Disobedient sediments can feedback on their transportation, deposition and geomorphology

Abstract

Most sediments are obedient to the winds, waves and currents, which direct their transportation and deposition. It has long been recognized however, that the grain size, and/or grain kind, of sediments can feedback on the processes of their own transportation, deposition and geomorphology as well as that of succeeding deposits. This note is to review three examples of marine sediments in which a single grain size or grain kind produces multiple feedbacks. Tidal bars of Holocene ooid sands on Great Bahama Bank are an example of multiple feedbacks of one grain kind on tidal currents, wave action and accumulations. These feedbacks are responsible for the distinctive pattern of elongated bars and channels, which in turn amplify tidal currents. The near constant movement of grains on the shallow bars and in the channels is where pellet nuclei are coated to form ooids; thus the development and growth of bars feeds back to produce more ooids. Regional encrinites, which consist predominantly of the disarticulated highly porous skeletons of crinoids, are common from Ordovician to Jurassic during blooms of these filter feeders. The resulting grains, which are equivalent to quartz grains a tenth of their size, can be entrained by currents as low as a knot (ca 0.5 m/s). The resulting mobile substrate deters other invertebrates (taphonomic feedback) and results in the prevalence of layering produced by traction transport of low velocity. The belt of mud extending for some 1600 km between the Amazon and Orinocco rivers is a special example of the feedback of mud on depositional processes, sedimentary structures and geomorphology of the accumulations. The clay-rich mud from the Amazon produces fluid mud which dampens and transforms wave action from the open sea to promote its own accumulation in giant bars in the inner shelf and shoreline with a variety of familiar laminations. The result is a wedge of mud-rich deposits some 24 m thick and 30 km wide capped with cheniers of sand, which has prograded seaward in some 6000 years.