Marc Behrendt |
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The Weeks Formation - by Marc Behrendt
Out in the desert hills of the House Range in western Utah lies a formation unique to the area. The trilobite fauna collected from this area is diverse and ranges from the fantastic to the ubiquitous. The matrix is famous for its coloration. It is a favorite of many collectors. It is the Weeks Formation.
The Weeks Formation is an Upper Cambrian unit found solely in the House Range of Utah. The exposed layers occur as isolated pockets around the Marjum Pass with the most notable exposure being in North Canyon, which was previously known as Weeks Canyon. The Weeks Formation lies directly above the Middle Cambrian Marjum Formation and lies directly beneath the Big Horse Limestone Member of the Orr Formation. Charles Walcott in 1903 and 1905 was the first to investigate the Weeks, including the quarry that most collectors are familiar.
The bottom layer of the Weeks Formation, ranging from 30 – 80 meters thick, is a black mudstone. Lamination (layering or thin bedding planes) alternates between mud-rich and mud- poor sedimentation. Fragments of trilobites and echinoderms are rare, though burrows may be seen. In some sections, bedding may be indistinct or absent. These layers may have an occasional shale layer that may contain rare complete agnostid trilobites or sponge spicules. Likely, this was a deep water, low energy environment.
The following rock types alternate through the Weeks above the bottom layer. The skeletal-wackestone (mud-supported) and packstone (grain-supported) lithofacies range from well-laminated to bioturbated to homogeneous (no bedding planes). When exposed to the weather this rock will turn a reddish color. Sponge spicules are common, lingulid brachiopods, trilobites and echinoderms are present. This rock was deposited in a typical open ocean or subtidal environment.
The Peloidal-packstone and grainstone lithofacies is a thinly
laminated to moderately homogeneous rock containing fragmented trilobites
and echinoderms. Freshly exposed dark grey rock turns to a light grey or
orange color upon weathering. Lingulid brachiopods and rarely trilobites
may be found on bedding surfaces. These sediments were deposited on a
marine shelf where the depositional environment fluctuated between high and
low energy.
The Ooid-grainstone lithofacies section is dark grey on a fresh surface and weathers to a mottled orange and grey. This unit, when found, is only about 3 meters thick. Fragmented fossils are uncommon. It was likely a shallow higher-energy environment than the other lithofacies. Occasionally, a bedding plane may contain ripples.
The silty lime-mudstone lithofacies is a major constituent rock of the Weeks Formation. Limey mudstone beds are 1-2 cm thick and when freshly exposed are medium grey with orange bedding layers. Weathered rock turns to a dull reddish orange. Lingulid brachiopods are common on the bedding surfaces and trilobites are uncommon. These sediments were deposited in a shallow, restricted shelf area that periodically yielded poor living conditions. Occasionally, a bedding layer will yield many trilobites, but generally organisms were not numerous. BeeBe (1990) has suggested that the environment periodically experienced hypersalinic events. This is the facies that collectors are most familiar.
Barren lime-mudstone lithofacies consisted of 2-4 cm thick beds separated by reddish bedding planes. Burrows are evident in this rock; fossil fragments are very rare suggesting this was a shallow, low energy and stressed environment.
The lowest 30 meters of the Weeks Formation contains a distinctly different fauna than the upper sections. Agnostids are the sole trilobite representatives, including Lejopyge and Ptychagnostus. Sponge spicules may be common. Polymeroid trilobites are not found.
The upper levels contain some agnostid species but polymeroid trilobites are the predominant form. Also found are the aglaspid Beckwithia typa and an undetermined echinoderm. Inarticulate brachiopods are not found in the first 60 meters of the Weeks, but may be quite common in most of the above layers.
Collecting in the Weeks Formation is not difficult. The rock is hard and thick but lies in flat even layers that split easily with a hammer and chisel. Fossils are not exposed on a fresh surface, but usually remain under a layer of matrix requiring close examination. The collector must tilt the rock so shadows form, and then examine each and every shadow and suspicious lump. Some of the trilobites are very tiny; some are large, so one never is sure what to look for. However, the work is well worth it! When prepared, the trilobites are often complete with thick, beautiful caramel-brown shell. At times, a trilobite impression will have no shell present, so one must be prepared for some duds.
The most common of the trilobites at the Walcott quarry is Cedaria, followed by Densonella, Modocia, Menomonia, Dresbachia, Weeksina, Democephalus, and Tricrepicephalus. Among the rarest of the trilobites are Olenoides, Chiasmacantha, Maryvillia, Meteoraspis, Norwoodia, Selenocoryphe, plus a small host of agnostid trilobites. Other species are known, but only from a few specimens. Non-trilobite fauna include the aglaspid Beckwithia, brachiopods, and soft body animals.
Agnostid trilobites, among the rarest of the Walcott quarry fauna, were known only from a few complete specimens and several fragments. Recently, 3 of the species from the Weeks - Triadaspis, Ammagnostus, and Proagnostus, were found in a new Weeks Formation exposure locality. Complete Lejopyge are exceptionally rare, though a hash layer of fragmented Lejopyge specimens can be found at the base of the Weeks Formation.
Recently, a private group is leasing and quarrying the famous Walcott quarry for its lovely reddish-brown shale and selling the rock as patio stone! The Bureau of Land Management who manages this public land, does not allow commercial collecting of fossils. However, a local BLM representative stated that since this quarry is mining only the rock and not the fossils, there is no reason to stop their operation! The BLM was unimpressed that historically, the site has been a paleontological site since Walcott discovered and worked it up in the early 1900s. Throughout our discussion, the BLM employee continued to remind me, the quarry outfit is only taking the rock, not the fossils!
Collecting the Weeks formation fossils can be astonishingly fantastic or agonizingly frustrating. The quarry operation has made collecting difficult at times, but when the opportunity allows itself, a day can pass very quickly while splitting limestone and searching for the shadowy impressions of the Upper Cambrian’s marine denizens.
(Acknowledgement: I thank Ron Rea for his knowledge and guidance as I tried to define Dr. Walcott’s geological terminology. I also thank Ken Karns and Shanan Peters Ph.D for their suggestions and advice)
References: Beebe, Matthew, May 1990, Trilobite Faunas and Depositional Environments of the Weeks Formation (Cambrian), Utah, University of Kansas, 111 pages.
Plummer, Charles C. and McGeary, 1988, David, Physical Geology, Wm. C. Brown Publishers, 535 pages.
Bentley, C.B., 1958, Upper Cambrian Stratigraphy of Western Utah. Brigham Young University Research Studies. Geology Series, 70 pages.
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