GEOLOGICAL EXCURSIONS TO A
TRANSMOUNTAIN PRECAMBRIAN ADVENTURE

Written by Dr. Michael J. Whitelaw
Edited by Dr. William C. Cornell

INTRODUCTION

The Franklin Mountains divide the city of El Paso into two distinct halves. Until 1970, the only route from one side to the other was via I-10. In order to ease the commute from one side of El Paso to the other side, the city decided to build a road across the mountain. This road was to become Trans-Mountain Road (State Highway 375). From the east side, it follows Fusselman Canyon to the crest of the Franklin Range at Smuggler's Pass. Fusselman Canyon was named after a Texas ranger called Fusselman who was bush-wacked in the canyon by rustlers trying to move cattle over Smugglers Pass. Trans-Mountain Road then runs down the west side of the range and across part of the Rio Grande valley before it intersects I-10.

The east-west trend of Trans-Mountain Road cuts across the general north-south trend of the rocks in the Franklin Mountains. This, together with the fact that the rocks in the Franklin Mountain Range are tilted down to the west, means that as you drive up Fusselman Canyon to Smugglers Pass you are effectively driving up through the rock units that make up this part of the Franklin Range. These rock units are all ancient, the oldest is about 1,250 million years (MY) old and the youngest is about 1,100 MY old. They include a marble, which marks the location of a shallow beach platform similar, perhaps, to the modern day Bahamas, a thin sequence of rocks that were originally basalt lavas and then a thick sequence of sediments deposited in lagoons and as beach sands. This sequence of rocks was intruded (melted into and baked) by a granite magma about 1,100 MY ago. The remains of this cooled magma chamber can still be seen along Trans-Mountain Road. Some of the magma erupted to the surface and formed volcanic rocks which can now be seen at Smugglers Pass.

This booklet is written so that you can take yourself on a guided tour through this entire sequence.


SAFETY CONSIDERATIONS ON TRANS MOUNTAIN ROAD

A few common sense safety precautions can ensure that everybody enjoys this geologic outing. Be aware that Trans-Mountain Road is a busy thoroughfare and that cars generally travel on it at high speeds. When driving, give yourself plenty of space to get on and off the road and make sure you pull your vehicle entirely off the road when parking. When walking along road-cuts be aware of passing traffic, you should also look up! There are many areas along the road-cuts where rock-falls can occur. When walking off-road there are other potential dangers to consider. The Franklins are home to an assortment of snakes and stinging insects so walk carefully and kick over any rock before you try picking it up. Also be aware that much of Fusselman Canyon was part of the Fort Bliss Castner Artillery Range. Unexploded artillery shells are occasionally found in the range. Under no circumstances should these be touched. These shells may look old but please remember that explosives become less stable (more likely to explode) with age.

STRATIGRAPHY

Precambrian rocks are rocks older than 570 MY. Since the rock units in Fusselman Canyon exceed 1,000 million (a billion) years in age, they easily qualify as Precambrian rocks. The various rocks units or strata that occur along Trans-Mountain Road will be described in order from oldest to youngest:

Castner Marble
The Castner Marble generally occurs as a series of alternating layers of marble and hornfels. It has a maximum known thickness of 1,390 ft (424 m). The marble layers are generally white, grey or pinkish-orange and are often exhibit a coarse crystalline structure. They are carbonate-rich (limestone) rock layers. The hornfels layers are generally green to black and are mud rich layers. The original limestones and muds were deposited as a series of alternating layers on a shallow marine platform that was subject to tide and wave activity. The limestones and muds now exist as marbles and hornfels, respectively, because they were baked to approximately 600o C by the surrounding granites. Occasional volcanic ash units called felsic tuffs also occur within the Castner Marble. These units tell geologists that there was volcanic activity in the area during deposition of the Castner Marble. Some of these felsic tuffs have been found to contain zircons which can be age-dated by radio-isotopic methods. These zircons indicate that the Castner Marble is approximately 1,250 MY old.

