Abstract The Jurassic sediments of the southern Alberta plains, known only from subsurface work, are described as to lithology, thickness, distribution, stratigraphic relationships, and geologic history. The three-fold subdivision of the Ellis group into Swift formation, Rierdon formation, and Sawtooth formation, established in Montana, is recognizable and applicable in Alberta. A major unconformity between the Jurassic beds and the underlying Paleozoic limestone influences the distribution of the Sawtooth formation. Late Jurassic or early Cretaceous erosion truncating the Jurassic formations from south to north has determined the present extent and northern boundaries of these formations in southern Alberta.

A sub–Middle Jurassic unconformity is exhumed at Swift Reservoir, in the Rocky Mountain fold-and-thrust belt of Montana. The unconformity separates late Mississippian Sun River Dolomite of the Madison Group (ca. 340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (ca. 170 Ma). North-northwest–trending, karst-widened fractures (grikes) filled with cherty and phosphatic sandstone and conglomerate of the basal Sawtooth Formation penetrate the Madison Group for 4 m below the unconformity. The fractures link into sandstone-filled cavities along bedding planes. Clam borings, filled with fine-grained Sawtooth sandstone, pepper the unconformity surface and some of the fracture walls. Sandstone-filled clam borings also perforate rounded clasts of Mississippian limestone that lie on the surface of the unconformity within basal Sawtooth conglomerate. After deposition of the overlying foreland basin clastic wedge, the grikes were stylolitized by layer-parallel shortening and then buckled over fault-propagation anticlinal crests in the Late Cretaceous–Paleocene fold-and-thrust belt. We propose that the grikes record uplift and erosion followed by subsidence as the Rocky Mountain foreland experienced elastic flexure in response to tectonic loading at the plate boundary farther to the west during the Middle Jurassic. The forebulge opened strike-parallel fractures in the Madison Group that were then karstified. The sandstone-filled karst system contributes secondary porosity and permeability to the upper Madison Group, which is a major petroleum reservoir in the region. The recognition of the fractures as pre–Middle Jurassic revises previous models that have related them to Cretaceous or Paleocene fracturing over the crests of fault-propagation folds in the fold-and-thrust belt, substantially changing our understanding of the hydrocarbon system.

Abstract Sediments of Middle Jurassic and Upper Jurassic age are present in the extreme southern plains and in the foothills and mountain areas of Alberta. On the plains the northern boundary of the Jurassic is roughly crescentic in shape, approximately coinciding with the South Saskatchewan River and its tributary, the Oldman River, and from there trending northwestward, passing a few miles east of Calgary. On the Alberta plains, Jurassic strata are readily divisible into three formations, which were originally described and named in Montana, and which are, in ascending order, Sawtooth, Rierdon, and Swift. In the Alberta foothills, Jurassic strata are called the Fernie group, which is roughly the equivalent of the three formations of the plains. Jurassic strata in southern Alberta rest unconformably on the Rundle formation of Mississippian age. On the plains they are overlain unconformably by the basal sandstones of the Blairmore formation which have been correlated with the Cutbank and Sunburst sands of Montana. Post-Jurassic erosion truncated the formations so that they wedge out northward; consequently, the Swift being the uppermost has a very limited distribution in Alberta, having been eroded completely from the crest of the Sweetgrass arch and remaining only in the extreme southeastern corner of Alberta and in a narrow belt paralleling the foothills. The Rierdon and Sawtooth formations extend northward approximately to the limits of the Jurassic as defined above. In the western foothills the Fernie is overlain conformably in most places by Kootenay sandstone and shale of Lower Cretaceous age. The Sawtooth formation consists of two sandstone members, separated by green, pyritic, non-calcareous shale. The upper sand or its equivalent is a reservoir for oil and gas in several small fields in Alberta. The formation has a maximum thickness of about 235 feet. The Reirdon formation is made up of alternating gray calcareous shale and greenish-gray limestone with some pyrite, glauconite, and ironstone, and has a maximum thickness of 250 feet. The Swift formation lies unconformably on the Rierdon and has a maximum thickness of 150 feet. It consists of glauconitic sandstone, siltstones, concretions, dark gray shale, and generally has chert pebbles at the base. Facies change from west to east in the Fernie group of the foothills to the Ellis group of the southern Alberta plains; but the changes are not so pronounced as to preclude the making of satisfactory correlations. On the east flank of the Sweetgrass arch the Ellis group can be readily correlated with the Gravelbourg, Shaunavon, and Vanguard formations of Saskatchewan. However, they are not the exact equivalents of these formations as some overlapping occurs.

Abstract The Sweetgrass arch in northern Montana has yielded nearly 500 billion cu ft of natural gas since the first discovery in 1915. The 24 gas fields which have been found are mainly on the flanks of the arch. Most of the accumulations are in stratigraphic traps, but closures and faulting are controlling factors in some cases. Several fields have multiple pay zones. The producing formations, from oldest to youngest, are the Sun River Dolomite of Late Missis- sippian age; the Sawtooth Formation of Middle Jurassic age; the Swift Formation of Late Jurassic age; and the Cut Bank Sandstone, Sunburst Sandstone, and Blackleaf Formation of Early Cretaceous age. All of the reservoir rocks are sandstone except the Sun River Dolomite. The Lower Cretaceous reservoirs are the most important gas zones in the area, having contributed more than 90 per cent of the cumulative yield. The giant Cut Bank field alone has yielded about 70 per cent of the gas obtained from the Sweetgrass arch. All of the gas produced from the area is marketed in western Montana. The current yield of about 16 billion cu ft annually is less than one-half that needed to supply present market demand.