An NPR report this morning reports that New Orleans is built over a thousand feet of loose soil. It sinks at the rate of two feet per century. In the future, it may sink even faster at the rate of one meter over the next century. The report is that gas and oil removal do have an effect on subsidence,
I recall, during my mudlogging experience, that the drillers went thought 4,000 feet before they reached rock. That was near Baton Rouge. Still, I think that some sort of consolidation or bedrock may take effect at 80-90 feet. The reference that I read seems to have been a good one. I will have to do more research. Of course, much of the area probably has deeper bedrock levels.
Also see: http://www.salon.com/news/feature/2005/ ... ex_np.html
http://www.orlandosentinel.com/orl-mike ... stemailedl ink
Uncle Flo, Another Correction of My Reports
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Re: Uncle Flo, Another Correction of My Reports
Old Softee--
There is a lot of variation in subsurface strata over short distances in some areas where there has been a lot of river deposition and erosion, and there are also variations in lithification processes. There are records of sandstone forming from unconsolidated sediments, right on the surface, in a few areas, though that is rare. There could also have been some uplift and erosion in the New Orleans area, related to isostatic rebound. As you must know, the sediments offshore in the gulf are tens of thousands of feet thick, and much of that was deposited in a relative short span of geological time. The added weight of sediment can cause uplifting in immediately adjacent areas. This would be similar to rebound in glaciated regions, where areas sometimes rose a couple of hundred feet after the ice melted at the end of the Wisconsin glacial stage. I once lived in an area that was still thought to be rising very slowly, ten thousand years after the last glacial ice melted.
The igneous rock usually described as "basement" is probably a very long way below the surface along the Gulf Coast.
There is a lot of variation in subsurface strata over short distances in some areas where there has been a lot of river deposition and erosion, and there are also variations in lithification processes. There are records of sandstone forming from unconsolidated sediments, right on the surface, in a few areas, though that is rare. There could also have been some uplift and erosion in the New Orleans area, related to isostatic rebound. As you must know, the sediments offshore in the gulf are tens of thousands of feet thick, and much of that was deposited in a relative short span of geological time. The added weight of sediment can cause uplifting in immediately adjacent areas. This would be similar to rebound in glaciated regions, where areas sometimes rose a couple of hundred feet after the ice melted at the end of the Wisconsin glacial stage. I once lived in an area that was still thought to be rising very slowly, ten thousand years after the last glacial ice melted.
The igneous rock usually described as "basement" is probably a very long way below the surface along the Gulf Coast.
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Re: Uncle Flo, Another Correction of My Reports
When I took geology during the early 60s, physical geologists still understood mountain building as the result of isostatic rebound. I took geology just as tectonic theories about plates emerged and the textbooks were already out of date.
I have a strong memory, though I cannot cite a source that there bedrock about 80-90 feet beneath New Orleans. I assume that is not the case over the entire area.
From my mud logging experience, I do not know how deep bedrock was. The geologists knew what was there at least down to 6,000 feet. We were drilling in an old gas field. The people who paid for the drilling wanted to know what was at 14,000 feet. I suspect that the geologists knew well what was there down to 14,000 feet but were not certain exactly were the find gas in every situation. I never saw anything much except limestone and sandstones or mixtures of the two. I do not recall mudstones. We did find gas. The well became, I believe, a producing well. It still functions--I think.
I have a strong memory, though I cannot cite a source that there bedrock about 80-90 feet beneath New Orleans. I assume that is not the case over the entire area.
From my mud logging experience, I do not know how deep bedrock was. The geologists knew what was there at least down to 6,000 feet. We were drilling in an old gas field. The people who paid for the drilling wanted to know what was at 14,000 feet. I suspect that the geologists knew well what was there down to 14,000 feet but were not certain exactly were the find gas in every situation. I never saw anything much except limestone and sandstones or mixtures of the two. I do not recall mudstones. We did find gas. The well became, I believe, a producing well. It still functions--I think.