Distribution, Microfauna, and Source-Rock Potential of Mississippian Delle Phosphatic Member of Woodman Formation and Equivalents, Utah and Adjacent States
Distribution, Microfauna, and Source-Rock Potential of Mississippian Delle Phosphatic Member of Woodman Formation and Equivalents, Utah and Adjacent States
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发表时间：
1984
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通讯作者：
C. Sandberg;R. Gutschick
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作者：
C. Sandberg;R. Gutschick
Woodman 组的密西西比 Delle 磷酸盐岩层及其类似物在犹他州南湖滨山脉的典型剖面以及犹他州奥奎尔山脉、帕旺特山脉和克劳福德山脉的参考剖面中进行了命名和描述。 Delle 还形成了 Deseret 石灰岩、Brazer 白云岩、Little Flat 地层、Aspen Range 地层、Deep Creek 地层和 Chainman 页岩的基部岩层，其范围从内华达州东南部、犹他州西部和怀俄明州最西部延伸 750 公里（4.65 亿英里），一直到爱达荷州东南部 Snake River 火山平原的南缘。该岩层主要包括富含有机磷的暗色泥岩、泥晶质灰岩结核、层状黑色放射虫燧石（利铁矿）、薄层磷矿、深色燧石泥晶岩厚互层和透镜体、以及一些来源不明的红色粉砂岩、黄棕色前三角洲粉砂岩和粗粒泥石流积灰岩。德塞雷特饥饿盆地和鹿角前陆海槽东半部碳酸盐台地邻近的低前坡上，德尔磷酸盐层缺乏明显的带壳巨型动物群，但含有丰富的微型动物群，主要由牙形动物、放射虫、凝集有孔虫组成，以及少量的大型浮游、浮游、游动和底栖动物。这种多样化生物群的知识主要是从大型泥晶石灰岩结核中回收的，对于了解富含有机物的磷酸序列中的碳氢化合物的来源非常重要，一种新的古生态模型解释了德尔和牙形刺生物相中的生物群的起源，该物种具有至少由七对元素组成的装置。模型解释了暴露的经过深度风化、淋滤和生物降解的磷酸盐页岩的有机碳和碳氢化合物含量，这些值通常明显低于地下的预期值，而地下相同的岩石已经产生或正在产生石油，但从精心挑选的露头样品中获得了以下有机碳产量：1.50％至7.95％。磷酸盐页岩的浓度为 0.67% 至 5.1%（中值 2.64），微晶岩的浓度为 0.25% 至 3.17%（中值 1.25）。在牙形刺 CAI 值范围为 1.5 至 4 的区域，碳氢化合物分析范围为 50 至 300 ppm。地图显示了 Delle 层的厚度和分布。有机碳和 CAI 值在内华达州中东部和犹他州中西部划定了一个含有低 CAI 值的“%老点”，表明碳氢化合物的成熟度最适合石油生产。
The Mississippian Delle Phosphatic Member of the Woodman Formation and equivalents is named and de- scribed at its type section in the South Lakeside Mountains, Utah, and at reference sections in the Oquirrh Moun-tains, Pavant Range, and Crawford Mountains, Utah. The Delle also forms the basal member of the Deseret Limestone, Brazer Dolomite, Little Flat Formation, Aspen Range Formation, Deep Creek Formation, and Chainman Shale in an area extending 750 km (465 mil from southeastern Nevada, ihrough western Utah and extreme western Wyoming , to the south edge of the Snake River Volcanic Plain in southeastern Idaho. The member comprises mainly dark organic-rich phosphatic mudstone, shale, and siltstone containing large micritic limestone concretions, bedded black radio- larian chert (lydite), thin beds of peloidal phosphorite, thick interbeds and lenses of dark cherty micrite, and some tongues of red siltstone of unknown origin, yellowish-brown prodeltaic siltstone, and coarse debris-flow encrinite. The Delle Member was deposited in mid-Mississippian (Osagean to Meramecian) time in the Deseret starved basin and on the adjacent lower foreslope of a carbonate platform within the eastern half of the Antler foreland trough. The Delle Phosphatic Member lacks conspicuous shelly megafaunas but contains a rich microfauna, consisting mostly importantly of conodonts, radiolarians, agglutinate foraminiferans, and fewer large planktonic, epiplanktonic, nektonic, and benthic animals and floating marine and transported land plants. Knowledge of this diverse biota, which was recovered mostly from the large micritic limestone concretions, is important to an understanding of the source of the hydrocarbons in the organic-rich phosphatic sequence. One conodont species, Bispathodus utahensis, which has an apparatus composed of at least seven pairs of elements, is described as new. A paleoecologic model inter- prets the origin of the biota in the Delle and a conodont biofacies model interprets the distribution and paleoecology of the conodonts. Organic-carbon and hydrocarbon contents of outcropping phosphatic shales that have been deeply weathered, leached, and biodegraded are difficult to evaluate. Analyses generally produce values that are significantly lower than values that can be expected in the subsurface, where the same rocks have generated or are generating petroleum. Nevertheless, the following organic-carbon yields have been obtained from carefully selected outcrop samples: 1.50 to 7.95 percent (4.29 median) for phosphatic shales, 0.67 to 5.1 1 percent (2.64 median) forphosphorites, and 0.25 to 3.17 percent (1.25 median) for micrites. Hydrocarbon analyses range from 50 to 300 ppm in areas where conodont CAI values range from 1.5 to 4. Maps are presented to show the thickness of the Delle Member and the distribution of organic-carbon and CAI values. A ‘%old spot” containing low CAI values that indicate optimum maturation of hy- drocarbons for petroleum generation is delineated in east-central Nevada and west-central Utah.