by Keith Young
Scattered over Central Texas at a number of localities are remnants of a terra rossa (paleosol). Usually, the A-horizon and upper part of the B-horizon have been removed by erosion. But the B-horizon, especially its lower part, can be seen in many areas. As is typical of terras rossas, the base of the profile seldom has a C-horizon, is usually diagenetically altered, and is often truncated. The age of formation of the Central Texas Terra Rossa is between 0.73 and 2.0 m.y. B.P.
Figure 1: Four areas containing major outcrops of Central Texas Terra Rossa. Terra rossa is not continuous since it has been completely removed by erosion in some areas. Silicified fossils and remnants of red clay indicate that terra rossa was once very widespread.
Figure 2: Profile of Central Texas Terra Rossa overlying Person Formation on Loop 1604 with U.S. Highway 287. The A-horizon and the upper part of the B-horizon have been removed by erosion.
Figure 3: Alignment of sinkholes along faults. Many of these sinkholes have been plugged with red clay from the terra rossa and are full of water at least during wet weather. This figure is somewhat reduced from that of the U.S. Army Corps of Engineers (1933). This area is part of the high karstic plain of Woodruff and Abbott (1979) and lies between the Blanco River and the Guadalupe River.
Figure 4: Doline on the old Wegner Ranch. This is one of the sinkholes along the Freeman Ranch-Bear Cree Fault Zone just off Purgatory Road, Comal County, Texas.
Figure 5: Section of Person Formation along Loop 433 South, New Braunfels, Comal County, Texas. The rock is approximately 50 % cave-deposits.
Figure 6: Etched and embayed chert nodule from the Fort Terrett Formation, about 20 feet below the collapse zone of the Kirschberg Gypsum, 2.0 to 2.5 km. southeast of Junction, Kimble County, Texas, on Interstate 10.
Figure 7: Two views of geologic organs (solution cavities and widened fractures filled with red clay derived from overlying terra rossa) in the Person Formation, north San Antonio, along Loope 1604 just east of its intersection with Interstate 10, Bexar County, Texas.
Larger images: (A; 227K) : (B; 217K)
Relict to parts of Central Texas is a terra rossa. Perusing the literature, I was surprised to discover how seldom it is mentioned, even in theses that were mapped in areas with terra rossa. The purpose of this paper is to increase interest in this terra rossa by discussing its distribution, geology, and age. Only incomplete conclusions can be drawn, because of insufficient study.
The concepts of terra rossa (from the Italian term "terra rossa," terre rosse, pl.; sometimes misspelled "terra rosa") have varied considerably over the years since the first discussion of the subject by Fuchs in 1875, some 26 years after the term was introduced in the Italian literature (Joffe, 1949). Terras rossas in parts of the United States have recently been discussed by Ruhe (1975), Hall (1976), and Quinlan (1978), but before this the attitude of American pedologists had generally been one of ignoring the term, because in their opinion the term was used inconsistently (Baldwin et al., 1938). But if that were a good reason, we would not be able to use any early names in many fields.
Terras rossas were first named and described in Italy (Joffe, 1949). The present Mediterranean climate is xerophytic in many areas where there are now terras rossas, and even the rendzinas in Serbia occur in a climate with only 800 to 900 mm of rainfall per year (Tanasijevic et al., 1966). This is true also of terras rossas (sometimes called rendzinas) of Spain.
Joffe (1949) suggests that the true terras rossas developed on limestone are paleosols formed under a more humid climate, usually considered to have had 1500 mm or more of rainfall per year, although even this is not a consistent figure among various authors. Whether Joffe's usage is the first suggestion of paleosols for these terras rossas I do not know, but Joffe (1949) gives credit for the idea to no one before him, and his historical summary, though short, is fairly good. Shaw (1974) refers to the Central Texas Terra Rossa as "residual terra rosa," and Quinlan (1978) refers to the midcontinent (United States) terras rossas as paleosols, even though they are interpreted to have had fluvial products added to the residual insolubles of the limestone.
Duchanfour (1970) uses the term terra rossa only for paleosols formed on limestone terrains under a climate of greater rainfall and higher temperature than the present Mediterranean climate. Much past confusion in America seems to have resulted from trying to produce these soils within the present climatic regime of a particular area. Duchanfour (1970) lists several characters of terras rossas:
1) A1- and A2B-horizons are frequently removed by erosion.
