ABSTRACT: Understanding of very thick Late Devonian shelf strata in Idaho is hindered by formation terminologies. Interpreted genetically, and in combination with lower accommodation settings in Montana, strata reveal craton-to-basin geometries and analogues similar to other western Laurentian basins. The Jefferson Formation Birdbear Member and Three Forks Formation in Montana are correlated to the Jefferson Grandview Dolomite in Idaho using regional sequence stratigraphic surfaces. A new stratigraphic framework defines three widely deposited latest Frasnian sequences and Early Famennian intrashelf basin paleogeography. Peritidal to marine mixed siliciclastic and carbonate rocks of the Middle Devonian lower Jefferson Formation in Idaho are overlain by the Frasnian Dark Dolomite. These rocks are overlain by similar lithologies, including thick evaporite solution breccias of the latest Frasnian and Early Famennian upper Jefferson Formation. Latest Frasnian sequences are similar to Nisku–Winterburn sequences in western Canada. Overlying Famennian successions are correlatives to the Three Forks Formation Logan Gulch Member in Montana and the Palliser–Wabamun units of Alberta. Biohermal Dark Dolomite in the central Lemhi Range and Borah Peak area of the Lost River Range was deposited west of the Lemhi Arch, with buildups also established on ramps near the shelf break in the Grandview Canyon area (Grandview Reef). During onset of the Antler Orogeny, prior to deposition of the Middle Famennian Three Forks Trident Member and widespread disconformities, a latest Frasnian outer shelf barrier formed above the Grandview Reef. Cyclic, heterolithic, peloidal western Grandview Dolomite facies were deposited and are ~330 m thick, although correlative facies of the Jefferson D4 through D6 members are twice as thick behind the shelf edge in the central Lemhi and Borah Peak area. Lower Grandview Dolomite black subtidal carbonate and Nisku buildups (Gooseberry Reef) formed in three late Frasnian sequences and under a basal Famennian sequence boundary. At this time, the Lemhi Arch foundered, but remained unstable—it was termed the “Beaverhead Mountains uplift.” An intrashelf basin dominated midshelf paleogeography during the Early Famennian, accommodating thick shallow water barrier sandstone, solution-collapse breccia, and restricted marine dolostone and limestone of the upper Grandview Dolomite. Crinoid packstone beds near the top of the Jefferson Formation occur below the Three Forks Trident Member in the Lost River Range. Similar nodular, crinoidal limestone with cephalopods occurs under an unconformity with the Sappington Formation in the Beaverhead Mountains. These rocks were previously called the False Birdbear and were grouped with the Jefferson Formation; however, they comprise their own ~15-m-thick sequence and are unrelated to the rarely fossiliferous and dolomitized upper Grandview Dolomite. Open marine shale–limestone sequences of the 80-m-thick Trident Member were deposited on the Idaho shelf above and below regional surfaces and hiatuses. These rocks were variably accommodated on reactivated paleohighs and in local seaways on the craton margin. An unconformity developed on the outer Idaho shelf in the latest Devonian during deposition of the Sappington Formation on the Lemhi Arch and in the Central Montana Trough. Sappington strata were either not deposited on the western shelf or accumulated under deep water conditions and were eroded during regional Mississippian basin inversion and turbidite deposition.

