Abstract Central Texas contains Precambrian rocks, which were exposed due to the Llano uplift. This paper looks further into the Devil’s Waterhole of Ink’s Lake to get a better understanding of the area. First the geology of the Llano uplift is studied to get a better understanding of how the rocks in the area were formed. The rocks in the area, which consists of Valley Springs Gneiss and Town Mountain Granite, are studied even further to see the composition of each and how each was formed. The Valley Spring Gneiss is split into amphibolite, biotite gneiss, quartz-feldspar gneiss, and quartzite. Each of the five different rock samples were observed and studied in the area to give us a better understanding of their compositions, how they were formed, and what their protoliths are.

The Devil’s Waterhole of Ink’s lake is the area of focus in studying Precambrian rock. Throughout the Ink’s Lake region we will take focus on the Precambrian metamorphic rock that was exposed due to the Llano uplift. The Llano uplift contains some of the oldest rocks in North America and is exposed in Ink’s Lake, Texas. This is exposed due to the oceanic-continent collision, which drove the Llano uplift upward. After the weathering away of the younger rock on top, the currently exposed metamorphic rock is more easily observed. Following the Llano uplift exposure, there was an intrusion of the Town Mountain Granite, which we will also look further into. We also observe the areas reactions such as the development of the sillimanite and intrusions of pegmatites and dikes. A better understanding can be achieved with a description of the Llano uplift.
The Llano uplift consists of metavolcanic, metaplutonic, and metasedimentary rocks that have been polydeformed with a moderate to high-pressure and is approximately 1360 to 1232 ± 4 Ma, and consists of upper amphibolite to lower granulite facies regional metamorphism (Mosher, 1998). In the area of the Devil’s Waterhole of Ink’s Lake we will look more closely at these upper amphibolite facies as well as the gneiss and other Precambrian rocks in the area. These rocks were intruded by younger syntechtonic to post-tectonic granites, approximately 1119 +6/–3 to 1070 ± 2 Ma (Mosher, 1998). The Llano uplift’s geology is in its name. The term uplift comes from the fact that the Precambrian rock strata were previously buried, eventually uplifted, and the younger strata on top were eroded away, thus exposing the Precambrian rock. Figure 1 on the next page will help us further understand what happened to develop the Llano Uplift. Figure 1 (Morelock, 2005)
We can see that approximately 1.3 billion years ago the Valley Spring sediment gathered just on shore. 0.2 Billion years later the Packsaddle sediments were deposited and buried the Valley Spring sediment. Due to ancient tectonic activity, in this case a subduction, the oceanic crust drove itself under the continental crust and uplifted the two sedimentary deposits. This subduction allowed for the development of the Valley Spring gneiss. Also, rising magma later developed the younger granitic intrusion. Understanding how the Llano uplift was formed also provides us with a little in sight on how the rocks in the area were developed. The region was metamorphosed under conditions of relatively high temp and low pressure, which was caused by folding and faulting of the region followed by the intrusion of granite (McGehee, 1963). This further solidifies our thought process on Figure 1. From here we can further discuss the rocks in the area as well as what was observed in the Devil’s waterhole region.
While mapping the area of the Devil’s Waterhole, we came across three sections that had an important significance worth noting and mapping. Section one of our map lays in the western section of the Devil’s Waterhole. This was located at the center of the river bed where Biotite Gneiss was noted. From this point we can see that the nearby strike and dip measurements are dipping in opposite directions. Upon closer examination of section one, we can see a small yet impactful anticline. The anticline consists of Biotite Gneiss and is only observed to be two feet tall. It’s noted to be impactful because as mentioned above, the surrounding strike and dips are dipping away from the anticline up to approximately three hundred feet away. The metamorphic rocks of the Valley Spring Domains are generally deformed (Mosher, 1998). This anticline was developed by the process that created the Llano Uplift, which caused much folding and faulting of the area.
Station two is approximately 400 feet east of station one, and still lies in the center of the river bed. It’s at this point that there is a pigmatitic boundary between gneiss and granite that is worth noting. The gneiss shows specs of granite within it. The meaning behind the pigmatitic boundary is that the section heated up at some point high enough to melt the protolith, granite, which in turn created these blobs of granite that are mixed in the gneiss. In the Valley Spring Domain, the protoliths are mainly granitic in composition and are usually volcanic or plutonic granitic rocks (Mosher 1998). It is also further understood that the area consists of protoliths such as rhyolite and arkosic sediments (Rougvie et al, 1997). Due to the granitic composition we can further conclude that at this station we are observing a pigmatitic boundary between the gneiss and rhyolite, which was caused by the granitic intrusion some time after the uplift.
Station three was approximately nine hundred feet northwest from station two. Here it was noted that a quartzite outcrop contains mats of sillimanite and muscovite. It is know that sillimanite is created through the chemical reaction: muscovite + quartz ↔ sanidine + sillimanite + water. This reaction shows that the quartz and muscovite reacted at very high temperature and pressure to create the sillimanite. We know that it was under high pressure and temperature because sillimanite will only form under these conditions. Muscovite was also noted in these mats, which could be created through the process of a retrograde reaction. As the quartzite cooled and the pressure was released slowly, it approached a more stable condition. With the presence of water there is a rare case that a retrograde reaction could occur to revert the created sillimanite back into the muscovite parent. Another possibility is that all the muscovite did not completely turn into sillimanite; therefore we see both muscovite and sillimanite in the same quartzite sample. It is also noted that there is development of sillimanite in an amphibolite sample near station two.
