Subject: Marine Sediments
This question is from my 8th grade daughter who has a science project on geology. Question is: How would an area get from mudstone to limestone to shale since the depositional environment of all those rocks is deep ocean?
The answer to your question lies in not only understanding where the
sediments finally accumulate, but also what their sources are. As I
explain below, another factor is when they accumulate.
Limestones are composed of calcium carbonate and most all are formed from the accumulation of oceanic organisms that make their shells of calcium carbonate. There are shallow water marine limestones and deep water marine limestones. Shallow water limestones are mostly made up of the shell material of reefs, that is corals, bivalves and the like that live in warm shallow waters. In the open ocean, tiny micro-organisms called coccoliths (microscopic plants) and foraminifera (microscopic animals) live in surface waters and they too make their shells of calcium carbonate. When they die, their shells sink to the seafloor to accumulate as a "carbonate ooze". After this ooze is buried, the ooze becomes compacted and cemented to form a chalk. That's right, the same stuff that you use on a blackboard! In the very, very deep ocean (deeper than 4 to 5 kilometers) all the calcium carbonate dissolves away, and the oozes cannot accumulate there. This is because the deep old waters there contain excess dissolved carbon dioxide which makes them slightly acidic (lower pH). For your science project, place a piece of chalk in a glass of tap water and another in a glass of water containing carbon dioxide (like Perrier mineral water you can buy from the store; make sure its the one with the gas bubbles). The glass one that rapidly effervesces (fizzes) and dissolves the chalk is rather like that deep sea. Thus, deep sea carbonate oozes accumulate atop submarine highs, plateaus, seamounts, and the midocean ridges. Why does that old, deep sea water contain carbon dioxide? Because of the constant input and decay (oxidation) of organic matter (CH2O) derived from the organisms that inhabit the sea. CH2O + O2 = CO2 + H2O.
Mudstones and shales are essentially one in the same. These are rocks made of mud. They form in many environments, on land and in the ocean. Because mud comes from the weathering of mountains, most mudstones either accumulate on the continents or along the oceanic margins of the continents. The mud is transported by rivers and dumped at the shoreline to form marine deltas (like the Mississippi Delta). Other muds make it past the deltas and accumulate on the continental shelf. Others bypass the continental shelf, travel by gravity as heavy, turbid mixtures of mud and water (called turbidites) to accumulate on the continental slopes and rises as "deep sea fans." The turbidites can move very quickly down the continental slopes (sometimes up to about 40 mph) and a few make it out onto the abyssal plains of the true deep sea.
Most of the mud that reaches the deep sea is transported there during times of global sea level lowering, when the ice caps expand, take up water, and the sea level falls. When the sea level falls far enough (say 100 meters), the continental shelf becomes dry land, and the rivers that supply the mud travel all the way out to the edge of the continental shelf. Thus, during a glacial stage with lowered sea levels, the transfer of mud to the abyss becomes much more effective. We are not in a full glacial stage right now, but the earth was in one only 19,000 years ago.
Craig R. Glenn, Associate Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822