Abstract The Boot Inlet Formation (Reynolds Point Group, Shaler Supergroup) is an early Neoproterozoic (< 1077 MA, >723 Ma) succession that crops out within the Minto Inlier on northern Victoria Island in the Canadian Arctic archipelago, and consists of strata that accumulated on a carbonate ramp. Inner-ramp facies comprise molar-tooth lime mudstone and current-bedded ooid grainstone (locally herringbone cross-laminated) with scalloped erosional surfaces. Ooid shoals ( 3–4 m thick) and sheets (0.5–1.0 m thick) are interbedded with 10–15 m thick stromatolite bioherms and biostromes forming complexes 0.5 to 5.0 km wide. The most common mid-ramp facies is parted to ribbon-bedded limestone with conspicuous ripples, gutter casts, hummocky cross-stratification, and intraformational breccias readily interprétable as storm deposits; these finegrained rocks form shallowing-upward, meter-scale cycles capped by oolitic limestone and small reefs. Outer-ramp facies comprise shale with large carbonate concretions. Reefs are most common in the lower half of the succession, where overall sea-level rise combined with higher-order transgressions to produce maximum accommodation space. A pronounced zonation of reef types occurs across the ramp. A current-oriented biostrome of Baicalia? is the only reef type on the inner ramp. Patch reefs and table reefs characterize the inner- to mid-ramp transition, and consist of stacked meter-scale bushes of Tungussia that pass upward into broad domal sheets of parallel, columnar stromatolites (Baicalia) oriented at a high angle to the sheets. Overall upward decrease in diversity of growth form is accompanied by evidence for increasing wave and current energy. Concentric-sheet bioherms up to 60 m in diameter and 15 m high, composed of sheets of closely spaced “pencil stromatolites” (Jurusania), grew in outer-ramp facies during rapid transgression. The Boot Inlet reefs are similar to other Prolerozoic reefs in being composed entirely of stromatolites, including some of the same forms as characterize other early Neoproterozoic patch reefs. Calcimicrobes are conspicuously absent, despite their abundance in coeval deeper-water reefs in the Mackenzie Mountains. The presence of kalyptra-like stromatolitic structures in the Boot Inlet reefs is similar to that of Early Cambrian calcimicrobe-archaeocyathan reefs, and lends support for the view that the Phanerozoic reef archetype originated during the Neoproterozoic.

Abstract Cryogenian correlation in Australia is based on an extensive data set from the Centralian Superbasin and Adelaide Rift Complex and integrates biostratigraphy and isotope chemostratigraphy to provide a three-dimensional interpretation based on outcrop and drill holes. Studies are ongoing, but newer data are consistent with the distributions discussed here. From the chemostratigraphic and biostratigraphic viewpoint, the first appearance of the acritarch Cerebrosphaera buickii , coupled with a large negative isotope excursion at c. 800 Ma, supported by the first appearance of the stromatolite Baicalia burra , seems to have potential for boundary placement. It is widely recognized across Australia and seems to have potential globally.

Abstract In Tasmania, Neoproterozoic glaciogenic deposits were laid down in one or more epicratonic basins, probably situated at the eastern margin of the Australian–Antarctic craton. Rifting and volcanism took place in the late Cryogenian to early Ediacaran. On King Island, north of Tasmania, the Cottons Breccia consists of 50–200 m of diamictite, conglomerate and sandstone. Limestone and dolostone clasts are abundant in the diamictite, although carbonate is unknown in the underlying successions. The Cottons Breccia is overlain by 10 m of laminated dolostone and limestone with a negative, upward-decreasing δ 13 C profile. Rift volcanics and shallow intrusives higher in the sequence are dated at c. 575 Ma. In NW Tasmania, two diamictite units are found in the Togari Group. The Julius River Member, 200 m thick, contains dominantly dolostone clasts and overlies a shallow-marine dolostone unit with vase-shaped microfossils and C-isotopes consistent with a mid-Cryogenian age. Some clasts in the Julius River Member contain a stromatolite ( Baicalia cf. B. burra ) very similar to a form that is abundant in the middle part of the Burra Group, Adelaide rift basin. The Julius River Member is immediately overlain by black shale and impure carbonate dated by Re–Os at 641±5 Ma. The younger diamictite in the Togari Group is the Croles Hill Diamictite, 70 m thick, with predominantly volcanic clasts, underlain by a shale and mafic-volcaniclastic succession and overlain by thin mudstone followed by thick rift tholeiites. At one locality this diamictite is underlain by a rhyodacite flow dated at 582±4 Ma. In southern Tasmania, diamictites are found in the Wedge River Beds and in the Cotcase Creek Formation (Fm.) (Weld River Group). Laminated siltstone with dropstones is associated with the diamictites in the Cotcase Creek Fm. The southern Tasmanian deposits are poorly constrained in age. Chronometric and other evidence suggests correlation of the Julius River Member, Cottons Breccia and Croles Hill Diamictite with the Sturt, Elatina and Gaskiers glacial phases, respectively. However, a glacial origin for the Julius River Member and Croles Hill Diamictite remains uncertain.

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.