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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 or commercial 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 and many lines of excellent prose from the original authors has been rewritten in my stilted style to maintain a continuity. 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 have also included the reverse mileage from the end point back to Llano. 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
I hope that I have offended no author by changing and using their material - I have tried to include all sources in the reference, if I missed yours, please tell me. If you object to having material from your paper included here; or want it revised please contact me.
This site is under construction.
Some of the trips have been logged, milage, photos and GPS done and the material organized and revised. Some of the trips will not even open. Since the class will travel on some of these roads, I thought that you might like a guide to the roadside geology - even incomplete.
Mileage in bold is Llano to the end point, i.e. Burnet - mileage in regular type is from the end point to Llano. Yes, that means start at the bottom and work up.
An earlier version of this field guide is at: UPRM Geology Department
The rocks and a large amount of mineralization has resulted in production of building stone, talc, graphite, serpentine, iron ore, copper, lead, silver, and gold in the past, the area is known as the "Central Mineral Region." In reality, despite a variety of minerals having been formed, mining has been something less than successful. Building stone has been the main output from this region and granites, sandstones and limestones have been used in construction projects.
The Earth's crust is thicker and composed of relatively light rocks under the Llano Uplift. This thick light crust "floats" high on the dense rocks of the Earth's mantle, bringing very old rocks up to the surface. The crust probably thickened when tectonic plates collided to form mountain ranges, but these are usually linear. It is the location of a bend in the ancient continental margin, and areas like that have increased intrusion of granitic rocks.
The preCambrian core rocks are more than 1.3 billion years old. Sediments from mountain ranges of rhyolitic volcanic rocks and tuffaceous sediments accumulated as rivers discharged onto a coastal plain and continental shelf to form the Valley Spring wedge. In turn these sediments were covered with dark, fine-grained muddy sediments that were the Packsaddle. These sediments were in a tectonic collision zone more than one billion years ago and metamorphosed the Valley Springs Gneiss and the Packsaddle Schist. The collision built a new mountain range and produced magma to form granite batholiths.
The Hickory Sandstone (Cambrian) was deposited on the eroded surface of the preCambrian rocks. The bottom sections are coarse-grained fluvial and alluvial deposits. The preCambrian surface has topographic variations of up to 800 feet, resulting in variations in thickness of the Hickory Sandstone. In some places the preCambrian rocks were high enough to stand above Hickory Sandstone deposition. These are local variations and the overall thickness of the sandstone does not suggest uplift.
The Lion Mtn. Sandstone of the Cambrian Riley Fm. is the first unit that has isopach variations that suggest a high spot correponding to the current uplift shape. The Cambrian Wilberns Fm. also is 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 several times after the Cambrian.
During the Ouachita mountain building episode, the uplift was cut by a series of NE-trending faults with normal to oblique slip. The large graben fault blocks of Paleozoic rocks were formed.
Units as old as the Precambrian were being eroded at the start of Cretaceous deposition, and the erosion produced a series of basal sandstones as the ocean advanced over the paleo-Llano Uplift. The Llano Uplift and all of the rocks west of the Balcones Fault Zone were uplifted when the Balcones Fault moved during the Cenozoic. Increased erosion once again exposed the preCambrian rocks.
| Mesozoic Era 248 to 66 mya |
Cretaceous 66 to 144 mya Lower Cretaceous rocks virtually blanket the center half of the state, and limestone cliffs, caverns, canyons, springs, abundant fossils, and dinosaur tracks are all part of the "Cretaceous scene." The continents continued to pull apart in Cretaceous time. The Rocky Mountains underwent their major push (the Laramide orogeny) and shallow seas on the new continents' margins advanced and retreated repeatedly. Some sea advances filled the trough in front of the Rocky Mountains, creating a connecting seaway all the way from the Artic Ocean to the Gulf of Mexico. The shallow Cretaceous seas over Texas were filled with calcareous-shelled organisms, and thick deposits of limestone were laid down. On the sandy shorelines and mudflats of these seas dinosaurs roamed freely, leaving evidence of their passing in fantastic fossilized footprints and trackways all across Texas. The Hill Country around San Antonio and Kerrville is carved in Cretaceous rocks, as are the Colorado River Canyon north of Austin and the Devils River-Rio Grande Canyon west of Del Rio. |
| Paleozoic Era 543 to 248 mya The Paleozoic is bracketed by two of the most important events in the history of animal life. At its beginning, multicelled animals underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90% of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy. Roughly halfway in between, animals, fungi, and plants alike colonized the land and the insects took to the air. During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole. These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Large limestone outcrops are evidence of these periodic incursions of continental seas. |
Pennsylvanian (323 to 290 mya) Ordovician (490 to 443 mya) The Ordovician period began approximately 490 million years ago, with the end of the Cambrian, and ended around 443 million years ago, with the beginning of the Silurian. At this time, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern super-continent Gondwana. Throughout the Ordovician, Gondwana shifted towards the South Pole and much of it was submerged underwater. Extensive dolomite and limestone deposits, with less extensive chert deposits, were laid down in shallow seas that covered Texas in Ordovician time. Remnant Ordovician outcrops are best seen in the Llano uplift northwest of Austin.