The oldest fossils in the El Paso area are preserved within the Castner Marble. These appear as dome-shaped structures that are the remains of blue-green algae communities. They are called stromatolites. These stromatolites were one of the dominant life-forms on the planet 1,250 MY ago. At this stage in the earth's history there was no life at all on land and only single celled rocks organisms existed in the seas. These stromatolites pre-date the first land plants by 800 MY!

Mundy Breccia
The Mundy Breccia directly overlies the Castner Marble. It consists of fragmented basalt (lava) rocks that occur in a layer up to 250 ft (76 m) thick. The Castner Marble rocks directly below the Mundy Breccia have undergone soft sediment deformation. This folding and buckling of the Castner Marble layers occurred when the Mundy Breccia was deposited directly on top of it and before the Castner Marble sediments had a chance to dry out and harden. This suggests that the Mundy Breccia is essentially the same age as the Castner Marble. The origin of the Mundy Breccia is a source of controversy among geologists. Some geologists think that the Mundy Breccia is a blocky lava flow similar to some of the flows that erupt on the island of Hawaii today. Other geologists think that the Mundy Breccia formed as a sedimentary deposit. They suggest that the Mundy Breccia was formed when a basalt lava flow in the area was eroded and the blocks were transported and dumped by stream activity. It is even unclear as to whether the Mundy Breccia was erupted/deposited in a terrestrial (land) or marine (undersea) environment. In large part, this controversy occurs because textures within the rock that would help resolve this problem have been obscured by the same 600oC baking event that affected the Castner Marble.

Lanoria Formation
The next youngest rock unit is the Lanoria Formation, sometimes called the Lanoria Quartzite. The Lanoria Formation disconformably overlies the Mundy Breccia and Castner Marble. This means that it is separated from the underlying units by an erosion surface. The Lanoria Formation consists of 2,600 ft (793 m) of quartzites, quartzitic siltstones and baked shales. The quartzites and siltstones have recently been interpreted as beach sand and/or river deposits while the shales have been interpreted as lagoon deposits. These rocks have all been baked by the same event that affected the Castner Marble and Mundy Breccia and so have metamorphic rock names instead of sedimentary rock names. The alternating layers of quartzites and meta-shales have very different hardnesses that is reflected in the rock outcrops. The quartzite units in the Lanoria Formation resist erosion and tend to form cliff faces on the east side of the Franklin Range while the meta-shales are softer and form slopes.

Current flow indicators such as dune and ripple marks are commonly preserved in the Lanoria Formation. These indicate that the dominant stream flow directions were from south to north during Lanoria time, approximately 1,150 MY ago. This is of great interest to geologists because México did not exist at this time and the El Paso area probably marked the southwest coastal margin of North America. Rivers transported zircon minerals found within the Lanoria Formation have been uranium-lead age-dated at 1,800 MY. This implies that during Lanoria time there was a continent at least 1,800 MY old located off the south coast of North America. This continent was being eroded and its sediments were being transported to and deposited on the North American continent. There is no sign of this ancient continent today.

Thunderbird Group
Lanoria Formation is overlain by three formations that comprise the Thunderbird Group. These are well exposed in the Smugglers Pass road-cut at the crest of the Franklin Range. It is not yet clear whether there was much time between termination of Lanoria deposition and initiation of Thunderbird deposition but it appears likely that the change was transitional. The oldest formation in the Thunderbird Group directly overlies the Lanoria Formation and is called the Coronado Hills Conglomerate. It is approximately 90 feet (27 m) thick and contains cobbles and pebbles of well-rounded quartzite, siltstone, shale, jasper and volcanic rocks. It is overlain by the Smugglers Pass and Tom Mays Park formations. These formations contain some sedimentary deposits but are dominated by volcanic rocks. The volcanic rocks include ash-fall (tuff) deposits, ignimbrites (welded ash), and lavas of trachytic and rhyolitic composition. Uranium-lead and potassium-argon age dates indicate that this volcanic activity occurred approximately 1,130 MY ago. These rocks also have been baked by the same 600o C heating event that affected the Castner Marble, Mundy Breccia and Lanoria Formation.

