Elk Point Basin Research Articles

Calcium Pumping and Anhydrite/Halite Relationships in Silurian A Unit of Michigan Basin: ABSTRACT

Observed relationships between anhydrite and halite in the A unit of the Michigan basin are not easily explained by classical evaporite depositional models. Within the Northern Reef trend, productive Niagaran pinnacle reefs are surrounded by A unit halite, which commonly exceeds 100 m in total thickness. However, A unit evaporites consist of thick anhydrite deposits on reef flanks and above reefs in the A-1 and A-2, respectively, Stratigraphic data suggest that the anhydrites surrounding reefs are contemporaneous with off-reef halite deposits. This reef-evaporite relationship poses three problems. (1) Why would gypsum precipitate from a halite-saturated brine (2) Why are anhydrites associated with the reefs (3) Why are anhydrites significantly thicker than predicted by evaporation models In a normal marine evaporation sequence (Hardie-Eugster model), gypsum is deposited from a brine until calcium is depleted. Upon further evaporation, the resultant halite-saturated brine would precipitate gypsum only in contact with a calcium source. The authors propose a calcium pumping mechanism whereby calcium-rich water associated with pinnacle reefs is responsible for gypsum precipitation around these reefs contemporaneous with off-reef halite. The additional supply of calcium also explains the anomalous thickness of these anhydrite deposits. Similar anhydrite halos around pinnacle reefs have been observed in more » the Devonian Elk Point basin. « less

Fluid inclusion and isotopic evidence on dolomitization, Devonian of Western Canada

The Presqu'ile and Manetoe Facies are diagenetic features developed in Lower and Middle Devonian Formations of the Elk Point Basin (Presqu'ile) and the Mackenzie Shelf (Manetoe). Both facies contain coarsely crystalline dolomite and white sparry dolomite cement. Less extensive diagenetic phases, in order of paragenesis, include fluorite, anhydrite, barite, calcite, quartz, sphalerite and galena. Conditions of dolomitization are outlined from core and outcrop examination, thin section and cathodoluminescent petrography, fluid inclusion analyses, and C and O isotopic data. Fluid inclusion and stable isotope analyses from dolomite are combined to determine the isotopic composition of the dolomitizing fluids. Chemical analyses of fluid inclusion waters are compared with formation water analyses to derive a proposed origin for the diagenetic fluids. Dolomite formed from hot, high salinity fluids early in the diagenetic history of the basin. Limestone was dolomitized as a result of the same process that created white sparry dolomite cement in the Manetoe Facies. Remnants of the dolomitizing fluids are present as formation waters in some Devonian formations in Alberta. Subsequent invasion of meteoric waters produced more dilute diagenetic fluids which resulted in the precipitation of calcite and quartz cement in the Manetoe Facies. These fluids are present in fluid inclusions and are responsible for the low 18O content of the calcite cement. The temperatures of calcite and quartz formation differ as a function of burial depth within the Manetoe Facies during the Mesozoic and Cenozoic, while the temperature of formation for dolomite does not.

Genetic Sequence Relationships of Winnipegosis Platform Carbonates, Southern Elk Point Basin, North Dakota: ABSTRACT

Genetic Sequence Relationships of Winnipegosis Platform Carbonates, Southern Elk Point Basin, North Dakota: ABSTRACT

Genetic Sequence Relationships of Winnipegosis Platform Carbonates, Southern Elk Point Basin, North Dakota: ABSTRACT

Examination of cores and well log data from the Winnipegosis Formation (Givetian) within a study area of approximately 11,500 mi/sup 2/ (30,000 km/sup 2/) in northern North Dakota allows recognition of seven time-stratigraphic progradational units within the Winnipegosis Formation. Together with the underlying Ashern Formation, these units are arranged in landward-stepping, vertical stacking, and seaward-stepping geometric patterns, which reflect changes in relative sea level. Abrupt juxtaposition of shallow over deeper water lithologies, evidence for subaerial exposure, and onlap geometries further suggest that these progradational units form two larger, Vail-type sequences separated by regionally persistent unconformities or their correlative conformities. Sea level rise during the early Eifelian caused southeastward onlap of the Ashern Formation onto Middle Silurian carbonates of the Interlake Formation. Maximum flooding, expressed by deepest marine facies and a hardground surface, suggests the existence of a condensed section at the top of the Ashern Formation. This was developed during the maximum rate of sea level rise. A decrease in the rate of sea level rise resulted in aggradation of lower Winnipegosis units on a gently dipping ramp. These are represented by nodular and burrowed open marine limestones with scattered stromatoporoid patch reefs and grainstone shoals. During the subsequent seamore » level fall, represented by Temple units, a shelf margin with pronounced depositional topography and adjacent starved basin were developed. Temple strata include coral-brachiopod-stromatoporoid reefs and productive fore-reef talus deposits along the shelf margin rim.« less

Reservoir development and hydrocarbon potential of Winnipegosis (Middle Devonian) pinnacle reefs, southern Elk Point Basin, North Dakota

Winnipegosis (Middle Devonian) pinnacle reefs have been a major exploration target within the southern Elk Point Basin for over 20 years. To date, ten pinnacle reefs have been drilled in North Dakota with commercial quantities of oil being conspicuously absent. Many of these pinnacle reefs have excellent reservoir potential as indicated by the recent discovery of the Home-Tableland reef in southeastern Saskatchewan.

Manetoe Facies--A Gas-Bearing, Megacrystalline, Devonian Dolomite, Yukon and Northwest Territories, Canada

The Manetoe facies is a broadly stratiform and regionally developed (38,000 km2 or 15,000 mi2) body of white sparry dolomite hosted in Lower and Middle Devonian strata, and is the northwestward continuation of the similar, but better known, Presqu'ile facies dolomite. Several large gas fields have been discovered within anomalously thick developments of the Manetoe dolomite close to the northwest limit of the Devonian Elk Point basin of Alberta. The external geometry of the Manetoe facies indicates that it may have been a cavern or karst system excavated in the late Middle Devonian by a large, vertically confined coastal aquifer. The local regions of anomalously thick Manetoe facies may have developed where the aquifer stoped upward into strata above the confining aquiclude of the Headless Formation shale. Dolomite cementation may have occurred during any of three possible episodes of large-scale fluid movement. Fluid-inclusion and strontium isotope data favor the hypothesis that Manetoe dolomites precipitated from heated (150°-210°C or 300°-410°F) hypersaline evaporitic brines that have interacted with the underlying Precambrian crust, probably in the late Devonian. Lead-isotope data support the general concept of pre-Mesozoic dolomite cementation, and largely preclude a late Early Tertiary deep burial setting. Spectacular zones of detrital dolomite cement within the Manetoe are most readily accommodated by a near-surface, very early Middle Devonian aquifer mixing-model origin, but the presence of these zones does not preclude the other possible origins. Carbon and xygen isotope data from the Manetoe facies support the concept that the Presqu'ile and Manetoe facies are physically continuous and had a common origin.

Some Musings on The Devonian Elk Point Basin, Western Canada

ABSTRACT There is a large volume of literature on the Elk Point Basin; it is probably the best-known body of Devonian rocks in Canada, possibly in North America. There are, however, still many unanswered questions; also, some answers long taken for granted are neither the only ones possible nor necessarily the best. These six rhetorical questions stress the uncertainties: 1) Where was the eastern shoreline? Unknown, but certainly well east of the present edge. 2) What circulation barrier isolated the Elk Point brines? A carbonate barrier in part, but other factors may have been involved. 3) Was there a land area to the west, the West Alberta Ridge - Peace River Arch? At times, certainly, but not necessarily at all times. 4) What happened to switch off Hume - Keg River - Winnipegosis platform carbonate production? A eustatic pulse is but one of several possibilities. 5) What, in the south, corresponds with the Watt Mountain break? Probably the 6) The Meadow Lake Escarpment, a misnomer? Probably. The concept of a paleoescarpment of high relief is hard to defend, and is quite unnecessary in interpreting the geology of the area. QUELQUE MEDITATION AU SUJET DU BASSIN DE ELK POINT DE L'OUEST CANADIEN RESUME Un bon nombre d'etudes on ete publiees sur le Bassin de Elk Point; c'est probablement l'assemblage geologique d'age devonien le mieux connu au Canada et meme en Amerique du Nord. Cependant il reste encore plusieurs questions sans reponse; et meme certaines reponses longtemps considerees comme acquises ne sont plus les seules possibles ni necessairement les meilleures. Les six questions qui suivent font bien ressortir les incertitudes qui demeurent: 1) Ou se trouvait la ligne de rivage a l'est du bassin? Sa position n'est pas connue mais elle devait etre certainement beaucoup plus a l'est que la limite actuelle du bassin. 2) Quel type de barriere de circulation a permis d'isoler les evaporites du Elk Point? Une barriere carbonatee a certainement joue un role mais d'autres facteurs pourraient etre consideres. 3) Y avait-il a l'ouest une terre emergee (crete de l'Alberta de l'ouest - Arche de Peace River)? Surement qu'a certains moments il y avait une terre emergee mais pas necessairement tout le temps. 4) Qu'est-il arrive pour que le developpement de la plateforme carbonatee Hume - Keg River - Winnipegosis s'interrompe? Un changement eustatique est une des possibilites a envisager parmi d'autres. 5) Qu'est-ce qui, au sud, correspond a la disparition du Watt Mountain? Probablement les Second Redbeds. 6) L'Escarpement du Lac Meadow, une fausse appellation? Le concept d'un paleoescarpement a haut relief est difficile a justifier et est, de plus, tout a fait superflu pour interpreter la geologie de la region. Traduit par Helene Gignac

Paleotectonic Controls on Sedimentation in Northern Williston Basin Area, Saskatchewan: ABSTRACT

The Williston basin lies within the so-called stable cratonic interior and would not be expected to have had the same intensity of tectonic activity as is generally considered to be characteristic of cratonic margin sedimentary basins. Local paleotectonic controls on sedimentation are much more subtle, thus sediment distribution patterns are commonly related to major regional structural elements such as the ancestral Williston basin and the Sweetgrass arch. From time to time, however, other structural features appear to have been effective controls on sediment distribution patterns. In southern Saskatchewan, one of the most active of these was the Swift Current platform. This feature appears to have been sufficiently positive during early Paleozoic time to have caused a d stinct thinning of those sediments over it. But it is in early Middle Devonian time that it plays a more prominent role. During the time the Winnipegosis Formation was being deposited, the platform acted as a core for an accreting carbonate wedge that forms the southwestern margin of the Elk Point basin. Progressive inundation of the platform during the remainder of the Devonian made it a part of a broad, shallow shelf sea. However, it was a site of active local salt solution during that time, and may have been as well the locale for repeated accumulations of anhydrite deposits, particularly during the time the Duperow Formation was being laid down. The platform was mildly positive during other periods of sedimentation, as well as during periods of erosion. It was a site of widespread sa t solution during Mesozoic time, which was also its time of major tectonic fluctuation, as well as being the period when it had the most significant influence on sedimentation. Southeastern Saskatchewan is the locale for some significant regional gravity and magnetic anomalies which appear related to exposed structural zones in the Precambrian Shield. A major gravity anomaly on the extreme eastern side of the province is on trend with the Nelson River zone of Manitoba and a magnetic anomaly (Camfield-Gough conductor zone) can be traced to the Wollaston trend in north-central Saskatchewan. The existence of these two trends suggests that the regions proximal to them may have been tectonically active during various geologic periods. The Camfield-Gough zone is particularly significant in that it lies along the axis of the Hummingbird trough, an area affected by basement-controlled early salt solution, and it extends southward into the United States, where it is lanked by a number of local multizone oil-producing structures in North Dakota and Montana. The proximity of these structures to that conductor, which is thought to be a suture between two Proterozoic plates, suggests there is a relationship between them. Isopach and isochron maps have been used to show that these structures have been active during various geologic periods. Similar mapping techniques may show that similar structures are present in southeastern Saskatchewan. End_of_Article - Last_Page 1345------------

