W. Galloway

Department of Geological Sciences, University of Texas at Austin, Austin, TX

ABSTRACT: Stratal Architecture of Linked, Sand-Rich Shore-Zone and Shelf Depositional Systems: Utsira Sequence, North Sea Basin

Sand-bearing shelf depositional systems remain a sparsely documented and contentious theme of sedimentary geology. Much of the recent controversy revolves around the question of whether or how sand can be transported onto a marine shelf in sufficient quantity of form significant sand bodies. In general, two main paradigms exist. (1) Sand is brought onto the shelf platform during periods of relative fall and low-stand. It is then reworked to varying degrees by marine coastal and shelf processes during subsequent transgression, and wholly or partly remolded into shelf sand bodies. (2) Sand can, given appropriate shelf geography, sediment supply, and marine energy, be stripped from the shoreface and swept onto the shelf, where it nourishes constructional shelf systems under long-term conditions of relatively stable, falling, or rising sea level. This latter paradigm has seen little application in studies of modern (which are inherently strongly influenced by the Holocene transgression) and ancient shelf systems, particularly since the advent of the sequence stratigraphic paradigm.

The Cenozoic fill of the North Sea Basin contains several examples of both mud- and sandrich shelf depositional systems (Galloway et al. 1993). Examples include units of the Grid and Mousa Sandstones (middle to late Eocene) and most of the Oligocene and Miocene fill. The objective of this paper is to demonstrate that the Upper Miocene Utsira Formation, of the Viking Trough, is a regional marine shelf sand deposit that is well circumscribed by well and seismic data, is fully preserved, and records a relatively stable paleo-oceanographic setting that was ideal for shelf sand transport and accumulation. The Utsira shelf system persisted for several million years, creating a sand-rich second-order stratigraphic sequence whose composition, lithofacies associations, external geometry, and internal architecture reflect its fully marine origin.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado