Idaho History Sept 12, 2021

Central Idaho Volcanoes

Idaho and Valley Counties

Mount Idaho

1881 Idaho Territory Map

1881-Idaho-MtIdaho— — — — — — — — — —

August 16, 1881
Idaho_SpokaneTimes_08-16-1881-a
Volcano In Idaho

Lewiston, I.T., Aug. 16. — A volcanic eruption occurred in the mountains about twenty miles east of Mt. Idaho on the 9th inst, sending forth fire and smoke hundreds of feet in hight [sic], and throwing rocks into the air which lit several miles from the scene of eruption. A column of black smoke is said to be still rising from the mouth of the volcano, which is visible fifty miles away. The shock attending the eruption was distinctly felt on Salmon river, seventy-five miles from the place. No one has as yet approached the scene of eruption.

— Blake. Spokane Times

— Newspaper Source found at: Washington Secretary of State Website Database, 2007
source: U.S. Volcanic Eruptions: “Non-Volcano Eruptions” Newspaper Clippings
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Daily Gazette – New Jersey August 18, 1881

A Volcano in Idaho

A dispatch from Lewiston, Idaho, says that there was a volcanic eruption in the mountain south of the South Fork of the Clearwater, about twenty miles east of Mt. Idaho, on the 9th instant. The mountain sent forth a column of fire and smoke several hundred feet in height, and a rock, which fell at a distance of several miles from the place of eruption. The shock was distinctly felt at Mt. Idaho, on the extreme west of the Camas Prairie, and at the mouth of Salmon River, a distance of about 75 miles.

Later news from Camas Prairie says that a column of smoke is coming from the opening, which is distinctly visible from the prairie. No one as yet has approached the place. Evidence of volcanic action at some former periods exist in many places in the immediate vicinity. So far as appears the opening is less than a thousand feet above the bed of the South Fork of the Clearwater, and within three miles of the Milner trail, between Mt. Idaho and Florence.
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Northern Christian Advocate – New York August 25, 1881

Volcano

There are reports of a volcanic eruption in Idaho. August 9th a column of fire and smoke is said to have burst from a mountain summit about 1000 feet above the south fork of the Clearwater, about 20 miles east from Mt. Idaho; the smoke continued to pour forth in great volume and to rise several hundred feet. A rock of considerable size was also thrown a number of miles from the mountain’s base. There were in the region distinct evidences of former volcanic action.
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The Owyhee Avalanche September 10, 1881

Idaho Volcano

The Nez Perce News of the 1st instant says: “Our big brother, Col. F.J. Parker, returned from the inside last night, and says that the supposed volcano is simply a chemical eruption at the head of a ridge of high altitude known as Devil’s back, a divide between the waters of the Salmon and Clearwater rivers. The trees on the mountain sides are shattered into kindling wood by the force of the explosion, and also set the woods on fire. The formation in the vicinage of the eruption is quartzite and limestone, and is terribly broken up. There were two explosions, the last twenty hours after the other, but not so loud.”

source: © PBC Idaho County GenWeb – Miscellaneous Published Articles and Newspaper Items From Idaho County and the Vicinity, compiled by Penny Bennett Casey
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Spokane Times – September 13, 1881 Idaho

Journalistic Push

— Fortunate is Walla Walla in the possession of a live journalist. Such an one is Col. Frank J. Parker, editor of the “Statesman.” He doesn’t allow nature to put on too much style without giving a pen picture of the “very latest.” On a recent occasion, a volcano was reported away over in Idaho, an account of which was first sent to THE TIMES by telegraph. We hadn’t time to run over and investigate the matter, and are very sorry to say that we were compelled to leave our readers in dreadful suspense till Brother Parker had resolved to be “the first to be there.” Always ready to go where duty calls, this enterprising journalist tore himself away from the endearments of civilization and pushed rapidly toward the frontier. His speed was like that of the wind. Passing Dayton, Lewiston and Pharoah’s Hill, he made the quickest time to Mt. Idaho. From there he traveled with a pack horse, and wore out the seat of a government pack-saddle. No one but an enterprising newspaper man would have suffered thusly. When Col. Parker reached the ragged edge of the greatest labyrinth of tangled forests ever designated on map or chart, and where no white man had ever been, he could learn less about the volcano than while at Walla Walla. Here he left the remnants of his pack train, and crawled into and through the primeval forest, with glory only seventeen miles away. On and on, manfully he climbed the mountain side, sparring himself onward and upward with the encouraging word, “Excelsior!”. He became weary, yet fainted not; lost the soles to his boots, yet halted not; hungry, and ate naught; but still he pressed manfully on with that peculiar trait of hoping against hope — so common with newspaper men — leading him on.

