In: Geologica Carpathica, vol. 64, no. 5
Vladimír Šimo - Adam Tomašových
Trace-fossil assemblages with a new ichnogenus in “spotted” (Fleckenmergel—Fleckenkalk) deposits: a signature of oxygen-limited benthic communities
Year, pages: 2013, 355 - 374
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.
How to cite:
Šimo, V., Tomašových, A. 2013. Trace-fossil assemblages with a new ichnogenus in “spotted” (Fleckenmergel—Fleckenkalk) deposits: a signature of oxygen-limited benthic communities. In Geologica Carpathica, vol. 64, no.5, pp. 355-374.
Šimo, V., Tomašových, A. (2013). Trace-fossil assemblages with a new ichnogenus in “spotted” (Fleckenmergel—Fleckenkalk) deposits: a signature of oxygen-limited benthic communities. Geologica Carpathica, 64(5), 355-374.