Red-Bluff Granite Complex
The Red Bluff Granite Complex consists of at least seven major intrusive bodies. These magma bodies that melted their way into the overlying sedimentary and volcanic sequence 1,130 MY ago. The Red Bluff Granites are exposed along much of the eastern base of the Franklin Range, particularly in the Fusselman and McKelligan Canyon areas. These granites are subdivided, according to mineral content, into alkali feldspar rich, biotite rich and biotite-hornblende types. In addition to the large granite bodies there are many smaller intrusive units that run parallel to (sills) or cut across (dikes) the surrounding rocks. The minerals in the Thunderbird Group volcanic units are identical to those in the Red Bluff Granites. This leads geologists to think that the two rock bodies are co-magmatic and formed of the same material. Some of the magma erupted onto the surface to form the Thunderbird volcanic rocks and some of it cooled and hardened underground to form the intrusive Red Bluff Granites. If this is true, then it is possible for a motorist to drive from the magma chamber through the sequence of overlying rocks and into the volcanic sequence simply by driving up Trans-Mountain Road to Smugglers Pass!

Trans-Mountain Road it is possible to see the force that magma can exert on rocks that surround the magma chamber. Large blocks of these rocks have been wedged off of the magma chamber roof and have fallen into the melt. These blocks are called xenoliths (Latin for "strange-rock"). The exposures of Castner Marble, Mundy Breccia, and some of the lowermost Lanoria Formation all exist as a giant xenoliths within part of the Red Bluff Granite Complex.

The effects of this hot body of magma which intruded into the area 1,130 MY ago were extensive. It melted a considerable amount of surrounding rocks and contact metamorphosed (baked) the rest to temperatures in the order of 600o C. It was this event that was responsible for turning the Castner limestones and muds to marbles and hornfels, for baking the Mundy Breccia, for turning Lanoria sandstones to quartzites, and for baking the rocks of the Thunderbird Group.

GEOLOGIC HISTORY OF EL PASO
AREA AS REPRESENTED BY THE
TRANS-MOUNTAIN ROAD SEQUENCE

The oldest rocks deposited in the El Paso area are of the Castner Marble. It was originally deposited as a series of alternating or laminated limestone and mudstone layers in a tidal, shallow marine environment. This interpretation is supported by preserved sedimentary structures such as ripple marks, teepee structures, oolites and the laminated character of the bedding. The presence of stromatolites, modern examples which are normally found in tidal marine environments, also supports this interpretation. All this evidence suggests that the El Paso area existed as a quiet coastal area on the southwest margin of the North American continent. Rare felsic tuffs may also be found within the Castner Marble. These are formed by the deposition of ash from volcanic eruptions and suggest that some volcanic activity was also occurring in the area. Zircons removed from these tuffs indicate that the Castner Marble is approximately 1,250 MY old.

By the end of Castner time, geologic activity had begun to increase in the El Paso area. The upper units of the Castner Marble show signs of this activity in the form of increased occurrence of conglomerate horizons, often developed on areas of heightened topography, and bed deformation. This increased activity is further shown by the presence of the Mundy Breccia, a definite indication of volcanic activity in the area. Whether it is an actual lava flow or the remanent of a flow which had been eroded, transported and dumped by stream activity is uncertain. However, it was rapidly deposited on the Castner Marble sediments before they had a chance to lithify or harden and so it too must be approximately 1,250 MY old.

The boundary between the Mundy Breccia and the Lanoria Formation is marked by an erosion surface. Since erosion most commonly occurs on dry land, it seems likely that the El Paso area was above sea-level some time after the deposition of the Mundy Breccia and before deposition of the Lanoria Formation. The Lanoria Formation preserves a thick sequence of alternating sandstone and mudstone(shale) units that have been interpreted as lagoon, beach and shallow marine coastal deposits. The fact that these rock types alternate up the sequence suggests that sea-level rose and fell several times during Lanoria time, and that the shoreline probably advanced and regressed across the El Paso area several times during Lanoria time.

