Lithostratigraphy - Stratigraphy based on the identification and correlation of similar rock units.
- Bed: a distinct layer in a rock sequence
- Member: a group of beds united by certain common characters
- Formation: a group of members, again united by characters with features in common. The formation is the primary unit of lithostratigraphy and is most useful in geological mapping. Hence it is formations that are normally represented by different colors on geological maps and cross sections, and a formation is normally defined for its mapping applications. A formation is not defined by its possession of a single rock type, but typically is limited to rock types that are found in close association in a mappable environment.
- Group: a group ranks above a formation; it is composed of two or more formations and is often used for simplifying stratigraphy on a large scale map.
- Contiguous: All rocks belonging to the unit are in physical contact, at least in underground.
- Contemporaneous: Contiguous in time. (This doesn't mean that all rocks in a unit were laid down simultaneously. They can. for instance, be time transgressive (aka diachronous).)
It's subjective. Once you are above the "bed" level there is no objective, physically real property of "memberness", "formationness", or "groupness". These designations are to some degree arbitrary conventions that are discarded or maintained by geologists based on their utility.
Facies and Walther's Law:
Facies: The sum of the lithological and paleontological characteristics of a deposit in a given place.
Lithofacies: a consistent lithologic character within a formation excluding its fossil content. Usually indicative of the character of the depositional environment (rather than just description) as the lithologic response to a depositional system.
Walther's law: After Johannes Walther - The principle that facies that occur in conformable vertical succession of strata also occur in laterally adjacent environments is known as Walther's law of correlation of facies.
Facies in marginal marine environments most often move landward, seaward, or remain in place as a result of:
- Sediment supply
- Sea level
- Tectonic uplift, subsidence, or stability.
- the sediment supply is overwhelmed by a relative rise in the ocean,
- or when the basin margin subsides tectonically.
- the sediment supply increases overwhelming available accommodation space,
- by a relative drop in the ocean,
- or when the basin margin is uplifted tectonically.
Sea level rarely holds still. When the shoreline moves seaward due to enhanced sediment supply (progradation) the facies are regressive. Transgressive-regressive facies patterns form most of the marine stratigraphic record.
- Biostratigraphic events
- Magnetostratigraphic boundaries
Note: Walther's law doesn't apply if unconformities are present! (A good way to identify unconformities is to observe that Walther's Law is violated.)
Complication I: Asymmetry of transgressions and regressions
- rapid erosion in high-energy shoreline environments
- consequent reworking of sediments
- sediment starvation (because a diminishing continental area is being exposed to erosion.)
The traditional model of symmetrical transgression and regression often taught at the introductory level is an unlikely scenario due to net erosion, reworking and progradation of sediments during sea level rise. What we actually see preserved looks like:
- Rapid transgression (flooding) and abrupt facies changes.
- Protracted regression with more gradual facies change.
Preservation of the sedimentary record is the rare exception. Sea level rise may temporarily preserve sediments, but ultimately preservation depends on net subsidence.
Complication II: Unconformities everywhere
It appears that only during times of sea level rise without subsequent erosion that sediments can actually accumulate, which occurs when accommodation space increases because the basin floor subsides rapidly or sea-level rises rapidly.
Cyclothems in the Pottsville Group
- angular unconformity: between tilted and undeformed sediments
- nonconformity: between crystalline rocks and sediments
- disconformity: between sedimentary rocks showing visible indication of depositional hiatus (right)
- paraconformity: a cryptic disconformity for which there is not immediate evidence of missing sediment, but abrupt changes in fossil fauna indicate adjacent beds are of significantly different ages.
What does the Grand Canyon tell us about unconformities and the base level of erosion?
- basal conglomerates or breccias
- erosional surfaces which show relief
- Truncation of bedding planes, dikes, or faults
- ferruginization or chertification in carbonates
- paleontological data (E.G. burrows in paleosol.)
- Discordant dip angles.
The time missing in an unconformity is known as a hiatus or lacuna. The origin and history of an unconformity may be revealed by the study of temporally calibrated cross sections from the edge of sedimentary basins.
Since no single section likely records continuous sedimentation it is important to demonstrate equivalence of widely separated sections through the process of correlation to piece together a complete history of the planet.
- ash falls (right)
- glacial diamictites
- other indicators of synchronous global environmental changes.