I read this morning that on April 19th, 1980, REM played their first gig as REM. They'd played for a friend's birthday party on April 5th, but the band was nameless at that time. (This seems to support that contention) So in honor of one of the iconic 80's bands, here's a few clips. Cheers, and thanks for the musical memories!
One notable feature of the Depoe Bay basalts is the way the pillows are interbedded with sediments. In some spots, such as this one, the pillows appear to be completely surrounded by, and supported in, the sandstone. My best guess would be that discrete pillows formed on the sea floor, then foundered into the underlying unconsolidated sediment. Alternatively, they might be invasive- that is the pillows may have formed by intrusion of lava into the sediment below- but that seems less likely, to me at least, to form such nice, round, and normal-looking pillows. It seems quite unlikely that this was a simultaneous deposition of such quantities of both lava and sediment.
This area is not very accessible; we're standing on a sidewalk about 15 or 20 feet above the outcrop, with a vertical wall dropping down to the exposure. But it might be possible to come in from the south, near the passage into the harbor, and walk up to get a closer look at the nature of this deposit. Both of my preferred explanations would require soft sediment deformation, evidence of which might or might not be visible with a closer look. I'd also want to take a closer look at the composition of the finer material, with an eye toward sorting out terrigenous clastics from basaltic breccia fragments, and also to look for evidence of hydrothermal alteration. The latter might support the idea of invasive pillows. Until then, though, the geology from a distance isn't bad.
Somewhere in the OR/WA/ID tri-state area, a spring surfaced in the Miocene. The liquid did as liquids will, and flowed downhill, rushing across the landscape. Gurgling, bubbling, and chortling across the territory later to become known as Oregon, it charged toward its confluence with the great basin of saltwater we now call the Pacific Ocean. When the torrent finally reached that shore, it playfully plunged in.
And froze solid.
Because this was no ordinary, aqueous, spring, but a vast cauldron of molten rock, likely in the range of 1200 degrees Celsius (~2200 F).
The two likely products of a basaltic lava entering water are breccia- shattered fragments- and pillows. Both are evident at Depoe Bay and in this photo. Below, I've highlighted a few of the pillows, balloon-like structures that form as lava enters the water, are quenched on the outer surface, then continue to fill for a bit.
Looking more or less north from the southern end of the city of Depoe Bay, in the foreground we see the basalt of Depoe Bay. The buff cliffs under the buildings in the distance are composed of sandstone, referred to as the sandstone of Whale Cove (a small cove just to the south), and the darker rocks out on the point are made of Cape Foulweather basalt. Overall, this represents a pair of Columbia River Basalt flows separated by an interval of sedimentation. It wouldn't surprise me to find that work done in the 40+ years since the geological sketch map I posted Monday has reorganized current understanding and nomenclature of these rocks. For example, the sandstone could have logically been demoted to "member" status in the larger and more extensive Astoria Formation. It's source and environment are probably very similar to the latter formation, with the only real distinction being that it was deposited after a basalt flow reached the area.
Looking south-southwest from under the Depoe Bay Bridge, you can see the narrow notch the fleet must navigate to get into and out of the harbor. As hair-raising a prospect as that sounds to me, the lack of major currents- even those of tides, due to the restricted size of the basin- means that it's probably a less complicated and risky proposition than in larger estuaries. I'm pretty much guessing here, but I've heard stories about "crossing the bar" from several other Oregon locations, especially the Columbia River Bar, and I'd bet for an experienced operator, Depoe Bay presents minimal risk. The basalt forming the sides of the channel are Columbia River Basalt, formerly referred to as the "Basalt of Depoe Bay" (PDF). I guess my concern arises from the fact there's not a lot of room for error, and it doesn't look as if it would be very forgiving to any ship foolish enough to run into it.
Under the Highway 101 bridge, at the southern end of the Depoe Bay metropolitan area, one can see the narrow neck of the channel into the eponymous bay. I can't tell, at the resolution of the photos I have, what form the basalt is taking here. I suspect it's predominantly breccia, perhaps with some isolated pillows, based on the patchy white spots. The latter are almost certainly composed of zeolites and/or calcite, which are signature accessory/secondary minerals with mafic to intermediate lavas in wet environments.
Not much "geology" to be seen in this photo, aside from a knob of basalt in the lower left. But make no mistake, there's a lot of geology going on here. I had long assumed (due to pillow basalts I'll show in coming days) that in this area, it was Siletz River Volcanics, the basement rock of the Coast Range. However, given the proximity of this area to Otter Rock and the ring dikes there, it should come as no surprise that this is actually Columbia River Basalt, of Miocene rather than Eocene age. There are flows of two distinct ages, separated by an interval of sedimentation referred to as the sandstone of Whale Cove. Up until the 80's, these Miocene basalts were thought to be local eruptions, though that started to look problematic with the development of Plate Tectonic theory, and even at the time, the close chemical affinity to the CRB was recognized. By the mid-late 80's, a number of the more northerly coastal occurrences were confirmed as CRB, and now, the Miocene basalts along the northern coast, down to Seal Rock, are accepted as flows that made it all the way from eastern Oregon to the coast.
The bay itself is something of an oddity: it's quite small, at six acres, with a very tight entry, as we'll see in coming days. It's clear that this was not eroded by wave energy, and there's no major drainage through it, as is the case with most of the larger estuaries on the coast, but only a pair of fairly modest creeks. A key insight for grasping the nature of most of Oregon's harbors is that during the Pleistocene, sea levels were roughly 100 meters (~330 feet) lower than now, as ice sheets covered much of the northern continents. "The shore" would have been miles outboard from its current location. The implication is that erosion by the two creeks- the confluence of which is submerged by the modern bay- could have a steeper gradient and greater competence than one might assume given all the standing water that one sees today. At the end of the last glaciation as ice sheets melted off, the rising sea swamped what I picture as a steep, narrow canyon, creating a small, but very well protected, pocket bay.
There is a publication in The Ore Bin, from 1971 (PDF), that outlines the geology of the area. With the caveat that the "Miocene volcanoes"are now interpreted as flows from hundreds of miles away rather than local eruptions, it looks pretty good today. The following map is on page 14 of that document.