LAB 1. TECTONIC STRUCTURES OF TEXAS

The purpose of this lab is to build on the material on global geomorphology covered in the last lecture and to provide examples of large-scale tectonically-influenced regions in Texas. The lecture illustrated that continents have "global-scale" tectonic regions, such as orogenic belts, coastal plains and basins and domes. In this lab, we focus down to a smaller area - the State of Texas (still pretty big!). Texas also has large-scale tectonic regions that create landscapes with distinctive geomorphology - compare, for example,  mountainous far-west Texas (part of an orogenic belt) with the flat, low-lying area around Houston (part of a passive margin coastal plain).

This lab introduces the large-scale tectonic structures of Texas by examining aspects of the geological evolution of the state over time, from the Precambrian (approximately 1.1 billion years ago) to the present-day.

Figure 1. Tectonic Map of Texas. 

The Llano Uplift (find this feature on Figure 1).
Old cratonic rocks of the North American craton are exposed in only a few places in Texas. The most prominent, and a dominant geologic feature of the state, is the Llano Uplift in the center of the Texas Hill Country (Fig. 1). The uplift is equivalent to a dome (part of the craton that is high enough to stick up through surrounding platform rocks). Part of the Llano Uplift forms Enchanted Rock - a granite exfoliation dome (Fig. 2).

       
Figure 2. Enchanted Rock. This Precambrian (Before the Cambrian Time Period or before ~542 million years ago) granite (1.1 billion years old) is part of the North American craton exposed in the Llano Uplift (see people on top for scale).

It appears that the craton is thicker here than in the surrounding region. This is why this part of the state is an "uplift" (the thicker continental crust "floats" higher on the underlying mantle).

The Ouachita Tectonic Front (find this feature on Figure 1).
The Ouachita Tectonic Front is an orogenic belt formed during the Ouachita Orogeny (an orogeny is a period during which orogenesis occurred; Fig. 1). This orogeny resulted from the collision of North America and South America in the late Paleozoic Era (about 290 million years ago). The collision formed a large mountain chain along the southern margin of North America (Fig. 3). 

Figure 3. The collision of North America and South America about 290 million years ago. 

Although much of these mountains are now eroded and buried, remnants form the Ouachita Mountains of Arkansas/Oklahoma and the Marathon Mountains of Texas (Fig.4). 

Figure 4. Shaded Relief Image of Texas. OM: Ouachita Mountains; MM: Marathon Mountains; EP: Edwards Plateau; B&R: Basin and Range; PB: Permian Basin; LE: Llano Estacado

The Balcones Escarpment (coincides with the Balcones Fault Zone - find this feature on Figure 1).
The Balcones Escarpment (a relatively steep "step" in the landscape) marks the line of contact between North and South America during the Ouachita Orogeny. The Balcones Escarpment separates the uplifted region to the northwest of the Balcones Fault Zone from the subsided region to the southeast (Fig. 1). Vertical displacement between these two regions has occurred along a series of high-angle normal faults that probably formed during the Ouachita Orogeny, the rifting of North and South America following the orogeny and subsidence of the Gulf Coastal Plain due to sediment loading. In general, rocks to the northwest of the escarpment are more resistant to erosion than rocks to the southeast and so differential erosion also has played a role in creating about 300 feet of elevation change across the escarpment (Fig. 5).

Figure 5. Satellite view to the southwest across Texas, showing Austin (A) and San Antonio (B) at the base of the Balcones Escarpment (arrowed). Corpus Christi is seen in the distance (C).

The Edwards Plateau (find this feature on Figures 1, 4, 5 & 6).
The Edwards Plateau (Fig. 4) is a platform region capped by hard Cretaceous (~100 million years old) limestones. Some of the relief of this region is due to uplift along the Balcones Fault Zone, some is due to the resistance of the limestones to erosion (Fig. 6).

