What is an Onlap sequence?

What is an Onlap sequence?

Onlap is a particular pattern of reflections in seismic data that, according to principles of sequence stratigraphy, occurs during periods of transgression. See: apparent dip, downlap, pinch-out, retrogradation, sequence stratigraphy, transgression. Onlap, downlap, and progradation.

What is Onlap and Offlap?

It is generally associated with a marine transgression. It is a more general term than overstep, in which the younger beds overlap onto successively older beds. The opposite is offlap, in which each younger rock bed pinches out short of the full extent of the underlying older bed, typically due to a marine regression.

What is an Offlap sequence?

offlap A conformable sequence of inclined strata, deposited during a marine regression, in which each stratum is succeeded laterally by progressively younger units, marking the direction in which the sea retreated.

What is Onlap Downlap?

Onlap: Reflector termination on surfaces with greater dips than that of the overlying beds; lapping onto a structural high. Downlap: Reflector termination on surfaces which dip less than that of the overlying beds; lapping onto a structural low.

What is Onlap in seismic?

Onlap: A base-discordant relationship in which initially horizontal strata progressively terminate against an initially inclined surface, or in which initially inclined strata terminate progressively updip against a surface of greater initial inclination.

What is Downlap?

1. n. [Geology] The termination of more steeply dipping overlying strata against a surface or underlying strata that have lower apparent dips; a term used to describe a particular geometry of reflections in seismic data in sequence stratigraphy.

What is Aggradation and progradation?

Progradational: shore and nearshore deposits move outward into the ocean and overlie deeper water deposits. Retrogradational: deeper water deposits move towards land and overlie shallow water deposits. Aggradational: facies remain in the same general location and stack atop others of the same facies.

What is fining upward sequence?

Fining- and coarsening-upward sequence Fining upward sequence means that the grain size is decreasing upward, whereas coarsening upward means that the grain size is increasing upward.

What is Downlap in stratigraphy?

1. n. [Geology] The termination of more steeply dipping overlying strata against a surface or underlying strata that have lower apparent dips; a term used to describe a particular geometry of reflections in seismic data in sequence stratigraphy. See: apparent dip, dip, onlap, sequence stratigraphy.

What is truncation in geology?

Truncation is a termination of strata or seismic reflections interpreted as strata along an unconformity surface due to post-depositional or structural effects. Truncation occurs along the upper limit of stratigraphic cycles (sequences).