New
stratigraphic section to define the Cambrian-ordovician boundary in Eastern Cordillera, northwest Argentina
MOYA, M. C AND ALBANESI, G. L.
The Eastern Cordillera and the Famatina
System, northwestern Argentina, are the unique regions of South America where
the Cambrian-Ordovician boundary has been documented by means of trilobites,
graptolites and conodonts. In the Eastern Cordillera, the most complete
paleontological record is found in the Cajas Range, and the
Cambrian-Ordovician boundary is defined at the Cardonal Formation. In the
Famatina System, the late Cambrian - early Ordovician succession is
represented by the Volcancito Formation. The present work proposes a
biostratigraphical analysis for a new section, located at El Angosto del
Moreno, western flank of the Eastern Cordillera, south of the Cajas Range,
Jujuy province.
El Angosto section does not present
structural complications and its stratigraphic development allows us to
analyze geological and faunal events occurred during the late Cambrian - early
Ordovician span of time. Exposed deposits, consist of a marine-platform
clastic succession, main1y made up of alternating lithostratigraphical
intervals of sandstones and shales, whose development has been probably caused
by eustatic fluctuations that affected the marginal environments of
Gondwanaland. In El Angosto area, the lithostratigraphical units are
identified as Padrioc (sandstones), Lampazar (shales), Cardonal (sandstones)
and Saladillo (shales) formations. The Padrioc Formation is unconformably
overlying the Middle?-Upper Cambrian sequences (Iruya unconformity), while the
Saladillo Formation underlies the shaly Parcha Formation, by means of the
Tumbaya unconformity, bearing typical graptolites of the Tetragraptus
approximatus Zone (early Arenigian). The Ordovician succession ends up with
A synthesis on the litho-faunistic
characteristics recognized in El Angosto critical interval includes: The
Lampazar Formation, ca. 70 m thick, is represented by wackes and grey-black
shales, interbedding carbonate coquinites in its upper half, which were
processed for conodonts. This formation bears an abundant trilobite fauna that
corresponds to the Parabolina (Neoparabolina) frequens Zone, including the index fossil and the
following taxa, Parabolinella argentinensis Kobayashi,
Angelina
hyeronimi (Kayser), Rossaspis rossi Harrington and Leanza, Plicatolina
scalpta Harrington and Leanza, Lloydia
(Leiostegium)
douglasi Harrington,
and numerous agnostids. Interbedded coquinites yielded conodonts from the Hirsutodontus
hirsutos Subzone
of the Cordylodus proavus Zone (upper Cambrian), which is represented by
Cordylodus
proavus Müller,C. cf. proavus
Müller, C. primitivus
Bagnoli et al., C.
andresi Viira et al., Eoconodontus
notchpeakensis
(Miller), Hirsutodontus hirsutos Miller, Proconodontus
muelleri Miller, Furnishina
sp. and Prooneotodus
sp. In diverse localities
for global reference, particularly from Nórth America, this subzone is
developed through the transgressive cycle following the Lange Ranch Eustatic
Event, whose regressive peak could correspond to the Iruya unconformity in the
Eastern Cordillera.
Overlying the Lampazar Formation,
quartzites of the Cardonal Formation, ca. 100 m thick, did not produce
identifiable fossils at the moment. Over this unit, the Saladillo Formation,
ca. 70 m thick, begins with green shales and siltstones, and fequent
quartzite strata. Trilobite faunas from this interval define a transitional
biostratigraphic interval between the P. (N.) frequens and
the Kainella meridionalis zones, being absent both index species. Registered
trilobites include, Angelina
byeronimi
(Kayser), Rossaspis
rossi (Harrington and Leanza), Lloydia
(Leiostegium) douglasi Harrington,
Parabolinella argentinensis Kobayashi, Leptaplastides marianus (Hoek),
Apatokephalus exiguos Harrington and Leanza, diverse species of the
genera Onycopyge, Shumardia, Asaphellus, and agnostids. The first appearance of planktic
graptolites, e.g. Rhabdinopora parabola
(Bulman), R.
flabelliformis cf. R. f.
socialis
(Salter) and species of the genus Anisograptus
are recorded from the
same interval. A coquinite sample processed for conodonts produced
unidentifiable fragments. However, the existence of these microfossil
remains promote new detailed sampling throughout the critical interval for
the establishment of the Cambrian-Ordovician boundary, by means of index
conodont species, as it is
currently
accepted for global correlation. The appearance of Kainella meridionalis
Kobayashi occurs in
the upper portion of the Saladillo Formation, through the uppermost 20 m,
underlying the lower Arenigian shales. In this part of the section, the
Saladillo Formation is represented by a conspicuous facies of carbonate
sandstones, shales and abundant coquinite levels. The identified fauna
includes: K. meridionalis, Angelina kayseri Harrington
and Leanza, Pseudokainella lata
(Kobayashi), Shumardia erquensis Kobayashi, Leptoplastides marianus (Hoek), Asaphellus catamarcensis Kobayashi and diverse
forms of the Rhabdinopora and Anisograptus genera. The interbedded levels
of carbonate coquinites yielded conodonts of the Cordylodus
angulatus Zone
(late Lower Tremadocian), representing a low abundance and diversity fauna,
that includes the eponymous species, Monocostodus
sevierensis Miller,
Teridontus nakamurai (Nogami) and T. obesus Ji and Barnes.
It is important to
note the excellent preservation of collected fossils, that permits a precise
taxonomic study. Particularly, conodonts present CAI values of 1.5-2,
differing from those contemporaneous elements previously documented from Cajas
and Volcancito sections, which exhibit black color alteration (CAI 5) and
frequently show textural alteration, indicating high diagenetic (anchizone) or
very low metamorphic processes. This preliminary biostratigraphic survey of El
Angosto section suggests new sampling, and detailed paleontologic analysis,
shall allow to propose it as a reference section (hypostratotype) for South
America to establish the Cambrian-Ordovician boundary by means of high
resolution biostratigraphy.