Tuesday, March 30, 2010

Summer, 2009 Paleomagnetic Studies

During the summer of 2009 we made some progress using paleomagnetic field studies to better illustrate what volcanic units and events we have in the Owyhee River country, and which ones should have better (or any for that matter!) age control.  I will describe the volcanic units in order of increasing age, each followed by thoughts on what I might sample this next summer.  Feedback from you all re: what I should, or should not do, are welcome.  The equal area plot that Kyle put up earlier can be viewed while reading what’s below.

West Crater lava field – from previous years we already had 6 sites, but we added one more taken near Lava Sink Reservoir, on the upper W flank of the flow field.  We never really had a site that was undeniably “in” West Crater flows, and this site solves that small aspect.  It averages with the previous sites quite adequately.

What’s Next? – adding a site to the NW, in the SW corner of Sect. 33 where Kyle and Cooper tell me that West Crater flows thin against what was originally the W side of the Owyhee. 

Saddle Butte lava field – we didn’t add any new sites.  Never got on that side of the river.

What’s Next? – in the averaging of the 6 sites we have in this unit, I leave out the two sites we did on the river trip at Sand Springs Camp (in “old” Saddle Butte), and at river km 38.75.  For different reasons I fear they are not my best work.  I want to replace these two sites by driving and walking over to the river from the W.  I also want to add a site in E Granite Creek to better sample “old” Saddle Butte, to see if it has discernibly different magnetic directions than “young” Saddle Butte.

Forgotten Butte (vent 4526’) – this was one time thought to be a rootless vent of Clarks Butte.  We added three sites that make it clear now that it is different in magnetic direction, and younger stratigraphically.  A site located just N of Bogus Lake, and another located between where the young West Crater lavas and Lava Butte (sometimes known as Rocky Butte) lavas almost touch agree with sites in Forgotten Butte, and not with Clarks Butte.  Because the center of Bogus Creek valley is a patch of higher standing older flows this means Clarks Butte is covered by these flows in the area.  Forgotten Crater is therefore younger.  We know Clarks Butte flows go W through this valley, because we have the terraces downriver.  We have no evidence in hand that Forgotten Crater makes it to the Owyhee River, but we should keep our eyes open.

What’s Next? – adding a site along a W to E road which ends at Dry Lake Reservoir, to the NW of Lava Butte.  I have a N-S contact drawn on the topo map separating Forgotten  and Clarks Buttes, and it ends at about Dry Lake Reservoir.  I want to prove it out.

Clarks Butte lava field – we added a new site along the N part of the field, just S from the Jordan Craters flows.  It was rather badly struck by lightning, and didn’t clean up very well.  Alas!  We have 4 sites in good agreement with one another.

What’s Next? – adding a site along the S margin of the field near Dry Lake Reservoir as described above.  Also, sampling the rectolinear “uplifted” outcrop, parallel to and directly below Bogus Point.  I’m betting this is Clarks Butte.  There are several “old” bulgey outcrops which protrude up from the floor of Bogus Creek valley.  Are these Clarks Butte, or just West Crater flows with a bit of inflated “attitude”?  Or are they Forgotten Butte?Also, we have an argon age experiment in play for this unit.  Age by June!!

Greeley Bar lava field – we added two more sites to this unit.  They average together pretty well with earlier work, and while the mean remanent direction is similar to Deer Park they are arguably slightly different.

What’s Next? – adding one more site up top to the NW near Riley Horn Reservoir where Greeley Bar overlaps the old shield volcano which has collapsed northward into the Hole in the Ground.  Also, getting down onto the Greeley Bar bench and sampling whatever units we can find there, that we can sample (I mean actually stand next to and drill), and get the sun to shine on the sun compass.  Not too much to ask, is it?  We have an argon experiment in play for this unit, data by June, we hope.

Deer Park vent – we took our first two sites in this unit.  An ordinary sort of reversed direction is found, that is similar, but perhaps not the same as Greeley Bar.  The two units can be petrographically differentiated in the field; Greeley Bar is quite phyric in plagioclase, while Deer Park is more aphanitic.  Deer Park underlies Greeley Bar; it made a dam across the Owyhee River first, perhaps numerous decades earlier.  Does anything we have seen from above, or from river level confirm that Greeley Bar flows into a lake not of its own making?

What’s Next? – adding another site at the N base of the Deer Park lava cascade, just above where it turns into a cliff.  Will it have the same petrography?  Will it have the same direction?

