Date of Award

December 2020

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

John L Isbell

Committee Members

Julie A Bowles, Kathryn N Pauls

Keywords

Ansilta, Argentina, Geochronology, Sedimentology, Stratigraphy

Abstract

The Ansilta Formation, located several kilometers east of the Astronomical Observatory El Leoncito near Barreal, San Juan Province, Argentina was deposited during the late Mississippian to early Pennsylvanian. This succession includes glacimarine, nearshore, and fluvial systems deposited at the mid-Carboniferous portion of the late Paleozoic ice age within the Calingasta—Uspallata Basin on the western margin of the Protoprecordillera. The lower member of the Ansilta Formation is equivalent to the nearby Leoncito, Majaditas, and Hoyada Verde formations, and is composed of diamictite, conglomerates, sandstones, pebbly mudstones, and laminated mudstone. Glacial environments are unique in that facies changes occur as a result of glacial fluctuations that may be independent from changes of relative sea-level. For this reason, an alternative conceptual framework outside of the traditional sequence stratigraphic approach is necessary to understand these strata. The lower glacial member (0-540 m) records at least five glacial advances, where glacial dynamic signatures are interpreted using glacial sequence stratigraphy and glacial systems tracts (GST). The upper member of the Ansilta Formation (540-700 m) consists of progradational shallow marine and fluvial strata with conglomerates, sandstones, and mudstones. This assemblage shows no direct evidence of continued glaciation in the basin due to the absence of glacially faceted clasts and diamictites, representing a major deglaciation phase in western Argentina and is part of the stepped deglaciation across Gondwana that ended elsewhere in the late Permian. Additionally, samples were collected from various sandstones throughout the formation for detrital zircon U-Pb geochronology to determine sediment provenance as a way of isolating different glacial sources and to assist in bracketing sequences. Results indicate multiple glacial advances, with at least three distinct source areas. The lowermost stage includes locally sourced and recycled underlying Ordovician and Silurian basement, represented by similar Famatinian (460-500 Ma) and Mesoproterozoic peaks (1050-1400 Ma) peaks, with the Mesoproterozoic source likely reworked locally from the underlying basement or derived from the Sierra de Pie de Palo range in the western Sierra Pampeanas. The middle stages of the upper glacial member show a population distinct unto itself, with a peak during the Mississippian (330-360 Ma). Pebble counts recorded along the section showed increasing quantities of plutonic and metasedimentary rock clasts and decreasing amounts of sedimentary clasts from the bottom to the top of the glacial interval. This is attributed to the un-roofing of sedimentary strata and subsequent erosion of the crystalline core of the Protoprecordillera, resulting in the influx of Carboniferous first-cycle sediments. The uppermost stage shows relations based on K-S statistical results to formations within the Paganzo basin to the northeast, which could represent a temporal link between the Calingasta-Uspallata and Paganzo basins during the late Carboniferous, manifested as distal outwash from braided fluvial systems.

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