Print this pageWhy Do Siliciclastic Shelves Exist? How Do They Differ from "Ramp Margins"? New Sequence Stratigraphic Aspects Vital for Petroleum Exploration
Elaborating on Seilacher (1982) and Swift & Thorne (1991), the floor of a sea or lake at stillstand aggrades to an equilibrium shelf profile governed by storm erosion (wave + tide + wind drift + hyperpycnal flow), except near deltas where progradation exceeds erosion. Each storm shaves the surface back down to this ideal profile, leaving a subsidence-accommodated increment, transporting the surplus seaward to prograde the adjacent slope. (In contrast, under falling sea level, storms progressively lower the shelf.) The shelf thus widens until the next sea-level rise, unless structurally confined (subduction, transform). Seaward fining (sand to fluid mud) reflects decreasing bottom wave power with increasing water depth, governing the maximum grain size remobilizable at any point along the profile. Thus, each grain size has its own storm wavebase, toward which it moves incrementally; its absolute value (m), controlling shelf gradient, depends on tidal regime and largest storm-wave height (basin specific). Sea-level changes make shelves cyclically drown, build and emerge, forming stacked sequences, e.g. the present continental shelf is the last interglacial highstand shelf, modified by falling-stage and lowstand erosion (submarine, subaerial), then planed by ravinement and drowned by postglacial rise. A ramp margin sensu Exxon has no shelf break. But, in a wider context, a ramp is one flank of a foreland gulf or strait ("closed shelf"), both of which do have a shelf break, at one or both ends, adjoining a flysch trough or ocean (e.g. modern Gulf of Carpentaria-Arafura Sea; Cretaceous Western Interior Seaway). Hence, both of Exxon's Type 1 sequences (ramp, shelf) must occur on ramps and "open shelves"; ramps contain just the proximal part of shelf Type 1 sequences, i.e. shelf-edge LST is remote. Given also the disuse of Type 2 sequences, a new terminology is proposed: Suprashelf vs Shelf-Edge sequences. Drowning, whether of a ramp or an open shelf, initiates a new, stillstand shelf-slope, perched on the old one, producing a parasequence with clinoforms (e.g. modern innermost shelf). Clinoform height (neglecting compaction) reflects the sum of: (1) pre-drowning water depth; (2) eustatic rise height; and (3) rise duration, which determines deepening by subsidence while the shelf starves (sediment trapped inshore). For example, the long-lasting rise from 75 to 73Ma (Haq), though only c. 50m, produced clinoforms c. 400m high in the Western Interior (Lewis Shale).
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil