Cause of Northern North Sea Jurassic Unconformities: ABSTRACT

Abstract

Thirteen unconformities and their correlative conformities (sequence boundaries) divide the strata of the Jurassic of the northern North Sea into twelve cycles of coastal onlap. Comparison of charts showing regional relative change of coastal onlap, unconformity age, stratal patterns, and facies relations from the northern North Sea with global charts of relative changes of coastal onlap and eustatic sea level indicates that the unconformities are global and are caused by eustatic changes of sea level. Nine of the global unconformities are believed to be caused by rapid eustatic falls of sea level, three by slow falls followed by rapid rises, and one by an increased rate of sea level fall. In addition, four marine hiatuses were identified that are interpreted to be relate to rapid rises of sea level. Fault block rotation or differential block subsidence occurred almost continuously throughout the Jurassic, causing tilting of beds. Unconformity recognition is enhanced by periodic truncation of tilted strata by lowstand erosion and/or onlap during the subsequent rises, but the tectonics do not cause the unconformities. Northern North Sea unconformities and coastal onlap are demonstrated on two seismic sections tied to well control. One section is from the United Kingdom part of the north Viking graben; the second is from the inner Moray Firth. Sequence boundaries and charts showing chronostratigraphy, relative change of coastal onlap, and ages of unconformities are shown for each seismic section. End_of_Article - Last_Page 1003------------