Thomas R. Fairchild

Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): Confirmation of rapid icehouse to greenhouse transition in snowball Earth

Stratigraphic and isotopic data identify the lower 45 m of the Araras Group, on the southwest margin of the Amazon craton, as a Neoproterozoic platform cap carbonate deposited below wave base upon Varanger glacial diamictites of the Puga Formation. The basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube-like structures and fenestral nonmicrobial planar laminites with tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans (pseudomorphs after aragonite). The basal contact of the Puga cap exhibits soft-sediment deformational structures (principally load casts) that are here attributed to rebound-induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite. These features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball Earth model of Neoproterozoic glaciation.

Integrated correlation of the Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil): palaeogeographic, palaeoclimatic and palaeobiologic implications

Abstract The Corumba Group of SW Brazil and the Arroyo del Soldado Group (ASG) of Uruguay are correlated on the basis of litho-, bio- and chemostratigraphy. Both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf. In the Corumba basin, sedimentation began in the Varangerian, represented by the glaciomarine Puga Formation. A series of sea-level fluctuations coupled with climatic changes are recorded up section. While uppermost deposits of the ASG are of lowermost Cambrian age, sedimentation ceased in the latest Vendian in the Corumba basin. An assemblage of six species of organic-walled microfossils dominated by Bavlinella faveolata and Soldadophycus bossii, three species of vendotaenids and two species of skeletal fossils (Cloudina and Titanotheca) is described from the Corumba Group. The vendotaenid Eoholynia corumbensis sp. nov is described from siltstones of the Guaicurus Formation. An important diversity of skeletal fossils in the Corumba, Arroyo del Soldado and Nama groups points to favourable Vendian palaeoclimatic conditions in SW-Gondwana. Preliminary carbon isotopic data show a series of alternating positive and negative excursions, corroborating the upper Vendian age indicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age. It is postulated that the Corumba and ASGs were deposited onto the same shelf, which opened to the east. The Rio de la Plata Superterrane (Craton) extends farther to the north than previously expected, or it was already amalgamated with the Amazonian Craton by Vendian times. Collision of the platform with the Parana Block caused closure of the basin during the Cambrian-Early Ordovician. Finally, models of Neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed.

The dawn of animal skeletogenesis: Ultrastructural analysis of the Ediacaran metazoan Corumbella werneri

Although the basic morphology of the Ediacaran metazoan Corumbella werneri (the type species of the genus) is well established, little is known about its skeletal tissue. Carbonaceous fragments of this fossil from the Itapucumi Group (Paraguay) reveal details of the ultrastructure of its carapace, providing an unprecedented opportunity to understand a paradigmatic issue of the evolution of skeletogenesis in early metazoans. Corumbella was a sessile predator whose carapace consisted of organic polygonal plates with pores and papillae similar to features observed in some conulariids. Its occurrence with the shelly fossil Cloudina suggests that the acquisition of protective structures in metazoans involved penecontemporaneous processes of biomineralization and secretion of organic walls.

Recent discoveries of Proterozoic microfossils in south-central Brazil

Abstract Twenty-two Precambrian microfossiliferous localities, seven newly reported here, are known from the Sao Francisco Block and associated Brasilia fold belt of south-central Brazil, reported from outcrop and drill core samples of Mesoand Neoproterozoic cherts, carbonates, and siliciclastics of the ∼1170 to ∼950 Ma old Paranoa Group and the ∼950 to ∼700 Ma old Sao Francisco Supergroup (composed of the Jequitai Formation and the overlying Bambui Group). Representatives of nearly 30 genera of Proterozoic microfossils, chiefly cyanobacteria and subsidiary acritarchs, have been reported from these strata, preserved mostly within carbonaceous black chert in flat-laminated and high-relief stromatolites, and represent a wide spectrum of environments evidently ranging from nearshore lagoonal to supratidal, intertidal, subtidal, and offshore marine settings. Contrary to previously held views, the microfossils are generally more abundant and better preserved in the older unit (Paranoa Group) than in the younger one (Sao Francisco Supergroup). Nevertheless, among the newly reported assemblages, one from the lower part of the Bambui Group stands out as probably the best preserved and most diverse microbiota known from any Precambrian unit of South America. Although paleobiologic studies of the Brazilian Proterozoic are still in a formative stage, recent discoveries are highly promising, such as the detection in the work presented here of possible differences: (1) between the microfossil sizes in stromatolites of the Paranoa and Bambui Groups; (2) in the abundance and size of acritarchs with respect to lithology and facies; and (3) in the microfossil assemblages preserved in morphologically distinct stromatolites.

