Geology 111G/Lecture 12

Stratigraphy and Correlation

Original Horizontality

Superposition

Cross-cutting relationships

Unconformities

Correlation

I. Stratigraphy: Study of stratified rocks (strata). Knowledge of the ways by which layered rocks accumulate and are subsequently altered provides clues to the fundamental principles of physical geology. The major principles of stratigraphy provide a means of establishing the timing of geologic events and give the science a historical aspect unique to the physical sciences. Strata: term for stratified rocks.

A. Original horizontality. Most sedimentary and volcanic rocks are deposited parallel to Earth's surface, or roughly horizontally.

B. Stratigraphic superposition: If a series of sedimentary rocks is not overturned, the uppermost layer is the youngest and the bottom layer is the oldest; young rocks are deposited on old rocks.

1. Nicolas Steno, an Italian physician, 1669.

2. Overturned: layered succession is upside down.

C. Cross-cutting relations: A rock is younger than any rock it cuts across.

II. Unconformites: Surfaces of non-deposition and/or erosion that separate two rock masses. These represent intervals of time not represented by the deposition of sediment. Hiatus: time not recorded, distinct from the physical surface.

A. Angular unconformity. Surface separates sedimentary rocks that dip at different angles. Significance is that the rocks were tilted or uplifted and eroded prior to resumption of deposition.

B. Disconformity. Surface separates sedimentary layers that are parallel above and below.

C. Nonconformity. Surface separates sedimentary layers above from metamorphic or igneous rocks below. Signifies large amount of uplift and erosion to juxtapose rocks formed at depth in crust with those formed at surface.

III. Correlation: determination of equivalence in age and position in the geologic record. In order to establish a record of continuous events, rock successions from different places need to be correlated to create a composite column of rock.

A. Geological record (rock record): term for all the events and time preserved by rocks.

B. Correlation based on physical characteristics and position in sequence of strata.

C. Correlation based on fossil content.

D. Principle of Faunal Succession: strata may be recognized based on their fossil content because there is a distinctive lineage of fossils in the stratigraphic succession.

1. William Smith, England, 1793.

2. This conceptual advance permitted division of geologic time into formally designated intervals based on their fossil content: geologic time units. These could then be arranged into their proper order based on the stratigraphic principles above. Boundaries between units were based on disappearances of fossil organisms.