Abstract The Bitter Springs Formation constitutes the upper part of Supersequence 1, an 830 Ma carbonate succession deposited in the intracratonic Centralian Superbasin. The Superbasin covered 2 million km 2 of the Australian continent during Neoproterozoic times. We have integrated new carbon, oxygen, sulfur, and strontium isotopic information, and new biomarker information, with existing results, to test and extend previous studies of the formation. The Gillen Member of the Bitter Springs Formation was deposited in a marine environment. The evidence for this interpretation includes isotopes of strontium, carbon, and sulfur, and biomarkers. 87 Sr/ 86 Sr ratios in the Gillen Member are comparable to the lowest ever recorded from the Shaler Supergroup of Canada, of similar age. These low strontium isotopic ratios are associated with comparable secular change in seawater δ 13 O carb . Steroidal hydrocarbon biomarker assemblages, seawater δ 34 S sulfate values, and low δ 34 S pyrite values further indicate a marine origin. The lower part of the Loves Creek Member (Units 1 and 2) was also deposited in a marine environment. Facies-independent secular change in seawater δ 13 O carb and low 87 Sr/ 86 Sr ratios are comparable to those in the upper Shaler Supergroup of Canada. Seawater δ 34 S sulfate , values, low δ 34 S pyrite , and evidence for algal biomarkers are also consistent with marine deposition. The upper part of the Loves Creek Member (Unit 3), and lower part of Unit 2, is nonmarine. Rare pyrite, heavy δ 34 S pyrite , abundant irregularly branched C 25 and C 30 isoprenoids from methanogens, and abundant pseudomorphs after halite indicate carbonate precipitation in sulfate-poor hypersaline lakes. Heavy and variable δ 13 S carb and δ 18 O indicate that continental groundwater brines, which were low in dissolved carbonate, drained into the lakes. Evaporation of the lake waters increased the concentration of dissolved carbonate, and increased still more the δ 18 O values, and may have had a small positive effect on δ 13 C carb . Dolomitization of the continental redbeds occurred by the evaporative reflux of groundwater. Cyanobacteria formed mats on the lake margins, where they were quickly silicified. Domical and irregular stromatolites formed within the lakes. The occasional presence of C 30 desmethyl steranes and 4-methylsteranes in the lower parts of Unit 3 indicates that either there were marine incursions or that marine algae were transported to the lakes, possibly by winds. Nonmarine Neoproterozoic algae may have synthesized these sterols.

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.