CARTWRIGHT, JOSEPH A., Imperial College, London, England

ABSTRACT: How Laterally Extensive Are Sequence Boundaries?

Sequence boundaries are the foundation on which modern sequence stratigraphic concepts have been erected. They are widely portrayed as discrete surfaces that are correlatable on the scales of basins. They comprise surfaces of stratal discontinuity (onlap, downlap, toplap, and erosional truncation) linked by uniquely defined correlative conformities. They are distinguished from local discontinuity surfaces by their property of regional extent. Onlap surfaces are absolutely central to the definition of type 1 and type 2 sequence boundaries, and set the chronostratigraphic clock by which the correlative conformity for any given sequence boundary can be uniquely defined.

This paper examines the conditions necessary for the development of onlap onto a discrete surface. Drawing extensively from Van Hinte's conception of condensed onlap sections and apparent seismic onlap, a simple geometric argument is presented that establishes a rigorous constraint on the development of a discrete onlap surface. The main conclusion is that a discrete onlap surface can form only if there is no net sediment accumulation beyond the lap-out position. Consideration of depositional processes in a range of environments suggests that this condition is rarely met, and consequently, laterally extensive onlap onto a discrete surface is extremely rare. It is more common to find that onlap develops onto families of closely related surfaces that may appear as a single surface on se smic data, and that may be mistakenly correlated over large areas. An extensive correlation of apparent onlap surfaces will cross time lines and should not be used to erect a chronostratigraphic framework.

The areal extent of sequence boundaries is a prerequisite for the eustatist's view of stratigraphy. If it is assumed that sequence boundaries have an eustatic origin, then there is a tacit expectation that they should be correlatable over large areas. It is concluded that the apparent areal extent of sequence boundaries owes more to the map-driven need to correlate over large areas and to the limited vertical resolution of seismic data than to stratigraphic reality.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.