|Best road cut ever? Quite possibly. Fall out from several episodes of activity at Chimborazo volcano, Ecuador|
There are three basic types of contacts:
(1) depositional contacts, where a sediment layer is deposited over preexisting rock.
(2) fault contacts, where two units are juxtaposed by a fracture on which sliding has occurred.
(3) intrusive contacts, where one rock body cuts across another rock body.
In this article , we consider in more detail the nature and interpretation of depositional contacts.
Relatively continuous sedimentation in a region leads to the deposition of a sequence of roughly parallel sedimentary units in which the contacts between adjacent beds do not represent substantial gaps in time. Gaps in this context can be identified from gaps in the fossil succession.
The boundary between adjacent beds or units in such a sequence is called a conformable contact. For example, we say, “In eastern New York, the Becraft Limestone was deposited conformably over the New Scotland Formation.” The New Scotland Formation is an argillaceous limestone representing marine deposition below wave base, whereas the Becrft Limestone is a pure, coarse-grained limestone representing deposition in a shallow-marine beach environment.
|The rock formation above shows an angular unconformity found on the coast of Portugal at Telheiro Beach,|
copyright by: Gabriel Gutierrez-Alonso.
If there is an interruption in sedimentation, such that there is a measure able gap in time between the base of the sedimentary unit and what lies beneath it, then we say that the contact is unconformable.
F I G U R E 2 The principal types of unconformities:
( c) nonconformity, (d) buttress unconformity.
For example, we say, “In eastern New York, the Upper Silurian Rondout Formation is deposited unconformably on the Middle Ordovician Austin Glen Formation,” because Upper Ordovician and Lower Silurian strata are absent. Unconformable contacts are generally referred to as unconformities, and the gap in time represented by the unconformity (that is, the difference in age between the base of the strata above the unconformity and the top of the unit below the unconformity) is called a hiatus. In order to convey a meaningful description of a specific unconformity, geologists distinguish among four types of unconformities that are schematically shown in Figures 1&2 and defined in Table .
Unconformities represent gaps in the rock record that can range in duration from thousands of years to billions of years. Examples of great unconformities, representing millions or billions of years, occur in the Canadian shield, where Pleistocene till buries Proterozoic and Archean gneisses. In the classic unconformity between Paleozoic sedimentary rocks and Precambrian gneisses is shown and many introductory geology books show this contact in the Grand Canyon.
F I G U R E 3 Angular unconformity in the Caledonides at Siccar Point (Scotland).
The hammerhead rests on the unconformity, which is tilted due to later deformation.
It is a special experience to put your finger on a major unconformity and to think about how much of Earth’s history is missing at the contact. Imagine how James Hutton felt when, in the late eighteenth century, he stood at Siccar Point along the coast of Scotland(Figure 3), and stared at the Caledonian unconformity between shallowly dipping Devonian Red Sandstone and vertically dipping Silurian strata and, as the present-day waves lapped on and off the outcrop and deposited new sand, suddenly realized what the contact meant. His discovery is one of the most fundamental in field geology.
How do you recognize an unconformity (Figure 4) in the field today? Well, if it is an angular unconformity or a buttress unconformity, there is an angular discordance between bedding above and below the unconformity. A nonconformity is obvious, because crystalline rocks occur below the contact.
At a disconformity, beds of the rock sequence above and below the unconformity are parallel to one another, but there is a measurable age difference between the two sequences. The disconformity surface represents a period of nondeposition and/or erosion (Figure 1. a).
At an angular unconformity, strata below the unconformity have a different attitude than strata above the unconformity. Beds below the unconformity are truncated at the unconformity, while beds above the unconformity roughly parallel the unconformity surface. Therefore, if the unconformity is tilted, the overlying strata are tilted by the same amount. Because of the angular discordance at angular unconformities, they are quite easy to recognize in the field. Their occurrence means that the sub-unconformity strata were deformed (tilted or folded) and then were truncated by erosion prior to deposition of the rocks above the unconformity. Therefore, angular unconformities are indicative of a period of active tectonism. If the beds below the unconformity are folded, then the angle of discordance between the super- and sub-unconformity strata will change with location, and there may be outcrops at which the two sequences are coincidentally parallel (Figure 1. b).
Nonconformity is used for unconformities at which strata were deposited on a basement of older crystalline rocks. The crystalline rocks may be either plutonic or metamorphic. For example, the unconformity between Cambrian strata and Precambrian basement in the Grand Canyon is a
nonconformity (Figure 2. c).
A buttress unconformity (also called onlap unconformity) occurs where beds of the younger sequence were deposited in a region of significant predepositional topography. Imagine a shallow sea in which there are islands composed of older bedrock. When sedimentation occurs in this sea, the new horizontal layers of strata terminate at the margins of the island. Eventually, as the sea rises, the islands are buried by sediment. But along the margins of the island, the sedimentary layers appear to be truncated by the unconformity. Rocks below the unconformity may or may not parallel the unconformity, depending on the pre-unconformity structure. Note that a buttress unconformity differs from an angular unconformity in that the younger layers are truncated at the unconformity surface (Figure 2. d).
Disconformities, however, can be more of a challenge to recognize. If strata in the sequence are fossiliferous, and you can recognize the fossil species and know their age, then you can recognize a gap in the fossil succession.
|F I G U R E 4 Some features used to identify unconformities: (a) scour channels in sediments, (b) basal conglomerate, (c) age discordance from fossil evidence, and (d) soil horizon or paleosol.|
Commonly, an unconformity may be marked by a surface of erosion, as indicated by scour features, or by a paleosol, which is a soil horizon that formed from weathering prior to deposition of the overlying sequence. Some unconformities are marked by the occurrence of a basal conglomerate, which contains clasts of the rocks under the unconformity.
Recognition of a basal conglomerate is also helpful in determining whether the contact between strata and a plutonic rock is intrusive or whether it represents a nonconformity.
Try this Cross Section Interpretation Exercise
Try this Cross Section Interpretation Exercise