EVALUATING THE ROLE OF ORGANICS, MINERALOGY AND PETROPHYSICAL PROPERTIES ON EARLY DIAGENESIS AND CONCRETION FORMATION IN VOLCANICLASTIC HOST ROCK COMPOSITION

The Brushy Basin Member of the Jurassic Morrison Formation was deposited in an ephemeral alkaline saline lake system with copious input of volcanic ash that resulted in a highly reactive chemical system during early diagenesis. Three broad diagenetic facies – defined by iron oxidation states – are interpreted: oxidized, intermediate and reduced. Diagenetic facies reflect meter-scale paleotopography; oxidized facies represent shallow water to subaerial conditions and reduced facies reflect deeper water deposition. Three categories of concretions are characterized based on mineralogy: carbonate, iron (oxyhydr)oxide and phosphate concretions. Comparisons of concretion and host rock chemistry shows that most examples are enriched in calcium, iron and manganese by ~ 100%, but only carbonate concretions in more permeable, relatively coarse-grained sandstone channels are greatly enriched (>>100%) in calcium, manganese and strontium relative to the host rock. Variation in concretion mineralogy and morphology within the Brushy Basin Member suggests alkaline saline fluid chemistries create micro-diagenetic environments within a larger lake system to affect concretion precipitation. However, petrophysical properties of porosity and permeability are more important factors affecting concretion size, morphology, and even mineralogy than host rock composition. Diagenetic fluid-rock interactions in volcaniclastic sediments reacting with alkaline saline fluids can be quite spatially variable even on an outcrop scale. Broader diagenetic questions of iron cycling and concretion formation on a pore- to outcrop-scale are illuminated for both Earth and Mars in similar volcaniclastic, iron-rich sediments of the Burns formation.