distributions, geometries, and accumulations. Based on extensive fieldwork, we surmise that both advective and diffusive mass-transfer processes were responsible for the observed distribution patterns in Navajo Sandstone concretions. Iron was derived from grain coatings of eolian sands. Either banded or nodular precipitation occurred depending on Fe-enriched water flux rates.

Computational modeling using Sym.8 water-rock interaction simulator shows self-organizing patterns consistent with field observations. Results indicate competition between advective and diffusive mass-transfer rates as an important factor that determines precipitation and nucleation patterns. Liesegang-type banding and sparse prograding nucleation/reaction fronts are obtained using chemical and physical parameters simulating likely conditions in the Navajo Sandstone.

The concretion-forming principles derived from the terrestrial model can be applied to the Mars counterpart. In addition to the mass-transfer mechanism, iron bleaching and mobilization from neighboring, or deeper sediments is also a necessary mechanism. Although organic acids (e.g., methane and acetates) were the important Fe-reducing agent in Navajo Sandstone, volcanic gas or acids may have been the mobilizing agent on Mars.

Study of concretion formation has an unexpected benefit to reservoir quality prediction. In particular, cements occur when solutes imported into the porous media via diffusive and advective mass-transfer processes interact with locally derived solutes. Thus, principles and knowledge of sediment-water interactions gained through studies of concretion diagenesis can be broadly applied to reservoir models.