The end of Lanoria Formation deposition and the onset of Thunderbird Group deposition marks the beginning of a vigorous episode of geologic activity in the El Paso area. The initial activity is preserved by the Smugglers Pass Conglomerate, a mixture of pebbles and boulders that was deposited on top of the Lanoria Formation. These sediments are typically laid down in highland stream channels and other areas where there is steep topography. Therefore, the Coronado Hills Conglomerate preserves evidence for uplifting of the area above sea-level and initiation of strong erosional activity. Much of the uplift was also associated with the onset of strong igneous (volcanic and plutonic) activity. This is preserved in the volcanic Smuggler's Pass and Tom Mays Park formations. Eruptions were in all probability explosive and included thick lava flows as well as ash-fall deposits. The ash-falls were probably similar to those recently seen during the eruption of Mount Pinatubo in the Phillippines. Radioisotope age studies of these rocks suggest that the eruptions occurred approximately 1,130 MY ago.

At the same time that the Thunderbird Group volcanic rocks were erupting, the magma chambers which supplied the lava were melting their way toward the surface. Before the magma chambers melted all the way to the surface they cooled and formed the Red Bluff Granite Complex. This effectively sandwiched the Castner Marble, Mundy Breccia and Lanoria Formation rocks between the two igneous rock bodies. Evidence for the upward melting of the Red Bluff Granites into the surrounding rocks is preserved by the many xenoliths that occur in the granites and in the dike and sill complexes that criss-cross the area. The heat from the magma chambers was high enough to bake(metamorphose) all of the surrounding rocks to temperatures of approximately 600oC. This baking was responsible for changing the Castner carbonates and muds to marble and hornfels, the Lanoria sandstones to quartzites and the Thunderbird conglomerates and rhyolites to meta-conglomerates and meta-rhyolites.

After this intense igneous activity, the El Paso area appears to have undergone a long period of erosion. No rocks between the 1,130 and 500 MY are known in the El Paso area. They either were never deposited or, if they were formed, were totally destroyed by erosion. Since 500 MY ago a series of sandstones and limestones have been deposited in the El Paso area, all associated with the advance and regression of various shallow seas over the North American continent. These rocks are readily visible along Scenic Drive but are almost totally absent from the Fusselman Canyon area. These rocks, together with the Precambrian rocks now exposed in Fusselman Canyon, remained flat lying until perhaps 5 MY ago. Since then the rocks have been uplifted and tilted toward the west to form the Franklin Mountains as we now know them.


ROAD STOPS

This self guided tour of the Franklin Mountain Precambrian rocks starts on the east side of the range at the entrance to Fusselman Canyon and will finish at Smugglers Pass. Mileages are given starting from the intersection of the North-South(Patriot) Freeway and Trans-Mountain Road. Before the first stop you will pass the Franklin Mountains Wilderness Museum. The museum has an excellent display of both the rocks and archeological objects that may be found in the area. You may consider stopping by the museum before you begin your tour

SIMPLIFIED ROAD LOG

STOP MILEAGE DESCRIPTION
0 0.00 Intersection of Trans-Mountain Road/Patriot Freeway
0.00 0.65 Wilderness Park Museum
1 1.65 Red Bluff Granite/Pegmatite Dike/Xenoliths
2 2.35 Lower Granite Sill and Castner Marble Stromatolites
3 2.55 Castner Marble and Mundy Breccia
4 3.00 Upper Red Bluff Granite Sill
5 3.40 Lanoria Formation quartzites
6 4.75 Smugglers Pass -- Thunderbird Group volcanics

STOP 1: LOWER RED BLUFF GRANITE SILL
Directions:
Proceed 1.65 mi west up Trans-Mountain Road. Cross the central median strip at the turn around a lane located at the west end of the first road-cut and enter the park located south of the east bound lane. From this park it is possible to observe the large scale features preserved in the cut on the opposite side of the road.

