Geol 135 Sedimentation

J Bret Bennington

Updated 10/99

Stratigraphy

Stratigraphy is the study of sedimentary layers: their composition, geographic distribution, and geological and chronometric significance.

Lithostratigraphy ? the study of the physical nature of sedimentary layers and the geographic correlation of layers.

Chronostratigraphy ? the study of the age relationships of rock layers and the use of strata as temporal markers.

Biostratigraphy ? a specialization of chronostratigraphy that uses the fossil content in strata to define their age relationships.

Although rock layers and geologic time are obviously linked (the former is the basis for the later) it should be remembered that rocks are real, physical entities whereas time is an abstract concept. This is reflected in the fact that the Code of Stratigraphic Nomenclature recognizes two different systems of units: chronometric units (divisions of time) and chronostratigraphic units (divisions of rock layers):

Time Units Time ? Rock Units

Eons Eonathems
Eras Erathems
Periods Systems
Epochs Series
Ages Stages

Early, Middle, Late Lower, Middle, Upper

Material stratigraphic units

These are stratigraphic units based not on time, but on material packages of rock. They are defined in five ways:

Lithostratigraphic Unit ? a body of rock defined on the basis of its lithological characteristics and its stratigraphic position relative to other bodies of rock.

Biostratigraphic Unit ? a body of rock defined on the basis of its fossil content.

Allostratigraphic Unit ? a body of rock defined by its position relative to bounding unconformities or other correlatable surfaces that reflect changes is base level during deposition.

Magnetostratigraphic Unit ? a body of rock defined on the basis of remnant magnetism polarity.

Chronostratigraphic Unit ? any body of rock defined on the basis of isochronous surfaces. Note that both biostratigraphic and magnetostratigraphic units may also be chronostratigraphic units.

The Hierarchy of lithostratigraphic units

Deposition of sedimentary layers is discontinuous by nature at a variety of temporal scales. A hiatus in deposition will produce some kind of change in the nature of the sediment, resulting in two stratigraphically adjacent units that can be distinguished

Laminae (lamination) - < 1 cm in thickness (example, tidal laminae caused by twice daily changes in flow direction and velocity, or varves caused by annual cycles of sedimentation in lakes).

Beds (bed) ? a layer of sediment recognizable in outcrop by its separation from other beds along a bedding plane. Beds often represent single depositional events such as storms, floods, or flows. Indistinct or massive bedding can result if original beds are intermixed.

Member ? a distinctive bed or set of beds within a formation Members may have wide geographic extent, but are usually thin relative to formations.

Formation ? a stratigraphic unit defined on the basis of its relatively homogeneous lithological characteristics as well as other physical and fossil attributes, and by its stratigraphic position. It must be mappable at the surface or traceable in the subsurface. Formations have no strict chronometric parameters and may be diachronous across their extent.

Type Section ? the geographic location where an exposure provides the basis for the originally published description of a formation. All formations are named after the geographic location or a prominent named geographic feature of their type sections. A type section should include both the upper and lower boundaries of the formation. By convention, only the lower boundary is strictly defined. The upper boundary is simply taken to be the bottom of the lower boundary of the next formation. This way, if another section is located that includes additional layers above the top of the formation at the type section, these can be incorporated without altering the original stratotype definition.

Group / Supergroup ? Groups are collections of genetically related formations. A supergroup is a collection of formations or groups that are related by some major geological or tectonic episode in earth history.

Members, formations, groups and supergroups are defined by geologists. Formal recognition of a new named stratigraphic unit requires publication of the name and its stratotype or rational in a peer-reviewed publication. Previously defined names can be changed or amended in a similar fashion.

Unconformities

An unconformity is a gap in the stratigraphic record where the passage of time is not recorded by sedimentary record. Unconformities can result from both erosion as well as nondeposition. There is no defined minimum length of time for an unconformity, although usually they are recognized by the absence of biozones (intervals of time defined by the presence of one or more species) which are usually at least a million years in length. Small, obscure unconformities that do not represent detectable lengths of missing time are often called diastems.

Types of unconformities:

Angular Unconformity ? deposition, folding/erosion, deposition, with strata in angular contact.

Nonconformity ? deposition of sedimentary rock on eroded metamorphic or igneous rock.

Disconformity ? deposition of sedimentary rock on eroded but tectonically undisturbed sedimentary rock. Some geologists refer to a disconformity in which there is no evidence of significant erosion as a paraconformity.

Up indicators in sedimentary rocks

Trace fossils

Cross beds

Sole marks (flute marks, tool marks, current lineations)

Mudcracks

Geopetal structures

Wave ripples