Impact of Early Diagenesis of Eolian Reservoirs, Great Sand Dunes National Monument, Colorado: ABSTRACT

Abstract

Dune and associated alluvial and playa deposits at Great Sand Dunes National Monument, Colorado, provide an excellent opportunity to study early diagenetic development of vertical and horizontal permeability barriers in recent eolian deposits (> 10 ka). Cements observed include calcite, aragonite, protodolomite(?), amorphous silica, iron hydroxide, smectite, trona, and halite. Cementation is controlled by the availability of water, with several hydrologic subenvironments producing different cements. Evaporative cementation in dunes adjacent to playas is commonly dominated by trona and halite, but calcite, aragonite, and amorphous silica also bind the sediment. These cements are generally most concentrated in fine laminations where capillary action has pulled water into dunes. Iron hydroxides, calcite, and amorphous silica precipitate at the interface between ground water and streams or lakes, where the pH gradient may exceed 5 pH units (pH 5.7-11.5). Subsequent movement of the ground-water table can result in cross-cutting cement zones. Early cementation in dunes prevents deflation and provides a mechanism for preservation of the reservoir unit. Intense cementation may permanently occlude porosity, or leaching may reestablish well-interconnected porosity. An understanding of the extent and composition of early cement zones can be used to improve hydrodynamic models for production and enhanced recovery. End_of_Article - Last_Page 276------------