An unconformity is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion. Figure 11 shows how an unconformity forms. ….

A gap in the sequence of rock layers is called an unconformity.

An Unconformity is a gap in the geologic record. This shows where some rock layers have been lost because of erosion. An Inclusion is a piece of rock that is contained in another rock.

Uncomformity

*The Law of Superposition is that an undeformed sedimentary rock layer is older than the layers about it and younger than the layers below it. According to the Law of Superposition, layer 1 was the first layer deposited, and thus the oldest layer.

Scientists often determine the correct sequence of sedimentary rock layers using the fossils found within them. They compare the fossils to fi gure out if two layers are from the same geologic time period, or if one layer is older than the other.

radiometric dating

bottom layer

Geologists put all unconformities into three main groups: disconformities, nonconformities, and angular unconformities. A disconformity is a place where part of a sequence of parallel rock layers is missing. Disconformities can form when a sequence of rock layers is pushed up because of forces inside the Earth.

Explain why some rock layers can be missing from the sequence in some outcrops. They were weathered and eroded or never deposited. reconstruct a sequence of events? We de not need to walk, dig, or identify layers or fossils.

Weathering is the process that changes solid rock into sediments. Sediments were described in the Rocks chapter. With weathering, rock is disintegrated.

Angular unconformity also causes geologic tilting. Angular unconformity occurs when parallel strata of sedimentary rocks are deposited on layers that are tilted, perhaps as a result of erosion.

sedimentary rocks is deposited in flat layers. Over years, the sediment becomes deeply buried, hardens, and changes into sedimentary rock. According to the law of superposition, in horizontal sedimentary rock layers the oldest is at the bottom. Each higher layer is younger than the layer below it.

There are three kinds of rock: igneous, sedimentary, and metamorphic.

On the seafloor, sediments are thinnest near spreading centers (young seafloor) and thicker away from the ridge, where the seafloor is older and has more time to accumulate. – Sediment layers are thickest near the continents, the source of lithogenous material, and thinner farther out to sea.

Shales are often found with layers of sandstone or limestone. They typically form in environments where muds, silts, and other sediments were deposited by gentle transporting currents and became compacted, as, for example, the deep-ocean floor, basins of shallow seas, river floodplains, and playas.

Shale is a hardened, compacted clay or silty clay that commonly breaks along bedding planes some of which are no thicker than paper. The best exposures are found beneath ledges of harder more resistant rocks such as limestone and sandstones. Most shales are soft enough to be cut with a knife and can be very brittle.

Shale is a fine-grained rock made from compacted mud and clay. The defining characteristic of shale is its ability to break into layers or fissility. Black and gray shale are common, but the rock can occur in any color. Shale is commercially important.