My final document for my geology capstone at IUPUI (G420).

1. Geology of
the
Northwestern
Part of the
Pony
Quadrangle
By: Ian Marrs

2. Ian Marrs
G420
1
Geology of the Northwestern Part of the Pony Quadrangle
Abstract
Brownback Gulch in Montana offers a unique perspective into cratonal sequences and how
cratons change through time. The area records pre-Cambrian metamorphic rocks, an unconformity
between them and Paleozoic sedimentary rocks through the Mississippian Era, and deformation that
affected them. Using field observations, the depositional history of the Paleozoic rocks can be
understood primarily through cratonic sequences of oceanic transgression and regression. An analysis of
the deformation events recorded in the area describes events which led to the folding and faulting of
units in the area. A detailed understanding of these processes and their inter-relationships allows for an
understanding of paleo- and modern deposition and tectonic stresses as they occur throughout the
world.
Study Area
The specific areas studied in this report are a northwestern portion of the Pony Quadrangle and
a southwestern portion of the Jefferson Island Quadrangle (see Plate 1). The study area is located near
Cardwell, MT (Figure 1) and the most intensely surveyed portion of the area was in Brownback Gulch.
These areas provide a unique perspective on how the North American craton formed, what
sedimentation occurred along its western margin, and what kinds of deformation can be expected in
this area of the world.
Introduction
Overall cratonic formation, sedimentation, and deformation are crucial to the understanding of
modern processes, particularly along the margins of continents. This is due to cratons having the
capacity to preserve long stretches of time in the geologic record which allows a variety of depositional
environments and alteration/deformation processes to be studied in relatively contained areas. The
area discussed below displays evidence of having experienced orogenic stresses and deformation
processes which have affected the local rocks greatly.
Stratigraphy
Pre-Cambrian
Surviving records of cratonization show craton genesis’s on Earth in the late Archean (Montagne
et al.) (~3.3 Ga (GSA, 2018)) during the last major crust-forming event. Such Archean cratons necessitate
a structure which extends to the mantle in order to survive convective mantle forces for billions of years
(Meuller and Frost).
These basement rocks of the North American and other associated cratons were originally
crystalline igneous rocks, likely mafic in composition. This is evidenced by the complexes mainly
comprised of gneiss, amphibolite, pegmatite, and weakly foliated granofels observed in the field.
Pegmatite formation occurs when quartz-rich fluid is emplaced into a rock unit. The protoliths of the
gneisses, amphibolites, and granofels were likely intrusive igneous in the case of the gneiss and granofel
while the amphibolite was likely a mafic magma intrusion. Such intrusive rocks have been observed ad
dated to ~2.7 Ga (Foster et al., 2006) and specifically mafic dikes at 1.2-1.1 Ga (Whitmeyer and
Karlstrom, 2007). However, a lower temperature metamorphic event may have been able to produce

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conditions that would form weakly-foliated granofels from extrusive rocks as well. All the rocks detailed
here have mineral assemblages (garnet, plagioclase, hornblende, and biotite) corresponding to the
amphibolite facies which is further evidenced by the presence of hydrated minerals present within the
observed rocks.
The amphibolites and pegmatites are often present as dikes, sometimes cutting through pre-
existing gneiss foliations many of which are folded, occasionally multiple times. The folding does not
affect all the dikes observed in the field which implies that there was a potential repeating sequence of
folding and dike emplacement (emplacement, folding, emplacement, etc.). One such sequence is
observed to have occurred in an outcrop of gneiss near the study area in similarly aged Archean
gneisses, thus such sequences are inferred to have occurred in the study area’s rocks as well.
Between the Archean and the Cambrian periods, widespread exhumation of the Archean
basement rock occurred. Subsequently, an unconformity is observed between the Archean basement
and Paleozoic sedimentary rock units in the study area. The exact timing of the unconformity’s source is
unclear but the occurrence of two collisional events, the collision of the Wyoming craton with Laurentia
and the Big Sky orogeny, may have supplied the necessary stress regime to uplift the Archean rocks.
These events may have also fueled the magmatic processes necessary for the previously mentioned
intrusions due to the presence of the Great Falls Tectonic zone, the conversion of the cratonic passive
margin to an active margin, in the area during this time.
Field observations lend further evidence to the existence of an unconformity in the area as
metamorphic foliation attitudes and sedimentary bed strikes are perpendicular at the contact between
these units. Such behavior is typical of an unconformity. The timing of this unconformity and the nature
of the units involved correspond to ‘The Great Unconformity’ famously observed at the Grand Canyon,
AZ. This relationship between the study area and other areas on the craton is significant for
understanding how cratonal deposition and erosion sequences have occurred on Laurentia through
time.
Paleozoic
Deposition during the Paleozoic in North America can be understood as a series of sequences controlled
mainly by sea level. As ancient sea levels rose were able to intrude onto the craton sediment was carried
in and deposited, this is known as a transgressive sequence. Such sequences are often well preserved in
outcrop and offer insight into the depositional environments of times in North America’s past. The
opposite is true when the sea levels lower, or regress, from the craton. Regressive sequences are often
poorly preserved, if at all, in outcrop. Transgressive and regressive sequences can be detailed across
much of the lateral expanse of the North American craton (Figure 2).
Cambrian
In the lower portion of the middle Cambrian (~520 Ma) deposition of pebble conglomerates and
arenites (quartz and feldspathic) occurred. This sequence of conglomerates into arenites represents the
environmental shift from near-shore to off-shore deposition. The surveyed area of the craton was a
quartz-sand dominated region, likely a beach due to the lowermost units being in proximity to shallow-
and deep-marine facies. Following this, layers of carbonate mudstone, shale, and siltstone were
deposited, which likely represents the transgression of a seaway across the North American craton. As
the sea continued to transgress, units of shallow-marine facies such as carbonate mudstone and coated