When Col. Parker had reached the summit of the mountains, where the volcano was supposed to be, imagine his great astonishment to find right before his eyes the very spot which might be the one he was looking for, and yet it might not. The mountain was charred and torn, was steaming a little, and sent up a peculiar and unpleasant odor. “Bravo! bravo!” shout Stanley; “Whooplah! it is the Devil’s Hole!” and as such it is known to this day.

— Newspaper Source found at: Washington Secretary of State Website Database, 2007

source: U.S. Volcanic Eruptions: “Non-Volcano Eruptions” Newspaper Clippings
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Fire (Not Volcano)

The fire having the widest repercussions for the area was one that occurred in August 1881 near Buffalo Hump. A settler accidentally got a patch of timber burning and, being all alone, he decided that the best way to attract attention and get help would be to set off a powder blast. The earth-shaking explosion and the leaping flames apparently brought him the assistance he wanted, but it probably attracted more attention than he had bargained for. In due course, the rumor had found its way into various newspapers throughout the West that there had been a volcanic eruption and an earthquake at Buffalo Hump.

source: A History of the Nez Perce Forest, page 71.  [h/t Kelsey McCartney FB]
see also:  Sister M. Alfreda Elsonsohn’s book “Pioneer Days in Idaho County” – per Kevin Norwood FB
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Thunder Mountain

1902 Thunder Mountain Mining District Map

1902-Idaho-ThunderMtn— — — — — — — — — —

Thunder Mountain Caldera

TowardsThunderMountain2016Looking towards Thunder and Red mountains showing the rim of the old caldera.

Photo: by Local Color Photography August 2016.
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Thunder Mountain Cauldron Complex 50-30 million BCE

ThunderMountainCauldronComplex(Cropped from Table 1 From Link and Janecke, 1999.)

citation: Challis Magmatic Episode By Laura DeGrey and Paul Link of Idaho State University Digital Geology of Idaho
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Temporal Evolution of the Thunder Mountain Caldera and Related Features, Central Idaho

by B. F. Leonard and R. F. Marvin USGS 1982

Abstract

The eruption of latite ash flows 50 million years ago began an episode of volcanic activity that lasted about 7 million years. Ash flows and minor lava flows, first latitic, then rhyolitic, built the Thunder Mountain field of Challis Volcanics to a height of 1,500 meters before subsidence of a nearly circular cauldron block 60-65 kilometers in diameter. Subsidence of the Quartz Creek cauldron, dated at 47 million years, was accompanied by development of the Cougar Basin caldera, within which the Sunnyside rhyolitic ash flow (an informal, locally recognized unit) was erupted from a central vent 46-47 million years ago. The eruption of the Sunnyside rhyolite was attended by development of the Thunder Mountain caldera, to which most of the Sunnyside was confined. The caldera was filled to shallow depth with volcaniclastic sediments containing plant remains of Eocene age. The caldera floor was tilted, perhaps by diapiric(?) emplacement of the Casto pluton about 44 million years ago. A minor vent near the caldera edge spilled latite lava across the tilted caldera floor. The highest latite flow has a date of about 43 million years: younger dates obtained from the flow very likely reflect argon loss from glass. The outpouring of latite lava is the last volcanic event recorded in the caldera. The dates of the earliest intrusives (47-million-year-old dikes of the cauldron margin) and of the latest (37-million-year-old dike of the Little Pistol swarm) indicate that intrusive activity began within the explosive stage of local volcanism and apparently ceased some few million years after extrusion of the youngest lavas of the field. Within this interval, the eastern margin of the cauldron was invaded by the Casto pluton, and myriad small stocks and dikes of rhyolite and latite were emplaced, mainly along the margins of the cauldron and its nested calderas. Twenty-eight new potassium-argon dates document the history summarized here.

