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Nearshore and Shelf Systems
Lateral Variations in Tidal Deposits of the Upper Cretaceous Neuquen Basin, Western Argentina
Shallow marine to coastal, clastic and evaporitic sediments of the Allen (Loncoche) Formation were deposited during the late Campanian-early Maastrichtian in the Neuquen foreland basin (western Argentina). On the basis of several stratigraphic sections, four facies associations representing different tidal subenvironments have been recognized: A) crossbedded sandstones; B) massive mudstones with dolomites and volcanic ashes; C) sandstones and mudstones, and D) evaporites. Facies association A is composed of quartz-felspathic crossbedded sandstones, with epsilon and sigmoidal crossbedding, and clay drapes being the principal tide-related sedimentary structures. A subtidal to intertidal depositional environment is inferred for facies association A. Facies association B is composed of montmorillonitic mudstones with intercalations of volcanic ash, stromatolites and dolomites, containing a brackish to marine fauna. Facies association B was deposited in an intertidal setting. Facies association C, characterized by mixed montmorillonite-rich mudstones and feldspathic sandstones with lenticular, wavy, and flaser bedding, is considered to be a subtidal to intertidal lagoonal deposit. Facies association D is composed of evaporites and stromatolites formed in an upper intertidal to supratidal environment.
The arrangement of facies associations on the east side of the foreland basin resembles the classical tidal-flat examples of the North Sea, while toward the northwest, paleoenvironments correspond to modern, tide-dominated estuarine environments. Differences in the depositional paleoenvironments are attributed to differences in the tidal regimes, which in turn are related to the basin’s geometry. A microtidal regime affected the depositional environments of the low-relief coast to the southeast, while a macrotidal regime influenced the depositional environments of the funnel-shaped basin toward the northwest. Additional factors affecting the distribution of facies associations include paleoslope and sediment input. Gentle paleoslopes and reduced sediment input affected deposition on the eastern, cratonic side of the foreland basin, whereas increased sediment input and steeper paleoslopes affected deposition in the western, retro-arc side of the basin.
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