Abstract

ASTINI, Ricardo A. Striated pavements in the don braulio formation and the nature of the hirnantian glaciation (Late Ordovician) in the andean region. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.1, pp.1-25. ISSN 1853-6360.

Stratigraphic evidence indicates that the Late Ordovician glacial record of the Argentine Precordillera correlates with the glacial maximum that affected Gondwana during the Hirnantian Stage. This glaciation, anchored in west-central Africa, had a strong effect along the proto-Andean region due to the existence of local prominent relieves related to its active geodynamic setting. A peripheral position for the Precordillera as well as for the rest of the proto-Andean basins, and a slightly warmer thermal regime allowed complex ice-contact dynamics and development of an ice cap with partially floating outlet glaciers that recorded several stages of advance and retreat. Erosive unconformities, striated pavements, and the various additional sedimentologic features previously recorded in the Don Braulio Formation indicate the existence of at least three stages of advance and retreat of the ice. A strongly erosive base and regional angular stratigraphic boundary with the underlying units allow suggesting initial subaerial exposure related to the rapidly expanding ice phase and consequent glacioeustatic falling stage. Glaciotectonic folding and meter-scale boulders of the underlying formation with continuous glacial striae record the first glacial advance. Inferred relief may be related to active tectonics along the Western Gondwana margin. Intratill and intertill type pavements record two additional stages of advance, where as two massive matrix-supported diamictites are interpreted as subglacial tills deposited during retreat stages. The final retreat, representing the waning stage, is recorded by few dropstones embedded in the lowermost part of the open-marine shales (Upper Member of the Don Braulio Formation) that onlap onto the glacial horizon along the entire Andean region. However, a first transgressive interval is required to explain the veneer conglomerate that caps the glacial Dmm, which in turn, was striated during a last ice advance. The first typical Hirnantia fauna has its appearance in the veneer conglomerate (interpreted a the ravinement surface), few meters under the first finding of Normalograptus persculptus. Previous to this, there is no authochthonous fauna in the glacial Dmm as to suggest open glaciomarine environments. The complex facies associations and lateral facies and thickness variations allow considering the glacial horizon preserved along the proto-Andean margin as representative of proximal subglacial and restricted proglacial environments. This is a major difference with open glaciomarine settings where less variable facies and thickness are preserved. The glacial influence mainly bracketed to the Hirnantian stage is compatible with the previously inferred paleogeographic position of the Precordillera terrane as being part of Gondwana by the Late Ordovician.

Keywords : Glacial sedimentation; Striated pavements; Hirnantian glaciation; Advance and retreat stages; Glacial dynamics; Late Ordovician; Gondwana; Andean region; Precordillera.