Abstract The Eagle Ford Group crops out in a series of spectacular cut-bank exposures within Lozier Canyon region in Terrell County (west Texas). These outcrops provide an unparalleled opportunity to examine the Eagle Ford Group and gain valuable insights into explaining and predicting the vertical and lateral variability, as well as the thickness changes that can occur regionally within an unconventional source rock play. In the subsurface of south Texas, the Eagle Ford Group is typically divided into an organic-rich Lower Eagle Formation and a carbonate-rich Upper Eagle Ford Formation. Both formations are petrophysically distinct, especially on gamma ray (GR) and sonic logs. When geochemically analyzed, the basal portion of the Upper Eagle Ford Formation also contains a unique positive carbon isotope δ 13 C excursion interpreted as the Ocean Anoxic Event 2 (OAE2). The peak of this isotope excursion is the assigned proxy for the base of the Turonian Stage. Within the Eagle Ford outcrops of west Texas a vertical succession of five informal lithostratigraphic units, referred to as units A to E from the base up, are fairly obvious. Unit A consists of interbedded grainstones and carbonate mudstones. Unit B is dominated by organic-rich black carbonate mudstones. Unit C consists of packstone beds interbedded with light gray carbonate mudstones. Unit D consists of bioturbated marls, while Unit E consists of grainstones interbedded with carbonate mudstones and bentonites. By incorporating petrophysical and geochemical data, the Lower and Upper Eagle Formations from the subsurface of south Texas can also be defined in the Eagle Ford outcrops of west Texas. Our work suggests that outcrop units A and B represent the Lower Eagle Ford Formation, while outcrop units C, D, and E represent the Upper Eagle Ford Formation. Similar to the subsurface of south Texas, a distinct positive carbon isotope δ 13 C excursion also occurs in the basal portions of the Upper Eagle Ford Formation (unit C) in outcrop. More detailed analysis of the outcrop and subsurface data from the Eagle Ford Group in west Texas indicates that the five informal lithostratigraphic units can be further divided into a vertical succession of 16 subunits. This more detailed vertical facies succession was used to define four genetically related depositional sequences each with distinctive geochemical and petrophysical characteristics which make them particularly suitable for regional subsurface mapping. For nomenclature simplicity, these four sequences are herein termed the lower and upper (allo-) members of the Lower Eagle Ford Formation and the lower and upper (allo-) members of the Upper Eagle Ford Formation. The lower member of the Lower Eagle Ford Formation is an organic-rich, high-resistivity, uranium-poor mudstone-dominated sequence. A distinctive clay-rich, low-resistivity zone also marks its base. This sequence appears to be the primary unconventional reservoir interval in the subsurface of south Texas. The upper member of the Lower Eagle Ford Formation can be characterized as a uranium- and bentonite-rich, mudstone-dominated sequence. The lower member of the Upper Eagle Ford Formation is a uranium-poor interbedded mudstone and limestone succession characterized by an overall (low) blocky GR pattern, the presence of a distinctive positive carbon isotope δ 13 C excursion, and a clay-rich, low-resistivity zone at its base. The upper member of the Upper Eagle Formation is a bentonite-bearing, low-TOC interval that is more bioturbated toward its base and interbedded toward its top. It is characterized by the presence of a high GR, low resistivity, and low velocity mudstone at its interpreted maximum flooding surface. Regional correlations of the four defined Eagle Ford depositional sequences (allomembers) reveal that the unconformities at the base of each of the four sequences, as well as the one at the base of the overlying Austin Chalk, modify the thickness and distribution of underlying strata. Thus any attempt to explain and predict the distribution and thickness variations of any of the four sequences (allomembers), especially the organic-rich lower member of the Lower Eagle Ford Formation, is highly dependent on the recognition and regional mapping of these unconformities.

Abstract Dating of detrital zircons from well cuttings is a useful technique to constrain stratigraphic ages and structural interpretations in complexly deformed terranes. This technique was applied in the Helena Salient of the Montana Disturbed Belt to determine whether the Norcen, Kimpton Ranch 1-11 and Buckhorn, Federal 2-24 wells penetrated Phanerozoic strata beneith the allochthonous Precambrian rocks carried on the Lombard thrust. Since some Phanerozoic strata have reservoir potential, their presence below the Lombard thrust has important implications for the oil and gas potential of this structural province.

