Onlap

onlap

[′ȯn‚lap]
(geology)
A type of overlap characterized by regular and progressive pinching out of the strata toward the margins of a depositional basin; each unit transgresses and extends beyond the point of reference of the underlying unit. Also known as transgressive overlap.

Onlap

 

the bedding of layers of sedimentary rock that results from the advance of the sea onto land, a process called transgression. In onlap, the upper, younger layers are more widely distributed than the lower horizons, called the base horizons, which were deposited earlier. The latter overlie more ancient rocks with a stratigraphic break. The presence of such a break indicates that dry land existed for a more or less protracted period after deposition of the rock in the given area, after which it subsided. The advance of the sea onto land is accompanied by the abrasion and leveling of the land surface, on which the sediments of the transgressing marine basins lie horizontally. Onlap is often combined with angular unconformities. The identification of onlaps is very important in paleogeographic investigations and in studying the history of the oscillatory movements of the earth’s crust.

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Analysis of the integration of this seismic data with regional well data identified gas prospects AO1 and AO2 in the Aptian clastic onlap play.
These Neogene to Quaternary sediments unconformable onlap onto the Paleozoic rocks and sometimes, reactivation of thrust faults resulted in over-thrusting of the younger units.
In the extreme east, the Muthaymimah Formation directly overlies ophiolite but appears to progressively onlap the Simsima Fms.
The southern and southeastern margins of the basin are marked by onlap onto hanging-wall rocks of the Meguma terrane and overlying late Palaeozoic strata.
Regional transgression onto the subsiding passive margin led to onlap of continental to shallow marine facies followed by deeper water facies, which reached a maximum in the Late Cretaceous (Turonian--Coniacian) and is reflected in the Cretaceous stratigraphic record by a gradual change from siliciclastic dominated sedimentation in the Early Cretaceous to more pelagic shale and carbonate sedimentation in the Late Cretaceous.
En las secciones sismicas interpretadas (Figuras 7, 8 y 9), se observan terminaciones tipo onlap, downlap, toplap y truncaciones erosionales que permiten reconocer cambios eustaticos.
Sobreyace discordante en onlap una secuencia sedimentaria conformada en su base por un nivel de brecha (Figura No.
Los materiales de esta secuencia se superponen a los de la secuencia 2 continuando el proceso de onlap sobre las partes altas de los mounds (Fig.
Onlap 50 Hamiltonmadehis move, reclaiming the lead again - and for good.
The Middle sequences correspond to onlap transgressive sequences and downlap basinward.
1603), who report only a slight onlap of the Laurentide Ice Sheet against the north coast of Gaspesie.
Aqui aparecen sistemas de canales complejos con terminaciones en onlap y downlap hacia el limite de secuencias, downlap hacia el abanico submarino y facies de canal/levee, slump y slide de gravedad (Figura 7).