Upper Cretaceous Sequence Stratigraphy of the Rock Springs Uplift, Wyoming

K. Rudolph, W. Devlin, J. Crabaugh
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引用次数: 18

Abstract

Three scales of sedimentary cyclicity are observed in the Campanian to Maastrichtian-aged strata of southwestern Wyoming based on sedimentary stacking patterns, facies successions, paleontological data and geometric criteria from outcrop, and subsurface data. The largest scale of sequence stratigraphic interpretation is of 10-15 million years duration per cycle and is designated as second order. Second-order cyclicity is represented by profound sequence boundaries and maximum-flooding surfaces that are related to regional tectono-subsidence drivers. The 2nd-order sequence boundaries at the base of the Ericson Formation (Moxa Unconformity) and the base of the Fort Union Formation (Laramide Unconformity) locally erode thousands of feet of section on discrete structural highs, but also show evidence of uplift on a broader scale. Both unconformities terminate long successions of marine shoreline progradation. Second-order maximum flooding surfaces are present within thick marine mudstone successions of the Baxter-Niobrara and Lewis formations. These intervals represent the deepest water deposits in the Upper Cretaceous and are associated with important regional seals and source rocks. The large-scale tectono-cyclicity is composed of an aggregate of smaller-scale, 3rd- and 4th-order sequences and their component systems tracts. In addition to the second-order sequences described above, third-order sequence boundaries are interpreted at the base of the Blair Formation, base of the Chimney Rock Member of the Rock Springs Formation, and at the base of the Canyon Creek Member of the Ericson Formation. Third-order maximum flooding surfaces are in the middle Blair Formation, middle Black Butte Member of the Rock Springs Formation, and in the upper part of the Rusty Member of the Ericson Formation. At least 7 additional 4th-order sequences can be interpreted within these successions. The higher-order cycles are shorter in duration and smaller in magnitude (i.e., thinner and with a smaller degree of change in environments or bathymetry within a cycle). The sequence stratigraphic interpretation approach is observationally based and consistent across these scales. However, finer-scale sequence stratigraphic interpretations, especially at the 4th-order scale, are subject to additional subjectivity. An important challenge is to separate apparent cyclicity related to factors such as local to regional shifting of depocenters (autocyclicity) from regionally correlative cyclicity (allocyclicity) within the high-frequency sequences. Examples of this are provided, most clearly for the lower portion of the Blair Formation. Such considerations are important economically, as this is the scale that controls sandstone reservoir and mudstone seal architecture within petroleum fields. Standard systems tracts criteria using parasequence stacking patterns are extended to non-marine strata by use of the degree of amalgamation (net/gross) of the fluvial sandstones. This approach is demonstrated to be plausible via physical ties to shoreline systems using regional correlations and paleontology, and via estimates of accommodation change from geohistory analysis.
怀俄明州岩泉隆起的上白垩纪层序地层学
根据沉积叠置模式、相序列、古生物资料、露头几何标准和地下资料,对美国怀俄明州西南部坎帕阶-马斯垂阶地层进行了3个尺度的沉积旋回。层序地层解释的最大尺度是每旋回持续1000万年至1500万年,称为二级。二级旋回性表现为深层层序边界和与区域构造沉降驱动因素有关的最大淹没面。埃里克森组(Moxa不整合)底部和联合堡组(Laramide不整合)底部的二级层序边界局部侵蚀了数千英尺的离散构造高点,但也显示出更大范围内的隆升证据。这两种不整合终止了长期的海洋岸线进积序列。在Baxter-Niobrara和Lewis组的厚海相泥岩层序中存在二级最大泛洪面。这些层段代表了上白垩统最深的水沉积,并与重要的区域盖层和烃源岩有关。大尺度构造旋回是由小尺度的三、四级层序及其组成体系域的集合。除上述二级层序外,还在布莱尔组底部、岩泉组烟囱岩段底部和埃里克森组峡谷溪段底部解释了三级层序边界。三阶最大驱油面位于Blair组中部、Rock Springs组Black Butte组中部和Ericson组Rusty组上部。在这些序列中至少可以解释另外7个4阶序列。高阶旋回持续时间较短,幅度较小(即,在一个旋回内,较薄,环境或水深变化程度较小)。层序地层学解释方法是基于观测的,在这些尺度上是一致的。然而,细尺度层序地层解释,特别是四阶层序地层解释,受制于额外的主观性。一个重要的挑战是在高频层序中区分与沉积中心局部到区域移动相关的明显旋回性(自旋回)和区域相关的旋回性(异旋回)。提供了这方面的例子,最明显的是布莱尔组的下部。这些考虑在经济上是重要的,因为这是控制油田内砂岩储层和泥岩密封结构的尺度。采用准层序叠置模式的标准体系域标准通过河流砂岩的合并度(净/粗)扩展到非海相地层。通过与海岸线系统的物理联系,利用区域相关性和古生物学,以及通过地质历史分析对适应性变化的估计,这种方法被证明是合理的。
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