Boreskov

OP‐38THE OLDEST EVIDENCE OF BIOTURBATION ON EARTHRogov V.I.Trofimuk Institute of Petroleum‐Gas Geology and Geophysics, Novosibirsk, RussiaWe documented intensely bioturbated ichnofabric and associated discrete, identifiabletrace fossils in the Khatyspyt Formation cropping out on the Olenek Uplift in the northeasternpart of the central Siberia. Stratigraphic sections of the Khatyspyt Formation werelogged for ichnofabric indices (percentage of original sedimentary fabric disrupted bybiogenic reworking). The upper part of the Khatyspyt Formation appears to be moderately tointensely bioturbated (see Figure). There are two styles of ichnofabric preservation in theKhatyspyt Formation: three‐dimensional and compressed. Three‐dimensional preservation isattributed to early diagenetic silicification of discrete sedimentary layers and providesinsights into the structure of both individual burrows and the entire ichnocoenosis. Eachburrow demonstrates a terminal backfill structure which is a result of active displacement ofa tunnel within the substrate and emplacement of the material by an animal posteriorly as itprogressed through sediment. There is no trace of wall lining in the tunnels. Branchingburrows have never been observed. Backfill menisci are composed of little‐altered substratesediment. Intervening menisci consist of early diagenetic microcrystalline silica (withdispersed grains of dolomitic mudstone) that could be selectively replacing areas withelevated levels of organic matter. In compressed preservation the intervening menisciconsist of silicified mudstone with dispersed pyrite globules; the color contrast in thesemenisci can be greatly enhanced by diagenetic processes during fossilization. Theappearance of ichnofabric can be severely disturbed by diagenetic overprint. For example,the menisci can be connected with each other by a string of silicified material or pyriteglobules. In other specimens the original saucer‐like shape of the menisci has beenexaggerated and transformed beyond recognition, or the entire burrow is surrounded by ahalo of microcrystalline quartz. Most burrows have width 0.5–3.0 mm, with the maximumreaching 6.5 mm. Depth of bioturbation measured from silicified sedimentary layers thatwere subject to minimum sediment compaction reached 5 cm. Meniscate backfill representsa locomotion structure (repichnion). The lowest stratigraphic occurrence of the ichnofabric isat the base of the Khatyspyt Formation 185 m below the first appearance datum (FAD) ofCambrotubulus decurvatus and 335 m below the FAD of Treptichnus pedum. By all means,the Khatyspyt ichnofabric is of late Ediacaran age which makes it the oldest reliablepaleontological evidence of bioturbation. This study was supported by RFBR grants no. 09‐05‐00520 and 10‐05‐00953, RAS Program “Biosphere Origin and Evolution” and NationalGeographic Society.96