Facies Variation in Relation to Miocene Paleoclimate Change in the Andean Foreland, Northwestern Argentina
The Subandean thrust belt in NW Argentina has advanced eastward since the early Miocene. The fluviatile environment developed in the Subandean foreland basin is controlled by topography and water availability, which are highly coupled to the precipitation. Hence, any climatic variability affects the sediment discharge. The Miocene sediment record from outcrops along Iruya River, Peña Colorada River, and La Porcelana River shows a series that cycle between mudstones and sandstones. Previous studies identified three tectonic progradational mega-cycles. We observe many more oscillations (on the order of tens) within the tectonic megacycles which may reflect climatic cyclicity. We are testing the hypothesis that there is climate control on the cyclicity between mudstones and sandstones. Mudstone unit thickness varies from older (~14 Ma) to younger (~4.5 Ma) horizons between 5m and 50cm, respectively. Mudstones are infrequent in the upper Miocene and Pliocene strata. The mudstone goes from red-yellowish to red-brown, especially where paleosols are identified. The calcium carbonate is present in the matrix of these mudstones, including calcium carbonate nodules in the ones with paleosols. Sandstones become more dominant up section, from ~50cm up to >10m in thickness. These are quartz of light-grey and yellowish color in Iruya River, but brown-yellow, red-brown, and dark-gray in Peña Colorada and La Porcelana Rivers. The difference in coloration is due to the mud incorporated in the red-brown sandstones, and hydrocarbons in the dark-grey ones. To date, we have completed extensive field observations, characterizing floodplains with frequent paleosol development, and channels belts with heterogeneous deposits at Iruya River, but more homogeneous sandstones in the Peña Colorada and La Porcelana River sections. In progress is documentation of the oxygen and carbon isotopic fractionation of the calcium carbonate in the paleosols. Next we will identify seismic stratigraphic sequences, to be correlated with the isotopic and facies data. The preliminary results show a coarsening upward sequence as well as a shift from -12‰ δ 13C C3 plants to -9‰ δ 13C C3 plants, and a stable trend of the oxygen values oscillating around -8 ‰ δ18O. These data imply an increase of sediment grain size correlating with a decrease of vegetation cover. This paired increase may be due to a climatic control on the erodibility of the surface.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009