|Visit a site on the Mojave Desert, southeast of Death Valley, where many trilobites occur in the Lower Cambrian Carrara Formation, some 518 million years old.|
|Click on the image for a larger picture: Here's a slab of slightly metamorphosed, heat and pressure-altered shale from the Lower Cambrian Carrara Formation, collected in the Nopah Range, Inyo County, California; it bears several head shields, or cephalons, from a variety of trilobite called Olenellus clarki.|
One of the more prolific producers of trilobites in the Mojave Desert-Great Basin region of eastern California is the Lower to Middle Cambrian Carrara Formation, a sedimentary rock deposit that has yielded more than 95 species of trilobites distributed among 38 genera. The Carrara was first described in the geological literature from excellent and characteristic exposures in Carrara Canyon, at Bare Mountain, a few miles south of Beatty, Nevada, where metamorphosed carbonates in the youngest periods of sedimentary deposition yielded vast quantities of high-grade, commercially exploitable marble, productive deposits that were extensively mined in the early portion of the 20th century. By a curious twist of fate, though, the Carrara Canyon outcrops of the Carrara yield few identifiable fossil remains. Trilobites are virtually nonexistent there, save for a few poorly preserved fragments of the exoskeleton, such as spines and free cheeks from the cephalon, or head shield. It is therefore a very disappointing region to explore, at least in a paleontological sense.
More productive exposures can be visited in the Funeral Range of eastern California--that impressive hulk that guards the eastern borders of Death Valley National Park. The problem here, obviously, is that unauthorized fossil collecting within the borders of the national park is not permitted. Yet, such classic sites as Echo Canyon, Titantothere Canyon and Pyramid Peak--all tucked away within the rugged and wild Funeral Range--continue to lure amateur fossil seekers, curious to observe in situ, with hands obediently kept off the rocks, the beautiful trilobites preserved along the bedding planes.
In western Nevada, most of the classic trilobite-bearing beds in the Carrara Formation occur on the Atomic Energy Commission Nuclear Test Site, which lies east of Highway 95 from the vicinity of Scotty's Junction all the way south to Las Vegas. Paleontologists privileged enough to gain access to the site report beautiful trilobite specimens, some of them complete, from a number of Carrara Formation exposures at Striped Hills, Jangle Ridge and the Spectre Range.
In addition to Carrara Formation exposures lying within Death Valley and the nuclear test site, amateur fossil collectors face yet another obstacle. Much of the Mojave Desert is currently a federally protected wilderness area. For example, one of the more frequently visited outcrops of the Carrara used to be Eagle Mountain, south of Death Valley Junction (just east of the the Death Valley National Park border), where abundant, identifiable trilobites had been collected for decades. The locality now lies within the federally designated Eagle Mountain Wilderness and it is completely off limits to any manner of unauthorized collecting.
Another broad band of productive trilobite-bearing Carrara Formation exposures can be visited in the Nopah Range, Inyo County, California. While it's true that most of the Nopah Range has been swallowed up by the recently established Nopah Wilderness, there is still one productive place where trilobites still occur on Public Lands--all from a single unit of slightly metamorphosed shale that is uppermost lower Cambrian in geologic age (or roughly 518 million years old). The locality lies in the Nopah Range and has attracted much attention of late from many amateur fossil enthusiasts, since this particular site represents one of the few accessible places remaining in all the southwest where unauthorized explorations of the Carrara Formation are allowed to take place.
And now for the obligatory words of caution. Endemic to the Mojave Desert of California, including the Las Vegas, Nevada, region by the way, is Valley Fever. This is a potentially serious illness called, scientifically, Coccidioidomycosis, or "coccy" for short; it's caused by the inhalation of an infectious airborne fungus whose spores lie dormant in the uncultivated, harsh alkaline soils of the Mojave Desert. When an unsuspecting and susceptible individual breaths the spores into his or her lungs, the fungus springs to life, as it prefers the moist, dark recesses of the human lungs (cats, dogs, rodents and even snakes, among other vertebrates, are also susceptible to "coccy") to multiply and be happy. Most cases of active Valley Fever resemble a minor touch of the flu, though the majority of those exposed show absolutely no symptoms of any kind of illness; it is important to note, of course, that in rather rare instances Valley Fever can progress to a severe and serious infection, causing high fever, chills, unending fatigue, rapid weight loss, inflammation of the joints, meningitis, pneumonia and even death. Every fossil prospector who chooses to visit the Mojave Desert must be fully aware of the risks involved.
