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NARROW
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ICHNOLOGY OF MUDDY SHALLOW-WATER CONTOURITES FROM THE UPPER JURASSIC–LOWER CRETACEOUS VACA MUERTA FORMATION, ARGENTINA: IMPLICATIONS FOR TRACE-FOSSIL MODELS
Infaunalization and resource partitioning during the Mesozoic marine revolution
REPLY: BIOGENIC STRUCTURES OF UNIONIFORM BIVALVES IN WET-INTERDUNE DEPOSITS (LATE MIOCENE–EARLY PLIOCENE, ARGENTINA)
BIOGENIC STRUCTURES OF UNIONIFORM BIVALVES IN WET-INTERDUNE DEPOSITS (LATE MIOCENE–EARLY PLIOCENE, ARGENTINA)
Abstract In the Miocene beds of Cabo Viamonte, deep-marine hyperpycnal channel levee complexes occur at the toe of the depositional slope in sigmoidal clinoforms. The hyperpycnal channel fill is composed of thick packages of transitional and recurrent (vertical and lateral) sedimentary structures without rheologic boundaries, with variable textural ranges, recurrence of inverse-to-normal grading, multiple internal reactivation surfaces, and abundant organic content. This complex internal arrangement allows recognition of three facies associations (FAs) in the hyperpycnal channel fill. The FA1 comprises the coarsest clastic fractions, composed of intraformational boulders, matrix- and/or clast-supported conglomerate, and gravelly sandstone, which results from the combination of inertial flows and turbidity currents generated during the circulation of hyperpycnal flows. The FA2 comprises conglomerate, gravelly sandstone, and coarse- to fine-grained sandstone that results from rapid aggradation from a hyperpycnal turbidity current capable of transporting gravel as bed load and particles up to the size of coarse sand in turbulent suspension. The FA3 comprises heterolithic deposits that result from progressive accumulation by traction and suspension processes of low-density hyperpycnal turbidity currents. The coarsegrained lower channel fill is interpreted as a single sedimentation event. This is suggested by multiple internal erosion surfaces without lateral continuity, the lack of lithologic contrasts between sediment below and above the reactivation surfaces, and the absence of biogenic structures. On the contrary, the fine-grained upper channel fill, deposited mainly during the waning stages of the hyperpycnal flow, may involve more than one sedimentation event. This interpretation is supported by the presence of isolated beds with low-diversity trace fossils within the associated mudstones.
Abstract Combined ichnologic and sedimentologic analysis of Miocene deep-marine hyperpycnites in the Austral foreland basin, Tierra del Fuego, Argentina, allows recognition and characterization of different trace fossil assemblages from proximal to distal within this depositional system. In proximal and middle settings, hyperpycnal channel and inner levee deposits contain trace fossils in climbing-ripple and parallel-laminated sandstone facies; dwelling structures of suspension-feeding organisms (e.g., Diplocraterion ) dominate. In outer levee depos-its, fine-grained heterolithic intervals display a suite of trace fossils related to op-portunistic producers composed of concentrically laminated burrows of detritus or deposit feeders and also locomotion and grazing traces (e.g., Protovirgularia, Gordia ). The general low intensity of bioturbation and the dominance of structures pro-duced by opportunistic organisms suggest a stressful environment probably be-cause of episodes with high sedimentation rates and fluctuating salinity. In addition, in the middle slope setting, escape structures of imported bivalves occur in isolated sandstone bodies that accumulated at the base of the depositional slope.