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grain packstone were deposited. The cross bedding seen within these units shows a paleoflow direction
to the West which corresponds to features seen in the Flathead Formation and Wolsey Shale (See Plate
2). This paleoflow data is relevant because it may indicate that the study area was depositing sediment
into the Williston Basin (Sandberg, 1962).
In the upper portion of the middle Cambrian units of shale, sparry mud/packstone and,
carbonate mudstones were deposited. These units were then overlain by coarse-grained wackestones
and intraformational conglomerates in the lower portion of the upper Cambrian (~490 Ma). These two
formations likely represent a high point in the sea’s intrusion onto the North American craton in the
Cambrian, known as the Sauk sequence (Sloss, 1963).
At the end of the Cambrian, deposition occurred of a relatively small shale unit sometimes
grading into larger grained beds of sandy limestone, some of which were later oxidized and/or
dolomitized. This unit also experience widespread bioturbation. Dolomitization occurs in modern tidal
flats in arid climates, such as the Middle East and the known pathways of the continents during the
Devonian suggests this widespread dolomitization may have occurred due to the North American
craton’s near-equatorial position (Figure 3) during this time. This pattern of regression and
dolomitization is observed to repeat in the study area.
Devonian
Following the Cambrian, an unconformity (late-upper Cambrian to upper Devonian; ~110 Ma) is
observed in the study area and no outcrops of Ordovician, Silurian, or early Devonian rocks are
observed. However, the Kaskaskia sequence began in the Devonian, marking another transgression of
the sea onto the North American craton. The source of the unconformity is unclear.
In the upper Devonian units of limestone were deposited and later dolomitized. The resulting
dolostones are well preserved in outcrop along with calcareous shales which appear more resistant to
erosion than units surrounding them. The Devonian dolostones are found to be immensely thick in the
study area (see Plate 3) which may indicate lack of subsequent erosion processes, unobserved igneous
intrusions beneath the units, or a unique characteristic about the dolomitization process for these units
which makes them more resistant than other carbonates in the area. The younger of the Devonian units,
the Jefferson Formation, gives off a pungent smell of sulfur when a fresh rock face is exposed. Such
smells are not uncommon among evaporites or dolostones, but the strength of this unit’s smell is
characteristic and may be important for understanding why this unit is so thick in this area.
In the transition between the upper Devonian and the lower Mississippian there was a sudden
influx of terrigenous material, particularly mature sand onto the paleo-carbonate shelf which dominated
the western United States at the time. These terrigenous quartz arenite sands are interbedded with
sandy limestone and shale including. Some carbonates in the unit were later dolomitized. The source of
this siliciclastic sediment is a point of contention and the exact source is unknown currently. However,
the Antler and Caledonian Orogenies were taking place during this time period which, if conditions were
adequate, may have supplied such sediment across the carbonate shelf to the study area.
Mississippian
During the lower Mississippian, the units comprising the Madison Group (the Lodgepole and
Mission Canyon formations) were deposited. Thin bedding lower in the unit overlain by thicker bedding

