ASBOG Chapter 3--Geologic Map and Cross-Section Interpretation

Allochthon

A fault block or rock mass moved from its place of origin

Autochthon

A rock mass underlying a thrust fault that has not moved from its location of origin

Blind Thrust

A thrust fault that dies out in the subsurface as it loses slip and stratigraphic separation

Detachment Fault

A low-angle normal fault associated with regional extension

Dip-slip fault

A fault in which the movement is parallel to the dip of the fault plane, such as a normal, reverse, or listric fault

Fenster

A "window" into rocks underlying a thrust fault

Footwall Block

The block that underlies a non-vertical fault

Growth Fault

A fault that forms contemporaneously with deposition in sedimentary rock. The throw increases with depth and the sedimentary units on the downthrown side are thicker than the corresponding units on the upthrown side

Hanging-wall Block

The block that overlies a non-vertical fault

Heave

The amount of horizontal displacement on a fault

Klippe

An outlier of a thrust sheet completely surrounded by the exposed footwall

Key Beds

A well-defined, easily identified strata that is distinctive enough to be useful in correlation in mapping

Listric Faults

A fault with a curved fault plane. Near the surface the fault plane is steeply dipping, but it becomes progressively flatter with depth. Listric faults may be normal or reverse.

Ma

Abbreviation for Mega annum, one million years

Nappe

A sheetlike rock unit that has moved over other rocks (allochthonous) on a predominately horizontal surface

Net Slip

The distance between two formerly adjacent points on either side of the fault, measured on the fault surface

Nonconformity

An unconformity formed by older igneous rocks in contact with younger sediments, indicating a missing time after the igneous intrusion

Normal Fault

A dip-slip fault in which the hanging wall has moved down relative to the footwall. The dip of the fault is between 45° and 90°

Oblique-slip fault

A fault in which movement is not parallel to the strike or dip of the fault plane

Offset

The horizontal component of displacement measured perpendicular to the strike of the disrupted unit

Reverse fault

A dip-slip fault in which the hanging wall has moved up relative to the footwall. The dip of the fault is between 45° and 90°

Rule of V's

The outcrop pattern of a formation as it crosses a valley forms a V shape (as viewed on a map). The V points in the direction that the formation underlies the valley.

Strike separation

The horizontal distance between a stratigraphic unit offset by a fault, measured along the strike of the fault. It is an apparent displacement

Strike-slip Fault

A fault in which movement is parallel to the strike of the fault plane

Throw

The amount of vertical displacement on a fault, also, the vertical component of net slip

Thrust fault

A reverse fault in which the fault plane dips less than 45°

Type Locality

The place where a geologic feature (such as a fossil species) was first recognized and described. It contains the type section.

Type Section

The originally described strata that constitutes a stratigraphic unit to which other parts of the unit may be compared. It is preferable to describe the location where the unit attains its maximum thickness and where the top and bottom units are exposed.

Unconformity

A gap in the geologic record; an interruption in the depositional sequence that implies uplift and erosion have removed part of the geologic record or non-deposition has occurred.