Peter E. Isaacson

Permian marine sedimentation in northern Chile: new paleontological evidence from the Juan de Morales Formation, and regional paleogeographic implications

Abstract Permian marine sedimentary rocks that crop out in northern Chile are closely related to the development of a Late Paleozoic magmatic arc. A study of Upper Paleozoic units east of Iquique (20°S) identified three members within the Juan de Morales Formation, each of which were deposited in a different sedimentary environment. A coarse-grained terrigenous basal member represents alluvial sedimentation from a local volcanic source. A mixed carbonate-terrigenous middle member represents coastal and proximal shallow marine sedimentation during a relative sea-level rise related with a global transgression. Preliminary foraminifer biostratigraphy of this middle member identified a late Early Permian (late Artinskian–Kungurian) highly impoverished nodosarid–geinitzinid assemblage lacking fusulines and algae, which is characteristic of temperate cold waters and/or disphotic zone. The upper fine-grained terrigenous member represents shallow marine siliciclastic sedimentation under storm influence. The Juan de Morales Formation consists of continental, coastal and shallow marine sediments deposited at the active western margin of Gondwana at mid to low latitudes. A revised late Early Permian age and similar paleogeography and sedimentary environments are also proposed for the Huentelauquen Formation and related units of northern and central Chile, Arizaro Formation of northwestern Argentina, and equivalent units of southernmost Peru.

LOWER CARBONIFEROUS BRACHIOPODS FROM SIERRA DE ALMEIDA, NORTHERN CHILE

A low-diversity brachiopod assemblage from the upper member of the Zorritas Formation in the Sierra de Almeida, northern Chile, further confirms the presence of Lower Carboniferous (Tournaisian) strata in that region. Key taxa include Schuchertella? sp., Chilenochonetes anna new genus and species, Paurorhyncha chavelensis (Amos), and Septosyringothyris covacevichi new species. Other elements of the assemblage, including Paraconularia sp., Posidoniella sp., Bellerophon (Bellerophon) sp., Eocanites sernageomin- us, and Imitoceras? sp. support this Early Carboniferous age assignment. Local distribution of taxa suggests small-scale environmental control. Rhynchonelloid and syringothyroid shells, many articulated, occur in lenticular sandstone bodies that were deposited in deltaic distributary channels; chonetids, schuchertellids and mollusc-bearing, iron-rich concretions are found in siltstones and mudstones that reflect interdistributary quiet-water settings and possibly a delta-front setting. Certain elements of this fauna are also present in north- western Argentina and Peru, indicating a regional nearshore clastic regime. The closest correlation elsewhere is with the Tournaisian faunas of southeastern Australia, although analogous big-shell assemblages are found in lower Mississippian clastic strata of the central Appalachians, Ohio, and Indiana.

A new Carboniferous brachiopod genus from South America

Azurduya gen. nov. (Brachiopoda: Camarotoechiidae) is described from Early Carboniferous sequences in the Argentine Precordillera and northern Chile. Marine assemblages and the palynoflora associated with this genus suggest a Tournaisian age. The type species Azurduya chavelensis (Amos, 1958) is reviewed and redescribed from material from the type locality. Additional material from equivalent localities in the Rio Blanco Basin (La Rioja and San Juan provinces, Argentine Precordilera) has been used to understand ontogenetic changes as well intraspecific variation. Azurduya cingolanii sp. nov. is proposed.

Global biostratigraphic comparison and correlation of an early Cisuralian palynoflora from Bolivia

This paper presents new data about Early Permian (Cisuralian) strata, palynostratigraphy and absolute dating from the Copacabana Formation in central Bolivia. Recent stratigraphic and palynologic data from marine and transitional rocks at Apillapampa refine the age of Cisuralian palynomorphs in South America. Twelve samples interbedded with five volcanic ashes (processed and productive) yielded 94 palynomorph species arranged in two informal palynoassemblages: the lower assemblage Vittatina costabilis corresponds to one sample near the base of the Copacabana Formation and the upper Lueckisporites virkkiae assemblage occurs in overlying marine and coal-bearing transitional intervals. Ages were also independently refined by a modern review of conodonts, fusulinids, along with those U–Pb radiometric ages (Isotope Dilution Thermal Ionization Mass Spectrometry [ID-TIMS] of zircon-bearing interbedded tuffs). These data suggest that the lower marine member of the Copacabana Formation at this location is Asselian and Sakmarian. Lueckisporites virkkiae is a key species of palynomorph utilised in South American and global Permian biostratigraphic reconstructions. Hence, a thorough global comparison of these palynofloras and correlations is addressed in this contribution, considering first appearances of mainly cosmopolitan diagnostic taxa. Correlations are established with many similar Permian palynofloras, some also constrained with radiometric data, in South America (Brazil, Uruguay and Argentina) and elsewhere.

