Structural controls of hot-spring

28 Pages·2010·0.52 MB·English

UNITED STATES
DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY
STRUCTURAL CONTROLS OF HOT-SPRING SYSTEMS
IN SOUTHWESTERN MONTANA
By Robert A. Chadwick and Robert B. Leonard
Open-File Report 79-1333
Helena, Montana
September 1979
CONTENTS
Page
SI units and inch-pound equivalents ............... Ill
Abstract. ............................ 1
Introduction. .......................... 1
Regional structural setting ................... 3
Regional structural controls of hot springs ........... 4
Local structural controls of hot springs. ............ 8
Intersection of faults ................... 8
Intersection of fault and anticlinal axis. ......... 12
Step faults or other intravalley faults. .......... 15
Intersection of fault and sedimentary aquifer. ....... 17
Minor faults and fracture zones in crystalline rock. .... 18
Thermal-water circulation .................... 18
Conclusions ........................... 20
References cited. ........................ 21
ILLUSTRATIONS
Figures 1-4. Maps showing:
1. Geologic setting of hot springs in southwestern
Montana ..................... 2
2. Hot springs at or near intersections of faults. . . 9
3. Hot springs at or near intersections of faults
or vein systems and anticlinal axis ....... 13
4. Range-front faults of the upper Jefferson Valley. . 16
TABLE
Table 1. Geologic setting of hot springs,
II
SI UNITS AND INCH-POUND EQUIVALENTS
The following factors can be used to convert the International System
(SI) of metric units in this report to the equivalent inch-pound units.
Multiply metric unit By To obtain inch-pound unit
kilometer (km) 0.6214 mile (mi)
meter (m) 3.281 foot (ft)
temperature, degrees Fahrenheit (°F) = 1.8 °C +32
III
STRUCTURAL CONTROLS OF HOT-SPRING SYSTEMS
IN SOUTHWESTERN MONTANA
by Robert A. ChadwickV and Robert B. Leonard
ABSTRACT
Thermal waters that issue as hot (more than 38°C) springs in south
western Montana appear to circulate to depth along Cenozoic block faults,
deep fractures penetrating the dominantly crystalline rock crust, or major
structural lineaments. At individual hot springs, rising thermal waters are
transmitted along conduits formed by the intersection of a major fault with
other faults, fracture zones, anticlinal axes (which may be faulted or frac
tured), or sedimentary aquifers. Step faults and other intravalley faults
may influence circulation at some springs. At others, fracture zones alone
may provide the necessary vertical permeability.
Normal regional heat apparently is sufficient to maintain the hydro-
thermal systems without enhancement from cooling igneous bodies. The thermal
gradient normally is higher in low thermal conductivity sediments of the
block-fault valleys than the 30°C per kilometer average for crystalline
rock. To attain reservoir temperatures of 60° to 120°C indicated by chemical
geothermometers, waters would have to circulate to depths of about 2 to 4
kilometers in crystalline rock and about 1 to 2 kilometers in valley sediments
INTRODUCTION
Southwestern Montana, defined herein as the area south of 47° N.
latitude and west of 110° W. longitude, is a region of numerous but scattered
thermal springs. Within this region the temperatures of 25 "hot" springs
(fig. 1), including several having multiple vents, equal or exceed 38°C
(100°F). The temperatures of most of the other thermal springs generally
range from 20° to 30°C. The well known thermal region of Yellowstone
National Park adjoins this region on the southeast.
The purpose of this report is to discuss the regional and local geologic
controls that govern the occurrence of the hot springs and their related
hydrothermal systems. The report summarizes the results of surface geologic
mapping and shallow direct-current resistivity, seismic, and soil-temperature
surveys conducted by Montana State University (Chadwick and others, 1978)
and coordinated with related hydrogeologic and geochemical investigations
conducted by the U.S. Geological Survey (Leonard and others, 1978) during
1975-77. The information is useful for the appraisal and development of
geothermal resources in Montana and may be applicable to similar geologic
settings elsewhere. Montana State University participation was made possible
by Grants Nos. 14-08-0001-G-238 and 334 from the U.S. Geological Survey
under the Extramural Geothermal Research Program.
l_f Professor, Montana State University, Bozeman, Mont.
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Figure 1. Geologic setting of hot springs in southwestern Montana,
REGIONAL STRUCTURAL SETTING
Southwestern Montana is underlain primarily by crystalline rocks Precam-
brian metamorphic rocks and Cretaceous to Tertiary batholiths. It contains
the only hot (above 38°C) springs in the State, with the exception of two
(Quinn and Camas) associated with crystalline rock in northwestern Montana.
Therefore, knowledge of the structural conditions of this southwestern Montana
crystalline province is essential to an understanding of the hot-spring
systems.
The crust of the region is considered to be a basement complex of Precam-
brian W metamorphic rocks intruded in part by Precambrian X gneiss (King,
1976). The basement is overlain in places by infolded Precambrian Y metasedi-
mentary rocks of the Belt Supergroup and Paleozoic to Mesozoic sedimentary
and volcanic rocks. During Cretaceous and Tertiary time the basement was
intruded by major granitic batholiths (Boulder, Idaho, Philipsburg, Pioneer,
Tobacco Root) and smaller intrusives.
