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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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.
The Kamar-1 well is expected to test the Kamar Subcrop, as well as the Kamar Onlap prospect (stratigraphically above the Kamar Subcrop) to which GCA estimated 232 Bcf of natural gas and 13.
In Block 665 a large Cretaceous onlap play has been identified.
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.
The nickpoint concept and its implications regarding onlap to the stratigraphic record.
This inference is further supported by (1) greater abundance and larger sizes of coral colonies on the south end of th e outcrop, (2) onlap of cross-stratified skeletal grainstone on the south side of the mound, and (3) lesser concentrations of microcrystalline matrix in the Skeletal Lithofacies on the south side of the mound.
Those geologic aspects which define Woodson Ridge are: 1) an anomalous broad surface topography reflected as the ridge crest; 2) a lower Claibome-age fault along the southwest flank; 3) stratigraphic onlap of the Lower Claibome (Meridian Sand) and Middle Wilcox (basal Tuscahoma Fm.
Trap Types: Neogene inversion anticlines and pre-Oligocene compressional structures; stacked Eocene stratigraphic onlap traps.
Maps currently under construction will show the interval-by-interval depositional limits that document the onlap that occurred during early Pennsylvanian transgression in the Illinois Basin.
Several play types have been identified, including rotated fault blocks, stratigraphic onlap pinchouts, truncation traps, strike slip associated traps and deepwater fans.