2) The B-horizon is very argillaceous, as is the A2 B-horizon, and passes into a BC-horizon with pockets of clay and corroded limestone.
3) A C-horizon is sporadically present in deeper fissures.
4) Siliceous (quartzose) constituents are common.
5) Pulverulitic limestone is common.
6) At depth the soil was constantly humid during all seasons.
7) At some localities the dominant clay is kaoite; at others it is illite.
8) In the forested areas, the color may be more brownish.
9) Frequently the soils are truncated at the base.
Duchanfour (1970) emphasizes that these terras rossas date from a former Pleistocene interglacial, are therefore paleosols, and have undergone a large amount of diagenesis, especially in the horizons below the upper part of the B-horizon. That the soils Duchanfour (1970) and most other European pedologists now refer to as terras rossas are largely eolian is refuted by their occurrence only on limestone terrains. If the soils are browner, a forested canopy may be indicated at the time of their formation.
Under these definitions, avoiding arguments of in situ or eolian origins, terras rossas are diagenetically altered paleosols that formed on limestone terrains under humid and tropical or nearly tropical, climatic regimes of approximately 1500 mm or more of rainfall per year.
DISTRIBUTION OF THE CENTRAL TEXAS TERRA ROSSA
The areal extent of the Central Texas Terra Rossa is incompletely known because the boundaries have never been mapped. Figure 1 shows four areas in Central Texas within which terra rossa can be viewed, but the boundaries of these areas are approximate, and there are smaller, internal areas from which the terra rossa has been removed. Much of the high, karstic plain of Woodruff and Abbott (1979, fig. 6) is covered with terra rossa, but terra rossa extends beyond some of the areas they have denoted and has subsequently been removed from other areas.
Associated with much of the Central Texas Terra Rossa is extensive Pleistocene silicification of Cretaceous fossils. Some other areas, not within the localities of Figure 1, yielding silicified Cretaceous fossils (Ikins and Clabaugh, 1940; Stanton, 1947; Moore, 1964), may represent Pleistocene silicification, with other evidence for terra rossa either less obvious or unrecorded. On the divide between Kerrville and Fredericksburg along State Highway 16, there is terra rossa. This terra rossa seems to be associated with the collapse of the Kirschberg Gypsum described by Barnes (1946); it may be Pleistocene. Other terras rossas, such as that on the Ellenberger Limestone just a few miles southeast of Brady on the divide between Brady Creek and the San Saba River, are probably Paleozoic. Studies of all of these terras rossas would lead to a greater understanding of the geology of Central Texas.
The soil profile for a terra rossa is not easily defined. This apparently accounts for terms such as "BC-horizon" in Duchanfour (1970). Figure 2 represents a profile from the Central Texas Terra Rossa along Loop 1604 north of San Antonio, 1.4 miles east of its intersection with U.S. Highway 287. The profile is developed on Person Formation, the upper formation of the Edwards Group (Rose, 1972), and it is not truncated at the base as are some profiles. A truncated soil is a soil in which the red clay of the B-horizon rests directly on unaltered limestone. Frequently, the larger karren are not developed on bare rock, but on the limestone beneath the B-horizon. With such a truncated soil, it is generally assumed that there has been some movement of soil into or onto these areas (Duchanfour, 1979), and that the base of the profile is not a natural product of mechanical or chemical weathering.
It would appear from Figure 2 that the A-horizon and at least part of the B-horizon have been removed by erosion. Most of the B-horizon is red, somewhat plastic clay. The lower part of the horizon is apparently B2, with limestone and caliche nodules; the latter could, of course, be post-terra rossa. If there is a C-horizon, it is now represented by a cave-fill-like breccia composed largely of pseudosparite at the base of the profile. This pseudosparite is almost identical to the pseudosparite, which is cave-fill associated with redeposited, red clay, within the underlying Person Formation. According to Duchanfour (1970), such diagenetic alterations are typical of the basal parts of profiles of terras rossas.
At other localities, the Central Texas Terra Rossa can be seen to be truncated, and on the Kainer Formation there may be well-developed C-horizon.
