Diagenesis and Porosity
Evolution of the Flathead Sandstone (Middle Cambrian), Wyoming and Montana
Ozkan, Aysen1,
Earle F. McBride2 (1) University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX
The Flathead is a
shallow marine sandstone deposited during middle Cambrian transgression
on the Precambrian basement rocks over much of Wyoming and southern Montana. It is composed of
coarse to medium grained sandstones. Polycrystalline quartz types, feldspars, biotite, metamorphic and granitic
rock fragments indicate that underlying crystalline rocks and similar rocks
outside the basin of deposition were the major source rocks. Composition ranges
from quartzarenite to arkose
(average Q92 F7.4 R0.6). The Flathead has
undergone physical and chemical compaction, recrystallization,
dissolution, oxidation, reduction, grain fracturing, and generation of
different authigenic cements and replacement minerals
in the course of burial from 5 to 7 km. Authigenic
phases include quartz, Fe-oxide, kaolinite, calcite,
siderite, ankerite, dolomite, chalcedony, and opal,
with trace amounts of K-feldspar, pyrite, Ti-oxide, barite, and tourmaline.
According to fluid inclusion data from Teton Pass locality, minimum
estimate of Tmax (maximum formation temperature)
reached is 150-160 oC. Silica for the
quartz cement was derived mainly from intergranular
pressure solution, and stylolites, and also
dissolution of silicate framework grains. S/I transformation within the shale
layers and overlying formation were minor silica sources. Oxygen and carbon
stable isotope data, microprobe analyses, textural, and CL studies indicate
that calcite and dolomite cement formed during deep burial, but later were recrystallized in meteoric water. Total porosity ranges
from absent to 11.5% and averages 3.6%. Calculations indicate that compaction
was more important than cementation in reducing the porosity of the sands.
Permeability ranges from 0.5 to 439 mD, with a
geometric mean of 5.8 mD.