1	Geologic Time & Stratigraphy
2	Geologic Time Scale Based on appearance of life forms Earth= ~4.6 billion years old Life= Been around for ~ 2.5 billion Animals with hard parts= only been around the last 570 million! Modern humans, < 1 million years! Time scale: Eons (biggest unit) Eras Periods Epochs (smallest unit)
3	Dating Rocks Age of rocks, fossils or geologic events is determining in two ways: Relative Age- Rock, fossil or geologic event defined relative to other fossils, rocks, features or events Numerical (Absolute) Age- Given in units of time, usually years Ma-millions Ga- billions
4	Basic Principles of Relative Dating Principle of Uniformitarianism present is the key to the past Law of horizontality Sediments are deposited in a horizontal layer Law of superposition Topmost layers are younger Lateral continuity of sedimentary layers Deposition will extend laterally for some distance before pinching out Correlation Uses similarities in lithology or fossil to establish sameness of rock units over a distance Principle of Cross-cutting relationships Intrusions or faults that cut across sedimentary layers are younger
5	Relative Age & Stratigraphy To understand the relative relationships between rock units, we must understand the basics of stratigraphy:
6	Formation Facts Have relatively uniform lithology Continue laterally for some extent Can grade from one-to-another by intertonguing Lithologies overlap
7	Sedimentary Facies Facies- characteristic environments defined by lithology Examples: sandy/sandstone facies=beach environment muddy/shale =offshore marine limestone facies =deep marine Facies Fossil- used to ID environment Important for reconstructing paleoenvironments Transgressions vs. Regressions of seas Transgressions- seas advance Regression- seas retreat
8	Transgression & Regressions
9	Walther’s Law of Depositional Environments Depositional environments change their lateral position with time Sedimentary facies of adjacent depositional settings overlap one another in a vertical sequence. Caveat: does not apply to strata w/unconformitities
10	Unconformities Gap/break in geologic record Represents erosion or non-deposition Show lateral and vertical differences Amount of time missing may vary from place-to-place along one surface May disappear completely into a continuous sequence of strata 3 types: Angular, Disconformity, Nonconformity
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