Geology 3120 Unconformities and Contacts 1 Outline The

Geology 3120 Unconformities and Contacts 1

Outline • The “contact” that started it all… • Steno, Hutton, and Lyell • Conformities and Unconformities • The Grand Canyon and Contact Corner • Intrusive contacts • Fault contacts • Shear zone contacts • Exercise - timing of geologic events 2

The “Contact” • Siccar Point, Berwickshire, Scotland • James Hutton (1788) 3 • Vertical Silurian rocks overlain by Devonian Old Red Sandstone

Steno’s Principles • Nicolaus Steno (1669) Principle of Original Horizontality - sediments are deposited in horizontal layers • Principle of Superposition - the highest material in a vertical section is the youngest • Principle of Original Continuity - sediments are deposited in laterally continuous layers • Also consider cross-cutting relationships - if Rock A crosscuts Rock B, then Rock A is younger than Rock B • B A 4

Hutton and Lyell • Charles Lyell (1830) Principle of Uniformitarianism - “the present is the key to the past”, or that present processes can be used to infer past processes • • Based on observations by James Hutton (1785) • Rates of geological processes are not necessarily constant • Catastrophic processes may occur 5

Conformable contacts and Unconformities Conformity Disconformity Angular Unconformity Nonconformity 6

The Grand Canyon Paleozoic sedimentary rocks (248 -540 Ma) • • Angular unconformity Proterozoic sedimentary rocks (800 -1250 Ma) • • Nonconformity • Vishnu Group (1400 -1500 Ma) 7

The Grand Canyon Unconformities 8

Contact Corner (disconformity) Boulder Creek Granodiorite (1700 Ma) Silver Plume Dikes (1400 Ma) Fountain Formation 9 (320 -300 Ma)

Intrusive Contacts Igneous - plutons, dikes 10

Intrusive Contacts Sedimentary - salt structures & diapirs 11

Fault Contacts Hurricane normal fault, Utah and Arizona (western boundary of Colorado Plateau) • Slickenlineations (probably quartz growth) - see crosssection of growth below: 1. • 2. QTZ 3. 12

Shear Zone Contacts 13

Exercise - timing of geologic events The following exercise is designed to introduce you to how sediments and intrusive rocks can be used to constrain the timing of deposition, faulting, folding or other deformational processes. The figure is a block diagram of a sequence of rocks that have undergone a period of folding, thrust faulting, normal faulting and intermittent magmatic events (dike intrusion). See if you can determine the geologic history of deposition, intrusion, and deformation by examining the contact relationships between the sediments, igneous rocks, and faults. 14

Block model for exercise 15

Geologic History • 12 Ma dike • 15 Ma dike • Normal fault • 20 Ma sed • 22 Ma sed • Erosion • Thrust fault • Folding • Layer B • 60 Ma sed • Layer G • 70 Ma sed • 80 Ma 16 sed

References Slide 6 Busch, R. M. and D. Tasa, Laboratory Manual in Physical Geology, 3 rd. Ed. , American Geological Institute and National Association of Geology Teachers, 260 p. , 1990. Slide 11 Twiss, R. J. and E. M. Moores, Structural Geology, W. H. Freeman & Co. , New York, 532 p. , 1992. 17