Publication: Architecture and development of the alluvial sediments of the Upper Jurassic Tordillo
Formation in the Cañada Ancha Valley, northern Neuquén Basin, Argentina
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2009
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Elsevier
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Abstract
The Upper Jurassic Tordillo Formation at Cañada Ancha area, northern Neuquén Basin, Argentina, comprises a
multi-stage suit of predominantly alluvial sediments that is heterolithic in nature. In that suit, several
lithofacies, architectural elements, and bounding surfaces of different order have been identified and their
lateral and vertical distribution characterized. This analysis allowed the differentiation of 3 main units (lower,
middle and upper), 20 subunits (C-1 to C-20), and the characterization of their alluvial styles.
The lower unit (which comprises subunits C-1 to C-4) is mainly formed by fine- to medium-grained
sandstones, which become medium- to coarse-grained towards the top. These sandstones characterize
settings ranging from floodplains with isolated, unconfined flows, to more complex, vertically stacked, multistorey
sheet sandstones of braided fluvial systems. The middle unit (C-5 to C-10) is dominated by pale browngrey
fine-to coarse-grained sands and medium size subangular to angular conglomerates, which reflect
amalgamated complexes of sandstone sheets and downstream accretion macroforms. Remarkably, this
alluvial sedimentation was episodically punctuated by volcaniclastic flows. The upper unit (C-11 to C-20)
consists of finer sediments, mainly pink to white fine-to medium grained sandstones and red to green
siltstones. Towards the top, bioturbation becomes important, and also the presence of volcanosedimentary
flows is noticeable. Fluvial settings include braided sheet sandstones with waning flood deposits evolving to
isolated high-sinuosity fluvial systems, with flash flood deposits. At the top of this unit, facies may suggest
marine influence.
Vertical changes in the fluvial style result from both climatic and tectonic controls. A semiarid to arid climate
and the active tectonism linked to the eastward migration of the Andean volcanic arc determined major
bounding surfaces, fluvial style evolution and the presence of the volcano-sedimentary deposits. Different
stages of high and low subsidence rates has been deduced from the vertical stacking of sediments.