Tertiary intrusions of Black Hills

1
Tertiary intrusions of Black Hills

• And some sed rx too..

2
Paleozoic sed units

• Deadwood
• Cambrian
• Sandstone/shale/conglomerate
• Transgressive shoreline and shallow marine
• Basal units deposited on PreC basement
• Upper sandstone with skolithos
• Winnipeg
• Ordovician
• Shale/siltstone
• Generally covered

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Paleozoic sed units

• Whitewood
• Ordovician
• Hackley surface/cliff former
• Equivalent of the Bighorn Dolomite
• Can lump Winnipeg with it
• Englewood
• Devonian to Mississippian
• Dark pink to buff limestone/shales
• Slope former
• Similar to 3 Forks/Jefferson in WY

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Paleozoic sed units

• Pahasapa
• Mississippian
• Same as Madison Limestone
• Cliff former
• Minnelusa
• Pennsylvanian
• Sandstone/shale/limestone
• Mostly a pinkish sandstone
• Same as Tensleep/Amsden formation in WY

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• Previous geology
• 2.5 Ga Little Elk Creek
• 2.2 and 1.9-2.0 Ga sed depostition
• 1.9-1.7 Ga Trans Hudson orogen
• 1.7 Ga Harney Peak granite
• Paleozoic sed rocks
• 40-60 Ma, Tertiary intrusions
• Related to Laramide uplift

6

• Previous geology
• 2.5 Ga Little Cottonwood
• 2.2 and 1.9-2.0 Ga sed depostition
• 1.9-1.7 Ga Trans Hudson orogen
• 1.7 Ga Harney Peak granite
• 40-60 Ma, Tertiary intrusions
• Related to Laramide uplift

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Bear Butte

• Previous geology
• 2.5 Ga Little Cottonwood
• 2.2 and 1.9-2.0 Ga sed depostition
• 1.9-1.7 Ga Trans Hudson orogen
• 1.7 Ga Harney Peak granite
• 40-60 Ma, Tertiary intrusions
• Related to Laramide uplift

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• Laramide
• Shallow subduction angle
• Subduction zone in W. Idaho
• How do we get magmatism in South Dakota?
• Too far inland to be back-arc basin

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Phaneritic rock with gt10 felsic minerals
Q
.
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
Grano-
Tonalite
Granite
Alkali Feldspar Granite
diorite
Alkali Fs.
Qtz. Diorite/
20
20
Quartz Syenite
Qtz. Gabbro
Quartz
Quartz
Quartz
Monzonite
Syenite
Monzodiorite
Alkali Fs.
5
Diorite/Gabbro/
5
Syenite
Syenite
Monzonite
Monzodiorite
Anorthosite
35
90
10
65
A
P
(Foid)-bearing
(Foid)-bearing
(Foid)-bearing
Syenite
Monzonite
Monzodiorite
(Foid)-bearing
10
10
Diorite/Gabbro
(Foid)-bearing
Alkali Fs.
Syenite
(Foid)
(Foid) Syenite
(Foid)
Monzosyenite
Monzodiorite
(Foid) Gabbro
60
60
(Foid)olites
F
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Classification of Igneous Rocks

• Aphanitic rocks
• Base on phenocrysts
• In Black Hills
• Rhyolite
• Latite
• Trachyte
• Phonolite
• Prepare to encounter all!
• Lots with aegirine (Na-pyroxene)

Figure 2-3. A classification and nomenclature of
volcanic rocks. After IUGS.
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Classification of Igneous Rocks

• Are all aphanitic rocks extrusive?

Figure 2-3. A classification and nomenclature of
volcanic rocks. After IUGS.
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What are these intrusions?

• Stock
• Sill
• Laccolith

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Why so many sills and laccoliths in Black Hills?

• Which way does magma move?
• What if a horizontal plane of weakness is there?
• Which rock type has lots of horizontal planes of
  weakness?
• Which sed unit will you expect to find a lot of
  sills/laccoliths in?

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Depth of intrusion

• Shallow
• but how do we know?
• Aphanitic
• Few phenocrysts
• Some quenched margins
• Minor baking of host rock
• P lt 1 kbar or 3 km

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Relative ages of intrusives

• Phonolites are freshest, likely youngest?
• Can be cut by rhyolites
• Most geochron on phonolites
• No good minerals in rhyolites

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Source of magma

• Quartz bearing and non-quartz bearing rocks
• Hard to differentiate a quartz saturated rock to
  a quartz undersaturated rock
• 2 sources likely
• Qtz-bearing partial melt of felsic cont. crust?
• Phonolite upper mantle melt?

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Tomorrow

• Introduction/review of Paleozoic sed units
• Familiarize yourself with intrusions/intrusion
  style in Black Hills
• Sunday night Intro to your final project!!

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Monday

• HOT!!
• DONT drink from Whitewood creek
• Watch for poison ivy, ticks, and roos!
• Map boundaries and completed southern portion in
  lounge