14 research outputs found
Biostratigraphic significance of the latest Cambrian-earliest Ordovician agnostoid trilobites from Northwestern Argentina
The biostratigraphic significance of the latest Cambrian-earliest Ordovician trilobite agnostoids from northwestern Argentina is summarized. A characterization of the faunas recognized below and above the Cambrian-Ordovician boundary is presented on the bases of information from sierra de Cajas (Jujuy Province, Eastern Cordillera; Lampazar and Cardonal Formations), rÃo Volcancito (La Rioja Province, Famatina range; Volcancito Formation) and other localities assigned to the Parabolina frequens argentina Biozone. In addition, the stratigraphic meaning of each agnostoid species from the P. frequens argentina Zone is updated. The uppermost Cambrian is characterized by a distinctive agnostoid assemblage composed of Lotagnostus (Lotagnostus) sp., Lotagnostus (Semagnostus) zuninoi (HARRINGTON and LEANZA), Micragnostus vilonii HARRINGTON and LEANZA, M. calviformis HARRINGTON and LEANZA, Strictagnostus? micropeltis (HARRINGTON and LEANZA), Pseudorhaptagnostus (Machairagnostus) tmetus HARRINGTON and LEANZA, Pseudorhaptagnostus (Machairagnostus) cf. tmetus HARRINGTON and LEANZA, Pseudorhaptagnostus (Machairagnostus) sp., Gymnagnostus bolivianus (HOEK), Gymnagnostus perinflatus HARRINGTON and LEANZA, and Leiagnostus turgidulus HARRINGTON and LEANZA. This fauna is commonly recorded together with the polymeroid Beltella ulrichi (KAYSER), Onychopyge HARRINGTON, Plicatolina scalpta HARRINGTON and LEANZA, Parabolina frequens argentina (KAYSER), Parabolinella coelatifrons HARRINGTON and LEANZA, and Angelina hyeronimi (KAYSER). Based on this agnostoid-polymeroid fauna, a Pseudorhaptagnostus (Machairagnostus) - Gymnagnostus Assemblage Subzone (lower part of the P. frequens argentina Zone) is formally proposed. Agnostoid diversity drastically declines through the Cambrian-Ordovician boundary, probably as a consequence of a regional regressive-transgressive event. In the lowest Tremadoc the agnostoid records are mainly restricted to Trilobagnostus chiushuensis (KOBAYASHI) and some forms described under open nomenclature (Anglagnostus? sp., Micragnostus sp.). The upper part of the P. frequens argentina Zone is better defined by polymeroid trilobites. Jujuyaspis keideli KOBAYASHI is its most characteristic species
El Alisal: a new locality with trace fossils of the Puncoviscana Formation (late Precambrian-early Cambrian) in Salta Province, Argentina
The Puncoviscana, Suncho and Las Aguaditas/Negro Peinado formations represent a thick siliciclastic folded succession that crops out on a strip about 800 km long and 150 km wide in northwestern Argentina (Jujuy, Salta, Tucumán, Catamarca and La Rioja provinces). These sequences are lithologically characterized by very lowgrade metamorphic sediments grading from slates to schists; turbidites, pelagic clays, limestones and volcanic rocks. On the basis of paleoichnological data, these units have been assigned to the late Precambrian-early Cambrian. A new locality bearing well preserved trace fossils from the Puncoviscana Formation is described herein. The outcrop is located about 45 km west of Salta city (Salta Province), displaying an alternation of colored differentiated grayish-bluish slates and fine sandstones. Ichnofossils include Cochlichnus anguineus HITCHCOCK, Helminthoidichnites tenuis FITCH, Helminthoida isp., Monomorphichnus lineatus CRIMES, LEGG and MARCOS ARBOLEYA, Planolites isp. and Torrowangea? isp., as well as some limb marks and a few slightly bended smooth trails (indet.). An Early Cambrian age for the succession at El Alisal as well as some environmental considerations are allowed by the ichnological association. In addition, the ichnogenus Planolites is mentioned for the first time from the "red shales and conglomerate type" facies at the neighbor locality of Chorrillos
El Alisal: a new locality with trace fossils of the Puncoviscana Formation (late Precambrian-early Cambrian) in Salta Province, Argentina
The Puncoviscana, Suncho and Las Aguaditas/Negro Peinado formations represent a thick siliciclastic folded succession that crops out on a strip about 800 km long and 150 km wide in northwestern Argentina (Jujuy, Salta, Tucumán, Catamarca and La Rioja provinces). These sequences are lithologically characterized by very lowgrade metamorphic sediments grading from slates to schists; turbidites, pelagic clays, limestones and volcanic rocks. On the basis of paleoichnological data, these units have been assigned to the late Precambrian-early Cambrian. A new locality bearing well preserved trace fossils from the Puncoviscana Formation is described herein. The outcrop is located about 45 km west of Salta city (Salta Province), displaying an alternation of colored differentiated grayish-bluish slates and fine sandstones. Ichnofossils include Cochlichnus anguineus HITCHCOCK, Helminthoidichnites tenuis FITCH, Helminthoida isp., Monomorphichnus lineatus CRIMES, LEGG and MARCOS ARBOLEYA, Planolites isp. and Torrowangea? isp., as well as some limb marks and a few slightly bended smooth trails (indet.). An Early Cambrian age for the succession at El Alisal as well as some environmental considerations are allowed by the ichnological association. In addition, the ichnogenus Planolites is mentioned for the first time from the "red shales and conglomerate type" facies at the neighbor locality of Chorrillos
