The ca. 1.460 Ga Revett Formation is a gray and purple quartzite lithosome in northwestern Montana, and it interfingers eastward into red argillite of the Grinnell Formation in Glacier National Park. The Revett Formation was analyzed in northwestern Montana by identifying sedimentary structures in stratigraphic sections and by interpreting flow processes of the structures using the standard flow regime model (e.g., Simons et al., 1965). The sedimentary structures and thicknesses of the event beds were then organized into eight sediment types (lithofacies) that were grouped into three sediment complexes: the playa complex, the antidune complex, and the sheet sand complex. The arrangements of the sediment types and complexes within the stratigraphic framework of the lower informal Revett member indicated the configurations of the depositional environments in space, and the vertical configurations of the sediment types revealed the depositional history of the lower Revett member. The lower Revett member lithosome interfingers eastward into the red argillite of the Grinnell Formation lithosome, and has eight through-going descriptive, stacked, lithic units, called lithostromes. Lithostromes 2, 4, 6, and 8 (from the bottom up) are composed of the sheet sand complex and extend into playa complexes of the Grinnell Formation. They were deposited by sandy sheetfloods that flowed at grade and terminated as the water sank into the sand substrate. Between lithostromes 2, 4, 6, and 8 are lithostromes marked by playa lakes of the playa complex that spread from the east across western Montana during humid periods. They were overlain by sheetfloods of the antidune complex that built eastward over the playa complex as the playa lakes retreated with increasing aridity. The antidune complex was overlain by the sheet sand complex of a vast sand plain deposited by sheetfloods from the southwest that flowed at grade level across western Montana during arid periods. The sheetflood deposits of the Revett Formation were mostly deposited by the upper-flow regime element of the established fluvial facies model.

Crinkle cracks are sand-filled cracks up to 5 mm wide in plan view that pinch at their ends. In cross section, they are canted and crinkled. They cut mudstone beds that underlie hummocky cross-laminated sandstone lenses. They are here described from the Piegan Group, Proterozoic Belt Supergroup, but they are widespread in Proterozoic and Phanerozoic marine and lacustrine rocks. However, they represent a distinctive, descriptive style of mudcracks, not attributed to inferred syneresis processes, although they have been commonly attributed to syneresis. In plan view, crinkle cracks closely resemble cracks formed where oscillatory waves striking viscous mud banks are transformed into fluid solitary-like waves that open surface cracks on their trailing limbs and close the cracks on their leading limbs as they pass through the viscous mud. Crinkle cracks preserved in rocks are hypothetically attributed to oscillatory waves moving sand over viscous mud. The oscillatory waves are transformed into solitary-like waves as they pass down into the mud, forming the cracks. The surface sand falls down into the cracks, preserving them. With burial, the water escapes, and the viscous mud compacts, crinkling the sand-filled cracks.

The redox state of the mid-Proterozoic oceans, lakes, and atmospheres is still debated, but it is vital for understanding the emergence and rise of macroscopic organisms and eukaryotes. The Appekunny Formation, Belt Supergroup, Montana, contains some of these early macrofossils dated between 1.47 Ga and 1.40 Ga and provides a well-preserved record of paleoenvironmental conditions. We analyzed the iron chemistry and mineralogy in samples from Glacier National Park, Montana, by pairing bulk rock magnetic techniques with textural techniques, including light microscopy, scanning electron microscopy, and synchrotron-based X-ray absorption spectroscopy. Field observations of the Appekunny Formation combined with mineralogical information allowed revised correlations of stratigraphic members across the park. However, late diagenetic and/or metasomatic fluids affected primary iron phases, as evidenced by prevalent postdepositional phases including base-metal sulfides. On the west side of the park, pyrrhotite and chlorite rims formed during burial metamorphism in at least two recrystallization events. These complex postdepositional transformations could affect bulk proxies for paleoredox. By pairing bulk and textural techniques, we show primary records of redox chemistry were preserved in early diagenetic and often recrystallized framboidal pyrite, submicron magnetite grains interpreted to be detrital in origin, and red-bed laminae interpreted to record primary detrital oxides. Based on these observations, we hypothesize that the shallow waters of the mid-Proterozoic Belt Basin were similar to those in modern marine and lacustrine waters: fully oxygenated, with detrital reactive iron fluxes that mineralized pyrite during organic diagenesis in suboxic, anoxic, and sulfidic conditions in sedimentary pore waters.

