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The Louann Salt of the Gulf of Mexico: How Long Does it Take to Deposit a Giant Salt Deposit?


The present is the key to the past.... except when the geological event has no modern equivalent. Existing models for deposition of giant salt deposits, such as the Mid-Jurassic Louann Salt of the Gulf of Mexico, have been built on observation of modern evaporite environments, and this has led many to believe that salt deposition was slow, spread over a very long time (many millions of years), and in shallow water; but the modern analogs are misleading. The Louann Salt formed at 170Ma (Bajocian) in a supercontinent on the point of break-up during a global hot period; there is no modern analog because these conditions are not found today. The center of the supercontinent was subject to climate extremes. A set of long, narrow basins formed along the incipient break, creating a narrow, deep, seaway, surrounded by hot and dry desert, connected at one end to the ocean. Conditions were perfect for salt deposition. Our analysis suggests that salt precipitation was very rapid - perhaps of the order of meters per year - so that deposition of the entire 3-5km thick salt mass could have been achieved in as little as a few thousand years. When water supply was cut off, the basin dried out, leaving a hot, dry and in places very deep hole, surrounded by a global scale desert. High temperature and air pressure amplified the effect of regional winds; extreme wind conditions are indicated by sand dunes large enough to be seen on seismic data, that are more than three times the height of the largest dunes seen on the modern earth's surface. This environment set up unique conditions for the deposition of both source and reservoir rocks.