Characterization of Navajo sandstone hydrous ferric oxide concretions

In Utah's Grand Staircase Escalante National Monument (GSENM), abundant spheroidal hydrous ferric oxide (HFO) concretions (cemented mineral masses) of the Jurassic Navajo Sandstone are present both in situ and loose in topographic lows. The HFO cement phases are typically amorphous HFO, goethite and hematite. Concretions are a significant record of diagenetic history, iron cycling and precipitation/mobilization events. The physical characterization of the concretions provides data to infer the processes affecting precipitation geometries and timing of precipitation events throughout the diagenetic history of the sandstone reservoir. Physical properties of the concretions also provide data to propose a model for formation of spheroidal HFO concretions. Three precipitation geometries are present in GSENM: 1. spheroidal (aspect ratio ranges from 1.00 to 1.06) to discoidal (aspect ratio >1.06) macro concretions >5 mm diameter, 2. spheroidal to discoidal micro concretions <5 mm diameter, and 3. HFO-lined joints and related asymmetrical loopy mineralization and banded precipitation patterns (Liesegang bands). Three end members of Utah macro concretions (rind, layered and solid) and two end members of micro concretions (rind and solid) are distinguished by internal structure. Different precipitation geometries result from a combination of diffusive and advective mass transfer. Multiple generations of cement textures and minerals suggest a pervasive growth model similar to some carbonate concretions where HFO concretions are precipitated with a set radius from multiple nuclei throughout the concretion. Chemical and mineral phase gradients detected via QEMSCAN and visible to near infrared (Vnir) reflectance spectroscopy suggest that concretions form initially from an amorphous HFO gel that dehydrates with age to more stable phases (goethite, hematite). Some internal structures of concretions are a result of overprinting from late-stage precipitation events. The Utah concretions interest planetary scientists because similarities with spherules in Meridiani Planum suggest that Mars "blueberries" also form via diffusive reactant transfer during groundwater diagenesis and Mars hematite likely had precursor HFO phases. Multiple mineralization events have probably occurred on Mars although variability of Utah concretions represents a more complex history of diagenesis. Concretions preserve a record of diagenetic events. Reservoir evolution is better understood by determining how concretions grow and change over time.