What to Look For:
As you have driven up Trans-Mountain Road you have driven into the Red Bluff Granite Complex. You are now standing within one of the magma chambers which cooled and hardened 1,130 MY ago. The rock exposed in the road-cut is granite, as are most of the rocks that you have driven past since the Wilderness Museum. Granites form when mineral crystals grow in a cooling magma chamber. The minerals formed in this granite include quartz (clear and glassy), feldspar (white to grey-green) and biotite (black). This granite outcrop also includes a variety of other features. At the west end of the road-cut blocks of white and green striped rock occur within the granite. These are blocks of the Castner Marble, an originally sedimentary rock which once formed the roof of the magma chamber. At some stage the roof collapsed and these blocks fell into the chamber. As the magma cooled and crystallized the blocks were frozen into the magma and preserved as xenoliths. You may also note that black colored xenoliths also occur in the magma. The original source of these blocks is not clear but seem related to basalt type lava rocks. In addition to the xenoliths a series of large veins called dikes and sills criss-cross the entire rock face. These formed during late cooling phases of the granite when the remaining liquid material was forcefully injected into the surrounding rock.

After seeing the "big-picture" from the south side of the road it is useful to cross the road and examine the road-cut in detail. When you get close to the outcrop, you will notice that many of the veins contain large crystals. Rocks that contain such large crystals are called "pegmatites." One such pegmatite, which occurs toward the east end of the road-cut, is known locally as the "Riebeckite Dike." It contains spectacular crystals of a black colored mineral called Riebeckite (a member of the amphibole mineral family). It also contains large crystals of quartz (clear), sodium feldspar (white) and minor amounts of mica muscovite (silver-gold), and biotite (shiny black)). If you walk to the west end of the outcrop, you can view the Castner Marble xenoliths in detail. The striped colors in these blocks are caused by alternating layers of white to pink calcite (originally limestone) and green-black hornfels (originally mudstones).

STOP 2. RED BLUFF GRANITE AND CASTNER MARBLE STROMATOLITES
Directions:
Cross back onto the west bound lane of Trans-Mountain Road and proceed 0.7 miles west to the next stop. Stop at the west end of the road-cut where the guard rail starts. A small trail heads north from the guard rail. Follow this trail down into a small valley and then up the other side for approximately 80 yards.

What to Look For:
As you walk up this track you will cross from granites of the Red Bluff Granite Complex into the Castner Marble. This exposure of the Castner Marble occurs at the base of a giant xenolith (called a roof pendant). This xenolith occupies the entire valley and the next ridge immediately to the west of your current location and includes the Castner Marble, the Mundy Breccia and part of the Lanoria Formation. Much of the Castner Marble shows a rib-like appearance. This occurs because the mud rich hornfels layers are more resistant to erosion than the calcite rich marble layers. Calcite is easily dissolved by rain. At this locality the Castner Marble contains excellent examples of stromatolites -- dome shaped sedimentary structures created by colonies of single-celled blue green algae. Stromatolites generally form in shallow marine tidal areas. Blue-green algae are photosynthetic and therefore, like plants, need sunlight to live. They also secrete a slimy or sticky substance on the surfaces on which they grow. A constant wave and tide activity wash sediments over the algae and this becomes stuck in the algal mats. In order to keep living the algae are forced to grow up through the sediments to reach sunlight again. This process repeats in a long series of cycles which lead to the algae forming dome shaped structures with many laminations in each one. Stromatolites are the oldest known macroscopic fossils on the planet. These stromatolites have an age of 1,250 million (one and a quarter billion) years!

STOP 3. CASTNER ROAD-CUT
Directions:
Return to Trans-Mountain Road and drive 0.2 miles to a gravel car park. Park at the west end of the car park. This stop will consist of inspection of the Castner Marble in the road-cut extending to the west.

What to Look For
In addition to the Castner Marble, the ridge that runs north from the west end of the car park contains exposures of the Mundy Breccia and the Lanoria Formation. The Mundy Breccia exists as a rubbly grey unit that parallels the ridge about one third of the distance from its top. The Lanoria Formation forms the large cliff faces that appear up the valley, to the north. All these rocks are part of a gigantic xenolith. The granites of the magma chamber which surround this block can be seen on the ridges at the north end of the valley you are in, in the next road-cut up on Trans-Mountain Road and, of course, in the road-cut you just left at stop 2.