Introduction

The Thunder Mountain caldera is the central feature of an irregular field of Challis Volcanics (Figure 1) that formed during the Eocene at the geographic center of the Idaho batholith. The caldera is old, has been deformed repeatedly almost from its inception to the present, and is virtually indistinguishable topographically from the jumbled fault blocks of the Salmon River Mountains in which it lies. Nevertheless, the caldera has evolved in much the same way as the better exposed and better studied calderas of Oligocene to Pleistocene age in the western United States. We present here a geologic sketch of the local volcanic field and an outline of the temporal evolution of the Thunder Mountain caldera and related features. The main conclusions of this report were given earlier as an abstract (Leonard and Marvin, 1975).

Formality requires the naming of two subsidence features that were glossed over in the abstract. The Thunder Mountain caldera nests within a larger caldera, here named the Cougar Basin caldera (Figure 1). The northern and eastern sectors of the Cougar Basin caldera are hard to reach and poorly known, and our attempts to date the subsidence of the caldera isotopically have been frustrated by the difficulty of preparing suitable mineral separates. For these reasons, the Cougar Basin caldera receives little more than its name in this report. Both calderas are outlined by remnants of their wall rocks, but part of the subcircular outline of the calderas is now masked by a northeast-trending linear structure, too complex to be labeled a medial graben, that passes through the calderas and ends near the north margin of the cauldron. The cauldron – the large volcanic subsidence structure that delimits the Challis Volcanics of the Thunder Mountain field – is here named the Quartz Creek cauldron (Figure 1). The northern sector of the cauldron nearly coincides with the principal trough of the 1,800-gamma contour of the aeromagnetic map of Cater and others (1973, plate 1). The southwest sector coincides approximately with the smoothed 1,600-gamma contour of an aeromagnetic map kindly made available by Don R. Mabey, U. S. Geological Survey.

Figure1Geologicsketch-aFigure 1. Geologic sketch of Thunder Mountain caldera and related features. Compiled by B. F. Leonard from mapping at various scales by Cater and others (1973, plate 1), B. F. Leonard (unpublished), and J. G. Brophy and Gordon May (unpublished).
(for larger size go to link to paper)

Principal place names used in the text are shown on published topographic maps of the U. S. Geological Survey, chiefly on the Big Creek and Yellow Pine 15-minute quadrangles and the Challis and Elk City 2-degree quadrangles. The name Land Monument Mesa is not shown on published maps. The name is for the mesa 1.7 kilometers north-northeast of the Dewey mine, Thunder Mountain district. Most of the sample locations (Figure 2) are referred to maps available when the fieldwork was done. Since then, new maps have replaced or supplemented some of the old ones. As anyone who wishes to visit the sample sites will soon find only the newer maps available, the principal changes are noted here. The former 15-minute quadrangles have been subdivided into the Wolf Fang Peak, Big Creek, Edwardsburg, Profile Gap, Yellow Pine, Stibnite, Big Chief Creek, and Chilcoot Peak 7 1/2 minute quadrangles; the Monument, Safety Creek, and Rainbow Peak 7 1/2 minute quadrangles now supplement the 2-degree quadrangles for the area northeast and east of Thunder Mountain. The elevations of sample sites are reported in feet because elevations are so shown on the topographic maps of the region. Horizontal distances have been converted from English to metric units. Plate 1 of Cater and others (1973) shows reconnaissance geologic information for all but the western part of the area treated in our report.

Figure2Locationofsamples-aFigure 2. Location of samples. Hare from topographic map of the 3late of Idaho, U. S. Geological Survey, scale 1.500,000
(for larger size go to link to paper)

Geologic Sketch

The Quartz Creek cauldron that delimits the Thunder Mountain field is crudely circular in plan, 60-65 kilometers in diameter, bounded on the west and northwest by a system of ring faults and an attendant swarm of dikes, and along its eastern sector invaded by the Eocene Casto pluton (Figure 1). Radial faults and subsidiary ring fractures dissect the cauldron block, but their pattern is largely obscured by a host of younger faults produced by regional rotational stress and by recent regional and local subsidence. Within the cauldron block, vertical displacement along major subsidence faults exceeds several kilometers, causing plutonic Precambrian intrusives, low- to high-grade Precambrian metamorphic rocks, and various facies of the Cretaceous Idaho batholith to lie in disarray against one another and against the Challis Volcanics.

The Challis Volcanics of the Thunder Mountain field comprise (1) two units of regional extent, (2) a pyroclastic and volcaniclastic filling unit, confined to the Thunder Mountain caldera and its environs, and (3), as a minor part of the filling unit, late flows confined to the caldera but separated from other components of the caldera filling by an erosion surface of at least local extent. The Thunder Mountain caldera and its environs, and (3), as a minor part of the filling unit, late flows confined to the caldera but separated from other components of the caldera filling by an erosion surface of at least local extent.

