ABSTRACT: Transgressive Systems Tracts: Insights from Modern Continental Margins

SWIFT, DONALD J. P., Old Dominion University, Norfolk, VA, and JULIAN A. THORNE, Richardson, TX

On modern shelves, transgression drives the equilibrium shelf profile landward. The shoreface is eroded and the resulting debris accumulates beneath the rising lower limb of the profile. Depositional systems on transgressive coasts are therefore organized around a diachronous ravinement surface, cut by erosional shoreface retreat.

The ravinement is a source diastem, whose incision provides sediment for two transgressive systems tracts. A back-barrier wedge is a mosaic of onlapping barrier, inlet, lagoonal, and estuarine depositional systems. The back-barrier wedge forms below the ravinement and is progressively truncated by it. It rests on the transgressive surface, a class of surface whose time range encompasses the turnaround event, when progradational parasequences are succeeded by backstepping parasequences. The transgressive surface may take the form of a fluvial entrenchment surface, or depositional conformity. A backstep shelf wedge forms above the ravinement and progressively onlaps it. It is a single depositional system, with a basal sand facies, and (if sediment input is high enough) an interbedded fa ies and distal mud facies. It is capped by a maximum flooding surface. These two subsystems together constitute the backstep wedge systems tract, partial equivalent of the Vailian transgressive systems tract.

On modern shelves, the rates of accommodation variables (sea level change, fluid power), relative to rates of supply variables (sediment input, grain size), control the formation of bounding surfaces and the resulting transgressive parasequence architecture.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)