Environment of Deposition of Winnipegosis Formation (Middle Devonian), Williston Basin, North Dakota: ABSTRACT

The Winnipegosis Formation (Middle Devonian) is the major carbonate unit of the first transgressive-regressive pulse of the Kaskaskia sequence. The sea invaded the narrow, elongated Elk Point basin which extended from northern Alberta southeastward to North Dakota. The southeastern end of this basin corresponds to the present-day Williston basin. In North Dakota, reworking of red beds and deposition of restricted argillaceous carbonates occurred (Ashern). Winnipegosis deposition began after a brief hiatus. Initially, there was a widespread establishment of a clear quiet shallow-marine environment. Subsequently, the basin differentiated into three distinct environments of deposition: (1) scattered pinnacle reefs, (2) a deeper interreef basin, and (3) an encompassing carbonate platform. Carbonate production in the pinnacle-reef and platform environments End_Page 616------------------------------ was able to keep pace with rising sea level as the transgression continued. In the pinnacle-reef environment, several lithofacies developed through time. Of special importance, due to recent production, is an upper porous dolomite in which the original limestone has undergone extensive fabric-obscuring dolomitization. In the platform environment, there developed a patch-reef lithofacies and several quiet-water shallow-marine lithofacies which illustrate a vertical subtidal regressive sequence. In addition, the pinnacle-reef and platform environments grade into an uppermost intertidal and/or supratidal regressive series of dolomites and anhydrites. Carbonate production did not keep pace with rising sea level in the interreef environment resulting in topographic relief. Subaqueous lamin ted lithofacies were deposited throughout the basin and between the pinnacle reefs. During the regressive phase, barrier reefs formed in northern Alberta which restricted the basin and resulted in the deposition of evaporites (Prairie) which eventually filled the basin. End_of_Article - Last_Page 617------------

Reef Facies of Winnipegosis Formation (Middle Devonian), Williston Basin, North Dakota: ABSTRACT

The Winnipegosis Formation is a carbonate unit deposited in the Elk Point basin during the Middle Devonian. The Elk Point, a narrow, elongate basin extending southeastward from northern Alberta-Northwest Territories through central Saskatchewan and southwestern Manitoba to northeastern Montana and most of North Dakota, was flooded during the Kaskaskia transgression. The Winnipegosis Formation in the North Dakota part of this basin represents a transgressive-regressive stratigraphic sequence. The Winnipegosis has been studied on the basis of well cores from the Williston basin area of North Dakota. The following depositional environments are recognized: a carbonate platform in west and southwest, a platform-restricted basin in north-central, and a reef in east-central Nort Dakota. The reef environment, where highly productive, has been studied in south-central Saskatchewan (Winnipegosis Formation) and northern Alberta (Keg River, an equivalent formation). To date there has been no production from the reef facies in North Dakota and only limited production from the platform environment. The reef facies is a porous and permeable dolomitized limestone; dolomitization is so extensive that the original fabric can seldom be discerned. Organisms that were present are now recognized as ghosts, as distinctive porosity patterns, and rarely as recognizable skeletons. In many parts of the reef facies, it is difficult to reconstruct the original fabric. Interpretation of the reef environment in the Winnipegosis Formation is based on comparisons of highly dolomitized fossils in the reef facies with fossils found in other facies, with fossils better preserved in less altered parts of the reef facies, with previously described reefs in Canada, and with fossils present in the reefs that crop out in Manitoba. End_of_Article - Last_Page 969------------

Potash salts in the Williston Basin, U.S.A.

