The upper part of the Brushy Basin Member of the Jurassic Morrison Formation in the Four Corners regions of the U.S. was deposited in an ephemeral alkaline saline lake system with copious input of volcanic ash that resulted in dynamic fluid–rock interaction during early diagenesis. Three broad diagenetic facies—defined by color and associated bioturbation features—are interpreted: red, green, and intermediate. Diagenetic facies reflect meter-scale paleotopography: red facies represent shallow water to subaerial, oxidizing conditions; green facies reflect saturated conditions and reducing pore-water chemistry shortly after deposition, and intermediate facies represent a combination of—or rapid transition between—the previous two conditions. Three categories of concretions are characterized based on mineralogy: carbonate, iron (oxyhydr)oxide, and phosphate concretions. Variation in concretion mineralogy and morphology in the Brushy Basin Member suggests that alkaline saline fluid chemistries created diagenetic microenvironments within a larger lake system to influence concretion precipitation. However, porosity and permeability are more important factors affecting concretion size, morphology, and mineralogy than host-rock composition. Diagenetic fluid–rock interactions in volcaniclastic sediments can be spatially variable even on tens- of-centimeters scale.