Trace-fossil tiering in flysch sediments: examples from the Guipúzcoan flysch (Cretaceous-Paleogene), northern Spain

Abstract

Distribution of trace fossils in flysch sediments is in great part conditioned by tiering; that is, the vertical partitioning of trace-producing organisms below the sediment-water interface. Ichnologic studies in the Guipúzcoan flysch (Upper Albian-Lower Eocene), northern Spain, show that tiering in flysch sediments is strongly controlled by sediment character, mode and rate of deposition, and degree of oxygenation of pore waters. Depending on the mode of deposition, trace-fossil tiering in flysch developed and was preserved in two radically different styles: (1) In event layers (chiefly turbidites), tiering is preserved without secondary distortion, owing to synchronous colonization and the lack of vertical ascent through time. Vertical tiering structure depends primarily on sediment composition, sequence and thicknesses of structural divisions, and oxygen profile. Variations in tiering are interpreted to reflect the particular oxygen tolerances, penetration potentials, and nutritional requirements of tracemakers; (2) In background sediments (hemipelagites and pelagites), tiering resulted from successive colonizations that were influenced by background sedimentation rates, sediment composition, and oxygen levels of pore waters. Tiering of actively or passively filled burrows is reflected by complex ichnofabrics and the superposition of different tiers by ascension. Tiering structure was frozen upon deposition of superjacent event layers. The original tiering of graphoglyptids (open burrows) may be reconstructed from their cross-cutting relationships on turbidite soles. These contrasting styles of tiering development are characteristic of flysch and other sediments composed of alternating event and background deposits.