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Authigenic magnesium‐clays have been observed and documented in the Green River Formation, specifically as ooidal stevensite grains. Magnesian clays are a valuable proxy for reconstructing shallow‐water, saline–alkaline lake palaeoenvironments due to their responses to chemical changes. Magnesium‐rich clay minerals are relatively common components in modern and ancient lake systems. A rare interaction of lacustrine magnesian clays, microbialites, carbonates and volcanoclastic deposits of the Eocene Green River Formation crop out in Sanpete Valley, Utah, USA. The characterization of this interaction, Mg‐rich clay and carbonate minerals genesis, the environmental controls on Mg‐rich clay minerals distribution and accumulation is still poorly understood. This study has identified various species of Mg‐clay minerals (stevensite, mixed‐layer kerolite–stevensite and sepiolite), along with calcite and dolomite, found in four Mg‐clay‐bearing lithofacies: (i) Mg‐clay stromatolites; (ii) Mg‐clay arenites; (iii) Mg‐claystone; and (iv) intraclastic Mg‐clay hybrid arenites and conglomerates. Magnesium‐clay bearing rocks from the Sanpete Valley area were deposited in a south‐western, shallow embayment of Eocene Lake Uinta along the margin of the Sevier fold and thrust belt. Sanpete Valley's Mg‐clay‐bearing section shows syngenetic mineralogical paragenesis with neoformation of magnesian clays accompanied by calcite and silica. Regional chronostratigraphic correlation of Sanpete Valley's Mg‐clay‐bearing section with the Uinta Basin was made by zircon U–Pb dating (laser ablation–inductively coupled plasma–mass spectrometry) of two tuff beds. Stevensite and kerolite are precipitated under very specific conditions. This study presents a depositional model, considering evidence from the authigenic mineralogy, facies, fossil evidence and basin context. All of these findings allow a comprehensive analysis of the lithostratigraphic and chemostratigraphic evolution of this isolated bay system, as well as texture classification. This study provides analogue for detailed correlation and comparison to other basins containing similar Mg‐clay and microbialite‐bearing deposits. [ABSTRACT FROM AUTHOR]

Fossil and living stromatolites are abundant around the margins of Lake Tanganyika, Africa, and provide a wealth of paleolimnologic and paleoclimatic information for the late Holocene. Six lines of evidence show that stromatolites and cements are precipitating in the lake today: (1) carbonate saturation state calculations, (2) documentation of living stromatolites and their depth distribution, (3) new stable isotope data showing the lake's present mixing state and ancient evaporation and inflow balance, (4) new radiocarbon data and a reevaluation of apparent 14C ages derived from Lake Tanganyika carbonates, (5) the presence of modern Mg-calcite cements derived from lake waters, and (6) the presence of modern, biologically mediated Mg-calcite precipitates in the lake. Lake Tanganyika's lake levels have been remarkably stable over the past 2800 yr, fluctuating around the marginally open to marginally closed level through most of this time period. Lake lowstands and high [Delta]18O values from the ninth century B.C. to the early fifth century A.D. indicate that the lake basin was comparatively dry during this time. However, the period prior to the most recent opening of Lake Kivu into the Lake Tanganyika basin (ca. A.D. 550) was not marked by major lake lowstands, nor was this opening accompanied by a dramatic lake-level rise. The Kivu opening was roughly coincident with a significant shift toward isotopically lighter ([Delta]18O and [Delta]13C) lake water, which persists today. The lake remained close to its outlet level between the sixth and thirteenth centuries A.D. Lake levels rose between the fourteenth and sixteenth centuries. At some time between the late sixteenth and early nineteenth centuries, lake level fell to perhaps its lowest level in the past 2800 yr. By the early nineteenth century, lake level had begun to rise to the overflow level, apparently the result of a regional increase in precipitation/evaporation ratios. Weak [Delta]18O/[Delta]13C covariance for late Holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level, with only occasional periods of closure. However, there is no simple relationship between solute input from Lake Kivu, isotope input from Lake Kivu, and lake levels in Lake Tanganyika. Lake Kivu waters are the primary source of major ions in Lake Tanganyika, but are much less important in controlling the [Delta]18O and the lake level of Lake Tanganyika. Because the Ruzizi River's discharge into Lake Tanganyika is largely derived from sources other than Lake Kivu, the overflow events in the two lakes have been uncoupled during the late Holocene.

The 25- to 30-m-thick Algal Member of the Mescal Limestone (middle Proterozoic Apache Group) contains two distinct stromatolitic units: at the base, a 2- to 3-m-thick unit composed of columnar stromatolites and above, a thicker unit of stratiform and pseudocolumnar stromatolites. Columnar forms from the first unit belong to the Group Tungussia, and two new Forms are described: T. mescalita and T. chrysotila. Among the pseudocolumnar stromatolites of the thicker unit, one distinctive new taxon, Apachina henryi, is described. Because of the low stromatolite diversity, the biostratigraphic value of this assemblage is limited. The presence of Tungussia is consistent with the generally accepted isotopic age for the Apache Group of 1200 to 1100 Ma. The Mescal stromatolites do not closely resemble any other known Proterozoic stromatolites in the southwestern United States or northwestern Mexico. Analyses of sedimentary features and stromatolite growth forms suggest deposition on a stable, flat, shallow, subtidal protected platform during phases of Tungussia growth. Current action probably influenced the development of columns, pseudocolumns, and elongate stromatolitic ridges; these conditions alternated with phases of relatively quiet water characterized by nonoriented stromatolitic domes and stratiform stromatolites. Stable conditions favorable for development of the Mescal stromatolites were short-lived and did not permit the development of thick, stromatolite-bearing units such as those characteristic of many Proterozoic sequences elsewhere.

Large acanthomorphic acritarchs have been found in chert of the Ediacaran Doushantuo Formation at several localities in South China, including the East Yangtze Gorges of Hubei Province, Weng'an area of Guizhou Province, and elsewhere. However, their potential for biostratigraphic subdivision and correlation of Ediacaran successions is limited by facies control, taphonomic biases, and taxonomic problems. In the Yangtze Gorges, the Doushantuo Formation is generally subdivided into four lithologic members. However, in the Weng'an area, the Doushantuo Formation comprises just a lower and upper part separated by a mid-Doushantuo erosional surface. In the Yangtze Gorges at the Zhangcunping section, the Doushantuo succession is similar to that at Weng'an. So far, the correlation between the Yangtze Gorges and Weng'an area, and elsewhere has been an issue of debate. To resolve the debate, we selected eight sections in the Yangtze Gorges area and systemically sampled chert nodules of the Doushantuo Formation, focusing in particular on the upper Doushantuo Formation. Our data confirm two different assemblages appearing separately in the second and third members, which are separated by a negative δ13C excursion (EN2). The lower assemblage is characterized by Tianzhushania and a diverse suite of large acanthomorphic acritarchs. The upper assemblage is distinguished from the lower assemblage, by (1) absence of Tianzhushania; (2) occurrence of abundant, 100-150 μm, smooth-walled spherical microfossils; (3) occurrence of highly diverse acanthomorphic acritarchs including species extending from the second member and new forms in this member; (4) occurrence of unnamed new forms of protist; and (5) occurrence of the tubular microfossil Sinocyclocylicus guizhouensis. Since the Tianzhushania-dominated assemblage is not present in Australia, it seems that only the upper acanthomorph assemblage is present and thus the lower Doushantuo acanthomorph assemblage is missing in Australia. [ABSTRACT FROM AUTHOR]