Owyhee Butte – we took our first three sites in this unit.  We found normal polarity, so these flows cannot correlate to Bogus Point, as originally mapped.  The 1.86± argon age is within the Olduvai Normal Polarity Subchron so our age work seems valid.  Owyhee Butte flows W forming a lava delta seen from the rim and from river level.  The topographic continuation of the transition between subaerial lava, and subaqueous pillows and fragmental basalt seems to be just below 3800’ elevations.  You can follow it SE to Eiguren Reservoir #2, and NE to near the bend in White Rock Creek.  To the W Owyhee Butte seems to overlie reversed polarity Bogus Point lava flows, and reversed polarity Little Owyhee Butte lava flows.  There is a fair possibility that yet another (older?) basalt underlies Owyhee Butte between Bogus Point and Little Owyhee Butte.  It would be the E rim of the Owyhee canyon from about the falls of While Rock Creek to the latitude of Eiguren Reservoir #1.  Owyhee Butte crowds S onto Little Owyhee Butte and deflects W and SE from that topography, and possibly makes it all the way to the N side of Jordan Creek W of Arock.  Where Owyhee Butte lavas rim the W side of Hidden Valley they form a scarp not dissimilar in elevation to the scarp formed to the W.  I don’t know if any of this margin has pillows and fragmental outcrops, but does this possibly mean that Owyhee Butte was flowing eastward into standing water also?  Or just across relatively dry mud flats?

What’s Next? – adding two sites on the E side of Owyhee Butte, NE and SE and investigating the nature of the eastern rim above Hidden Valley.  Any evidence of water interaction?  Also add a site just E of the main N-S road, and E of Eiguren Reservoir #2.  SW margin of Owyhee Butte loses simple topographic expression; do flows make it to the Owyhee River Rim?

Arock summit volcano – we took a site at the summit of the Arock shield.  I had loudly trumpeted last Spring that it was the vent for the Bogus Point lava flows.  WRONG!!!!  It has ordinary normal polarity, and thus can’t correlate.  I loosely imagine these flows to be Olduvai in age, but without much conviction; they could be much older.  I believe the Bogus Point lavas come out from under this shield.  The Arock lavas seem to encroaching southward onto an older vent I am calling the Juniper Tree Reservoir vent, which is just N of the Merrill Springs Rim.  It is this vent that is my current best guess as the source of Bogus Point flows.  But we already know how good my guesses are!

What’s Next? – finding where the margins of this volcano are.  I will take on site on its NW margin in the ephemeral drainage next to West Crater, probably at about the 3900 to 4000’ elevation level.  I will also take a site up NE from the road through White Rock Creek valley, probably again at about the 4000’ elevation level.  Lastly, I will take a site or 2 E and SE of Bogus Creek Well.  Only a poor, possible topographic signature of the W edge of Arock lavas exists.  Do those flow lap onto the gravels at and N of Bogus Creek Well, or are they more likely lapped by the gravels?

Little Owyhee Butte – we took 6 sites from lava flows from this vent.  It has an ordinary reversed polarity magnetization.  They group adequately, but better when I exclude the site done on the main road coming up from the bridge (possibly rotated southward at edge of rim), and a site done NW of Eiguren Reservoir #1, that may be in a different, older(?) unit.  The K/Ar age for this vent is near identical to Owyhee Butte at 1.87± Ma, and must be too young, and at least 1.94 Ma in age, the age of the onset of the Olduvai Subchron.  At a site we did above the rim of Jordan Creek that I call the “gooseneck” we saw a glorious expanse of subaerial lava flows over a southward directed pillow sequence.  The transition of subaerial to subaqueous is at or below 3700’ there.  Cooper says pillows exist at the SW margin of Little Owyhee Butte flows where they rim the Owyhee River opposite where Crooked Creek joins the Owyhee.  They would also seem to transition, at or below 3700’.  We need to get some real measurements of these elevations!

What’s Next? – we already have a good population of sites.  I will add one at the small “vent” located where the main road splits from the road to Owyhee Butte.  I am guessing it is a Little Owyhee Butte “rootless” vent, but I am (0 for 1) on those guesses.  I will also investigate the W side of this flow field, as it rims the Owyhee River.  Is it lapping onto another reversed polarity flow from another source?

Threemile Hill lava field – we have one site in this shield, and didn’t add any last summer.  Its’ argon age of 1.86± Ma again has to be incorrect, and probably too young because the flow has reversed polarity.  This volcano seems a long way from the river.  But, it is relatively young and may be part of an episode of volcanism that includes other shields which do interact with the Owyhee River and “lake”. 

What’s Next? – adding a site at the far N end of this shield volcano. 

Bogus Rim lava field – we added 4 sites within this field, and they average together fairly well.  The argon age for this flow at 1.92± is nearly correct, considering the uncertainty level.  Our final new site was at the rim in Rinehart Ranch, so we have paleomagnetic confirmation the flows go at least that far.  Bogus Rim seems to be the nth player of a sequence of big flows, and we need to know how much time they all take.  Restated, the Owyhee River had done a fair job of downcutting by (??) Ma, and a sequence of basalt flows and sediments, ending with Bogus Rim, lifted the rivers base level and said “please start again!”.  How long did that take? 

What’s Next? – adding numerous new sites including; S side of river opposite Rinehart Ranch mesa, drilling whatever can be reached on the NE side of Jackson Hole (top flow must be Bogus Rim), near Cave Reservoir in Mud Creek, near lower Fort Creek Reservoir, just W of the “other” Cave Reservoir in the middle of Bogus Rim, and finally the outcrops perched above Birch Creek Ranch.  We should submit our own argon sample for this unit; it is too important stratigraphically to us to leave our age understanding where it is now. 

