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Abstract:

This paper reports the facies analysis and ichnology of a logged section in the lower part of the Mulichinco Formation (Early Valanginian), at Vega de Escalone in the Neuquén Basin, western Argentina. Four facies associations (A-D) have been defined, and they record deposition below the storm wave base (A), mainly as gravity-flow deposits (coquinas) devoid of any evidence of bioturbation. Hummocky cross stratification, wave and current ripples, and mud intervals record the offshore transition (B). Wave and current ripples and heterolithic stratification record the shoreface and a shallowing-upward trend (C). The uppermost part of the succession exhibits tidal influence (D). Trace fossils documented in the shoreface (C) include abundant Asteriacites lumbricalis, Lockeia isp., Gyrochorte comosa, Planolites isp., and fugichnial trace fossils that indicate a stressed environment affected by changes in sedimentation rate and probably salinity. In heterolithic ripple troughs, abundant Asteriacites lumbricalis are well preserved, protected by mud drapes, and they include evidence of horizontal displacement and vertical superposition. Lockeia and Planolites intersect Asteriacites, and Gyrochorte comosa is a facies-crossing trace fossil. Trace fossils in the tidally influenced interval (D) are restricted in diversity and distribution. Some bedding planes contain monospecific assemblages of Asteriacites lumbricalis; others contain J-shape burrows and Gyrochorte comosa. Trace fossils can be assigned to the Cruziana ichnofacies while vertical burrows in hummocky cross stratification are interpreted as records of the Skolithos ichnofacies. This apparent inversion in the trace-fossil distribution according to the marine ichnofacies can be a response to the change from an opportunistic to a background fauna.

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