International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Trace-fossil assemblages with a new ichnogenus in “spotted” (Fleckenmergel—Fleckenkalk) deposits: a signature of oxygen-limited benthic communities

Pages: 355 - 374

Authors: VLADIMÍR ŠIMO, ADAM TOMAŠOVÝCH

Abstract: Highly-bioturbated “spotted” limestones and marls (Fleckenmergel—Fleckenkalk facies) of the Early Jurassic, which were deposited in broad and recurrent deep-shelf habitats of the Northern Tethys, are characterized by rare benthic carbonate-producing macroinvertebrates. To address this paradox, we analyse trace-fossil assemblages in a ~ 85 m-thick succession of Pliensbachian spotted deposits (Zliechov Basin, Western Carpathians). They are dominated by infaunal and semi-infaunal deposit-feeders, with 9 ichnogenera and pyritized tubes of the semi-infaunal foraminifer Bathysiphon, being dominated by Chondrites, Lamellaeichnus (new ichnogenus), and Teichichnus. Lamellaeichnus, represented by a horizon- tal basal cylindrical burrow and an upper row of stacked convex-up gutters, was produced by a mobile deposit-feeder inhabiting shallow tiers because it is crossed by most other trace fossils. We show that the spotty appearance of the deposits is generated by a mixture of (1) dark, organic-rich shallow- and deep-tier traces (TOC = 0.16—0.36), and (2) light grey, organic-poor mottled or structurless sediment (TOC = 0.09—0.22). The higher TOC in shallow-tier burrows of Lamellaeichnus demonstrates that uppermost sediment layers were affected by poor redox cycling. Such conditions imply a limited mixed-layer depth and inefficient nutrient recycling conditioned by hypoxic bottom-waters, allowed by poor circulation and high sedimentation rates in depocenters of the Zliechov Basin. Hypoxic conditions are further supported by (1) dominance of trace-fossils produced by infaunal deposit feeders, (2) high abundance of hypoxia- tolerant agglutinated foraminifer Bathysiphon, and (3) high abundance of Chondrites with ~ 0.5 mm-sized branches. Oxygen-deficient bottom-conditions can thus simultaneously explain the rarity of benthic carbonate-producing macroinvertebrates and high standing abundance of tolerant soft-shell and agglutinated organisms in spotted deposits.

Keywords: Jurassic, Western Carpathians, community paleoecology, dysoxia, bioturbation, ichnofacies, trace-fossil assemblage

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