9/4/14 Chapter 4: Stratigraphic principles ● Humans classify things ○ classification of phenomena is inherent ○ necessary for reasoning ● 4 kinds of strata graphic classification ○ lithostratigraphic: rocks, by lithology ○ biostratigraphic: rocks, by fossils ○ chronostratigraphic: rocks, by age ○ geochronologic: time (abstract) ● Lithostratigraphy ○ lithostratigraphic unit ■ local unit of rock defined by lithology: ■ color, composition, texture, stratification, genesis (origin) ○ formation is basic unit ■ boundaries may be sharp transitional or arbitrary ○ group > formation > member > bed (large to small) ○ named for geographic feature ● Diachronous: ○ cutting across time, timetransgressive ○ not exactly the same age everywhere ○ upper and lower boundaries are not time horizons ○ lithographic units never correspond exactly to chronostratigraphic units ○ because, a local environment occupies just a small area at any time ○ migrates across a much larger area through time ○ sediment deposited in this environment is of different ages in different places ○ it is not important to distinguish lithographic and chronostratigraphic units ● Biostratigraphy ○ basic unit = bio zone, fossil zone ■ unit of rock defined by fossils ■ includes all rocks in which an index fossil occurs ■ named for that fossil ○ examples named for Cretaceous oysters, Texas: ■ Exogyra ponderosa zone ■ Exogyra cancellata zone ○ Index fossils lived only short time ○ Purpose of biostratigraphy is to provide evidence for chronostratigraphy ○ Good index fossils: ■ evolve rapidly ■ diverse (many species) ■ short stratigraphic ranges ■ planktonic, float at surface ■ independent of seafloor environment ■ regional to global distribution ■ large population ■ good for longdistance correlation ● Facies = environmental aspect of a rock ○ Good facies fossils: ■ conservative, evolve slowly ■ not diverse ■ long strata graphic ranges ■ Benthic, live on sea floor ■ environmentally restricted ■ local to global distribution ■ useless for longdistance correlation ■ useful for recognizing environments of deposits ● Chronostratigraphy: ○ chronostratigraphic unit: ■ by any evidence: index fossils, isotopic dates, superposition, crosscutting relations ■ global: all rocks of that age everywhere ○ upper and lower surfaces are isochronous (same age everywhere) ○ basic chronologic unit = stage ■ subdivisions: lower, middle, upper (L, M, U) ○ names = geologic time scale ■ Maastrichtian stage, Eocene Series, Cambrian system, Paleozoic Erathem, Phanerozoic Eonothem ● Sedimentary facies ○ facies = environment aspect of a rock ■ information about environment of deposition ○ Anton Lavoisier 1789 ■ different environment, organisms and sedimentary exists next to each other at same time ■ different facies may be contemporary, of same age ■ some facies may repeat vertically ● Walther’s Law of Sedimentary facies ○ some facies are arranged in gradient ○ “only those facies that are geographically contiguous (next to each other) can succeed one another vertically” ○ if sea level rises or falls, facies migrate inland and off shore together, in same order ○ in rock record, migrating environments produce vertical and lateral faces changes ○ verticle succession of facies = horizontal succession of facies ● Stratigraphic column, local crosscutting show: ○ vertical change in facies ● Restored stratigraphic crosssection shows: ○ persistence, migration, regional relationships of environments through time ● Marine transgression ○ advance of sea over land ■ preceded by erosional unconformity ■ landward shift of facies through time ■ grain size fines upward ○ Local, relative, apparent sealevel rise may be caused by: ■ global sealevel rise ■ local crustal subsidence ■ local coastal erosion ● Marine Regression ○ retreat of sea, exposing land ■ land area increases ■ grain size coarsens upward ○ local relative, apparent sealevel fall may be caused by: ■ global fall in sea level ■ local crustal uplift ■ local influx of sediment (as streams build delta) ● Stratigraphic onlap ○ during marine transgression ○ each younger layer reaches farther inland ● Stratigraphic offlap ○ during marine regression ○ each layer reaches less far inland than earlier years ● Transgression, regression ○ local, relative phenomena ■ with local or far away cause ■ tectonic, climatic, erosional, depositional ■ sedimentary supply from uplift, mountainbuilding ○ Eustatic changes in sea level are global ■ glacioeustatic: growth or melting of ice sheets ○ strata record is better for marine transgression than for regression ● L.L. Sloss 1963 ○ Sequence classification ○ sequence = package of strata, bounded above and below by regional unconformities ○ 1 transgression and regression ○ boundaries diachronous ○