Stratigraphy the study of layered rocks. Within strata (or individual stratum) we can study the thickness
1. Chapter 5 Rocks, Fossils, and Time -… Earth events are recorded in the geologic record. Page 72 photo shows 14 million years of geologic history. The horizontal layers of the Grand Canyon show approx. 300 m.y. of history, including a 100 million year gap.
Example: a 7,000 yr. history of El Niño events has been reconstructed from core samples from a lake in the Galapagos Islands.
2. Stratigraphy – the study of layered rocks. Within strata (or individual stratum) we can study the thickness & nature of sediments, presence and types of organics, sample for certain isotopes. This proxy data can be used to determine past environmental conditions.
Within vertical successions of strata, individual layers are separated by bedding planes. The existence of the bedding plane suggests a minor, short-term environmental change.
3. The presence of bedding planes, fluctu-ations between sediment types, changes within given beds offer suggestions of environmental changes.
4. Vertical strata – Principle of Superposition, Prin-ciples of Inclusions, Cross-cutting Relation-ships, Unconformities useful in determining relative ages (see pp. 74-75).
5. Unconformities – interruptions in the geologic column.
Angular unconformity – strata below are folded or tilted
Disconformity – layered rocks above and below are parallel (undeformed), and a noticeable eroded surface is present.
Paraconformity – layered rocks above & below, with a subtle erosion surface.
Nonconformity – layered rocks deposited over igneous or metamorphic rocks.
6. Lateral changes in layered rocks.
Principle of Lateral Continuity – Deposition occurs across the basin (topographic low), but environments change due to varying distances from the shore, water depth, currents, etc.. Each environment yields a different sedimentary facies, i.e., a sediment with distinctive characteristics related to the depositional environment.
8. Marine Transgressions and Regressions
World-wide Sea Level affected by size of Polar Ice Sheets & Oceanic Rift (Divergent Zone) activity.
Local “rises and falls” of the land affect the geologic record also.
Tapeats Sandstone records the flooding of the continent in Early/Mid Cambrian time. Sand facies migrated inland – Bliss SS – El Paso – Late Cambrian/Early Ord.
9. In the Grand Canyon, as the Transgression (Sea Level Rise) continued, deposition of the Bright Angel Shale and then the Muav Limestone followed (Figure 5.8), as the water progressively deepened. Figure 5.9 (left column) shows “Transgressive Sequence”
When conditions change and a Regression (Sea Level Decline) occurs, the facies boundaries migrate seaward and exposed sediments are subject to erosion. Figure 5.9 (right column) shows “Regressive Sequence”.
10. When a significant enough change in sea level & facies shift occurs, different litho-logies “stack up”.
When sea level is stable for a long period of time, continued sedimentation causes a progradation of facies into the basin, similar to regression.
Lateral changes are generally gradual, e.g., the transition between a silt and clay facies would progress from a clayey silt to a silty clay. The transition from clay to limestone would be a limey shale to a clayey limestone.
11. At the top of the Muav Limestone, there is a significant disconformity of approx. 100 m.y.. The entire Ordovician and Silurian Periods and most of the Devonian Period are missing.
12. Fossils and Fossilization
Remains or traces of past life. Fossils useful for determination of relative ages of rocks & ash, environments of deposition, and evidence of evolution.
Types of Fossils – Body and Trace
Body Fossils – Remnants of organic parts (hard or soft) or sediment impressions of hard parts.
Trace Fossils – Evidence of life activities, even if no tangible evidence of fossil exists.
14. Altered (continued)
Carbonization – preserved plant fossils in shale
Molds and Casts – shell left impression in sediment, shell material dissolved, void refilled with mud or other minerals.
Trace fossils – ichnofossils - preservation of life activity – footprints, trails, burrows, droppings (copralites), nests.
Univ. of Calif. Berkeley Museum of Paleontology
16. Fossils and Time estimates
William Smith (1769 – 1839) – independently observed superposition (but didn’t name it) and reasoned that the same would occur with fossils, i.e., noticed succession of species and relationships between strata and fossils, i.e., certain fossils were restricted to certain strata.
Rock types (from similar environments) may be repeated through time, but particular fossils are not.