Dwight, Great write up. I assume you're aware of the subsidence issues in the Houston area. All of the processes you discuss below seem to be occurring down there with the addition to the removal of oil and gas and now water causing a compaction of the clays as water is removed and resulting in subsidence. It has resulted in the flooding and abandonment of some subdivisions and ruptures of public service piping such as sewer lines and water lines were they cross the fracture planes.
The USGS and the Harris Galveston Subsidence District have been doing leveling down there for years. While there isn't much that can be done about the general subsidence of the gulf on a geologic time line, they are very considered about over pumping and subsidence related to groundwater removal and have pretty much forced the entire area off of groundwater. A couple feet loss of elevation in the Houston area could be significant when the next Class 5 hurricane comes ashore. Very interesting problems and I guess if your house happened to have been built on one of the faults, we'll, it probably wouldn't be so good. I need to find someone down there to give me a tour some time. Geary -----Original Message----- From: dirt...@comcast.net [mailto:dirt...@comcast.net] Sent: Friday, August 24, 2007 11:01 AM To: John P. Brooks; Texascavers@texascavers.com Subject: [Texascavers] Texas Gulf Coast Faults Texas Gulf Coast Faults Are real faults. The Earth has broken and rocks on either side of them have moved relative to each other. There are a lot of them. They are not mud cracks! A mud crack is a shrinkage phenomenon where the surface layer of the mud has contracted as it shrank. Cooling hot rocks can do the same thing and produces columnar jointing seen in igneous rocks (columnar basalt as at Fingal's Cave, the Devil's Postpile, or Devil's Tower). A fracture or joint is where the rocks are broken but not displaced. Fracture traces are where there is a swarm of fractures closely spaced. Tens or a hundred instead of just one. But still no relative motion across the fracture, just an incipient opening. A good place to drill your water well as the yield will be better. If faulting occurs, joints and fracture traces are weaknesses in the rock and an actual fault may propagate along a series of fractures. A fault is where the earth is displaced and there has been relative motion across the fault. It means that there is differential stress on either side and movement along the fault releases that stress. It is like bending a stick until it finally breaks. The sense of motion can be vertical or lateral, or a combination. When the opposite sides of the fault actually move, an earthquake occurs. That said, most faults are not the gigantic ones like the San Andreas where there has been continued motion of hundreds of miles of displacement over millions of years. Most faults along the Texas coast are places where adjustment (relatively small movements along the faults) has taken place over geologic time as the coast is gently raised and lowered, loaded by the accumulation of additional sediment, unloaded by erosion, or it is alternatively loaded and unloaded as sea level rises and falls. Drilling and geophysical data show the faults quite well, but they are hard to see at the surface. The surface fault traces are usually places where there is slightly more ground moisture and a few more blades of grass per acre. If you look carefully, you can see that! I remember when I worked for the Texas Bureau of Economic Geology we laid out a large mosaic of air photos on the gym floor. If you got up on a step-ladder, you could see the fault and fracture traces delineated by slightly greater soil moisture (or something). They were shown as subtile lines most probably caused by more grass, lines of better tree and shrub growth, and slight alignments of small drainages. It takes subsurface data to determine if actual fault displacement has occurred along any given line (linement). We did that exercise to locate possible faults in relation to proposed siting of a nuclear power plant. That said, these are not locations where there should be great concern. However, when the inevitable adjustments of the Earth's surface occurs, breaks will most likely occur again along one of these faults, where it has broken before. There is not much problem for individuals, unless your house happens to be on top of one of them when does move. Not very probable in your lifetime, but it could happen -----. All things are relative, and driving to a cave is a LOT more dangerous! DirtDoc --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com For additional commands, e-mail: texascavers-h...@texascavers.com --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com For additional commands, e-mail: texascavers-h...@texascavers.com
