Relative Geological Time Concept that a specific sequence of events have resulted in the evolution of the Earth to its current state. The relative timing of events may be unraveled by careful examination of the “Rock Record”.
Relative Geological Time • Principles: • Principle of Uniformitarianism:“The present is the key to the past”. • Studying processes today gives insight into past events. • Principle of Original Horizontality: Beds of sedimentary rock are deposited in a horizontal orientation. • Principle of Superposition:In a sequence of undisturbed sedimentary rocks, the layers get younger upwards. • Principle of Lateral Continuity:Original sedimentary layers extend laterally until they thin at its edges.
Relative Geological Time • Principles (cont.): • Principle of Inclusions:Inclusions are older than the rock in which they are contained. • Principle of Cross-Cutting Relations:A disrupted pattern is older than the cause of the disruption. • Principle of Fossil Succession:Fossils contained in strata are related to the age of the rocks.
Principles of original horizontality, cross-cutting relations, & superposition. Block diagram of Miner Canyon contains: 1) Flat-lying sequence of sedimentary rocks; 2) Tilted sequence of sedimentary rocks; 3) Angular unconformity between 1 & 2; 4) Two igneous intrusions – granite and dike. NOTE: granite is cut by an angular unconformity, so it is therefore older than the event that created the unconformity.
Fig. 08.02 Step 1: Deposition – horizontal
Fig. 08.03 Step 1 (cont.): Deposition – horizontal
Fig. 08.04 Step 2: Intrusion of granite
Fig. 08.05 Step 3: Titling of sequence Step 4: Erosion to create unconformity
Fig. 08.06 Step 5a: Submergence and deposition of Larsonton Formation and…..
Fig. 08.07 Step 5b: ….. possibly additional formations
Fig. 08.08 Step 6: Intrusion of dike
Fig. 08.09 Step 7: Erosion of Larsonton Formation – disconformity
Fig. 08.10 Step 8: Submergence and deposition of Foster City, Hamlinville and Skinner Gulch sequences
Fig. 08.11 Step 9: Erosion due to stream
Fig. 08.12 Principle of Inclusions
Fig. 08.13 Principles of Fossil Succession and Lateral Continuity
Unconformities Surface that represents a break in the geologic record. The rock unit(s) immediately above the break is/are much younger than that/those below. Usually represent buried erosional surfaces – parcel of geology is missing!
Unconformities Three types: 1) Disconformity: unconformity is parallel to layering, but there is a gap in the geologic record. Typically erosional and hard to spot. Look for weathering surfaces, boulders/pebbles of older rock in younger. 2) Angular Unconformity: Younger strata overly older rocks that were tilted/folded. 3) Nonconformity: A contact in which an erosional surface on a plutonic or metamorphic rock has been covered by younger sediments or volcanic rocks (i.e., unconformity separates different rock types).
Reasons for Unconformities • Formation of unconformity involves: • Horizontal or conformable strata or beds are formed • Break in sedimentation or deposition • Happens due to tectonic movements, that causes uplift or subsidence of land surface • Next phase of Deposition or sedimentation cycle, where new sedimentation produce another set of conformable beds
Fig. 08.14a-d Angular Unconformity
Fig. 08.15a-f Nonconformity
Fig. 08.13 Disconformity
There is an unconformity. Where is it? What type is it? http://epod.usra.edu/archive/images/telheiroweb.jpg
There is an unconformity. Where is it? What type is it? http://www.cerritos.edu/earth-science/images/ESCI_110_Lecture_notes_images/Sandia_unconformity.jpg
There is a fold. What type? How do you know? There is an unconformity. Where is it? What type is it? How do you know? http://www.kgs.ku.edu/Publications/Oil/primer05.html
There are several unconformities. Where are they? What types are?