The potash deposits of North Dakota and Montana are extensions of the rich deposits now being mined in Saskatchewan. These deposits underlie 28,500 km 2 (11,000 sq miles) in northwestern North Dakota and 7,800 km 2 (3,000 sq miles) in northeastern Montana. The Middle Devonian potash deposits occur in beds within the Prairie Formation, an evaporitic sequence composed primarily of halite. The Prairie Formation evaporites were deposited within the Devonian Elk Point Basin, which extended from northwestern Alberta to the northwestern North Dakota-northeastern Montana area. On the U.S. side of the border, the Prairie Formation salt reaches a maximum thickness of 168 m (550 ft) in Burke County, North Dakota, whereas in Saskatchewan it exceeds 213 m (700 ft). North Dakota's potash ranges from 1,707 m (5,600 ft) deep near its eastern limit in northwestern Bottineau County to over 3,660 m (12,000 ft) in eastern McKenzie County. In Montana, the deposits range from 2,530 m (8,300 ft) in Daniels County to 3,500 m (11,500 ft) in Richland County. Three potash zones can be identified in Saskatchewan and North Dakota; two, in Montana. In North Dakota, the two major beds, the Esterhazy and Belle Plaine, reach a combined gross thickness of 25 m (83 ft) and a net thickness of 17 m (55 ft). In Montana they reach a combined thickness of 19 m (61 ft) gross and 13 m (43 ft) net.

Geology of Paleozoic Strata in West-Central Saskatchewan: ABSTRACT

The study area is located just south of the Precambrian shield between the Meadow Lake escarpment and the Alberta-Saskatchewan boundary. It is approximately 350 km long and 70 km wide and constitutes part of the Middle Devonian Meadow Lake basin, which is the southeastern portion of the early Elk Point basin. The Paleozoic strata comprise clastic rocks of the Cambrian Deadwood Formation and dominantly carbonate rocks End_Page 826------------------------------ of the Middle Devonian Elk Point Group. The Deadwood clastics are quartzose sandstone, sand, subordinate shale, and conglomerate. Locally, in the northernmost part of the area, they have been removed by pre-Devonian erosion. The carbonate sequence of the Elk Point Group is divided into lower and upper subgroups. The Lower Elk Point subgroup contains the Meadow Lake Formation (new) which is subdivided into lower and upper members. The basal formation of the Upper Elk Point subgroup, the Winnipegosis is present in the area. The younger formations of these southwest-dipping Devonian strata are absent owing to erosion or nondeposition. The Devonian carbonate rocks are overlain unconformably by clastic sediments of the Lower Cretaceous Mannville Group. The common occurrence of fractured and brecciated carbonate rocks within the Meadow Lake Formation indicates salt solution and can be correlated with the extensive rock salt deposits of the Lower Elk Point subgroup in central Alberta. The depositional edge of one of these salt deposits, the Cold Lake Salt, can be traced. Accessory copper, lead, and zinc minerals are present locally in clastic rocks of the Deadwood Formation and also in the Devonian carbonate rocks. Gas shows have been reported from the upper member of the Meadow Lake Formation. Meadow Lake limestone of economic significance was recently discovered near Pinehouse Lake. End_of_Article - Last_Page 827------------

Ancient Anhydrite Facies and Environments, Middle Devonian Elk Point Basin, Alberta

ABSTRACT The association of evaporites with a large percentage of the world's oil and gas reservoirs makes these rocks economically significant. This study was intended to increase our knowledge of environments of deposition and diagenesis of these rocks in a basin in which good core control was available -- the Middle Devonian Elk Point basin, Alberta. We have recognized four major environments of anhydrite deposition -- supratidal, shallow-water, deep-water, and replacement. Supratidal anhydrite includes gypsum pseudomorphs and most of the nodular, nodular-mosaic, and mosaic types. These are identified by comparison with the anhydrite in the sabkha sequence of the Trucial Coast. Bedded-massive anhydrite is interpreted to have been deposited in shallow water under conditions comparable to those in the Pekelmeer of Bonaire Island. Tiny nodules and thin laminae of anhydrite which are usually within or interbredded with laminated limestone are interpreted as deep-water in origin, from their stratigraphic position in the deeper parts of the Elk Point basin. No modern analogues of these types are known. Some massive, nodular, nodular-mosaic, and mosaic anhydrites are considered to be postlithification replacement types. They are commonly located on the edges of shoals, banks, and reefs and are characterized by their relatively greater thickness and by the inclusion of carbonate fragments of the host rock within the anhydrite. By detailed core studies of the anhydrite environments and their order of occurrence in three subbasins, we have been able to recognize eleven major sedimentary events. By correlating these events between subbasins, with the aid of additional core and density-log control, the history of the Middle Devonian basin has been reconstructed.

Origin of Laminated and Graded Sediments, Middle Devonian of Western Canada

Beds composed of laminated calcite, dolomite, and anhydrite form the basal unit of the Muskeg and Prairie evaporitic succession in the Middle Devonian Elk Point Basin of western Canada. These laminated rocks (laminites) are restricted to the basal intermound succession between carbonate mounds and banks of the Keg River and Winnipegosis Formations. Laminae occur as couplets composed of a microcrystalline mineral layer and a non-skeletal organic detrital film; bioclasts of benthonic organisms are absent. Individual dolomitic and anhydritic laminae in various sectors of the Elk Point Basin in northern Alberta have been correlated visually and statistically over distances up to 25 km. Graded clastic carbonate beds with eroded basal contacts, conformable tops, tabular clasts, and occasional benthonic bioclasts are common in some dolomitic laminite sections and apparently are more frequent close to the flanks of carbonate buildups. By comparison with modern depositional basins of various size and water chemistry, the Devonian Elk Point laminites are interpreted as having accumulated subaqueously in a physically and chemically stratified water mass in which the lower water stratum was anoxic. The probable generative mechanism was precipitation within the upper stratum, seasonal in effect and possibly biochemically controlled. Bioturbation and physical disruption of the deposited laminae were prevented by the anoxic bottom environment and the physical stratification of the water mass. The interbedded graded beds are interpreted as turbidites or debris flows related in source to proximity of carbonate buildups. As both carbonate and sulfate laminae are involved in this interpretation, and as these laminae grade upward into bedded anhydrite and halite deposits, the same subaqueous environmental interpretation, albeit with evolving hypersaline water chemistry, is suggested for the overlying evaporites.

Ancient Anhydrite Facies and Environments--Their Role in Reconstructing Geologic History of Middle Devonian Elk Point Basin, Alberta: ABSTRACT

The association of evaporites with a large percentage of the world's oil and gas reservoirs makes these rocks economically significant. Good core control for study of environments of deposition and diagenesis of these rocks is available in the Middle Devonian Elk Point basin, Alberta. We have recognized 4 major types of anhydrite formation--supratidal, shallow-water, deep-water, and replacement. Supratidal anhydrite includes gypsum pseudomorphs and most of the nodular, nodular-mosaic, and mosaic types. These are identified by comparison with the anhydrite in the sabkha sequence of the Trucial Coast. Bedded anhydrite is interpreted to have been deposited in shallow water under conditions comparable with those in the Pekelmee of Bonaire Island. Tiny nodules and thin laminae of anhydrite, which are usually within or interbedded with laminated limestone, are interpreted as deep-water in origin, from their stratigraphic position in the deeper parts of the Elk Point basin. No modern analogues of these types are known. Some massive, nodular, nodular-mosaic, and mosaic anh drites, considered to be postlithification replacement types, are located on the edges of shoals, banks, and reefs, and are characterized by their relatively greater thickness and by the inclusion of carbonate fragments of the host rock within the anhydrite. By detailed core studies of the anhydrite environments and their order of occurrence in the subbasins, we have recognized 11 major sedimentary events. By correlation of these events End_Page 769------------------------------ between subbasins, with the aid of additional core and density-log control, the history of the Middle Devonian basin has been reconstructed. End_of_Article - Last_Page 770------------

