In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability.Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall.Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.

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相关的一致性还是细微的不一致性？美国犹他州亨利山区上白垩统曼科斯页岩层序边界的远侧表现

在硅质碎屑层序地层模型中，地层以泥岩为主的远端海洋环境中的层序边界通常被认为是相关整合——陆上不整合向海的延伸。尽管在文献中广泛使用，但相关整合的客观识别标准仍然缺乏，这主要是由于直接检查近海泥岩主导环境中层序边界特征的案例研究数量有限。本研究侧重于犹他州中南部出露的 Mancos 页岩组的 Tununk 页岩段和 Ferron 砂岩段之间以泥岩为主的过渡层段，以扩展我们对在远东陆架环境中发育的层序边界特征的理解。斜坡设置。进行了综合的沉积学、岩石学和层序地层学分析，以表征将 Tununk 与 Ferron 沉积系统分开的层序边界（以下称为 TF 层序边界）及其横向沿沉积走向的变异性。泥岩-泥岩接触，三个测量剖面的 TF 层序边界均为细微的不整合面，以下方侵蚀截断和上方上超为特征，标志着相组合明显向盆地移动。TF 序列边界也标志着从 Tununk 近海泥浆带系统到 Ferron Notom 三角洲系统的变化，因此代表了一个划分两个不同成因沉积系统的表面。基于包围 TF 序列边界的两个不同的标记床，TF 序列边界可以自信地在研究区域中追踪。沿沉积走向的 TF 层序边界特征的横向变化表明它是由同种异源基准水平下降产生的。海上陆架泥岩地层可能包含比目前认识的更高的类似于 TF 层序边界的细微不整合发生率。仔细的沉积学和岩石学分析，结合受可靠年代地层标记层限制的横向相关性，对于识别陆架泥岩层序中的细微不整合面至关重要。准确识别泥岩地层中的细微不整合面对于将层序地层学方法适当地应用于远端陆棚环境至关重要，