怀俄明大角山古近系白河群沉积物物源及地层对比

J. Malone, J. Craddock, D. Malone
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引用次数: 1

摘要

白河群凝灰质砂岩和砾岩的碎屑锆石年龄为了解美国西部落基山脉北部拉拉米破碎前陆盆地体系的古近系盆地演化、岩浆活动和古排水提供了新的思路。始新世-渐新世上部白河群的非海相沉积物不规则地保存在怀俄明州北部和南达科他州西部。残余的拉腊米隆升和活跃的岩浆中心为40 Ma左右开始的宽阔的低起伏山谷提供了碎屑和火山碎屑沉积物。白河群的亚水平地层暴露在高海拔的大角山(~ 2300 ~ 2800 m),其中~ 10 ~ 50 m厚的剖面沿着中至低起伏(高达150 m)的表面不整合地停留在前寒武纪-古生代岩石上。我们对大角山三个地点(Darton’s Bluff、Hazelton Road、Freeze Out Point)的7个样品(3个凝灰质砂岩、2个砾岩、2个砂岩)的碎屑和火成岩(灰落)锆石进行了U-Pb年龄测定。每个地方都有砾岩层,有当地结晶基底的碎屑,和互层的凝灰质砂岩。4个样品的碎屑锆石年龄谱显示,峰值年龄在2.9 Ga左右,与大角山太古宙结晶基底年龄相符,最大沉积年龄(MDAs)为27 Ma(样品20BH15;渐新世)和35 Ma(样品FO-2;晚始新世)。在古近纪,大角山地区接受了来自当地结晶基底的沉积物和来自火成岩和沉积源的长距离河流向西输送。始新世早期,大角山的显生宙盖层被挖掘和剥离,表明古-中生代地层的后Laramide输入可能来自Cordilleran (Sevier)褶皱冲断带的残余高地,而不是局部的Laramide地块隆升。此外,据推测,圣胡安山脉和大盆地的新生代岩浆省通过喷发的火山灰云和从北到东北流向怀俄明州东北部的河流系统贡献了火山碎屑沉积物。白河群代表了晚始新世-渐新世沉积在Laramide基底上的局部沉积,与相邻大平原的区域沉积高度同步。这两个地区的沉积物都来自冲积扇和河流排水网络,这些网络利用了拉拉米基底隆起、科迪勒拉冲断带和前陆源,以及美国西部内陆的新生代火成岩中心。
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Sediment provenance and stratigraphic correlations of the Paleogene White River Group in the Bighorn Mountains, Wyoming
Detrital zircon ages for tuffaceous sandstones and conglomerates of the White River Group provide insights on Paleogene basin evolution, magmatic activity, and paleodrainage throughout the Laramide broken foreland basin system of the northern Rocky Mountains in the western United States. Nonmarine deposits of the upper Eocene-Oligocene White River Group are preserved irregularly across northern Wyoming and western South Dakota. Residual Laramide uplifts and active magmatic centers supplied clastic and volcaniclastic sediment to broad, low-relief valleys beginning around 40 Ma. Subhorizontal strata of the White River Group are exposed in the elevated Bighorn Mountains (∼2300 to 2800 m), where sections ∼10-50 m thick rest unconformably on Precambrian-Paleozoic rocks along a surface of moderate to low relief (up to 150 m). U-Pb ages were obtained for detrital and igneous (ash-fall) zircons from seven samples (3 tuffaceous sandstones, 2 conglomerates, 2 sandstones) spanning three localities in the Bighorn Mountains (Darton’s Bluff, Hazelton Road, Freeze Out Point). Each locality contains conglomeratic layers, with clasts of local crystalline basement, and interbedded tuffaceous sandstones. Detrital zircon age spectra for four samples reveal peak ages around 2.9 Ga, matching the age of Archean crystalline basement within Bighorn Mountains, and maximum depositional ages (MDAs) of 27 Ma (sample 20BH15; Oligocene) and 35 Ma (sample FO-2; late Eocene). During the Paleogene, the Bighorn Mountains region received sediment from local crystalline basement and long-distance river transport from igneous and sedimentary sources to the west. The Bighorn Mountains were exhumed and stripped of Phanerozoic cover strata by early Eocene time, suggesting that post-Laramide input from Paleozoic-Mesozoic strata was likely from relict highlands of the Cordilleran (Sevier) fold-thrust belt rather than local Laramide block uplifts. In addition, Cenozoic magmatic provinces in the San Juan Mountains and Great Basin are inferred to have contributed volcaniclastic sediment through both eruptive ash clouds and north- to northeast-flowing fluvial systems that reached northeastern Wyoming. The White River Group preserved in the Bighorn Mountains represents localized late Eocene-Oligocene sediment accumulation atop a Laramide basement high coeval with regional deposition across the adjacent Great Plains. Both regions were supplied sediment from alluvial fans and fluvial drainage networks that tapped Laramide basement uplifts, Cordilleran thrust-belt, and foreland sources, along with Cenozoic igneous centers of the western U.S. interior.
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