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is characteristic. The Lodgepole formation often displays regular banding between more and less
dolomitized material which presents as alternating beds of grey and gold/brown carbonate material.
Bryozoans, corals, crinoids, and brachiopods are common along with bioturbation record deposition on
a shallow carbonate shelf.
At approximately 340 Ma deposition of very thickly bedded carbonates (fossiliferous limestone
and calcic dolostone) and siltstones occurred. The bottom of the Mission Canyon formation is
interbeded with the underlying Lodgepole formation. This interbedding may have resulted from sea
levels fluctuating rapidly, leading to interbedding at outcrop scale and such fluctuations would not be
uncommon during the Kaskaskia sequence. These beds also contain fossils common to the Mississippian
along with chert nodules. Following this unit’s deposition, another unconformity is observed between
the Mississippian and the Permian. This is likely due in part to the Cordilleran orogeny (Fuentes et al.,
2009).
Deformation
The presence of multiple folds which are cut by dikes and faults implies a complex tectonic
regime in the area. The study area has experienced at six five orogenic events; the Trans-Hudson, Big
Sky, Antler, Cordilleran, Sevier, and Laramide orogenies. Such orogenic events have been known to
cause massive deformation in pre-existing rock units. As mentioned below, all the discussed
deformation events are not uncommon in the Laramide orogeny, which occurred during the Cretaceous
and Tertiary, so it can be assumed to have been responsible for all of them that involve the Paleozoic
units in the area.
Folding
Folded rock units are observed in both the Archean metamorphic and Paleozoic sedimentary
units observed in the study area. The exact source of the folds is unclear but, as discussed below the
folding of the metamorphic units can be shown to have occurred during a different stress event than
that which folded the Paleozoic units. At least following the deposition of the Lodgepole Formation in
the lower Mississippian (~350 Ma), deformation led to a folding event in the area. This is evidenced by
field observations of bedded carbonates belonging to the Jefferson, Three Forks, and Lodgepole
formations being folded in the area having attitudes which describe a synclinal fold in Brownback Gulch
(Figure 4A). Minor folds are observed within the limbs of these units but are not described further. The
syncline fold can be said to have occurred due to a different stress regime than folds observed in
metamorphic foliations in the study area though the latter of these folds likely occurred much earlier.
This is evidenced by the different trends and plunges of the fold hinges (Figure 4B).
Faulting
The Brownback Gulch fold was subsequently cut by a normal fault. This fault cuts across
Brownback Gulch from the northwest to the southeast at approximately a 70° dip and intersects the
Lodgepole, Three Forks, and Jefferson formations. Such a dip angle for a normal fault is not impossible
but uncommonly close to vertical. There is potential for the fault to have actually been a reverse fault
which was subsequently rotated and now appears as a normal fault in outcrop. Such an event is not
unprecedented and may explain the higher than typical angle for a normal fault that is observed. More
mapping in the northwestern section of the study site would likely yield a more complete explanation of
this fault.

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This fault is evidenced by the field observation of outcroppings of these formations being found
directly next to Archean gneisses with foliation attitudes parallel to the strikes of the sedimentary beds.
Faults which occurred during the Laramide orogeny typically cut to the basement rocks in affected
areas, which is the case in the study area.
Tilting
In the northeastern portion of the studied area are a series of formations (Flathead to Mission
Canyon) that are observed to be tilted at approximately 41°. The exact cause of the tilting is not clear.
However, stresses on these units must have at least occurred following the deposition of the Mission
Canyon formation as it and the underlying units are involved in the tilting. Such tilting may have either
occurred during an uplift event, such as igneous intrusion, or the previously noted rotation of a faulted
area but the exact cause is unclear and not discussed further.
Conclusion
The study area is one of cyclic change and has been subjected to tectonic and depositional
events throughout its history. Such changes observed in the geologic record allow for an understanding
of what future changes to the planet’s oceans and its geography may hold. There are still several
questions that remain unanswered such as the timing of the Brownback Gulch fold, the source of certain
unconformities, the cause of uniquely large sedimentary units observed in the area and the source of
terrigenous material observed in the Three Forks formation, and the cause of abnormal fault angle.
References
Foster, D. A., Meuller, P. A., Mogk, D. W., Wooden, J. L., Vogl, J. J., 2006, Proterozoic evolution of the
western margin of the Wyoming Craton: implications for the tectonic and magmatic evolution of
the Northern Rocky Mountains: Canadian Journal of Earth Sciences v. 43, 1601-1619.
Fuentes, F., Decelles, P., Gehrels, G. E., 2009, Jurassic onset of foreland basin deposition in northwestern
Montana, USA: Implications for along-strike synchroneity of Cordilleran orogenic activity:
Geology, v. 37, 379-382, DOI: 10.1130/G25557A.1.
Geologic Society of America, 2018; GSA Geologic Time Scale v. 5.0.
Meuller, P. A., and Frost, C. D., 2006, The Wyoming Province: a distinctive Archean craton in Laurentian
North America: Canada Journal of Earth Science, v. 43, p. 1391-1397, doi:10.1139/E06-075.
Montagne, J., Goering, J. and Vaniman, C., Idealized Stratigraphic Column Northern Gallatin and
Madison Ranges Montana. After: Hall, 1961; McMannis and Chadwick, 1964; Witkind, 1969;
Kehew, 1971; Montagne, C., 1972; Montagne J., 1975.
Sandberg, C. A., 1962, Geology of the Williston Basin, North Dakota, Montana, and South Dakota, with
reference to subsurface disposal of radioactive wastes: United States Department of the Interior
Geological Survey, TEI-809.
Sloss, L. L., 1963, Sequences in the Cratonic Interior of North America: Gcologicul Society of America
Bulletin, v. 74, p. 93-114, https://doi.org/10.1130/0016-7606(1963)74[93:SITCIO]2.0.CO;2.

