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| Titel |
Biostratigraphy of the Santa Rosita Formation (Furongian-Tremadocian) in its type area, Eastern Cordillera, NW Argentina |
| VerfasserIn |
G. G. Voldman, G. L. Albanesi, G. Ortega, C. R. Monaldi, F. J. Zeballo, M. E. Giuliano |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061219
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| Zusammenfassung |
| The Cambrian - Lower Ordovician stratigraphic units from the Eastern Cordillera were
originally defined in the Santa Victoria Range, Salta Province, NW Argentina. At the
southern margin of the Santa Victoria River, near the homonymous locality, the Santa Rosita
Formation is conformably overlain by the Acoite Formation. These stratigraphic units are
dated by means of conodont, graptolite and trilobite biostratigraphy in the type
sections. The upper interval (ca. 700 m) of the Santa Rosita Formation consists of
sandstones interbedded with purplish to grayish shales and occasional coquinas,
which were sampled for microfossils. 15 carbonate samples (35 kg) were processed
following standard laboratory techniques for conodont recovery. A number of species
from the genera Acanthodus, Acodus, Decoriconus, Drepanodus, Drepanoistodus,
Iapetognathus, Kallidontus, Paltodus, Utahconus, Teridontus and the protoconodont
Phakelodus were recovered from 5 productive samples. The conodont elements
exhibit a CAI 3 and correspond to the Paltodus deltifer deltifer Subzone of the P.
deltifer Zone (middle Tremadocian, Tr2). Pelites of the Acoite Formation, at 300 m
from its base, bear Araneograptus murrayi and Thysanopyge sp., whose ranges
span the Tremadocian – Floian boundary. Conodonts and graptolites were also
yielded by outcrops of the Santa Rosita Formation at the Nazareno area, 30 km
to the south of the Santa Victoria type locality. The conodont associations were
recorded from calcareous levels of the Alfarcito and Rupasca members, including
Drepanodus arcuatus, Drepanoistodus chucaleznensis, Teridontus gallicus, Utahconus
humahuacensis, Acanthodus sp. and Utahconus sp. They also integrate the eponymous
subspecies from the Paltodus deltifer pristinus and P. d. deltifer subzones of the P.
deltifer Zone (middle Tremadocian, Tr2). The Santa Rosita Formation correlates
with a thick heterolithic succession at the Zenta Range, 120 km to the southwest
of the Santa Victoria type area. This mostly siliciclastic succession intercalates
calcareous concretions, coquinas and calcarenites bearing a significant conodont and
graptolite fauna. Exposures of gray shales and subordinated coquinas near Abra de
Santa Ana, produced a low diversity conodont fauna that incorporates Acodus apex,
Drepanoistodus chucaleznensis, and Drepanoistodus costatus, which are referred to the
lower part of the Acodus deltatus – Paroistodus proteus Zone. The Araneograptus
murrayi Zone is suggested by a graptolite assemblage that includes A. murrayi,
Kiaerograptus cf. K. supremus, and Kiaerograptus? sp., which was collected from
strata overlying the conodont-bearing levels. These conodont and graptolite faunas
indicate a late Tremadocian age (Tr3). Another locality on the road to Abra de Zenta,
west of Santa Ana, contains Hunnegraptus cf. H. novus, Hunnegraptus spp. and
Paradelograptus sp., allowing for the recognition of the Hunnegraptus copiosus
Zone that typifies the uppermost Tremadocian. The studied assemblages include
elements of wide intercontinental distribution as well as endemic forms from the
Central Andean Basin, characterizing a faunal province with a particular signature. |
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