Fluid-Mud Deposits of the Lower Jurassic Tilje Formation, Offshore Mid-Norway

Aitor A. Ichaso and Robert W. Dalrymple
Dept. of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON, Canada

The Jurassic-aged Tilje Formation (120-300 m thick) consists predominantly of heterolithic deposits and is though to have accumulated in tide-dominated estuarine and deltaic environments in an active rift setting. Individual mud layers that are anomalously thick (> 1 cm) and internally homogeneous are widespread. Bioturbation is generally absent from the mud layers and occurs only as a result of post-depositional, top-down colonization. These mud layers, which are interpreted to represent fluid-mud deposits, occur in several different environmental settings. (1) Tidal-fluvial channels contain sharp-based, 1-5 cm thick homogeneous and structureless mudstones, some of which contain disseminated organic matter. Thick fluid-mud layers are generally restricted to channel-bottom positions where they occur in association with cross-bedded and current-rippled sandstones and mud-pebble conglomerates. They are replaced upward in the channel successions by thinner mud layers. (2) Delta-front deposits and distal distributary channels contain sharp-based 0.5-2 cm thick homogeneous mudstones with rare internal lamination, accompanied by dispersed grains of silt to very-fine sand. They occur in association with anomalously coarse river-flood deposits composed of thin- to medium-bedded heterolithics that thin and fine upward as a result of decreasing river flow that was modulated by tidal activity. (3) Storm deposits consist of sharp-based 0.5-3 cm thick weakly laminated mudstones that contain dispersed very-fine sand grains. They occur in association with thick very-fine sand layers containing wave-generated ripples and HCS. The widespread presence of fluid-mud deposits within the Tilje Formation may have been favored by the tectonically active rift-basin setting. The diversity of environments in which fluid muds occur is consistent with recent observations in modern environments.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas