Abstract

Seismic sequence stratigraphy draws heavily upon analysis of stratal discordances to decipher the depositional history of a basin. Onlap and downlap, which constitute two different classes of baselap, are the most important types of discordances used to identify sequence boundaries. In the absence of a regional grid of seismic lines from which the total stratal geometry can be documented, however, baselapping relationships are often misleading and misinterpreted. Class 1 baselap, or onlap, is frequently identified along seismic sequence boundaries, but it may also he the most ambiguous lapout pattern to interpret. Onlap includes four subclasses: Subclass 1a--platform proximal onlap--onlap in the direction of the source of sediment supply; Subclass 1b--basinal proximal onlap in the direction of the sediment source; Subclass 1c--distal horizontal onlap--onlap of horizontal bottomset in a direction away from the source of sediment supply; and Subclass 1d--distal inclined onlap--inclined bottomset onlapping an inclined surface. Class 2 baselap, or downlap, includes three subclasses: horizontal downlap, or Subclass 2a, describes initially inclined toeset strata that downlap an initially horizontal, bottomset surface; Subclass 2b, or climbing downlap, in which toeset strata downlap a toeset and foreset surface inclined upward in the direction of lapout; and Subclass 2c, or descending downlap, describes toeset strata downlapping a foreset-toeset surface inclined downward in the direction of sediment input. The distinction between these types of baselap is important in aiding the reconstruction of relative sea-level and depositional history, as for example, in resolving whether a carbonate platform bounded by baselapping strata was subjected to a sea-level fall or a sea-level rise. A Mississippian carbonate ramp exposed along the late Paleozoic southern margin of North America demonstrates a dual history of lowstand exposure and transgressive drowning. A sequence boundary (SB345) developed at the end of Osagean time by subaerial exposure at the top of a basinward-prograding Early Mississippian carbonate ramp. The resulting hiatus lasted as much as 10 m.y. and left a chert gravel residuum across the shelf. In addition, deltaic and shoreface systems prograded from the northeast and occupied a lowstand accommodation zone along the basinward margin of the ramp near the present Mississippi embayment. These lowstand deposits form a relatively thick downlapping wedge of siliciclastic strata above the Early Mississippian ramp and SB345. A late Meramecian to Chesterian sea-level rise followed, forcing the siliciclastic depositional systems to retrograde landward. Concomitantly, carbonate sedimentation resumed across the flooded shelf and formed ooid and skeletal grain shoals. Carbonate sedimentation was short-lived, however, because large quantities of mud, derived from the Ouachita orogen to the south, encroached the shelf during the Chesterian sea-level rise, forming an onlapping succession of black shale. The rapid increase in accommodation space and the influx of suspended sediment and nutrients combined to submerge, suffocate, and poison the carbonate factory. Subaerial exposure and drowning led to formation of two baselap surfaces: (1) a lowstand unconformity, and (2) a drowning surface. They lie so close to each other on the shelf, however, that seismic discrimination is impossible. At lower frequencies (35-50 Hz) synthetic seismic sections show reflections baselapping only one surface, the lowstand unconformity when in fact there are two surfaces of very different origin. At higher frequencies (75-100 Hz), baselap is visible at the drowning surface. The paradox is that although the Kinderhookian-lower Chesterian carbonate ramp was terminated by drowning, the visible seismic baselap was due to lowstand exposure.

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