Download Granite Weathering and Sandstone Grain Cementation

Granite Weathering and Sandstone Grain Cementation in Jemez Springs, Sandoval County, New
Mexico.
Liz Bunin and Christine Doman
Abstract
The granitic basement rocks of the Jemez Springs area of Sandoval county, New Mexico
have been weathered. In these rocks, hornblende has been replaced by chlorite, which is being
replaced by iron oxides (magnetite). Zoned zircon crystals and cerium-lanthium oxides were also
found in the granite. Nearby, the permian Abo and Yeso sandstone formations are cemented with
hematite cement. Attempts were made to determine the provenance of the Abo and Yeso
sediments, but were inconclusive.
Introduction
Our samples were collected from two locations in Sandoval County, New Mexico. The
granite samples came from an area known as the Guadalupe Box, in a side canyon of San Diego
Canyon. The canyon has steep walls cut by the Guadalupe River. The basement rock of this area
is a Precambrian granite, metamorphosed to various degrees roughly 1.6 billion years ago. The
rock is pink to dark red in handsample and has many crystals large enough to be visible with the
naked eye. These crystalline basement rocks were uplifted and eroded due to extensive regional
faulting related to the uplift of the Sierra Naciamentio; the general complexity of the geology
makes determining the cause of the uplift of this particular section of canyon difficult, though it
can be attributed to the Jemez fault zone (Laughlin and Eddy). The sandstone samples were
taken from an outcrop of Permian age Yeso formation stone in the valley near Jemez Pueblo. The
Yeso formation is a Quartzarenite Aeolian dune formation with evident and extensive
crossbedding.
The presence of iron in both samples and their geographic relationship may indicate that
that the granite could have weathered and contributed iron, quartz or other minerals to the
sandstone. If any continuity between the samples is evidence for this relationship.
Materials and Methods
Thinsection slides were made of each sample. The sandstone was stabilized in resin
before being cut. Our samples were initially analyzed using optical microscopy between 40x and
400x magnification in plane-polarized light and cross-polarized light. This method was used to
preliminarily identify mineral constituents of the granite and sandstone and identify any specific
features to investigate further. The samples were then more thoroughly investigated using the
scanning electron microscope and energy dispersive spectroscopy. EDS in particular allows
specific and precise identification of the elemental constituents of a particular crystal.
Results
The granite sample contains minerals consistent with most granites and with the
descriptions of the basement rock from the literature and the geologic maps (Laughlin and Eddy;
Trimmer 2006). The constituent minerals are feldspar, zoned zircon crystals, clay, chlorite,
magnetite, and trace amounts of cerium-lanthium oxides. The quartz in the granite exhibits
undulatory extinction which indicates that the rocks have undergone some amount of diagenetic
strain. The minerals in the granite exhibit intermediate weathering as feldspars weather to clay
and hornblende weathers to chlorite and then to iron oxide. This process can be seen in figure 1
where the black iron oxide crystals are replacing the chlorite. Figure 2 shows twinning preserved
from a feldspar crystal as it weathers to microcrystaline clay.
The sandstone sample is an aeolian quartzarenite. It contains no feldspar, only quartz and
cement. The quartz grains are rounded indicating transport from a considerable distance. The
grains exhibit unit extinction an so are likely to have not have been subjected to diagenetic strain.
The red color of the rocks is due to the iron oxide cement between the grains. The exact
composition of the cement could not be determined.
Discussions
There was no apparent physical or chemical relationship between the two samples. Iron
oxide cement in the sandstone could be sourced from many places, particularly any overlying
beds contributing dissolved iron through meteoric water (Friedman et al, 1992). The quartz
grains in the sandstone do not display undulatory extinction as do the quartz grains in the granite.
This indicates that the granite has undergone strain where the sandstone has not. Assuming the
granite is the source of the quartz in the Yeso sandstone, the granite would have had to have
been subjected to strain after the quartz was eroded. This is unlikely because the ages of the
rocks suggest the granite was buried at at least 500 meters at the time of deposition of the
sandstone (Goff et al, 1996).
In the granite, the chlorite and magnetite are likely the result of an iron-rich silicate
(likely hornblende) weathering. The clay likely resulted from the weathering of feldspar and
much of the twinning can still be seen in intermediately weathered feldspar grains (Philpotts,
1989; Perkins et al, 2000). Zoned zircons with calcium replacement are consistent with the
crystallization of zircon around a pure core (Liati et al 2009). In the sandstone, the composition
and shape of the grains mean that the sandstone is both compositionally and texturally mature
and suggest that it is extensively weathered and far from its source. This is evidence against any
relationship between the Yeso sandstone and the granite.
Conclusion
Determining the provenance of the Abo and Yeso sandstone formations has been
complicated by the small numbers of samples accessible to us. Additionally, the use of the
electron microprobe would have been helpful in determining the compositions of the feldspar
and clay in the granite, and the iron oxide cement in the sandstone. It is unlikely that the
provenance of the Abo and Yeso sandstones is the granitic basement rock of this region, but
future research could determine this more conclusively. To more certainly determine the
provenance of the Abo and Yeso sandstones, electron microprobe analysis could be used to
analyze trace heavy metal elements in larger sized samples taken from multiple locations at
various depths in the granite and from both the thin, horizontal and larger, crossbeds in the
sandstone. Specifically, attention could be paid to the cerium-lanthium oxides present in the
granite and attempts could be made to source the iron using isotope ratios.
Figure 1
Figure 2
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