![[an image from another site showing the Ouachita orogenic belt]](http://www.lib.utexas.edu/geo/ggtc/figs/fig.1.gif){kind=link}
![[an image from another site showing the situation around the middle of Cretaceous deposition]](http://www.lib.utexas.edu/geo/ggtc/figs/fig.5.gif){kind=link}
This section is dedicated to Mary Stewart,
the only person to ever come by my office to complain about my web site.
To paraphrase, she, and a lot of other people, have wanted to know why the Llano Uplift is "uplifted" and how long it has been that way.
The Llano Uplift is called that because the rocks are "uplifted" relative to rocks of the same age that occur outside of the Llano Uplift. Precambrian igneous and metamorphic rocks that are deeply buried under other rocks in the rest of Texas are exposed in the Llano Uplift. The uplift also exposes a nice rim of lower Paleozoic rocks that are typically hidden under the ubiquitous Cretaceous limestone.
It turns out that there is some disagreement and uncertainty about just how long the Uplift has been an uplift. There is a simple answer which I think handles most of the answer, and then a bunch of complex details that surround it.
Simple Answer: 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. A professor formerly at UT-El Paso (Randy Keller) found a gravity anomaly caused by relatively low density granites in the subsurface. This anomaly is centered on the current 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 "when" is a bit more complex. Since the crust seems to have been thickened during the Precambrian Era around 1.1 billion years ago, the Llano Uplift seems to have been a relative high spot since that time.
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 commonly have increased intrusion of granitic rocks.
Complex Details 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 sides of 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 whole Llano area at this time. 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 showing 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.
(this discussion is short on detail and references in spots, however, I can't afford to spend a lot of time on it, so if anything was going to get put on the Web, this is all it can be for now).
Crustal thickening did occur during the Proterozoic (aka late Precambrian), as evidenced by a positive Bouger Gravity anomaly documented by Randy Keller from UT-El Paso. 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 depth.
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 (Barnes and Bell, 1977), 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 showing no evidence for a specific "uplift".
When the thicknesses of other sediments deposited in the various Cambrian units are examined (Barnes and Bell, 1977) the thickness do not always seem to 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 isopach map which suggests a high spot corresponding 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.
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 positive relief he suggests is only about 9 meters.
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.
[an image from another site showing the Ouachita orogenic belt]
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.
[an image from another site showing the situation around the middle of Cretaceous deposition]
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.
Apatite fission track thermochronology (which I'm not going to explain) shows that the L.U. has not been hotter than 90°C since the Permian (Corrigan and others, 1993). This places limits on how deeply buried the rocks of the L.U. could have been since that time.