Young Basalt Fields in and near the Rome Basin

A continuation of the Owyhee River Project, into a somewhat older time domain, offers an opportunity to use the same mix of riverene hydrology, geologic mapping, sedimentology and slope stability, volcanic geology, chronology, and lastly paleomagnetism studies, that were used in a rather younger timeframe. Understanding of the evolution of the Owyhee River system from some network draining a Miocene volcanic upland toward a large freshwater lake in SW Idaho, to an entrenched stream following the Snake River toward the Columbia would be the target we would be after. Lava flows and landsliding, perhaps at an initial “monumental” scale could have impounded lakes along the course of the Owyhee River that might have persisted for some time. I am interested in this work just to figure out the number of separate volcanic events that may be involved. Their separate directions of magnetization are of use to me. Hopefully, you will also find reason(s) to wish to pursue this work.

I have Kyle’s air photos now, and am looking into various “situations” I describe below. I will have additional comments within a couple of weeks to add to those below. So get your 7.5’ maps out, and follow along with my thoughts.

I will discuss these “young” lava fields, as they seem to group/align spatially (and probably temporally as well!) from the NW toward the SE.

NW of Saddle Mountain

There is a SW to NE alignment of young basalt shield volcanoes, to the W and N of Saddle Butte that morphologically seem quite young. The pressure ridges and original topography of the flow field are still evident, and remind me of the general appearance of the Greeley Bar flow field. There are 4 different vents named (in the absence of any real geography) vent 4528’, vent 4556’, vent 4521’, and vent 4373’. They appear in the SE corner of the Turnbull Peak quad, across the Saddle Butte quad, the S half of Mustang Butte quad, and the SW quarter of the Sacramento Butte quad. Their lava flows collectively move S and E to form the N margin of the 144 ka Saddle Butte lava field, embay Saddle Butte proper, and flow off to the NW and NE making the SE margin of “Barren Valley”. I believe they embay an older Mustang Butte, but I need to look at the air photos to be sure. Brennan Jordan argon dated one of these flows along the road to Rinehart Ranch, before you even get as far in as Crowley. He obtained an age of 1.40 ± 0.04 Ma from BGC. Pretty damn young!

Why should you care? – How these flows “finish” to the E is not clear to me. They may have interacted with “lake(s)”of the Owyhee River in the Wrangle Butte quad area. One flow tries to sneak S of Saddle Butte, but seems be be pinched off by Saddle Butte topography W of Rattlesnake Cave.

Another flow clearly is moving SE past the SW of Wrangle Butte, but what happens to it in the Bull Creek drainage is again unclear. The topography between Wrangle Butte and Chanis Rock also seems odd. It is kind of an E facing scarp, and a continuation of the more pronounced E facing scarp to the E of Saddle Butte. I wonder that an W-E “bread loaf” of topography between Top Hat Reservoir and Bull Creek is an eastward continuation of these “young” lava flows. Farther E a platform of flows, now S of Bull Creek canyon, form the W and SW margin of Chalk Basin. This platform is asymmetric across Bull Creek canyon, with the S bank breaking over at about 3940’, and the N bank breaking over rather higher at 4060’. Is the end of the “younger” flows pinned to a lower(younger) base level? The platform slopes to the S another 4 miles, ending in a S-facing scarp just N of Ryegrass Creek. There these lava flows are asymmetric across Ryegrass Creek, with the 144ka Saddle Butte lava flows on the lower S side. These flows also shoal against a SW-NE ridge of older topography located just N of Ryegrass Creek, and just W opposite Lambert Rocks.

Lastly, the topography just SE of Saddle Butte catches my eye. Are these Saddle Butte flows forming the prominent SE-facing scarp N of the Saddle Butte lava field? Or, is there a younger shield, broadly centered in the SW corner of Sect. 17, T29S, R40E. I hope the air photos will help me decide, because in Google or Bing satellite images it is not clear. I believe that Saddle Butte is composed of somewhat evolved lava flows and not actually basaltic. Perhaps we can tell just from hand specimen where these flows come from.

