Geologic principles for defining relative age. & What is an unconformity?

Nonconformity

A nonconformity is a type of unconformity at which sedimentary rocks overlie generally much older intrusive igneous rocks and/or metamorphic rocks (Fig. 10.5b). These igneous or metamorphic rocks underwent cooling, uplift, and erosion prior to becoming the substrate, or basement, on which new sediments accumulated. NONCONFORMITY: SEDIMENTARY OVER IGNEOUS OR METAMORPHIC

The principle of inclusions:

A rock containing an inclusion (fragment of another rock) must be younger than the inclusion. For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone

The principle of baked contacts

An igneous intrusion "bakes" (metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusion

The principle of uniformitarianism

As we've seen, the principle of uniformitarianism means that physical processes we observe operating today also operated in the past, at roughly comparable rates. Put concisely, "the present is the key to the past"

The principle of cross-cutting relations

If one geologic feature cuts across another, the feature that has been cut is older. For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers. But if a layer of sediment buries a fault, the sediment must be younger than the fault.

Disconformity

Imagine that a sequence of sedimentary beds has been deposited beneath a shallow sea. Sea level drops, exposing the beds for some time. During this time, no new sediment accumulates, and some of the pre-existing sediment gets eroded away. Later, sea level rises, and a new sequence of sediment accumulates over the old. The boundary between the two sequences is a disconformity (Fig. 10.5c, d). Even though the beds above and below the disconformity are parallel, the contact between them represents an interruption in deposition. DISCONFORMITY: SEDIMENTARY OVER SEDIMENTARY, PARALLEL, BUT TIME IS MISSING

The principle of superposition

In a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect. Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest

angular unconformity

Rocks below an angular unconformity were tilted or folded before the unconformity developed (Fig. 10.5a). An angular unconformity cuts across the underlying layers, and the orientation of layers below an unconformity is different from the orientation of the layers above. ANGULAR UNCONFORMITY: SEDIMENTARY OVER SEDIMENTARY, BUT AT DIFFERENT ANGLES

The principle of lateral continuity

Sediments generally accumulate in continuous sheets within a given region. When you see a sedimentary layer cut by a canyon, you can assume that the layer once spanned the area, and was later eroded by the river that formed the canyon

The principle of original horizontality

Sediments on Earth settle out of fluids in a gravitational field. The surfaces on which sediments accumulate (such as floodplains or the seafloor) are fairly flat. Therefore, layers of sediment, when first deposited, are fairly horizontal (Fig. 10.1b). If sediments collect on a steep slope, they typically slide downslope before lithification, so they will not be preserved as sedimentary rocks. With this in mind, we conclude that folding, tilting, and faulting of sedimentary beds must occur after the beds were deposited

Unconformity

boundary surface between two units, which represents a period of nondeposition and possibly erosion, as an unconformity. The gap in the geologic record that is reflected in an unconformity is called a hiatus.