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Guidebook to the Geology of Travis County

Chapter 4 : Pilot Knob - a Cretaceous Volcano near Austin

D. L. Parker

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The word volcano usually summons images of snowcapped, towering mountain peaks, or remote, exotic islands in the minds of most people. Few of us living in Austin would suspect that an old volcano, its shape now extremely modified by erosional processes, lies only seven miles south of central Austin, near Bergstrom Air Force Base.

A volcano, as defined by Bullard (1962), is a vent or chimney that, at one time, has connected a reservoir of magma in the depths of the earth with the surface. The edifice of lavas and pyroclastic rocks built up around the vent is merely an expression of the activity of the volcano.

A geologic map shows that the Pilot Knob volcanic complex is about 2 miles in diameter (Fig. 50). A cluster of four, small, rounded hills (including Pilot Knob proper) form a core area of the old volcano composed of trap rock, which is resistant, fine-grained mafic volcanic rock. The core area stands topographically elevated above a surrounding circular lowland, drained by Cottonmouth Creek, that is underlain chiefly by volcanic ash and other pyroclastic debris. Several smaller bodies of trap rock occur in the volcanic ash (Fig. 50). A topographic rim surrounding the Cottonmouth Creek lowland to the north is formed by sedimentary rock, mainly lithified beach sediments composed of shell fragments and reworked volcanic ash that accumulated in the shallow waters around the volcano.

Figure 50 Geologic Map of the Pilot Knob Complex
Geologic Map of the Pilot Knob Complex (BIGGER)

Figure 51
North-South Section Through Pilot Knob
North-South Section Through Pilot Knob (BIGGER)

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HISTORY OF THE VOLCANO

In Late Cretaceous time the area that is now central Texas was a vast marine shelE on which carbonate rocks were beiny deposited, the entire area gradually subsidiny as the sediments were laid down. Imagine what would happen if hot, molten magma, working its way to the surface, were to encounter water-laden, unconsolidated sediments. The water would be quickly heated and vaporized into steam. A large amount of water suddenly converted into steam could result in an enormous explosion. Weiss and Clabaugh (1955) hypothesized that the Pilot Knob complex was, in essence, an explosion crater, created by such a process as described above (Fig. 51). Explosive eruptions continued at Pilot Knob as new magma, injected from below, encountered more water in the volcanic ash. Gradually, a mound of ash was built up over the explosion crater. Eruption of ash continued until the mound grew above the level of the shallow sea. Ash beds, now altered to clay, occur interbedded with limestone and marl of the Austin Group around Pilot Knob; these ash beds provide evidence for subaerial eruptions at Pilot Knob. The ash mound at Pilot Knob eventually built up an unstable slope on the sea bottom, and some of the ash and carbonate mud slid downhill as mudflows, ripping up the underlying carbonate mud in places and injecting itself into the carbonate mud at other places. The formation of an ash mound above sea level at Pilot Knob permitted intrusion of magma into the mound without contact with sea water, and, therefore, without explosive eruption of ash. Such magma was cooled and solidified to form the core and satellite areas of trap rock. Some of the trap rock bodies are the erosional remnants of lava flows, judging from their apparent dip away from the central core area. Cooling joints exposed on a small, rounded hill about 1,500 feet west of Pilot Knob proper suggest an original dip of that trap rock body towards the center of the core area, possibly indicating that it is the erosional remnant of a cone sheet injected outwards from a central, discordant intrusive body of magma. Exposures at other bodies of trap rock are not generally good enough to determine their exact implacement, but some, at least, are probably plugs of solidified intrusive magma. Magnetic anomalies on the northeast flank of the core area suggest a buried trap rock body within the ash mound, possibly a cone sheet or lava flow (Barker, 1970, written communication).

North-South Stratigraphic Correlation Through Austin (BIGGER)
Figure 52
North-South Stratigraphic Correlation Through Austin


East-West Stratigraphic Correlation Just North of Pilot Knob Vicinity (BIGGER)
Figure 53
East-West Stratigraphic Correlation Just North of Pilot Knob Vicinity

As volcanic activity diminished, beaches developed around the ash mound. One such beach deposit, now lithified and resistant to erosion, extends several miles to the north of the volcano. It crops out along Onion Creek, where it is responsible for both Upper and Lower McKinney Falls. The entire shelf continued to subside after cessation of volcanic activity, and muds of the Taylor Group gradually covered the entire volcano. During the Tertiary the central Texas area was uplifted, exposing the volcano as younger sedimentary rocks were stripped off the Cretaceous volcanic rocks. Today the terrain we observe at Pilot Knob is a reflection of the relative resistance to erosion of the different rock types that crop out in and around the volcanic complex.

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DATING THE VOLCANIC ACTIVITY

Two different approaches have been used by geologists to date the volcanism at Pilot Knob. The first approach was through the stratigraphy of the rocks at and around Pilot Knob and the fossils they contain. The second approach was by isotopic age dating of the igneous rocks. The two means are very different, but supplement each other nicely, as we shall see.