As one gazes to the mountain slopes in the Nopah Range in the vicinity of the fossil site, the Carrara Formation is the essentially recessive interval, some 1,200 feet thick in stratigraphic thickness. (It is considerably thicker than that in actual area of outcrop, due to faulting and repetition of some of the sedimentary beds.) The formation lies between two massive layers of widely differing lithologies--a dark bluish carbonate layer at the top called the Bonanza King Dolomite, and pale brown to dark brown quartzite below representing the lower Cambrian Zabriskie Quartzite. The Carrara is such a distinctive unit in the field, a varied mixture of tan, brown and greenish shales interbedded with several massive beds of bluish to gray-blue limestone, that it can be followed with ease throughout the Nopah Range. But don't touch anything within the Nopah Wilderness (a detailed map delineating the geographic extent of the Nopah Wilderness can be obtained from the BLM). At the trilobite locality, the Carrara Formation can be observed lying below the massive bluish carbonate accumulations of the middle to upper Cambrian Bonanza King dolomite. The Bonanza King yields few trilobites, but is known to contain locally abundant oval algal structures called Girvanella signifying deposition in a warm, shallow Paleozoic Era sea. Click Here for an image which depicts the dramatic contact between the Lower-Middle Cambrian Carrara Formation and the Middle-Upper Cambrian Bonanza King Dolomite in the Nopah Range.
Throughout its area of outcrop, the Carrara can be separated into nine easily distinguished stratigraphic subunits, or members. The youngest member, just below the overlying Bonanza King Dolomite, is the Desert Range Limestone. It can be recognized from afar due to its distinctive lithologic mixture of thin-bedded black silty limestone interbedded with orange dolomitic partings. The Desert Range is noted producer of Glossopleura trilobites, representing a middle Cambrian geologic age, but almost all of the productive beds lie on the nuclear test site in western Nevada.
Immediately below the Desert Range Limestone is the Jangle Limestone Member, which is the uppermost, or youngest, of the three major carbonate units in the Carrara Formation. It is characterized by one to as many as five massive layers separated by thin argillaceous partings. In the Grapevine Mountains of eastern California, the Jangle yields a diverse and abundant middle Cambrian trilobite fauna consisting of Mexicella grandoculus, Mexicaspis radianis, Nyella climlimbata, Ptarmiganoides hexantha and Volocephalina connexa. Exposures of the Jangle Limestone Member in the Nopah Range yield only sparse trilobite fragments and occasional algal nodules of the Girvanella variety. The algal nodules are usually referred to by sedimentologists as oncolites, and were theoretically formed by direct precipitation of calcium carbonate from Cambrian sea-waters, unlike modern algal bodies from the Bahamas that develop directly through accretionary capturing of the surrounding oceanic muds.
In descending order of geologic age, the next oldest unit in the Carrara is the Pahrump Hills Shale Member. It consists of a heterogeneous accumulation of red-and-green mudstone, tan siltstone, silty limestone and dolomite. Typically, the lowermost exposures produce abundant invertebrate tracks and trails preserved on the bedding planes of an orange-brown siltstone, while strata higher in the section often yield abundant oncolites embedded in a cryptalgal limestone. Even though the Pahrump Hills Shale reveals abundant trace fossils--including problematic trilobite feeding grooves, scratch marks and profuse tracks clearly allied with arthropodal life movements--trilobite fossils are scarce to nonexistent at most outcrops. The most productive trilobite-bearing sites include the Grapevine Mountains in California and the Groom, Desert, Spectre and Belted ranges in western to central Nevada. Trilobites identified from those localities include Albertelloides rectinmarhinatus, Caborcella pseudaulax, Caborella reducta, Chancia venusta, Kootenia germona, Pachyaspis gallagari, Pagetia resseri, Sysacephalus obscurus, Volocephalina connexa, Zancanthoides sp., Albertellina aspinosa and Elrathina antiqua. All of the specimens suggest a middle Cambrian age for the Pahrump Hills Shale Member.
Underlying the Pahrump Hills Shale is the Red Pass Limestone Member, named for Red Pass, which lies roughly three-quarters of a mile east of Titantothere Canyon in Death Valley National Park. The Red Pass is easily distinguished in the Carrara section since it forms a prominent carbonate cliff face in a section dominated both above and below the interval by more recessive-weathering shales. Limestones in the Red Pass produce invertebrate tracks and trails, in addition to occasional oncolites, sometimes found in a superior state of preservation. (Thin sections of the algal material yield actual filaments from the original plants, an extinct species of blue green algae.) Other varieties of fossils remains are generally rare, occurring only in the uppermost and lowermost layers. These include such trilobites as Kochaspis augusta, Kochaspis lilian, Kochiellina groomensis, Kochielina janglensis, Plagiura extensa, Plagiura vetracta, Plagiura cercops and Schistometopus sp. Paleontologists agree that the Red Pass Limestone Member is entirely middle Cambrian in geologic age.