Origin and impact of the oldest metazoan bioclastic sediments

The emergence of soft-bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of Ediacaran environments, leading to the genesis of biogenic hard-part deposits for the first time in Earth’s history. The impact of bioclast origin on sedimentary processes is analyzed herein, focusing on the sedimentology and taphonomy of shell concentrations dominated by the Ediacaran index fossil Cloudina from the Itapucumi Group, Paraguay. Skeletal concentrations include both dense accumulations of parautochthonous, disarticulated specimens (“Type 1 deposits”) and in situ specimens preserved as loosely packed assemblages (“Type 2 deposits”). At that time, Cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts. The simple fabric and geometry of these accumulations are typical of Cambrian-style shell beds. Despite their Precambrian age, these deposits indicate that the establishment of the Phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes.

A record of Permian subaqueous vent activity in southeastern Brazil

The remarkable occurrence of more than 4,500 conical siliceous mounds in an area of less than 1.5 square kilometres has been reported in the Paraná basin, near Anhembi, São Paulo, in southeastern Brazil. These structures, which are up to two metres high, are thought to have been formed at the margin of a very shallow, broad but waning internal sea, and it was originally suggested that they are stromatolites. Yet their restricted occurrence, unusual abundance and nearly pure siliceous composition have never been satisfactorily explained by this hypothesis. Here we report field and laboratory observations on their shape, construction, composition and mineralogy. On the basis of our data we suggest that the conical mounds are the result of subaqueous Late Permian vent activity in southwestern Gondwana. The present siliceous cone field differs considerably from other Palaeozoic siliceous hot spring deposits, such as those at Rhynie, Scotland, and the Drummond basin, Australia, and therefore represents an unusual occurrence of vent activity.

Proterozoic microfossils from subsurface siliciclastic rocks of the São Francisco Craton, south-central Brazil

Abstract Compressed organic-walled microfossils attributable to five filamentous taxa, and ten coccoidal eight, (taxa originally spheroidal forms and two originally ellipsoidal, colonial forms), all having long time-ranges in the Proterozoic, have been identified in fine-grained siliciclastic sediments in cores from four bore holes cutting subsurface equivalents of the Mesoproterozoic Conselheiro Mata Group and Neoproterozoic Bambui Group on the Sao Francisco Craton in south-central Brazil. This is the first documented report of organic-walled microfossils from subsurface Proterozoic rocks of Brazil, including the first reference to occurrences of microfossils in the Conselheiro Mata Group and the Serra de Santa Helena, Lagoa do Jacare, and Serra da Saudade Formations of the Bambui Group. Surprisingly, microfossils in the Conselheiro Mata Group are better preserved, more diverse, and more abundant than in the younger, extremely depauperate assemblages of the Bambui Group, possibly reflecting contrasting biostratinomic and paleoclimatic conditions. The Conselheiro Mata assemblages are dominated by fragments of filamentous microbial mats and small to medium-sized (≤200 μm), unornamented, simple acritarchs, none of which yet provides conclusive biostratigraphical evidence as to the age of the microfossiliferous successions. The morphologic simplicity, relatively small size, and low diversity of the microfossils in the Conselheiro Mata Group, however, are consistent with the Mesoproterozoic age attributed to this unit. Additionally, the relative proportions of planktic (sphaeromorphs) versus benthic (tubular filaments) microfossils are suggestive of fluctuations in sea level along part of the most fossiliferous bore hole.