The crystalline rock complex is broken by numerous fractures that appear
to control occurrences of thermal springs. Halbouty (1976) described two
dominant fracture directions in Montana, northwest and northeast, that prob
ably reflect trends of weakness in the basement.
Thrust faulting during Cretaceous to early Tertiary time followed by
block faulting further altered the regional fracture pattern. Cenozoic
volcanics and sedimentary rocks drape the crystalline-sedimentary crustal
blocks or fill the valleys formed within them. Present-day earthquake
zones (Smith and Sbar, 1974) tend to follow the north- to northwest-trending
pattern of thrust and block faulting, indicating that some faults are still
active. Figure 1 illustrates the distribution of crystalline rock, the
boundaries of major block-fault valleys as indicated by surface geology and
by gravity surveys (Burfeind, 1967), and selected major structural elements
in the region.
The predominantly metamorphic-granitic province is bounded on the north
by the Montana lineament, a major crustal discontinuity (Weidman, 1965),
which separates it from the Precambrian Y folded sedimentary rocks of north
western Montana. Reynolds (1977) extends the Montana lineament eastward to
the White Sulphur Springs area and postulates that late Cenozoic movement
along this zone stretched the crust to allow development of the Smith, Town-
send, and Helena block-fault valleys. Rhyolitic volcanism took place along
or near the lineament intermittently during the period 44.5 to 19.6 m.y.
(million years) ago (Williams, 1975; Chadwick, 1978). Thus, the Montana
lineament is a major crustal discontinuity that has persisted as a zone of
structural adjustment and igneous activity for an extended period.
A major northwest-trending basement zone of weakness, the Bismark-Gardiner
fault zone (fig. 1), has been intermittently active during geologic time
(Reid, 1957). This zone forms one of the major linear features of the north
western United States and is at various places along its length a steep
reverse fault, a wide crush zone, and a belt of igneous intrusive and volcanic
centers. In Montana, the Bismark fault extends southeastward from the Jeffer
son Valley across the Madison Valley (where it is termed the North Meadow
Creek fault). Continuing southeastward, the zone of weakness is buried
beneath Eocene volcanic rocks, but reappears east of the Yellowstone Valley
as a zone of Eocene volcanic and intrusive centers forming the Western Absaroka
Belt (Chadwick, 1970) and the parallel Gardiner fault. Total length from
the Jefferson Valley to the southern Absaroka Mountains of Wyoming is about
350 km.
Results of numerous studies suggest that during Cenozoic time south
western Montana was a region of extensive block faulting that developed
graben or half-graben intermontane basins generally elongated north-northwest
to north-northeast (see, for example, Pardee, 1950; Kuenzi and Fields, 1971;
Scholten and Ramspott, 1968). The province probably reflects extension of
basin-and-range type structural activity northward from the Basin and Range
province proper (Reynolds and Kleinkopf, 1977).
Kuenzi and Fields (1971) believe that basins began to be delineated in
Eocene time and that sedimentation began in late Eocene to early Oligocene
time. Oligocene sedimentation was extensive and was accompanied by ash-fall
deposits indicating one or more episodes of silicic volcanism. Rhyolitic
volcanism within Montana may have furnished much of the ash to the basins.
Rhyolitic flows and intrusive bodies in the northern Boulder batholith region
are dated as 36 to 37 m.y. (Chadwick, 1978).
Extensive erosion of basin sediments in Oligocene to middle Miocene
time produced widespread unconformities (Kuenzi and Fields, 1971). Subsidence
and renewed sedimentation during late Miocene to Pliocene time were followed
by extensive block faulting and local uplift during late Pliocene-early
Pleistocene time. Movement continued on some faults during late Pleistocene
and Holocene time as evidenced by displacement of glacial deposits and allu
vial fans (Pardee, 1950; Witkind, 1975; Weinheimer, 1979).
REGIONAL STRUCTURAL CONTROLS OF HOT SPRINGS
Most of the hot springs in the region are (a) in deeply fractured crystal
line rock terrain, or (b) marginal to or within Cenozoic block-fault valleys
underlain and bounded by predominantly crystalline rock. At some of the hot
springs both conditions are present. Several hot springs also lie along
regional faults or shear zones. Locations of the hot springs in relation to
lithology and structure are shown in figure 1, and the geologic settings are
summarized in table 1.
Eleven hot springs issue directly from deeply fractured crystalline rock
masses mainly the Boulder, Idaho, or Pioneer batholiths or related intrusive
or metamorphic rocks. Broadwater and Lolo Hot Springs are located along the
southern part of the Montana lineament system, which forms the north boundary
of the Idaho and Boulder batholiths. Alhambra, Norris, Sleeping Child, and
other hot springs are situated along minor faults within the crystalline
rock. At Elkhorn, Gallogly, and Medicine fractures rather than faults
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