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Jim Bradshaw: South Louisiana tapped remnants of a salty sea

During the first week of May in 1862, workmen enlarging briny springs at Avery Island unexpectedly ran into a wall of solid salt. They didn’t know it, but they’d hit the core of the island itself, and run up against a piece of ancient geology
It was the first discovery of solid rock salt in North America, and eventually caused the curious or the far-sighted to poke into some of the other “islands” pushing up through the south Louisiana marshland and discover that they were also made of salt.
That discovery came as s bit of surprise.
Even though Indians had evaporated salt from the spring on Avery Island since prehistoric times, nobody seemed to have wondered what made the water so salty until John Marsh Avery tried to clean and deepen the springs and recover more salt.
Up until that time, most geologists thought the places we know today as Avery Island, Jefferson Island, Weeks Island, Côte Blanche Island, and Belle Isle were the remains of ancient volcanoes.
Upon further review, they found that the islands are evidence that what today is solid land was once beneath the sea that we call the Gulf of Mexico.
We know that thousands of years ago, the Gulf was far larger than it is today, stretching north nearly to Oklahoma, but scientists argue over just how the basin that became that sea was first formed and filled with water.
There are several theories. The first is that 500 million years ago the Gulf basin was part of a solid land mass called Llanoria, and that it began to subside when a crack opened in the earth stretching from south Texas to Mobile Bay.
The second theory is that the Gulf was formed when a huge continent called Pangea began to fall apart.
According to this theory, North and South America were once part of one big continent. Then about 250 million years ago they were split apart, and part of the space between them became the Gulf.
The third theory is that the Gulf basin has always been with us; it was created early in the formation of the earth’s crust.
I’m sure that you’ve probably read them already, but for not-too-technical explanations of all of this you might want to look again at the Corps of Engineers report, “Geology of the Mississippi River Deltaic Plain Southeastern Louisiana,” (Technical Report No. 3-483, July 1958) or the “General Geology of the Mississippi Embayment,” by E,M. Cushing and some colleagues (U.S. Government Printing Office, 1964).
As these and other studies point out, there are also arguments over when the Gulf permanently filled with water. The answer to that question depends, at least in part, on which theory of its basin formation is correct. According to the most popular scenario, water poured from the Pacific Ocean into the developing Gulf through small gaps that opened and closed from time to time in the mountains of Mexico.
Water flowed in when rising sea levels or earthquakes or some other phenomenon opened the gaps and it was trapped in the Gulf when they were closed.
The water evaporated during the long eras when no new water was flowing in, creating salt deposits. Eventually, somewhere between 65 and 150 million years ago, the Gulf basin formed a relatively water-tight crust and it was permanently filled.
Some of the salt deposits formed during the filling-and-unfilling period ended up beneath what is now coastal Louisiana.
Pressures created by heat and shifting material far inside the earth eventually pushed some of these salt piles up through the surface.
When those Avery Island workmen clanked their shovels onto solid salt in 1862, they were hitting the hardened remnants left millions of years before by the Gulf of Mexico’s struggle to be born.
A collection of Jim Bradshaw’s columns, "Cajuns and Other Characters," is now available from Pelican Publishing. You can contact him at jimbradshaw4321@gmail.com or P.O. Box 1121, Washington LA 70589.

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