Estudo comparativo entre estromatólitos do tipo Conophyton das faixas Ribeira e Brasília

The Conophyton stromatolites were described in the Ribeira (C. garganicum) and Brasilia (C. metulum and C. cylindricum) belts, in lithostratigraphic unities attributed to the Mesoproterozoic. Conophyton garganicum occurs in the Ribeira Belt (Itaiacoca Group) in the Itapeva region (Sao Paulo). In the Itaiacoca Group, the Conophyton garganicum forms have also been found at Abapa (Parana). They are similar to the other coniform stromatolites described in Itapeva, but differ from the other forms in the Brasilia belt, including C. cylindricum and C. metulum. In the Brasilia Belt two occurrences of Conophyton are described in the Vazante Group in Minas Gerais, in the Cabeludo (C. cylindricum) and Lagamar (C. metulum) regions and one in the Paranoa Group near Cabeceiras (Goias) (C. cylindricum). Conophyton from Cabeludo differs from the Lagamar´s in laminar thickness, as well as the regularity degree and laminar heritage of the axial zone. Otherwise, the Conophyton from Cabeceiras is similar to the Cabeludo´s in laminar thickness and configuration. The significant differences between the Conophyton from Brasilia and Ribeira belts indicate distinct depositional environments and/or ages. Conophyton shows Late Mesoproterozoic and Early Neoproterozoic ages from Vazante, Paranoa and Itaiacoca groups, which is consistent with available radiometric ages.

Microfossils associated with silicified Stratifera undata Komar 1966 from the late Proterozoic Bambuí Group, south-central Brazil

Abstract Small, partly silicified, laterally linked stromatolites exhibiting a high degree of inheritance in the shape of successively stacked laminae from the lower Bambui Group (late Proterozoic), near Unai, Minas Gerais, south-central Brazil, are here described and classified as Stratifera undata Komar 1966. Rare, moderately preserved microfossils and stratiform concentrations of poorly preserved probable microfossils consist essentially of coccoidal forms, there being no convincing evidence of filamentous forms. If, in fact, coccoidal micro-organisms were the overwhelmingly dominant mat-formers, this represents an unusual situation among silicified microflorules from morphologically distinct Precambrian stromatolites yet is quite similar to that observed in another form of Stratifera ( S. biwabikensis from the Gunflint Iron-Formation), which, like the Unai stromatolites, also formed under permanently submerged conditions. S. undata is now known in Brazil from similar upper Proterozoic settings in two very widely separated localities, which suggests its potential use in regional stratigraphic correlation. As a result of this study, we recommend that greater attention be paid to the ‘simpler’ stromatolite morphologies, especially when micro fossiliferous, since such forms commonly comprise the basal portions of biostratigraphically significant stromatolites and may share a common microstructure with these same, more complex forms. Inasmuch as microstructure is generally acknowledged as the stromatolite property most closely controlled by biological factors, study of silicified microflorules within simpler forms may permit inferences regarding not only the microbial communities responsible for more complex stromatolites having the same microstructure but also possible biological reasons for the succession of distinct stromatolite assemblages observed in upper Precambrian rocks.

Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization

Abstract. Vase-shaped microfossils (VSMs) occur in dolomitic extraclasts of indeterminate provenance within the basal diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group) of west-central Brazil, having an age constrained between 889 ± 44 Ma (K-Ar; basement rocks) and 587 ± 7 Ma (40Ar/39Ar age of early metamorphic cryptomelane in overlying manganese ore). Early isopachous carbonate cement entombed these VSMs, preserving rare direct evidence of original wall composition that is carbonaceous (now kerogenous) in practically all specimens. Some tests are siliceous or composed of a quartz-kerogen mixture; secondary replacement explains some features of these tests, but original biomineralization seems more likely for others. This interpretation, coupled with test morphology, suggests affinity to arcellinid testate amoebae. Five VSM taxa are recognized in the deposit: Cycliocyrillium simplex Porter, Meisterfeld, and Knoll, 2003, and C. torquata Porter, Meisterfeld, and Knoll, 2003, originally described in the Chuar Group (USA), and three new monospecific genera—Palaeoamphora urucumense n. gen. n. sp., Limeta lageniformis n. gen. n. sp., and Taruma rata n. gen. n. sp. Most of the taxonomically important characteristics of these VSMs occur also in extant testate amoebae, but the combinations of some characters, such as organic-walled tests having exceptionally long necks that exhibit terminal apertures (L. lageniformis n. gen. n. sp.), are evidently novel additions to the known diversity of Neoproterozoic VSMs. Evidence of glacially influenced deposition in the conformably overlying Santa Cruz Formation may indicate that the Urucum Formation slightly preceded or was penecontemporaneous with a major Neoproterozoic glaciation, although the VSM-hosting extraclasts must be older, possibly rivaling the age of the testate amoebae of the Chichkan Formation (766 ± 7 Ma) that are currently regarded as the oldest record of protists in the geological record.