Upper Eocene ejecta of the New Jersey continental margin reveal dynamics of Chesapeake Bay impact

Evidence for the Chesapeake Bay Crater as the source for New Jersey continental margin ejecta is provided by fine-grained tektites and coarse-grained unmelted ejecta. The Upper Eocene ejecta deposit, now demonstrated to be part of the North American strewn field, occurs on the New Jersey continental margin at Ocean Drilling Program (ODP) Sites 904 and 903. The mineralogy, major oxide composition of the ejecta materials, and biostratigraphic age of the enclosing sediments link the origin of these ejecta to the recently recognized Chesapeake Bay impact crater, located only 330 km away. Sediments associated with the ejecta provide information about the dynamics of impact events. The 35-cm-thick ejecta-bearing layer can be subdivided into three subunits that indicate a sequence of events. Bottom subunit III documents sediment failure and deposition of gravel-sized fragments, middle subunit II records deposition of abundant sand-sized ejecta by gravity settling, and upper subunit I contains a 12-cm-thick sedimentary deposit containing rare silt-sized tektites and evidence of waning currents. These events are interpreted by linking sediment deposition to seismic ground motion and subsequent tsunami waves triggered by both the Chesapeake Bay impact and slope failures.     

对“论‘太湖冲击坑溅射物’”一文的答复

      本期刊登了黄志诚和刘冠邦有关太湖“溅射物”的讨论文章。该文从沉积学观点,对太湖某些沉积物及结核开展了 研究工作,确定了沉积成因菱铁矿结核的存在, 并据此对我们在2009 年提出的太湖“溅射物”冲击成因观点提出质疑。这 些非溅射物结核的新资料,为进一步深入对比研究溅射物提供了某些参考。本文根据溅射物的外形、内部特征以及同位素 资料,与非溅射物结核进行了初步对比,结果显示对太湖“溅射物”的不同结论可能与研究样品差异有关, 相关研究工作 还值得进一步深入下去。     

琼北火山喷发物风化特征的研究与应用

通过X光衍射、扫描电镜和能谱分析发现 ,琼北雷虎岭 -马鞍岭地区火山喷发物风化形成的次生黏土主要是球状和不规则球状富铁埃洛石 ,随着风化时间的增长 ,埃洛石的转变顺序为 10 埃洛石→ ( 10 + 7 )埃洛石→ 7 埃洛石 ,据此推测该地区火山喷发的先后顺序为雷虎岭东南→儒红村北→儒红村东。喷发年代在 6 0 0 0～ 12 0 0 0aBP之间     

Contrasting volcanic-tectonic processes during the past 2 Ma on the Mid-Atlantic Ridge: submersible mapping,petrological and magnetic results at lat. 34°52′ N and 33°55′ N

In August–September of 1995, 20 Nautile dives and detailed magnetic surveys (spaced every 1.8 km) were undertaken on two segments of the Mid-Atlantic Ridge between the Oceanographer and Hayes fractures zones. These two segments are only 65 km apart and show strong morphology and gravity contrasts. OH1 is shallower and has a large mantle Bouguer anomaly (MBA) bull's eye, whereas OH3 is deeper and has a smaller MBA bull's eye.Thirteen dives were devoted to segment OH1. The Median Ridge (MR) located on the central high (1700 m deep) is topped by 100 to 300 m high circular volcanoes. The volcanics consists mainly of porphyritic and/or vesicular pillows and volcaniclastics. The NVZ (2200 m deep), located in the valley floor east of the MR, consists of near aphyric fluid lava flows. A chain of off-axis volcanoes, displaying a magnetic continuity with surroundings, extends on both sides of the axis. Three volcanoes on the east side and one on the west side of the axis were explored and sampled by submersible. The off-axis increase of weathering, Fe-Mn coating and magnetic signature suggest that the volcanoes were built at or near the ridge axis. The spacing of NS elongated hills bearing circular volcanoes and separated large magnetic signature (2 to 4 km) depressions suggests that several similar volcanic events occured during the past 2 Ma. The last 1 Ma episode involves (1) the construction of an axial ridge (MR) by fissure eruptions and the formation of circular summit volcanoes by focused volcanism, and (2) the extrusion of fluid magma in the depressions formed by further fissuring and faulting of the MR.     

