1 SEDIMENT PROVENANCE AND TECTONIC SIGNIFICANCE OF THE CRETACEOUS PIRGUA SUBGROUP, NW ARGENTINA by Scott McBride A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2008
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SEDIMENT PROVENANCE AND TECTONIC SIGNIFICANCE OF THE CRETACEOUS PIRGUA SUBGROUP, NW ARGENTINA
by
Scott McBride
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA 2008
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STATEMENT BY THE AUTHOR This thesis has been submitted in partial fulfillment of requirements for the Master of Science degree at The University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the Department of Geosciences when the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. Scott McBride 12/12/2008 (author’s signature) (date) APPROVAL BY RESEARCH COMMITTEE As members of the Research Committee, we recommend that this thesis be accepted as fulfilling the research requirement for the degree of Master of Science. Peter DeCelles Major Advisor (type name) (signature) (date)
Paul Kapp (type name) (signature) (date)
George Gehrels (type name) (signature) (date)
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ACKNOWLEDGEMENTS
Funding for this research was provided by the ExxonMobil-University of Arizona
Convergent Orogenic Systems Analysis project, the Geological Society of America, and
the University of Arizona Department of Geosciences Summer Research Fund. I am
grateful to Ryan Rodney for field assistance in Salta, and also to Joel Saylor for providing
valuable experience on his Zada Basin project. U/Pb zircon analyses were conducted at
the Arizona Laserchron Center; petrographic slides were prepared by Quality Thin
Sections of Tucson.
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TABLE OF CONTENTS
LIST OF FIGURES AND TABLES....................................................................................5
quartz-rich with almost no plagioclase or volcanics. Local compositions of the various
sub-basins reflect the adjacent basement lithologies (Sabino 2002).
Detrital zircon data are broadly consistent with this view of locally-derived
continental sediments. The primary signal in all samples is the recycling of the
Puncoviscana Formation, with age peaks of ~550 Ma and ~1 Ga. The younger of these
peaks correlates to the Pampean Orogeny, the local expression of the greater
Braziliano/Pan-African orogeny during the assembly of Gondwana (Finney et al. 2003,
Loewy et al. 2004). The second corresponds to the Proterozoic Grenville-Sunsas orogeny,
present to the north in Bolivia and to the south in the Sierras Pampeanas (Finney et al.
2003, Ramos 2008). The presence of Ordovician zircons in all post-Cambrian samples
are attributed to the Famatinian arc, which emplaced plutons into this region during the
collision of a microcontinent (Otamendi 2003, Hongn and Ritter 2007). A source for the
very small number of Permian grains is undetermined (Choiyoi rhyolite province?). The
Jurassic zircons found in the Pirgua Subgroup could be derived from the incipient
Andean arc (Scheuber et al. 1997), or from local Jurassic plutons (Cristiani et al. 1999).
The absence of Cretaceous grains in the Pirgua Subgroup is not unexpected due to the
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mature character of these sediments, and the mafic nature of the local volcanics present in
the basin (Marquillas et al. 2005). Yet, their absence combined with paleocurrents
flowing from the west implies a significant paleo-divide between the Pirgua Subgroup
and the magmatic arc during the Cretaceous.
In sharp contrast to the Pirgua Subgroup, the Purilactis Group samples were
derived from a much more active tectonic setting. An immature, plagioclase and volcanic
rich composition suggests a magmatic arc provenance. In addition, the prominent detrital
zircon age peak at 78 Ma suggests rapid input of first-cycle igneous grains, potentially
approximating the depositional age. In addition, the presence of a weak signature of
Permian, Ordovician, and Puncoviscana-equivalent grains suggests the Purilactis Group
could share a secondary source with the Pirgua Subgroup; this deduction is weakened
somewhat by the presence of adjacent basement provinces of similar age: Pampia,
Antofalla, and Chilenia. Nonetheless, this common secondary sediment source, and the
lack of arc-derived sediments in the Salta Group, together reinforce the suggestion of a
Cretaceous age Pampean/Famatinian remnant upland between the Salta and Purilactis
Groups. One explanation for this upland could be a passive orogenic remnant; another
that it was a rift shoulder of the Salta Rift.
The depositional age of the Purilactis Group sample overlaps with the age of the
Pirgua Subgroup. Therefore, the Salta Rift was active at the same time as the Purilactis
thrust belt, yet located only 400 km to the east. But could both compressional and
extensional systems exist simultaneously in the backarc of the early Andes? It seems
more plausible that the two basins were separated chronologically: either Pirgua
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Subgroup before Purilactis Group, or else intermittent deposition in each of the basins
through the Cretaceous.
In the first scenario, the Pirgua Subgroup would be the entire Salta Rift
sedimentary package. The stratigraphy described in this study would have been deposited
between 130 Ma and sometime after 90 Ma, the age of the Las Conchas basalts in the Las
Curtiembres Formation. The overlying Los Blanquitos Formation could have been
deposited quickly afterwards as a syn-rift unit, or more gradually through the remainder
of the Cretaceous as a post-rift thermal subsidence package. The latter is suggested by its
greater areal extent and lesser degree of normal faulting (Marquillas et al. 2005). Either
way, in this scenario backarc contraction began shortly after 90 Ma, leading to the
deposition of the Purilactis Group in the foredeep of a fold and thrust belt, including the
sampled interval at 78 Ma. Then, the Balbuena and Santa Barbara subgroups of the
Pirgua Subgroup would be unrelated to the Salta Rift, but could instead be distal foreland
basin deposits (DeCelles et al., 2008). Predictions of this model include a major
unconformity above the Pirgua Subgroup, and a distal foreland basin character for upper
Salta Group sediments. This model fails to account for the presence of the lower
Purilactis Group before 90 Ma.
