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 In these rocks, hornblende has been replaced by chlorite, which is being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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