Although limestone-marl bedding couplets in the Cenomanian-Turonian Bridge Creek Limestone (BCL) have been widely attributed to changes in environmental conditions ultimately driven by Earth's orbital cycles, the causes of short-term variations in organic matter (OM) enrichment and composition (i.e., types and proportions of different macerals) in the bedding couplets through the BCL have rarely been examined in detail. To fill this gap, this study examined the BCL through an integrated sedimentological and organic petrology analysis. With the well-developed depositional context, organic petrology analysis was conducted on 17 samples from seven limestone-marl bedding couplets consisting of different sedimentary facies types in the USGS #1 Portland Core to systematically examine differences in the maceral composition among different expressions of the limestone-marl bedding couplets. The BCL in the #1 Portland Core has overall low thermal maturity (∼0.60 % vitrinite reflectance). All BCL samples contain dominant marine OM including bituminite (dominantly micrinized), alginite, and liptodetrinite and minor but persistently present terrigenous OM including vitrinite and inertinite. The OM composition and characteristics, combined with sedimentary facies characteristics and TOC data, suggest that the OM enrichment and preservation through the BCL is subject to various processes such as bottom current reworking and burial, bioturbation, early diagenesis, and pulses of volcanic ash input. The interplay of these processes led to changes in sedimentation rate, which can be associated with short-term relative changes in sea level and episodic volcanic input. Direct examinations of the composition and texture of OM in fine-grained sedimentary rocks can provide valuable insights into the causes of short-term variations in depositional conditions on a process basis, which should be integrated with other datasets (e.g., sedimentology, geochemistry) to fully resolve the specific mechanism(s) that modulated sedimentation in similar fine-grained marine systems characterized by apparently cyclic lithological alternations.

中文翻译：

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通过综合沉积学和有机岩石学分析，解开格林霍恩组 Bridge Creek 石灰岩段中石灰岩-泥灰岩层理对联的原因


尽管 Cenomanian-Turonian Bridge Creek 石灰岩 （BCL） 中的石灰岩-泥灰岩层理对联被广泛归因于最终由地球轨道周期驱动的环境条件变化，但通过 BCL 对层理对联中有机物 （OM） 富集和组成（即不同浆料的类型和比例）的短期变化的原因很少被详细研究。为了填补这一空白，本研究通过沉积学和有机岩石学的综合分析来检查 BCL。在发达的沉积背景下，对 USGS #1 波特兰岩芯中由不同沉积相类型组成的 7 个石灰岩-泥灰岩层理对联的 17 个样品进行了有机岩石学分析，以系统地研究石灰岩-泥灰岩层理对联不同表达之间岩浆组成的孔质组成的差异。#1 波特兰核心中的 BCL 总体热成熟度较低（∼0.60% 镜质体反射率）。所有 BCL 样品均含有主要的海洋 OM，包括沥青矿（主要晶硅酸盐）、褐藻酸盐和脂黄石，以及少量但持续存在的陆源 OM，包括镜质矿和惰性石。OM 的组成和特征，结合沉积相特征和 TOC 数据，表明通过 BCL 的 OM 富集和保存受到各种过程的影响，例如底流返工和埋藏、生物扰动、早期成岩作用和火山灰输入脉冲。这些过程的相互作用导致了沉积速率的变化，这可能与海平面的短期相对变化和偶发性火山输入有关。 直接检查细粒沉积岩中 OM 的成分和质地可以为沉积条件在过程基础上短期变化的原因提供有价值的见解，这应该与其他数据集（例如沉积学、地球化学）相结合，以充分解决在以明显的循环岩替为特征的类似细粒海洋系统中调节沉积的特定机制。