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Tertiary intrusions of Black Hills. And some sed rx too…. 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

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tertiary intrusions of black hills

Tertiary intrusions of Black Hills

And some sed rx too…..

paleozoic sed units
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
paleozoic sed units3
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
paleozoic sed units4
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
slide5
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
slide6
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
bear butte
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
slide8
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
phaneritic rock with 10 felsic minerals

Q

.

Quartzolite

90

90

Phaneritic rock with >10% felsic minerals

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

Monzodiorite

Monzonite

Anorthosite

90

35

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

classification of igneous rocks

Q

60

60

Rhyolite

Dacite

20

20

Trachyte

Latite

Andesite/Basalt

35

65

P

A

(foid)-bearing

(foid)-bearing

(foid)-bearing

Trachyte

Latite

Andesite/Basalt

10

10

Phonolite

Tephrite

60

60

(Foid)ites

F

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.

classification of igneous rocks11

Q

60

60

Rhyolite

Dacite

20

20

Trachyte

Latite

Andesite/Basalt

35

65

P

A

(foid)-bearing

(foid)-bearing

(foid)-bearing

Trachyte

Latite

Andesite/Basalt

10

10

Phonolite

Tephrite

60

60

(Foid)ites

F

Classification of Igneous Rocks
  • Are all aphanitic rocks extrusive?

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

what are these intrusions
What are these intrusions?
  • Stock
  • Sill
  • Laccolith
why so many sills and laccoliths in black hills
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?
depth of intrusion
Depth of intrusion
  • Shallow
    • but how do we know?
  • Aphanitic
  • Few phenocrysts
  • Some quenched margins
  • Minor baking of host rock
  • P < 1 kbar or ~ 3 km
relative ages of intrusives
Relative ages of intrusives
  • Phonolites are freshest, likely youngest?
  • Can be cut by rhyolites
  • Most geochron on phonolites
    • No good minerals in rhyolites
source of magma
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?
tomorrow
Tomorrow
  • Introduction/review of Paleozoic sed units
  • Familiarize yourself with intrusions/intrusion style in Black Hills
  • Sunday night: Intro to your final project!!
monday
Monday
  • HOT!!
  • DON’T drink from Whitewood creek
  • Watch for poison ivy, ticks, and roos!
  • Map boundaries and completed southern portion in lounge