369

Geology of the Magic Reservoir Area, Snake River Plain, Idaho

by William P. Leeman’

ABSTRACT

The Magic Reservoir area occurs along the northern margin of the Snake River Plain, Idaho, in a struc- turally complex zone where the western and eastern limbs of the plain converge. Volcanic activity since middle Miocene time follows the general trend else- where in the Snake River Plain. Rhyolitic ash-flow and air-fall tuffs, subordinate basalt flows, and inter- bedded sediments were deposited intermittently be- tween at least 10 million to about 5 million years ago; culminating eruptions of extensive rhyolite lavas filled a large depressed area which is inferred to be a buried caldera. Extensive mafic lavas of a hybrid or contaminated character were extruded over much of the inferred caldera, shortly following rhyolitic activity there, and were in turn followed by relatively minor pyroclastic eruptions and emplace- ment of domes of rhyolite as recently as 3 million years ago. Subsequent volcanic activity was basaltic, and vent loci apparently were controlled largely by older normal faults and caldera structures.

INTRODUCTION

Geologic mapping and geochronology of Tertiary rocks along margins of the Snake River Plain in Idaho can provide useful constraints on models for development of this province. The area near Magic Reservoir is of considerable interest in this respect because it lies in the zone where the northeast- trending eastern part of the plain and northwest- trending western part converge. Previous mapping in the area that includes the eastern Mount Bennett Hills and the Timmerman and Picabo Hills (Figure 1) provides a reasonably good view of the stratigraphic relations there (Schmidt, 1961; Malde and others, 1963; Smith, 1966; LaPoint, 1977; Struhsacker and others, 1982 this volume). In this paper a summary of

lDepartment of Geology, Rice University, Houston, Texas 77001.

Figure 1. General location map for the Magic Reservoir area. Major highways and towns are shown. Two prominent rhyolite domes are identified (triangles) as are Quaternary basaltic vents (stars). Inferred caldera boundary is shown by dotted line (see Figure 2).

geologic relations is presented, and evidence is cited for the presence of a partly buried caldera in the area of Magic Reservoir.

GEOLOGY OF THE MAGIC RESERVOIR AREA

Schmidt (1961) and Smith (1966) provide the most detailed descriptions of geologic units in the area. The regional geology is summarized on the Hailey 2O sheet (Rember and Bennett, 1979). A composite geologic map based on their work and on reconnaissance mapping by Malde and others (1963) and the author is shown in Figure 2. Stratigraphic relations are illustrated in Table 1. In the map area local outcrops of late Paleozoic sediments (MPs), Cretaceous granodioritic to quartz dioritic intrusive rocks (Kg) of the Idaho batholith, and erosional

370 Cenozoic Geology of Idaho

Sediments

E::

Basalt lavas and vents

c3 Qbr r-3

Qbs

e Qbm

m Qb’v Qbb

i Trmb

m Tb

Rhyolite lavas. domes. and tuffs

m Tpc

u Tmr

Tiv

Older units

l Magic Hot

Springs

i Vents

=-= Faults

H Inferred

coldera limits

Figure 2. Geologic map for the Magic Reservoir area. Compiled mainly from Schmidt (1961), Malde and others (1963), and Smith (1966). Geologic units are as follows: undivided Quaternary and late Tertiary sediments (Qs and Ts); unnamed Recent hasalts (Qbr,, Qbh, Qhr,, in increasing age sequence); Schooler Basalt (Qbs); Macon Basalt (Qbm); younger and older unnamed Bruneau basalts (Qbby and Qbb); Square Mountain Basalt (Tsmb); undivided Banbury basalts (Tb); Poison Creek Tuff (Tpc); undivided Moonstone Rhyolite (Tmr); undivided ldavada Volcanics rhyolitic volcanic rocks (Tiv); Challis Volcanics (Tcv); undivided granitoid rocks of the Idaho batholith (Kg); and undivided Mesozoic-Paleozoic sedimentary rocks (MPs). See text and Table I for further details.

