Abstract: In Bolivia, a marked climatic paleogradient (from west northwest to south) is visible in the Carboniferous depositional systems. In the northwest is the Pangean trend, a warm-water Pennsylvanian and Permian succession (preceded by a Late Devonian glacially derived rock assemblage). To the south is the cold climate Gondwanan trend, a succession of Late Devonian and Pennsylvanian cold-water siliciclastics with glacially influenced deposition. Whereas Devonian through (limited) Mississippian strata are comparable in overall character, a sharp climatic gradient in western South America is established by the earliest Pennsylvanian. The Pangean trend in northwestern Bolivia and Peru continues with warm-water Pennsylvanian and Permian carbonates, evaporites, and mixed siliciclastics of a semiarid, open seaway association (Copacabana Formation). This unit was deposited by marine transgression north (northern Bolivian subsurface and Lake Titicaca area), reaching central Bolivia by the Early Permian (Early Cisuralian). Regionally, the warm Pangean pattern continues into the younger and more restricted overlying Cisuralian and younger Permian and Triassic rocks characterized by restricted marine deposits of both humid and arid association (including red beds). To the south, Early Pennsylvanian rocks in the Gondwanan trend record continental and lacustrine glacial deposition as far north as central Bolivia, with glacial influence strongest in southern Bolivia and northern Argentina. By the Late Pennsylvanian, glacial influence has waned and is restricted to southern Bolivia near the Argentine border. The Copacabana Formation is enigmatic because of the following: (1) its autochthonous succession over cold-water, glaciogenic deposits of the Late Devonian and Mississippian and (2) its apparent coeval deposition with Pennsylvanian (and Permian) glacial diamictites. Although the former can be attributed to paleolatitudinal shift, or a clockwise rotation of Gondwana, what is not easily explained (and much discussed) is the autochthonous continuity of northeastern and central Bolivian carbonate deposits of the northern Peru–Bolivia Basin with southern Pennsylvanian and Permian glaciogenic deposits, which accumulated in the Tarija–Chaco Basin. Given that these cold and warm-water deposits were coeval in time, a severe climate gradient must have existed across Bolivia beginning in Pennsylvanian time. Western Gondwana records steady movement from high latitudes (~55°S) in the Late Devonian to midlatitudes (~40°S) by Pennsylvanian time. Glacial deposits seen in the northwest during the Late Devonian become restricted to the southern Tarija–Chaco Basin by the Late Pennsylvanian. By Early Pennsylvanian (Bashkirian) time, carbonates, evaporites, and siliciclastics were deposited in northwest Bolivia. In central Bolivia, Mississippian diamictites, undated Pennsylvanian siliciclastics, Copacabana lithofacies, and carbonates of the Vitiacua Formation are vertically stacked at a few locations.

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 Sappington Formation is a Late Devonian to Early Mississippian age, primarily siliciclastic, formation that outcrops in southwestern Montana. The Sappington Formation has been presented in the past as broadly analogous in lithologic character and age to other formations in the region, the Bakken and Exshaw Formations. The Sappington Formation was the focus of several studies in the 1950s through 1970s that extensively analyzed the fauna and distribution of the formation and established a lithostratigraphic and biostratigraphic framework. This study provides a needed present day look at the Sappington Formation, by incorporating extensive previous work with present day field observations and modern geologic concepts. The methodology applied to this work includes literature review, outcrop observations, and interpretations on lithofacies, depositional environments, and sequence stratigraphy. Results from this work include a sequence stratigraphic model, depositional models, and a summary of the effects of allocylic processes on deposition. Three sequences were identified using previous biostratigraphy and correlated throughout the study area using lithostratigraphy. Recently, intracratonic Late Devonian and Early Mississippian age formations have become of economic importance as “unconventional” hydrocarbon plays, owing to success in the Bakken Formation.

There are several isolated outcrops (outliers?) of Devonian rocks in the Pacific Northwest (Washington and Oregon), USA. A locality in northeastern Washington, Limestone Hill, is considered in detail, and other small outcrops in northwest Washington and central Oregon are discussed. Limestone Hill is a Paleozoic outlier. The locality has Ordovician and Silurian (Llandovery and Wenlock) strata, Lower Devonian (Lochstone conglomerate, and Upper Devonian (Frasnian) carbonates with fossils. It has long been known that the area has many allochthons, and it has been assumed that Limestone Hill represents lithologies deposited much farther west. More recent data suggest that Limestone Hill is parautochthonous. Several brachiopod taxa, previously unknown from the Frasnian portion of Limestone Hill, have been found recently and are described herein. The brachiopod faunule consists of Emanuella sp., “ Allanella ” engelmanni , Cyrtina sp., Thomasaria sp., and an athyridid. These brachiopods appear to be like coeval faunas in Idaho, Montana, Utah, and Nevada, although more species assignments must be made. Frasnian brachiopods are in serious need of updates, as Famennian miospore and acritarch data suggest significant basin restriction and reduced seaway connectivity, with at least ephemerally extensive land areas with ubiquitous land plant taxa.

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.