UNCONFORMITIES

• Unconformities are depositional contacts that overlie rocks distinctly older than they are.

• They are often called “gaps” in the sedimentary record.

• The contact represents time when no sediment was deposited.

• Usually they also represent time when the underlying rock was being eroded.

NONCONFORMITY

Sedimentary rocks (which necessarily form at Earth’s surface) Intrusive igneous rocks (which necessarily form deep in Earth’s crust). How can they be directly superposed?

One way of course is for the sediments to be the country rock or host rock for the intrusion. The other way, and the one that now interests us, is for the original host rock and part of the intrusion to be eroded away, at the surface, before the sediments are laid down above them. The unconformity (symbolized by the wavy line) represents the time of erosion of the host rock and intrusion. This type of unconformity is called a nonconformity. Sediments overlie igneous (or metamorphic) rocks. WHY METAMORPHIC ROCKS?

NONCONFORMITY

1-2°

Igneous and Metamorphic Rocks (Precambrian and Paleozoic) Sediments and sedimentary rocks (Cretaceous and younger)

Geologic map of the Fall Line north of Americus, GA

The Fall Line is the northern boundary of the Coastal Plain province. At this locality it is also the southern boundary of the Piedmont province.

Sumner Butte

Tapeats Sandstone Bright Angel Shale

Muav Limestone Redwall Limestone

Supai Formation

View NE from Bright Angel Trail Brahma Temple

Brahma Schist

ANGULAR UNCONFORMITY

A second type of unconformity is shown here. In this case sediments are both above and below the contact, but the ones below meet the contact at an angle. (the ones above are roughly parallel to it). This is called an angular unconformity because the dips of beds above and below it are different.

In this case the unconformity represents the time during which the underlying beds were tilted (folded, actually) and eroded. The overlying beds were then deposited on top of that flat erosional surface.

Angular Unconformity

Geologic map of the Fall Line near Tuscaloosa, AL. The Fall Line is the northern boundary of the Coastal Plain Province. At this location it is also the southern boundary of the Valley and Ridge Province.

1-2°

Coastal Plain outliers

Vishnu Schist

Bass Limestone Hakatai Shale

Yellow Lines – Base of Tapeats Sandstone (C) Orange Lines – Bedding in underlying Formation (pC)

Tapeats Missing Shinumo Quartzite Dox Sandstone

View NNE from Grandview Point

DISCONFORMITY

In this case, once again, there are sedimentary rocks above and below the unconformity. Unlike the angular unconformity, however, the beds on both sides are essentially parallel to the contact.

The unconformity in this case represents either a simple lapse in the continuity of sedimentation OR that plus erosion of the underlying layer. The first type can be devilishly hard to recognize, that latter type is easier.

Disconformity

The Fall Line in Western Alabama and eastern Mississippi. Here the Fall Line swings northward into the Mississippi Embayment. It is still the boundary of the Coastal Plain (now to the west). Here the older sedimentary rocks to the east are not structurally deformed. (They were too far from the collision zone that folded them to the east). Because the Paleozoic rocks are virtually flat and the Cretaceous ones are virtually flat, the Fall Line is a disconformity.

Sediments and sedimentary rocks (Cretaceous and younger)

Sediments and sedimentary rocks (Pennsylvanian and older)

1-2°

Shown on earlier slide

1-2°

Muav Limestone (Cambrian) Bright Angel Shale (Cambrian)

Devonian Silurian MISSING! Ordovician

View SW from Bright Angel Trail

Providence Sand (Cretaceous)

highly weathered Clayton Formation(Paleocene)

The Fall Line is an unconformity, with Cretaceous and younger sediments overlying Paleozoic and Younger rocks.

Over ~flat sedimentary

rocks

Over folded sedimentary

rocks Over igneous and metamorphic rocks

What kind of unconformity is it in each of the three zones indicated?