It is generally stated that a minimum of 1500 mm of rainfall per year is necessary to produce terra rossa. In addition to the minimum rainfall, Mediterranean terras rossas also required yearly dry-wet cycles, but not sufficiently dry to remove moisture from horizons below the upper part of the B-horizon (Duchanfour, 1970). I have seen no horizon of plinthite in the Central Texas Terra Rossa to indicate a marked seasonal change in the level of the water table. The above requirements already place certain restrictions on the age of the Central Texas Terra Rossa, because 1500 mm or more of rainfall is not normal to Central Texas.
The Central Texas Terra Rossa is sufficiently ancient in the Pleistocene that it has been dissected, sometimes removed completely, and diagenetically altered. In addition to the Central Texas Terra Rossa shown in figure 1, other areas once covered by it, but not shown, may be indicated by accumulations of red clay in caves and shallow subsurface or by the occurrence of extensive silicification of Cretaceous fossils. The area mapped as high karstic plain by Woodruff and Abbott (1979) in the Cibolo Creek drainage was probably covered with terra rossa at one time, since the caves and collapse zones in the Person Formation contain much red clay and red-stained rock and pseudosparite (Newcomb, 1971). Some of this area has not been studied closely, and there may still remain outcrops of terra rossa.
Between Purgatory Creek and the Guadalupe River along much of Purgatory Road (locality B of Figure 1), the Central Texas Terra Rossa is associated with a pediment cutting across Kainer, Person, Georgetown, Del Rio, Buda, and Dessau formations. Much of this area is the high, karstic plain of Woodruff and Abbott (1979). The Kainer and Person formations and the Buda and Dessau formations are in fault contact. At some localities along this pediment, the terra rossa has been removed. Along the Freeman Ranch-Bear Creek fault zone and along the Bat Cave fault, there are rows of dolines (figure 3 and figure 4) (Noyes and Young, 1960). These faults have the reverse drag on the down-thrown block (Bills, 1957; Tucker, 1968), which allowed water to flow laterally along the faults toward Purgatory Springs instead of down the regional dip. This further resulted in a series of dolines, probably collapse dolines, along the faults. One of these dolines, illustrated in figure 4, is on the old Wegner Ranch on the northeast side of Purgatory Road.
The silicified, fossil trees described as coming from an unconformity between the Edwards and Georgetown Formations by Cronin (1932) are actually Pleistocene. They come from the red clay that accumulated in the dolines along Purgatory Road, including the doline pictured in figure 4. Such fossil wood should not be confused with Cretaceous wood found around the Devils River Trend.
Along Loop 1604, in San Antonio (locality A, figure 1), the Central Texas Terra Rossa occupies an area that is nearly flat but dissected locally. The streams in this area with rejuvenated meanders (Shaw, 1974) may have originated on this surface.
Although there is no visible terra rossa on the Welch Ranch (now southwest Round Rock, Williamson County), terra rossa occurs just southwest of the area (locality D, FIGURE 1). On the Welch Ranch, grainstone at the top of the Edwards Limestone contained several cenotes that had been fenced so that cattle could not fall in and drown. One cenote was full of water to within one meter of the surface of the ground in 1965 when I mapped the area.
That lithology was important in the development of the Central Texas Terra Rossa is suggested by its rarity on the Glen Rose Formation and its dominance on rocks on the Edwards Group. Even within the limestones of the Edwards Group, karstification of the Person Formation is much more thorough than karstification of the Kainer Formation, even though the Central Texas Terra Rossa extends uninterruptedly across their contacts (usually fault contacts). Terra rossa has not been observed on the formations of the Austin Group, but outcrops of Dessau Formation on the edge of the Edwards Plateau are so small and uncommon (Young, 1985, 1986) that this may have no meaning.
Silica, usually microquartz, is commonly associated with the Central Texas Terra Rossa as with other terras rossas (Duchanfour, 1970). Along Purgatory Road at localities where the terra rossa has largely been removed, the surface of the pediment is commonly strewn with fragments (mostly from 5 to 30 cm in size) of silicified rudists, largely of specimens of the genera Caprinuloidea and Texicaprina.
The section of Person Formation (figure 5) along Loop 433 South, New Braunfels, Comal County, contains about 50 percent pseudosparite and associated red cave-clay and cave-breccia. Other beds show that at least some of the rock was originally cross-bedded, coarse grainstone. Although no terra rossa is on the surface, red cave-fill in Inner Space Caverns (Woodruff et al., 1985) near Georgetown, Williamson County, indicates a source of red clay (terra rossa) not too distant.
