CALCITE CEMENTATION IN SANDSTONE: FROM CONCRETIONS TO CEMENTED FORMATIONS

For years the prevailing model for the formation of concretions was one in which cementation progressed by the growth of the outer edge of the concretion. Zonation in such concretions would therefore be concentric, similar to the structure of an onion. In the past decade, however, this model has been called into question. Detailed petrographic analysis of concretions by numerous authors suggests that a more complex mode of growth involving multiple nucleation sites (i.e., pervasive growth) is far more common. Such concretions form at numerous isolated sites throughout the concretion body, with late-stage cements filling in around previously deposited early-stage cements. Our study of fluvial deposits in the Rio Grand Rift, and recent work by others, suggest that cementation progresses in a similar manner at larger scales. Cementation of sandstone beds seems to be principally accomplished by the amalgamation of numerous concretions. Our detailed outcrop mapping indicates that these amalgamated concretionary beds merge together to form “megaconcretions”, which can extend for over a half a kilometer in length. We speculate that total cementation of sandstone units is probably accomplished in some, or perhaps all, cases by amalgamation of such megaconcretions. Thus, cementation seems to progress though the coalescence of numerous cemented bodies in a manner that is scale independent. We hypothesize that such cementation is most common in nature because it maximizes the surface area available for precipitation to proceed (i.e., reactive surface area), and at larger scales allows flow to proceed despite high percentages of cement.