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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 465

Last Page: 465

Title: Diagenesis of Tertiary Arkoses, Santa Ynez Mountains, Santa Barbara and Ventura Counties, California: ABSTRACT

Author(s): Kenneth P. Helmold

Article Type: Meeting abstract


The stratigraphic distribution of authigenic minerals in feldspar-rich sandstones from tectonically active settings is important in determining reservoir quality. Scanning electron microscopy (SEM), optical microscopy, and X-ray diffractometry were utilized to determine the types and distribution of secondary minerals in Paleogene sandstones of the Santa Ynez Mountains, and to determine their paragenesis. Four main types of authigenic minerals are present: (1) early-stage phyllosilicate pore linings and pore fillings; (2) silicate overgrowths on quartz, plagioclase, and K-feldspar grains; (3) middle-stage zeolite (laumontite) replacement; and (4) late-stage calcite replacement. The honeycomb structure and high iron and magnesium content indicate that the clays are montmor llonite or chlorite type or possibly a close mixture of the two. Microanalyses show that regardless of the composition of the nucleus all plagioclase overgrowths are chemically pure albite. Authigenic albite also occurs as fracture fillings in plagioclase grains. K-feldspar overgrowths were recognized on both microcline and perthite grains. SEM photographs of quartz overgrowths reveal the same paragenetic sequence as described by E. Pittman; isolated incipient overgrowths with well-developed rhombohedral and prism faces which coalesce to partly or completely envelop the nucleus. Within the most deeply buried sandstones laumontite occurs as (1) patches interstitial to framework grains; (2) alterations within plagioclase grains; and (3) displacive patches along the cleavage of detrital bio ite. Some interstitial laumontite may be pore filling, but most is believed to be replacement of calcic plagioclases. The analyzed laumontite is a pure Ca-Al hydrous silicate with only minor amounts of sodium, potassium, and iron. The first appearance of laumontite in the eastern and central parts of the basin is at vitrinite reflectivities of 1.1 and 1.3% Ro respectively, corresponding to an estimated paleotemperature of 200°C. Calcite ranges from less than a few percent to almost 50% in some samples and is the latest authigenic mineral, replacing quartz, feldspar, and in rare instances laumontite. The authigenic minerals present in the lower Paleogene sandstones of the Santa Ynez Mountains render them ineffective as reservoirs.

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