I observed that there was a section of amphibolite near station two that also seems to contain sillimanite as well. Here the situation is quite different than that of the quartzite because sillimanite developing in quartzite under the right conditions is more common than that of amphibolite. My conclusion to such a case is that there must have been an inclusion of minerals from the surroundings such as a dike. There were no faults nearby, however there is a quartzite outcrop right next to the amphibolite section that was being observed. There is likelihood that the minerals needed for the sillimanite to develop were transferred from the quartzite to the amphibolite to help create the sillimanite. The development of the Llano Uplift was already discussed and it can be seen how the uplift affects the geology of the area. Now let us take a look at the types of rocks present in the Devil’s Waterhole due to this Llano Uplift.
Valley Spring Gneiss and Town Mountain Granite were all observed in the area. Of the Valley Spring Gneiss there were amphibolites, biotite gneiss quartz-feldspar gneiss, and quartzite. Village Spring Gneiss is well exposed all along the shore of Ink’s Lake Devil’s Waterhole. The first to be mentioned was the amphibolite. Hand samples appear dark grey to black in color and are coarse grain in size. It is noted that the rocks seem to be 80% amphibole, 10% feldspar, and about 10% hornblende. The area did not consist of near as much amphibolite as it did with the gneiss. No slides were provided of the amphibolite as I did not view any amphibole in any of the samples.
The biotite gneiss was pink in color with a black to light grey foliation in it and is also noted to be coarse grained. A hand sample of the gneiss seems to be approximately 70% feldspars and 30% micas. This was further reviewed in thin sections. On average the grain sizes were about 0.30mm. Of the many slides provided there seems to be on average 30% K-feldspar, 30% quartz, 20% plagioclase, and 20% other minerals such as chlorite, biotite, and muscovite. A couple slides can be viewed in Figure 2, 3, and 4 in which the pictures were taken in lab: Figure 2 Figure 3
Figure 4
The k-feldspar can be seen in Figure 2 with the tartan twinning present. In Figure 3 and 4 there is a lot more activity with much more to see. We can observe the biotite and muscovite as well as chlorite. The difference between the two samples is that Figure 2 is that of quartz-feldspar gneiss while Figures 3 and 4 are biotite gneiss. The composition of the quartz-feldspar gneiss is about 60% quarts and 40% feldspar. The composition of the biotite gneiss is about 60% quartz, 20% plagioclase, and 20% micas. The protolith of the granitic gneiss would be that of rhyolite due to its composition. The biotite gneiss protolith will be that of rhyolite as well as other sedimentary inclusions, which is what gives us the biotite, muscovite, and chlorite in the present samples. The quartzite hand samples were reddish pink in color with a shade of white. The shade of pink must come from a source of iron. It is nonfoliated and fine grained. The composition was quite difficult to determine until the slides were observed. A quartzite slide is shown below in Figure 5, in which this picture was also taken in lab: Figure 5
From the slide we can see that the composition is approximately 60% quartz and 30% k-feldspar with 10% micas. The crystal size is about 0.5 mm in width. The protolith of the quartzite must have been some sort of quartz sandstone, which was heated up and morphed into the samples observed today. The Town Mountain Granite is the youngest of the Precambrian rocks that were studied in the area. It was pink in color with a hint of black shading, which was most likely caused due to weathering. A fresh surface indicates that the granite is mainly pink in color. The little foliation that is observed seems to be contorted, but for the most part there is little to no foliation to be noted, which is the case since it is a part of the granitic intrusion that occurred after the uplift. The slides were observed to help get a better idea on the composition of the granite. Figure 6 is a picture of the Town Mountain Granite slide that was taken in lab: Figure 6
The composition is about 50% k-feldspar, 40% quartz, and 10% biotite. Grain size is coarse grained and about 0.5 mm in size. The protolith of this granite can very well be rhyolite. After the rhyolite intruded the Llano uplift and reacted with its surroundings, it became exposed and out Town Mountain Granite formed. The Devil’s Waterhole, a part of Ink’s Lake of central Texas, contains much of the evidence of the tectonic activity that occurred with the Llano uplift. From the anticline to the development of the sillimanite in the parts of Valley gneiss spring, we can see that there were times of high pressure and high temperatures. The further understanding of the Llano uplift helped solidify some of the thought processes on how some of these things were formed in the mapping area. Also understanding the process of the igneous intrusion, the Town Mountain Granite, we can see why and how the pigmatitic rock of gneiss and granite formed. The protoliths of the rocks observed became more easily determined as well.

Bibliography
McGehee, R.V., 1963, Precambrian Geology of the Southeastern Llano Uplift, Texas [Dissertation]: Austin, University of Texas.

Morelock, J, and Ramirez, W. 2004. "Llano Uplift Mineral page." Web page. Available at http://geology.uprm.edu/Morelock/thcgeol.html

Mosher, S., 1998, Tectonic evolution of the southern Laurentian Grenville Orogenic Belt: Geological Society of America Bulletin, v. 110, p. 1357-1375.

Rougvie, J.R, Carlson, W.D., Copeland, P., Connelly, J.N. 1997, Late Thermal Evolution of Proterozoic Rocks in the Northeastern Llano Uplift, Central Texas: Precambrian Research, 94, 49–72.