SEQUENCE STRATIGRAPHY OF 6TH-ORDER (41 KY) PLIOCENE-PLEISTOCENE CYCLOTHEMS, WANGANUI BASIN, NEW-ZEALAND - A CASE FOR THE REGRESSIVE SYSTEMSTRACT

This study is based on a late Pliocene and early Pleistocene (approxim ately 2.6-1.7 Ma) succession about 1 km thick of 20 sixth-order (41 k. y. duration) cyclothems of shelf origin exposed in the Rangitikei Rive r valley in the eastern part of Wanganui basin, The cyclothems correla te with delta(18)O isotope stages 100-58, and each 41 k.y. glacial-int erglacial stage couplet is represented by an individual depositional s equence comprising transgressive, highstand, and regressive systems tr acts, Unlike most examples inferred from the stratigraphic record, the se systems tracts were deposited during phases of known sea-level cycl es indicated by the contemporary oxygen isotope ice-volume curve, Beca use of the high rate of subsidence of Wanganui basin, glacioeustatic s ea-level falls during most cycles were not of sufficient magnitude to expose the outer shelf, Thus, the Rangitikei section provides an excep tional example of regressive strata deposited landward of the contempo rary shelf break, Simple one-dimensional modeling shows that moderate to high rates of basin subsidence (1-2 mm/yr) and low rates of sedimen tation (<0.2 mm/yr) during transgressions combined to produce an accom modation surplus at the relative highstand. This surplus accommodation was infilled during the late highstand and ensuing fall partly by agg radational, highstand systems tract shelf siltstone, and chiefly by st rongly progradational shoreface sediments of the regressive systems tr act, Rangitikei regressive systems tracts are distinguished from force d regressive systems tracts (sensu Hunt and Tucker, 1992) by their dif ferent stratal geometry, By definition, forced regressive systems trac ts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of down-stepped disjunct shorel ine wedges stranded on the shelf and/or slope, in contrast, regressive systems tracts exhibit a gradational lower contact, above which paras equences are stacked in a strongly progradational pattern terminated b y the superjacent sequence boundary, Cyclothems display two types of m oth termed Rangitikei(dt) (depositional transgression), and Rangitikei (nt) (nondepositional transgression), which include the following arch itectural elements in ascending stratigraphic order: (1) a basal seque nce boundary that is coincident with either the transgressive surface of erosion, which displays small-scale (up to 50 cm) erosional relief and may be penetrated by the ichnofossil Ophiomorpha, or its deeper wa ter correlative conformity; (2) either a thick (5-30 m) transgressive systems tract comprising a deepening upward nearshore to inner shelf, mixed carbonate-siliciclastic lithofacies succession (depositional tra nsgression), or a thin (<2 m) transgressive systems tract comprising c ondensed fossiliferous facies deposited on the sediment-starved offsho re shelf (nondepositional transgression); (3) a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract; (4) a highstand systems tract comprising a 10-20-m-thick inter val of aggradational, shelf siltstone; and (5) a thick (up to 45 m) pr ogradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract, Condensed shell beds are associated w ith intrasequence and sequence-bounding discontinuities, and, together with the sedimentological and stratal characteristics of the sequence s, are important indicators of stratigraphic architecture, Four types of shell bed are associated with surfaces formed by four different typ es of stratal termination; onlap, backlap, downlap, and flooding surfa ce shell beds (cf, Kidwell, 1991) are associated, respectively, with t he transgressive surface of erosion, ''apparent truncation'' at the to p of the transgressive systems tract, the downlap surface, and local m arine flooding surfaces, A fifth shell-bed type, termed a compound she ll bed, forms in offshore environments where the downlap surface conve rges with the sequence boundary, and elements of both the downlap and the backlap shell beds become mixed or superposed, The shell beds mark zones of stratal attenuation and can be used as surrogates for seismi c discontinuities when applying sequence stratigraphic concepts at out crop scale.