Characterization of Navajo Sandstone Concretions: Mars Comparisons and Criteria for Distinguishing Diagenetic Origins

Introduction: Careful characterization of terrestrial concretions and Mars “blueberries” yields important criteria for distinguishing their diagenetic origin in comparison to other models for small, spherical forms. The Jurassic Navajo Sandstone in the Grand Staircase Escalante National Monument (GSENM), southern Utah, contains many hydrous ferric oxide mineralization geometries (HFO phases including ferrihydrite, goethite, and hematite). Spheroidal concretions have been suggested as an analog for hematite concretions found at Meridiani Planum in the Burns formation [1, 2]. Calvin et al. [3] characterize Martian “blueberries” and their research provides a basis for our comparison to the Navajo Sandstone concretions. Methods: Characterization of Navajo Sandstone concretions utilized field and laboratory strategies. In the field, concretion diameters were measured at their largest width with a digital caliper to differentiate size populations. Concretions that had weathered out and collected in topographic lows (n= 1400) were measured to 0.1 mm. In the laboratory, QEMSCAN, visible near infrared (VNIR) reflectance spectroscopy, and whole rock analysis are used to determine the mineral phases and adsorbed species present in the cements of the concretions. Results and Discussion: Navajo Sandstone concretions have many similarities with Martian “blueberries” (Table 1) in the following characteristics: a finegrained, homogenous host rock [2, 4]; spheroidal geometries; interior and exterior cement structures; HFO mineralogies (with some differences in mineral phases and crystal size); enrichment in Ni relative to the host rock; distinct size populations; in situ self-organized spacing and distributions within the host rock; and conjoined forms. The strength of the similarities from detailed characterizations all point to an origin of groundwater diagenesis in a porous and permeable eolian sandstone. Some notable differences exist in: 1) host rock composition; 2) more complex mineral phases in Utah concretions including the presence of silicates; 3) internal structure (more varied in Utah); and 4) HFO crystal size. The notable differences and range of variability can be attributed to the more complex diagenetic history of the terrestrial examples. At least seven different mineralization and mobilization events are documented in the GSENM study area [5]. These distinct events are Table 1. Comparison of characteristics of Burns formation concretions and Navajo Sandstone concretions. Mars data from Calvin et al. [9]. X= present