Recent Advances in Sequence Stratigraphy: The Lowstand and Transgressive Systems Tracts

On basin margins characterized by a ramp physiography, relative sea level fall induces basinally isolated, shelf-perched, forced-regression shoreline and deltaic deposition rather than deep-water submarine-fan turbidite deposition. These deposits comprise the early lowstand systems tract. In proximal locations, the sequence boundary at the base of these lowstand deposits is expressed as an erosional unconformity, whereas in distal settings this surface occurs as a correlative conformity. The stratigraphic discontinuity at the top of these lowstand deposits is a ravinement surface and in places can be more striking than the sequence-bounding surface at the base. significant erosion due to fluvial processes during forced regression and subsequent shoreface processes during transgression may be common at the tops of these lowstand deposits. The transgressive systems tract commonly comprises, proximally to distally, backstepped barrier beach deposits, sheet-like lag deposits, and [open quotes]healing phase[close quotes] deposits. This latter unit has not been described widely, but volumetrically may contain a significant part of the transgressive systems tract. Healing phase deposits comprise a wedge of sediment that onlaps the last clinoform of the underlying progradational phase. These sediments commonly are derived by cannibalization from the top or edge of the subjacent progradational phase deposits (i.e., either highstand ormore » lowstand), during and immediately after transgression. These deposits are referred to as the healing phase because their deposition tends to [open quotes]heal over[close quotes] the relatively steep clinoform slope of the underlying progradational phase after transgression has resulted in landward shift of the depocenter. Although sometimes misinterpreted as lowstand deposits, these depositional units commonly do not contain significant reservoir facies.« less