A walk up the road-cut that starts at this car park will allow you to see the best exposures of the Castner Marble in the Franklin Mountains. As you walk up the cut, you will see alternating layers of marble and hornfels units that are so repetitive that they are called "rythmites." You may see patches of a red-brown mineral, particularly at the start of the road-cut. These are garnet crystals. Although these are not gem quality, they can be quite beautiful when you find a complete twelve faced crystal. Garnet crystals found along the road are generally quite small but crystals up to six inches across occur in the valley to the north of the car park.

Note that the layering in the Castner Marble is all tilted down to the west. Like all sedimentary units the Castner Marble sediments were originally deposited in a flat lying position. The tilting, which occurs in nearly all sedimentary units in the Franklin Mountains is a result of uplifting and tectonic activity during the last 5 MY. This activity created the mountain range that we see today.

Approximately, 80 yards up this road-cut, a red colored breccia dike occurs. The magmatic origin of this dike is confirmed by the presence of the igneous mineral plagioclase and by the fact that the dike bifurcates (splits into two dikes) as it goes up through the outcrop. The magma was clearly exploiting weaknesses in the Castner Marble as it made its way toward the surface. The dike includes fragments of a wide variety of rock types collectively called breccia. The magma must have been injected into the Castner Marble with great force in order to have been able to carry all of this breccia as well. You may have noticed that a series of holes has been drilled in both the dike and the surrounding marble. Samples of rock were collected from these holes for study of the magnetic fields preserved in the rock. These magnetic studies suggest that the dike is probably between 5 and 30 MY old. Therefore, it is much younger than the surrounding 1,250 MY old marble.

As you continue to walk up the road-cut you should look out for flat pebble conglomerate layers that occur within the Castner Marble. These are horizons of the marble that consist of torn up and jumbled fragments of the originally flat lying, laminated, limestones. These horizons have been variously interpreted as storm induced deposits, where wave action ripped up previously deposited sediments, or as packages of sediment that have slumped off a shallow water platform, into deeper ocean waters, perhaps because of earthquake activity.

As you get close to the west end of this road-cut the character of the laminated Castner rocks begins to change. Gentle bends or folds begin to become more prevalent in the layering. These are soft sediment deformation features caused when the Mundy Breccia was rapidly deposited on top of the Castner before it had a chance to completely lithify or harden. The weight of the overlying rock both bent and buckled the Castner sediments.

The Mundy Breccia is exposed about 5 yards west of the end of the road-cut. In this area, it is best seen on the slope directly above the road and consists of large blocks of basalt which are surrounded by a fine grained basalt matrix. This unit has been heavily altered by the 600o C Red Bluff baking event and so is extremely hard. This alteration is also responsible for obscuring the true origin of the Mundy Breccia.

STOP 4. UPPER RED BLUFF GRANITE SILL
Directions:
Return to your vehicle and drive 0.8 miles to the west end of the next road-cut. It is possible to safely park behind the concrete road barriers at this stop.

What to Look For:
By the time you reach this stop, you have driven out of the giant Castner Marble/Mundy Breccia/Lanoria Formation xenolith and into the upper part of the Red Bluff Granite magma chamber. Because this granite body separates the xenolith from the rest of the Lanoria Formation, it is called a granite sill. It is estimated to be 1,245 ft thick. This road-cut contains fresh, non-weathered exposures of the granite and so is the best place to see what it should look like. The granite is grey to green in color and contains large crystals of quartz (clear and glassy) and alkali feldspar (milky grey) as well as minor amounts of biotite mica and hornblende (both black). This road-cut also exhibits a complex sequence of fractures that were probably developed when the magma chamber cooled, hardened and shrank. These fractures are called joints.

STOP 5. LANORIA QUARTZITE
Directions:
The west end of the Red Bluff sill road-cut affords an excellent view of the Lanoria Formation. It stretches from this location all the way up Trans-Mountain Road to the base of Smugglers Pass. The quartzite units in the Lanoria Formation form the spectacular cliff walls that are visible on the east side of the range while the mud-rich shale units form the intervening slopes. From this stop drive 0.55 mi to a small exposure of the Lanoria Formation located on the north side of Trans-Mountain Road.

