Sequence stratigraphy of sixth-order (41 k.y.) Pliocene-Pleistocene cyclothems, Wanganui basin, New Zealand: A case for the regressive systems tract
Sequence stratigraphy of sixth-order (41 k.y.) Pliocene-Pleistocene cyclothems, Wanganui basin, New Zealand: A case for the regressive systems tract
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新西兰旺加努伊盆地六阶（41 k.y.）上新世-更新世旋回层序地层学：海退体系域的一个案例
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发表时间：
1997
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通讯作者：
P. Kamp
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作者：
T. Naish;P. Kamp
这项研究是基于晚上新世和早更新世（约2.6 - 1.7 Ma）约1 km厚的20个六阶（41 k.y. Wanganui盆地东部Rangitikei河流域出露的陆棚成因旋回层。旋回层与δ 18 O同位素阶段100 - 58相对应，每个阶段41 k. y。冰期-间冰期组合由海进、高水位和海退体系域组成。与大多数从地层记录中推断的例子不同，这些体系域是在当代氧同位素冰体积曲线所示的已知海平面旋回阶段沉积的。由于旺加努伊盆地的高沉降率，在大多数旋回中，冰川海平面福尔斯下降的幅度不足以暴露外大陆架。因此，Rangitikei部分提供了一个例外的例子，海退地层沉积向陆地的当代陆架坡折。简单的一维模型显示，海侵期间中等到高的盆地沉降速率（1 - 2 mm/年）和低的沉积速率（<0.2 mm/年）相结合，在相对高位产生了可容纳空间盈余。这种过剩的住宿在后期高位和随后的下降部分加积，高位体系域陆架粉砂岩，主要是强烈的海退体系域的residational滨面沉积物填充。Rangitikei海退体系域与强制海退体系域（Hunt和Tucker，1992）的区别在于它们不同的地层几何形状。根据定义，强制海退体系域显示与下伏高位体系域的侵蚀接触，并且通常作为一系列滞留在大陆架和/或斜坡上的下台阶分离的海岸线楔形体出现。相比之下，海退体系域表现出递变较低的接触面，在其上方准层序以强进积模式堆叠，终止于上覆层序边界。旋回层显示两种类型的基序，称为Rangitikei dt（沉积海侵）和Rangitikei nt（非沉积海侵），其中包括以下建筑元素在上升地层顺序：（1）与海侵面或侵蚀面一致的层序底界面，（高达50 cm）侵蚀起伏，可能被遗迹化石蛇形目或其深水相关整合所穿透;（2）厚（5 - 30 m）的海侵体系域，包括向上加深的近岸至内陆棚、碳酸盐-硅质混合岩相序列（沉积海侵），或薄（<2 m）海侵体系域，包括沉积在缺乏沉积物的近海陆棚上的浓缩含烃相（3）一个尖锐的下超面，将海侵体系域的凝聚相和超高压体系域的陆源粉砂岩分隔开来;（4）高位体系域，包括10 - 20 m厚的加积陆棚粉砂岩层段;（5）厚（达45 m）的海退体系域的加积内陆棚至滨面岩相组合。凝析壳层与层序内和层序边界不连续面有关，并且与层序的沉积学和地层学特征一起，是地层结构的重要标志。四种类型的壳层与由四种不同类型的地层终止形成的表面有关;上超、后超、下超和泛水面壳层（参见图1）。Kidwell，1991）分别与海侵侵蚀面、海侵体系域顶部的“明显削截”、下超面和局部海泛面有关。第五种壳层类型，称为复合壳层，形成于下超面与层序边界会聚的近海环境中，下超和后超壳层的元素混合或叠加。壳层标志着地层衰减带，在露头尺度上应用层序地层学概念时，壳层可用作地震不连续面的替代物。
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 δ 18 O isotope stages 100‐58, and each 41 k.y. glacialinterglacial 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 downstepped 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 Rangitikei dt (depositional transgression), and Rangitikei nt (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.