Cambrian The Cambrian Period marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. |
| Precambrian Time Proterozoic Era (2500 to 543 mya) The geologic record begins in Texas a little over a billion years ago, when thick sequences of coarse, then fine sediment were dumped into an ancient sea bordering a continent. Eventually the continent collided with either another continent or an ocean margin in a plate tectonic event that buried, squeezed and heated the borderlands, including the sediment piles. The collision built mountains and created metamorphic schist and gneiss out of the deeply buried sediments and generated molten magmas which cooled to form granite bodies. Erosion then flattened this range to a table-top by Cambrian time. All these rocks are displayed in the Llano country of central Texas. |
Town Mountain Granite The Town Mountain Granite suite (TMG) of the Proterozoic Llano Uplift of central Texas consists of numerous voluminous intrusions with similar characteristics and ages. TMG is generally pink, very coarse grained, porphyritic granite with accompanying pink coarse-grained non-porphyritic granite. Mineralogically, TMG consists primarily of plagioclase feldspar, potassium feldspar (microcline), and quartz with biotite and/or hornblende Packsaddle Schist The Packsaddle Schist is one of the two major Precambrian metamorphic units in this area. Isotopic ages for many of these rocks fall in the range of about 1150 Ma to 1170 Ma (mid to late Proterozoic) which is approximately equivalent to Grenville age rocks from the eastern U.S. and Canada. The metamorphic rocks throughout the Llano Uplift are folded into broad anticlines and synclines that generally plunge gently to the southeast. The foliation is generally parallel to lithologic layering and here it can be seen that the layers are dipping to the southeast. Crenulations on the foliation layers trend parallel to major fold axes and can be clearly noted in the roadcut. As originally described, the Packsaddle Schist measured over 20,000 feet thick and consisted of various rock types formed by low pressure, medium grade metamorphism. There are many different rock types within this unit, including: graphite, mica, amphibole, and cordierite schist; marble; quartz-feldspar rocks (commonly referred to as "leptites"); various syntectonic meta-igneous gneisses; and serpentinite with metagabbro. Valley Spring Gneiss Valley Spring Gneiss shows the greatest variety in lithologic composition. Quartz-feldspar-mica gneiss, quartz-feldspar-epidote gneiss, quartz-feldspar-amphibole gneiss, and quartzite alternate with mica schist, amphibolite, calc-silicate rock, and marble. The rocks range from very well foliated to poorly foliated. They seem to originally have been a complex mixture of volcanic and intrusive rocks with interlayered sedimentary rocks. The total thickness is unknown, but probable exceeds 20,000 feet. |
The landforms that make the hill country so attractive are the result of tectonics that have shaped and formed the rocks, and the rocks and climate acting on them to erode and reshape the landscape. On the left, below, you can see the changing continent pattern. The movements produced the tectonics. On the right, is the change in geomorphology produced by erosion of the broad Edwards Plateau.
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The basic geologic patterns across the state have a strong bearing on other environmental elements such as weather, topography, and vegetation. The observed topography is the product of two factors, rocks and erosion. Guadalupe Peak, in west Texas, stands high because the rocks were uplifted, but equally important, because the reef limestones are hard, and resist erosion better than surrounding rocks. The topography of the uplifted, north-south ranges in West Texas stands out clearly because of the faulted structure of these ranges, but again, they contain hard rocks which differentially resist erosion. It is also easy to spot the distinctive dome-like topography of the volcanic intrusions around Big Bend National Park. The hard basalt bodies started out roughly dome-shaped, and erosion has had a tougher time attacking these than the less resistant shales surrounding them. This interaction of fundamental geologic structure with erosion to create topography is repeated many times across Texas.