According to the literature, the Trujillo sediment was material
transported from the northmost Texas panhandle and from the
Triassic-age mountains of central and south Oklahoma. Because of
the prozimity of those mountains, the sediment is relatively immature,
containing mica and
feldspar along with significant rock fragments. The mica is
mostly restricted to the shales.
I took two small hematite cones, a fat one and a thin one to study
the nature of the rock they formed in. Small cones were chosen as
they were much easier to cut into without access to good rock cutting
equipment. Because the analysis was done on only two samples it
is not thorough enough to conclusively quantify the rock but does
provide some indications of rock properties and variation as ot relates
to the cones. The two cones are pictured here.
In crossection, the two cones showed a characteristic difference
usually seen between fat and thin cones, notably the degree of hematite
cement. These views show
crossections in both indirect and direct
reflected light. As seen, thin cones have less hematite.
Crossections of larger thin cones usually show concentric three
dimensional cones but I have no photos of these at present.
A close look at the tips of each cone hints at their properties. The
sandstone of each is very immature with median grain sizes around the
0.5 through 1.2 mm. size. Sorting is fairly poor, grains are
subangular usually, sometimes subrounded. See photos here.
Contrast between the two cone
types are reflected in contrasts in the rock properties; sorting,
roundness, and sediment maturity. Thin cones have grains which
are subangular to subrounded, moderately sorted, and quartz content of
40 to 70 percent. Fat cones have subangular poorly sorted grains
composed of 20 to 50 percent quartz. See the cone tips here.
Within the fat cones The grain mineralogy can be seen. This image
shows the high percentage of pink potassium feldsapar found in fat
cones. There is so much feldspar, the rock sparkles in strong
light, giving a false indication of mica. The sparkle is cleavage
surfaces of the feldspar grains, mica occurring mostly in the shales
much less abundant in the cone-bearing sandstones.
Within the thin cones there is much less evidence of feldspar.
This view of a thin cone interior
shows the decreased feldspar presence, lower hematite amounts,
and lower matrix material in general.
This look at only two cones supports the assumption thin cones are
formed in rock of better vertical permiability than the rock containing
fat cones. The better vertical permiability however looks to be a
result of better sorting and cleaner, more mature lower matrix
sandstone, rather than grain size.