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Tertiary intrusions of Black Hills

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

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  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

  3. 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

  4. 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

  5. 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

  7. 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

  8. 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

  9. 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

  10. 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.

  11. 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.

  12. What are these intrusions? • Stock • Sill • Laccolith

  13. 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?

  14. 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

  15. Relative ages of intrusives • Phonolites are freshest, likely youngest? • Can be cut by rhyolites • Most geochron on phonolites • No good minerals in rhyolites

  16. 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?

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

  18. 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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