Preston Cloud

The ediacarian period and syste: metazoa inherit the Earth.

The Ediacarian, here defined as the initial period and system of the Phanerozoic Eon, is characterized by the oldest known multicellular animal life. The distinctive biotal assemblage comprises naked Metazoa, represented in the type region by 26 species in 18 genera and 4 or more phyla, plus simple metazoan surface tracks. Elements of this unique biota appeared worldwide at low paleolatitudes, following terminal Proterozoic glaciation. Ediacarian history lasted from about 670 million to 550 million years ago. This interval, plus Early Cambrian, was the time during which metazoan life diversified into nearly all of the major phyla and most of the invertebrate classes and orders subsequently known.

Giant Stromatolites and Associated Vertical Tubes from the Upper Proterozoic Noonday Dolomite, Death Valley Region, Eastern California

Stromatolitic domes having apparent growth reliefs up to 100 m or more characterize the lower member of the Noonday Dolomite, late Proterozoic, Death Valley region, eastern California. Long to short, subparallel, initially subvertical tubes penetrate the bedding of these structures at angles that range from 90° where stratification was initially horizontal to downslope angles up to 110° (and acute upslope angles) as initial dips increase. Tubes generally are missing where initial dips exceed 20°. They are common in the larger buildups and most abundant in the upper and central parts of these. They may be open or filled with coarsely sparry secondary dolomite, medium-gray ultra microgranular dolomite, silty brown dolomite, or drusy quartz. They pinch and swell irregularly along their length from a mean diameter of 1.5 cm and are subcircular to irregular in horizontal cross section. Secondary crystalline growths similar to those in many tubes are common along lamination surfaces and as irregular vug fillings. Weathering along intersecting fracture sets locally produces a subvertical lineation in the same orientation as associated tubes. Possible origins considered for the tubes and found improbable are: (1) metazoan burrows, (2) interspaces of columnar stromatolites, (3) columnar stromatolites, (4) solution pipes, and (5) root casts. Instead the spaces now represented by filled vugs, sparry lamination surfaces, and tubes are interpreted as having, in general, a common origin caused by the upward movement of fluids through the stromatolite mounds.

Physical limits of glauconite formation

Available records indicate that the formation of glauconite requires marine water near normal salinity, reducing conditions, and appropriate source materials. It is favored by high organic content of the bottom sediments in which it forms, and by slow or negative sedimentation. It may form under a fairly wide but not unlimited range of temperatures and depths. This variety of limiting factors combines with the general ease of field recognition to make glauconite much more useful in paleoecology than other accessory minerals which are so far known to provide clues to depositional environment.

Proterozoic Age of the Bambui Group in Brazil

Conophyton aff. C. metula Kirichenko is abundant in dolomites of the Paraopeba Formation (Bambui Group) north of Vazante, Minas Gerais, Brazil. That suggests a middle Riphean (Proterozoic) age for the Bambui Group of this region. Such an age, in the range of 950 to 1,350 m.y., and probably closer to the younger number, is consistent with the probable ages of other now known Bambui stromatolites, as well as with the sparse and equivocal radiometric data available. It is older, however, than the Silurian age given on the 1960 geologic map of Brazil and the “upper Eocambrian” age now generally accepted.

Pseudofossils: A Plea for Caution

The geologic account of ancient life is plagued with reports that do not withstand critical assessment. This is a special problem in the older rocks, where reports of spurious records continue to dilute the authentic record of evolution on the primitive Earth. The problem is illustrated by brief analyses of three timely examples of widely different ages. These are (1) desiccation curls interpreted as arthropods, (2) drag marks resembling “worm tracks,” and (3) pyrite rosettes interpreted as medusae. It is left to the reader to draw conclusions about similar instances not here alluded to; the record of alleged pre-Phanerozoic life is full of them.

Algal and cryptalgal structures and platform environments of the late pre-Phanerozoic Noonday Dolomite, eastern California

