Journal of Sedimentary Research, 2006, v. 76, 0–0 Research Article DOI: 10.2110/jsr.2006.034 DELTAS AND SEA-LEVEL CHANGE SZCZEPAN J. PORE ˛ BSKI 1 AND RONALD J. STEEL 2 1 Polish Academy of Sciences, Institute of Geological Sciences, Krako ´w Research Centre, Senacka 1, 31-002 Krako ´w, Poland 2 The University of Texas at Austin, Geological Sciences Department, 1 University Station C1100, Austin, Texas 78712-0254, U.S.A. e-mail: ndporebs@cyf-kr.edu.pl ABSTRACT: Sea-level shift from the innermost shelf out to the shelf edge produces bayhead, inner-shelf, mid-shelf, and shelf- margin deltas. We suggest that these delta types are distinguishable in the ancient record and that such distinction has advantages as compared to the conventional, entirely process-based classification. Bayhead and inner-shelf deltas tend to form thin clinoforms (a few meters to tens of meters amplitude, respectively), and as they aggrade with rising relative sea level they generate a ‘‘tail’’ of thick paralic deposits. Mid-shelf deltas produce clinoforms as high as the mid-shelf water depth, tend to follow a subhorizontal trajectory, generate little or no paralic tail, and are commonly thinned by transgressive ravinement. Shelf-edge deltas in a stable-to-falling relative sea level usually have no paralic tail, create by far the highest clinoforms, and can have a thick succession of sandy turbidites on the delta fronts. If sea level falls below the shelf margin, the shelf-edge delta becomes incised by its own channels and large volumes of sand can be delivered onto the slope and the basin floor. Many deltas require a strong fluvial drive to attain a shelf transit, though as they approach the outer shelf they commonly become wave dominated. Tidal influence can increase on the outermost shelf if relative sea level is falling, if the shelf-break is poorly developed, and if basinal water depth is shallow. During transgression, the system tends to be tidally and/or wave influenced. Deltas that transit back and forth on the shelf on short time scales (tens of kiloyears to 100 ky) and that are driven largely by sea-level fluctuations are referred to here as accommodation-driven deltas. Deltas that can reach the shelf edge without sea- level fall are termed supply-driven deltas. These highstand deltas deposit thick, sandy, stacked parasequences during their shelf transit, and they tend to have an extensive muddy delta front on reaching the shelf-edge area. Such deltas would not normally be incised at the shelf edge, and they would produce a progradational, shelf-edge attached, sandy slope apron (Exxonian shelf- margin systems tract) rather than basin-floor fans. Sequence boundaries are best developed on accommodation-driven deltas, and are likely to be represented on a variety of time scales (third, fourth, and fifth order). Sequence boundaries in supply-dominated deltas may be identifiable only at lower- order time scales, or they may be non-existent. STEADY-STATE VERSUS EVOLUTIONARY DELTA CLASSIFICATION Deltas have traditionally been classified in terms of process–product reaction, where delta type is defined by the relative contribution of fluvial-, wave-, or tidal-energy flux that was dominant during deposition at the seaward edge of the delta (Galloway 1975). In this actualistic (based on present highstand of sea level) and uniformitarian (steady-state) approach, the delta system is seen as the outcome of intrabasinal processes, whereas external controls are held constant. However, studies of Quaternary shelves have shown that deltas also vary greatly in their external geometry and internal characteristics in response to falling and rising of sea level (e.g., McMaster et al. 1970; Suter and Berryhill 1985; Kolla et al. 2000; Tesson et al. 2000). End members in this sea-level- driven family are bayhead deltas, inner-shelf (or shoal-water platform) deltas, mid-shelf (or shelf-phase) deltas, and shelf-margin (shelf-edge, or deep-water) deltas. There is also an analogous family of deltas in a ramp (non-shelf–slope break) setting (Fig. 1). The regressive–transgressive transits of deltas during repeated sea-level cycles produce the shelf platforms and shelf margins known to occur around the edges of deepwater basins. If a delta reaches a platform-margin position, it also has the potential to deliver, directly or indirectly, significant volumes of sand into the deep-water areas (but see Steel et al. 2003). Although it has long been known that inner-shelf and middle-shelf deltas are different from shelf-edge deltas (Edwards 1981; Winker 1982; Suter and Berryhill 1985; Suter et al. 1987; Elliot 1989), there is surprisingly little mention of these delta types from pre-Pleistocene successions. We suggest that this type of delta classification and the recognition of this family of deltas in the ancient record is important; because: (1) it emphasizes mixed energy systems rather than the conventional end-member energy categories, and places deltas within a more dynamic sequence stratigraphic context, (2) it is a powerful tool for prediction of sand partitioning across the shelf, onto slope and basinal settings, and (3) it can help in choosing the best location for the sequence boundary. We are not suggesting the above delta types as alternatives to the process-based, conventional classification (Galloway 1975 and its modifications) (Orton and Reading 1993; Postma 1990), but we believe that both of these approaches should be integrated to achieve better understanding of deltas. We are following the concept outlined by Boyd Journal of Sedimentary Research sedp-76-02-06.3d 30/11/05 07:55:42 1 Copyright E 2006, SEPM (Society for Sedimentary Geology) 1527-1404/06/076-xxx/$03.00