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

ANDRADA; LAZO, DARÍO G. ; BRESSAN, GRACIELA S.

Viena

Simposio; 8th International Meeting on Taphonomy and Fossilization; 2017

University of Vienna

 A total of 10 cm-sized ellipsoidalcarbonate concretions containing axiid decapod specimens were studied. Theycome from a thin interval of fine sandstones of the Agrio Formation (NeuquénBasin, west-central Argentina) of early Hauterivian age, based on associatedammonoids. All specimens belong to Protaxiusisochela Woodward (Axiidea, Axiidae). A total of 25 different specimenswere identified including nearly complete individuals (n=5), disassociationunits (n=13) and isolated elements (n=7). Half of the concretions contained twoindividuals and half had three. Chelipeds were the most abundant piece whilecarapaces and abdomen were present in half of the specimens only. Carapaces areoften poorly-preserved showing diffuse contour and grooves. Some of the specimensare preserved in Salter?s position and thus they are interpreted as exuviaewhile others are identified as carcass remains. SEM and EDAX analysis ofcuticle revealed presence of massive calcium phosphate and isolated smallpyrite crystals (>100 µm) in its inner surface. Thin sections of concretionsshow a sedimentary matrix composed of fine-grained calcareous sandstoneincluding in some cases parallel lamination and ripple cross-lamination. Theobserved disarticulation pattern agrees well with patterns previously describedin Astacidea, Caridea and Brachyura. It is inferred that the quality ofpreservation is related to rapid burial of specimens, based on the overall goodtaphonomic condition, low disarticulation and presence of flagella. The sequenceof fossilization of the best preserved specimens would have been as follows: 1)immediately post-mortem phosphatization of cuticle in terms of days probablywithin burrow systems but before the burrows were infilled with sediment; 2)rapid shallow entombment of carcasses within the first centimeters of infillingsediment and closure of the phosphatization window; 3) anaerobic decay anddisarticulation of carcasses and pyrite precipitation; and 4) precipitation ofcarbonate cement around the remaining carcasses during a pause in sedimentationin the early diagenetic stage. This occurred before compaction of the sedimentbecause the specimens are preserved in 3D. Sin-sedimentary carbonateconcretions are usually formed a little below the sediment-water interfaceduring times of low or zero sedimentation rates allowing the carbonate to beproduced and inhibiting dilution with siliciclastics. An almost zerosedimentation rate also probably enhanced the phosphatization of cuticlesbefore burial, pointing to alternating episodes of low and high clasticsedimentation.