Devonian brachiopods from northern Chile

Fossiliferous marine Devonian has been known in Chile for less than a decade. The initial discovery from south of the Salar de Atacama region, in the Sierra de Almeida, northern Chile is described together with its brachiopods. The brachiopods indicate an age span of Emsian-Eifelian or Givetian, shallow-water conditions in the Benthic Assemblage 2 range, and a biogeographic boundary region between the cool climate Malvinokaffric Realm and warmer region extra-Malvinokaffric Realms, including the Eastern Americas Realm and the Rhenish-Bohemian Region of the Old World Realm. The recent discovery of a Malvinokaffric Realm trilobite far to the south in Chile, at Buill in the Andes of Chiloe, serves to underline our very preliminary knowledge of the Chilean Devonian, whereas the alleged Devonian brachiopods from the Chonos Archipelago far to the south are probably bivalves of uncertain age.

Integrated study of fauna and microflora from the Early Devonian (Pragian–Emsian) of northwestern Argentina

The Devonian System in northern Argentina has been broadly analysed, but details of its lithologies, biostratigraphy and fossil content have not been presented in a comprehensive study. We performed the first integrative analysis of the palynological and macrofossil content from the Pescado Formation at the Zenta Range, Argentina. We define a new species of cryptospore and extend the stratigraphic record of the ichnogenus Psammichnites isp. for South America. The stratigraphic ranges of the palynomorphs suggest a time span from the ?late Lochkovian to Pragian–earliest Emsian, but the co-occurrences of key invertebrates narrow the age of the beds to the late Pragian and early Emsian. Moreover, sedimentary analysis indicates a proximal shoreface–foreshore depocenter during this time range for the Zenta region. The contraction phase of the basin during the middle Pragian and Emsian is evidenced by the presence of sand bodies at the top of the column and the higher supply of terrigenous components. During this regression event, a low diversity Malvinokaffric Realm brachiopod assemblage occurs, with dominance of Australospirifer hawkinsi. The predominance of the latter species during this event is coeval with the first decline of the Malvinokaffric Realm in the neighbouring Paraná basin.

Palynologic delineation of the Devonian–Carboniferous boundary, West-Central Montana, USA

ABSTRACT Cratonic depositional systems in the Central Montana Trough involve the Devonian--Carboniferous boundary (DCB), and reflect both subtle regional epeirogeny and significant global glacioeustatic controls. A palynologic analysis of the upper Three Forks, Sappington and lower Lodgepole formations was carried out at the classic Logan Gulch location in Horseshoe Hills. The lower Trident Member of Three Forks Formation yielded low-diversity cosmopolitan, long ranging phytoplankton and few spores species (LAs1), attributed to the middle Famennian. The upper part of the same green seaway shale yielded only leiosphaerids and Botryococcus (LAs2), along with an external mold of a clymenid ammonoid. Age-diagnostic spores Retispora lepidophyta, Verrucosisporites nitidus and Vallatisporites vallatus from middle Sappington siltstone (LAs3) indicated a Strunian LN Zone. Two more assemblages from upper Cottonwood Canyon Member (LAs4) and false Bakken (LAs5), in the lower Lodgepole Formation yielded scarce, poorly preserved spores. The presence of Waltzispora polita in LAs4 indicated a Tournaisan-Visean age.

Biostratigraphic Significance of Lower Cisuralian Palynoflora from Apillapampa, Bolivia

Recent stratigraphic and palynological data from marine and transitional rocks of the Copacabana Formation at Apillapampa in central Bolivia refine the age of Cisuralian palynomorphs in South America. Samples interbedded with volcanic ashes yielded palynomorph taxa arranged into two informal palynoassemblages: the lower assemblage Vittatina costabilis (Vc) occurs in one sample at the base of the Copacabana Formation, and the upper Lueckisporites virkkiae (Lv) assemblage is found in overlying marine and coal-bearing transitional intervals. Ages were also independently refined by a modern review of microfossil (conodonts and fusulinids) and five U–Pb radiometric ages (ID–TIMS of zircon-bearing interbedded tuffs), which confirm that the Copacabana Formation at this location is Asselian and Sakmarian in age. Lueckisporites virkkiae and other palynomorphs are key species of palynozones utilized in South American and global Permian biostratigraphic reconstructions. Hence, a thorough global comparison of these palynofloras and correlations is made in this study, considering the first appearances of mainly cosmopolitan diagnostic taxa. Correlations are established with many similar Permian palynofloras, some also constrained by radiometric data, in South America (Brazil, Uruguay, and Argentina) and elsewhere (Africa, Australia, Oman, and Saudi Arabia).