The units of regional extent are a lower, latitic unit and an upper, rhyolitic unit. Both are largely ash-flow tuffs and their welded equivalents. The lower unit of latite tuff and breccia is 400-500 meters thick. It rests on rocks of the Idaho batholith and older metamorphic complexes, locally contains consolidated mudflow debris rich in blocks of batholithic granodiorite and grus derived therefrom, contains at least two thin flows of latite, and in composition ranges without evident order from latite to quartz latite. The latitic unit is exposed only at the cauldron edge and at the periphery of the main area of Challis Volcanics; its extent throughout the field is conjectural. The upper unit of rhyolite tuff and welded tuff is 1,000 – 1,100 meters thick. It rests on the latite unit but laps onto the adjacent plutonic and metamorphic terrane. In addition to the predominant rhyolitic pyroclastics, the upper, rhyolitic unit contains one or more thin flows of rhyolite, two or more of andesite, and a few lenses of volcanic sandstone and granodiorite-bearing mudflow debris. The disposition of the two units of regional extent (Figure 1) indicates that they were formed before major subsidence of the cauldron, but the vent or vents from which they issued have not been identified.

The filling unit, confined to the Thunder Mountain caldera and its environs, has three subunits. The lowest is the Sunnyside rhyolite.

The informal name Sunnyside rhyolite is adopted in this report for a subunit of local interest in the Challis Volcanics of the Thunder Mountain field. Mining men and geologists familiar with the Thunder Mountain district customarily use the name Sunnyside rhyolite, and it is appropriate to accede to their custom. The informal name Sunnyside rhyolite corresponds to the equally informal rhyolite of Sunnyside (Leonard, in Cater and others, 1973, p. 46) and to the Sunnyside rhyolite crystal tuff, Sunnyside rhyolite tuff, Sunnyside tuff, and Sunnyside welded tuff of Shannon and Reynolds (1975).

The Sunnyside rhyolite is a crystal-rich tuff, evidently the product of a single ash flow more than 200 meters thick, that is dominantly a single cooling unit with local, thin cooling units near the top. The major cooling unit is thought to have a thin, basal vitrophyre (seen only as slivers along faults), a thick medial zone of welded tuff with discontinuous vapor-phase zones near its upper contact, and a capping of nonwelded tuff, perhaps 10-20 meters thick, that thickens northeastward toward the distal end of the flow. The distribution of pumice fragments and biotite flakes in the Sunnyside rhyolite suggests that the ash flow issued from a vent near Thunder Mountain, at the center of the volcanic field.

Current work by E. B. Ekren and Gordon May (oral communication, 1982) leads them to conclude that the Sunnyside rhyolite of Leonard’s usage comprises deposits from two major ash flows, and that multiple cooling units may be present in the lower ash flow. Thus our treatment of the Sunnyside rhyolite in this report may be too simple.

The Sunnyside rhyolite is overlain by the Dewey beds. The informal name Dewey beds is adopted in this report for a subunit of local interest in the Challis Volcanics of the Thunder Mountain field. Mining men and geologists familiar with the Thunder Mountain district customarily use the name Dewey beds, and their name for the subunit is conveniently adoptable here. The name corresponds to the volcaniclastic subunit of Leonard (in Cater and others, 1973) and to the Dewey beds, Dewey conglomerate, Dewey conglomerates, Dewey strata, Dewey unit, Dewey volcaniclastic beds, and Dewey volcaniclastics of Shannon and Reynolds (1975).

The Dewey beds consist of water-laid volcanic conglomerate, volcanic sandstone, mudstone, carbonaceous shale, a little lignite, and at one place lake beds of laminated carbonaceous mudstone and airfall(?) tuff. This largely volcaniclastic subunit, rich in material eroded from the Sunnyside rhyolite, is well exposed only at the Dewey mine, where its drilled thickness exceeds 90 meters. The outcrop, near site 5 of Figure 2, is too small to show on Figure 1. Fragmentary plant fossils from the volcaniclastic subunit are Eocene (J. A. Wolfe, written communication, 1967; oral communication, 1968, 1981). Associated with the volcaniclastic rocks is a black, carbonaceous breccia of uncertain stratigraphic position and irregular distribution, interpreted as ancient mudflow debris. Carbonaceous mudstone, perhaps representing an ancient swamp deposit, was exposed by mining in 1981, after fieldwork for this report was finished.

