The widespread unconformity(T

7

) in the Pearl River Mouth Basin(PRMB)

the origin of which is closely related to dynamic basin evolution

eustatic variation and hydrocarbon accumulation

is one of the most important unconformities of the basins located at the northern margin of South China Sea.The provenances and sedimentary environments above and below the interface show significant differences.Based on seismic and logging data

we documented the distribution and structural features of the unconformity in the PRMB during the late Early Oligocene T

7

(32~30 Ma)

and discussed the mechanism of the unconformity and the basinal evolution features indicated herein.Results show that the T

7

boundary is a stage boundary unconformity with most faulting terminating at T

7

or the fault displacement diminishing markedly across the unconformity

while the underlying Enping Formation formed in the shape of a wedge with its stratum thickness obviously controlled by faulting.The T

7

unconformity can be divided into three zones:the angular unconformity zone

local unconformity and onlap zone

and conformity zone in the north

corresponding to the strong erosion zone

weak erosion zone

and non-erosion zone respectively.The angular unconformity zone is characterized by truncation at the shoulder part due to fault block rotation; the local unconformity and onlap zone shows onlap features and local denudation that is significantly gentle compared with angular unconformity zone; the conformity zone is located on the south of the southern uplift in PRMB and characterized by onlapping at the depression margin and conformity contact in the depression center.The distribution pattern of the stage boundary unconformity in this study area is controlled by tectonism and palaeogeomorphology in different zones of the basin.The angular unconformity zone in the north is mainly controlled by fault rotation and uplifting

but in local unconformity and onlap zone

the erosion is mainly caused by diapir or local uplifts.The conformity zone was located under paleowater level and at the depression center.The pattern indicates that the formation mechanism of the stage boundary unconformity is related to the erosion on shoulder uplift structure at the basin margin at the rifting stage.