Middle “gray” flow @ Rinehart Ranch – we took a site in the so called “middle” flow of the rim at Rinehart Ranch.  In the gigapan that Kyle posted of that outcrop you can see Kevin Geipel, my field assistant, and I leaving that site.  It has an ordinary reversed magnetic polarity, that turns out to be quite similar to the overlying Bogus Rim direction.  I could interpret the directional proximity to mean time proximity of only a few decades or so.  But, we believe the direction similarity is a coincidence, as the colluvial wedge, with capping soil that separates them must have taken 1000’s of years or more to form, plenty of time for the geomagnetic field to go wherever it wanted to go (maybe several times!), and then come back.  But it does mean that an earlier stratigraphic unit than the Bogus Rim lava, has a magnetic direction difficult to tell it from.  The “gray” aspect of that flow arises from the profusion of plagioclase in the basalt; hopefully that aspect will stay systematic and assist differentiation.

What’s Next? – when we are on the NE side of Jackson Hole, I hope to run into this plag-phyric flow under the Bogus Rim lava flows there.  At Iron Point, Kyle and I could look down to a lighter gray outcrop, with rounded corners bounding joint surfaces, that I think might be the middle “gray” flow.  Also, on the W side of the Owyhee River are outcrops of Bogus Rim, underlain by something else.  Kyle was showing Cooper and I pictures from the upper parts of Jackson Creek of these two units.  Sites could be located in these units if the logistics of getting there isn’t too adverse.  We have an argon experiment in play for this unit.  By inference it will suggest what a good age for Bogus Rim should be also.

shield volcano under Arock – Bogus Point lava flows – we added three sites to this unit.  They average together well, having yet another ordinary direction of reversed polarity.  I estimate the age of this unit to be 2.1±0.2 Ma, but without a lot of conviction; it could be rather older.  A unit just to the E, that Ninad Bondre called “Bernie’s Bench”has an age of 2.8Ma, and that might be the appropriate age frame to be thinking of.  This unit is in for an argon age date, and that is very important.  We need to figure out from which vent this unit comes from.

What’s Next? – numerous additional sites to be taken.  An area I am calling the Juniper Tree Reservoir vent, may be the source of the Bogus Point lava flows.  It seems to be under the Arock shield.  Also to their S, the Merrill Springs Rim may be Bogus Point in character.  We also want to sample the unit under Owyhee Butte lava flows 2 mi NW of Eiguren Reservoir #2 to see if they are still Bogus Point, or some other unit.  Lastly, we want to sample the basalt ledge on the N side of Bogus Creek, under the white sediments, under Bogus Rim.  These almost have to still be Bogus Point lava flows, but we want to make absolutely sure.

“Tertiary” basalt above Rome – we took a single site along Hwy 95, just E of the Owyhee River, where the road begins its steep descent to cross the bridge over the river.  The rock was coarsely diktytaxitic, oxidized, and looked more than a bit tired, so I am guessing an age of about 4 Ma for it.  It has a rather odd reversed polarity magnetic direction, but needs other sites to confirm this is not an outcrop stability issue.  This all may be important with regard to several vents in the Rome basin area, which have 30-year-old K-Ar ages between 1 and 2 Ma that Bill Hart and others generated.  These experiments all need to be replaced, but they still likely provide quality age inferences.  These “young” basalts may well have interacted with the Owyhee “lake” post-Bogus Rim time, possibly for numerous 100,000’s of years.  I will be sending another letter regarding this idea/opportunity.  To the E and SE of Rome is one of these “young” basalts which seems to vent from Round Mountain, in the Scott Reservoir quad.  These flows are really topographically above the Rome basin, but the present course of the Owyhee River is determined largely by the southern and western margin of this flow field.  The western rim of this field above the Owyhee River seems to be relatively horizontal and just below 3700’, which is within the basin. 

What’s Next? – adding sites to confirm that the site I took IS in older basalt flows, and that “young” Round Mountain flows lap northward against them before being deflected NE toward Dry Creek, and W toward the area just S of the Owyhee River bridge.  And, some sort of inspection of the edge of Round Mountain flows should be conducted.  Can we see evidence of flowing into standing water?  Interaction with flowing water?  Did these lava flows fill the preexisting canyon of the Owyhee, and move it S and W?  By its’ morphology, Round Mountain is a fairly young volcano, so its’ impact on the Owyhee River is really quite recent.  More on this topic later.

2 comments:

  1. Posted for Duane Champion, USGS

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  2. Duane,
    This is an impressive review of your observations and interpretations to date! I look forward to assisting with the collection of (at least some) of the many samples from the additional sites on your to-do list. Perhaps, with your guidance, we could also develop a somewhat parallel list of recon priorities that Kyle, others, or I could visit to scout out some of these sites for access and suitable sample locations in advance of your drill. Thanks for sharing!

    -Spud

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