Wednesday, April 28, 2010

Young Basalts in Yeehow-ville: Even more ruminations from Duane!

[all...once I clone myself, I will add annotated images to these discussions...Kyle]

The Junction of Crooked Creek, and the Owyhee River

The air photos help to see more details in the vicinity of where Crooked Creek joins the Owyhee River. This is an area that Kyle, Cooper, and I have briefly mused over. I think I see three (3) different young basalt flows in this area. There is an odd “comet-shaped” lobe of lava which enters Sect. 31, T30S, R41E located in the SW corner of the Owyhee Butte quad from the SSW. There it has an omni-directional scarp which rolls over at about 3640’, which is lower than adjacent surfaces to the N and SE of the lobe. This lobe seems to be a northerly extension of the lower of the double outcrop that Bill Hart argon dated just N of the State Wayside Park, at 1.25 ± 0.28 Ma. The younger of the two E-facing scarps that Hart dated at 0.91 ± 0.36Ma, is located just a half mile to the W of this odd shaped outcrop, and forms a surface that rolls over at the higher elevation of 3720’. A third surface which enters this same immediate area from the E, coming from Little Owyhee Butte across the present course of the Owyhee River, seems to directly overlie the odd shaped lobe, and rolls over at a higher elevation of just lower than 3700’, and is separated from it by thin, light colored sediments.

So what do we have here? I think the odd shaped lobe is stratigraphically oldest, probably the Little Owyhee Butte lavas from the E are next, and that the E-facing scarp and surface to the W is the youngest. There is an odd meander in the course of the Owyhee in Sect. 32, T30S, R41E, rather square in map view, that is created by the eastward continuation of the lava flows of the odd shaped lobe, passing underneath the Little Owyhee River lava flows. I have a paleomagnetic site, in what I thought was Little Owyhee Butte lava flowsjust N of this area, that doesn’t fit so well with the other sites. I now wonder that it samples not Little Owyhee Butte, but the lava flows which come into the area from the W at the highest level. More sites on both sides of the Owyhee River will help decide this. Although surface degradation is the general process of this area through the last 4 Ma, these 3 units stack during a period of aggradation. I wonder how long it lasted? The NE projection of the oldest odd shaped lobe, and the W projection of the Little Owyhee Butte lava flows I believe is telling us about the location of the junction between Crooked Creek and the Owyhee River at the time of these eruptions. Not really that far from the current junction, located just to the SE.

Lastly, another curiosity for investigation to the south. In the area just N and E of where Crooked Creek crosses Hwy 95, the creek flows in a fairly sharp canyon for about 3 miles. The E and W rims of the canyon are at very comparable elevations, and we know that Bill Hart sampled the W rim from basalt flow outcrops for his argon age experiment. So I guess the E rim will probably be basalt lava flow outcrop too. But, that dark surface and dark layer passes laterally to the E all the way to the Cliffs of Rome, where I believe Caitlin showed me the odd erosional sediment columns of that area. I half remember the dark layer, but I remember it as a coarse fragmental layer, and not as any kind of lava flow. Can we see the transition of one of these basalt flows becoming deltaic, then fragmental, proceeding to a cemented black basaltic sand layer pro-delta? Is the transition from flow to fragmental about where a N-S canyon has be cut through the layer just SSE from Crooked Creek Ranch? The Rome 7.5’ quad shows quarries in this layer. What are they quarrying? Broken basaltic fragments just the perfect size for road metal? I can’t wait for the roads to dry out!

Area East of Saddle Butte

The air photos have clarified my thinking about “young” vents E and N of Saddle Butte. I believe the W-E “breadloaf”of lava located just S of Wrangle Butte is in fact a badly eroded isthmus between a lava flow field from vent 4373’ in the extreme NE corner of the Saddle Butte 7.5’ quad, and a S-sloping lava flow field bounded by Bull Creek Canyon, the Owyhee River, and a S-facing scarp located just N of Ryegrass Creek. Some field inspection S of Wrangle Butte near Top Hat Reservoir is required. The S margin of the flow field from vent 4373’W of this constriction is either Bull Creek, or a parallel drainage just to its south. Near Ryegrass Creek the lowest scarp from this flow rolls over at about 3780’ which is low, but not as low as other “young” scarps. Therefore, rather young, but perhaps not as young as 2 Ma. This lava flow field looms over Chalk Basin, and projects eastward forming the Basin’s S boundary in the Lambert Rocks 7.5’ quad. The last basalt flow outcrops appear again at about 3780’ in this area, but E and forward of those outcrops are broad, smooth patches of strongly oxidized sediment(?) that I wonder are palagonitized fragmental basalt and basalt sand. Is this another place where we can inspect the transition from subaerial basalt flow to lake sediments?