Abstract The Lower Cambrian (Waucoban Series; Fallotaspis–Olenellus Biozone) carbonate-siliciclastic units of western Laurentia outcrop from Sonora, Mexico, to western Nevada, and from northeastern Washington to east-central Alaska. The allochthonous Cassiar terrane in northwestern British Columbia was originally deposited between these two segments, forming a widespread large carbonate platform along this margin during the initial Paleozoic flooding (basal Sauk megasequence) of North America. The Lower Cambrian carbonate-siliciclastic units of western Laurentia commonly are subdivided into two second-order or composite third-order depositional sequences. The lowstand systems tracts to these sequences commonly are marine siliciclastics that grade upward into transgressive systems tracts composed of interbedded shaly carbonates or carbonate-rich shales that grade upward into carbonate-dominated highstand systems tracts. The carbonates commonly record deposition on a gently sloping ramp; however, areas of localized syndepositional extensional faulting created locally faulted carbonate margins. Large allochthonous blocks of shallow-water carbonates and siliciclastics were deposited basinward of these active fault zones. The facies of the western Laurentian Lower Cambrian carbonate ramps, arranged from land toward the basin, are fluvial nearshore siliciclastics, carbonate tidal flats, mixed carbonate-siliciclastic lagoons, high-energy ooid grainstone shoals containing or surrounded by algal-archaeocyathan mounds, downslope nodular facies, and interbedded calcisiltite and shale deposited below a fair-weather wave base. The algal-archaeocyathan mounds constructed small isolated biostromes (<90 m [<295 ft] thick), not large continuous reefs. The continuity of the ooid grainstone shoals along the western Laurentian margin indicates that they formed a barrier separating restricted facies to the east from open-marine facies to the west that stretched from present-day east-central Alaska to northwestern Mexico.

Abstract Lower Cambrian–Lower Ordovician passive-margin sediments were deposited across northeastern Washington, northern Idaho, and western Montana. Lower Cambrian strata record the initial transgression onto Laurentia in northeastern Washington. Middle Cambrian–Lower Ordovician units were deposited across a much broader area and record the establishment of a western ooid-algal shoal complex that restricted water circulation in an intrashelf basin that formed between the shoal and craton. Long-standing topographic highs in the region include Montania, which may have controlled the location of the ooid-algal shoal complex, and the Lemhi arch, which served as a western source area for siliciclastic sediment input. The Sauk megasequence sediments were deposited in multiple grand cycles that are regionally correlative. Each grand cycle consists of a shale and sandstone base deposited as sea level transgressed onto the craton. The basal siliciclastic units are gradationally overlain by carbonate, which was deposited as sea level continued to rise and an extensive carbonate platform developed. Some of the boundaries between the grand cycles are unconformities, recording rapid changes in sea level. Meter-scale shallowing-upward cycles are common within grand cycles, but they have not yet been regionally correlated.

Abstract A regional study, based on detailed descriptions of 17 outcrops across east-central Idaho and southwestern Montana, provides a dip-oriented cross section in which to better understand the distribution of Upper Mississippian (Chesterian) stratigraphy on the distal margin of the Antler foreland basin. Chesterian strata constitute an eastward-thinning wedge of mixed carbonate and siliciclastic rocks that formed on a west-facing ramp. Foreland-basin tectonism subdivided the ramp into three distinct depositional settings: the western, central, and eastern ramp. The western ramp records nearly continuous Chesterian deposition, whereas the central and eastern ramps have significant unconformities. Mud-rich subtidal carbonate predominates on the western ramp, but this interfingered during the late Chesterian with tidally influenced siliciclastics. The central ramp contains an intraramp basin with deep subtidal siliciclastics and carbonate that formed adjacent to shallower-water facies to the west and east. The eastern ramp has mostly peritidal carbonate and shallow marine to fluvial siliciclastics, but a transgression from the north during the late Chesterian inundated this portion of the ramp with open marine carbonate. New conodont biostratigraphic constraints indicate that these Chesterian strata are a second-order megasequence (10-12 My duration) composed of more than seven third-order depositional sequences (S0-S7), each having a duration of 1-5 My. The sequences are grouped into three composite sequences (I, II, and III) that define long-term changes in accommodation controlled by syndepositional tectonism. Composite sequence I was deposited during a period of tectonic loading that partitioned the ramp via subsidence loading and extension. Composite sequence II records a period of tectonic stabilization and deposition during the most extensive eustatic flooding, whereas composite sequence III is dominated by a localized subsidence event in the Big Snowy Trough. Higher-frequency (fourth- and fifth- order) parasequences are common throughout the study interval, but they are only locally correlative. A change from thick-bedded carbonate- dominated parasequences in the early and middle Chesterian to thinner-bedded mixed carbonate and siliciclastic parasequences in the late Chesterian likely reflects the onset of moderate- to high-amplitude, high-frequency eustatic fluctuations caused by the initiation of Gondwanan glaciation.