The thick (1.3 km) and virtually complete Middle Cambrian platformal succession of the southern Rocky Mountains consists of four Grand Cycles, each beginning with a formation of the inner-detrital facies belt and ending with a formation of the middle carbonate facies belt. The first three cycles, Mount Whyte/Cathedral, Stephen/Eldon, and Pika, share a common depositional style and are the main subject of this bulletin. The terminal, Arctomys/Waterfowl, cycle is of a distinctly different style and will be presented in a separate paper. Basinal equivalents of the platformal formations, the lower and middle parts of the Chancellor Group, have not been studied in detail and receive only brief mention. The description of a terminal Lower Cambrian carbonate unit known as the Peyto and Hota formations, clarifies some Middle Cambrian problems. Dominant influences on Middle Cambrian sedimentation in the region were the thermally subsiding Cordilleran Miogeocline; a hinge-line near the present mountain front;; a source of fine grained detritus to the north; and the Kicking Horse Rim, a persistent paleogeographic "high". The Mount Whyte Formation is subdivided into two new members, the Weed and Chephren. The established Ross Lake Member, the new Trinity Lakes Member, and a wedge of deep-water carbonates, the Takakkaw Tongue, are recognized in the Cathedral Formation. The Takakkaw Tongue extends basinward from the platform-margin reef. The Stephen Formation is subdivided into four new members, the Narao and Waputik members on the platform, and the Amiskwi and Wapta members in the basin. A tongue of outer detrital siliciclastic strata that penetrates the platform-margin shallow-water carbonates of the Eldon Formation is formally recognized as the Field Member. One new member, the Tershishner Member, is recognized in the Pika Formation of the Front Ranges. Formations and members of the Mount Whyte/Cathedral/Stephen succession are recognized only as members where they pass northward into the more shaly Snake Indian Formation. Outer-detrital (slope facies) equivalents of the Mount Whyte Formation are recognized as the Naiset Formation.

The acritarch assemblages of strata from the base of the Upper Proterozoic Sheepbed Formation to the base of the Lower Cambrian (Atdabanian) Sekwi Formation are described. The sections sampled are in southwestern (internal) structural units where erosion beneath the "sub-Cambrian"(?) unconformity is least evident. Problems of lithostratigraphic correlation of post-Sheepbed, pre-Backbone Ranges formations remain. Acritarchs indicate the age of the Sheepbed Formation and the Blue-flower Formation above it is latest Proterozoic (Vendian), whereas that of the Vampire Formation is Early Cambrian (Atdabanian). The Backbone Ranges Formation has not yielded datable acritarchs, but it is for the most part Cambrian in age, based on other fossil evidence. Comparisons are made with the Russian Platform and southern Canadian Rocky Mountains successions. The total number of acritarch genera increases markedly across the Precambrian–Cambrian transition.

No abstract

Much of the existing literature on the base of the Cambrian System in the southern Rocky Mountains tends to de-emphasize the importance of the stratigraphic hiatus represented. Although a sub-Cambrian regional unconformity has been reported by several authors, conclusive evidence of the unconformity has not been published.This paper documents profound truncation of Miette Group strata of Windermere age at the base of the Gog Group of Early Cambrian age, and draws attention to several localities at which Precambrian structures are truncated at the base of the Cambrian. The sub-Cambrian unconformity is comparable in style and magnitude to the sub-Devonian unconformity of the same region.

Sinemurian phosphorites in southeastern British Columbia and southwestern Alberta conform with the "West Coast type" phosphorite depositional model. The model indicates that they were deposited on or near the Early Jurassic western cratonic margin, next to a sea or trough from which cold water upwelled. This suggests that the allochthonous terrane Quesnellia lay well offshore in Sinemurian time. The sea separating Quesnellia from North America was partly floored by oceanic crust ("Eastern Terrane") and partly by a thick sequence of rifted, continental terrace wedge rocks comprising the Purcell Supergroup and overlying Paleozoic sequence. This sequence must have been depressed sufficiently that access of upwelling deep currents to the phosphorite depositional area was not impeded.Les phosphorites du Sinkmurien dans le sud-est de la Colombie-Britannique et le sud-ouest de 1'Alberta ont une origine en accord avec le modele skdimentaire des phosphorites du << type de la cBte Ouest B. Le modkle indique que la skdimentation a eu lieu sur ou prks de la marge cratonique occidentale d'bge jurassique pdcoce, au voisinage d'une mer ou d'une fosse ou se produisait une remontke d'eau profonde froide. Ceci suggkre que le terrane allochtone de Quesnellia, au temps sinkmurien, se trouvait localis6 bien au large. Le fond de la mer qui skparait le terrane de Quesnellia du continent de 1'Amkrique du Nord ktait partiellement occupk par une crotite ockanique (N terrane de 1'Est m) et aussi partiellement par une s6quence puissante de roches formant un prisme sur une terrasse continentale effondke par distension, incluant le Supergroupe de Purcell et la skquence palkozoique sus-jacente. Cette skquence a dO etre dkprimke suffisarnment pour permettre aux courants de remontke des eaux profondes d'acckder i l'aire de dkpBt des phosphorites.[Traduit par la revue]Can.