Lithostratigraphy - Stratigraphy based on the identification and correlation of similar rock units.

French chemist, Antoine Lavoisier, first applied uniformitarian concepts of geology in a comparison of modern depositional systems with their ancient rock equivalents. By 1789 Lavoisier recognized that gravel can only be moved by waves near the shore whereas finer sediment can be carried into deeper water, and that each environment had distinctive organisms. He further reasoned that these environments would transgress (shift towards the continent) or regress (shift basinward) if sea level rose or fell, respectively.

As a practical matter, we use a hierarchical system for organizing the identity of rock units. These include (from least to most inclusive):

Higher-order rock units (member and up) are expected to be: How do you distinguish a member from a formation from a group?

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:

Suppose sea level were constant but the floor of the basin were subsiding tectonically. (D-world w/o sea level change) In principle, facies would aggrade - build directly on top of one another in a subsiding basin (right) but this rarely happens. Instead...

Facies are transgressive (move shoreward) when

Facies are regressional (move seaward) when:

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.

Time lines are depositional surfaces of equal age. These may be: When they cut across facies boundaries the facies deposit is considered to be diachronous, or time transgressive.

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

Repetition of trangressive and regressive cycles is often asymmetric. Trangressive sequences are often masked by: A full trangressive cycle is only accumulated on a rapidly subsiding margin where sediments sink and preserve before they can be significantly reworked. Thus, what we typically see are stacked successions of shoaling-upward sequences.

Rock layers that laterally thin to a point where they vanish are known as pinch outs. The result of a series of pinch outs due to facies migration during transgression or regression are interfingering relationships.

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:


While we have discussed the base level of erosion previously as being sea level, but one could apply this concept more broadly to mean the level above which sediments cannot accumulate permanently, otherwise known as the base level of aggradation. It is an imaginary surface of equilibrium between the forces of erosion and deposition.

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

While radiometric age constraints imply that on average ancient sediments accumulate on the order of meters per thousands of years, modern observations indicate orders of magnitude greater accumulation rates (meters per year to hundreds of years) implying that the stratigraphic record is very incomplete. Indeed every bedding plane could represent some cryptic surface of missing time, a kind of unconformity called a diastem. Because sea level fluctuates up and down, most geological time is not represented in the sedimentary record. The Barrel diagram at right plots thickness of deposit against time. Note that the thickness diminishes during episodes of erosion. Net accumulation is in bold lines.

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
Unconformities are defined as temporal breaks in a stratigraphic sequence. These include:


What does the Grand Canyon tell us about unconformities and the base level of erosion?

In the field, unconformities are often recognized by:

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.

Time slices and marker beds: Remember that the rock units we correlate across a basin may be tine transgressive or contain paraconformities - cryptic disconformities. We therefore crave marker beds, key lithologies or sedimentary structures that represent isochronous unique events such as: The first two examples are lithostratigraphic, but the last could require the methods of paleontology, geochemistry, magnetostratigraphy, etc. We take these up in turn in the coming lectures.