L. L. Sloss

Sequences in the Cratonic Interior of North America

The concept of major rock-stratigraphic units of interregional scope was introduced in 1948 (Longwell, 1949). It is now possible to restate the concept and to define more explicitly the sequences delimited by interregional unconformities in the continental interior of North America. The sedimentary record of the North American craton from late Precambrian to present is characterized by six major unconformities. These interregional unconformities subdivide the cratonic stratigraphic column into six sequences—major rock-stratigraphic units (of higher than group, megagroup, or supergroup rank) which can be identified, where preserved, in all cratonic interior areas. At the cratonic margins the bounding unconformities tend to disappear in continuous successions, and the cratonic sequences are replaced by others controlled by events in the marginal basins and eugeosynclinal borders. Although the time values of the unconformities vary widely because of differences in degree of nondeposition and amount of erosion, the approximate dates of the regressional maxima represented are: (1) very late Precambrian, (2) early Middle Ordovician, (3) early Middle Devonian, (4) “post-Elvira” Mississippian, (5) early Middle Jurassic, and (6) late Paleocene. A seventh major regression is now in progress.

Stratigraphic Models in Exploration

The S. E. P. M. [Society of Economic Paleontologists and Mineralogists] is tending to become the stratigraphic arm of exploration geology and its greatest contributions have been made through integration of individual specialties into the broad field of stratigraphy. One small part of this integration is to consider stratigraphic models which express, in a systematic form, the interaction of a defined set of variables which operate to produce an observable result. The variables are geological processes and the result is a sedimentary response; consideration is limited to terrigenous clastic sediments deposited at or near marine strand lines. The shape of a body of sedimentary rocks varies as a function of the quantity of material supplied to the depositional site, the rate of subsidence at the site, the rate of dispersal, and the nature of the material supplied. Stratigraphic models based upon these variables are shown for cratonic sources with equal subsidence for regression and transgression, linear subsidence, nonlinear subsidence, and external sources producing clastic wedges and turbidites.

Early Proterozoic Sedimentary Basins of the Lake Superior Region

Studies of the complexly folded and metamorphosed early Proterozoic strata (1.8 to 2.1 b.y. B.P.; Van Schmus, 1976) of the Lake Superior region (Fig. 1) have revealed features indicating a complex sedimentary-tectonic evolution. These rocks, termed “the Marquette Range Supergroup,” and which include stratigraphic equivalents in Minnesota and Ontario (Cannon and Gair, 1970), have been divided into four groups (James, 1958), which, in ascending order, are the Chocolay, Menominee, Baraga, and Paint River Groups. In Minnesota and Ontario, the stratigraphic equivalents of the Chocolay, Menominee, and Baraga Groups are the Mille Lacs (Chocolay Group equivalent in the Cuyuna area), and lower (Menominee Group equivalent) and upper (Baraga Group equivalent) Animikie Group (Morey, 1978a) (Table 1). The stratified rocks of the Marquette Range Supergroup and stratigraphic equivalents occur in four structural settings (Fig. 1A): (1) elongate, generally synclinal, structural troughs bounded by Archean rocks; (2) extensive homoclines; (3) outliers on granitic basement; and (4) mantling rims around early Proterozoic gneiss domes (Sims, 1976). Some rims also form steep narrow troughs (for example, the Michigan River trough; Klasner and Cannon, 1974), which are here grouped with gneiss-dome rims as opposed to the other structural troughs (type 1 above) which appear to show no genetic relation to gneiss domes. Most of the data discussed herein concern the first two types of structural settings. It is the purpose of this paper to suggest a relationship between the four types of structural settings and the mode of original sedimentation. Specifically, we propose that elongate early Proterozoic sedimentary basins existed in eastern Upper Michigan at the sites of what became structural troughs later in the early Proterozoic; that is, the eastern Upper Michigan structural troughs are simply appressed sedimentary basins. As will be discussed, the Animikie basin (as reported by Morey, 1967; Morey and Ojakangas, 1970; and Alwin, 1979) represents a more “stable” type of sediment receptacle (types 1, 2 structural exposure) with a sedimentary history broadly comparable to the eastern Upper Michigan basins. Finally, it is noted that individual basin history was complex, with some more areas showing evidence of both basinal and platformal sedimentation.

The Significance of Evaporites

ABSTRACT Interpretation of the lithologic and paleontologic associations of which evaporites form a part makes possible classification and mapping of the environments involved in evaporite occurrence. Consideration of stratigraphic sequences containing evaporites leads to recognition of a major cycle made up of successive environmental conditions from normal marine, through stages of increasing salinity, to evaporitic conditions. Analysis of the relationship between evaporite occurrences and the elements of the tectonic framework suggests a classification into intra-basinal and basin-margin occurrences. Further classification is based on the nature of the sill responsible for restriction of the evaporitic environment.

Microstylolites in Sandstone

ABSTRACT Stylolites developed on a microscopic scale along grain boundaries were found in the chert facies of the Cut Bank sandstones, Cut Bank Oil Field, Montana, and the interpenetrating columns of chert and dark residue along the seam are similar to other stylolites. Associated with the microstylolites are quartz grains imbedded in chert grains and secondarily enlarged quartz grains. The development of microstylolites and enlargement of quartz grains has affected the porosity and permeability of the producing horizon in the Cut Bank Oil field. The authors view the evidence here presented as support to the pressure-solution theory of origin of stylolites.

Paleozoic Sedimentation in Montana Area

The record of Paleozoic sedimentation in Montana and adjacent states and provinces reveals the influence of the tectonic framework of the area on the lithologic character, distribution, and thickness of the systems. Isopach and lithofacies maps of each system are presented, with interpretations of the positions and times of activity of positive and negative elements. The study indicates that the complex Cordilleran geosyncline, Williston basin, Central Montana trough, and Wyoming shelf areas were important factors in controlling sedimentation by their tectonic behavior during deposition. The Sweetgrass arch appears to have had little influence during sedimentation, but pre-Middle Devonian and pre-Middle Jurassic uplift and erosion of the element broadly affected the prese t distribution and thickness of Paleozoic systems.

High-Speed Digital Computers in Stratigraphic and Facies Analysis

Rapid expansion of facilities for high-speed computation at reasonable cost on a service basis makes it possible for the geologist to free himself of the routine computations involved in making facies maps. Some of the conventional procedures for compiling data and designing data cards are described, and a program is given for computing facies percentages and ratios. This program may be used directly on any IBM 650 computer with a standard wiring board. Suggestions are offered for application of the basic program to a variety of three and four end-member systems.

Devonian System in Central and Northwestern Montana

The Devonian strata of central and northwestern Montana are confined to an Upper Devonian age. In central Montana these rocks are divided, in order of increasing age, into the Three Forks formation, predominantly shale; the Jefferson formation, composed of an upper dolomite member, including some anhydrite or evaporite-solution breccia, and a lower dense limestone member; and an unnamed basal unit of shale and shaly dolomite which bears a transgressive relationship to the underlying Ordovician and Cambrian. In northwestern Montana these terms are not applicable and the Devonian is divided, in descending order, into arbitrary units A, B, and C. Unit A is dolomite and anhydrite, or evaporite-solution breccia, unit B is dense limestone, and unit C is a red shale and shaly do omite sequence resting on the channeled surface of the Upper Cambrian. Oil is produced from the Devonian of south-central Alberta and gas has been encountered in it in Montana. Petroleum possibilities appear to be confined to the dolomites of the Jefferson formation and unit A, and it is suggested that these possibilities may have been enhanced by favorable depositional environments and post-depositional effects.

Paleozoic Stratigraphy of Southwestern Montana

Within this area all Paleozoic systems excepting the Silurian are represented. Cambrian strata are similar to those of better known areas on the north and east, but differ in the presence of several intrasystemic unconformities. The Ordovician Kinnikinic quartzite occurs along the Idaho-Montana border and a thin edge of Bighorn dolomite is present along the eastern margin of the area in Gallatin County. Elsewhere, Cambrian strata are overlain by Upper Devonian. Devonian strata of the Jefferson and Three Forks formations are typically developed and widespread, with little lateral variation. The Mississippian system, on the other hand, is characterized by extremes of facies expression, including rapid lateral variation among redbeds, evaporites, fragmental limestones, dolomites, and black shales. Mississippian rocks range from 1,000 to more than 4,000 feet in thickness. Pennsylvanian strata are dominated by the quartzose and well cemented sandstones of the Quadrant formation which ranges in thickness from approximately 100 feet to nearly 3,000 feet. The Permian system is represented by the Phosphoria formation, composed of quartzites, limestones, dolomites, black shales, phosphorites, and bedded cherts. End_Page 2135------------------------------ Fig. 1. Regional index map, showing location of area discussed and other areas with which correlations are made: (1) southeastern Idaho, (2) central Idaho, (3) southwestern Montana, (4) central Montana, (5) northwestern Montana, (6) northern Wyoming and Big Horn Basin, (7) Black Hills, (8) Williston Basin. Fig. 2. Index map of southwestern Montana, showing location of topographic features referred to in text. End_Page 2136------------------------------

Big Snowy Group: Lithology and Correlation in the Northern Great Plains

The Big Snowy group consists of a middle and upper Mississippian series of shales, sandstones, limestones, and evaporites. Recently drilled deep wells and new interpretations of older wells in Montana and the Dakotas yield information which makes possible further considerations on the subsurface extensions, correlations, and lithology of the Big Snowy sediments in the Williston basin. A lateral tracing of persistent lithologic units, the recognition of the removal of certain units by Mississippian erosion, and the addition of a basal unit not present in outcrop areas make possible satisfactory correlation of the subsurface Big Snowy in the Williston basin. Paleogeographic studies reveal the influence of the ancestral Sweetgrass arch on the character of clastic sediments, in the Big Snowy group, and indicate the possibility of favorable reservoir conditions in untested areas adjacent to that positive element.