Use of Genetic Sequence Stratigraphy in Defining and Determining a Regional Tectonic Control on the "Mid-Cimmerian Unconformity"—Implications for North Sea Basin Development and the Global Sea Level Chart
John R. Underhill, Mark A. Partington, 1993. "Use of Genetic Sequence Stratigraphy in Defining and Determining a Regional Tectonic Control on the "Mid-Cimmerian Unconformity"—Implications for North Sea Basin Development and the Global Sea Level Chart", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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Detailed analysis of Jurassic stratigraphic relations in the North Sea highlights the occurrence of numerous, distinctive, and correlatable marine shale horizons. These horizons are often characterized by condensed sections in subbasin depocenters and shales marking the maximum extent of marine flooding into nonmarine environments on their margins. Biostratigraphic data indicate that these intervals are correlatable and isochronous within the level of resolution currently employed. Correlation of these maximum flooding surfaces allows the Jurassic stratigraphy to be subdivided into genetic stratigraphic sequences with chronostratigraphic significance. Mapping of stratigraphic relations using this approach from offshore areas of northwest Europe and onshore exposures in Britain, Greenland, and continental Europe allows the nature of Middle Jurassic ("mid-Cimmerian ") unconformities to be identified. The earliest and most significant of these unconformities occurs consistently in the Aalenian and is characterized by progressive truncation of stratigraphy throughout the North Sea area in a broadly concentric but asymmetric subcrop pattern over an area with a diameter greater than 1250 km. The oldest rocks subcrop immediately adjacent to the rift arm triple junction. Strata above the unconformity show that progressive marine onlap occurred from late Aalenian to early Kimmeridgian times with the central area covered by the youngest strata. These relations are interpreted to be consistent with widespread late Toarcian–Bathonian regional tectonic uplift (Central North Sea Dome) that was synchronous with crustal thinning in areas that later became the rift arms, followed by progressive onlap onto a gently dipping slope as the dome deflated during the Callovian–Kimmeridgian. Knowledge that igneous rocks of the Forties Province were emplaced after the development of the unconformity is consistent with ideal models of rifting consequent upon the impingement of a mantle plume head on the base of the lithosphere. However, since recent studies have demonstrated that the excess temperature in such a plume must have been minimal to be compatible with the limited melting observed, it cannot be considered to have been sufficiently hot to have caused widespread melting. This conclusion and the Callovian-Kimmeridgian deflation make it likely that the driving force was a transient plume head or "blob" rather than a focused, long-lasting "hot" jet. Simple comparison with the present global chart of coastal onlap and eustatic sea level change shows that the same intra-Aalenian "mid-Cimmerian unconformity" equates with the chart's most significant regressive changes in coastal onlap (the 177 Ma sequence boundary separating the Absaroka and Zuni first-order megacycles). However, this part of the curve appears to be based exclusively upon stratigraphical sections in Dorset and Yorkshire, associated with the geographically restricted region affected by regional, thermal-driven doming. Consequently, the study highlights the impracticality of using two relatively closely spaced sections to define a global eustatic signal for the Jurassic and suggests that there remains a need to test the global interpretation using sections drawn from outside the uplifted area. To the natural concern that local tectonic events have generated an apparent eustatic event must now also be added the possibility that asthenosphere-driven regional uplift may produce correlatable behavior over distances greater than 1000 km.
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Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,“Entitled Carbonate Sequence Stratigraphy: New Developments and Applications.”