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|Title:||Sedimentology and Paleogeography of a Devonian Turbidite Basin in Argentina|
|Abstract:||<p>The area studied lies in central western Argentina, just east of the Cordillera of the Andes, in the geological province names the Precordillera. This thesis describes the sedimentology and paleogeography of the Punta Negra Formation (Middle to Upper Devonian). This formation is well exposed over the central Precordillera by a series of thrust faults which trend north-south. The Punta Negra Formation was deposited on a continental borderland and its distribution and lithofacies were controlled by two north-south trending rises: the Zonda-Villicum arc, on the east, and the Tontal arc, on the west.</p> <p>The Punta Negra consists of gray shales and conglomerates, on the east, and of olive-green graywackes and shales. with few paraconglomerates, on the west. The eastern lithofacies is considered to have been deposited on a slope bordering the mainland and the western lithofacies - which includes most of the outcrops of the Punta Negra Formation - is though to be a submarine fan deposit. The submarine fan occupied a small basin restricted by the two rises above mentioned. This submarine fan does not fit well in proposed models because the upper and middle fan sections - with deep channels and numerous conglomerates - seem to be absent. Only the outer fan morphology is recognizable. This deviation from the model is attributed to the effect of submarine topography. The source area lat to the southeast, on the mainland.</p> <p>The graywackes that form the submarine fan were transported mainly as high density turbidity currents. But in the last stages of deposition, grain flow and debris flow mechanisms also acted on the sediment. The result of this combination of mechanisms was a complex grading pattern. In most beds one finds: (i) a thin fine-grained layer at the base; the grain-size rapidly increases and, then, follows (ii) a slight inverse grading which extend up to the middle of the bed, (iii) the central portion of the beds are usually structureless and, finally, follows (iv) a normal grading into the shale layer on top.</p> <p>A reconnaissance field study of the Regional Geology of the Precordillera was carried out, in order to better establish the paleogeographic setting of the Punta Negra Formation. The rock sequence studied extends from the Precambrian to the Devonian, inclusive, with three intervening unconformities. The Precambrian is barely exposed on the east and consists of schists. The Cambro-Ordovician consists, on the east, of 2000m of shallow water limestones unconformably overlying the Precambrian basement. On the west, shales, turbidites and a few conglomerates probably represent slope, rise and abyssal plain sediments; paleocurrents are mainly from the north. The Ordovician on the west contains pillow lavas and ultrabasic bodies but there is no evidence of an underlying continental basement or of western sediment sources. The Cambro-Ordovician assemblage suggests the existence of a passive, Atlantic-type continental margin. Following a Late Ordovician tectonic disturbance, Silurian and Devonian sedimentation was wholly clastic and controlled by submarine rises. One such rise, the Tontal arc, acted as a barrier to the westward movement of much of the clastic sediment, which paleocurrent and petrography indicates was derived from the shield to the east. Marine sedimentation over the old continental shelf, east of the Tontal arc, was brought to a close by the deposition during the Devonian of thick turbidite beds in at least two submarine Fans, with paleocurrents predominantly from the east and south.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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