Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden
Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden
复制标题
瑞典哥特兰志留纪中期穆尔德事件期间碳酸盐台地演化和牙形刺地层
作者：
M. Calner;L. Jeppsson
从沉积学和牙形石生物地层学的证据被用来重新解释中霍默（晚文洛克）在哥特兰，瑞典的继承。新的牙形石分带包括Ozarkodina bohemica longa带（包括5个亚带）、Kockelella ortus absidata带和Ctenognathodus murchisoni带（命名为Ozarkodina bohemica longa和Pseudooneotodus linguicornis）。这些新的区域与相相结合，以对比地层和推断主要的沉积环境和沉积的控制在中期Homerian Mulde事件。在事件前后，即在最上部O. S. sagitta区和C区。murchisoni区这些时期的特点是扩张的珊瑚礁和浅滩相的泥灰岩在最顶端的Slite组东部哥得兰岛和西部哥得兰岛的Klinteberg组的下部，分别。中间的O. B. longa和K. O. Absidata带最初的特征是快速的相变，包括硅质沉积，以及后来碳酸盐沉积体系的稳定。沉积物的组成和沉积速率与可容纳空间的产生和破坏密切相关，反映了碳酸盐和硅质岩沉积体系沉积偏置的典型情况。根据海岸线迁移、地层边界和主要礁带的地层位置，几种相组合可以拟合成台地演化的层序地层模型。高水位体系域（HST）的情况普遍存在之前，并在事件的早期部分;上部Slite组，包括较低的Fröjel组。这HST的特点是多产的骨骼生产和区域礁的发展，除了在最后阶段，碳酸盐生产下降的穆尔德事件的开始。平台的增长受到抑制，在随后的海退体系域（ESTA）时，区域硅质沉积占主导地位; Gannarve成员。随后的低水位导致了区域性的露出和岩溶作用，即平台的完全终止。后灭绝海侵体系域（TST）完全由非骨架碳酸盐组成;哈拉组的巴拉段。珊瑚礁的重新出现和多产的骨骼生产标志着平台在第二次HST期间的恢复;剩余的Halla和下Klinteberg地层。高分辨率生物地层学和层序地层学的整合表明，对平台演化的主要物理控制是穆尔德事件早期的五级海平面变化，并且大部分地层是在平台加积和进积时堆积的。在高位体系域。因此，志留纪的海洋事件和相关的海平面变化对浅海碳酸盐系统产生了深远的影响。哥得兰的中晚期荷马海平面曲线显示出两个快速回归，都导致高位体系域的截断。第一次低位发生在C.第二个在公司的末尾。ludensis Chron.间隔期的特点是stillstand或可能缓慢的海侵。
Evidence from sedimentology and conodont biostratigraphy is used to reinterpret the mid-Homerian (Late Wenlock) succession on Gotland, Sweden. A new conodont zonation includes from below: the Ozarkodina bohemica longa Zone (including five subzones), the Kockelella ortus absidata Zone and the Ctenognathodus murchisoni Zone (two taxa are named, Ozarkodina bohemica longa and Pseudooneotodus linguicornis). These new zones are integrated with facies in order to correlate strata and infer the major depositional environments and the controls on deposition during the mid-Homerian Mulde Event. Reef-associated and skeletal carbonate deposition predominated before and after the event, i.e. during the uppermost O. s. sagitta Zone and, again, in the C. murchisoni Zone. These periods are characterized by the expansion of reefs and shoal facies across marls in the topmost Slite Group on eastern Gotland and in the lower parts of the Klinteberg Formation on western Gotland, respectively. The intervening O. b. longa and K. o. absidata zones are initially characterized by rapid facies changes, including siliciclastic deposition, and later stabilisation of a carbonate depositional system. The composition of sediments and depositional rates are closely related to the creation and destruction of accommodation space and reflects a classical case of depositional bias of the carbonate and siliciclastic depositional systems. Based on coastline migration, stratal boundaries, and the stratigraphic position of major reef belts, several facies associations can be fitted into a sequence stratigraphic model for platform evolution. A highstand systems tract (HST) situation prevailed prior to, and during the early part of the event; the upper Slite Group including the lower Fröjel Formation. This HST was characterized by prolific skeletal production and regional reef development except for during the latest stage when carbonate production declined at the onset of the Mulde Event. Platform growth was inhibited during a following regressive systems tract (RST) when regional siliciclastic deposition predominated; the Gannarve Member. The subsequent lowstand resulted in regional emersion and karstification, i.e. a complete termination of the platform. The post-extinction transgressive systems tract (TST) is exclusively composed of non-skeletal carbonates; the Bara Member of the Halla Formation. Re-occurrence of reefs and a prolific skeletal production marks platform recovery during a second HST; the remaining Halla and the lower Klinteberg formations. Integration of high-resolution biostratigraphy and sequence stratigraphy reveals that the major physical control on platform evolution was a 5th order eustatic sea-level change during an early part of the Mulde Event, and that the bulk of the strata accumulated when the platform aggraded and prograded during the highstand systems tracts. Thus, Silurian oceanic events and associated sea-level changes had profound impact on the neritic carbonate system. The Gotland-based middle and late Homerian sea-level curve shows two rapid regressions, both leading to truncation of highstand systems tracts. The first lowstand occurred at the very end of the C. lundgreni Chron, and the second at the end of the Co.? ludensis Chron. The intervening interval was characterized by stillstand or possibly slow transgression.