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Whitmeyer, S. J., Karlstrom, K. E., 2007, Tectonic model for the Proterozoic growth of North America,
v.3, p. 220-259, doi: 10.1130/GES00055.1.
Figures
Figure 1. Map showing Archean craton locations and relationships to surrounding orogens in the
northwestern United States and portions of Canada. Red star indicates study area for this report.
Adapted from Meuller and Frost (2006).

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Figure 2. Record of depositional sequences on the North American craton through time. X-axis
represents a geographical span from around central Nevada on the left to the Appalachian belt on the
right. Black sections represent areas lacking deposition across space while the white/stippled sections
represent those areas of recorded deposition. White/stippled pattern only used to differentiate cratonic
sequences, not changes in lithology, type of deposition, etc. Adapted from Sloss (1963).

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Figure 3. Position of the continents during the late Devonian. Adapted from Vernikovsky, V. A., Metelkin,
D. V., Yu, T., Malyshev, N. A., Petrov, O., Sobolev, N. N., and Matushkin, N. Y., 2013, Concerning the issue
of paleotectonic reconstructions in the Arctic and of the tectonic unity of the New Siberian Islands
Terrane: New paleomagnetic and paleontological data: Russian Geology and Geophysics, v. 54, DOI:
10.1134/S1028334X13080072.

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A.

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B.
Figure 4. A) Stereographic projection of Brownback Gulch syncline. B) Stereographic projection of
metamorphic synform fold.

12. PLATE 1

13. Mmc
76
7460
65
48
48
74
61 41
65
6855
55
53
33
40
38
39 40
3
21
35
47 49
43
27
53
45
35
60
66
36
21
69
70
74
70
74
39
65
62
60
52
Agn
Agn
Cf
Cw
Cm
Cpa
Cp
Dj
Dtf
Ml
Cf
Cw
Cm
Cpa
Cp
Dj
Cf
Cw
Cm
Dj
Dtf Ml
Legend
Dj JEFFERSON FM.
PARK SH.
MEAGHER LS.
FLATHEAD FM.
WOLSEY SH.
PILGRIM LS.
Cf
Cd
Cpa
Cm
Cw
Cp DRY CREEK SH.
Strike and dip of bedding
300
67
CRYSTALLINE
AND META-
MORPHIC
ROCKS
Agn
Dtf
Ml
Mmc MISSION CANYON FM.
LODGE POLE FM.
THREE FORKS FM.
56
Strike and dip of foliation
70
Fault contact with dip
Less well-located fault
contact
Trace of syncline fold
Geologic Map of the Northwestern Part of the Pony Quadrangle
Compiled by: Ian Marrs
1km
N

14. PLATE 2

15. Crystalline and Metamorphic Rocks
Flathead Formation
Wolsey Shale
Meagher Limestone
Park Shale
Pilgrim Limestone
Dry Creek Shale
Jefferson Formation
Three Forks Formation
Lodgepole Formation
Mission Canyon Formation
Thick beds of gneiss, amphibolite, pegmatite, and weakly foliated granofels. Oldest areas include mostly gneiss, sometimes folded and cut by dikes of amphibolites and pregmatites. Differing grain sizes and fabrics between outcroppings is observed. Feldspars, bio-
tites, hornblende, epidote, quartz, magnetite, and garnet are often present. Folds are observed in outcrop and dikes may be interfolded or cut across both folding and foliation.
Thick beds and laminae of quartz arenite and shale respectively. Fresh arenite color appears white and orange on fresh surface but dark grey and dark orange on weathered. The quartz grains are medium-coarse sand in size. Glauconite also appears on erosional
surfaces and within beds as a green, fine sand sized grain. Wavy, non-parallel cross bedding. Quartz granule conglomerates are found near the bottom of the unit.
Thick beds of carbonate mudstone and siltstone with thick laminae of shale. All are planar parallel. Fresh color of the mudstone and shale is dark grey while the weathered color is a lighter grey. Fresh color of the siltstone is predominantly grey with dull red hue
while the weathered color is a brighter red with grey undertones. Tool marks and trace fossils appear but are uncommon.
Thick beds of predominantly carbonate mudstone with coated grain packstone interbedded. The carbonate mudstone has fine sand inclusions but is dominated by silt grains. The coated grain packstone has grains ranging from fine to medium sand with millime-
ter sized coated grains whose centers appear spary. Burrows, mottled bedding, and cross bedding appear throughout. The mottled bedding often covers most bed-faces. Fractures that in-filled with calcite are also present.
Thick beds of spary and carbonate mudstone and green shale interbedded with fine-grained sandstone and a lens of spary packstone laminae. The lens appears as a grey, fine-sand bed with laminae of orange, oxidized minerals with fine sand sized grains. Mottled
bedding present in the lens. Fresh color of the mudstone and shale is a dark grey while the weathered color is a lighter grey. Fresh color of the green shale is a dark green/grey while the weathered surface appears as a light grey with noticeable green tones. Planar
breakages of the shale and carbonate mudstone present due to erosion of glauconite.
Very thick beds of grey, coarse-grained wackstone and dolostone with silt to sand sized grains. Beds are planar, non-parallel containing very thin wavy, parallel laminae. Intrusions of calcite with mottled bedding and intraclasts included. Clasts range from cm to
10s of cm in length and width. Pinch outs present.
Wavy, parallel beds of green shale alongside black and brown shale with oxidized surfaces. Fractures appear throughout and are often in-filled with calcite. Mottled bedding and vertical burrows present. Grey, spary arenite with calcite cement and dolomitized
limestone present. Spary arenite contains fine grained sand grains.
Planar, parallel, and continuous dolostone beds of varying, usually very thick, size often with laminae present. Grey, fresh dolostone surfaces and dark grey weathered. Laminae are usually planar, parallel, and discontinuous. Varying degrees of dolomitization and
in-filled areas are seen throughout. Intraclasts of the unit’s bed may be present along-side sandy lenses. Oxidized calcite nodules and quartz nodules are common. Some beds may have a mottled appearance where dolostone and coarser-grained spary dolomite
are interbedded. Algeal fossils may be present. Argillaceous texture, solution breccia (especially near the top of the unit), and smell of sulfur is indicative of the formation.
Typically, thinly bedded planar, parallel, discontinuous beds of calcic dolostone. Oxidized calcite concretions may be present. Near the top of the unit gradation into sandstone or inclusions of fine quartz sand may appear and beds may become wavy and discontin-
uous. The unit may retrograde and form sequences of argillaceous carbonate material and sandstone. Also, black, calcareous shale with thin, dark grey laminae with often discontinuous beds. Planar, parallel, and continuous thin laminae are present
Wavy, parallel, continuous very thin beds of grey carbonate mudstone and fossiliferous limestone with dark grey interior and argillaceous texture. Areas may display blue-golden color patterning in bands. Gold banded areas are thin planar, parallel, continuous
beds, are highly oxidized and may contain terrigenous material as clasts. Blue bands are thick beds of calcic dolostone often with parallel, planar, continuous laminae present. Bryozoan, horn coral, crinoid, brachiopod fossils, and bioturbation found throughout.
Rip-up clasts and turbidite sequences may occur near bottom. When beds are tilted near vertical, porpising may occur. Interbedding of overlain formation may occur near top.
Light grey thickly bedded wavy, parallel, continuous beds of carbonate bearing siltstone and interbedded argillaceous fossiliferous limestone. Horn corals and crinoids present. Presence of black chert is indicative of formation and presents as nodules. Chert ap-
pears as irregular black protrusions which cross bedding planes due to replacement followed by differential erosion of carbonate. Nodules may be amorphous concretions multiple centimeters across or long strands ranging up to 10s of centimeters.
Note: this thickness is to the top of the unit measured in the area, the actual top of the unit is not exposed.
Lorem ipsum
Pre-
cambrian
Paleozoic
Cambrian
MiddleUpper
Devonian
Upper
Mississippian
Lower
Stratigraphic Column of the Northwestern Part of the Pony Quadrangle
Compiled by: Ian Marrs
ABBREVIATED LITHOLOGY
TIME UNITS
ERA EPOCHPERIOD
FORMATION
THICKNESSINDEF.
405’
1458’
729’
1296’
51’
324’
81’
729’
81’
324’

16. PLATE 3