Biogenic Sedimentary Structures in Outer Bengal Fan Deposits Drilled during Leg 116

Abstract

Bioturbation in the outer Bengal Fan is influenced by two major factors, bottom-water oxygenation and turbidite deposition. Biogenic traces reflect changing bottom-water oxygenation over the last 17 m.y.; in the Miocene clear oxygenation cycles are developed in the hemipelagically deposited sections. Since the Pliocene these cycles are less pronounced and an intermediate level of oxygenation is documented by the trace fossils. As only three closely spaced sections were studied, it is uncertain whether the change in oxygenation is a regional phenomenon or just a local effect resulting from changing bottom topography due to intraplate deformation.Turbidite deposition also strongly influences bioturbation at the sites studied. In the outer Bengal Fan, fine-grained parts of turbidites are often burrowed. In contrast, biogenic reworking is less intense or nonexistent in the sandy parts of turbidites. A considerable number of the fine-grained sections are bioturbated by Phycospiphon, and the thickness of the reworked section can exceed the penetration depth of this trace. In this case, we infer that bioturbation and sediment accumulation were in equilibrium and that sedimentation was relatively slow for normal deposition by a turbidity current of the order of 1 cm per day during an event.

Variation of Trace Fossils and Ichnofacies in Neogene and Quaternary Pelagic Sediments from the Eastern Equatorial Pacific Ocean (Leg 138)

“…Trace fossils constitute a major portion of the observable structures within deep-sea cores. Several descriptive accounts of the variety of trace fossils present in DSDP cores (e.g., Ekdale, 1977Ekdale, , 1980Fütterer, 1984) have been written, yet relatively few ODP/DSDP authors have attempted to document systematically their occurrence during routine core description (e.g., Wetzel, 1987;Wetzel and Wijayananda, 1990;Droser and Bottjer, 1991). The 5500 m of sediment recovered from the eastern equatorial Pacific during Leg 138 provided an ideal opportunity to document the spatial and temporal variations in trace-fossil diversity and abundance in a wide range of deep-sea pelagic sediment facies from ooze to clay.…”

Ichnology of deep‐sea fan overbank deposits of the Ganei slates (Eocene, Switzerland)— a classical flysch trace fossil locality studied first by Oswald Heer

“…This estimate is based on: (1) the occurrence of trace fossils that probably represent a longterm adaptation of their producers to such a deepsea environment; (2) the occurrence of a complete tiering structure, which is not dominated by opportunistic trace-fossil producers; and (3) comparisons with modern counterparts (Wetzel, 1984;Wetzel and Wijayananda, 1990).…”

Sedimentary and Diagenetic Structures and Textures in Turbiditic and Hemiturbiditic Strata as Revealed by Whole-Core X-Radiography, Middle Valley, Northern Juan de Fuca Ridge

“…This assemblage of ichnofauna has several different patterns of occurrence in these cores. Ichnofauna assemblages in deep marine strata are indicators of many environmental variables, including differences in sediment accumulation rates, oxygen levels, and depositional environments (Ekdale et al, 1984;Wetzel and Wijayananda, 1990). Thick-bedded, sandy turbidites are seldom bioturbated because they accumulate rapidly and because they drastically change the substrate, making long-term adaptation difficult for the benthic infauna.…”

“…Where small-sized Skolithos, Planolites, Thalassinoides, Chondrites, and Zoophycus occur together in thin bioturbated sections (Fig. 4), a high oxygenation level and hemipelagic depositional conditions with relatively constant sedimentation rates are indicated (Berger et al, 1979;Wetzel and Wijayananda, 1990). In such cases, sediments accumulated slowly relative to bioturbation rates.…”