Sacramento Butte

The topography of this butte also catches my eye. It forms a prominent W-facing scarp, framing the E side of the Barren Valley. We drive partly over its surface on the way to Rinehart Ranch. It did not strike me as particularly young, but it did not seem as old as Deer Butte either. While its source vent is clear, the topography N and E of that vent suggests either significant erosion, or that another unnamed vent area is 2 miles ENE of the vent. Are these “two” vents allied in time? That “vent” forms scarps with breakovers at and above 4300’. While high, this is still lower in elevation than Deer Butte, which is only 4Ma old. Is this vent younger? Sacramento Butte’s W-facing scarp breaks over about 4160’; does this mean it’s younger, or was this just the sort of relief on the “upland” surface? I am curious about these volcanoes. They are probably older, but perhaps by not that much.

W of Iron Mountain

Several shield volcanoes in the vicinity of Iron Mountain may be rather young. Farthest W is vent 4838’ which is located at the W side of the kipuka within the 144 ka Saddle Butte lava flows that we cross on Hwy 78. Bill Hart may have an argon experiment on these lava flows. These flows move SE and end in the area of Palomino Lake. The relief of this flow field surface strikes me as rather young.

Another vent, called vent 4257’ in the SE corner of the Palomino Lake quad, is perhaps a major source of “young” lava flows. It passes N of Iron Mountain, and bifurcates into two E directed arms around some old topography. The northern arm seems to end on a broad E slope to the S of a minor flow projecting E out of the Saddle Butte lava field. The southern arm continues E well past Iron Mountain, into the Owyhee Butte quad where I fear it hides under thin sediments, possibly all the way to the present course of the river. This southern arm has N-facing and E-facing scarps that break over at about 3840’ that both seem to die out as they pass into the Owyhee Butte quad. Again, are they masked by sediments, or do the flows pinch out?

Another arm of flows from vent 4257’ passes W and S of Iron Mountain, with Palomino Creek probably forming its SW margin, going all the way to Burns Junction. This arm bifurcates around Scott Butte, passing eastward between Scott Butte and Iron Mountain, ending at an E-facing scarp the breaks over at about 3840’. Isn’t that interesting? SE of Burns Junction, the flows go some distance down Drought Creek, and pass E past Burns Junction, probably ending in some odd topography where Palomino Creek dumps over a broad E-facing scarp. Some maps put a N-S fault through this topography. In any case, vent 4257’ is a bit of a brute.

Iron Mountain itself, may be a relatively young basalt shield, though it seems clearly stratigraphically older than the vent 4257’ lava flows. Lava flows from the vent move SE, and end at an E-facing scarp that breaks over at the same elevation as the vent 4257’ lava flows N and S of it have. Can you say “Graded to the same level?” Perhaps Iron Mountain and vent 4257’ are part of a single time episode, with two vents; first Iron Mountain, then vent 4257’.

A vent called “Dome”, in the Palomino Lake quad, which is almost nested within the Saddle Butte lava field, may or may not be a young vent. It probably bears some inspection on the ground, and at least a single paleomagnetic site. Hart also believes that Scott Butte is about 1Ma in age. Morphologically, I can believe this, but I want to see the air photos, and eyeball it from the ground also.

In the Rome quad, Bill Hart has generated two K-Ar ages on flows on the W bank of Crooked Creek. I thought these were within the State Wayside Park, but they are actually several miles N of there. He found an age of 0.91 ± 0.36 Ma on the “rim HAOT” and an age of 1.25 ± 0.28 Ma on the “lowest exposed flow” about 30m lower in elevation. Although both these experiments need to be replaced with 40/39 work, these experiments say these scarps are 1-2 Ma in age, and NOT 3-4 Ma in age. The upper date seems to be in flows from vent 4257’. Lower age is less clear. Some topography, E of Scott Butte, may be a local basaltic vent. It seems to be embayed by the vent 4257’ lava flows, but could be the source of the lower ledge. DRILL THEM ALL! And let God sort them out!

