Cambrian faunas of the Snowy Range formation, Southwestern Montana and Northwestern Wyoming
Creators: name: Grant, Richard E., 1927-1994
Publisher: name: The University of Texas at Austin
Year: 1958
Subjects: subject: Paleontology--Montana--Cambrian
subject: Paleontology--Wyoming--Cambrian

description: Fossils and rock samples were collected from the Snowy Range formation (Cambrian) at 24 sections measured in the Horseshoe Hills and Bridger Mountains of Montana and 8 sections in the vicinity of Yellowstone National Park in Montana and Wyoming. Where the Snowy Range formation is overlain by the Maywood unit (Devonian), both were measured and sampled, although the Maywood proved to be unfossiliferous. Lowermost of the 3 members of the Snowy Range formation is the Dry Creek shale which lies conformably on the Pilgrim limestone. It consists of about 50 feet of purplish, thin bedded, fissile to slightly plastic shale with a few irregular beds of brownish gray, platy, dolomitic siltstone in the lower three-fourths, and some thin beds of silty limestone or limestone-pebble conglomerate in the upper quarter. No evidence is present in this area for subaerial erosion between the Pilgrim and Snowy Range formations, although beds of siltstone in the Dry Creek may reflect uplift and erosion at the source of terrigenous sediments. The middle member is the Sage, which averages about 200 feet thick where complete. Its base is marked at most localities by a one to 20 foot bioherm of columnar algal limestone; the remainder is a fairly regular alternation of one to two foot beds of limestone or limestone pebble conglomerate with 2 to 4 foot beds of green shale or very argil laceous greenish gray limestone. The upper part contains beds of non columnar algal limestone. This member was deposited far from shore in shallow turbid water that contained abundant calcium in solution. Microcrystalline calcite ooze, along with fine fragments of fossils arri fecal pellets, collected in ripple troughs and other depressions in the mud of the sea floor. These small accumulations became sufficiently consolidated to maintain coherence when excavated by currents that swept away clay particles and concentrated these consolidated limestone "pods" into beds that later were cemented to form limestone-pebble conglomerate. The uppermost member, the Grove Creek, is nowhere complete in this area; the upper part was removed by erosion at some localities and by faulting at others. The member consists of about 25 feet of dolomitized limestone-pebble or cobble conglomerate with intercalated beds of dolomitized gray splintery shale. This member owes much of its distinctive character to weathering that took place before deposition of the Upper Ordovician Bighorn formation. The Maywood unit is a Devonian soil or weathered zone that overlies some part of the Snowy Range formation in the northwestern part of the area of study, but overlies formations as low as those in the Precambrian Belt Series or as high as the Upper Ordovician Bighorn formation in other parts of Montana. It consists of a greatly varying thickness (averaging about 55 feet) of reddish orange or brown, thin bedded, silty dolomite or dolomitic limestone. At a few localities it is coarse grained, thick bedded, light brown dolomite. No fossils were found in the Maywood in the area studied, but Devonian fossils have been collected from it in other parts of Montana. Fossils described from the Snowy Range formation are trilobites (80 species in 48 genera: Aphelaspis whitfordi, Pinctus artus, P. pullus, Taenicephalus gallupensis, Saratogia carita, Monocheilus demissus, and Homagnostus insolitus are new species, Comanchia amplooculata lippa is a new subspecies, and Simbaltea is a new generic name for Kendallia Raasch, which is preoccupied); brachiopods (20 species in 14 genera: Angulotreta glabra. A. vescula and A. catheta are new species, Huenella taxana var. fortis and Eoorthis remnicha var. A are new varieties); gas tropods (4 species in 4 genera, none new); sponges (one species); graptolites (one species); unidentifiable pelmatozoan fragments; miscellaneous problematica; algal limestone (two form-species). Fossil species in the Snowy Range formation are assigned to local zones, based on ranges of genera and species of trilobites, which can be correlated with standard zones of the Cambrian Correlation Chart or local zones of other areas. A few species that occur near the base of the formation belong to the Aphelaspis and "post-Aphelaspist" zones of the Dresbachian Stage and are equivalent to the Aphelaspis zone of the Chart and the Aphelaspis and "post-Aphelaspis" zones of Texas. Most species in the formation belong to one or more zones of the Franconian Stage. Lowermost of these is the Elvinia zone (with Camaraspis subzone and Irvingella major zonule in the upper part) which corresponds to the Elvinia zone of the Cambrian Correlation Chart. Next higher is the Taenicephalus zone (with Parabolinoides subzone at base) which correlates with the Conaspis zone of the Chart. Above this is the Idahoia zone (with basal I. wyomingensis subzone, middle I. wisconsensis sub zone and upper I. serapio subzone) which correlates with the Ptychaspis subzone of the Ptychaspis-Prosaukia zone of the Chart. The uppermost zone of the Franconian is the Prosaukia zone, which is equivalent to the Prosaukia subzone of the Chart. Species of the Trempealeauan Stage are assigned to the Illaenurus zone, which corresponds to the Lower Trempealeauan of Texas or the lower part of the Saukia zone of the Upper Mississippi Valley. An alternative zonation is offered, based on the ranges of genera and species of brachiopods. Boundaries of most brachiopod zones fall at the levels of the boundaries of trilobite zones, but some do not. The Apsotreta expansa zone is equivalent to the "post-Aphelaspis" zone of the Dresbachian Stage. The Linnarssonella zone and the Angulotreta tetonensis zone (with Ceratreta-Eoorthia subzone at base) correspond respectively to the Elvinia and Taenicephalus zones of the lower part of the Franconian Stage. The Angulotreta vescula zone corresponds to all but the uppermost part of the Idahoia zone, and the A. catheta zone (with Finkelnburgia osceola subzone at top) is equivalent to this uppermost part of the Idahoia zone and the entire Prosaukia zone of the Franconian Stage as well as the totality of the Illaenurus zone of the Trempealeauan Stage. Thus no Franconian-Trempealeauan boundary is apparent in the brachiopod zonation