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Book Chapter

The Argentine Precordillera: A Little American Carbonate Bank

By
Martin Keller
Martin Keller
Institut für Geologie und Mineralogie, Erlangen, Germany
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Published:
January 01, 2012

Abstract

The Precordillera of western Argentina is an exotic terrane along the western edge of South America. Abundant, but not unequivocal, evidence shows that the Precordillera originated outboard of the western Ouachita Basin of west-central Texas and that it shared some of the major sedimentary events of that part of the Laurentian margin and adjacent areas during the Cambrian-Middle Ordovician. Consequently, the Precordilleran carbonate platform can be regarded as part of the great American carbonate bank (GACB); however, as it originated as a marginal plateau to Laurentia with a distinct tectonosedimentary history, it is better regarded as a relative of the GACB—the little American carbonate bank.

The rise and fall of the little American carbonate bank are reflected in the formation of several supersequences. Their recognition is based on physical criteria that focus on stratal patterns and accommodation signatures. The sedimentologic changes and associated unconformities mostly coincide with formational boundaries of the lithostratigraphic framework.

Supersequence PC I (La Laja Formation, ∼500 m [∼1640 ft] thick) is composed of alternating fine-grained siliciclastic and carbonate rocks, arranged in depositional grand cycles. The base of this sequence is not exposed, compromising the recognition of the base of the Sauk megasequence in the Precordillera. The Lower and Middle Cambrian rocks represent inner detrital and carbonate belt settings similar to those of the coeval Laurentian margin. The depositional environment had a ramplike configuration with successive onlap of strata onto the marginal plateau. Sedimentologically, this supersequence reflects the protracted Early-Middle Cambrian sea level rise following the final shaping of Laurentia. Tectonically, it represents the initial flooding of the terrane following Neoproterozoic and Early Cambrian rifting with the concomitant formation of the western Ouachita Basin and the Precordilleran marginal plateau.

Supersequence PC II (Zonda and La Flecha Formations) is represented by the Upper Cambrian rocks of the Precordillera. Peritidal dolomites, in the supersequences upper part with well-developed small-scale cycles, testify to the almost complete flooding of the terrane and the elimination of a siliciclastic source area. The very thick (∼1250 m [∼4101 ft]) Upper Cambrian rocks are typical of a probable Bahamas-type, isolated, wide carbonate bank with high accommodation.

During deposition of supersequence PC III (La Silla Formation, ∼400 m [∼1310 ft] thick), the carbonate bank was still keeping up with relative sea level rise. An increase of subtidal facies at the expense of supratidal facies, however, demonstrates that this sequence formed during accelerating accommodation. The very uniform distribution of La Silla rocks attests to an Upper Cambrian-Lower Ordovician isolated carbonate shelf configuration and the total flooding of the marginal plateau.

Supersequence PC IV (most of San Juan Formation, ∼300 m [∼980 ft] thick) records the reestablishment of a subtidal carbonate ramp system. Restored to its presumed original position outboard of the western Ouachita Basin, the depositional surface was dipping toward that basin. Bordering this basin on all sides was a conspicuous and coeval association of sponge-algal mounds that seems to be unique to this part of the Laurentian margin at that time. The uppermost part of the San Juan Formation is a deepening-upward succession with very irregular thickness formed as a response to renewed crustal extension preceding the final separation of the marginal plateau from mainland North America. This rifting compromises recognition of the stratigraphic top of the Sauk megasequence. These basal Whiterockian deposits and the overlying siliciclastic rocks record the demise of the little American carbonate bank and the birth of an independent terrane.

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Contents

Memoir

Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia

James Derby
James Derby
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Richard Fritz
Richard Fritz
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Susan Longacre
Susan Longacre
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William Morgan
William Morgan
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Charles Sternbach
Charles Sternbach
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American Association of Petroleum Geologists
Volume
98
ISBN electronic:
9781629810201
Publication date:
January 01, 2012

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