In this section only the shallow subsurface is considered--sufficiently shallow that there could be evidence of an overlying terra rossa or of a former terra rossa. In 1969 a coring program was carried out for a proposed quarry along Alligator (Geronimo) Creek just above the Balcones scarp, south of Hunter, Hays County. (Samples of the cores are in the collections of the Texas Memorial Museum, University of Texas at Austin.) Cores verified some of the previously mentioned surficial observations, because the Person Formation was much more highly altered, diagenetically, than was the Kainer Formation. These cores also demonstrated three intervals of formation of cave-fill. Of course, there were more than three, but they cannot all be demonstrated readily. In these cores it was possible to see (1) an older, unstained cave-fill, which was cut by (2) an iron-stained (red) cave-fill related to the Central Texas Terra Rossa, which in turn was cut by (3) a younger, unstained cave-fill, deposited after the period of formation of the Central Texas Terra Rossa. However, the ages are relative because, at present in areas of terra rossa, cave-fill that is being deposited may be red, whereas in areas of no terra rossa the cave-fill is unstained.
During coring, the bit would frequently drop the full length of the kelly, and the travelling block would bounce on the rotary table. Most often, however, the caverns had been collapsed, and the recovered core would pass from limestone into red cave-clay, clay-breccia, or pseudosparite.
According to Duchanfour (1970), there has been some diagenesis of all terras rossas. The diagenesis of the Central Texas Terra Rossa has not been studied, but one can assume that it is generally one or more of the types attributed by Ellis (1985) to meteoric water. Certainly, the superficial appearance of what may be the C-horizon (figure 2) is similar to the pseudosparite that represents ancient cave-fill (probably derived from the terra rossa) in the underlying Person Formation.
That silica (usually microquartz) is a common constituent of terras rossas (Duchanfour, 1970) does not mean that all silicification of the Cretaceous fossils occurred during the formation of the Central Texas Terra Rossa, but the many occurrences of silicified fossils in red cave-clay and in the profile of the terra rossa itself suggest high mobility of silica at the time of formation of the terra rossa.
In the highway cuts on Interstate Highway 10 in the hills southeast of Junction, Kimble County, there is exposed a collapsed zone where the Kirschberg Gypsum has been removed by solution. Removal of the gypsum would have been most active during the period of higher rainfall represented by formation of the terra rossa. At this particular locality, for depths up to 10 meters below the base of the Kirschberg level, the undersides of the chert nodules have been deeply etched and embayed (Figure 6). This embayment presumably occurred with the abnormal salinities produced by solution of the gypsum by meteoric water. Certainly at this time, silica seems to have been mobilized and redeposited by the silicification of fossils.
Many fractures, widened by solution (figure 7), and other geologic organs appear to be associated with, or were just subsequent to, the formation of the Central Texas Terra Rossa.
ATTEMPTS TO DATE THE CENTRAL TEXAS TERRA ROSSA
Dating the Central Texas Terra Rossa has not been easy. Approaches can be made from the following disciplines: (1) geomorphology, (2) paleoclimatology, (3) diagenesis, (4) redeposition, (5) paleontology, and (6) paleomagnetism.
The relation of outcropping areas of Central Texas Terra Rossa to earlier Pleistocene channels of the Guadalupe and Blanco Rivers (Koenig, 1940; Woodruff, 1977) indicates that this terra rossa formed prior to the capture of these streams. These captures may also correlate chronologically with primary drainage change on the Brazos River as described by Stricklin (1961) and Hibbard and Dahlquest (1967). Hibbard and Taylor (1960) consider major changes of drainage in Kansas and adjacent Oklahoma to be at the end of the deposition of the "Yarmouthian" (Crooked Creek Formation of Kansas = Seymour Formation of North Texas).
Since "Yarmouthian" and "Illinoisian" do not mean the same to everyone, I should point out that I am using the terms as I read them in Hibbard (1970), Hibbard and Dahlquest (1967), and Hibbard and Taylor (1960). Hibbard and Dahlquest (1967) suggested that the climate of North Texas in the Late "Yarmouthian" was subhumid, mesothermal, frost-free, and maritime. Later, Hibbard (1970) dated most of the drainage changes of the Great Plains, including those of North Texas, as occurring with a change of climate from subhumid to much drier at or near the end of the "Yarmouthian."
