Monday, April 14, 2014

Geo 730: April 14, Day 470: Depoe Bay Geology

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.
Photo unmodified. July 10, 2012. FlashEarth location.

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