Stratigraphic architecture and gamma ray logs of deeper ramp carbonates (Upper Jurassic, SW Germany)

The objective of this paper is to contribute to the development of sequence stratigraphic models for extensive epicontinental carbonate systems deposited over cratonic areas. Epicontinental carbonates of the SW German Upper Jurassic were analysed in terms of microfacies, sedimentology and sequence stratigraphy based on 2.5 km of core, 70 borehole gamma ray logs and 24 quarries. Facies analysis revealed six major facies belts across the deeper parts of the carbonate ramp, situated generally below fair-weather wave base, and mostly below average storm wave base but in the reach of occasional storm events. Observed stratigraphic patterns differ in some aspects from widely published sequence stratigraphic models: 1. Elementary sedimentary cycles are mostly more or less symmetrical and are, thus, referred to as genetic sequences or genetic units [AAAPG Bull. 55 (1971) 1137; Frazier, D.E., 1974. Depositional episodes: their relationship to the Quaternary stratigraphic framework in the northwestern portion of the Gulf Basin. University of Texas, Austin, Bureau of Economic Geology Geologicalo Circular 71-1; AAPG Bull. 73 (1989) 125; Galloway, W.E., Hobday, D.K., 1996. Terrigenous Clastic Depositional Systems. 489 pp., Springer; Cross, T.A., Baker, M.R., Chapin, M.S., Clark, M.S., Gardner, M.H., Hanson, M.S., Lessenger, M.A., Little, L.D., McDonough, K.J., Sonnenfeld, M.D., Valasek, D.W., Williams, M.R., Witter, D.N., 1993. Applications of high-resolution sequence stratigraphy to reservoir analysis. Edition Technip, 1993, 11-33; Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine 16 (1992) 357; Homewood, P., Mauriaud, P., Lafont, F., 2000. Best practices in sequence stratigraphy. Elf EP Mem. 25, 81 pp.; Homewood, P., Eberli, G.P., 2000. Genetic stratigraphy on the exploration and production scales. Elf EP Mem. 24, 290 pp.], in contrast to the asymmetrical, shallowing-upward parasequences of the EXXON approach. 2. Neither sequence boundaries nor maximum flooding surfaces could be clearly delineated. Cycle boundaries are generally not represented by sharp stratal surfaces but are always transitional and, thus, referred to as turnarounds [Nor. Pet. Soc. Spec. Publ. 8 (1998) 171]. Several types of genetic sequences were delineated. Both major types of facies and sequences show characteristic gamma ray log signatures. Based on the cycle stacking and the gamma ray patterns, a hierarchy of sequences was recognized, probably driven in part by 100,000- and 400,000-year Milankovitch signals. The cyclicity allowed regional correlations across various depositional environments such as sponge-microbial bioherms and coeval basins. The basin-wide correlation revealed evidence for a subtle clinoform-type stratigraphic architecture along very gentle slopes, rather than a so far assumed simple layer cake pattern. (C) 2002 Elsevier Science B.V. All rights reserved.