An angular unconformity of a few degrees separates the Dewey beds from a pair of thin, vesiculartopped, anomalously young-looking latite flows derived from a small volcanic center within the caldera at Lookout Mountain, 10 kilometers northeast of Thunder Mountain. The flows are indicated on Figure 1 as flows of Lookout Mountain.

The part of the filling unit below the latite flows is economically significant, for the upper part of the Sunnyside rhyolite, undetermined parts of the Dewey beds, and some of the black breccia contain the low-grade gold deposits of the Sunnyside and Dewey mines (Leonard, in Cater and others, 1973, p. 45-52).

Much younger than the Tertiary filling unit is water-laid Pleistocene gravel that may once have formed a thin veneer on the southern half of the caldera floor. The gravel is now found only as relics along minor faults that dissect the complex graben of the Thunder Mountain district.

Deposition of the gravel marked the end of the caldera as a topographic depression. Most of the former sump is now a high, rolling upland, a few hundred meters lower than the fringing alpine peaks and mostly separated from them by canyons formed in narrow grabens. The caldera floor is perched 1,500 meters above the main drainage from the Thunder Mountain district.

Tertiary intrusives of the area comprise dikes, stocks, and a plutonic mass of batholithic dimensions – the Casto pluton – which dominates the southeast sector of the cauldron. The dikes range in width from less than 1 meter to more than 100 meters, and in length from a few meters to more than 1 kilometer. Most of the dikes stand vertically. They are dominantly rhyolitic or latitic in composition, and commonly they are porphyritic, but their fabric varies widely. The dikes, if counted, would surely number several thousand, most of them concentrated in swarms, and most of them external to the two calderas. Two swarms, Smith Creek and Little Pistol, are labeled on Figure 1, but they are merely parts of larger arrays of dikes that extend beyond the limits of the illustration. Locally within these swarms the dikes lie side by side, without intervening relics of country rock, yet no dike is seen to cut its neighbors.

A few stocks are present within the dike swarms, but many more are clustered within the Challis Volcanics between the margins of the Thunder Mountain and Cougar Basin calderas, at sites 12 kilometers southwest and 15 kilometers northeast of Thunder Mountain (Figure 1). The stocks are nonequant in shape, seldom more than a few hundred meters in mean diameter, variable in fabric, commonly fault bounded, locally riddled by dikes, and – like the dikes – dominantly rhyolitic or latitic in composition. A few stocks, dikes, and less simple intrusives are dioritic or quartz dioritic.

The dikes, stocks, and pluton are related to structural settings or events in ways that make it desirable to treat the intrusives individually or as geographic and chronologic groups, according to the problems they present rather than as the class of all intrusives.

Two great silicified zones within the cauldron block but outside the Cougar Basin caldera contain many of the gold, silver, antimony, and tungsten deposits of the region. The silicified zones are some kilometers long and tens or hundreds of meters wide. The western zone is in part coextensive with the Smith Creek dike swarm and in part peripheral to the western margin of the cauldron. The eastern zone is associated with ring fractures of the Cougar Basin caldera (see subsequent discussion of the stocks of upper Indian Creek). The silicified zones and their mineral deposits receive scant attention in this report because they have not been adequately dated. …

Conclusion

The eruption of latite ash flows 50 million years ago began an episode of volcanic activity that lasted about 7 million years. Ash flows and minor lava flows, first latitic, then rhyolitic, built the Thunder Mountain field of Challis Volcanics to a height of perhaps 1500 meters before the subsidence of a nearly circular cauldron block having a diameter of 60-65 kilometers. Major subsidence, dated at 47 million years, was accompanied by development of the Cougar Basin caldera, within which the Sunnyside rhyolitic ash flow was erupted from a central vent 46-47 million years ago. Eruption of the Sunnyside ash flow was attended by development of the Thunder Mountain caldera, to which most of the Sunnyside ash flow was confined. Collapse of the Thunder Mountain caldera cannot be precisely dated. An upper limit is either the 47-million-year date of cauldron subsidence or more likely, we think, the 46 to 47-million-year date of eruption of the Sunnyside rhyolite. A likely lower limit is the 44.6-million-year date of the Century Creek stock, a small intrusive nearly central to the caldera. Water-laid volcaniclastic debris, derived mainly from the Sunnyside rhyolite, was spread thinly over the floor of the collapsed Thunder Mountain caldera, trapping plant remains of Eocene age. The floor of the Thunder Mountain caldera was tilted gently southwestward, perhaps by diapiric emplacement of the Casto pluton. A minor vent 8 kilometers northeast of the center of the caldera erupted latite cinders, bombs, and lava of the Lookout Mountain unit. Thin flows from this vent spread southwestward across the caldera floor. The highest latite flow has a date of about 43 million years; younger dates obtained from the flow very likely reflect argon loss from glass. The outpouring of latite lava is the last volcanic event recorded in the caldera, whose evolution extended from middle Eocene to late Eocene time. (The informal designations of Eocene time are keyed to the preferred radiometric dates, recalculated according to 1977 decay constants, of Harland and others, 1971.) During the Pleistocene, part of the defunct caldera again served as a depositional site, this time for gravels carried in by meltwater from small alpine glaciers of the bordering highlands.

At present levels of exposure, the earliest indication of Tertiary intrusive activity is given by quartz diorite and latite dikes of the outer ring-fracture zone of the Quartz Creek cauldron. The dikes have a date of 47 million years. The latest indication of intrusive activity is given by a latite dike, dated at 37 million years, from the Little Pistol dike swarm. Thus the evidence points to a beginning of intrusive activity during the explosive stage of local volcanism and a cessation some few million years after extrusion of the youngest lavas of the Thunder Mountain field. Between the extremes of 47 and 37 million years, the eastern margin of the cauldron was invaded by the Casto pluton, and myriad small stocks and dikes were emplaced along the margins of the cauldron and its nested calderas. Small rhyolite stocks grew near the center of the Thunder Mountain caldera.

The Casto pluton presents a special problem, different from that of the stocks and dikes. The most reliable date, analytically, for the pluton is 47.8 million years. That date may represent the time of crystallization of the core of the pluton at some considerable depth beneath the cover rocks. The pluton did not, we think, ascend to invade the cauldron margin till much later, perhaps 44-45 million years ago.

The dates of the Challis Volcanics of the Thunder Mountain field do not differ significantly from the dates reported by Armstrong (1974) for the Challis Volcanics of the type area near the town of Challis: about 43-50 million years for the former and about 45-50 million years (recalculated) for the latter. The comparison excludes dates of ours that we have discussed as aberrant and dates of Armstrong’s for which he expressed some reservations. However, the Challis Volcanics extend far beyond the areas whose rocks have been radiometrically dated in our study and in Armstrong’s Within the large expanse of the Challis Volcanics of central Idaho there are, we believe, at least seven major caldera-related volcanic fields whose extrusive products, similar in composition and locally lapping over from one field to another, may be nearly contemporaneous but not rigidly so. Until the local sequences have been established and dated, appropriate limits for “Challis time” are only approximately definable. Meanwhile, it is worth noting that although a good many Tertiary intrusives in central Idaho are coeval with the Challis Volcanics as currently datable, some intrusives are younger. The 37-million-year date of hornblende latite from the Little Pistol dike swarm, for example, is an Oligocene date if referred to Harland and others’ (1971) preferred date of 38 million years (39 million years, recalculated) for the base of the Oligocene, and is statistically excludable from the set of 43 to 50-million-year dates that represent reliably dated Challis Volcanics.

Acknowledgments

We thank these members of the U. S. Geological Survey for their contributions to our work: Jack A. Wolfe for examination of plant fossils; H. H. Mehnert and V. M. Merritt for assistance with potassiumargon analyses; and Alan J. Busacca, Bruce T. Brady, G. T. Cebula, Ezekiel Rivera, and Michael Sekulich for mineral separations. Mr. Busacca and former field associates Neil Dale, Stephen J. Reynolds, and John Schloderer helped collect the rock samples. Don R. Mabey provided supplementary geophysical data. E. Bartlett Ekren and David H. McIntyre examined a suite of thin sections. Mr. McIntyre and Frederick S. Fisher reviewed the report. Melvin and Jim Ed Biggers, Sweet, Idaho; Lafe Cox, Yellow Pine, Idaho; and Larry Rowe, Caldwell, Idaho, efficiently managed pack strings and provided support in base camps. Eleanor Leonard and Ruth Leonard O’Neil kindly served as unpaid camp cooks.

Full paper 19 pages:
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Geology Examples

Idaho Batholith Quartz Creek
092810Rock-b

Sunnyside Rhyolite – folded
111610Fold-in-Rhyolite-b

Tuff with quartz crystals
TuffQuartzCrystals2010

Photos: by Local Color Photography 2010
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Challis Magmatic Episode

By Laura DeGrey and Paul Link of Idaho State University

Extent and timing of the Challis Magmatic Episode

The Challis volcanic episode was active during the Eocene, between ~52-39 Ma, with the bulk of the eruption between 51-45 Ma. The Challis volcanic episode is part of a wide-spread Eocene volcanic belt that covered a large part of the Eocene northwestern United States and southwestern British Columbia

EoceneVolcanicBelt-aFigure 1. Eocene volcanic belt in the northwestern United States. From Moye et al.,1988.

Geologic History of the Challis Magmatic Episode

The Challis Volcanic Group covers approximately 25,000 square kilometers in Idaho , making it the largest of the Eocene volcanic fields (Figure 2). The Eocene geologic setting of the Challis volcanics is commonly accepted as an extensional basin related to the subduction of the oceanic Farallon Plate beneath the continental North American Plate. Prior to Eocene time, before ~60 Ma, the convergence rate of the Farallon and North American Plates was very fast.

ChallisVolcField-aFigure2. Extent of Challis volcanism in Idaho. From Link and Janecke, 1999.

The angle of subduction was shallow and the subducting slab of the Farallon Plate reached as far as eastern Wyoming underneath the North American Plate (Figure 3). This far-reaching subducting slab set the stage for widespread volcanism along the northwestern side of the United States and southwestern British Columbia.

Subduction-aFigure 3. Drawing showing shallow subduction and then migration and steepening of the subducting plate.

Around 56 Ma, the subduction rate slowed and the angle of subduction became much steeper. During the steepening of subduction, the subducting slab migrated westward (Figure 3). Following the westward migration of subduction, backarc extensional basins developed and triggered widespread igneous activity that youngs in a westward direction. The extensional zone is oriented northeast-southwest and contains many high-angle and low-angle normal faults, which commonly dip toward each other, and form rift-grabens (Figure 3). These normal faults make up the Trans-Challis Fault Zone.

Igneous activity occurred in two main phases. The first phase began around 52 Ma and included the intrusion of shallow granitic plutons 3 – 4 miles below the surface. The second phase included volcanic eruptions. The main phase of eruption occurred between 51 – 45 Ma, and consisted of effusive and voluminous intermediate and mafic lava flows as well as highly explosive silicic ash-flows. Less violent and less voluminous silicic eruptions continued until approximately 39 Ma. Intrusive activity in the form of rhyolitic domes, rhyolitic plugs, rhyolitic dikes, and rhyolitic stocks accompanied the later stage volcanism. The violent eruptions of rhyolitic ash tuffs and ignimbrites caused the collapse of many calderas between 49 – 45 Ma.

continued: Digital Geology of Idaho
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see also:

Regional Geologic Setting and Volcanic Stratigraphy of the Challis Volcanic Field, Central Idaho

Falma J. Moye / William R. Hackett / John D. Blakley / Larry G. Snider

Introduction

Early Tertiary geologic history of the northwestern United States was characterized by a short-lived but intense magmatic episode from 55 Ma to 40 Ma. That episode resulted in an areally extensive and compositionally diverse belt of volcanic and plutonic rocks extending from southern British Columbia across northeastern Washington and central Idaho and into Montana and Wyoming. Although the time equivalence of this belt of rocks has been recognized and some basic geologic, geochemical, and radiometric work has been completed, the Eocene magmatic event remains poorly understood within the context of Eocene tectonics of the northwest.

The Challis volcanic field of central Idaho is the largest and most diverse of the Eocene volcanic fields, both in composition and in variety of volcanic deposits. Because it is dissected to subvolcanic levels, geologists can study the geochemical relationships among cogenetic volcanic and intrusive units and the internal structures of volcanic and hypabyssal complexes.

In this paper we summarize current knowledge of the geology of the Challis volcanic field, including its regional stratigraphy, geochronology, and geochemistry.

continued: Guidebook to the Geology of Central and Southern Idaho
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