BASELAP PATTERNS AND THE RECOGNITION OF LOWSTAND EXPOSURE AND DROWNING - A MISSISSIPPIAN-RAMP EXAMPLE AND ITS SEISMIC SIGNATURE

Seismic sequence stratigraphy draws heavily upon analysis of stratal d iscordances to decipher the depositional history of a basin. Onlap and downlap, which constitute two different classes of baselap, are the m ost important types of discordances used to identify sequence boundari es. In the absence of a regional grid of seismic lines from which the total stratal geometry can be documented, however, baselapping relatio nships are often misleading and misinterpreted. Class 1 baselap, or on lap, is frequently identified along seismic sequence boundaries, but i t may also be the most ambiguous lapout pattern to interpret. Onlap in cludes four subclasses: Subclass 1a-platform proximal onlap-onlap in t he direction of the source of sediment supply; Subclass 1b-basinal pro ximal onlap in the direction of the sediment source; Subclass 1c-dista l horizontal onlap-onlap of horizontal bottomset in a direction away f rom the source of sediment supply; and Subclass 1d-distal inclined onl ap-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 initia lly horizontal, bottomset surface; Subclass 2b, or climbing downlap, i n which toeset strata downlap a toeset and foreset surface inclined up ward in the direction of lapout; and Subclass 2c, or descending downla p, describes toeset strata downlapping a foreset-toeset surface inclin ed downward in the direction of sediment input. The distinction betwee n these types of baselap is important in aiding the reconstruction of relative sea-level and depositional history, as for example, in resolv ing whether a carbonate platform bounded by baselapping strata was sub jected to a sea-level fall or a sea-level rise. A Mississippian carbon ate ramp exposed along the late Paleozoic southern margin of North Ame rica demonstrates a dual history of lowstand exposure and transgressiv e drowning. A sequence boundary (SB345) developed at the end of Osagea n time by subaerial exposure at the top of a basinward-prograding Earl y Mississippian carbonate ramp. The resulting hiatus lasted as much as 10 m.y. and left a chert gravel residuum across the shelf. In additio n, deltaic and shoreface systems prograded from the northeast and occu pied a lowstand accommodation zone along the basin-ward margin of the ramp near the present Mississippi embayment. These lowstand deposits f orm a relatively thick downlapping wedge of siliciclastic strata above the Early Mississippian ramp and SB345. A late Meramecian to Chesteri an sea-level rise followed, forcing the siliciclastic depositional sys tems to retrograde landward. Concomitantly, carbonate sedimentation re sumed across the flooded shelf and formed ooid and skeletal grain shoa ls. Carbonate sedimentation was short-lived, however, bemuse large qua ntities of mud, derived from the Ouachita orogen to the south, encroac hed the shelf during the Chesterian sea-level rise, forming an onlappi ng succession of black shale. The rapid increase in accommodation spac e and the influx of suspended sediment and nutrients combined to subme rge, suffocate, and poison the carbonate factory. Subaerial exposure a nd drowning led to formation of two baselap surfaces: (1) a lowstand u nconformity, and (2) a drowning surface. They lie so close to each oth er on the shelf, however, that seismic discrimination is impossible. A t lower frequencies (35-50 Hz) synthetic seismic sections show reflect ions baselapping only one surface, the lowstand unconformity, when in fact there are two surfaces of very different origin. At higher freque ncies (75-100 Hz), baselap is visible at the drowning surface. The par adox is that although the Kinderhookian-lower Chesterian carbonate ram p was terminated by drowning, the visible seismic baselap was due to l owstand exposure.