In the second scenario, the Salta and Purilactis Groups overlap in time at a broad
scale, but in detail alternate and are completely disparate. The incipient Andean orogeny
could have had a primarily contractional backarc, as evidenced by a temporally extensive
record of thrust faulting and syn-orogenic sedimentation from the mid-Cretaceous to the
present (Coutand et al. 2001, Horton et al. 2001, Arriagada et al. 2006, DeCelles et al.
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2007). Extension in the Salta Rift would be accommodated at discrete interludes when
the backarc stress state temporarily switched. Subduction erosion at the trench is a
tectonic process that is episodic throughout the Cretaceous in this region (Haschke et al.
2006). When a portion of the forearc is removed, the lost space must be accounted for
either by advancement of the subducting plate and trench towards the continent, or else a
trench-ward migration of the continental interior. In the latter case the motion could be
accommodated by extension in the backarc. This model predicts that the Salta Group was
deposited during times of subduction erosion and arc migration, which occurred during
the Cretaceous at 130-125 Ma and 90-78 Ma (Haschke et al. 2006). A major challenge to
this hypothesis is the lack of similar rifts in other parts of the Andes that experienced
subduction erosion.
Distinguishing between these two Cretaceous tectonic models requires better age
control of the Salta and Purilactis Groups. In the Pirgua Subgroup, there are possible
undated tuffs in the Quebrada de las Conchas and Brealito areas. In the Purilactis Group,
the abundance of juvenile arc material suggests that zircon U-Pb ages often reflect
depositional ages. In addition, a more detailed study of the provenance patterns of the
various sub-basins of the Salta Rift and the Purilactis Group would no doubt provide a
more detailed picture of Cretaceous paleogeography.
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APPENDIX – FIGURES AND TABLES
FIGURE 1 – Tectonic setting of the Salta Group Marquillas et al. (2005).. (1) Pirgua
Subgroup extent. (2) Balbuena Subgroup extent. (3) Santa Barbara Subgroup extent.
Purilactis Group labeled to the west of the Salta Rift. Sub-basins of the Salta Rift are
abbreviated: Tres Cruces (TC), Lomas de Olmedo (LO), El Rey (R), Metan (M),
Alemania (A), Brealito (B), and Sey (S).
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FIGURE 2 – (Left) Salta Group stratigraphy (Marquillas et al. 2005). (Right) Pirgua
Subgroup isopach map; isopachs in km (Saltify and Marquillas, 1994).
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FIGURE 3 – (top left) Flatirons in the Balbuena and Santa Barbara Subgroups, Hornocal Syncline. (top right) Taken from the top of HOR1 looking NE, shows the dip slope used to correlate to the base of HOR2. (bottom left) Evidence of thrusting in the Las Curtiembres Formation, Quebrada de las Conchas. (bottom right) Angular unconformity between La Yesera and Puncoviscana Formations, Quebrada de Las Conchas. 31
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FIGURE 4 – Geologic map of the Serrania de Hornocal, showing section and sample locations. Simplified from Kley et al. (2005).
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FIGURE 5 – Photo of study area looking south; measured sections HOR1 and HOR2 are marked: 2400 m total thickness. The
Hornocal Fault runs through the valley between the foreground (Puncoviscana Formation) and background (Salta Group). The
Hornocal syncline is 3 km across between the Yacoraite Formation of the Balbuena Subgroup, which is near the crest of the
ridge on both sides. Peaks are at ~5000 m elevation, valley ~3000 m. Locations of Balbuena (B) and Santa Barbara (SB)
subgroups in the western limb of the syncline are shown, and dip tadpoles indicate local bedding orientation.
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FIGURE 6 – Measured sections HOR1 and HOR2; lithofacies codes in Table 1; see insert
for full resolution.
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FIGURE 7 – (Left) Paleocurrent measurements taken on trough cross-bedding and
imbricated cobbles in sections HOR1 and HOR2. Reference measured sections (Figure 6
– see insert for full resolution) for stratigraphic heights and exact directions. (Right)
Example of a stereonet calculation to find a trough axis.
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FIGURE 8 – Point counting ternary diagrams. See Table 2 for grain codes. Labeled fields
are from Dickinson and Suzcek SP (1979): Recycled Orogen (RO), Continental Block
TABLE 5 – Sample database (continued) Sample Lat. (S) Lon. (W) Elev. (m) So (RHR) Lithology TS P C DZ sample ICPMS
Puna
no measured section
SM20070714-2 23°24.699' 66°32.050' 4099 sandstone Y Y
El Rey
no measured section
SM20070711-1 24°43.921' 64°41.887' 1063 sandstone Y Y
Misiones
no measured section
SM20070702-1 27°13.881' 55°32.657' 133 sandstone Y Y
Basement samples
no measured section
SM20070628-1 23°16.723' 65°12.735' 3704 J? sandstone Y Y Y Y
SM20070628-2 23°14.933' 65°12.769' 3282 pC quartzite Y
SM20070629-1 23°10.994' 65°03.490' 4607 Pz sandstone Y Y Y Y
SM20070629-2 23°10.692' 65°10.530' 3895 pC quartzite Y Y Y
SM20070629-3 23°11.812' 65°11.989' 4389 pC quartzite Y
SM20070708-1 25°18.907' 66°21.078' 2699 O? granite Y "Y"
SM20070714-1 23°54.628' 66°49.949' 4099 O sandstone Y Y
MESON - Pete 23°20.978' 66°6.185' 3776 Cam. Quartzite Y Y -Pete
PUNC1 - Pete 24°41.863' 66°43.753' 3813 Cam. Quartzite Y Y-Pete
Samples TS PC DZ sample ICPMS
Total 97 34 16 22 73742 m
Measured section
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