STRATIGRAPHY -- The volcano at Pilot Knob exerted a profound influence on the stratigraphy of the Upper Cretaceous sedimentary rocks of the Austin area. The volcanism occurred during the deposition of the upper part of the Dessau and the Burditt formations of the Austin Chalk. These strata are only about one-third as thick near Pilot Knob as they are in localities around Austin more removed from the volcanic site. The faunal zones of the two formations, that is zones within the formations where certain fossils are particularly abundant, are also telescoped in the vicinity of Pilot Knob (Fig. 52). Two beds of altered ash occur within the Dessau Formation on Rinard Creek near Pilot Knob (Durham, 1955). The exact relationship of the beach rock the northern flank of Pilot Knob with the normal Austin Chalk can be demonstrated to be part of the Exponderosa erraticosta zone, although it may extend down to an age equivalent to the upper part of the Dessau in some areas along Onion Creek. The beach rock is now known as the McKown Formation of the Austin Chalk (Young, this report). Because the volcanism at Pilot Knob was contemporaneous with the deposition of the Dessau and Burditt Formations, an age of Lower Campanian [83 to 79 million years before present (Gill and Cobban, 1973)] can be interpreted for the volcanism according to the ages of the formations in the Austin Chalk as correlated by Young (1963), correlated to radiometric dated sections in the Rocky Mountain Region (Gill and Cobban, 1973).

ISOTOPIC AGE -- Baldwin and Adams (1971) utilized the potassium-argon method to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive isotope of potassium (potassium 40) to argon 40, an isotope of argon, which is an inert gas. By knowing the concentration of potassium in a rock mineral and the amount of argon gas produced by radioactive decay trapped within the minerals, an age can be assigned to the rock because the decay rate of potassium 40 to argon 40 is known from experimental work (for more information on potassium-argon dating method and other isotopic age methods see Faul, 1966). The age of Pilot Knob volcanism given by Baldwin and Adams is 79.5 +/- 3 million years, and is in good agreement with the age derived by correlation with fossils to other radiometrically dated deposits (Gill and Cobban, 1973).

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WHERE DOES THE MAGMA COME FROM ?

One of the primary goals of petrology is to determine the origin of igneous rocks, and to decipher possible genetic relationships between different types of igneous rocks. Petrologists use a variety of different methods to answer these questions, including field relations of igneous rocks, and laboratory work on natural and "synthetic" rocks. Field relations include such observations as areal distribution, stratigraphic succession, and cross-cutting relations of different rock types. They are usually summarized by geologists in the form of geologic maps and measured sections and cross-sections. Discussion of the history of Pilot Knob presented in this report is largely based on such field observations. Laboratory work includes the study of rocks in thin section (where a thin slice of rock is examined under a microscope to determine its mineralogical composition), chemical analyses of rocks, isotopic studies, and high pressure and temperature experiments. The latter are made in specially constructed "bombs" -- high pressure and temperature vessels -- in which conditions deep within the earth -- the source of magma -- can be simulated.

The igneous rocks of the Pilot Knob volcano have not, as yet, been subjected to rigorous petrological study, although such studies are in progress. Similar rocks near Uvalde, Texas, of about the same age, have been studied (Spencer, 1969). The Uvalde rocks are of the same igneous province as the Pilot Knob rocks; both are centers in a chain of volcanic and intrusive complexes that parallel the buried Ouachita structural belt (Grunig, this report) in central and south Texas. The Ouachita belt may have provided a zone of crustal weakness where magma could rise from great depths through the crust to the surface.

Spencer (1969) determined that two types of magma were generated in the earth's upper mantle by partial melting of mantle rocks. One of these magma types is basaltic in composition, composed of plagioclase, titanium-rich pyroxene, and olivine. The other magma type crystallized to nepheline, melilite, pyroxene, and, in some of the rocks, plagioclase. Some nepheline-bearing igneous rocks contain rounded inclusions of spinel peridotite, an ultramafic rock that may represent fragments of the upper mantle incorporated into the magma during its ascent to the surface. Most of the igneous rocks we observe at the surface probably are not representative of the original magmas formed in the mantle; certain processes, such as separation of phenocrysts from the liquid from which they crystallized have altered their original chemical compositions at depths shallower than the mantle.

By inference, the rocks at Pilot Knob crystallized from partial melts of the upper mantle. Only rocks of the second series have been discovered at Pilot Knob; no basaltic rocks have been located (Barker, 1974, written communication). No mantle fragments have been located in Pilot Knob igneous rocks, but their presence is suspected, and some of the olivine phenocrysts in the rocks may be fragments derived from the upper mantle.

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HOW TO SEE PILOT KNOB

Pilot Knob can be reached by driving out Highway 183 past Bergstrom Air Force Base (soon to be Austin's-Bergstrom International Airport). As you cross Onion Creek the Knob is visible ahead to your right. A circuit of the volcanic complex can be made by turning off Highway 183 onto Elroy Road (to the right) and following Elroy Road to the intersection with Bluff Springs Road, where a left turn leads back to Highway 183. Elroy Road closely follows the northern topographic rim of the volcano formed by the escarpment of the Cretaceous beach rock mentioned earlier. The lowland drained by Cottonmouth Creek and underlain by soft, easily eroded volcanic ash and other pyroclastic deposits, and the central core area formed by the resistant trap rock are easily visible to the south of Elroy Road. About 2/3 of a mile east of the intersection of Bluff Springs Road with Elroy Road, the trap rock of south Pilot Knob crops out along the road. This rock is very similar to that which forms the center of the volcano, and may represent the eroded remnant of a lava flow erupted along the flanks of the volcano or having flowed down the south side from the apex.

Except for the road margins Pilot Knob volcano lies entirely on private land. Always obtain permission from landowners before entering private property. Please do not trespass, as this only serves to erode relationships with landowners and makes it difficult, and sometimes impossible, for people who come later to visit outcrops. In the example of Pilot Knob little can be gained by going to the top. The trap rock can be observed on Bluff Springs Road, and the topography and total aspect of the old volcano can be observed much better from Elroy Road. up next


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