PowerPoint Presentation - Tertiary intrusions of Black Hills

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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
  Generally covered
      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 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
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
 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
 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                    QuickTime™ and a
                            TIFF (Uncompressed) decompressor
                               are neede d to see this picture.




   1.7 Ga Harney Peak
    granite
 40-60 Ma, Tertiary
  intrusions
   Related to Laramide
    uplift
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
                                                                          Q
            .                                                                 Quartzolite


Phaneritic rock
                                                                  90             90




with >10%
                                                                  Quartz-rich
                                                                   Granitoid

                                                     60                                         60




felsic minerals                                                 Granite               Grano-
                                                                                      diorite


                   Alkali Fs.       20                                                                        20    Qtz. Diorite/
                 Quartz Syenite                                                                                     Qtz. Gabbro
                                               Quartz             Quartz                   Quartz
                  Alkali Fs.                   Syenite           Monzonite               Monzodiorite
                               5                                                                                    5 Diorite/Gabbro/
                   Syenite                   Syenite                                        Monzodiorite
                                     10                    35     Monzonite            65                    90        Anorthosite
                          A               (Foid)-bearing        (Foid)-bearing              (Foid)-bearing            P
                                              Syenite             Monzonite                  Monzodiorite
                               10                                                                                  10 (Foid)-bearing
                                                                                                                      Diorite/Gabbro
                 (Foid)-bearing
                Alkali Fs. Syenite
                                                       (Foid)                (Foid)
                                                    Monzosyenite          Monzodiorite




                                                     60                                         60


                                                                  (Foid)olites




                                                                          F
    Classification of Igneous
              Rocks                                                       Q




 Aphanitic rocks
 Base % on                                             60                              60

  phenocrysts                                                  Rhyolite          Dacite


 In Black Hills                            20
                                                                                               20
   Rhyolite                                       Trachyte
                                                              35
                                                                      Latite       Andesite/Basalt
                                                                                  65


   Latite
                                        A        (foid)-bearing (foid)-bearing     (foid)-bearing     P
                                                     Trachyte     Latite            Andesite/Basalt
                                                                                                 10
                                            10
   Trachyte
   Phonolite
                                                          Phonolite            Tephrite



 Prepare to encounter
  all!
                                                                                       60
                                                          60
                                                                    (Foid)ites

 Lots with aegirine
  (Na-pyroxene)                                                           F
                Figure 2-3. A classification and nomenclature
                of volcanic rocks. After IUGS.
   Classification of Igneous
             Rocks                                                               Q



Are all aphanitic
 rocks extrusive?                                              60                              60

                                                                      Rhyolite          Dacite


                                                   20
                                                                                                      20
                                                          Trachyte           Latite       Andesite/Basalt
                                                                     35                  65
                                               A        (foid)-bearing (foid)-bearing     (foid)-bearing     P
                                                            Trachyte     Latite            Andesite/Basalt
                                                                                                        10
                                                   10

                                                                 Phonolite            Tephrite




                                                                                              60
                                                                 60
                                                                           (Foid)ites


      Figure 2-3. A classification and nomenclature
      of volcanic rocks. After IUGS.
                                                                                 F
        What are these
         intrusions?
Stock
Sill
Laccolith               T   IF F
                               a r
                                     (
                                     e
                                         U
                                              Q u i
                                          n co m
                                         n e e d e
                                                   c
                                                   d
                                                    T   I




                                                   p re ss
                                                      t o
                                                         F F
                                                          a r




                                                     kT i m
                                                          s
                                                            e ™
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                                                                e




                                                            e d )
                                                            e e
                                                                  Un c
                                                                      Q u i c
                                                                       o m
                                                                  n e e d




                                                                    a n d
                                                                          e d




                                                                   d e co m
                                                                 t h i s
                                                                             a
                                                                              kT i m
                                                                           p re s
                                                                              t o




                                                                               p re
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                                                                                     sso r
                                                                         p i ct u re .
                                                                                            a n
                                                                                           d e c
                                                                                         t h i s
                                                                                                 d
                                                                                                 o m
                                                                                                 p i
                                                                                                    a
                                                                                                    c
                                                                                                      p r e s
                                                                                                     t u re .
                                                                                                              so r
    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
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
Phonolites are freshest, likely
 youngest?
Can be cut by rhyolites
Most geochron on phonolites
  No good minerals in rhyolites
       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
Introduction/review of Paleozoic
 sed units
Familiarize yourself with
 intrusions/intrusion style in Black
 Hills
Sunday night: Intro to your final
 project!!
           Monday
HOT!!
DON’T drink from Whitewood creek
Watch for poison ivy, ticks, and
 roos!
Map boundaries and completed
 southern portion in lounge

				
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