Paleoenvironments and sequential stratigraphy of Ordovician marginal marine deposits of Sierra de Zapla (Sierras Subandinas, northwestern Argentina) and their relation with the Central Andean Basin

Abstract

The best and most complete palynologically constrained Ordovician section across the Sierras Subandinas crops out in the southern part of the Sierra de Zapla along the Capillas River (Subandean Ranges, Jujuy Province). The dominantly marginal marine setting explains the scarcity of invertebrates throughout the section. Palynomorphs are the main tool in stratigraphic studies of the area and trace fossils are important aids in paleoenvironmental analysis. Based on a detailed facies analysis, geometry and paleocurrent analysis, and recognition of key surfaces, four depositional sequences (S1-S4) with different internal arrangements and compatible with 3rd order cycles were identified. The Zanjón Formation (upper part of Global Stage 2 and Global Stage 3) with unexposed base, initiates the stratigraphic section and it is characterized by a heterolithic facies association with subordinate storm layers, thin-bedded phosphate-rich micro-conglomerates and inarticulate-rich shell beds. Common subaerial exposure features like truncated ripple tops and mud cracks suggest tidal flat environments. This interval yields a mixed Skolithos-Cruziana ichnofacies. Towards the top, an increase in mottling and a gradual change into dominant purplish-red colors (Laja Morada Member of the Labrado Formation) indicate a protracted subaerial emergence during a Darriwilian (Global Stage 4) relative sea-level drop, and a fluctuating water table in interfluves. This unit is covered in sharp erosive boundary by the Lagunilla Member of the Labrado Formation, suggesting development of composite fluvial incisions. Sandy fluvial to tide-dominated estuarine fills and thoroughly-bioturbated subtidal hetherolithic beds overlie the sequence boundary representing a typical transgressive systems tract. Development of surfaces with dominant Skolithos forms are interpreted as representing Glossifungites ichnofacies typically indicating by-pass erosion and firm-ground development previous to reworking of interfluves by advancing ravinement surface. The Capillas Formation (upper Darriwilian and lower part of Global Stage 5), sharply levels the previous estuarine complexes and represents a fine-grained wedge that gradually coarsens up. This unit contains the only truly marine shelly fauna concentrated in few storm beds above the fine-grained interval that characterizes a maximum flooding stage. This is the only interval that can be positively correlated with sections in the Cordillera Oriental to the west and Bolivia. A gradual coarsening to silty sandstones and the replacement of Cruziana by Skolithos dominated ichnofacies suggest a shallowing-upward section, capped by the thick-bedded, high energy quartz sandstones with pipe-rock structures of the Centinela Formation (middle and upper part of Global Stage 5). A shallowing-upward trend indicates the progradation of deltaic complexes, which in turn were erosively truncated by the waxing stage of the Gondwanan Hirnantian (uppermost Ordovician) ice-cap represented by the Zapla Formation (Global Stage 6) along the Central Andes. Above the diamictite-rich Zapla Formation the Lipeón Formation (Silurian) is interpreted as related to repeated transgressive ravinement surfaces that truncated Fe+2 saturated estuaries after a waning glacial stage and isostatic rebound, favouring the deposition of oolitic ironstones and succeeded by the development of a Zoophycos dominated muddy shelf.

Ordovician chitinozoans and marine phytoplankton of the Central Andean Basin, northwestern Argentina: A biostratigraphic and paleobiogeographic approach

“…1A). The 5000 meter thick Central Andean Basin has a vast area of continuous sedimentation, and is extremely important in the analysis of Gondwanan margin sequences (Astini, 2003).…”

“…This physiographic division in the NOA basin is a result of the tectonic effects principally derived from the Cenozoic Andean Cycle (Ramos, 2009). The Paleozoic and Mesozoic basins, situated along the pericratonic regions are the result of a foreland basin system (Astini, 2003). In describing the depositional history of the Central Andean Basin, Astini (2008) distinguishes three depozones in the Ordovician stratigraphy (Fig.…”

A Middle Ordovician (Darriwilian) Dysaerobic Brachiopod Assemblage from the Precordillera Terrane of Argentina: Implications for Early Colonization of Deep Waters

“…Upper Ordovician shales interbedded with sandstones, conglomerates and debris flows containing basement rocks and large boulders and olistoliths derived from the destroyed carbonate platform. This coarse clastic succession records deposition in a synorogenic foreland basin undergoing loading-induced flexural subsidence and deformation related to the collision of the Precordillera terrane against Gondwana (Astini et al, 1995;Astini, 1998Astini, , 2003. Regarding the black shale deposition, it seems likely that tectonic subsidence was enhanced by the sea-level rise documented around the Dapingian/Darriwilian boundary (Munnecke et al, 2010).…”

Assembly of Pampia to the SW Gondwana margin: A case of strike-slip docking?

“…The stratigraphy of this region consists of metasediments assigned to the Ediacaran-Early Cambrian Puncoviscana Formation (Turner, 1960;Buatois and Mángano, 2003;Schwartz and Gromet, 2004;Do Campo and Ribeiro Guevara, 2005;Adams et al, 2008a); Middle to Late Cambrian shallow platform quartzites, sandstones and red to greenish shales of the Mesón Group (Turner, 1960); Ordovician marine successions of sandstones interbedded with conglomerates and black shales best represented by the Santa Victoria Group (Turner, 1960;Astini, 2003); marine clastic sediments with iron beds of Silurian-Devonian age (Astini, 2003); fluvial and eolian deposits of Carboniferous-Permian age; sediments and volcanics of the Cretaceous-Lower Paleocene Salta Group rift basin and Cenozoic sediments of the Andean foreland basin (Mon and Salfity, 1995).…”

“…Non collisional tectonic models suggest that after deposition of the Puncoviscana Formation (and equivalent units to the south), on a (Astini, 2003;Ramos, 2004;Rapela et al, 2007;Favetto et al, 2008;Tohver et al, 2008;Ramé and Miró, 2010) and shear zones (dotted lines) of Martino (2003), Simpson et al (2003). c) Digital elevation model of Caspalá-Santa Ana locality and the paleomagnetic samples (white stars).…”