If the tectonics or climatic changes that altered the courses of the streams and resulted in rejuvenation were as regional as they seem to be, one would suspect that the streams of south-central Texas changed at the same time--that is, during the Late "Yarmouthian" of Hibbard (1970).
The climatic requirements of greater humidity and particularly of greater rainfall for a terra rossa do not tell us much about age, other than that the Central Texas Terra Rossa is not recent. However, the required greater rainfall (at least more than twice the present rainfall average of 700 mm per year at San Antonio) tells us that all of the Central Texas Terra Rossa was formed at the same time, because a consistent, long-term, high rainfall at different times in different local areas would be impossible. Furthermore, glacial stages would be excluded because of low temperature and probably insufficient rainfall. These data agree with those for the rejuvenation and change in stream courses mentioned above.
Duchanfour (1970) considers the amount of diagenesis of terras rossas of the Mediterranean region too great to have been completed in the Recent Interglacial.
At some localities red clay, reworked from the Central Texas Terra Rossa, has been deposited before modern drainage developed. There is a deposit of reworked red clay on Waller Creek, just below 51st Street in Austin, Travis County, that was transported across the present area of drainage of Shoal Creek prior to the development of this drainage area. Also, redeposition in caves occurred both during and after the formation of the Central Texas Terra Rossa.
From all of this evidence, then, the Central Texas Terra Rossa was formed between the Kansan and the Holocene. Glacial stages would be excluded because of climatic restrictions.
AGE OF THE CENTRAL TEXAS TERRA ROSSA
Recently, both paleontologic and geomagnetic data have greatly reduced the margin of error in dating the Central Texas Terra Rossa. From a small cave filled with red clay derived from the terra rossa in the Murchison Quarry (Fyllan Cave Local Fauna), northwest Austin, Texas, Taylor (1982, 1986) has described a Middle Irvingtonian fauna. This fauna is Late "Yarmouthian" and pre-"Illinoisian."
Furthermore, during deposition of the red clay in the cave there was a magnetic-reversal anomaly, since some samples are reversed and some are not. The reversal from Matuyama to Brunhes is known to have occurred in Late Middle Irvingtonian (Taylor, 1982). Also, the Jaramillo event just preceded the Matuyama-Brunhes reversal. If this anomaly represents reversal from Matuyama to Brunhes, then the fauna is about 0.73 m.y. B.P. (Taylor, 1986).
The paleo-ecological analysis of Taylor (1982) would indicate that the cave-fill for the Fyllam Cave Local Fauna was deposited after the formation of the Central Texas Terra Rossa, because the fauna seems to represent an environment of less humidity and rainfall than is required for the development of terra rossa. Both the paleomagnetic and paleoecologic data of Taylor (1982, 1986) agree with geomorphologic, climatic, and paleoecologic conclusions of Hibbard and Taylor (1960), Stricklin (1961), Hibbard and Dahlequest (1967), and Hibbard (1970).
Thus the age of the Central Texas Terra Rossa would appear to be older than Middle Irvingtonian (0.73 m.y. B.P.) and younger than "Kansan," probably Early and/or Middle "Yarmouthian."
The Central Texas Terra Rossa is a widespread paleosol with all the implications ascribed to terras rossas by Duchanfour (1970). It was widespread before partial removal. It represents a time of higher humidity and greater rainfall (1500 mm or more per year) than occurs in Central Texas at present (about 700 mm per year). The best dates for the formation of the Central Texas Terra Rossa are Early and/or Middle "Yarmouthian," that is, between 0.73 m.y. and 2.0 m.y. B.P., prior to the Brunhes Normal and the regional climatic change at the end of or within the Late "Yarmouthian."
I first became aware of the Central Texas Terra Rossa in the sping of 1949, while visiting outcrops northeast of Wimberly with F. L. Whitney. The Geology Foundation and the Research Institute, both at the University of Texas at Austin, have supported a number of projects in this area, and, therefore, have unknowingly supported this project. Geologic mapping for the Bureau of Economic Geology on other projects also aided this presentation, and Patrick L. Abbott and C. M. Woodruff, Jr., encouraged me.
Jeff Horowitz prepared Figure 1, and Rosemary Brant prepared the camera-ready copy.
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The Balcones Escarpment, Central Texas: Geological
Society of America, p. 63-70
The Balcones Escarpment, Central Texas: Geological
Society of America, p. 63-70