Middle Cenozoic diachronous shift to eolian deposition in the central Rocky Mountains: Timing, provenance, and significance for paleoclimate, tectonics, and paleogeography
Middle Cenozoic diachronous shift to eolian deposition in the central Rocky Mountains: Timing, provenance, and significance for paleoclimate, tectonics, and paleogeography
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DOI：
10.1130/ges01218.1
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发表时间：
2016-12
期刊：
影响因子：
2.5
通讯作者：
J. Rowley;Majie Fan
中科院分区：
文献类型：
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作者：
J. Rowley;Majie Fan
新生代中后期，落基山脉（此处指落基山脉）普遍存在风成沉积。虽然风成沉积环境的变化对构造、古气候和古地理具有重要意义，但对落基山脉这些风成沉积岩的形成时间和物源却知之甚少。本文研究了中新生代落基山脉中部及邻近的大平原地区向风成沉积环境过渡的时间，并利用砂岩岩石学和碎屑锆石U-Pb年代学对风成沉积岩的物源进行了约束。样品组成为Qm44F27Lt29 （Qm为单晶石英，F为长石，Lt为总岩屑）、Q47F27L26 （Q为总石英，L为总非石英岩屑）和Qm64P28K8 （P为斜长石，K为钾长石），锆石年龄群分别为17-44、45-218 Ma、220-708 Ma、948-1326 Ma、1332-1816 Ma和1825-3314 Ma。最年轻的锆石群来自北美西部和西南部的远端火山活动，其他锆石群则直接来自Laramide山脉的局部前寒武纪基底，以及分布在Laramide山脉两侧和Sevier腹地的古生代-下新生代地层。根据最年轻的锆石颗粒群的平均U-Pb年龄得出的最大沉积年龄与始新世晚期至中新世早期样品的可用火山灰辐射年龄基本一致，证实了碎屑锆石最大沉积年龄可用于约束不存在火山灰层或火山灰层中可测年矿物以及同步岩浆活动强烈时的沉积年龄。始新世晚期至渐新世早期，风成沉积的发生时间较晚，向东逐渐变短，表明落基山脉中部发生向东递进式干燥。这种历时性干燥可能是始新世晚期科迪勒山脉腹地和落基山脉中部的重新隆起和始新世-渐新世边界的全球变冷共同作用的结果。本文的物源资料表明，始新世晚期至渐新世早期，西风带和干燥的夏季季风将未岩化的河流沉积物和火山碎屑物质向东和东北移动，在落基山脉中部和邻近的大平原形成了大量的风成沉积。风成沉积作用持续到中新世，并在很大程度上覆盖了拉拉玛山脉的前寒武纪基底岩心。渐新世未岩化的风成期沉积物进一步被侵蚀并再循环成渐新世-中新世风成期晚期沉积岩。
Eolian sedimentation was widespread in the Rocky Mountains (Rockies herein) during the middle and late Cenozoic. Although changes to eolian depositional environment have significance for tectonics, paleoclimate, and paleogeography, little is known regarding the timing of initiation and the provenance of these eolian sedimentary rocks in the Rockies. Here we study the timing of a transition to eolian depositional environments in the central Rockies and the adjacent Great Plains during the middle Cenozoic, and use sandstone petrography and detrital zircon U-Pb geochronology to constrain the provenance of the eolian sedimentary rocks. Our samples have compositions of Qm44F27Lt29 (Qm is monocrystalline quartz, F is feldspar, and Lt is total lithics), Q47F27L26 (Q is total quartz, L is total non-quartzose lithics), and Qm64P28K8 (P is plagioclase and K is potassic feldspar), and zircon age populations of 17–44 , 45–218 Ma, 220–708 Ma, 948–1326 Ma, 1332–1816 Ma, and 1825–3314 Ma. The youngest zircon population was derived from distal volcanism in western and southwestern North America, and the other populations were derived directly from local Precambrian basements on Laramide ranges and recycled from Paleozoic–lower Cenozoic strata distributed along flanks of Laramide ranges and on the Sevier hinterland. The maximum depositional ages, based on the mean U-Pb ages of the youngest clusters of zircon grains, are generally consistent with the available ash radiometric dates for the latest Eocene–early Miocene samples, confirming that detrital zircon maximum depositional ages can be used to constrain depositional ages when ash beds or dateable minerals in ash beds were not present and when synchronous magmatic activity was intense. The occurrence of eolian deposition initiated during the latest Eocene–early Oligocene and became younger eastward, suggesting eastward progressive drying in the central Rockies. The diachronous drying may have resulted from the combined effect of renewed uplift of the Cordilleran hinterland and central Rockies during the late Eocene and global cooling at the Eocene-Oligocene boundary. The provenance data presented here suggest that during the latest Eocene–early Oligocene, the westerlies and possibly the dry summer monsoon winds transported unlithified fluvial sediments and pyroclastic materials eastward and northeastward, and formed massive eolian deposits in the central Rockies and the adjacent Great Plains. The eolian sedimentation continued into the Miocene and largely blanketed the Precambrian basement cores on Laramide ranges. The unlithified Oligocene eolian sediments were further eroded and recycled into the latest Oligocene–Miocene eolian sedimentary rocks.