Chicxulub impact ejecta from Albion Island, Belize

Impact ejecta from the Albion Formation are exposed in northern Belize. The ejecta come from the outer portion of the continuous ejecta blanket of the Chicxulub crater, which is located 360 km to the northwest. The basal unit of the Albion Formation is a 1-m-thick clay and dolomite spheroid bed composed of up to four discrete flows. The clay spheroids are altered impact glass, and the dolomite spheroids are accretionary lapilli. The upper unit is a 15-m-thick coarse diamictite bed containing altered glass, large accretionary blocks, striated, polished, and impacted cobbles, and rare shocked quartz. The abundance of accretionary clasts, evidence for atmospheric drag sorting, and the presence of multiple flows in the Albion Formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters.     

太湖冲击坑溅射物的发现及其意义

太湖冲击成因说由来已久,但始终未成定论。近几年在太湖及周边湖泊的淤泥层中发现了许多形态各异的奇石,经多种方法测试研究,确定为太湖冲击坑的溅射物。根据成份,溅射物分为两大类。一类富铁质,以菱铁矿及其胶结的碎屑为主,包括微小球粒、棍状及各种形态的块状和片状体 ;另一类贫铁质,以方解石及其胶结的长英质碎屑为主,碎屑为锐角状的石英晶屑及少量粘土和长英质岩屑。溅射物的大小从厘米级块体到毫米级球粒,再至微米级尘粉都有。溅射物外形多具有旋转扭曲形态及熔壳特征,显示了熔融、塑性- 半塑性特征。这些特征显示其成因经历了冲击震碎、熔融、挖掘抛射、空中飞行,最后溅落在冲击坑及其周围。溅射物的成分反映了太湖靶岩基岩岩性特征。太湖冲击坑溅射物的发现是继太湖诸岛石英砂岩中石英晶体的冲击变质微结构发现之后,又一重大突破。综合其它特征,可以确定太湖为一冲击坑。     

New records of Late Neoproterozoic Acraman ejecta in the Officer Basin

New occurrences of the Acraman impact ejecta layer were recently discovered in two drillholes, Giles 1 and Murnaroo 1, in the eastern Officer Basin, South Australia, using acritarch biostratigraphy and lithostratigraphy to predict the position. Fractured crystals were also observed in palynological preparations from drillhole SCYW 1a on the Stuart Shelf. These discoveries improve stratigraphic control of Late Neoproterozoic successions, especially in the eastern Officer Basin and will allow a test of the hypothesis that the Acraman impact event caused a global catastrophe.     

Discovery of a layer of probable impact melt spherules in the Late Archaean Jeerinah Formation,Fortescue Group,Western Australia

In the high‐grade (granulite facies) metamorphic rocks at Broken Hill the foliation is deformed by two groups of folds. Group 1 folds have an axial‐plane schistosity and a sillimanite lineation parallel to their fold axes; the foliation has been transposed into the plane of the schistosity by these folds. Group 2 folds deform the schistosity and distort the sillimanite lineation so that it now lies in a plane. Both groups of folds are developed as large folds. The retrograde schist zones are zones in which new fold structures have formed. These structures deform Group 1 and Group 2 folds and are associated with the formation of a new schistosity and strain‐slip cleavage. The interface between ore and gneiss is folded about Group 1 axial planes but about axes different from those in the foliation in the gneiss. On the basis of this, the orebody could not have been parallel to the foliation prior to the first recognizable structural and metamorphic events at Broken Hill. The orebody has been deformed by Group 2 and later structures.     

论“太湖冲击坑溅射物”——来自太湖现代沉积物中铁质结核的证据

针对太湖发现由菱铁矿组成的“铁质溅射物”和“贫铁溅射物”是太湖形成于陨石冲击的观点,本文对这些实为铁
质结核样品进行了详细的矿物学、地球化学和年代学测试。数据表明,这些样品是在太湖现代沉积淤泥层中形成的菱铁矿
和褐铁矿结核。结核中含有大量水生植物碎片、植物蛋白石、细菌和少量动物壳体碎片。菱铁矿高温灼烧后全部变成赤铁
矿,说明太湖地区不存在菱铁矿熔融状的高温相。菱铁矿14C测年结果表明各个样品形成于年代,不是瞬间形成的。δ13C 数
据指示为淡水无机碳来源,并非湖区碳酸盐岩直接来源。所谓的“贫铁溅射物”实际是黄土层的成岩钙结核。这些证据表明,
太湖发现的这些结核是沉积成岩期的产物,并非冲击高温溅射熔融形成的溅射物。