Recent. Pennsylvanian, Lookout Mt.

Transcription

1 UNIFORMITARIANISM

2 Recent Pennsylvanian, Lookout Mt.

3 Recent Mid Ordovician, Pigeon Mt.

4 Recent Late Ordovician, Taylor Ridge

5 THIN SECTION GRAINS +/- PORES CEMENT

6

7 Precambrian Sandstone, San Juan Mts, CO Between Silverton and Durango

8 These fossils were all found within about 50 miles of Americus. All have living relatives, most of them very close relatives. Their relatives all live in a particular environment which is not like the environment that presently exists in SW GA. Furthermore, no relative of any of these is physiologically capable of living in the sort of environment presently occurring in SW GA.

9 These two rocks are mined from Paleocene and Eocene strata near Andersonville in Sumter and Macon Counties. Both require intense chemical weathering of aluminosilicate minerals like feldspar to form. The required intensity only occurs in tropical rainforests. The closest tropical rainforest is in Jamaica or Cuba. Explain.

10 INCLUSIONS

11 A Garden Spider (Argiope aurantia) dispatching a wasp on my front porch.

12 Solid piece of acrylic with a solid Garden Spider inside it.

13 Glossopetrae ( tonguestones ) were well known objects in Medieval Europe. They were lumped as fossils, which comes from the Latin root that translates excavated, along with other curiously shaped things dug from the Earth. Well formed crystals, for example, were fossils. From Steno, 1669 Tonguestones were thought to be simply odd concretions that had formed in the rocks in roughly the shape of a tongue.

14 We know a little more about them now. Shark tooth partially excavated from a piece of Eocene Limestone. Morocco. (Collector unknown).

15 Nicholas Steno (Nils Stensen) was the first educated European to see inside a shark s mouth and realize what a tonguestone is. He published his observations (and From Steno, 1669 a lot of others that sprang from them) in 1669.

16 Solid piece of sandstone with a solid quartz grain inside it.

17 Solid piece of conglomerate with solid quartz gravel inside it. 2.5cm (1 )

18 Moxie Bald, ME Solid mass of granite with a solid mass of baked mudstone inside it (a xenolith ).

19 Near Elberton, GA Solid mass of granite with a solid mass of gneiss inside it (a xenolith ). (Water leaking from fracture porosity.)

20 Original Horizontality

21 Angle of repose: Damp Sand Dry Sand Saturated Sand Most sedimentary rocks are marine. Why? (This means they were deposited saturated with water).

22 Grand Canyon, AZ Canyonlands N.P., UT

23 These beds tilt underneath those beds! Horizon Bed Surface (Not horizontal!) UP Grand Canyon, AZ

24 Lookout Mt, near Cooper Heights, Walker Co., GA

25 Franklin Mts, El Paso, Texas

26 New Creek, WV

27 You are looking directly at the top of a package of sandstone beds at Seneca Rocks, near Franklin, WV. Which way is up?

28 Which way is up? Pinto, MD

29 Vertical Rocks like these are widespread in PA, MD, WV, and VA. This easily recognizable package of sediments is called Tonoloway Limestone, named for Big Tonoloway Creek in MD where they were first described. Which way is up?

30 UNDERLYING Rose Hill Sandstone lies in this direction along the outcrop. Which way is up?

31 Recent stromatolites dome upward. Fossil stromatolites presumably did as well when they were alive.

32 The beds are slightly overturned. horizontal

33 Fold between Huntersville and Minnehaha Springs, Pocahontas CO, WV.

34 Germany Valley, near Franklin, WV, ~50km (35 mi) north of Minnehaha Springs. Tuscarora Sandstone on North Fork Mountain

35 The shape of asymmetric ripples makes it easy to tell which way was up when they formed. You are looking at the tops of these ripples. Which way is up? San Juan Mts, CO

36 Original Lateral Continuity

37 Navajo Sandstone (Jurassic) Wingate Sandstone (Triassic) Moenkopi Shale (Triassic) Cutler Formation (Permian) Canyonlands N.P., UT Bed continuity has been disrupted by erosion.

38 Kaibab Limestone Redwall Limestone Tapeats Sandstone Grand Canyon, AZ. The beds all along either side of the canyon match. The erosion that created the canyon has cut through them.

39 Germany Valley, near Franklin, WV, ~50km (35 mi) north of Minnehaha Springs. Originally a continuous layer, the Silurian Tuscarora Sandstone has been breached and removed by erosion, along with some of the underlying Ordovician rocks.

