At one time in the 1960s, the rocks were only thought to be a monotonous set of dipping layers. The US Geological Survey visited the area and studied the old Salem Heights Water District water wells. This work, done jointly with the State Engineers Office (later spun off as the Oregon Water Resources Department), showed an interesting area where the lava flows dipped into a small lake and exploded into shards of black glass (obsidian) and yellow sand.
Soon after that study, water well drillers started using faster drilling methods and drove deep wells into and through the basaltic lava. The geology became better known to geologists and planners. The City of Salem took an interest in taking over the old water district as another source of drinking water. This groundwater resource area became the first or second ASR, (Artificial Storage and Recovery) project in the state.
Continued drilling of the entire Salem Hills area was spurred by rural residential development (Say it, "Rural Residential Development") which meant that more people could live out in and enjoy the Salem Hills. County planning requirements for this growth meant that my company and other geologists started to learn more about the Hills.
Here are a few of the lessons. Let me use the following figure, a cross-section, as an idea aid.
A. The blue part, represents ocean layers. This marine shale is located 50 miles inland, over a mountain range (the Oregon Coast Range) from the Pacific Ocean, but it is 450 feet higher than the present sea level. The rock has been lifted up that far.
B. The green parts are the four lower flows of an extensive lava flow sequence, the Columbia River Basalt Group. The pink piece is a remnant of a flow from the middle of the group. The names for these flows were decided in 1979 by geologists working on a nuclear waste tunnel storage system that was closed in favor of another nuclear waste depository site in the country. The geologists' study of lava flows went across the entire 2000 square mile area of the Columbia River Basalt Group from near the proposed depository at near the Wallowa Mountains in the northeast part of the state to the Pacific coast. The layers were recognized and mapped in the Salem area in 1979 and in 1995. There are six layers where the basalt group is more complete in the Salem Hills. The seventh layer, in this case, does not deliver water because it is weathered to clay. In this cross-section, the dark green zone represents three lava flows. The light green zone represents the weathered flow and a fresh flow below it.
C. The lava flow layers can be recognized and named by observing flow thicknesses, a marker bed and just be counting flows in some places. In this particular area, the marker bed is missing, except in a very few wells. The nearest location where the marker bed, the Vantage Member, is located is about 3 miles to the east along Sunnyside Road. The Vantage and the middle part of the Columbia River basalt was eroded away in the area of this shown cross-section. A small piece of the middle basalt is the pink piece, up the hill, but I haven't been up there to look at the rocks and, perhaps, to find the Vantage.
C. Hard lava flows contain water that can be pumped to private water wells. These particular flows deliver water to wells, more readily, because the water is not tied up in clay. The water issues from porous lava flow tops and through thin rock fractures. The lava flows start as small rivers of molten rock, that give off hot sulfurous fumes and steam. The gas and steam come out of the bottom of the lava flows and hang around in the top five or ten feet. Some of the gas and air bubbles are imprisoned in the cooling lava and form round molds or vesicles. The gas bleeds away leaving the round shapes. The "vesicular" top of the lava flow is weaker than the more solid lower part of the flow and is easily fractured by moving earth forces and, incidentally, by water well drilling. Groundwater is found in these zones to provide the water well supplies that make building a house possible in many parts of the Salem Hills.
No comments:
Post a Comment