MUD REDISTRIBUTION IN EPICONTINENTAL BASINS – EXPLORING LIKELY PROCESSES

Although muddy sediments were long thought as largely due to passive settling of small particles under low energy conditions, and thus by implication an indicator of distal and comparatively deep depositional settings, this temptingly simple concept is not in agreement with physical realities and known modern environments of mud deposition. The bulk of mud enters the oceans via rivers, and it drops out from freshwater plumes within tens of km’s from the river mouth. The stratigraphic record of multiple epicontinental basins indicates that muds were been redistributed over distances of 100’s and even 1000’s of kilometers, and to understand the underlying processes is essential to understand how epicontinental basins “work”.

Processes that are potentially involved to carry muddy sediments further are intermittent erosion of muddy surface sediments, river-fed hyperpycnites, bottom currents powered by wind driven circulation, storm wave reworking in combination with coastal downwelling or gravity flow processes (WESGF’s), and tidal circulation. The mud most likely travels in the form of floccules or reworked water-rich aggregates. In modern shelf seas, wave reworking and gravity driven fluid muds have been implicated in mid-shelf mud transport, but whether this mechanism is feasible in the case of very gentle slopes of expansive epicontinental seas is a matter of debate. Muddy hyperpycnites are increasingly recognized in the rock record, and in the vicinity of river deltas they are a viable source of cross-shelf mud transport. Bedload transport of flocculated muds and soft mud aggregates, respectively, produces fine lamination and lenticular fabric in shales and is commonly observed in the rock record. Fine lamination in shales can result from the majority of above processes, although careful examination of the rock record suggest that there are “multiple flavors” of laminated shales that may reflect unidirectional currents, as well as tidal circulation and storm wave reworking. Flume studies of oscillatory and reversing flow under conditions that allow mud deposition are in progress to address differences in the textural outcomes of these different processes.