The History and Geology of Rocks and Fossils in Helena, Montana
THE ROCKS AND FOSSILS COMPOSING HELENA'S GEOLOGIC HISTORY WERE LITHOLOGICALLY AND PALEONTOLOGICALLY CRUCIAL TO THE DEVELOPMENT AND EXISTENCE OF HELENA, MONTANA. The samples chosen for this project were located in the immediate Helena area or a short distance away. The samples reflect the numerous industrial and architectural uses along with the economic advantages of the gemstones found in and around the Helena, Montana, area.
The rocks and fossils were traced to their corresponding formation, mapped, and classified according to their lithology and classification. These various types of rocks occupy a large section of geologic time from Precambrian to Tertiary.
Geology of Helena, Montana
Because of the extensive paleogeology and geology of the area, it is crucial to use rock samples from a vast area surrounding Helena. Helena sits on many beds of rock that date back to the Late Precambrian period, about 1,400 million years ago. Helena sits at the northeastern most part of the Basin and Range geological province. Here, as in Utah and Nevada, the continental crust has been stretched, resulting in a series of nearly parallel mountain ranges and valleys. Mountain building began in the late Cretaceous period and continues today. Helena often experiences small seismic tremors.
Helena, with a population of 30,000, is a town that was built as a result of the mining boom of the 1800s, specifically the discovery of gold in 1864 near Last Chance Gulch. Although the geology of Helena gave it a head start, the preparation of its leaders led Helena, the state capital, to the thriving economic community in the beautiful state of Montana that it is today.
The major portion of the city overlies the Helena Dolomite of the Belt Super-group, a Late Precambrian formation of thick yellow-weathering dolomite. Deposits of Early Cambrian sediments have eroded, giving way to Middle Cambrian strata. Helena also sits on the northernmost zone of an immense granite or granodiorite field collectively known as Boulder Batholith. The main body of Helena is situated at the base of the northeast slope of Mount Helena on a pediment that extends northward into the Prickly Pear Valley. The valley itself was named by Lewis and Clark in 1805 for its abundant and irritating prickly pear cactus. The main topographical feature of the Helena valley is the ever-so-formidable Mount Helena, rising 1,400 feet above the valley. Mount Helena exhibits its strata like a layer cake, boasting seven decompositional layers.
The gold that ran from the veins in Last Chance Gulch made Helena a boom town; however, this was not its most prized mineral. Sapphires were also found in abundance, making Helena a miner's paradise. From its booming beginnings, and designation as the state capital, Helena has always been a cherished spot in Montana.
The first to map the area, geologically speaking, was Adolf Knoph (1957, 1963). Knoph placed the major portion of the city over several members of the Proterozoic Belt Super-group. He also mapped formations around the city, such as the 1,000 foot thick Empire Formation, found in an area known as the Scracligravel Hills, five miles north of the city. Overlapping the Empire Formation is the Helena Dolomite, a Late Precambrian 4,000-foot-thick formation composed of siliceous, buff to yellow weathering dolomite. The Snowslip (formally Marsh) Formation is interbedded with the Helena Dolomite. The Snowslip consists of reddish-purple, pale, argillaceous siltstone, and mudflake conglomerate with small beds of quartzite.
Close-up view of granodiorite
Rhyolitic breccia is a fragmental igneous rock. It is mainly composed of pink orthoclase feldspar, plagioclase feldspar, quartz, and ferromagnesian minerals with clasts of country rock, common opal, and other quartz varieties. Rhyolitic breccia deposits represent a very explosive eruption of volcanic material that geologists call a diatrene. Brownstone is light brown with other rock particles ingrained in the stone.
The first recorded usage of the breccia as a building stone was noted in the Helena Board of Trade Report for 1887, which acknowledged the breccia as "beautiful, pale, pink porphery." They also noted
it as "entirely indestructible, and were our whole city built of it, it would be as long lasting as the hills."
Section 2 Limestone
Limestone is a variety of sedimentary rock composed essentially of calcite (calcium carbonate). When burned or calcined, the limestone generates lime. More than a dozen old lime kilns, and the 100-year-old Ash Grove Cement plant still operating, attest to the industrial use of limestone in the area. With the great amount of limestone and a younger pluton intrusion, one can conclude that there is a great deal of marble, a crystalline metamorphosed form of limestone. Much of the limestone in Helena was formed by consolidation of seashells and chemical precipitation of calcium carbonate directly out of warm Paleozoic shallow seas. Two limestone formations have been utilized for industrial lime, the Cambrian Meagher Formation and the Mississippian Lodgepole Formation.
The limestone collected is of the Lodgepole Formation and dates to the early part of the Mississippian period. The Lodgepole limestone contains plenty of evidence that marine animals were present here when the sediment accumulated. Fossils such as horn coral and brachiopods, and bryozoans and crinoid fragments, are typical specimens brought out of the area. To date, few people have collected and analyzed the fossils found at Lodgepole. The formation contains nice batches of fossils that provide a window to the world of the Mississippian life form. The fossils found there are mostly broken shards of sessile filter feeders. The animals that lived here must have lived in a shallow marine environment less than 300 feet deep because algae fossils are also present. The limestone site is one of the best places in Montana for batches of fossils, shedding light on the life and times of the world 350 million years ago.
Section 3 Granodiorite
Grandiorite is a plutonic igneous rock intermediate in composition between granite and diorite. It has much less orthoclase than granite but more orthoclase than diorite. It is composed of white plagioclase, some pink orthoclase feldspar, and quartz, with lesser amounts of biotite and hornblende. In Helena and surrounding areas, granodiorite is quite an abundant rock. As for its megascopic characteristics, this rock is light to medium gray, with traces of rust and black specks, and is of phaneritic texture. The weathering of iron in the biotite and hornblende produce the rustic coloration.
Feldspar is an abundant aluminum silicate mineral. Plagioclase feldspar contains sodium and calcium deposits. Most feldspar minerals occur in blocky crystals. Biotite is a black variety of mica, a sheet-like silicate mineral. Hornblende is a common silicate mineral that crystallizes into glossy black needles. Hornblende is found in light-colored igneous and metamorphic rocks such as granite. Quartz is a very common mineral composed of silicon dioxide. The elements and composition of granodiorite greatly mimic the composition of granite, hence the name "granodiorite."
The specimen I collected came from Clancy, a small town 10 miles south of Helena on Interstate 15. These grandiorite rocks are abundant and run the length of the Boulder Batholith, between Helena and Butte, some 60 miles. The Boulder Batholith was once a molten pluton that intruded during the formation of the early Rocky Mountains about 70 million years ago during the Cretaceous period. Hot "salty" solutions of water seeping into cracks in the cooling, solidifying batholith generated the rich lode and placed ore deposits of gold, silver, copper, and manganese in the Butte-Helena region. The Boulder Batholith contains Cretaceous age granite that covers the hillsides with boulders. Granite-type rocks form joints and sheet jointing when overlying rocks have been eroded away for millions of years. This is called exfoliation and is often why granite formations are called "elephant heads."
Section 4 Flathead Sandstone
Sandstone is a variety of sedimentary rock formed through the accumulation of sand that has solidified into rock. The sandstone I collected rests unconformably above rocks of the Precambrian-age formation mostly consisting of the Spokane, Empire, and Helena Dolomite. The Flathead Sandstone is middle Cambrian in age and represents a sandy shoreline of a marine transgression sequence. Sandstone comes in a wide array of colors and types in North America, some of which include gray, red, argillaceous, and glauconitic sandstone. All of these types of sandstone can be found in Montana.
As with all sedimentary rocks, the sediment layers are usually clearly visible. The composition of sandstone is the same as sand; thus the rock is essentially composed of quartz. The bonding material that cements together the granules of sand is usually composed of silica, calcium carbonate, or iron oxide. Color of a rock is largely determined by the cementing material, iron oxides that cause a reddish-brown pigment in the sandstone.
Flathead Sandstone forms predominate knolls at the base of Mount Helena and Mount Ascension. Helena's landmark, "The Guardian of the Gulch," sits atop one of these Flathead Sandstone knolls.
The sandstone collected for this project was found southwest of Mount Helena. It is of surprising hardness, like most of the sandstone found in Montana. Many sandstones in Montana have a fair portion of quartz and are cemented very firmly together and not effortlessly scratched or broken.
Section 5 Pumice
Pumice is a form of volcanic igneous rock belonging to the rhyolite family, and is a white, finely grained vesicular silica mineral commonly found in Montana. Members of the rhyolite family usually have the same composition of granite. Pumice often contains crystals of quartz and feldspar. Pumice is formed by froth produced during an eruption of gas-rich magma. When the magma, then called lava, reaches the surface, the lava experiences a decrease of confining pressure, in turn producing froth which produces the pumice. The decreasing pressure is similar to the bubbles formed in carbonated beverages when their containers are opened. Froth bubbles formed by the eruption of a volcano make pumice light enough that it will float for weeks until the rock becomes water-logged.
Pumice is used for practical purposes such as abrasives and polishing compounds. This versatile rock is also used as insulators and in lightweight aggregates or compounds. Stucco, plaster, and cement also have pumice in their compounds. The pumice I collected appears to contain pieces of tuff. Tuff is composed of pyroclastic volcanic fragments such as crystalline rock but mainly contains fine ash. Ignimbrite is a rock formed when a volcano erupts a hot gas cloud (called a "nuee ardente") and forms
a hard, massive rock collectively known as welded tuff.
The pumice collected was found southeast of Helena on I-15 highway to Montana City. It is part of the Oligocene-age Renova Formation and is locally found as pumice deposits, welded tuff, tuffaceous sandstone and shale units. It is widespread throughout the Helena valley and forms the major aquifer atop the older, harder Precambrian and Paleozoic bedrock formations.
Section 6 Three Forks Shale
Shale is a flaky sedimentary rock largely composed of clay minerals. The shale I collected, on the other hand, is fossiliferous shale, meaning it contains fossils. Shale can contain a record of the local environment, in the form of ripple marks indicating current directions, or mud cracks. Shale is the raw material used in brick, tile, china, and pottery, and when mixed with limestone, it is used to make Portland cement. Clay minerals make up at least one-third of most shales; quartz, feldspar, and mica are other important minerals that make up the composition of the mineral. A notable property of shale is its fissility, the property of splitting easily along bedding planes into thin layers.
The fossiliferous shale collected for this project contained mainly brachiopod, nautiloid, and ammonite fossils. The fossils are part of a Devonian-age Three Forks formation. The collecting site sits at the base of Mount Ascension, a predominant outcropping of hills and tall mountains on Helena's southeast side. Brachiopods are marine bivalves with unequal valves but bilateral symmetry. One form of brachiopod collected appears to be Mucrospirfer mucronatus from the Middle Devonian age. Ammonites and nautiloids are a marine group of fast-moving invertebrates with eyes and other well-developed sensory organs and tentacles with sucker discs or cups.
Section 7 Stromatolite
Geologists originally considered the odd pattern of stromatolite to be some sort of animal fossil. Geologists now know that these foreign and bizarre structures were probably formed in the shallow water underneath bondable mats of blue-green and green algae. Stromatolites were the first certain photosynthesizers that appeared about 3 billion years ago. Stromatolites are basically flat-pressed structures that occur in many tidal-flat limestones. They are usually formed in shallow water and begin to grow when a layer of filament of blue-green algae binds sediment together to form bondable mats. The pads or mats of algae held captive carbonate sediment, and as the pads of algae grew upward, the sediment was incorporated in layers. After the organic material
decomposed, the layered carbonate sediment remained. Stromatolite is one of the few key geologic clues to the beginning of life on the planet Earth. Stromatolites are present in rocks throughout the rest of the Precambrian and on into Cambrian. Stromatolites make up the greatest percentage of Precambrian fossils and still occur today in very limited shallow water areas. Precambrian stromatolites are presumed to have formed in the same kind of environment. The structures have given important evidence of algal life into and beyond Precambrian time.
The collected specimen was part of a formation known as the Helena Formation, conformably placed at the base of Mount Helena, the most predominant feature in the Helena area. The formation is dolomitic limestone with interbedded stromatolite structures. These stones were quarried locally and used in the construction of walls and houses.
Section 8 Diabase
A diabase sill is a tabular plane of igneous rock intruded or sandwiched into previously bedded sedimentary or volcanic rock parallel to bedding planes. The molten magma injected between the sedimentary layers of the Helena (Siyeh) Formation formed this body of igneous sill, which is known as diabase. The very hot magma came up along fissures from deep inside Earth's crust. The enormous pressure and extreme heat exceeding 1,000 degrees Celsius forced the diorite into the sedimentary layers forming the sill seen today. Diabase is formed by cooling near the surface and crystallization into varied minerals such as hornblende. It has the same mafic or basic minerals as basalt, but slower cooling than basalt allowing larger crystal growth. It is a common rock found in sills, which are sheets of igneous rock that are parallel to the layers they intrude. They form when magma is forced along bedding planes between rock layers. They are usually basalt or diabase and can be hundreds of meters thick and many kilometers long.
The diabase was found in the Helena (Siyeh) Formation, a 2,500 foot thick formation primarily composed of dolomite with some limestone. The formation contains variable amounts of finely grained quartz and clay minerals. Much of the formation contains fossil algae (stromatolites). The upper part of the formation where I found the diabase contains intruded dark grayish-brown or black diabase forming a conspicuous sheet about 1 meter thick between horizontal layers of the Helena dolomite limestone. Local geologists believe the intrusion is of Tertiary or Cretaceous age.
Section 9 Argillite
Although argillite retains sedimentary characteristics such as mudcracks and ripple marks, the rock should be classified as a metasedimentary rock. This rock of sedimentary origin is composed of recrystallized clay minerals and still retains its sedimentary features. This metamorphic rock has been altered by heat and pressure, associated with deep burial over a long period and mountain-building forces.
The two colors of argillite tell of an interesting chapter in the early evolution of planet Earth. Argillite colors tell of how they formed and what conditions were present during their formation. The green-
colored rocks tell of an oxygen-deprived world without flora to filter carbon dioxide through and produce oxygen deep within the rock. The argillite that formed at a higher level with sufficient oxygen levels and carbon dioxide filters produced a dark red rock.
The samples found were located on an embankment of the Missouri River, some 17 miles from Helena. Indicative of argillite, these samples, although metamorphic, have retained their sedimentary characteristics such as mud cracks, bedding lines, and traces of erosion.
Section 10 Sapphire
Sapphires, with a Mohs hardness of 9, are a variety of extremely hard aluminum oxide mineral known as corundum. Used as gemstones and industrial abrasives, sapphires vary in color from the common blue/green to shades such as yellow and pink. The most valued sapphire color is a deep cornflower blue. The blue tint is thanks in part to the small amounts of iron and titanium.
The gold boom of the late 1800s brought miners and vendors alike in search of the elusive sapphire and the occasional gold vein. Sapphires occur with gold, so if there is gold there are usually sapphires, and vice versa. The first discovery of precious gems was made at the Eldorado Bar, a small bank along the Missouri River east of Helena in December 1865. In September 1873, The American Journal of Science called attention to the "existence of the ruby and sapphire in North Carolina and the Montana Territory." Later, Mr. George F. Kunz, a gem expert from Messrs. Tiffany & Co., contributed to the knowledge of gemstones in an 1883 publication entitled "Mineral Resources of the United States." Later Kunz wrote an article entitled "Precious Stones in the United States," in which he said "the finest sapphires for gems are collected by the miners from sluce-boxes of placer mines near Helena, Montana." During the early decades of this century, Montana was one of the largest sapphire-producing districts in the world, second only to Burma.
The sapphires I collected were from the French Bar Industries dredge near a pediment of the Spokane Hills. The main feature of the bar is the abandoned Korizek mine shaft, which was reported to have been some 100 feet deep.
Geology and paleontology are ever-changing and ever renewing sciences that depend on the effort and dedication of strong minded, patient individuals who love their work. I hope this paper demonstrates that there will always be questions and curiosity followed by understanding and guidance that will help us better understand our world. The geology of Montana is complex as a result of the overlaying impressions left after a long succession of geologic events. Deciphering and sorting out the different impressions in the proper order is an adventurous challenge, and I have enjoyed expanding my knowledge of geology.
Geology and paleontology are to me what astronomy was to Galileo. Understanding the geologic formation of Earth is fundamental to interpreting times long since past and will always give us more questions to ponder. The pursuit of science is key to understanding the mystery of how Earth works and what we think it does. For this reason I submit this paper to be judged among others with perhaps the same goal as mine, to understand how Earth works.
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Voynick, Stephen M. The Great American Sapphire. Missoula, Montana: Mountain Press Publishing Company, 1985.
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