The Entrada Sandstone (Jurassic), in the Ghost Ranch area of north-central New Mexico, is eolian in origin and is composed of subrounded to well rounded and mostly well-sorted quartz grains with few feldspars or rock fragments. Cementation of the Entrada is puzzling. Sandstones stratigraphically above (Dakota and Morrison) and below (members of the Chinle Formation) are well cemented with silica, but the Entrada in the study area is poorly cemented. Authigenic smectite, calcite, and kaolinite are cementing agents in the Entrada Sandstone.
The most common diagenetic features in the Entrada are grain coatings (smectite), pore filling (calcite, kaolinite), partially dissolved grains (feldspar), and replacement. Thin section and SEM analyses indicate the following diagenetic sequence: (1) smectite coating and bridgings, (2) calcite cementation and calcite-for-quartz replacement, (3) calcite and feldspar dissolution, and (4) kaolinite formation. Coatings effectively restrict or retard quartz cementation by inhibiting silica in pore waters from nucleating on coated quartz surfaces. An absence of quartz overgrowths is not necessarily due to coatings; temperature and availability of silica and other cations such as Al+3 and Mg+2 may be important factors to inhibit quartz overgrowth. The presence of a few ell-developed quartz overgrowth on uncoated parts of grains indicates that silica cementation was potentially favorable in the Entrada, but smectite coatings on the grains inhibited silica cementation. Early formation of calcite cement preserved preburial porosity. Later, acidic ground-water influx caused calcite dissolution, kaolinite precipitation, and formation of secondary porosity.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.