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Although many exciting field sites are no longer available, road cuts and parks give access to a pleathora of geology. The material presented in this site is not original. I have not been a stratigrapher since working in Wyoming in the late 1950's so I present the material from the referenced sources. To maintain a flow of text, the original authors are not cited in the text - this is not a scholarly presentation. I have scoured field trip guides, the internet and geology sources to compile information for general comments and road stops (see reference section on sources). I have visited these sites and present original photographs to help guide you to locations. Please feel free to use these photographs, crediting Jack Morelock for the photography.
The organization of material is mine. Each road trip originates at the intersection of highways 29-16-71 in Llano TX and flows outward to the end of the road log. I hope that this site will be useful to geologists and students planning to visit the Llano Uplift. If you have material that you would contribute, I would like to hear from you - email message - morelock
If you object to having material from your paper included here; or want it revised please contact me.
|highway 29 - Llano to Burnet||highway 71 - Llano to Brady|
|highway 29 - Llano to Mason||highway 71 - Llano to Marble Falls|
|highway 16 - Llano to San Saba||Shorter Highway Excursions|
|highway 16 - Llano to Fredericksburg||River Excursions|
|highway 2323 - Llano to Fredericksburg||References & Sources|
It turns out that there is some disagreement and uncertainty about just how long the Uplift has been an uplift. You could say the "why" is because of the granites. The Llano Uplift is an uplift because the earth's crust is thicker there than in the areas around the Uplift. Specifically the crust is both thick and composed of relatively light (probably granitic) rocks at depth. This thick light crust "floats" high on the dense rocks of the Earth's mantle, bringing very old rocks up to the surface.
The reason or reasons why the crust is thicker in the Llano Uplift is more difficult to explain. The crust can be thickened when tectonic plates collide and mountain ranges are formed (as in the Himalayas today). This may have played a part, but it is not clear why this would be localized where it is, instead of being spread out along the continental margin. The Llano Uplift is also the location of a bend in the ancient continental margin, and areas like that have increased intrusion of granitic rocks.
The rocks at the core of the Llano Uplift are 1.35 billion years old. They were buried, compressed and melted, uplifted and faulted. Sediments from old mountain ranges of rhyolitic volcanic rocks and tuffaceous sediments accumulated as a coastal plain and continental shelf to form the Valley Spring wedge. These were covered with dark, fine-grained muddy sediments of the Packsaddle. One billion years ago, these sediments were trapped in a tectonic collision zone and metamorphosed into new crystalline forms to be the Valley Springs Gneiss and the Packsaddle Schist. The collision built a new mountain range and produced magma forming granite batholiths.
Crustal thickening did occur during the Proterozoic (a.k.a. late Precambrian), as evidenced by a positive Bouger Gravity anomaly centered on the current Llano Uplift. However, examination of isopach (unit thickness) maps of sedimentary rocks deposited on the eroded igneous and metamorphic rocks during the Paleozoic (Barnes and Bell, 1977) show a variety of patterns that do not provide clear evidence of the uplift having always been a positive topographic feature, even relative to surrounding areas. However the units deposited in the uplift region throughout time have been shallow water facies. At no time since the Precambrian have the rocks of the Llano Uplift ever been at more than a kilometer or two of water depth. One consistent feature of the rocks deposited throughout the lower Paleozoic is that, where it is possible to interpret a depositional setting, the rocks are shallow marine (ocean) to terrestrial (land) in origin.
In the Cambrian, the Hickory Sandstone was deposited on the eroded surface of the Proterozoic rocks. The lowest portions of the unit are coarse-grained fluvial and alluvial deposits. Local topographic variations of the surface of the Proterozoic of up to 243 m. [800 ft.] have been documented, and account for much of the local thickness variation of the Hickory Sandstone. Deposition of the Hickory Sandstone was uneven, and some areas of Proterozoic rock were high enough not to be covered by the Hickory Sandstone. However, the variations seem local in nature, and overall the thickness of the Hickory Ss. thins to the northwest with no evidence for a specific "uplift".
When the thicknesses of other sediments deposited in the various Cambrian units are examined the thickness do not always bear a relation to the uplift area as delineated by the gravity anomaly. The Lion Mtn. Sandstone of the Cambrian Riley Fm. is the first unit that has an iopach map which suggests a high spot correponding to the current uplift shape. In younger units, the Cambrian Wilberns Fm. as a whole does seem thinner over the Llano Uplift, but the Ordovician Ellenburger Fm. has a more complex pattern. Rocks of the Llano Uplift seem to have been re-uplifted, as it were, several times after the Cambrian.
Adams (1954) suggested that in the Ordovician there existed a Florida-like peninsula running NNW-SSE in central Texas. He called it "the Texas Peninsula". Deeper water and thicker sediments occurred off the peninsula. However, the peninsula he suggested was NOT centered on the area defined by the gravity anomaly, the current Llano Uplift was on the east flank of the peninsula. Erosion of the Ordovician Ellenburger Fm. suggests tilting of the Llano area at this time.
Kier (1980) describes the Pennsylvanian Marble Falls Fm. as being a carbonate bank centered on the current area of the Llano Uplift. However, the amount of relief he suggests is only about 9 meters.
During the Ouachita Orogeny (mountain building event), which was most intense east and south of the Llano Uplift, the uplift was cut by a series of NE-trending faults with normal to oblique slip. The large grabens (fault blocks) of Paleozoic rocks which strongly influence the current topography were formed during this time. The Llano Uplift occurs at a bend in the orogenic belt. What role, if any, this orogeny played in elevating the basement rocks is unclear, but it probably contributed.
Units as old as the Precambrian were being eroded at the start of Cretaceous deposition, and the erosion products form a series of basal sandstones as the ocean prograded over the paleo-Llano Uplift. Bear Mtn., north of Fredericksburg, is an area where a Precambrian granite hill is surrounded by Cretaceous limestone. This area shows that the limestone was deposited around what was a granite hill (island) in the Cretaceous as well. Cretaceous limestones were thick enough to completely cover the all the older rocks probably to around 1,000 feet.
Along with all of the rocks west of the Balcones Fault Zone, the Llano Uplift was brought up in relative elevation when the Balcones Fault moved during the Cenozoic. This increased erosion, which once again exposed the Proterozoic rocks.
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