Lab 12 - Geologic Maps

2 Name__________________ ___________________ Section________________ Lab 12 : Geologic Maps Part 1. Geologic Time Scale Geologists cite a standard geologic time scale to reference different parts of Earth’s history along a timeline extending back 4.56 billion years to the present-day. Although the subdivisions of geologic time sound foreign to most, geologists are quite familiar with the sequence of geologic time as referenced by the names of the geologic Eons, Eras, Periods and Epochs. Strata are like the library of Earth’s history – they record past environments, preserve ancient life in the form of fossils, and retain the evidence of tectonic deformation in their orientation and structure. The stratigraphic record for the more recent past is more detailed (Think of a library - it would be easier to find a record of what happened yesterday than what happened 10 years ago on this day, or 100, or 1000 etc). Geologists commonly use more subdivisions for more recent times. Most of the geologic maps you’ll interpret in this exercise lump the three pre-Phanerozoic Eons into an informal Eon called the “PreCambrian.” Since the Cenozoic Era is the most recent, the periods within it are more finely divided into smaller Epochs. For the most up-to-date internationally agreed-upon time scale subdivisions and dates, see the International Chronostratigraphic Chart (v. 2020/01), published by the International Commission on Stratigraphy at https://98ca4554-1361-4fb1-a4d8-a1bb16d032e6.filesusr.com/ugd/f1fc07_c4d5c28dbe474fd89cef7cc009edd825.pdf?index=true EONS ERAS PERIODS EPOCHS ___Quaternary_____ Holocene Pleistocene Ng Neogene_____ Pliocene Cenozoic Miocene 66Ma-present Oligocene Pg Paleogene_____ Eocene Paleocene ___Cretaceous_____ Phanerozoic Mesozoic ___ Jurassic _____ 252-66Ma ___ Triassic _____ ___Permian_____ ___Pennsylvanian_____ ___ Mississippian _____ Paleozoic ___ Devonian_____ 541-252Ma ___Silurian_____ ___ Ordovician _____ ___ Cambrian _____ ___Precambrian 4560-541Ma (Includes: Proterozoic, Archean and Hadean Eons together)

3 1. Follow the web link above for the International Chronostratigraphic chart, and write-in the starting dates for each geologic time period in the Phanerozoic Eon on the chart in the blanks provided on to the RIGHT of each Period name. All dates are understood to be in Ma (millions of years). 2. Why do you think it is easier to find examples of Phanerozoic –aged strata than PreCambrian strata in most places on Earth? ____________________________________________________________________ 3. See p. 137 of the Lab 1 2 reading assignment to find the short-hand symbol used for each of the Periods in the Phanerozoic Eon, and for the Precambrian Eon, and write in the letter or letter- combination used as a symbol for each to the LEFT of each Period name. The short-hand for the Paleogene and Neogene Periods have been filled in for you. 4. On the International Chronostratigraphic Chart, a different time period name is used for the Mississippian and Pennsylvanian Periods combined. What is the name of this time period? ____________________________ The symbol for this period is “C” 5. Why do you think the symbol for Cretaceous is “K” instead of a “C” ? ____________________________________________________________________ Part 2. Stratigraphic Principles Geologists use fundamental stratigraphic principles, developed in the mid-1600’s- early 1800’s, to determine a sequence of geologic events based primarily on logic and observation of natural processes. Although these were discussed in the Lab Module 1 2 reading – they are worth another reading: A. Superposition – in a sequence of strata older layers are on the bottom of the sequence, and progressively younger layers are found toward the top of the sequence. Structural overturning of strata, which may happen in areas of severe crustal deformation close to plate boundaries, invalidates this principle. B. Original Horizontality – layers are deposited horizontally; if layers are not horizontal, some force (often tectonic or gravitational) has acted on the rock layers after they were deposited to change their orientation. C. Original Lateral Continuity – sedimentary layers, and some lava flows and volcanic ash layers are deposited over wide areas (i.e. marine strata deposited on floor of a large ocean, thick sand layers across a sandy desert, volcanic tuff or flood basalts across a continental surface, till deposits at the base of a continental glacier, floodplain deposits across a broad river valley). Layers can be traced laterally until they pinch out (thin to zero thickness) where the depositional medium ended. If a layer abruptly disappears, some event after deposition of the layer disturbed the original lateral continuity – erosion, faulting, igneous intrusion, etc.

4 D. Cross-Cutting Relationships – any geologic feature that cuts across another feature is younger than the feature it cuts. This applies to unconformities (erosional surfaces), faults and igneous intrusions (dikes, sills, batholiths). So any rock that has a fault running through it is older than the fault itself. Or any rock intruded by a dike, sill, batholith or other igneous intrusion is older than the intrusion itself. 6. Why do you think the geologic time scale is depicted with the Phanerozoic Eon on the t op and the PreCambrian Eon on the bottom? Why not the other way around? ____________________________________________________________________ Examine this block diagram showing both a geologic map view (top of block) and a cross-section view of a sequence of strata affected by faulting (at A and F), intrusion (at B and D), and erosion (squiggly line separating formation C from strata below). Answer the following using the information provided above. 7. Which principle tells you that fault F is younger than intrusion B and layer C? ______________________ 8. Which principle tells you that the folded layers intruded by batholith B were truncated by erosion AFTER they were deformed? ____________________________ 9. What principle tells you that layer C is older than layer E? ___________________________ 10. Referring to the geologic time scale, and using the principle of superposition, if intrusion B, and the layers it intrudes, date to the very end of the Ordovician Period, and layer E dates to the very beginning of the Mississippian Period, what time period does formation C and Intrusion D likely belong to?____________________________ 11. Only two formations would show up in a map view of this area – which two? B C D E Fig. 1. Source www.opengeology.org Part 3. Geologic Maps and Cross-Sections – Geologic maps depict the distribution of strata of different ages and rock types on the Earth’s surface. As such, they represent something like a birds-eye view of the geology of an area. Geologic maps also include interpretations of what the shallow crust would look like in side view based on the orientation of strata and structure types found on the surface. These are called cross sections.