Time and Geology Sir Charles Lyell Image source:

The Key to the Past Relative Time Principles Used to Determine Relative Age Unconformities Correlation The Standard Geologic Time Scale Absolute Time Principles of radioactive decay Instruments The age of the Earth

Important Historical Developments James Hutton ( ): Native of Edinburgh, Scotland. Educated as a medical doctor in Leiden (1749). Passionate about scientific inquiry. Father of modern Geology. Published “Theory of the Earth” in 1785 in which he outlined that geological features and ancient rocks could be explained by present-day physical and chemical processes. Charles Lyell ( ): Scottsman who attended Oxford University. Father was an avid naturalist. Rebelled against prevailing thought, which was rooted in Biblical interpretation and Catastrophism. Published “Principles of Geology,” which through succeeding editions came to address all the major geological processes recognized today. His main contribution was the development of Uniformitarianism (Actualism). He strongly supported an “old Earth” view. He was close friends with Charles Darwin. The present is the key to the past… Modern view holds that processes that operate today have shaped the Earth through Geological Time, but rates may not have always remained constant.

Important Relative Age Dating Principles Original Horizontality: all beds originally deposited in water formed close to horizontal Superposition: within a sequence of undisturbed sedimentary or volcanic rocks, layers young upward Lateral Continuity: original sedimentary layers extend laterally until it thins out at edges Cross-cutting Relationships: disruptions in any sequence occurred after the youngest established event in the undisturbed sequence

Complex Subsurface Geology

Sedimentary Deposition

Marine Sequence

Intrusion

Tilting & Erosion

Subsidence and New Marine Deposition

Missing Formation

Dike Event

Erosion and Exposure

Subsidence & Deposition

Fluvial Deposition

Contact Relations

Unconformities Unconformity: formation surface or contact that represents a gap in the geological record –Disconformity: the contact representing missing strata that separates beds that are parallel to each other. Often difficult to detect in the field. Must use fossils or other correlation methods. –Angular unconformity: younger strata overlie an erosion surface on tilted or folded rocks. Implies a specific sequence of events. –Nonconformity: contact with overlying sedimentary rocks on top of an erosion surface of plutonic or metamorphic rocks. Implies long-lived erosion prior to burial and re-deposition.

Index Fossils and Disconformity Animation

Correlation and Disconformities

Angular Unconformity

Folding & Erosion

Intrusion

Correlation of Units Correlation: usually refers to the determination of the time equivalence of a specific unit or formation. Can occur over a wide range of scales from local (outcrop) to continental to even global. –Physical Continuity: direct method for determining correlation. Trace beds over some distance away from local point of inspection. –Lithologic Similarity: may sometimes be used to infer that rocks were formed coevally (at the same time). Can be dangerous. Must evaluate super-position (e.g. Coconino vs. Navajo sandstones). –Fossils: common in sedimentary sequences. Most species observed in the rock record are now extinct (>99.9%). Faunal Succession: Fossil sequences progress in a definite and recognizable order. Older rocks contain fossils that are increasingly different from modern species. Strong evidence for evolution of species through time.

Grand Canyon Section Coconino Sandstone

Zion Section

Grand Canyon-Zion Geological Cross-section

More on Fossils Index Fossils: Extremely useful for correlation –Well defined morphological characteristics –Short-lived species –Geographically widespread distribution Fossil Assemblage: Several different species found together in a specific rock unit. –More useful for dating and correlation than a single index fossil –Also used to provide information about the environment of deposition

Index Fossils Global Correlation: Similar units found in India, Africa, S. America, Australia, Antarctica. Marks an important geological transition!

Index Fossils Index fossil: short-lived but geographically widely distributed.

Simple Geologic Time Scale