Tuesday, August 23, 2016

Geo 888: Stranded Terrace Deposits

Overlooking a small cove just north south of Sunset Bay, one can fairly easily spot the buff-colored layer of semi-consolidated sand, about six feet thick, overlying the tilted Eocene beds of the Coaledo Formation. It's a beautiful spot, and a nostalgic one for me. When I was young and hale, I clambered down into this cove a few times; there are some gorgeous sedimentary structures in the strata here. But at this point, with poorer balance and limited endurance, there's no way I'll be down there again unless a decent path is constructed. I have no reason to think that has been, or will be, done. The routes down were precarious, with tree roots and vines, mostly, as the only hand-holds, and the climbs back up were often scary, or worse if I had taken samples (one ten-pound block, in particular, comes to mind).

There's a modest fault running through the cove; this is particularly apparent in the ZoomEarth satellite image. I'm pretty sure the sandstone bed running from the middle toward the lower right is the same as the one on top of the tilted slab on the middle left.

However, the feature the sprang out at me on this trip was the isolated bit of terrace material on top of the anvil to the far right. There a tension between subsidence and uplift in this area. On a scale of millennia, there can be subsidence, as demonstrated by the dead tree stumps on the inner south shore of Sunset Bay. On a longer scale, tens of thousands to millions of years, terraces like this (and six more higher up tentatively identified) clearly show a pattern of tectonic uplift. That little pile of stranded terrace deposits will soon fall as the latest victim to the ongoing ups-and-downs of this area's coastal elevator.

Photo unmodified. July 21, 2016. ZoomEarth location.

Monday, August 22, 2016

Geo 887: Wave Refraction

As an ocean wave approaches shore, it reaches a point where the decreasing water depth causes it to slow and pile up. Wave frequency (number of waves in a period of time) remains more or less unchanged, but wavelength (distance between waves) is shortened, and amplitude (height between crest and trough) increases. Shortened wavelength and increased amplitude finally renders the wave unstable, and it breaks.

The seas on this day at Sunset Bay were quite calm, but the narrow opening to the bay at low tide meant that what waves were coming in off the ocean were well defined. Despite the curvature of the shore, you can see the waves are approaching it nearly perpendicularly across its length (despite the pesky botanical material in the way).

Photo unmodified. July 21, 2016. ZoomEarth location.

Sunday, August 21, 2016

One Year From Today

The blue line across the middle represents the mid-line of the August 21, 2017 total solar eclipse. The shaded area around that line represents the area of totality, where the sun's disk will be completely obscured for a period of time as the moon passes in front of it. The closer an observer is to the mid-line, the longer the sun will be obscured.

I've been excited about this for years; I've never seen a total solar eclipse. I've seen partials that were total or annular elsewhere, but I've never been in the path of totality. For viewing, all I really need to do is be awake and standing in view of the sun, but I'd like to get to a decent elevation. I understand that if you're in a spot with a good east-west vista, you can see the shadow of the moon approaching and receding before and after totality. In Corvallis, the umbral speed will be 1.310 km/sec, or 2929 miles per hour-- which is to say, the fastest predictable thing I will ever see. This interactive map (from which the above screen shot was taken) is the best resource I've found for planning, with extensive details about the event. Simply click the crosshairs on a point of interest, and a table of data will pop up, telling you everything you could want to know about the eclipse at that position.

I have some ideas about where to watch it, but I hope to have opportunities to do some scouting between now and then. It may be that I can get up to the Santiam Pass area and do geology for much of the remainder of the day. In the end, I suspect I'll play it by ear. This time of year, fires and smoke can muddle an otherwise glorious view. However, the chance of rain- or even heavy clouds- in mid-late August is next to nil.

Sunday, August 14, 2016

Geo 886: View of a Broken Shoreline

You can see (barely) the two beds from yesterday's photo behind Gary, and the fault in that photo is the nearly horizontal dark line extending east-west. Less obvious, but quite clear when you recognize what you're seeing, is a larger fault just behind him. The prominent sandstone bed behind Hollie is the same as the one on the far right side of the photo. If the offset on the closer fault is horizontal, and I'm not sure it is, it looks to be about 30-35 feet. Some three and a half years ago, I posted and annotated a shot of this same area, but looking approximately the opposite direction.

On that previous visit, in early March, the wind and chilly temperature combined to make not-so-pleasant conditions. On this visit, apparently, warmer temperatures and less energetic summer waves in July combined to deposit a couple inches of disgusting, sulfurous, black, slimy mud in certain areas of the intertidal zone. We had to walk through that to cross the larger foreground fault. I'm honestly not sure which "downside" was more unpleasant, but in both cases, it was well worth the discomfort. Though if I'd slipped and fallen in that awful mud, I have no doubt I'd prefer March.

Photo unmodified. July 21, 2016. ZoomEarth location.

Saturday, August 13, 2016

Geo 885: Lovely, But Not Without Fault

Sunset Bay is gorgeous, but not without fault. In fact, the whole thing is eroded around a major (inferred, but not directly observable, as far as I can tell) fault that slices through the middle of it. That major fault has quite a number of smaller, directly observable, subsidiary faults, such as the one above. This photo could have been framed a little more clearly; the prominent bed in the lower left correlates with the one on the far right. The bed running through the upper middle correlates with the tiny bit on the left middle. This spot is easily found in what is now called "Zoom Earth." (See crosshairs)

Photo unmodified. July 21, 2016. ZoomEarth location.

Sunday, August 7, 2016

Geo 884: Not Just a Pretty Face

As I predicted in yesterday's post, today's is a bit to the left, and without scale. The cobble in the lower right is the same one my foot was resting on in that photo. Just to the right of that cobble, there are some laminations with cross-cutting features, but they're obscure and poorly focused, so let's look at a crop from yesterday's shot.
Here I've highlighted a couple of the beds that cut across other beds. The principle of superposition tells us that younger beds are on top of older beds, and the principle of cutting tells us the feature doing the cutting-- whether that's bed forms, as here, a fault or a dike-- is younger than the rocks it cuts. The highlighted horizons are cutting the other beds nearby, so those horizons are younger. Since they're younger, we can infer they're on top. Thus stratigraphic "up" is the opposite of the orientation in these two photos. Which is to say (at the risk of being confusing) that "up" is down.

Back to today's photo, just a quick comment: who says rocks aren't pretty? There are some interesting details, but take a moment to enjoy to appreciate the esthetic quality of the exposure.

Here's a crop from above ("below") the cobble:
I'm guessing that the round clast on the upper left was able to roll into place, then draped with subsequent sediment. The bump (keep in mind, stratgraphically, that's a divot) in the right middle looks like a burrow of some sort. And toward the upper right, the tent-like form *may* be a very small flame loading structure. And I'd be remiss in failing to point out that all three of those are informed guesses, with varying degrees of confidence.

Another structure, which puzzles me a little, is visible in both yesterday's and today's shots. (The crop is from yesterday's.)
I *think* that's a dewatering structure; as saturated sediment compacts, the interstitial water can escape along a channel and disrupt the sediments it's passing through. However, I think it's also possible that it might be a burrow instead.

Photo unmodified. July 21, 2016. FlashEarth location.

Saturday, August 6, 2016

Geo 883: Riddle Me This

In this photo from Sunset Bay State Park, north is to the right, west toward the top, and so on. Which direction is stratigraphic "up?" This should be a pretty easy question if you have any experience with geology, but as a hint, look carefully at the cross-beds in the lower right.

I mostly took this photo because it's purdy, but it will reward close inspection. This one has my foot and boot for scale, but the one I'll post tomorrow overlaps this one on the left, albeit without scale. More details then.

Photo unmodified. July 21, 2016. FlashEarth location.

Sunday, July 31, 2016

Geo 882: More Ball and Pillow... But Wait! There's More!

The more I look at this photo, the more little details I'm spotting. Starting with the colorful band of sediments near the top, one can tell from cross-cutting relationships that stratigraphic "up" is to the right. That gray, elongate blob on the bottom of the colorful package looks like a rip-up, a clot of semi-consolidated mudstone ripped from one spot and redeposited in another. It looks awfully big for the flow regime indicated by the surrounding sand, but I can't think of a better explanation.

In the crop below, I've highlighted one of maybe a half-dozen of ball and pillow structures in the photo, and a flame structure- the latter tend to be pointy in the stratigraphic upward direction. While ball and pillow represents denser sediment sinking, flame structures represent less dense sediment rising or being displaced upwards as ball and pillow structures form. Also, above and below those two structures, tafoni or honeycomb weathering can be seen. (We'll be seeing a LOT more of this in days to come.)
If you open the top photo up to full size and look around, you can find numerous small examples of minor soft sediment deformation.

Photo unmodified. July 21, 2016. FlashEarth location.

Saturday, July 30, 2016

Geo 881: Ball and Pillow

Hey boys and girls! Remember me? I'm that guy who used to talk about rocks all the time! I finally got out last weekend to re-familiarize myself with some of them near Charleston, Oregon, at three state parks: Cape Arago, Shore Acres, and here, Sunset Bay.

The main feature I wanted to illustrate here is called ball-and-pillow structure, and makes up the downward lobate features above the lens cap. This happens when denser sediment is deposited on less dense sediment layers. The denser sediment sinks down into the less dense layers. The underlying sediments often push upward to form flame structures, but in this case, subsequent re-erosion has beveled off the upper layers, creating a disconformity. Above that horizon, you can see more or less laminar bedding, punctuated by imperfections created by burrowing creatures, referred to as bioturbation.

Below the lens cap, and just above the toe of my boot, there's another disconformity where flowing water carved out a small channel then refilled it. I should probably brush up on my sedimentary structures vocabulary, but I refer to these sorts of things as "cut-and-fill structures." There's a lot going on here.

My mindset on this trip was to take photos to show how beautiful the rocks and their colors are, but that certainly doesn't mean I missed the science!

By the way, I'm not going to commit myself  to the one-a-day regime I've done previously, but I figured I'd quit referring to years-worth of days, change the name to "Geo Series," and start the numbering after 880, which was the last of the Geo Series posts I finished, on May 30 of last year.

Photo unmodified. July 21, 2016. FlashEarth location.