Abstract The Helena and Wallace formations, currently of the “middle Belt carbonate”, were deposited in the block–fault Belt basin, within the Proterozoic Columbia continent, which filled from about 1480 to 1400 Ma. Dolomitic argillite–capped cycles of the Helena Formation were thought to represent a marine carbonate shelf deposit along the eastern margin of the Belt basin. Siliciclastic and calcitic rocks of the Wallace Formation were considered to be the western facies of the middle Belt carbonate, deposited in deeper water. This study shows that the Helena–type cycles form a unit across most of the Belt basin that is disconformably overlain by Wallace–type rocks. The Helena and Wallace formations are here revised to reflect the stacked stratigraphic relations. Both are inferred to be deposits of broad, shallow lakes. The Helena and Wallace are assigned to the resurrected and revised Piegan Group. The revised Helena Formation is characterized by cycles one to 10 m thick. The lower half-cycles are composed of light gray, thin, graded, siliciclastic layers 0.3 to 10 cm thick. Some continue upward and become mixed with tan-weathering dolomite in the upper half-cycles. In other cycles siliciclastic graded layers thin and fine upward but remain siliciclastic. The Helena Formation can be divided into lower, middle, and upper informal members. The lower and upper members have centimeter-scale bedded cycles, but the middle membercontains cycles with dark-gray, decimeter-scale hummocky cross-stratified arenite beds. The Grinnell Glacier section of Glacier National Park is selected as the revised Helena reference section. It is 500 m thick and contains 363 thin-bedded, dolomite-capped cycles, averaging 1.4 m thick. The Helena thins to 100 m on eastern thrust plates of the Front Range, thickens to 800 m north of Plains, Montana, but thins to 250 m in the Coeur d'Alene Mining District. Based principally on scattered halite casts, the crosscutting of the siliciclastic lithofacies by the dolomitic cycle caps, and the absence of significant scour at the cycle bases, the Helena Formation is interpreted to have been deposited in an underfilled, periodically hypersaline, broad, shallow lake. Its flat lake floor was everywhere above storm wave base. Stacking patterns of small-scale cycles indicate the Helena represents a large-scale expanding and contracting lake sequence. The revised Wallace Formation is characterized across northwestern Montana by gray-weathering siliciclastic upward-fining andthinning cycles, mostly 2 to 5 m thick. It can be subdivided into the following six members: (1) oolitic member, (2) molartooth member, (3) Baicalia member, (4) pinch-and-swell member, (5) microcouplet member, and (6) the full-cycle member. Across northern Idaho the Wallace members and cyclic patterns merge into a continuous unit of medium-gray arenite lenses in dark-gray argillite. Thinly laminated black argillite and dolomite units previously assigned to the upper Wallace in Idaho along with dolomitic argillite beds of the upper part of the Helena in Montana are here assigned to the lower Missoula Group. The Clark Fork section of northern Idaho is designated the Wallace reference section. It is 400 m thick and has 47 cycles. The Wallace Formation thickens eastward to more than 1,000 m in the Mission Range and thins to 300 m in Glacier National Park. Siliciclastic cycles of the Wallace Formation are similar to those of the Helena Formation. Cycle boundaries lack evidence of significant exposure and erosion, but only the lower and upper Wallace cycles have dolomite caps. For these reasons the Wallace Formation is interpreted to represent an underfilled and balanced-fill lake deposit that expanded and contracted, forming a genetic sequence. Widespread hummocky arenaceous beds indicate that the Wallace lake expanded westward, but its floor was flat and everywhere above storm wave base.

Abstract The distribution of Middle and Late Proterozoic sedimentary and metasedimentary cover that lies unconformably on Early Proterozoic and Archean crystalline basement has been known for decades, but recent work, employing techniques of paleomagnetic correlation, sedimentology, sequence stratigraphy, and analysis of tectonic subsidence has led to modifications of some long-accepted correlations and tectonic models. Within the context of both older classical studies and this new work, the stratigraphy, correlation, tectonic setting, fossil content, and mineral potential of Middle and Late Proterozoic rocks of parts of the Rocky Mountain, Colorado Plateau, and Basin and Range provinces of the United States are discussed. A problem common to interpretation of all Proterozoic strata is a widespread lack of fossil control on age and paleoecology, which makes correlations inherently uncertain and interpretation of depositional environments more difficult. We present current hypotheses about these topics and stress the uncertainty of some of our conclusions. The apparent polar wander path for the North American craton, as derived from the Middle and Late Proterozoic sedimentary cover, is central to our modifications of stratigraphie correlation, especially of Middle Proterozoic rocks. The reader is asked to view the work and summaries presented here in the light of ongoing scientific debate about strata that are chronically stubborn in yielding information. The authors of sections of this chapter include both those who have performed classical studies, which are the foundation of our present understanding, and younger geologists who have been busy refining and modifying early interpretations, using different methods of study. The treatment in this chapter is therefore variable depending on which generation of investigators is speaking.

Estuarine sediments from Croatan, Roanoke, and northernmost Pamlico Sounds in the Roanoke Island area of North Carolina demonstrate complex patterns of deposition and erosion. The sediments reflect the dynamic interplay between the fresh- and salt-water systems, as controlled by the shifting barrier island inlets from the Atlantic Ocean and the fresh-water discharge from the mainland. Utilizing the sediment data, detailed hydrographic surveys, and the historic maps of the region, the sedimentary processes of erosion and deposition within the estuaries of the Roanoke Island area have been related to the location and changes of the major inlets along the Outer Banks. During recorded history, and prior to 1817, inlets adjacent to or north of Roanoke Island were continuously open. Evidence suggests that during this time most of the Roanoke Island area was characterized predominantly by sediment deposition within a clear, saline, and productive estuarine system having a diverse benthic marine fauna. With the closing of the last of the northern inlets in 1817, the discharge from the Albemarle drainage system was diverted around Roanoke Island, widening and deepening Croatan Sound, and producing a highly variable, turbid, brackish water environment characterized by a restricted benthic fauna. Today, Croatan Sound is characterized in the central portions by active scour that exposes pre-Holocene peats and fossiliferous fine sands, erosion of the Recent salt marshes and peat beds along the shorelines, and slow deposition of very fine and fine-grained sediments over much of the rest of the area. Northernmost Pamlico Sound is characterized by a shifting erosional-depositional regime to which fine-grained sand is supplied by the tidal currents of Oregon Inlet. Roanoke Sound is characterized primarily by a depositional regime in which fine sand, derived from the Atlantic system by wind action and storm washover, is actively filling the shallow sound.