Biostratigraphic
significance of the Uppermost Cambrian-Lowest Ordovician agnostoid trilobites
from northwestern Argentina: a review
TORTELLO M. F.
The trilobite Parabolina (Neoparabolina) frequens
argentina Biozone (Harrington and Leanza, 1957) is widely
represented in northwestern Argentina (Jujuy, Salta and La Rioja Provinces).
Although this unit was originally assigned to the Lower Tremadoc, recent
analyses have demonstrated that its Lower part is assignable to the Uppermost
Cambrian (e.g. Aceñolaza, 1983; Salfity et.
al., 1984). Current interdisciplinary studies aim to determine the
exact position of the CambrianOrdovician boundary within the Biozone, as well
as to reconsider the biostratigraphic significance of its abundant trilobite
faunas.
According
to Aceñolaza (1983), the first appearances of the trilobite Jujuyaspis
keideli Kobayashi and the graptolite Rhabdinopora flabelliformis (Eichwald)
indicate the Lowest Tremadoc in South America. In addition, the conodonts Cordylodus
lindstromi and Iapetognathus
are worthy references of the Lowest Ordovician (e.g. Ortega and
Rao, 1995; Rao, 1999; Albanesi et.
al., 1999). These valuable biostratigraphic criteria have proved
useful in several sections from northwestern Argentina. For example, Sierra de
Cajas (Lampazar and Cardonal Formations) and Río Volcancito (Volcancito
Formation) constitute appropiate type sections for the Cambrian-Ordovician
transition in Eastern Cordillera and Famatina System, respectively (Tortello
and Aceñolaza, 1993; Ortega and Rao, 1995; Rao, 1999; Tortello et
al., 1999; Tortello and Esteban, 1999; Albanesi et al.,
1999).
Biostratigraphic
significances of the Uppermost Cambrian-Lowest Ordovician trilobite
agnostoids from northwestern Argentina are sunmarized here. Stratigraphic
position of each agnostoid species from the Parabolina frequens argentina Biozone
are up-to-dated. Based on information from Sierra de Cajas,
According to these data, Argentina appears
to have the richest Uppermost Cambrian agnostoid faunas from the Baltic
Province. The lower part of the Parabolina
frequens argentina Biozone is characterized by a remarkable
agnostoid assemblage composed of Lotagnostus
(Lotagnostus) sp., Lotagnostus (Semagnostus) suninoi
(Harrington
and Leanza), Micragnostus vilonii
Harrington and Leanza, Micragnostus
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 particular fauna, associated with conodonts from the Cordylodus
proavus Biozone (Rao, 1999;
Albanesi et al., 1999), is recorded
with olenid trilobites that are also represented in Ordovician levels of the
Biozone (e.g. Parabolina frequens argentina (Kayser), Parabolinella argentinensis Kobayashi,
Angelina
hyeronimi (Kayser).
On the other hand, agnostoid diversity
drastically declines in beds very Glose to the Cambrian-Odovician boundary. Trilobagnostus
chiushuensis (Kobayashi) crosses the boundary in the classic
section of Sierra de Cajas. Besides, in the Lowest Tremadoc (characterized by
the olenid Jujuyaspis keideli)
agnostoids are restricted to T.
chiushuensis, (?) Gymnagnostus bolivianus (Hoek) and some forms
described in open nomenclature (Anglagnostus?
sp., Micragnostus sp.). This faunal impoverishment could be
attributed to world-wide regressive-transgressive events that characterized
the Cambrian-Ordovician transition interval.
References:
Aceñolaza, F. G. 1983. The Tremadocian beds and the Cambrian-Ordovician boundary problems in Latin America. In: Papers for the Symposium on the Cambrian-Ordovician and Ordovician-Silurian, Boundaries: 88-93, Nanjing Institute of Geology and Palaeontology, Academia Sinica.
Albanesi, G. L., Esteban, S. B. and Barnes, C. R.1999. Conodontes del
intervalo del limite Cámbrico Ordovícico en la Formación Volcancito,
Sistema de Famatina, Argentina. Temas
Geologicos Mineros ITGE,
26: 521-526.
Harrington, H.J. and Leanza, A. F. 1957. Ordovician trilobites of
Argentina. Department
of Geology University of Kansas Specia! Publícation, 1:
1-276.
Ortega, G and Rao, R. I. 1995. Nuevos hallazgos de graptolitos (Ordovícico Inferior) en la sierra de Cajas, Departamento Humahuaca, Provincia de Jujuy, Argentina. Boletín de laAcademia Nacional de Ciencias, 60 (3-4): 293-316.
Rao, R.1. 1999. Los conodontes Cambro-Ordovícicos de la sierra de Cajas y del Espinazo del Diablo, Cordillera Oriental, República Argentina. Revista Española de Micropaleontología, 31 (1):23-51.
Salfity, J. A., Malanca, S., Moya, M. C., Monaldi, C. R. and Brandán, E. M.1984. El límite CámbricoOrdovícico en el norte de la Argentina. Noveno Congreso Geológico Argentino, Actas, 1: 568-575.
Tortello, M.
F. and Aceñolaza, G. F. 1993. Trilobites agnóstidos del límite Cámbrico-Ordovícico
de la Formación Lampazar, sierra de Cajas, Provincia de Jujuy, Argentina. Revista
Española de Paleontología, 8:177-184.
Tortello, M. F. and, Esteban, S. B.1999. La transición Cámbrico-Ordovícico
en la Formación Volcancito (sierra de Famatina, La Rioja, Argentina). Ameghiníana, 36(4):371-387.
Tortello, M. E, Rábano, I., Rao, R. I. and Aceñolaza, F. G. 1999.
Los trilobites de la transición Cámbrico-Ordovícico en la quebrada Amarilla
(sierra de Cajas, Jujuy, Argentina). Boletín Geológico
y Minero, 110-5: 555-572.