Lying directly below the Red Pass Limestone, in conformable fashion--that is, there were no apparent breaks in sedimentary deposition--is perhaps the most reliably fossiliferous unit in all the Carrara Formation--the fabulous Pyramid Shale Member, which was named for Pyramid Peak in the eastern sector of Death Valley National Park. Until 1994, when the California Desert Protection Act became law, Pyramid Peak could be found outside Death Valley National Monument. The prominent geographic landmark, and productive fossil locality, presently resides within the boundaries of Death Valley National Park: Fossil collecting there is obviously illegal without prior authorization from the Death Valley National Park Resources Division. If you think that you might possibly qualify for a collecting permit (a degree from an accredited institution of higher learning is necessary, in addition to a valid research project that can be verified through independent investigation by the petitioned authorities), contact the DVNP Resources Division at (760) 786-2331.
Not only is the Pyramid Shale Member fossiliferous at its type locality, but trilobites can be found at most of its exposures throughout the deserts of eastern California and western Nevada. It is in fact the most fossiliferous unit in the Nopah Range and is the member within which the trilobite locality discusssed here occurs in the Nopah Range. The fossils also show up near the main locality, within the Nopah Wilderness, but don't even think about keeping anything found there, because that area is presently under federal jurisdiction and administered by the Bureau of Land Management: it is completely off-limits to unauthorized collectors. Be sure to have an up-to-date, accurate map of the Nopah Wilderness while exploring the Nopah Range for fossil trilobite localities.
It should be noted that there is nothing in the original language of the Wilderness Act (circa 1964) that specifically allows hobbyists to excavate for minerals, fossils or any other natural resources within a designated wilderness area. The final approval to collect on wilderness lands likely rests with the individual BLM ranger in charge of his or her district. Therefore, always check with the local district ranger before collecting within a wilderness region: some rangers, for example, may permit only surface collecting within their particular jurisdiction, such as what's allowed to take place within the Southern Inyo Wilderness at Union Wash, near Lone Pine, California, where many freely eroded species of Early Triassic ammonoids can be gathered from surface exposures only--no digging into the bedrock is allowed there without a special use permit, which is issued only to professional paleontologists and geologists conducting formal, technical research projects. Wilderness areas administered by the U.S. Forest Service (in national forests, for example) are completely off-limits to any kind of unauthorized collecting--don't even bother to ask.
At the Nopah Range fossil trilobite locality, the specimens occur in greenish shales and maroon siltstones interbedded with a minor amount of barren quartzite and limestone in the lowermost 30 feet of the Pyramid exposures. Trilobites are relatively common at this site, appearing as fragmentary portions of the arthropod's original exoskeleton: Cephalons, thoracic segments and infrequent pygidia constitute the primary finds. Extended periods of assiduous hunting--that is, several hours spent splitting shales along their natural bedding planes of deposition (the blunt end of a rock hammer works best, in combination with a selection of well-tempered chisels for the more massive chunks one might yank out of the outcrops)--may net a complete specimen or two, but don't count on it. There are no guarantees of perfect trilobite remains from an early Cambrian locality. The fragile exoskeletons of the earliest trilobites in the fossil record tended to disassociate quite easily upon the death of the animal, or during the periodic molting process, when the arthropod discarded its outgrown cover for an external shield more suitable to its larger size. Typical early Cambrian trilobites identified from the Pyramid Shale at the Nopah Range site include Olenellus clarki; Olenellus gilberti and Olenellus multinoides. Higher in the section, trilobites become exceedingly rare, although the following middle Cambrian forms have been recognized from the Groom and Belted ranges in western Nevada: Poliella lomataspis, Sysacephalus longus, Oryctocephalus nyensis and Pagetia sp.
The Pyramid shale can be traced throughout the Nopah Range. While fossil-prospecting outside the boundaries of the Nopah Wilderness, simply watch for the greenish shales and maroon siltstones sandwiched between two massive layers of bluish limestone. Fossil prospectors here will likely observe numerous trenches in the Pyramid shales all along the Nopah Range, where it is permissible for amateurs to collect.
For the past 10 years or so, trilobite specialist Ed Fowler has been studying a key section of the Carrara Formation in the Nopah Range, a specific site that Fowler wrote, in a guide book to the various Cambrian stratotypes in the Great Basin, yields--and this is a direct quote-- "not uncommon" perfect trilobite specimens. A recent examination of that locality, which lies at the top of the proposed Dyeran Stage of the Lower Cambrian Waucoban Series, proved conclusively to this writer, at least, that the section Fowler has under study contains trilobites of no greater excellence of preservation, or even abundance, than at the fossil site mentioned here--the specimens are still virtually one-hundred percent fragmental at Fowler's study site, though one must suppose that if one could dedicate hundreds of man-hours to splitting shale there, one might turn up a stray perfect trilobite or two, eventually. Indeed, the Fowler section turned out to be a major disappointment. Of course, this statement will only serve to further drive the curiosity of many a fossil seeker, who will strive to track down the Fowler locality to dertermine on his/her own whether trilobites preserved there are in a better grade of preservation: be forewarned, though: you'll just have to trust the writer on this one.