40 Erosion has truncated the beds I-68 cut at Sideling Hill, near Hancock, MD. Photo by Chelsea Carter

41 Sideling Hill Lookout Mt, near Cooper Heights, Walker Co., GA

42 ? Pinto, MD Faults (dashed lines) will also disrupt the continuity of (or cut across) things that were present when the fault moved. Arrows indicate beds that were originally connected but were offset by the faults.

43 Eocene basaltic dikes cutting across Cambrian limestone. Highland Valley, VA

44 Basaltic dike of Triassic (Mesozoic) age cutting across Paleozoic Granite. Near Elberton, GA

45 The same dike as in the previous photo cutting across a slightly older basaltic dike.

46

47 Superposition

48 6. Glue with blue dye was added to the coarse sand. It infiltrated the pores and cemented the sand. (It also seeped downward into the permeable fine sand and partially stained that sand). 5. Coarse sand with fine gravel was added atop the medium sand. 4. Un-dyed glue was added to the fine sand, partially filling the pores and cementing it. 3. Medium grained white sand was added, forming a flat layer. 2. Dilute glue was added, partially filling the pores of the sandy clay and cementing the top. 1. Red sandy clay was added to the bottom of the jar. Not surprisingly, it formed an ~flat layer.

49 Last formed layer (youngest) progressively younger First formed layer (oldest)

50 Youngest Kaibab Limestone (Permian) Redwall Limestone (Mississippian) Oldest (Precambrian) Tapeats Sandstone (Cambrian) Grand Canyon, AZ

51 These beds tilt underneath those beds. Therefore they are older! Youngest Oldest Grand Canyon, AZ

52 PALEOZOIC MESOZOIC Grand Canyon, AZ

53 Younger Younger Younger Younger Younger Younger

54 Cross-Cutting Relationships

55 The erosional surface cuts across the youngest (highest) rock in the canyon (Permian), so it must be younger than that rock. In fact, the distant Mesozoic rocks are also eroded, so the surface must be younger than that.

56 The limestone is Cambrian, so the dikes must be younger than that.

57 The granite is very late Devonian and/or very early Mississippian so the dikes must be younger than that. The order of events is given on the diagram

58

59 Basic Principles of Relative Dating: Uniformitarianism The natural laws and processes in the universe are constant through time. The present is the key to the past. (If it looks like a ripple mark, it was a ripple mark). Original horizontality Sedimentary rocks are laid down in essentially horizontal layers. Lava flows, particularly those from fissure eruptions, tend to follow this pattern as well. Most igneous rocks do not. Metamorphosed sediments may preserve original sedimentary bedding. Original Lateral Continuity Sedimentary layers typically do not form with abrupt edges unless the sediment was filling a basin with a steep side. A rock can thin to not thickness over some distance or it can gradually change its character (from sand to mud, for example) but an abrupt termination generally implies a later event (erosion, intrusion, or faulting) that has truncated its original continuity.

60 Basic Principles of Relative Dating: (continued) Inclusion Any geologic feature or piece of a geologic feature that is entirely included within a rock is necessarily older than the rock that encloses it. The clasts (individual grains or pieces of sediment) in detrital rocks come from parent rocks that are older than the sediment they are in. Xenoliths in an igneous rock are from the (pre-existing) host rock into which the igneous rock was intruded or from some deeper, even older layer through which the magma passed. Alterations of contact metamorphism can be thought of as the intrusions heat included in the metamorphic rock.

61 Basic Principles of Relative Dating: (continued) Superposition Sedimentary rocks are deposited sequentially one atop another. Therefore they are progressively younger toward the top of the stack. Extrusive igneous rocks also follow this pattern. If rocks have been structurally shifted to a vertical position or overturned then indicators of original up are helpful in age determination. Intrusive rocks may not conform. Original sedimentary layering in metamorphic rocks may be recognizable, but the probability that they are not still upright is quite high.

62 Basic Principles of Relative Dating: (continued) Cross-Cutting Relationships Erosional surfaces, igneous intrusions, and faults all truncate the original continuity of sedimentary and other rocks. Any of these that cuts across rocks and other geologic features is younger than what it cuts. If one of these features does not cut a feature, but ends at that feature, then the feature that fails to cut is older than what it fails to cut. Cutting features may cut sedimentary beds, igneous or metamorphic rock bodies, or older cutting features.