Early Cambrian archaeocyathan limestone blocks in low-grade meta-conglomerate from El Jagüelito Formation (Sierra Grande, RÃo Negro, Argentina)
Massive grey limestone blocks containing a fairly diverse but poorly preserved archaeocyath fauna were recovered from a meta-conglomerate bed in the El Jagüelito Formation (Sierra Grande area, Eastern North Patagonian Massif, RÃo Negro, Argentina). This is the first documented reference of the presence of archaeocyaths in continental Argentina. Seven different taxa were identified, preliminary described and figured. Recrystallization of the skeletons due to regional low-grade metamorphism and deformation of the unit does not allow observation of key detailed features and prevents identification to genera and species. Nevertheless, the specimens studied show general affinities with archaeocyathan assemblages from the Australia-Antarctica palaeobiogeographic province and indicate a middle Early Cambrian (Atdabanian-Botomian) maximum age for the deposition of the El Jagüelito Formation protoliths. The similarities between the North Patagonian Early Paleozoic El Jagüelito Formation and those rocks from Antarctica suggest a geologic and biologic common history of these regions on the same southwest margin of Gondwana during that time
Microfacies, biota y paleoambientes sedimentarios del OrdovÃcico Temprano-Medio del Cerro Salazar, Sonora central, México
This work shows the results of facies/microfacies and paleoenvironmental analysis of the Pozo Nuevo
Formation outcropping at the Cerro Salazar, nearby the Rancho Las Norias (Central Sonora, Mexico).
The Pozo Nuevo Formation represents Early-Middle Ordovician deposits of a continental carbonate
shelf that developed in the southwestern tip of Laurentia. The conodonts and trilobites described from the
unit indicate a late Floian-early Dapingian age. The microfacies analysis enables identifying different
sub-environments within the carbonate shelf from minor to major depth: a coastal beach, represented
by quartz sandstone and laminated quartzite with desiccation cracks; an inner shelf which includes a
supratidal setting constituted by dolomitized wackestone and an intertidal-subtidal lagoon characterized by
diffusely laminated microbialites, oncoid mudstone, pelloid packstone grading to biopelloid and bioclastic
packstone containing foraminifera and ostracods; and a subtidal setting composed of grainstone and
packstone with crinoids and algae (Nuia) and detritic quartz and bioclastic packstone with bryozoans.
In the lagoon as well as in the subtidal platform, there are intercalated intraclastic - bioclastic packstone
levels, which are interpreted as proximal stormy events. The vertical variation of facies indicates a marked
trend toward shallowing. At the base of the section fine micritic sediments deposited in a quiet, low energy
lagoon, whereas a deeper subtidal environment with diverse faunas was developed in the middle part.
On the other hand, at the top of the succession there is an increased participation of detrital quartz with
deposition of coastal quartz sandstone culminating with thick dolomite levels of a vadose environment.En el presente trabajo se da a conocer el resultado de los análisis de facies/microfacies y
paleoambientes de la Formación Pozo Nuevo aflorante en el Cerro Salazar, en las cercanÃas del Rancho
Las Norias (Sonora central, México). Esta formación corresponde a depósitos de una plataforma
carbonática continental que se desarrolló en el extremo sudoeste de Laurentia en el OrdovÃcico Inferior-
Medio. Los conodontes y trilobites reconocidos indican una edad floiana tardÃa-dapingiana temprana.
Las microfacies permiten identificar distintos subambientes dentro de la plataforma carbonática: costero
de playa formado por areniscas cuarzosas y cuarcita laminada con grietas de desecación; plataforma
interna: supramareal constituido por facies dolomÃticas y laguna caracterizado por microbialitas
de laminación difusa, mudstone oncoidal, packstone peloidal que gradúa a biopeloidal y packstone
bioclástico con foraminÃferos y ostrácodos; plataforma media, submareal compuesto por grainstone y
packstone de crinoideos y algas (Nuia) con cuarzo detrÃtico y packstones bioclásticos con briozoarios.
Tanto en el ambiente de laguna como en la plataforma media, se intercalan niveles de packstone
intraclástico-bioclástico interpretados como eventos tempestÃticos proximales. La variación vertical de
facies, indica una tendencia marcada hacia la somerización con areniscas cuarzosas y dolomÃas en el
tope de la sección. En la base, las facies corresponden a sedimentos finos micrÃticos que se depositaron
en un ambiente tranquilo y de baja energÃa caracterÃsticos de laguna. En la parte media se desarrolló
una plataforma submareal con biota diversificada de crinoideos, briozoarios, algas, braquiópodos,
trilobites, nautiloideos y gasterópodos. Estratigráficamente sobre la sección inferior se incrementa la
participación de cuarzo detrÃtico que forma depósitos de arenas cuarzosas acumuladas en ambiente
costero de playa, registrando la migración de la lÃnea de costa hacia el depocentro de la cuenca. El
tope de la sección culmina con un importante espesor de dolomÃas originadas en un ambiente vadoso.
El modelo de sedimentación representa la gradual somerización de la cuenca desde carbonatos de baja
energÃa hasta de alta energÃa, con facies de areniscas cuarzosas de playa y dolomÃas vadosas