Test on Thursday, Feb 15 5 essays 75 minutes available paper provided

1. Test on Thursday, Feb 15 5 essays 75 minutes available paper provided Each essay will address a concept that has been emphasized in lecture and lab. Each answer should display two components: A short (one or two sentence), direct answer (in words) to the question. 2) Geologic examples and sketches that provide specific details and context for your answer.

2. Grain-supported fabric of calcite-cemented sandstone. Intergranular space is 35-40%; porosity near zero. 2

3. Sands compact with burial, but not as much as muds. The example below is an extreme case. wind-ripple strata grainflows These grainflows now dip 24-25degrees and have 31% intergranular space (includes both porosity and cement). Assuming that they were deposited with 32 degree dips, they have been compacted by 28%. Their initial porosity would have been about 43% (consistent with modern grainflows. 3

4. What is this? What is its chemical formula? Why is shaped this way?

5. Diagenesis: processes that take place after deposition and before metamorphism. Includes compaction, cementation, dissolution. Can be early (paleosols) or late (deep burial). Quartz overgrowth (diagenetic Monocrystalline quartz grain (detrital). Clast was derived from Mother Rocks, and rounded during transport. 5

6. Quartz overgrowths-- continuous crystal lattice shared by detrital grain and cement. 6

7. Mass Balance Problem • If initial porosity is 40% and cementation proceeds to completion (with little compaction), 40% of the rock mass was produced during diagenesis. Where did all this material come from??? • Typical cementing minerals like calcite and quartz are only weakly soluble in water, so only a tiny amount of cement can be precipitated by the volume of water that fills the pore. • Conclusion: Tremendous volumes of water must move through the pore spaces to generate all that cement. 7

8. 8 SEM image of quartz overgrowths on sand grains

9. With a regular light microscope, grain/overgrowth boundaries are very hard to see. 9

10. But wait, maybe there is a way around the mass balance problem that requires “tremendous volumes of water” to supply the quartz cement (false hope; this is a story in which new technology proves a new idea wrong): Riecke’s Principle • Solution of a mineral tends to occur most readily at points of contact where external pressure is greatest, and crystallization occurs most readily at points where external pressure is least. 10

11. Sutured contacts 11 Does this process explain the origin of most quartz overgrowths? No!

12. Quartz arenite under regular petrographic microscope 12

13. Quartz arenite seen with Cathodoluminescence Microscope (bombards thin section with electrons) 13

14. CL shows most quartz arenites with quartz overgrowths have an open fabric (few grain contacts); large volumes of water necessary! Rocks with open fabric were cemented early (before compaction), in a near- surface setting where abundant water moved through the sediment. So pressure solution and Riecke’s Principal don’t help in explanation. 14

15. Flaming Cliffs, Gobi Southern Mongolia Upper Cretaceous Red Sandstones Canyonlands Southeastern Utah Lower Permian 15

16. Hematite-rich clay rims on sand grains Red Cretaceous sandstone of Mongolia 16

17. Clay rims on sand grains can form in two distinct ways: • They can precipitate from pore-water solutions • (as a cement like quartz and calcite). • These clays (autochthonous) typically have crystals that • are oriented perpendicular to the surfaces of sand grains. • 2) They can be physically swept into position by moving • pore water. • These clays (allochthonous) are typically oriented parallel • to grain surfaces. 17

18. Practical Significance: Clay rims can “seal” the grain perimeter of a quartz grain, thus preventing overgrowths 18

19. The clays in most hematitic rims lie parallel to grain surfaces, so they were physically swept into place. Two main hypotheses for their origin: • 1) dust falls onto well-sorted sand deposits and infiltrates the sand via rainwater. • 2) suspended load is carried by water of losing streams into channel deposits. 19

20. Channel-fill conglomerate composed of fine-grained carbonate, Cretaceous of Mongolia. Could it be intraformational? 20

21. Caliche or Calcrete • Calcite that accumulates in soil profile, usually as fine-grained nodules or rhizoliths. • Develops best in semi-arid climate where there is insufficient rainfall to remove calcium ions from surficial materials. • Calcium may be supplied by atmospheric dust. • High preservation potential relative to many other types of soils (it is durable and relatively insoluble); • a very common type of paleosol (ancient soil) 21

22. In situ caliche nodules, Flaming Cliffs Rhizoliths (calcified plant roots), Canyonlands 22

23. Advancing flood of an ephemeral, losing stream, Namibia 23

24. Clay-filled burrows, Cretaceous of Mongolia 24

25. Clay-filled burrows, Cretaceous of Mongolia 25

26. The caliche and rhizoliths indicate red sandstones form in arid Climates. The clay-filled burrows support the idea that mud-rich floodwaters from losing streams percolated through the sand, depositing the clay rims. 26

27. 27

28. Most surface water is oxidizing; most subsurface water (below the water table) is reducing. 28

29. Conclusion: Pore waters of red sandstones have remained oxidizing for millions of years, probably because there was little organic matter in the original sands- not much life in the desert. oxidizing reducing 29

30. Conclusions on Red Sandstones • Hematite (Fe2O3) lies within clay coatings on sand grains. • Best developed in non-marine, arid or semi-arid paleoclimates (little organic matter). • Ultimate source of iron is silicates like biotite, hornblende, pyroxene in Mother Rocks (igneous, metamorphic). • In marine and lacustrine rocks, presence of organic matter means that iron is usually reduced to siderite or pyrite, so rocks don’t usually end up red. 30