What is dune bedding?
From the dramatic majesty of the modern day Sahara desert to the precision of beds recorded in sandstone units in Utah, dune bedding represents an impressive form of deposition. Dune bedding, a kind of bedform that preserves dunes and dune migration, is found within some sedimentary rocks. A bedform is the result of grains or sediment getting deposited in some unique way. In the rock record, dune beds appear as a series of inclined lines, or laminae, often truncated at the top and bottom.
Dune beds occur when wind or some other current blow across loose sand or some other sediment, generally small-grained. The sand moves in the direction of the wind, building on itself to create a little hill, called the crest. Then, due to gravity, at the peak of the hill the dune building collapses, forming the trough at the base of the crest. As the wind continues to blow, another crest begins to form; then at a brink point the sediment avalanches down, forming another trough. The process repeats itself consistently, forming a series of dunes stretching out under the wind’s discretion.
In this giant dune, the leeward side is shadowed.
Dune bedding can be identified by its appearance of asymmetrical ripples, looking like a wavy surface with waves all leaning in one direction. The direction in which the dunes lean indicates the direction of flow, or the direction in which the dunes migrate because of the force of wind. Dunes are made up of a crest and trough, and the side that sand avalanches down on, away from the wind, is called the dune’s leeward side.
Asymmetric current ripples recorded in a rock
The same forces that create dunes also form ripples. Ripples are simply smaller scaled and formed by a less powerful flow force. Dunes are commonly made of sand and shaped by the wind, whereas ripples can be made of silt or very fine sand, and formed via water. Technically, a bedform is considered a dune when the spacing from one dune crest to the next is just under 1 meter to over 1,000 meters. Dunes can be made of fine to very course sands and occasionally, gravel. Dunes made with the assistance of wind are called eolian dunes, and when developed under water, termed submarine.
Imagine a pool of shallow water with a sandy bottom. When the water gently moves in one direction, ripples begin to form. If the water were to move faster, with increased flow strength, the ripples would turn into dunes. If the flow regime got stronger still, the dunes would turn into a flat bed, and subsequently antidunes, and finally, a bedform called chutes and pools. Obviously such conditions of straightforward flow increase don’t happen in nature, but such experiments can be used by scientists to determine the relationship between flow strength and type of bedform.
While dunes appear as asymmetric waves during deposition, they generally look very different in the rock record. It is most common to find dune bedding along with cross bedding, the result of dunes moving in different directions over time. As the wind changes direction, the top of the dunes get eroded, and a new series of dunes in their migratory path get deposited on top. The resulting rock record will show thick horizontal layers, with inclined, truncated laminae found in each layer. Geologists can determine the direction of ancient air and water currents using the information deduced from these bedforms.