Abstract

This study is based on a late Pliocene and early Pleistocene (approximately 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 River valley in the eastern part of Wanganui basin. The cyclothems correlate with δ18O isotope stages 100–58, and each 41 k.y. glacial-interglacial stage couplet is represented by an individual depositional sequence comprising transgressive, highstand, and regressive systems tracts. Unlike most examples inferred from the stratigraphic record, these systems tracts were deposited during phases of known sea-level cycles indicated by the contemporary oxygen isotope ice-volume curve. Because of the high rate of subsidence of Wanganui basin, glacioeustatic sea-level falls during most cycles were not of sufficient magnitude to expose the outer shelf. Thus, the Rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break. Simple one-dimensional modeling shows that moderate to high rates of basin subsidence (1–2 mm/yr) and low rates of sedimentation (<0.2 mm/yr) during transgressions combined to produce an accommodation surplus at the relative highstand. This surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational, highstand systems tract shelf siltstone, and chiefly by strongly progradational shoreface sediments of the regressive systems tract. Rangitikei regressive systems tracts are distinguished from forced regressive systems tracts (sensu Hunt and Tucker, 1992) by their different stratal geometry. By definition, forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of down-stepped disjunct shoreline wedges stranded on the shelf and/or slope. In contrast, regressive systems tracts exhibit a gradational lower contact, above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary.

Cyclothems display two types of motif termed Rangitikeidt (depositional transgression), and Rangitikeint (nondepositional transgression), which include the following architectural elements in ascending stratigraphic order: (1) a basal sequence 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 water 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 transgression), or a thin (<2 m) transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment-starved offshore 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 interval of aggradational, shelf siltstone; and (5) a thick (up to 45 m) progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract. Condensed shell beds are associated with intrasequence and sequence-bounding discontinuities, and, together with the sedimentological and stratal characteristics of the sequences, are important indicators of stratigraphic architecture. Four types of shell bed are associated with surfaces formed by four different types of stratal termination; onlap, backlap, downlap, and flooding surface shell beds (cf. Kidwell, 1991) are associated, respectively, with the transgressive surface of erosion, “apparent truncation” at the top of the transgressive systems tract, the downlap surface, and local marine flooding surfaces. A fifth shell-bed type, termed a compound shell bed, forms in offshore environments where the downlap surface converges 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 seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale.

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