Stratigraphy and Sedimentology of the Morrison–Cedar Mountain Formation Boundary, East-Central Utah
Stratigraphy and Sedimentology of the Morrison–Cedar Mountain Formation Boundary, East-Central Utah
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犹他州中东部莫里森-雪松山地层边界的地层学和沉积学
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发表时间：
2007
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通讯作者：
B. Britt
中科院分区：
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作者：
Brent W. Greenhalgh;B. Britt
摘要雪松山组一直是众多古生物学和地质学研究的焦点，旨在了解早白垩世恐龙和构造。这些研究中的一个主要问题是，上侏罗纪莫里森组和下白垩纪雪松山组之间的接触是出了名的难以识别的巴克霍恩砾岩缺席。我们对犹他州和科罗拉多的这些地层进行的地层学和沉积学分析使我们能够准确地认识到这种以前神秘的接触。莫里森组顶部的特征是燧石-卵石层和沟道化砾岩的增加，这些砾岩被古土壤覆盖，表现出铁浓度、锰涂层颗粒、强烈的红-紫-绿色斑点和漂白。这些特征表明在提托阶有一段可容纳空间减少的时期。当底部的雪松山地层由Buckhorn砾岩或Yellow Cat段的含砾泥岩（碎屑相）组成时，包括稍微改造的、有角的莫里森地层风化层碎片，地层接触发生在这些段的底部。Buckhorn砾岩和碎屑岩Yellow Cat含砾泥岩相（1）相互交错，表明时间相等，（2）通常向上分级为砂岩，（3）组成一个沉积体系，局部去除了部分或全部覆盖莫里森组的古土壤。在没有鹿角和碎屑岩的情况下，细粒、栗色、钙质古土壤标志着雪松山组的底部。一个钙质结砾岩/硅质结砾岩古土壤复合体，指示了数百万年的沉积间断，通常覆盖在巴克霍恩砾岩的顶部，部分的黄色猫成员，在某些情况下，莫里森组古土壤。在钙质结砾岩之上，雪松山组由细粒漫滩沉积物组成，含钙质结核或钙质结砾岩层位（红宝石牧场段），并与混汞砂岩透镜体和岩层隔离。
Abstract The Cedar Mountain Formation has been the focus of numerous paleontological and geological studies aimed at understanding Early Cretaceous dinosaurs and tectonics. A major problem in these studies is that the contact between the Upper Jurassic Morrison and Lower Cretaceous Cedar Mountain Formations is notoriously difficult to identify where the Buckhorn Conglomerate is absent. Our stratigraphic and sedimentologic analysis of these formations in Utah and Colorado permits accurate recognition of this previously enigmatic contact. The top of the Morrison Formation is marked by increases in chert-pebble lags and channelized conglomerates overprinted by paleosols that exhibit iron concentrations, manganese-coated grains, intense red-purple-green mottling, and bleaching. These features indicate a period of reduced accommodation in the Tithonian. Where the basal Cedar Mountain Formation consists of the Buckhorn Conglomerate or a pebbly mudstone (debritic facies) of the Yellow Cat Member, comprising slightly reworked, angular fragments of Morrison Formation regolith, the formational contact occurs at the base of these members. The Buckhorn Conglomerate and the debritic Yellow Cat pebbly mudstone facies (1) interfinger with each other, indicating time equivalancy, (2) typically grade upsection into sandstones, and (3) comprise a depositional system that locally removes part or all of the paleosol that elsewhere caps the Morrison Formation. In the absence of the Buckhorn and debritic Yellow Cat facies, a fine-grained, maroon, calcareous paleosol marks the base of the Cedar Mountain Formation. A calcrete/silcrete paleosol complex, indicative of a multi-million-year depositional hiatus, commonly overprints the top of the Buckhorn Conglomerate, portions of the Yellow Cat Member, and in some cases, the Morrison Formation paleosol. Above the calcrete, the Cedar Mountain Formation consists of fine-grained overbank deposits with caliche nodules or calcrete horizons (Ruby Ranch Member) and isolated to amalgamated sandstone lenses and beds.