Abstract

A variety of diagenetic hematite and manganese oxide deposits occur within well-exposed Jurassic eolian and related deposits of southeastern Utah. Hematite concretions (millimeters to tens of meters in size) and strata-bound layers occur in the permeable Navajo, Page, and Entrada sandstones. Localized manganese oxide deposits without significant iron oxide occur in the overlying rocks covering the Summerville-Tidwell interval. Field, lab, and numerical modeling studies indicate the diagenetic deposits are related to the Moab fault. Fluid inclusion studies show salinities of fault fluids range from 0 to 19.7 NaCl equivalent weight percent. The δ18O (SMOW) and δ13C (PDB) values of cements and veins range from 7 to 27‰ and -12 to +5‰, respectively. The δ87Sr (SMOW) values of these cements and veins range from 0.210 to 2.977‰, values substantially more radiogenic than Pennsylvanian seawater. Saline brines formed from solution of Pennsylvanian salts by meteoric water and are interpreted to have flowed up the Moab fault and outward into adjacent permeable rocks. These brines are reducing from interaction with hydrocarbon, methane, organic acids, or hydrogen sulfide, and thus remove iron, manganese, and 87Sr, and bleach the sandstones near the fault. The isotopic evidence suggests multiple episodes of fluid flow up the Moab fault system. When saline, reduced brines mixed with shallow oxygenated groundwater, iron and manganese oxides were precipitated as cements to form concretions and tabular deposits in the porous sandstones. Multiple episodes of iron oxide mineralization and concretionary geometries are evident and can be explained as the result of permeability heterogeneities in the host rock, presence of favorable nucleii for precipitation, a self-organization process, or the influence of microbes.

This study emphasizes the nature of the reducing fluid that mobilized iron and its relation to the movement of saline or hydrocarbon fluids along the Moab fault system to precipitate hematite and manganese oxides as a result of oxidation of the fluid. The preservation of diagenetic concretionary iron and manganese oxides offers an excellent insight into permeability contrasts of sandstone units, an understanding of mixing fluid compositions, and solute transport along a major fault system.

You do not currently have access to this article.