GRADING PHENOMENA IN MUDSTONES – ANCIENT SHALES AND MODERN SHELF MUDS IN THE CONTEXT OF RECENT FLUME STUDIES

Ancient shale successions and modern shelf muds contain an abundance of subtly graded shale and mudstone beds (mm to cm thickness). These tend to show low-angle downlapping silty laminae in their basal portions, and in view of recent flume experiments on advective mud deposition, could potentially have been emplaced by bottom currents that carried flocculated mud in bedload. In the literature, the origin of such beds is typically ascribed to depositional events such as storms, hurricanes, flood fed river plumes, etc. On the basis of work on modern shelves, they have also been explained as the result of wave induced sediment gravity flows, a combination of wave induced mud fluidization (sediment concentration >10 g/l) and density flow.

Flume experiments were conducted at marine salinity and velocities between 10-50 cm/s, in order to explore formative mechanisms of above described beds. In uniform as well as decelerating flows, suspensions of clay and quartz silt mixtures (sediment concentration 2-6 g/l) produced deposits with low relief coarse silt ripples at the base, followed by low angle inclined silty laminae and an increasing clay component upwards. The clay-rich upper portion of the deposits shows internal laminae and cross-laminae as well, although those would easily be missed in both ancient and modern equivalents.

The sedimentary textures reproduce in these experiments are commonly ascribed to muddy tempestites, although these textures also showcase supposedly diagnostic features of fine grained wave enhanced sediment gravity flows. In the latter case, experiments produce the requisite features without the need for fluid mud conditions. These observations suggest that it will require more research before we can hope to understand the genesis of the various flavors of graded muds in the rock record. They are likely of multiple origins and might among others reflect (1) seasonal reworking of nearshore muds that are carried offshore by wind-driven circulation, (2) storm induced downwelling of muddy suspensions, (3) remobilization of bottom muds by tidal currents, as well as (4) being the result of wave-triggered gravity flows.