Depositional Sequence Mapping to Illustrate the Evolution of a Passive Continental Margin
Richard J. Hubbard, Joe Pape, David G. Roberts, 1985. "Depositional Sequence Mapping to Illustrate the Evolution of a Passive Continental Margin", Seismic Stratigraphy II: An Integrated Approach to Hydrocarbon Exploration, Orville Roger Berg, Donald G. Woolverton
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Application of the analytical techniques of deposition sequence mapping provides an in-depth understanding of the regional geology and evolution of passive continental margins. In this paper we illustrate the complete evolution of a passive continental margin using examples from offshore Newfoundland and the Beaufort Sea.
Four depositional megasequences are developed on the Newfoundland margin which record two phases of rifting and an early and late stage of passive continental margin subsidence. The first phase of rifting is represented by the Atlantic megasequence which had a duration of ca. 90 m.y.(Late Triassic to Late Jurassic) and is characterized by early evaporite and later carbonate basins. The Labrador megasequence resulted from the second phase of rifting. It had a duration of ca. 50 m.y. (Berriasian to Early Cenomanian) and is characterized by a transition from carbonate basins to normal clastic sedimentation. The margin became inactive during the late Cretaceous to Tertiary and passive post-rift sedimentation began. Early stage and late stage of the passive wedge are the type examples used in this paper.
The Eurekan megasequence of the Beaufort Sea provides a more illustrative example of a late stage passive sedge than the Newfoundland example. It is divided intofour deposition sequences, or delta cycles, primarily on the basis of the seismic recognition of depositional sequence boundaries.
The key to exploration success in the rift system offshore Newfoundland is the super-position of the Atlantic and Labrador syn-rift megasequences. Only where these two deposition systems interact is there a favorable combination of source, reservoir and structure.
The primary exploration uncertainty in the Eurekan megasequence concerns the presence and quality of reservoir sands within the deltaic depositional environments. It is stressed that depositional sequence mapping can only qualitatively high grade areas for enhanced reservoir development; direct well control is always required for accurate reservoir prediction.
Our observations, based on depositional sequence mapping, carry a number of implications for the various mechanisms that have been proposed to control the evolution of passive continental margins. We find general agreement with McKenzie's (1978) idealized model for rifting, and the important modifications made by Steckler and Watts (1981) and Cockran (1983) concerning the effects of, 1) local rift behavior, 2) sediment loading, and 3) changing sedimentary style with increasing age of the margin.
We find little support for Vail et al's proposal (in Schlee, 1984) that depositional sequence boundaries are largely controlled by global eustatic changes in sea level. Our observations suggest that unconformities are widespread rather than global and arise from two causes. The major unconformities, megasequence boundaries, develop at a time of major plate re-arrangement and corresponding orogeny. By comparison, individual sequence boundaries within a megasequence result from the local interplay between sediment supply and the rate of the various tectonic processes (rifting, thermal subsidence and sediment loading) which are active during each phase of passive continental margin evolution.