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

A 100-350-m- thick succession of centimeter-thick fine-grained turbidites f orm the lower, mud-dominated part of a prograding deep-sea fan in the Eocen e Beloveza Formation in the Carpathian Mountains of Poland. These strata co ntain over 50 ichnotaxa, only six of which are common (Phycosiphon, Nereite s, Chondrites, Scolicia, Halopoa, Ophiomorpha). Polished slabs and examinat ion of cross-cutting relationships show that post-event colonization was se quential. Using modem analogues, it can be shown that the colonization of d ifferent depth levels in the sediment at different times reflects changing geochemical conditions, especially the re-adjustment and re-establishment o f the redox boundary. Modern analogues allow an assessment of the timing of each colonization phase. The Phycosiphon producers entered first and occup ied the well-oxygenated muds, 5-40 mm below the sediment surface. As the re dox boundary reformed, Nereites producers entered and probably utilized mic robial matter just above the redox boundary, about 15-30 mm below the sedim ent surface. Subsequently, Chondrites producers penetrated down to the oxyg en-deficient layers of the event bed or even deeper levels. The Halopoa pro ducers entered with Phycosiphon, but reworked coarser layers. Contemporaneo usly with this sequential colonization, deep permanent bioturbators, which were unaffected by the event, cross-cut the post-depositional suite. The va riability of the resultant ichno-fabrics can be attributed in part to a pat chy distribution of the benthos and the relative frequency of turbidite: ev ents. Five ichnofabrics, which reflect the successive colonization episodes , are recognized; each was modified by elements of the permanent infauna. ( C) 2001 Elsevier Science B.V. All rights reserved.