Taphonomy of axiid decapods within early diagenetic carbonate concretions from the Agrio Formation (Lower Cretaceous), Neuquén Basin, west-central Argentina

ANDRADA, A.M.; BRESSAN, G.S.; LAZO, D.G.

Wien

Congreso; 8th International Meeting on Taphonomy and Fossilization (TAPHOS); 2017

University of Vienna

A total of 10 cm-sized ellipsoidal carbonate concretions containing axiid decapod specimens were studied. They come from a thin interval of fine sandstones of the Agrio Formation (Neuquén Basin, west-central Argentina) of early Hauterivian age, based on associated ammonoids. All specimens belong to Protaxius isochela Woodward (Axiidea, Axiidae). A total of 25 different specimens were identified including nearly complete individuals (n=5), disassociation units (n=13) and isolated elements (n=7). Half of the concretions contained two individuals and half had three. Chelipeds were the most abundant piece while carapaces and abdomen were present in half of the specimens only. Carapaces are often poorly-preserved showing diffuse contour and grooves. Some of the specimens are preserved in Salter?s position and thus they are interpreted as exuviae while others are identified as carcass remains. SEM and EDAX analysis of cuticle revealed presence of massive calcium phosphate and isolated small pyrite crystals (>100 μm) in its inner surface. Thin sections of concretions show a sedimentary matrix composed of fine-grained calcareous sandstone including in some cases parallel lamination and ripple cross-lamination. The observed disarticulation pattern agrees well with patterns previously described in Astacidea, Caridea and Brachyura. It is inferred that the quality of preservation is related to rapid burial of specimens, based on the overall good taphonomic condition, low disarticulation and presence of flagella. The sequence of fossilization of the best preserved specimens would have been as follows: 1) immediately post-mortem phosphatization of cuticle in terms of days probably within burrow systems but before the burrows were infilled with sediment; 2) rapid shallow entombment of carcasses within the first centimeters of infilling sediment and closure of the phosphatization window; 3) anaerobic decay and disarticulation of carcasses and pyrite precipitation; and 4) precipitation of carbonate cement around the remaining carcasses during a pause in sedimentation in the early diagenetic stage. This occurred before compaction of the sediment because the specimens are preserved in 3D. Sin-sedimentary carbonate concretions are usually formed a little below the sediment-water interface during times of low or zero sedimentation rates allowing the carbonate to be produced and inhibiting dilution with siliciclastics. An almost zero sedimentation rate also probably enhanced the phosphatization of cuticles before burial, pointing to alternating episodes of low and high clastic sedimentation.This is contribution C-133 of IDEAN.