Leeman-Geology of Magic Reservoir Area 371

remnants of andesitic to quartz latitic lavas and breccias and intercalated sediments of the Eocene Challis Volcanics (Tcv) form a basement to the younger rocks discussed in this paper.

STRATIGRAPHY AND VOLCANIC EVENTS

Late Miocene volcanic rocks and sediments lie unconformably on the basement rocks. The volcanic rocks consist mainly of rhyolitic ash-flow and air-fall

tuffs and subordinate basaltic lavas. These rocks are interbedded with fluvial and lacustrine sediments (Ts) containing abundant volcaniclastic components. A sequence of rhyolite ash-flow tuffs and thick tuffaceous sediments (Picabo Tuff) has been mapped east of Magic Reservoir (Schmidt, 1961). This sequence has not been studied in detail, but two distinct ash-flow tuff members, each containing multiple cooling units, have been discerned. Schmidt notes that the ash-flow tuffs become more densely welded and appear to increase in thickness toward the west. Similar ash-flow tuff sequences have been

Table 1. StratieraDhic relations in the western Snake River Plain. Idaho

“Chronostratigraphic epochs after Berggren (1972). %dicates magnetic polarity: normal (+), reverse (-).

372 Cenoro~ Geology of Idaho

mapped in the eastern Mount Bennett Hills (Smith, 1966). The latter rocks have been studied in greater detail, and four distinct periods of ash-flow tuff deposition can be inferred from intervening fluvial- lacustrine sediments and basalt flows (Table 1). Individual ash-flow tuff sequences comprise either multiple (Gwin Springs Formation and Fir Grove Tuft) or single cooling units (City of Rocks Tuff). The oldest tuff unit (older welded tuff, Table 1) is poorly exposed, and few details of its area1 distribution or internal zonation are available. Armstrong and others (1980) report a potassium- argon age of about 10 million years for this unit. The City of Rocks Tuff thickens southward and may be correlative with the rhyolite exposed at Shoshone Falls, near Twin Falls (compare, Stearns and others, 1938; Stearns, 1955; Smith, 1966), which has been dated at 6.5 million years old (Armstrong and others, 1975). This highly tentative correlation is based only on the petrographic and lithologic similarities of the units. On then map (Figure 2) all ash-flow tuff units and some of the associated sediments are designated as Idavada Volcanics (Tiv) after Malde and Powers (1962). Cumulative thickness of these units is at least 800 meters and 400 meters, respectively, east and west of Magic Reservoir. Sources for these rhyolitic tuffs are uncertain; some of those in the Mount Bennett Hills apparently were derived from more southerly sources as they thicken in that direction.

Late Miocene basalts interbedded with tuff units in the Mount Bennett Hills were designated as Banbury Formation by Malde and others (1963) but were individually mapped by Smith (1966). These units consist of one or more flow units that originated from vents to the north and covered wide areas. Their aggregate thickness averages about 60 meters but is greater to the north. East of Magic Reservoir older basalts are locally exposed in the Clay Bank Hills; although they are demonstrably younger than Miocene basalts in the Mount Bennett Hills, all of these units are shown as undifferentiated Tertiary basalt (Tb) in Figure 2.

A major rhyolite unit, Moonstone Rhyolite (Tmr), covers more than 250 square kilometers in the central map area, mainly southwest of Magic Reservoir. This unit is a heterogeneous accumulation of rhyolite lava flows, possibly some welded ash-flow tuff in the central area, and numerous exogenous domes. The type area, Moonstone Moun- tain, is a large dome from which a small lava flow issued. Schmidt (1961) noted the problems involved in distributing viscous rhyolite lavas over such a wide area if Moonstone Mountain were considered as the main vent area. Thus, it is likely that numerous vents are within the area between Magic Reservoir and the

Mount Bennett Hills. At least five domes of similar rhyolite occur south and southeast of Magic Reser- voir, and extensive outcrops extend southward almost to the town of Shoshone. The thickness of this rhyolite is estimated as a minimum 150 meters in the central area. Available potassium-argon dates (Armstrong and others, 1975; Stanley Evans, personal communication, 1981) range widely from about 6 million years old for rhyolite near Magic Hot Springs to 3 million years old for the dome at Wedge Butte, southeast of Magic Reservoir.

Shortly following deposition of the Moonstone Rhyolite lavas, a distinctive basalt was extruded over much of the map area. This unit, the Square Mountain Basalt (Tsmb), consists of one or more flows with a maximum thickness of 200 meters immediately west of Moonstone Mountain. How- ever, elsewhere this unit is considerably thinner (less than 10 meters). The great thickness of lava near Moonstone Mountain suggests its deposition in a local topographic or structural depression. This unit is notable because it contains abundant xenocrysts (embayed quartz, K-feldspar, anorthoclase) and xenoliths (Moonstone Rhyolite; granulitic and granitic gneiss, and rare gabbro all of which are partly fused). Xenoliths are exceedingly abundant in outcrops near Moonstone Mountain, and this area is considered to be a major vent area, although additional vents may have contributed to the wide distribution of the unit. It was postulated that this lava represents a hybrid mixture of basalt and rhyolite magma (Schmidt, 1961), but isotopic studies show that it has been heavily contaminated by Precambrian crustal rocks (Leeman, 1982a this volume). It is noteworthy that typical samples of Square Mountain Basalt contain about 59 percent SiO, and in most respects resemble ferrolatites from Craters of the Moon lava field (Leeman, 1982a this volume).

Additional rhyolite ash-flow and air-fall tuffs of the Poison Creek Tuff (Schmidt, 1961) were then deposited over restricted areas near Magic Reservoir. This unit consists of a local flow (or dome) and at least IS0 meters of tuffs whose major vents are inferred to lie near Magic Hot Springs (Schmidt, 1961; Struhsacker and others, 1982 this volume). Xenoliths of older Moonstone Rhyolite and Square Mountain Basalt occur in some of the tuffs. Potassium-argon ages determined for two rhyolites of the Poison Creek Tuff range between 5 and 6 million years old (S. Evans, personal communica- tion, 1981).

The Clay Bank Basalt, mentioned above with late Miocene basalts, is younger than the Poison Creek Tuff and consists of about 30 meters of olivine basalt flows. This unit outcrops only in the hills east of

Leeman-Geology a/ Magic Reservoir Area 373

Magic Reservoir where it forms prominent cliffs and landslide blocks. Its age is unknown but could be as young as late Pliocene.

Subsequent volcanic events included the extrusion of numerous Quaternary basalts as single or multiple flow units from widely dispersed vents in the map area. Most of these flow units can be distinguished readily (Schmidt, 1961; Smith, 1966; Malde and others, 1963; LaPoint, 1977) from outcrop patterns, geomorphology, and petrographic characteristics. They are complexly interbedded with Quaternary sediments of fluvial, lacustrine, or glacial origin. A detailed analysis of Pleistocene and Recent geomor- phic and geologic development is given by Schmidt (1961) for the area northeast of Magic Reservoir. Pleistocene basalts in this area were assigned by Schmidt to the Bellevue Formation which was considered equivalent to the Bruneau Formation (Qbb) of Malde and Powers (1962). However, paleomagnetic data show that late Bellevue basalts (Sonnars, Macon-Qbm, and Priest Basalts) and those in the Mount Bennett Hills (Timber Gulch and Schooler-Qbs Basalts) are normally magnetized and thus may be equivalent to older Snake River Group lavas (Malde and Powers, 1962). The Timber Gulch Basalt is denoted as younger Bruneau Formation (Qbby) on the map. Extensive measurements of paleomagnetic orientations for dated (0.7 to 1.5 million years old) Bruneau basalts from the western Snake River Plain are reversely magnetized (Arm- strong and others, 1975; Leeman, unpublished data).

Recent basalts (Qbr l-3) erupted from several vents in an east-trending zone across the east-central map area. All of these flows are sparsely vegetated and are correlative with the Snake River Group.

Stratigraphic relations for all of the described units are summarized in Table I where they can be compared with a generalized stratigraphy for the western Snake River Plain (Malde and Powers, 1962; Malde and others, 1963; Malde, 1972). Results of the latter studies have been modified slightly by the available potassium-argon dating (Armstrong and others, 1975; 1980) and more recent stratigraphic studies (for example, Kimmel, 1979; Neville and others, 1979; Swirydczuk, 1980; summarized in Leeman, 1982b this volume). In Table 1 and in previous discussions the chronostratigraphic epochs referred to are those of Berggren (1972). Most notably, the easterly time-transgressive nature of many volcanic and sedimentary units is now recognized. The stratigraphic succession from a late Eocene to middle Miocene depositional hiatus is generally similar throughout the western Snake River Plain and in the Magic Reservoir area in that early volcanism is dominantly rhyolitic with minor intercalated basalt flows. With time basaltic volca-

nism became dominant. However, the continuation of rhyolitic volcanism to as recently as 3 million years ago in the Magic Reservoir area appears to be somewhat atypical for this part of the Snake River Plain. It is noteworthy that the composition of the 3-million-year-old Wedge Butte rhyolite is relatively “evolved” compared with most Snake River Plain rhyolites (Leeman, 1982~ this volume), and it may represent highly differentiated magma erupted during the declining stages of activity of the Magic Reservoir volcanic center.

STRUCTURAL DEVELOPMENT

Structural development within the map area is partly known from the relations of geologic events, depositional history, and distribution of faults (Schmidt, 1961; Smith, 1966; Malde and others, 1963). In Miocene time, topographic highs ap- parently existed east of Magic Reservoir and in the eastern Mount Bennett Hills because alluvial gravels prograded northward from these areas and they influenced the distribution of sediments derived from the north. Early ash-flow tuffs were deposited on a dissected erosional surface of moderate relief. Later ash flows, apparently from southerly sources, thin to the north, which suggests that the deposi- tional surface was tilted toward the south. In support of this inference in the Mount Bennett Hills, flow structures in some of these ash flows indicate that they partly drained downslope (southward) after deposition, and late Miocene basalts also flowed to the south. Smith (1966) documented a complex history of faulting, based on stratigraphic and depositional evidence, in which normal faults were activated intermittently and partly controlled the distribution of volcanic and sedimentary units. During Miocene and Pliocene time two dominant sets of normal faults were active: (1) an east-trending set which produced minor horst and graben structures and (2) a northwest-trending set which accommodated the greatest vertical displacements and horizontal extension. Major uplift of the Mount Bennett Hills apparently occurred after deposition of City of Rocks Tuff and prior to extrusion of Bruneau age basalts (Malde, 1959; Smith, 1966). Bruneau age and younger faulting was relatively minor and is represented by small displacements on east-trending faults. Tilting toward the south continued after deposition of the Clay Bank Basalt (late Pliocene ?) because these lavas dip southward about 5 to 10 degrees.

Smith (1966) interpreted fault patterns in the eastern Mount Bennett Hills as a conjugate system caused by dominantly right-lateral wrench faulting.

374 Cenozoic Geology of Idaho

However, major extension normal to the northwest- trending fault set is also consistent with inferred southwest regional extension since middle Miocene time (Leeman, 1982~ this volume). Analysis of these structures is not sufficiently detailed at present to evaluate adequately the causative stress field or temporal changes in it.

MAGIC RESERVOIR VOLCANIC CENTER-A BURIED CALDERA?

