In this image we see folded and deformed cherts that were deposited above the pillow basalts. This picture is from the area west of the Roberts Mountain Thrust. The typical stratigraphy of the ocean floor has to so with the changing depth of the seafloor as oceanic crust is created at the spreading ridge and moves away. Near the spreading ridge, the crust is warm and buoyant and at shallow enough depths that carbonate deposits are preserved. As the crust cools and moves away from the spreading center it is less buoyant and is found at deeper depths. As the depth increases, the temperature of the bottom waters decrease. The sediments found in these deep-water regions are mainly silicious deposits, such as chert. This is because carbonate’s solubility increases with decreasing temperatures. Though throughout the ocean, carbonate detritus created by marine microorganisms rains out of the water column, the deep parts of the ocean (which are cooler) lack carbonate sediment because the falling detritus dissolves before it can be deposited. The depth at which this happens is called the CCD or the Calcite Compensation Depth. Thus the stratigraphy of ocean floor sediments typically starts out as carbonates, but later become silicious.-------------------------------So that completes the makeup of oceanic crust. If we find all these components present on land, then we can call it an Ophiolite Sequence. Lets review. Peridotites and Dunites with metamorphic shear fabrics are at the contact between the mantle and the crust. Above there are magna chambers with cumulate textures due to the progressive settling of crystals from the melt. Fed from the magma chamber, the sheeted dike complex is typically composed of many, many generations of near-vertical cross-cutting dikes. These dikes reach the surface and provide the magma erupted at the ocean’s floor as pillow basalts. Above the pillows are the marine sediments that start as carbonates but grade into silicious deposits as the ocean crust sinks, increasing the ocean floor depth below the CCD. It is on top of these marine sediments that we find the volcanic rocks and sediments created in Island Arcs.
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