Siliceous Cements and Their Timing Relations in Arkosic Arenites, Lobo Formation (Paleocene-Eocene), Southwestern New Mexico

W. C. James, Joseph Russo

Arkoses (Q₄₆F₅₃L₁) with subequal amounts of K-feldspar and plagioclase contain a variety of siliceous cements composing over 96% of those authigenic products present. Textural intergrowths and cement stratigraphy indicate siliceous phases overlap or postdate pure limonite and feldspar overgrowth formation and predate zeolites, albitization, and calcite constituents. Four siliceous phases are present: (1) impure siliceous cement, (2) microquartz, (3) chalcedony, and (4) quartz overgrowths.

The impure siliceous cement is 1-30 µm, subhedral to euhedral, equant to bladed microquartz intermixed with chlorite and/or limonite. The cement is pore filling and grain replacing. Microquartz is compositionally pure, equant, often euhedral, and less than 30 µm. It mainly lines pores. Bladed to fibrous habits with spherulitic fan textures are typical of chalcedony. Quartz overgrowths are larger sized and optically clear and possess optical continuity with host quartz grains.

Although siliceous phases overlap in timing, the impure siliceous cement was the first precipitated followed by a dissolution event and growth of pure microquartz or chalcedony. Often pore-filling chalcedony postdates an isopachous grain coating of microquartz. Euhedral quartz overgrowths postdate all other siliceous phases. Precipitation of siliceous components may in part be related to pH decreases and/or the increasing activity of iron, aluminum, and magnesium constituents. Although previous studies have suggested the proportion of impurities present as the principal determinant for morphology of resultant siliceous phases, nucleation site density and geometry likely had a greater influence in this specific case.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.