Sequential colonization of muddy turbidites in the Eocene Beloveža Formation, Carpathians, Poland

A 100–350-m- thick succession of centimeter-thick fine-grained turbidites form the lower, mud-dominated part of a prograding deep-sea fan in the Eocene Beloveža Formation in the Carpathian Mountains of Poland. These strata contain over 50 ichnotaxa, only six of which are common (Phycosiphon, Nereites, Chondrites, Scolicia, Halopoa, Ophiomorpha). Polished slabs and examination of cross-cutting relationships show that post-event colonization was sequential. Using modern analogues, it can be shown that the colonization of different depth levels in the sediment at different times reflects changing geochemical conditions, especially the re-adjustment and re-establishment of the redox boundary. Modern analogues allow an assessment of the timing of each colonization phase. The Phycosiphon producers entered first and occupied the well-oxygenated muds, 5–40 mm below the sediment surface. As the redox boundary reformed, Nereites producers entered and probably utilized microbial matter just above the redox boundary, about 15–30 mm below the sediment surface. Subsequently, Chondrites producers penetrated down to the oxygen-deficient layers of the event bed or even deeper levels. The Halopoa producers entered with Phycosiphon, but reworked coarser layers. Contemporaneously with this sequential colonization, deep permanent bioturbators, which were unaffected by the event, cross-cut the post-depositional suite. The variability of the resultant ichnofabrics can be attributed in part to a patchy distribution of the benthos and the relative frequency of turbidite events. Five ichnofabrics, which reflect the successive colonization episodes, are recognized; each was modified by elements of the permanent infauna.