Sequence Evolution and Hierarchy within the Lower Mississippian Madison Limestone of Wyoming
Sequence Evolution and Hierarchy within the Lower Mississippian Madison Limestone of Wyoming
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怀俄明州密西西比河下游麦迪逊石灰岩的层序演化和层次结构
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发表时间：
1996
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通讯作者：
M. Sonnenfeld
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作者：
M. Sonnenfeld
摘要麦迪逊石灰岩形成了一个大约12英里的二级层序。持续时间受岩溶不整合的限制，代表地理上从5英里到34英里的间歇期。麦迪逊存在一个复杂的五重物理层序地层体系。“基本”旋回(2-20英尺厚)和六个三级层序(30-200英尺厚)是比格伦盆地麦迪逊地区最容易识别的标尺。一般来说，麦迪逊被视为一个单一的进退循环。研究结果表明，这6个三级层序叠加成两个重要的长期复合层序，在规模上介于整个二级Madison层序及其组成部分的三级层序之间。对这一长期模式的认识对区域物理地层对比、评价从斜坡到陆棚体系的演化以及解决麦迪逊碳酸盐台地内相安排和层序样式的长期可容纳控制至关重要。本文讨论的层序地层格架为储集层描述提供了一种预测工具，使早期地层控制的、组构选择性的白云岩的储集层贡献与晚期成岩和构造叠置区分开来。麦迪逊盆地内识别的五个三级层序界面位于可容纳的“转折点”，通常代表进积旋回组的顶峰。五个内部层序界面中的四个还显示出麦迪逊地台大部分地区存在水下暴露和微岩溶作用的证据，可能是对海平面相对下降的反应。边界盖层序列III、IV和局部V出现在显著的溶液崩塌角砾岩的底部，与沉积旋回到退缩旋回集的开始相吻合。主要的坍塌角砾岩反映了蒸发岩的溶解作用，并被推断为代表了以前的蒸发岩与受限的泥质白云岩和微细粒砾岩互层。在三级海侵体系域的早期阶段，沉积速率几乎与可容空间的增加保持一致。这使得浅水、高盐度的状况能够在公海状况出现之前保持不变。这种蒸发岩产出的地层学背景与不对称的蒸发岩盖层旋回的传统范例形成对比。两个最古老的三级层序(I、II)是与西北方向的罗奇极建造相关的斜坡体系，并显示出总体的进积叠加几何结构，意味着可容/供沙比长期下降。与这一长期趋势相关联的是一种趋势，即递进堆积的、与海岸相连的砾岩杂岩直接被潮周至萨布卡沉积覆盖，而不夹杂泻湖岩石。层序II TST期间有限的泻湖发育反映了中尺度(4级)旋回的幕式沉积阶段。层序III，局限于使命峡谷组的大鹅成员，是一个高度加重的层序，非常缓慢地向上变浅。层序III开始的“分离的”砾石屏障复合体对于整个怀俄明州西北部厚厚的泻湖沉积是关键的，并对应于一种更平坦的陆架状地貌的出现。层序III代表了长期可容/沉积比接近平衡，构成了麦迪逊复合层序上部的下部。层序III的沉积过程中相对较高的可容纳性使其最终形成了比所有其他五个三级麦迪逊层序都要厚的浅水相沉积。层序界面盖层层序III显示出海平面相对下降的所有麦迪逊内部层序界面的证据最少，可能的岩溶作用仅限于内部地台。上麦迪逊复合层序的峰值海侵表现为广泛分布的小舌段骨架灰岩和砂砾岩(层序IV)。上奥萨基统至下梅拉米沙统层序IV-VI向上变薄，涉及岩溶层序界面，并与层序界面上方泥质侵入增加有关。这表明了住宿的长期减少，加上日益潮湿的条件，最终导致岩溶主不整合覆盖了麦迪逊。
Abstract The Madison Limestone forms a 2nd-order sequence of approximately 12 m.y. duration that is capped by a karsted unconformity representing a hiatus ranging geographically from 5 to 34 m.y. A complex five-fold physical stratigraphic hierarchy of sequences exists within the Madison. “Fundamental” cycles (2-20 ft thick) and six 3rd-order sequences (30-200 ft thick) are the scales most easily recognized within the Madison of the Bighorn Basin area. Generally the Madison is viewed as a single transgressive-regressive cycle. Results from this study show that the six 3rd-order sequences stack into two important long-term composite sequences that are intermediate in scale between the entire 2nd-order Madison and its component 3rd-order sequences. Recognition of this long-term pattern is critical to regional physical stratigraphic correlation, to appreciating an evolution from ramp to shelf systems, and to resolving the long-term accommodation controls of facies arrangements and sequence styles within the Madison carbonate platform. The sequence stratigraphic framework discussed in this paper provides a predictive tool for reservoir characterization that enables the reservoir contribution of early stratigraphically controlled, fabric-selective dolomites to be differentiated from later diagenetic and structural overprints. The five 3rd-order sequence boundaries identified within the Madison are placed at accommodation “turnarounds” that generally represent culminations of progradational cycle sets. Four out the five internal sequence boundaries also show evidence of subaerial exposure and microkarsting across most of the Madison platform and probably formed in response to relative falls of sea level. Boundaries capping sequences III, IV, and locally V occur at the base of prominent solution collapse breccias that coincide with the onset of aggradational to retrogradational cycle sets. Major collapse breccias reflect evaporite dissolution and are inferred to represent former evaporites interbedded with restricted argillaceous dolomudstones and micropeloidal grainstones. During early phases of 3rd-order transgressive systems tracts, sedimentation rates nearly kept up with increasing accommodation. This allowed shallow water, hypersaline conditions to be maintained prior to onset of open-marine conditions. This stratigraphic context for evaporite occurrences contrasts with the conventional paradigm of asymmetric, evaporite-capped cycles. The two oldest 3rd-order sequences (I, II) are ramp systems correlative with the Lodgepole Formation to the northwest and exhibit an overall progradational stacking geometry, implying a long-term decrease in accommodation/sediment supply ratio. Associated with this long-term trend was a tendency for progradationally stacked, “shore-attached” grainstone complexes to be directly overlain by thin peritidal to sabkha deposits without intervening lagoonal rocks. Limited lagoon development during the TST of Sequence II reflects episodic aggradational phases of intermediate-scale (4th-order) cycles. Sequence III, confined to the Big Goose Member of the Mission Canyon Formation is a highly aggradational succession that shallows upward very gradually. The onset of “detached” grainstone barrier complexes at the outset of Sequence III was critical to the aggradation of thick lagoonal deposits throughout northwest Wyoming and corresponds to the advent of a much flatter, shelf-like morphology. Sequence III represents near balance in the long-term accommodation/sediment ratio and forms the lower portion of the upper Madison composite sequence. The relatively high accommodation during deposition of Sequence III enabled it to culminate with a thicker accumulation of shallow water facies than did all five other 3rd-order Madison sequences. The sequence boundary capping sequence III shows the least evidence of all intra-Madison sequence boundaries for a relative fall of sea level, with possible karsting limited to the inner platform. The peak transgression of the upper Madison composite sequence is represented by the widespread skeletal lime packstone and grainstone of the Little Tongue Member (Sequence IV). Upper Osagean to lower Meramecian sequences IV-VI thin upward, involve karsted sequence boundaries, and are associated with increasing argillaceous influxes above sequence boundaries. This is indicative of the long-term decrease in accommodation, coupled with increasingly humid conditions that culminate in the karsted master unconformity capping the Madison.