New Mexico Geological Society Annual Spring Meeting — Abstracts

The Zia Formation (Miocene) consists of sandstones and mudrocks deposited in fluvial, eolian, and playa lake environments. Although much of the formation is poorly consolidated, resistant zones of calcite cementation are common. These zones range in size from isolated nodules to tabular cemented zones several meters thick that extend for over two kilometers laterally. The calcite cemented zones are highly complex, exhibiting a wide range of macroscopic and microscopic textures and geometries. Isolated or groups of nodules and rhizocretions with micritic fabrics and alveolar structures are inferred to be vadose carbonates. Individual or groups of ovoid or elongate concretions, characterized by blocky spar cements, and preservation of primary sedimentary structures are inferred to be phreatic carbonates. Most cemented units in the Zia reflect characteristics of both phreatic and vadose zone cementation (e.g., preservation of sedimentary structures plus rhizocretions and alveolar microtextures). σ¹³C values for vadose cements tend to be heavier and σ¹⁸O values tend to be lighter than phreatic cements. σ¹³C and σ¹⁸O values for units with mixed features tend to have intermediate values. Most cementation types that exhibit a mixture of features may reflect past fluctuations of the water table, where vadose cements were moved into the phreatic zone. Vadose zone cementation occurred principally in association with soil development, whereas phreatic zone cementation occurred preferentially in zones of high primary permeability. In many cases early vadose cements provided nucleation sites for later phreatic cementation. Tabular units in the Zia are often
laterally extensive, decreasing potential reservoir/aquifer quality by forming significant barriers to venical fluid flow . These barriers could result in compartmentalization of the reservoir/aquifer, and extensively reduced production if wells were screened on only one side of a cemented layer.