The photos have helped me decide that there is a basalt shield at the E base of Saddle Butte, with a vent at 4302’, that forms a lava flow field to the E and S, which rims the prominent SE-facing scarp that dominates the Wrangle Butte 7.5’ quad. This scarp breaks over at 4120’, is thus rather high and probably not all that “young”. As if I actually know what that means! The N end of this scarp is at Chanis Rock, which is the last place to actually see the rim basalt flow clearly. The N boundary of this flow field extends W from Chanis Rock, and is an important part of my reasoning that these flows come from vent 4302’, and not from Saddle Butte itself. There is thus a fat septum between this flow field, and the one described in the previous paragraph composed of rolling topography sloping E off Saddle Butte. This must be even older ground.

Lastly, the photos helped me note that S of Ryegrass Creek, in and amongst the terminating lobes of the 144 ka Saddle Butte lava field, is a low E-facing scarp. The 144 ka flow dump over this scarp, and mask it to some extent, but it extends all the way to the Owyhee River, and forms the walls of the unnamed canyon which contains Sand Spring, and a minor lobe of the 144 ka flows. Anyway,,,, off to the SW I lost view of a NE directed lobe of lava from the 4257’ vent in the Palomino Lake 7.5’ quad, because it was covered by the lava flow field of the 144 ka event. Is this scarp the terminus of the older flow lobe? This scarp rolls over at about 3700’, or perhaps a little lower. The lower elevation outcrops are softer, rounder, and subdued, making me wonder that again we can witness the transition from sharp, blocky subaerial basalt flow, to lava delta, to fragmental layer, to glassy, basalt sand fringe. Easy to get there and take a look. Other scarps associated with vent 4257’ lava lobes, roll over at elevation quite a bit higher than 3800’, so perhaps it is incorrect to associate these lower elevation scarps with that vent. Start the drill! Let’s see what correlates.

Sacramento Butte 

High ground 2 mi east of Sacramento Butte, that I am calling vent 4411’, seems to be the source of basalt flows with high elevation margins. Consequently, it may be rather old. Lava flows from it embay Sacramento Butte from the E, and pass both S and N of Sacramento Butte flowing W. An easily identifiable lobe is directly S of Sacramento Butte in the SE corner of the Sacramento Butte 7.5’ quad, and ends at a scarp rolling over at 4220’. Lower and Upper Clark Reservoirs are in the drainage that separates this lobe from Sacramento Butte. The vent 4411’ lava flows loom over, and form the W rim of Sand Basin and are found at variable elevations near 4300’. This scarp is erosional, and has to have retreated westward at least to some extent. Again, these flows are way up in the air, and may be 4 Ma old. The square shaped high mesa that looms over Rinehart Ranch on the W, may be a continuation of this flow field. My one-time belief that that mesa is just some “old stuff” up in the air is now gone. We will sample the square mesa, and tie it to one of these pre-Bogus rim flows.

So what to make of Sacramento Butte itself? Topography associated with it dominate about 6 sections that slope SW and end at Clark Ranch. A subsidiary knob at 4341’, on Sacramento Butte’s N flank, may be another different basalt vent. If that is the case then the Butte is even smaller aerially! This combined with George Walker’s Oregon state map data that Sacramento Butte is actually not a basalt, but somewhat evolved, makes me wonder that it is an old, masked volcanic center, and not anything of interest to our post-4 Ma geomorphic concerns.

The “Gooseneck” of Jordan Creek

It looks on the air photos that the lava flows from Owyhee Butte loop SE and behind Little Owyhee Butte, and may well make it all the way to the N side of Jordan Creek canyon, to the SW of Hidden Valley. When Kevin and I took the 3 paleomagnetic sites in the area, we just missed stumbling into this awareness, by only a couple hundred meters in one case. So we will go back and put a site into these flows and see what’s what. An odd flat bench, on the SE side of Jordan Creek, may be a continuation of these Owyhee Butte lava flows. They collectively have a SW slope, which does not make much sense for Little Owyhee Butte, but may for Owyhee Butte. An alternative possibility is that these are older flows, related to the NW sloping topography on the SE side of the creek. Doesn’t look hard to get there.

Diamond Butte area – Downstream from Birch Creek pullout 

I note something on the photos of this area, which I’m sure Kyle probably already knows. He knows everything first! [check these lavas out in one of my (Kyle) gigapans:] About a mile and a half downstream from the meadow at Birch Creek Ranch at which we pull out, are some very interesting outcrops on the E side of the Owyhee River. They are best expressed in the W half of Sect. 5, T27S, R43E. They are a doublet of basalt ledges, the upper one of which extends N almost 3 miles. One of them has to be Bogus Rim, but who is the other one? Better still, farther N on the extension of the upper ledge in Sect. 28, T26S, R43E is a knob with spot elevation of 3235’ that looks HIGHER than the upper ledge in the photos. Could we have three (3) ledges here?

