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The lower fine-grained interval of the Alfarcito Member of the Upper Cambrian-Tremadocian …


Biology Articles » Paleobiology » Paleoichnology » Ichnology of the Alfarcito Member (Santa Rosita Formation) of northwestern Argentina: animal-substrate interactions in a lower Paleozoic wave-dominated shallow sea » Stratigraphic framework

Stratigraphic framework
- Ichnology of the Alfarcito Member (Santa Rosita Formation) of northwestern Argentina: animal-substrate interactions in a lower Paleozoic wave-dominated shallow sea

The Upper Cambrian-Tremadocian Santa Rosita Formation overlies the upper Lower to Middle Cambrian Mesón Group and conformably underlies the Arenigian Acoite Formation. Both the Santa Rosita and Acoite formations are included in the Santa Victoria Group (Turner, 1960). Following the original interpretation by Moya (1998), the contact between the Mesón Group and the Santa Rosita Formation is regarded as an unconformity produced by a relative sea level fall (for discussion see Mángano and Buatois, 2004; Buatois and Mángano, 2005).
The stratigraphic nomenclature of Upper Cambrian to Tremadocian deposits in northwestern Argentina is confused. In most areas of Quebrada de Humahuaca, these strata have been included historically in the Santa Rosita Formation. A series of lithostratigraphic units were further defined by Harrington (1957) in the Cordón de Alfarcito area, including the Casa Colorada Shale, Alfarcito Limestone and Rupasca Shale. These units have been recently considered as members of the Santa Rosita Formation (Buatois and Mángano, 2003). Moya (1988) also proposed additional units, the Tilcara and Humacha formations, which were also regarded as members by Buatois and Mángano (2003). The Santa Rosita Formation has therefore been divided, from base to top, into five members: Tilcara, Casa Colorada, Alfarcito, Rupasca and Humacha members (Buatois and Mángano, 2003). This nomenclatural scheme preserves the traditional and widely used Santa Rosita Formation but at the same time recognizes its internal lithologic variability. However, ongoing research demonstrates a more complex stratigraphy and the current stratigraphic scheme is being revised at present.
The fine-grained lower interval of the Alfarcito Member overlies estuarine deposits. These estuarine deposits are laterally restricted, recording the infill of fluvio-estuarine valleys incised into the underlying open-marine deposits of the Casa Colorada Member. Valley fill commenced with deposition in braided fluvial systems restricted to the valley axis, but the bulk of sedimentation is represented by estuarine deposits that onlapped the valley margins and accumulated during a subsequent transgression. This transgression eventually led to the establishment of open-marine conditions due to basin-wide flooding that blanketed estuarine deposits within the valley, as well as interfluve areas. Estuarine deposits are therefore replaced upward by marine facies (transgressive systems tract) of the Alfarcito Member. This member consists of greenish grey mudstone and thinly interbedded rippled and hummocky cross-stratified sandstone. The maximum flooding surface of this depositional sequence lies within lower offshore deposits that occur in the lowermost interval of the Alfarcito Member. These strata are overlain by lower offshore to offshore transition deposits forming a progradational parasequence set, which represents a highstand systems tract that makes up a significant part of the lower interval of the Alfarcito Member. High-resolution sequence stratigraphic analysis also reveals the presence of a sequence boundary within the lower fine-grained interval of the Alfarcito Member. This sequence boundary is detected by a change in the stacking pattern of parasequences, from progradational to retrogradational parasequence sets. No lowstand deposits are preserved in the study area and the sequence boundary is, therefore, a co-planar surface or flooding surface/sequence boundary. The transgressive systems tract of this second depositional sequence consists of upper to lower offshore deposits stacked forming a retrogradational sequence set. The maximum flooding surface commonly occurs in lower offshore deposits, but it may lie within upper offshore deposits in more proximal locations. Highstand deposits occur above the maximum flooding surface and consist of a progradational parasequence set, which includes deposits that range from the lower offshore into the lower/middle shoreface. This interval is abruptly overlain by forced-regressive shoreface sandstone.
The age of the Alfarcito Member has been controversial due to its sparse fauna and the absence of detailed measured sections that would have allowed accurate placement of paleontologic data in an appropriate stratigraphic framework. A series of studies were recently undertaken in the Alfarcito area, providing valuable biostratigraphic information (Zeballo et al ., 2005a,b; Zeballo and Tortello, 2005). In particular conodont and graptolite faunas suggest that the bulk of the Alfarcito Member is of early Tremadocian age. In previous studies, fine-grained facies overlying estuarine deposits in the Quebrada de Moya and adyacent areas were included in the Casa Colorada Member (Aceñolaza, 1996, 2003; Mángano et al ., 2002; Rubinstein et al ., 2003; Buatois and Mángano, 2003; Mángano and Buatois, 2003). Ongoing research will result in a revised stratigraphic scheme for the area and indicates that these strata should be included in the Alfarcito Member.


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