Bouma sequence
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The Bouma sequence describes a classic set of sedimentary structures in turbidite beds deposited by turbidity currents at the bottoms of lakes, oceans and rivers.
Turbidite from the Devonian-age Becke-Oese Sandstone, Germany showing a complete Bouma sequence.
Bouma A interval of a turbidite showing dish structures with pillar structures between the dishes, Northern California.
Cretaceous turbidite showing Bouma A-D layers. Pigeon Point Formation, Pescadero Beach, California.
Bouma B and convolute-laminated Bouma C layers in a sandy turbidite. Cozy Dell Fm, Topatopa Mountains, California.
Turbidite
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A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Turbidites are deposited in the deep ocean troughs below the continental shelf, or similar structures in deep lakes, by underwater avalanches which slide down the steep slopes of the continental shelf edge. When the material comes to rest in the ocean trough, it is the sand and other coarse material which settles first followed by mud and eventually the very fine particulate matter. It is this sequence of deposition that creates the Bouma sequences that characterize these rocks.
Turbidite sequence. Carboniferous Ross Sandstone Formation (Namurian), County Clare, Western Ireland (USGS image)
Gorgoglione Flysch, Miocene, South Italy