What to Look For:
At this locality the Lanoria Formation consists of quartz sandstones, originally deposited as beach sands, but now altered to quartzites by the Red Bluff baking event. These occur as clean, clear to milky white colored rocks in the outcrop. Careful examination of these outcrops may also reveal the presence of cross-beds (buried sand dunes) and ripple marks that were generated by wave and current activity on this approximately 1,200 MY old beach. Dark black manganese oxide deposits on quartzite surfaces that look like ferms are dendrites, not fossils.

STOP 6. THUNDERBIRD GROUP VOLCANICS AND SEDIMENTS
Directions:
Continue 1.35 mi up the west bound lane of TransMountain Road to Smugglers Pass. Smugglers Pass separates South Franklin Mountain from North Franklin Mountain. North Franklin Mountain, with a peak at 7,192 ft, is the second highest point in the state of Texas. Between Stops 5 and 6, you effectively drive through the rest of the Lanoria Formation and into the formations of the Thunderbird Group. You may notice loose gravels on the side of the road as you head toward Smugglers Pass. These are the result of recent erosion in the Franklin Mountains and obscure the boundary between the Lanoria Formation and the overlying Thunderbird Group. Continue driving through Smugglers Pass until you reach the turning lane at its crest. Cross the median and pull into the car-park which overlooks the west side of the Franklin Range.

What to Look For:
The park is built on the Tom Mays Park Formation, the uppermost formation of the Thunderbird Group. The overlook at the car park marks the location of the western boundary fault of the Franklin Range and the most westerly extent of Precambrian rocks in the Franklins. Rocks of the Thunderbird Group, which is approximately 3,000 ft thick, are easily studied by walking back down the length of the Smugglers Pass road-cut. The first part of the road-cut contains layers of strongly welded volcanic ash (ignimbrites) and some river channel deposits. These units belong to the Tom Mays Park Formation. They give way to rhyolite and trachyte lava flows of the Smugglers Pass Formation approximately half way down the cut. These flows are extremely hard and glassy in texture and may contain white or pink colored feldspar minerals. The texture is due, in part, to the high silica content of these rocks, and in part, to the fact that these rocks were baked by the Red Bluff heating event. Keep in mind that these rocks are volcanic equivalents of the Red Bluff Granites. They represent molten material that managed to make its way from the Red Bluff Granite magma chamber to the surface. Like the Red Bluff Granites these rocks have been radio-isotopically dated at 1,130 MY old. The lower end of the road-cut contains conglomerates that belong to a formation called the Coronado Hills Conglomerate.

Most of these rocks have a strong red, red-brown or orange coloration to them. This is due to the amount of iron they contain and the fact that this iron has been oxidized (rusted) by exposure to air or groundwater. A fault or fracture system occurs approximately two-thirds of the way down the road-cut. It is marked by a collapse feature on the east bound side of the road and a large band of discolored rocks on the west bound side of the road. The yellow coloring is the result of extreme oxidation of the rocks in the fault zone by groundwater which had easy access to the rocks through the fault fracture system. Some of these rocks also have black colored markings that look exactly like tree fossil imprints. These are traces of manganese oxide crystals, called dendrites, that mimic plant features. Remember that these rocks were formed approximately 800 MY before the first land plants. They could not possibly contain plant fossils!


This concludes the tour of the Franklin Mountains Precambrian rocks. Return to El Paso by continuing west on Trans-Mountain Road to I-10. Turn south onto I-10 and you will be in down town El Paso in approximately 15 minutes. If you do return to town this way, be sure to look at the west face of the Franklins when you are between the Mesa and Sunland Park exits. You should see the shape of a red thunderbird formed by outcrops of the Thunderbird Group on the side of the range.

This concludes the tour of the Franklin Mountains Precambrian rocks. Return to El Paso by continuing west on Trans-Mountain Road to I-10. Turn south onto I-10 and you will be in down town El Paso in approximately 15 minutes. If you do return to town this way, be sure to look at the west face of the Franklins when you are between the Mesa and Sunland Park exits. You should see the shape of a red thunderbird formed by outcrops of the Thunderbird Group on the side of the range.



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