Figure 6. View from the top of Enchanted Rock. Llano Uplift Precambrian granite outcrops are seen to the right; the far horizon (EP) is the edge of the Edwards Plateau (Note: cratonic rocks of the Llano Uplift are exposed in a topographic basin within the Edwards Plateau).

The Permian Basin (find this feature on Figure 4).
During the Permian Period, about 280 million years ago, the Ouachita mountains bordered the eastern margin of several inland marine basins, including the Midland, Delaware, and Val Verde basins (see Figure 1 for the locations of these basins), which now collectively compose the oil-rich Permian Basin of West Texas (Fig. 4, PB; Fig. 7). These basins filled with marine, fluvial, deltaic and evaporite deposits, including gypsum, forming layers of sedimentary rocks that cover the craton in this area and also make this a platform region (Fig. 8).

Figure 7. View of the Guadalupe Mountains on the Texas/New Mexico border from the Permian Basin. The mountains are a fringing limestone reef that surrounded an inland sea.

Figure 8. Road cut exposing gypsum deposits in the Delaware Basin. Gypsum is an evaporite, formed by evaporation of sea water under a hot climate and usually where water circulation is restricted - for example, where mountains partially enclose an inland sea (these conditions existed in this part of Texas during the Permian Period).

The Basin and Range (find this feature on Figure 4).
Far west Texas lies in the Basin and Range Province of western North America. This region (Fig. 4, B&R) is part of the Laramide Tectonic Front (an orogenic belt). This orogeny, resulting from plate convergence along the western margin of North America, formed many of the major structures of the Rocky Mountains during the early Paleogene Period (30-60 million years ago). Orogenic activity also led to the emplacement of many volcanic rocks in the Big Bend region and the formation of numerous faults - movement along these faults created the basin and range topography (Fig. 9).

Figure 9. Numerous fault-block ranges (uplands) in Big Bend National Park.

Llano Estacado (find this feature on Figure 4).
During the middle to late Cenozoic Era (5-30 million years ago), streams originating in the recently elevated Rocky Mountains to the west transported large volumes of sand and gravel into the Texas Panhandle, which accumulated in large alluvial fans - an example of backarc basin sedimentation. These deposits form the Southern High Plains (Llano Estacado) - a nearly flat plateau with an average elevation approximating 3,000 feet (Fig. 4, LE). Between 5 and 10 million years ago, the sediment supply was cut off due to regional uplift in the west and the eastern edge of the plains began to retreat westward as rivers draining into the Gulf of Mexico caused headward erosion, as exemplified by Palo Duro Canyon (Fig. 10). The resulting escarpment (the steep cliff at the edge of the plateau) that extends in a north-south line in West Texas and the Panhandle is the Caprock Escarpment (Fig. 11). To the west, the Llano Estacado is bounded by the Mescalero escarpment, overlooking the Pecos Valley (Fig. 12). 

Figure 10. Palo Duro Canyon, carved by headward erosion of rivers cutting back into the Llano Estacado. 

   

Figure 11. The Caprock Escarpment in the Texas Panhandle (arrowed). The Llano Estacado lies to the left (west), numerous canyons and gullies mark the Caprock Escarpment and the eroded plains to the right (east). 

   
Figure 12. The Mascalero Escarpment (arrowed) on the western edge of the Llano Estacado in New Mexico (the Caprock Escarpment is a similar feature).

The Gulf Coastal Plain.
The Gulf Coastal Plain (Fig. 1) consists mainly of Cenozoic (post-65 million years ago) clastics (mud and sand) and is an example of passive margin sedimentation. Numerous faults parallel the coast, formed partly by rifting as North and South America separated after the Ouachita Orogeny, partly by Cenozoic sediment loading (Fig. 13).

Figure 13. Cross section of south Texas from the Llano Uplift to the Gulf of Mexico. LU: Llano Uplift; OOB: Ouachita Orogenic Belt; BFZ: Balcones Fault Zone; T--->: Base of Cenozoic sedimentation; C: coastline. Numerous high-angle faults cut the section (lines with arrows). Green columns are salt domes.

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