I think we can walk to these outcrops, but it may not be easy. There are two reaches of the Owyhee River that are cliffs plunging right into the water. A jeep trail would get us part way N, but we will have to walk. We can pick up the water late, only at the end of the walk, and carry it uphill to the ledges. Paleomagnetic work here, combined with magnetic work on the ledges W and directly above Birch Creek Ranch, should make quite a package.

vent 4569’ – SE Deer Butte Rim – backside of Bogus Bench 

This vent was featured to some extent in Bondre’s recent dissertation. It is in the NE corner of the Bogus Bench 7.5’ map. You can drive right past this vent on the way to Greeley Bar vent and hardly notice it on the ground. But, it has a wonderful, and I think youthful, expression in the 7.5’ topography, breached to the W. No basalt lava flow field to any easy witness, just the cone. The Miami University folks (I don’t remember who now) have a 40/39 age on this at about 5.5 Ma. My guts say no way! I believe that I have UTM coordinates where the sample comes from. I believe that I have loosely played with those coordinates, and that the sample site is W of the cone (which appears in photos to have NO outcrops) from the rim area to its W. If this is true, then the dating sample comes from a potentially scary place. The rim there is a continuation of the rim S of Deer Butte, and looms above Mud Creek at its SW base. Recent age dating puts Deer Butte at about 4 Ma, not 5.5 Ma. Possible that the age experiments are a bit screwed up. But it is also possible that the 5.5 Ma age sample comes from a lower ledge W of the 4569’ cone. S of the 4569’ cone the rim is the lower flow, N of the cone the rim is the Deer Butte flow field. Shannon and I sampled that rim very near Deer Butte Reservoir #2. I will plot the age sample location and see what I see. The lesson here: NEVER let your petrologist collect your stratigraphic and age samples! KEEP THEM IN THE LABORATORY generating data on 40 elements to two decimal places of precision. We will visit this cone this summer(?) and try to sample it. Inspection of the air photos does not lead to optimism.

SW corner of Bogus Bench Mesa 

Cooper has called attention to an odd “cascade” appearance to the SW corner of Bogus Bench Mesa. It looks like the Bogus Rim lavas(?) have dumped down over and subdued the sharpness of the rim there. The source of these flows would be knob 4432’, perched right above the edge. But that ridge really is a continuation of vent 4487’ a half mile to the NE. In my “mapping” of Bogus Bench, I separated vent 4487’ from the rest of the rumply relief which seems to be the vent area for the Bogus Rim lava flows. Those vents/flows seem to embay vent 4487’, and are thus younger. At the foot of Bogus Bench is a bench of lava flows that, with a equal grade mirror bench to the S, pinches the Bogus Creek drainage near Bogus Lake. One could entertain the idea that the cascade down from vent 4487(?) makes these benches, which roll over at about 4100’. This places then 150’ higher than the Bogus Rim outcrop that Cooper and I sampled in 2006 at Bogus Falls. OK, higher bench,, older flows,,, vent 4487’,,,, older than Bogus Rim vent!!!!!

ONLY PROBLEM is that the bench S of Bogus Lake is a continuation of topography extending 3 miles E that has been buried by Clarks Butte, Forgotten Butte, and finally West Crater lava flows. The bench really appears to have come from the E, and thus can’t have arisen from any cascade over the SW rim of Bogus Bench. If it comes from the E, it might come from the relatively young Threemile Hill vent, or possibly from the “other” Saddle Butte. I suppose the Arock summit vent is also a choice, by a somewhat convoluted flow path. I also suppose they could correlate to the Bogus Point lava flow, but their higher elevation here would have to be explained away, by a dramatic slope to the W of the topography of that time.

Older topography in this area scares me a bit, as we know that significant down to the N faulting occurs just to our S from this area. Is the old topography in place, or foundered in big blocks to some extent? Tough to know. The air photos give no hint that the bench closest to the SW corner of Bogus Bench is part of the stratigraphy UNDER that bench. I think it is perched against that rim from the S, and thus substantially younger.

I have lost track of the full count of all the possibilities up to this point. TOO MANY CHOICES. What I plan to do is sample the N bench at the Bogus Creek pinch point, and sample the the S bench of the pinch point where the road S to Arock summit crosses the younger, overlapping lava flows. A quarter mile walk W puts me on that topography. We’ll see if they seem to be the same, and what they might compare to. Then, we need to get up on the extreme SW corner of Bogus Bench to take a couple of sites. I am hoping that the road to Bogus Bench Reservoir has been extended since the 7.5’ map was put together in 1972, and that we will only have a short walk. Spot elevation 4371’ looks to me to be a place where we can sample the Bogus Bench which is older, and NOT Bogus Rim lava flows. Just across the ephemeral creek to the N is the foot of the topography from vent 4487’, and maybe we can find an outcrop there. If not, there seems to be a S-facing ledge just W of vent 4487’ that looks like a possibility. So many plans!!
Arock shield – Bogus Point lava field

I have looked hard at both sets of air photos, and for the life of me I can’t see how the Arock summit shield merges with the underlying Bogus Point lava flows. There are a couple of linear gashes to the E of Bogus Creek well, which offer the chance of getting into the flows. But, they seem to be continuations of the W to E faulting that crosses the NW flank of Owyhee Butte, and may be inappropriate places to sample. Kevin and I drove the entire W flank of the Arock shield, and while we saw piles of rocks, we did not have outcrops jumping in front of the truck. We will snoop around, but do not have any concrete plan of action at this point. 

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.