A late pre-Phanerozoic platform, almost continuously blanketed by algal mats, is recorded in the distributional pattern, composition, and textures of the Noonday Dolomite, Death Valley region, California. The platform, apparently L-shaped, was at least 160 km long. It lay west and south of the Nopah upland, established much earlier, and north of a fault-bounded east-trending depression containing units of the older Pahrump Group. A lower member of the Noonday consists of very finely crystalline, laminated, and relatively pure dolomite. The laminations, although commonly deposited horizontally, also outline mounds having a synoptic relief of a few metres to nearly 200 m. Algal or cryptalgal fabrics are common. Especially large mounds in the Nopah Range lie on the down-dropped south side of a contemporaneous fault, indicating tectonic control of size and location. The laminated mounds are characterized by large-scale vertical stacking, best explained as produced by differential growth of algal mats. The configuration of the laminations indicate that the mats grew to depths of at least 100 m but probably did not extend more than 150 m below the mound crests. The absence in the lower member of features attributable to moving water and the paucity of clearly detrital material indicate quiet water and very low relief in the bordering land. An upper member consists of (1) discontinuous bodies of thinly and evenly bedded siltstone and silty dolomite that ordinarily occupy the deeper of the intermound lows; (2) a laterally continuous unit of silty dolomite with wavy laminations and small, rippled to subhemispherical, laterally linked stromatolites; and (3) an upper unit of massive to laminated silty dolomite, containing large domal and bosslike stromatolites and associated with bodies of strongly cross-bedded quartz-dolomite sandstone. These features evidence the entry onto the platform of detritus supplied by increasingly stronger uplift of the Nopah Upland and the sweeping of the platform by increasingly stronger currents. We interpret the carbonate of the lower member and much of the carbonate of the upper member as having been precipitated from ambient waters by removal of CO2 during algal photosynthesis.

On microbial contaminants, micropseudofossils, and the oldest records of life

Abstract Microbial contaminants may be introduced on outcrop as well as en route to or in the laboratory. Micropseudofossils may be natural or man-made. It is possible to recognize such misleading objects and important that they are not allowed to dilute the growing record of authentic pre-Phanerozoic life. Filamentous microbial contaminants from minute cracks in samples of ancient carbonate rocks from Brazil (perhaps 1 Ga old) and South Africa (∼2.3 Ga old) are similar to occurrences previously described as fossils. Published records of supposedly Archean microbial life also include microcontaminants and laboratory artifacts. Although microstructures from sedimentary rocks of the Swaziland system could be fossils, they are not demonstrably so. The oldest structurally preserved fossils yet known seem to be the filaments described by Lois Nagy from stromatolitic limestone in the ∼2.3 Ga old Malmani Dolomite of South Africa. It will be difficult to establish unequivocal older records in the absence of definitive ultrastructural or micro-chemical evidence.

New microbial fossils from 2 Gyr old rocks in Northern Michigan

Abstract A diverse, abundant, apparently procaryotic microflora is here reported from the Tyler Fm., upper Baraga Group, Gogebic County, Michigan. In addition, numerous chains of spheroids and grains of hematite after pyrite from the Baraga Group, Baraga County, suggest a microbial presence. The Tyler microbiota occurs in spheroidal clasts and oncoids comprised of pyritic cherts along the Black River east of Ironwood. It includes nine recognizably different microbial forms, four of filaments, two of spheroids, one umbrellaform, and two stellate. Its nearest affinities are with the stratigraphically lower microbiota of the Gunflint Iron‐formation, with which it shares such distinctive fossils as Gunflintia, Metallogenium (Eoastrion), and Kakabekia cf. K. umbellata Barghoorn. The Baraga assemblage, considered as a single form of “dubiofossil,”; includes strongly curvate chains of hematized pyrite spheroids and grains arranged in configurations suggesting filamentous procaryotes. It is from a black phosphati...

New Pre-Paleozoic Nannofossils from the Stoer Formation (Torridonian), Northwest Scotland

Sphaeroidal unicellular fossils 10 to 62 μm in diameter are abundant in black shales referred to the Stoer Formation of lower Torridonian age at Stoer Bay, in northwest Scotland. The Rb-Sr whole-rock age is estimated as about 935 m.y. The fossils are assigned to the leiosphaeridacean acritarch genera Favososphaeridium and Protosphaeridium, common in younger pre-Paleozoic rocks of the U.S.S.R. Their biological affinities are obscure, but it is consistent with what is known so far of their associations and distribution to consider them as some form of marine phytoplankton.

New microbial fossils from ∼1.3 billion‐year‐old rocks of Eastern California

New types of microbial fossils and new occurrences of fossils previously reported only from the Beck Spring Dolomite of the Pahrump Group are now recognized from each of the three formations of the Pahrump Group (Crystal Spring Formation, Beck Spring Dolomite, and Kingston Peak Formation) approximately 1.3 X 10° years old. Comprising perhaps eight or nine distinctive forms, these fossils are characteristically preserved as faint ghostlike structures whose low‐contrast outlines are clearly revealed only when illuminated by a xenon lamp and recorded on high‐contrast film. They represent a distinctive, previously overlooked or neglected type of preservation that has significantly extended the known distribution of microbial fossils in the Pahrump. They include the oldest occurrence known to us of filaments designatable as Girvanella and apparently the first from rocks of pre‐Phanerozoic age. Similar fossils were also found, using the same techniques, in the Chuar Group of the Grand Canyon and in the Uluntui ...