Evaporite-Carbonate Relations in Basin Development: ABSTRACT

Petrographic studies of marine evaporites reveal that the initial assemblages of evaporite minerals were generated in 2 distinctly different ways. Some grew or accumulated at the sediment surface where they were evidently precipitated from an overlying brine; others were emplaced within a preexisting, unconsolidated, host sediment, and were formed in response to conditions that prevailed beneath the sediment surface within the interstitial waters of the sediment itself. The latter mode of origin characterizes supratidal sabkha-facies evaporites, which have been found to be an essential constituent of many marginal evaporite complexes. In the sabkha environment there is no overlying body of brine and the evaporite minerals are emplaced within the sediment from marine-deriv d groundwater. This same mode of genesis of evaporite minerals can be demonstrated for some of the anhydrite, halite, and potash deposits in several basin-facies evaporites. The possibility arises therefore that these units record phases of emergence and, as such, suggest that the whole evaporite assemblage was deposited in an environment of shoal water and emergent banks, and not beneath a deep body of brine as previously envisaged. In view of the evident relief within some of the basins, as inferred for example from the thickness of the carbonate reefs relative to the floor on which the evaporites were deposited, such an interpretation requires that profound changes in water level may have occurred within the basins. The evaporites of the Middle Devonian Elk Point basin are reviewed in terms of the various modes of evaporite-mineral genesis, as a background to discussion on the carbonate-evaporite relations within the complex. End_of_Article - Last_Page 870------------

EVAPORITES AND CARBONATES; TWO DEVONIAN BASINS OF WESTERN CANADA

ABSTRACT Evaporites and carbonates deposited in two Devonian basins of western Canada occur as (1) peripheral evaporites with basin-filling carbonate, or (2) peripheral carbonate with basin-filling evaporite, and reefs. The Upper Devonian (D-1) Stettler Formation provides an example of a limestone-filled basin on the periphery of which are evaporites (microdolomite, nodular anhydrite, some halite) having those characteristics which we associate with modern sabkhas of the Arabian Gulf. Deposits associated with the first stage of reef growth in the Upper Elk Point basin (Middle Devonian) provide an example of the second kind. The sedimentary units were deposited as follows: 1, platform; 2, reef, ending with laminar dolomite on both reef and basin-floor; 3, bituminous calcitic and anhydritic laminites on basin-floor (not on reef-sites); and 4, laminar halite filling interreef tracts nearly to reef-top level. During calcitic laminite and salt accumulation the reefs were sub-aerially exposed. Further reef stages supervened.

Regional Stratigraphic Analysis of Devonian System in Wyoming, Montana, Southern Saskatchewan, and Alberta

Devonian strata are traced throughout the Northern Rocky Mountain and Great Plains areas which include Wyoming, Idaho, Montana, southern Saskatchewan, and Alberta. The diverse Devonian nomenclatures between states and provinces are unified, and the areal limits of applicability of established formational names as cartographic units are demonstrated. Isopach and lithofacies studies are employed as a basis for interpreting the sedimentation processes and patterns responsible for the Devonian rocks now observed. In order to work out the dynamic picture of Devonian sedimentation, it is necessary to subdivide the total section into four operational rock units which are recognizable throughout the area on the basis of suitable lithologic criteria; the individual isopach and lithofacies maps of these units are interpreted in terms of the tectonic and environmental conditions responsible for the patterns exhibited. Finally, an integration of these four arbitrary stages of Devonian sedimentation presents the regional Devonian geologic history. The rock units include a basal Devonian unit, a lower limestone unit, a dolomite-evaporite unit, and a post-evaporite unit. Clastics (shale and sand), carbonates (limestone and dolomite), and evaporites (anhydrite and halite) constitute the three significant petrographic end members used in the statistical lithologic triangle. The two indices defining the lithologic variations are the clastic ratio and evaporite ratio. The important tectonic elements which were active during Devonian sedimentation include: Elk Point basin (Middle Devonian age) extending from east-central Alberta into southern Saskatchewan; Central Alberta basin; Southern Alberta arch; Central Montana positive axis; Southern Montana trough; Wyoming shelf; and Cambridge arch of Eardley in southeastern Montana and Wyoming (northwestern extension of Siouxia landmass). Evaporitic sediments are prominently developed in the basal and dolomite-evaporite units; and normal marine deposits, including reef-carbonates, are characteristic of the lower limestone unit.