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首页> 外文期刊>Marine Geology >Stratigraphic framework of sediment-starved sand ridges on a mixed siliciclastic/carbonate inner shelf; west-central Florida
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Stratigraphic framework of sediment-starved sand ridges on a mixed siliciclastic/carbonate inner shelf; west-central Florida

机译：硅质碎屑/碳酸盐混合内陆架上缺乏沉积物的沙脊的地层框架;佛罗里达州中西部

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Seismic reflection profiles and vibracores have revealed that an inner shelf, sand-ridge field has developed over the past few thousand years situated on an elevated, broad bedrock terrace. This terrace extends seaward of a major headland associated with the modern barrier-island coastline of west-central Florida. The overall geologic setting is a low-energy, sediment-starved, mixed siliciclastic/carbonate inner continental shelf supporting a thin sedimentary veneer. This veneer is arranged in a series of subparallel, shore-oblique, and to a minor extent, shore-parallel sand ridges. Seven major facies are present beneath the ridges, including a basal Neogene limestone gravel facies and a blue-green clay facies indicative of dominantly authigenic sedimentation. A major sequence boundary separates these older units from Holocene age, organic-rich mud facies (marsh), which grades upward into a muddy sand facies (lagoon or shallow open shelf/seagrass meadows). Cores reveal that the muddy shelf facies is either in sharp contact or grades upward into a shelly sand facies (ravinement or sudden termination of seagrass meadows). The shelly sand facies grades upward to a mixed siliciclastic/carbonate facies, which forms the sand ridges themselves. This mixed siliciclastic/carbonate facies differs from the sediment on the beach and shoreface, suggesting insignificant sediment exchange between the offshore ridges and the modern coastline. Additionally, the lack of early Holocene, pre-ridge facies in the troughs between the ridges suggests that the ridges themselves do not migrate laterally extensively. Radiocarbon dating has indicated that these sand ridges can form relatively quickly (approx 1.3 ka) on relatively low-energy inner shelves once open-marine conditions are available, and that frequent, high-energy, storm-dominated conditions are not necessarily required. We suggest that the two inner shelf depositional models presented (open-shelf vs. migrating barrier-island) may have co-existed spatially and/or temporally to explain the distribution of facies and vertical facies contacts.
机译：地震反射剖面和颤振岩表明，过去几千年来，一个内部架子，沙脊田地位于高架宽阔的基岩阶地上。这个露台延伸了主要岬角的海面，与佛罗里达州中西部的现代屏障岛海岸线相关。总体地质背景是低能量，缺乏沉积物，混合硅质碎屑/碳酸盐的内陆架，支撑着薄的沉积薄板。该饰面排列成一系列次平行，倾斜的海岸，并在较小程度上平行于海岸的沙脊。山脊下存在七个主要相，包括基底的新近纪石灰岩砾石相和蓝绿色黏土相，表明主要为自生沉积。一个主要的层序边界将这些较旧的单元与全新世时代的有机质丰富的泥质相（沼泽）分开，然后向上划分为泥质砂质相（泻湖或浅层的开放层架/海草草甸）。岩心显示，浑浊的陆架相或者是尖锐的接触，或者向上逐渐变成有贝壳的沙相（沟壑或海草草甸的突然终止）。贝壳状砂岩相逐渐向上混合成硅质碎屑/碳酸盐混合相，形成砂脊本身。这种混合的硅质碎屑/碳酸盐相不同于海滩和海岸面的沉积物，这表明近海脊与现代海岸线之间的沉积物交换微不足道。另外，由于在山脊之间的槽中缺乏早期全新世前山脊相，这表明山脊本身并没有在横向上广泛地迁移。放射性碳测年表明，一旦具备开放海洋条件，这些砂脊就可以在相对低能量的内层架上相对较快地形成（约1.3 ka），并且不一定需要频繁，高能量，以风暴为主的条件。我们建议，提出的两个内部层架沉积模型（开放层架与迁移的隔离岛）可能在空间和/或时间上共存，以解释相和垂直相接触的分布。

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