The Upper Cretaceous Niobrara Formation in the Denver-Julesburg (D-J) Basin, United States, records late Turonian–early Campanian transgressive–regressive cycles that governed the rhythmic character of pelagic and detrital sedimentation along the former carbonate ramp of the Western Interior seaway (WIS). The general stratigraphic and architectural complexity of the full Niobrara section has long been recognized; however, fine-scale subdivision of the formation from a sequence stratigraphic aspect has been lacking and is critical for basin-scale analysis. The timing and controls of significant changes in architectural and sediment-accumulation patterns have important implications for the Niobrara petroleum system. As part of this study, we present a sequence stratigraphic framework appropriate for the distal carbonate ramp of the WIS. We project published biozone mapping and radiometric ages into basin-wide well control to establish the timing of chronostratigraphic surfaces. Age-constrained isochore maps reveal a chronology of dynamic paleoseafloor morphology that is critical to understanding the stratigraphic and architectural evolution of the Niobrara.At a regional scale, thickness patterns suggest that the north-south–oriented flexural forebulge in the Sevier foreland basin had migrated eastward to the position of the future Rocky Mountains, a location coincident with the western edge of dominantly pelagic carbonate deposition in the backbulge of the WIS basin. At the D-J Basin scale, chronostratigraphic isochore maps of four third-order sequences, and their finer-scale subintervals, record two broadly distinct periods of depositional influence in the Niobrara. Between ca. 90.0 and ca. 85.7 Ma (upper Turonian–lower Santonian), relatively even deposition was dominated by a pattern of differential sediment accumulation. Systematic reversals of thicks and thins through time are indicative of compensational infilling patterns with no influence of tectonic uplifts along the seafloor. In contrast, from ca. 85.7 to ca. 81.7 Ma (lower Santonian–lower Campanian), a reorganization of earlier architecture was driven by the influence of sublinear uplifts along the trend of the emergent Transcontinental arch. As a result, sediment accumulation was dominated by patterns of draping over the long-lived paleohighs. Absolute timing of the architectural changes of the Niobrara suggests a link among Sevier thrusting events, a migrating flexural forebulge, and uplifts at reactivated basement shear zones in the distal foreland. Thus, both active tectonics and the inherited Proterozoic basement fabric of the WIS influenced the evolution of the Niobrara Formation. This example from the Cretaceous WIS shows that age-constraining sequence stratigraphy for regional isochore mapping can reveal relationships among the structural response to far-field tectonic events, oceanographic variations, and stratigraphic architecture.

中文翻译：

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Niobrara 组层序地层学：对美国丹佛-朱尔斯堡盆地年龄限制构造事件和地层复杂性的影响

美国丹佛-朱尔斯堡 (DJ) 盆地的上白垩统 Niobrara 组记录了土伦阶晚期 - 坎帕阶早期海侵 - 回归旋回，控制了沿西部内陆海道 (WIS) 前碳酸盐坡道的远洋和碎屑沉积的韵律特征）。整个 Niobrara 剖面的总体地层和建筑复杂性早已得到认可。然而，一直缺乏从层序地层方面对地层进行精细细分，这对于盆地尺度分析至关重要。建筑和沉积物堆积模式显着变化的时间和控制对 Niobrara 石油系统具有重要意义。作为这项研究的一部分，我们提出了一个适合 WIS 远端碳酸盐斜坡的层序地层框架。我们将已发表的生物区图和辐射年龄投射到盆地范围内的井控中，以确定年代地层表面的时间。受年龄限制的等距线图揭示了动态古海底形态的年表，这对于了解 Niobrara 的地层和建筑演化至关重要。在区域尺度上，厚度模式表明 Sevier 前陆盆地南北向的弯曲前隆起已经迁移向东到未来落基山脉的位置，该位置与 WIS 盆地后隆起中占主导地位的远洋碳酸盐沉积的西部边缘重合。在 DJ 盆地尺度上，四个三阶层序的年代地层等距线图及其更精细的亚区间，记录了 Niobrara 沉积影响的两个截然不同的时期。之间约。90.0 和大约 85.7 Ma（上Turonian-下Santonian），相对均匀的沉积以差异沉积物堆积的模式为主。随着时间的推移，厚层和薄层的系统逆转表明补偿性填充模式不受海底构造隆起的影响。相比之下，从约。85.7 到大约 81.7 Ma（下桑托阶-下坎帕阶），早期建筑的重组是由沿新兴横贯大陆拱门趋势的亚线性隆起的影响驱动的。因此，沉积物的积累主要是由长期存在的古隆起的悬垂模式决定的。Niobrara 构造变化的绝对时间表明 Sevier 逆冲事件、迁移的弯曲前隆和远端前陆重新激活的基底剪切带的隆起之间存在联系。因此，活动构造和 WIS 继承的元古代基底结构都影响了 Niobrara 组的演化。这个来自白垩纪 WIS 的例子表明，区域等距线测绘的年龄限制层序地层学可以揭示对远场构造事件、海洋变化和地层结构的结构响应之间的关系。