Several lines of evidence suggest the presence of a buried caldera near Magic Reservoir. A set of at least four domes of Moonstone Rhyolite define an arcuate trace along which Magic Hot Springs (Young and Mitchell, 1973) is located. This trace also includes inferred vent areas for Square Mountain Basalt and the Poison Creek Tuff and follows the prominent topographic scarp on the east side of Magic Reservoir. A set of four Quaternary basalt vents forms an arcuate extension of the trace to the south and west, and it is inferred that this trace continues northwestward into a zone of northwest-trending faults along which the north- eastern block was downdropped (Smith, 1966) near the terminus of the Mount Bennett Hills (see inferred caldera limits, Figure 2). It is more difficult to estimate the trace in the northwest sector where young basalt and sediment cover is extensive. Within the inferred caldera structure, the oldest rocks exposed are Moonstone Rhyolite which forms an extensive lava plateau; this relationship is reminiscent of the Plateau Rhyolites within the Yellowstone caldera and intracaldera rhyolites of the Bruneau- Jarbidge eruptive center (Bonnichsen, 1982b this volume). The central area within the inferred caldera is topographically higher than its margins, not unlike a resurgent dome. Challis and Idaho batholith basement rocks and Idavada tuffs outcrop very close to the west, north, and east margins of the structure at higher elevations, yet they are not exposed within its limits. It is particularly difficult to explain the absence of Idavada rocks, which were extensively deposited along the north margin of the Snake River Plain, unless they subsided or were removed during a caldera-forming event. Further evidence supporting the presence of a caldera collapse structure is the great thickness (200 meters) of Square Mountain Basalt just south of the inferred caldera fault near Moonstone Mountain. North of this structure the Square Mountain Basalt lavas are considerably thinner where they were deposited on granodiorite basement.

Assuming that a caldera is buried in this area, it is

of interest to review constraints on how and when it formed. Present data are too incomplete to assign this area as a source for any of the Idavada ash-flow tuffs. Many of those in the Mount Bennett Hills appear to thicken southward, suggesting that their source areas lie in that direction. Limited data for the Picabo Tuff (Schmidt, 1961) suggest that the lower member indeed thickens in the direction of Magic Reservoir. However, preliminary petrographic studies reveal that none of the Idavada tuffs is as porphyritic or contains phenocryst assemblages closely resembling Moonstone Rhyolite, which contains abundant phenocrysts of sanidine and quartz. More detailed chemical, mineralogical, and stratigraphic studies are required to firmly test a genetic relationship. The Poison Creek Tuff certainly originated from vents in the northeastern part of the inferred caldera, and these rocks are similar petrographically (though they are not as abundantly porphyritic) to certain Moonstone Rhyolite litholo- gies and contain xenoliths of Moonstone Rhyolite. The volume and area1 extent of Poison Creek Tuff are not large, and this unit may only represent a late-stage eruptive episode from the inferred caldera. Based on limiting potassium-argon ages, dominant eruptive activity probably occurred between 5 to 10 million years ago, but emplacement of late-stage domes apparently continued to at least 3 million years ago.

Much detailed work remains before the geologic history and even the existence of a buried caldera are firmly established. The locations of buried calderas in other parts of the Snake River Plain, initially based on geological inferrences such as presented here (compare, Citron, 1976; Kuntz and Covington, 1979; Prostka, 1980; Embree and others, 1982 this volume; Bonnichsen, 1982a this volume), have been strongly confirmed by geophysical studies and deep drilling (for example, Prostka and Embree, 1978; Doherty and others, 1979). Such studies are desirable in the Magic Reservoir area and may be justifiable in terms of the geothermal potential of such a relatively young magma system.

ACKNOWLEDGMENTS

The basis of this paper is the high-quality field work by D. L. Schmidt and C. L. Smith. The present paper essentially updates and summarizes portions of their work and provides a unified map compilation with my own interpretations concerning the likely existence of a buried caldera. This work is supported by National Science Foundation Grant EAR80-18580. Manuscript preparation was immeas-

Lmmon-Geology OJ Magic Reservoir Area 37s

urably aided by A. Elsweiler and A. Walters on the keyboards and D. Hicken on map compilation.

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