Why should you care? – We evidently have at least two young flow fields moving eastward and ending in E-facing scarps, possibly interacting with the Owyhee River. We should frame them stratigraphically, get better ages, and look at the stratigraphy they hold down and hold up. Might they have a pillowed margin, or some fragmental character? “Young” very clearly comes from the W and gets to, or gets near the Owyhee River.

S of Hwy 95

There are a couple of basalt lava fields to the S of Hwy 95 that catch my eye, but the flows from Jackies Butte, may be the only lavas that have interacted with “lakes” in the Rome basin. A smaller, and probably older vent, SE of Burns Junction may also be of interest. These flows are just E of Anderson Reservoir, so I call them the Anderson Reservoir vent. Crooked Creek is W of these flows, Rattlesnake Creek is E of them, and Drought Creek flows SSE before joining Crooked Creek and flowing NE through a funny little canyon cut across older terrain. George Walker mapped the young flows from vent 4257’ as flowing SE, down Drought Creek almost to its join with Crooked Creek. I am going to check if this is correct. Perhaps the vent 4257’ flows are the reason that Drought Creek turns abruptly S, from a former course to the NE, perhaps joining Palomino Creek S of Scott Butte.

The Anderson Reservoir flows seem heavily mantled by sediments, possibly suggesting greater age. And, no “vent” construct is easily visable either. But, I recall that Crooked Creek has glacial breakout floods from the Steens country, and perhaps they have carried enough sediment to prematurely age these flows. The eastern scarp of this flow field, where it can be seen, breaks over at about 3840’, and perhaps that means something to us.

The Jackies Butte flows have been K-Ar dated by Bill Hart at 0.36 ± 0.13 Ma at a site farther SE along Rattlesnake Creek. The age strikes me as a tad young, but again, it is not a 3 or 4 Ma flow field. If it is this young it should have normal polarity. Jackies Butte lava flows form a prominent N-facing scarp at the S end of the Rome basin. It breaks over at about 3890’, but may be relatively thin flow on a more than 100’ scarp mostly in sediments. We should inspect this margin to look for evidences of water-lava interaction. The scarp ends to the E in the vicinity of Rockhouse Reservoir.

Lastly, the basalt shield of Water Hole Butte may be of interest. It may be as old as Deer Butte, or even older. It backs SE onto even older topography, and abuts Jackies Butte over a more than 6 mile saddle between them. Its western flank is faulted rather severely which may be telling us something about its age. One of the fault traces cuts across the contact with Jackies Butte and breaks those lavas too. It is unlikely that this flow field interacts with the Owyhee River, and it may be one of the “upland” basalt volcanoes that preceed the downcutting witnessed over the past 4 Ma.

Why should you care? – I think clearly that Jackies Butte, and possibly the Anderson Reservoir vent have something to tell us about Owyhee downcutting and how Crooked Creek has adjusted to that arena. One of Kyle’s shoreline maps laps against the N scarp of Jackies Butte.

SE of Rome

A young flow field, on the E side of the Owyhee River may also help us understand the River’s history. The surface of the rim behind where we put into the Owyhee River is covered by flows from Round Mountain, which is about 5 miles off to the SE. Bill Hart has a K-Ar age of 1.49 ± 0.18 Ma on an outcrop of this lava flow field. This age needs to be replaced, but again these flows are no older than 2 Ma, from the age work in hand. These flows shoal northward against some older terrain (4 Ma (?)), that I think I sampled at the top of the grade before you drop down to the Owyhee bridge on Hwy 95. They also infill block-faulted terrain to the SE, having an edge that is difficult to pick up from Google or Bing. The elevation of the W rim of this flow field does not seem very constant. It is as low as 3700’ above the put-in, and climbs southeastward to almost 3900’ up river. It evidently is mantling sloping topography, and doesn’t seem to be “lake bound” in any easy way.

Why should you care? – The edge of this young flow field determines the course of the Owyhee River over a distance of 8 to 9 miles. Parts of this flow field are on the SW side of the river, meaning that this flow might have filled and buried the former canyon of the Owyhee River. Should we be looking for a filled channel in the eastern wall of the Owyhee S of the put-in park? Will we find evidences of lava-water interactions along its western margin? We should investigate this young flow.