岩相韵律数字化在旋回地层学研究中的应用——以我国西北新生代湖泊沉积为例

Digitization of lithofacies rhythms in the cyclostratigraphic research:A case study of Cenozoic lacustrine deposits in Northwest China

我国西北地区新生代广泛发育河湖相沉积,表现为红色泥岩为主体、夹有绿色泥岩和石膏薄层的岩相韵律性变化,目前已有多种古气候指标揭示千年至百万年尺度古环境、古气候演变规律。但是这些旋回地层剖面中红色泥岩和绿色泥岩(石膏)岩相韵律变化特征及其蕴含的轨道尺度湖泊水文演化并未受到广泛关注。本研究以天文解决方案的韵律化为基础,在酒泉、西宁盆地新生代旋回地层中开展岩相韵律数字化研究,明确了岩性等级和层偶指标所记录的地球轨道周期为相反相位,暗示二者代表不同类型的古气候指标,可能从不同方面记录了盆地水文循环和沉积过程。酒泉盆地中新世岩相韵律的等级和层偶指标共同记录了长偏心率和长斜率的地球轨道"节拍器"周期,西宁盆地始新世岩相韵律的等级和层偶序列主要受偏心率周期驱动、单个红色泥岩-石膏层偶的沉积限时以12万年为显著周期。目前西北地区新生代地层学研究中已有高分辨率古气候指标的数据支撑,然而岩相韵律数字化应该受到重视,其保存的轨道周期和相位变化,有助于深入理解从天文轨道驱动、古气候演化到沉积环境响应的关键过程。

The Cenozoic sedimentary sequences document fluctuations in regional hydrology by distinct alternations of red mudstones and green,gypsum-rich mudstones across the Northwestern China.Previous studies have performed multi-proxy methodologies to explore Cenozoic paleo-climate and paleo-environment evolutions in response to astronomical and tectonic forcing in the Northwest China.Our limited understanding of orbital-scale hydrological changes in the red mudstones and green(gypsum)mudstones rhythms is in part due to the scarcity of time series analysis of lithofacies rhythms.This paper has conducted two digitalization protocols of lithofacies rhythms,i.e.,lithologic rank and couplet,based on the astronomical solutions and Cenozoic cyclostratigraphic sections.In particular,we investigated the 245.28-meter-long green-red mudstone lithofacies rhythms from the West Hongliuxia section in the Jiuquan Basin and the 146-meter-long gypsum-red mudstone lithofacies rhythms from the Xiejia section in the Xining Basin,northeastern Tibetan Plateau.The digitalization protocols of Laskar 2004 solutions and lithofacies rhythms reveal that the rank series document full Milankovitch cycles from long orbital eccentricity,short orbital eccentricity,obliquity to precession index,which are in-phased with astronomical model.By contrast,the couplet series only show strong spectral power in orbital eccentricity band and weak power in obliquity and precession index cycles,which are anti-phased with astronomical model and rank series.Our results reveal that the lithologic rank and couplet series exhibit anti-phased orbital cycles between each other,which imply that they are different types of paleoclimate proxies and document regional hydrologic evolution and sedimentary process in different perspective.Compared to the rank proxy,couplet proxy shows strong spectral powers in 173 ka and 125 ka than those in 100 ka cycles,providing implications for understanding the preservation mechanism of amplitude modulations of insolation cycles.The Miocene(15.7 Ma to 11.6 Ma)rank and couplet series of green-red mudstone rhythms reveal the long eccentricity(405 ka)and long obliquity metronome(173 ka)cycles,indicating a 14 ka duration for a single couplet of half precession index forcing in the Jiuxi Basin.The Miocene lacustrine sediments document similar combinations of orbital cycles to coeval aeolian sediments in the Loess Plateau,i.e.,405 ka,173 ka,100 ka and 40 ka cycles,inferring dominant forcing of solar radiation on the regional hydrology and monsoon system.The Eocene to Oligocene(39.6 Ma to 33.2 Ma)rank and couplet series reveal dominant eccentricity forcing of gypsum-red mudstone rhythms,indicating a ca.120 ka duration for a single couplet in the Xining Basin.The Eocene lacustrine lithofacies rhythms reveal different combinations of orbital cycles to the coeval magnetic susceptibility variations of lacustrine sediments in the Lanzhou basin across the Eocene-Oligocene Transition(EOT).The EOT global cooling event has intensified regional aridification in the Northwestern China,which contributed to the disappearance of saline lacustrine environment and end of orbital forcing of lithofacies rhythms.By contrast,the regional aridification has promoted the pedogenesis and subsequently enhanced magnetic susceptibility contents in the floodplain environment,which could be more sensitive to climate changes under orbital forcing.To date,abundant and high-resolution paleoclimate data have driven the Cenozoic stratigraphic research in the Northwestern China,but the digitization approaches of lithofacies rhythms should gain momentum.The Milankovitch cycles and their phase relationships of lithofacies rhythms could enhance our knowledge on the key process from Earth orbital forcing,paleoclimate evolution to sedimentary environment response.