House Range Fossils: Wheeler Shale, Marjum Formation, and Weeks Formation
Fossil Sites
 

Fossils of the House Range in Utah

Wheeler Shale, Marjum Formation, and Weeks Formation, and other formations

Also see:
Pioche Formation

Related Interest:
Cambrian Explosion Fossils of Utah
Utah Trilobites
American Trilobites (where other Wheeler and Marjum trilobites, and other Utah localities are included)
House Range Photographs

 


Fossils from the House Range in Utah

Geological map of UtahArguably, Utah is America’s most fossiliferous state. In fact, a Utah geological map is a mosaic of colors, with formations spanning geological time from the Cambrian. The Cambrian period is especially well represented in Utah’s fossil record and its House Range is one of the most famous Cambrian fossil localities in the world. The House Range (see accompanying images) includes the Wheeler Shale and the overlying Marjum Formation, which are exposed throughout the House Range and nearby mountain ranges west of the town of Delta, Utah. The Wheeler Shale is named for a great bowl-shaped feature in the House Range known as the Wheeler Amphitheater, while the Marjum Formation is named for its outcrops at Marjum Pass, also in the House Range. While much of the House Range near Delta UtahWheeler Shale lacks fossils, certain layers contain abundant trilobites and other shelly fossils. The Wheeler Shale and Marjum Formation also contain a diverse biota of soft-bodied fossils, including many of the same taxa found in the more famous Burgess Shale, which qualifies the House Range as a Lagerstätte containing the evolutionary outcome of the Cambrian Explosion. The soft-body preservation found in the House Range fossils supports the inference that anoxic conditions sometimes existed. The central portion of the House Range also contains the Weeks Formation, a limestone that overlies the Marjum Formation and also contains well preserved trilobites.

Modocia weeksensis Weeks Formation TrilobiteDuring the Lower Cambrian (570 to 540 mya), western Utah was covered by a shallow sea. Sediments arrived from rivers flowing across the sandy lowlands of eastern Utah. Sand that was deposited near the shoreline eventually metamorphosed into quartzite, while finer sediments and silts that were deposited farther to sea eventually underwent lithification to become siltstone, sandstone, and shale. During the Middle to Late Cambrian (540 to 500 mya) the inland sea’s shoreline migrated east; this subsidence resulted in a relatively complete sequence in western Utah spanning the Cambrian. Utah was located near the equator during the Cambrian, so the water temperature was warm. The combination of warm, shallow water and nutrient-rich silt enabled a highly diverse fauna. The trilobites were particularly abundant and diverse due to the multiple environmental niches that they could occupy.

Pseudoarctolepis sharpi Rare Cambrian Phyllocarid Fossil from Wheeler ShaleThe House Range is most famous for its trilobites (Many House Range trilobites are here), with specimens residing in museums throughout the world. More than 500 species are described. The most famous trilobite found prodigiously throughout the House Range is Elrathia kingi. Trilobites of Order Agnostida are also diverse. Other invertebrates are also abundant, including sponges such as Diagonella and Choia, and Chancelloria, brachiopods, and echinoderms. Gogia, an early echinoderm of class Eocrinoidea (also called eocrinoid for “dawn crinoid”), are abundant. More than twenty species of non-mineralized arthropods have been described from the Wheeler Shale, as well as annelids, priapulids, and other worms, and the predator Anomalocaris, terror of the Cambrian. Brachiocaris and Pseudoarctolepis, enigmatic Phyllocarids, are probable crustacean arthropods. Emeraldella, which may be a basal Chelicerate, has similarities to Parapaleomerus sinensis from the Chengjiang Biota of China. Naraoia, soft-bodied trilobitomopha, often called soft-bodied trilobites, are found. Wiwaxia, a putative, primitive annelid first described from the Burgess Shale also occurs in the House Range.

Wiwaxia soft-bodied animal  from Marjum FormationExamination of Utah stratigraphy reveals fossiliferous rocks of every period from the Precambrian Proterozoic onward. Millard County alone has fossils from almost every period except, maybe the Jurassic and Cretaceous (though some old-timers claim the Cretaceous is represented). There are 10 distinct known Cambrian and Ordovician formations in the House Range; these include the lower Cambrian Prospect Mountain Quartzite, lower to middle Cambrian Pioche Formation, middle Cambrian Howell Limestone, Chisholm Formation, DomeLimestone, Whirlwind Formation, Swasey Limestone and then the well-known Wheeler Shale, Marjum Formations and Weeks Limestone. The Ordovician is represented by the Orr Formation, Notch Peak Formation, and the Pogonip Group, which comprises the House Limestone and the Fillmore Formation (Reference, personal communication with Delta, Utah resident, Robert Drachuk). The chart at the right shows Hintze’s and Robison’s 1975 representation of the Cambrian stratigraphy in the House Range.

Gaines, et. al. has recently studied the taphonomy of the Wheeler Shale delineating how some layers preserve soft-bodied organisms, normally with an absence of trilobites, while others preserve trilobites normally without other creatures with soft parts preserved. In particular, Elrathia trilobite laden biofacies and those with soft part preservation are mutually exclusive. The authors hypothesize the cause was differing degrees of bioturbation. When bioturbation was low, irrigation by seawater of impermeable clay-rich sediment was reduced allowing organic decay to proceed by means of sulfate reduction in anoxic sediments. The chemical process increased alkalinity and precipitated carbonate within pore spaces which further restricted bacterial activity; horizontal pyritized burrows are described that are typical of sediments formed under oxygen-depleted bottom waters. The Elrathia biofacies, in contrast, exhibit dolomite precipitated in trilobite carapaces, forming trilobite "nodules." The Elrathia biofacies, in contrast, probably represent deposition under dysaerobic consitions where bioturbation was greater, increasing bottom water oxygen content that, in turn, mediated more rapid decomposition of soft tissues and colonization by a benthic fauna.

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