The fossiliferous Pyramid Shale Member lies in sharp stratigraphic contact directly above the underlying Gold Ace Limestone Member. The bulk of the Gold Ace is an oncolite-bearing microspar limestone, an extremely fine-grained carbonate unit that was originally deposited in a shallow oceanic setting as a lime mud. Some bedding planes yield abundant fragmental trilobites, mostly unidentifiable, though sections at Titantothere Canyon in Death Valley National Park have yielded Olenellus puertoblancoensis, Olenellus howelli and Olenellus sp., all of which demonstrate an early Cambrian age for the Gold Ace Limestone Member.
The next oldest unit in the Carrara Formation is the Echo Shale Member, named for its distinctive occurrence in Echo Canyon, Death Valley National Park. It is predominantly a green micaceous platy shale, uniformly unfossiliferous, except for one rare occurrence at Titantothere Canyon, where paleontologists identified a lone trilobite, Olenellus clarki. What's intriguing about this particular member in the Carrara, though, is that its lateral correlative shale unit is none other than the spectacular lower Cambrian Latham Shale, exposed in the Providence and Marble Mountains of San Bernardino County, California. The Latham Shale has probably produced more trilobite specimens than any other lower Cambrian formation in the western states. The once heavily visited fossil trilobite quarry in the Marble Mountains is justifiably world famous, although the area has recently been included in the appropriately named Trilobite Wilderness, a federally protected place in which unauthorized visitors must keep their hands off the trilobites preserved there.
Next-oldest of the nine Carrara members is the Thimble Limestone Member, first described at Thimble Peak on the west side of Titantothere Canyon. The Thimble is chiefly an argillaceous dolomitic limestone that weathers to shades of orange, brown and black. Some limestones in its northwesternmost exposures yield abundant fragments of echinoderms, hyolithids (a problematic molluscan specimen sometimes noted in early Cambrian deposits worldwide--a conical test roughly a half inch long) and trilobites. At a few localities (not in the Nopah Range, unfortunately) abundant identifiable trilobites have been recovered, including Bristolia anteros, Bristolia brisolensis, Bristolia fragilis, Olenellus clarki, Olenellus euryparia, Olenellus fremonti, Olenellus howelli, Olenellus puertoblancoensis, Peachella brevispina and Peachella iddingsi. It is also interesting to note that the trilobite-bearing portions of the Thimble Limestone Member probably correlate with at least part of the Latham Shale, as well.
The oldest unit in the Carrara Formation, lying directly atop the lower Cambrian Zabriskie Quartzite (which yields vertical trace fossil worm borings paleontologists usually called Scolithus, which is usually considered a member of the Phylum Phoronida, or the Horseshoe Worms) in stunning fashion, is the terrigeneous Eagle Mountain Shale Member. This is a slope-forming silty shale unit that weathers out in shades of green and gray-brown. It was named for its typical exposures at Eagle Mountain, a few miles north of Shoshone, where it reaches its best topographic development. Though generally unfossiliferous, the Eagle Mountain Shale has nevertheless produced two identifiable trilobites, Olenellus arcuatis and Olenellus cylindricus, from green micaceous shales in the lowermost few feet of the sections exposed at Echo Canyon and Titantothere Canyon.
The single best reference work dealing with the Carrara Formation is Physical Stratigraphy and Trilobite Biostratigraphy of the Carrara Formation (Lower and Middle Cambrian) in the Southern Great Basin, United States Geological Survey Professional Paper 1047, by Allison R. Palmer and Robert B. Halley, published in 1979 (still available from the USGS for $7.50) In addition to naming and precisely detailing all nine members of the Carrara Formation, Palmer and Halley also describe and figure every one of the 95-some species of trilobites thus far identified; it is, indeed, a monumental contribution to paleontology and stratigraphy. As the authors note, the Carrara Formation is not richly fossiliferous, but it yields the most complete representation of early to middle Cambrian trilobites yet described from North America.
An added bonus for collectors is that this fossil locality in the Nopah Range is an easily accessible and a very productive trilobite-bearing site. Amateurs are still welcome to visit it, as long as the area continues to remain free from litter, graffiti and vandalism. The BLM reserves the right to close the place down without advance warning, and they will most certainly do just that if visitors abuse their privileges here. With so many exceptional fossiliferous exposures of the Carrara Formation already closed due to the Wilderness and California Desert Protections acts, it would be a shame to lose yet another, this time to our own boorish behavior.